STRUCTURAL AND TECTONIC ANALYSIS OF THESYLVESTER ALLOCHTHON, NORTHERN BRITISH COLUMBIA:

IMPLICATIONS FOR PALEOGEOGRAPHY AND ACCRETION

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Harms, Tekla Ann

STRUCTURAL AND TECTONIC ANALYSIS OF THE SYLVESTER ALLOCHTHON, NORTHERN BRITISH COLUMBIA: IMPLICATIONS FOR PALEOGEOGRAPHY AND ACCRETION

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STRUCTURAL AND TECTONIC ANALYSIS

OF THE SYLVESTER ALLOCHTHON, NORTHERN BRITISH COLUMBIA:

IMPLICATIONS FOR PALEOGEOGRAPHY AND ACCRETION

by

Tekla Ann Harms

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fullfillment of the Requirements For the Degree of

DOCTOR OF PHILOSOPHY

In the Graduate College

THE UNIVERSITY OF ARIZONA

198 6

THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE

As members of the Final Examination Committee, we certify that we have read

the dissertation prepared by ____ T_e_k_l_a __ A_" __ H_a_rm __ s __________________________ __

entitled Structural and tectonic analysis of the --------------------------~~----------------------------

Sylvester Allochthon, northern British Columbia:

Implications for paleogeography and accretion

and recommend that it be accepted as fulfilling the dissertation requirement

for the Degree of Doctor of Philosophy ------------------~~----------------------------

Date

Final approval and acceptance of this dissertation is contingent upon the candidate's submission of the final copy of the dissertation to the Graduate College"

have read this dissertation prepared under my that it be accepted as fulfilling the dissertation

STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgement the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED:

ACKNOWLEDGEMENTS

It is indeed a great honor and a sincere pleasure to acknowledge

the contribution of Peter Coney and Hubert Gabrielse to this work.

While they can, and may wish to be excused from association with the

conclusions presented here, they are responsible for p~oviding

extraordinary intellectual and economic support which allowed this

project to be conducted at all.

The co-operation of Mike Orchard of the Geological Survey of

Canada and of David Jones and Benita Murchey of the U.S. Geological

Survey in microfossil dating was crucial to the substantiation of ~his

thesis and is greatfully acknowledged. In addition, my many colleagues

at the Univer~ity of Arizona, and at the Geological Survey of Canada in

Vancouver provided stimulating exchanges and a conducive atmosphere

which greatly benefited my work. Stephen Wust, Lisel Currie, Jennifer

O'Brien and Susie Gareau all gave able, cheerful, unflagging and patient

field assistence under even the most trying conditions; they were

invaluable. Finally, I would like to acknowledge the hospitality of

Brinco Mining Ltd., Cassiar; Regional Resources Ltd., Midway; and

Erickson Gold Mining Corp., Cassiar and their employees.

This work was supported in.part by the Laboratory of

Geotectonics, Department of Geosciences, University of Arizona, and in

large measure by the U.S. Department of Energy through contract

DE-AC-21-84MC21l29. Significant critical logistical support was

provided by the Geological Survey of Canada, Cordilleran Division.

iii

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS. • v

LIST OF TABLES . • vi

ABSTRACT ••• .vii

INTRODUCTION • 1 Previous ·Work. . • • • 4 Age of Sylvester Allochthon Emplacement. • • • • • . • . . 5 Structural and Tectonic Analysis of the Sylvester Allochthon . . 8

LITHOLOGIC COMPOSITION OF THE SYLVESTER ALLOCHTHON • Lithotectonic Units. . . . . •

Package I ••. Package II • • . • Package III. • • Package IV ••• Units Outside the Detailed Study Area. .

The Sylvester Assemblage • • . . • . • • .

INTERNAL STRUCTURE OF THE SYLVESTER ALLOCHTHON • . Pre-Emplacement Thrusting.

BASAL SYLVESTER FAULT. • . . . • .

SUB-SYLVESTER AUTOCHTHON STRUCTURES.

CORRELATIVE TERRANES • • . • . . .

TECTONIC ANALYSIS: PALEOGEOGRAPHIC AND PALEOGEODYNAMIC INTERPRETATIONS . • . • .

StMt1ARY. • ••

REFERENCES • •

iv

• • • • • 9 · • . . • 11 · . • . • 11 · • • . • 12

• 15 • 17

· • . . • 18 o. 19

• 26 • 32

• 39

· 44

· 55

· 62

• 70

· 72

LIST OF ILLUSTRATIONS

Page

Figure

1. Location of Study Area and Regional Geologic Setting . • • • • • 2

2. View of a Portion of the Sylvester Allochthon and Cassiar Platform Autochthon. • • • • • • • . . • • • • • 3

3. Lithotectonic Map of the Sylvester Allochthon Study Area • .• • • • • • •

4. Chronostratigraphic Summary Chart. •

5. 'Sylvester Allochthon Structure Cross Section .

6. Characteristic Chert Deformation . .

7. Synoptic Stereonet of Sylvester Allochthon Lineations.

8. Schematic Sylvester Allochthon Structural Style. .

9. Pre-Emplacement Thrusting. . •

10. 206Pb*/238U_207Pb*/235U Concordia Diagram for Unit PIIt Tonalite

11. Autochthon Lineations. •

12. NE Verging Structures in the McDame Group below the Sylvester Allochthon • • . .

13. Cassiar"Platform Miogeocline Stratigraphic Column.

14. Sub-Sylvester Autochthon Duplex System •

15. View of the Sub~Sylvester Duplex System.

16.

17.

Duplex System Structures . . ".

Suite of Sylvester-Correlative Terranes in the Cordilleran Collage ....

v

• in pocket

23

27

29

31

33

35

36

42

43

45

48

49

50

56

LIST OF TABLES

Page

Table

1. Age Assignments from Microfossil Collections. . . . • . • . . . 10

vi

ABSTRACT

In northern British Columbia, the Sylvester Allochthon of the

Slide Mountain terrane is the most inboard of Cordilleran suspect

terranes, resting as a vast klippe upon miogeoclinal strata of the

Cassiar Platform. The Sylvester is oceanic; it comprises gabbro,

pillowed and massive basalt, banded chert, carbonate, argillite,

ultramafics and minor arenite, which range in age from Late Devonian to

Late Triassic.

Internal structure in the Sylvester Allochthon is characterized

as a stack of innumerable interleaved tectonic slices, bounded by

subhorizontal, layer-parallel faults. These lithotectonic units are an

order of magnitude smaller than the terrane itself and ma~' consist of

only a single or a few repeated rock types. The internal structure of

the Sylvester is complex but not chaotic; small numbers of slices occur

together in larger second-order packages which are also fault-bounded

and lensoidal. However, tectonic juxtaposition of unrelated lithologies

and older-over-younger faults are common. The "stratigraphy" of t·he

Sylvester assemblage is thus tectonic.

Sliver-bounding ~aulting within the Sylvester is known to have,

at least in part, predated its post-Triassic, pre-mid Cretaceous

emplacement. The Sylvester was emplaced onto North America as the roof

thrust to a foreland-style duplex within underlying North American

strata.

vii

viii

The Sylvester Allochthon is the most inboard of accreted

terranes, however it does not represent a simple marginal basin. New

microfossil dating demonstrates that most rock types occur through the

complete range of Sylvester ages. Coeval but depositionally

incompatable lithologies must have accumulated in separate ocean floor

paleoenvironments. Lithologies of the allochthon derive almost

exclusively from layer 1, only the surface of oceanic crust. Thus,

Sylvester slices are telescoped remnants detached from a vast area of

ocean crust which ranged in age and width through the upper Paleozoic

but which is now otherwise entirely consumed.

Similarities of rock type, internal structure, age range, and

regional tectonic setting have identified the Sylvester Allochthon as

broadly correlative with a discontinuous series of terranes extending

the length of the Cordillera. Together, these terranes may represent

the remnants of what was once the late Paleozoic proto-Pacific ocean

floor.

INTRODUCTION

The Sylvester Allochthon is a late Paleozoic assemblage of

oceanic rocks; basalt, gabbro, argillite, chert, carbonate and

ultramafic, which overlies Paleozoic miogeoclinal carbonate and clastic

strata of the Cassiar Platform in northern British Columbia. The

Sylvester Allochthon and Cassiar Platfonn are exposed in the Cassiar

Mountains, west of the Northern Rocky Mountain Trench, within the

Omineca Belt of the Canadian Cordillera (Figure 1). The Cassiar

Platform is a displaced part of the Proterozoic and Paleozoic North

American continental margin, offset approximately 900 km northward along

the dextral Northern Rocky Mountain Trench Fault in Late Cretaceous and

Eocene time (Gabrielse, 1985). Cassiar Platform stratigraphy flanks the

thin and elongate body of Sylvester rocks on all sides, and is

continuous beneath it: the Sylvester Allochthon is a vast rootless

klippe of exotic oceanic rocks (Figure 2).

It is now widely accepted that within the Cordillera,

assemblages which lie outboard of the ancient North American rifted­

margin, miogeoclinal wedge constitute accreted or suspect terranes

(Coney, Jones and Monger, 1980; Monger and Price, 1979). Each suspect

terrane is a discrete entity, with internally consistent stratigraphy,

structural character and geologic history which are discordant to that

of its neighbors and to that of North America (Jones, Howell, Coney and

Monger, 1983). Thus, the juxtaposition of exotic oceanic rocks of the

Sylvester above North American continental margin carbonates

1

FIGURE 1.

//////////////////////// ///////////////////////

~~~~~~~~~~~~~~~;.~:~~::~~:: ///////////////~////// //////////////v//////

////////////v////// ////////////~//////

""""""f""" //,//////// /////,

~~';';'~~~h;6i!;:: / / McDAME j / / / / / / ////// ////// ,MAP ARE!- / / / / / / //////,l////// /////'a////'/ '///// ////// '//// //////

// ///// //// //// ////

///// ///// /////

Location of Study Area and Regional Geologic Setting

//// //// //// //// /// /// ///

2

FIGURE 2. View of a Portion of the Sylvester Allochthon and Cassiar Platfona Autochthon. Looking NW. The darker oceanic rocks of the Sylvester Allochthon are juxt&posed against the underlying light grey carbonates of the aiogeocline across the horizontal basal Sylvester fault.

Co)

4

distinguishes it as an accreted terrane, and as the most inboard terrane

in the Cordilleran collage. Attempts to unravel Cordilleran history

through terrane analysis now must involve considerations of the pre­

emplacement history of a terrane, the relative, if not absolute,

paleogeography of the terrane through time, and the nature and timing of

terrane accretion and related deformation. However, few of these issues

have heretofore been addressed in the Sylvester Allochthon. Yet, as the

Sylvester is the m6st inboard of accreted terranes, answers ·to these

questions are not only of importance to continental margin tectonics,

but also impact upon paleogeographic interpretations of perhaps better

studied, more outboard terranes.

Gabrie1se (1963) first established the Sylvester Group in McDame

map area; Sylvester Group rocks were subsequently outlined in adjacent

Jennings River (Gabrie1se, 1970) and Cry Lake (Gabrie1se, 1979) map

areas. The Sylvester was recognized as distinct from underlying

carbonates and clastics on the basis of its eugeosynclina1 character,

however Gabrielse (1963) considered it to be in overall stratigraphic

continuity. While Gabrielse documented the range of rock types included

in the Sylvester Group, he established only one unit, the Nizi

Formation, which he believed lay at or near the top of the Sylvester

Group. When Mamet and Gabrie1se (1969) dated the Nizi Formation as

Chesterian, the Sylvester was felt to be bracketed between the Devonian

age of the underlying units, and the mid-Mississippian Nizi Formation.

5

During the initial identification of Cordilleran suspect

terranes, Monger (1977) recognized contrasting Paleozoic sequences in

the eugeosynclinal domain of the Canadian Cordillera and grouped the

Sylvester with a number of other inboard .oceanic units of similar age

into the Eastern Assemblages. Monger (1977) suggested some depositional

ties between lower, sedimentary Eastern Assemblage units with a

foundering continental margin, but gave evidence that overlying

volcanics were obducted oceanic rocks. Gabrielse and Mansy (1980), on

structural grounds, specifically suggested al10chthoneity of the

Sylvester Group. Subsequent more detailed mapping and sampling in a

transect in the Sylvester expanded its known age range and outlined

structural complexity and older-over-younger relationships in the

Sylvester substantiating its reinterpretation as an allochthonous

terrane (Gordey, Gabrie1se and Orchard, 1982). Monger's Eastern

Assemblages are now called the Slide Mountain Terrane (Monger and Berg,

1984). However, like Monger's original description of the Eastern

Assemblages, an inclination to associate the Slide Mountain Terrane with

the North American continental margin and to interpret the terrane

paleogeographica~ly as a marginal basin (Price, Monger and Roddick,

1985) has remained.

The time of emplacement of the Sylvester Allochthon is pooriy

constrained. The Sylvester assemblage, basal Sylvester fault, and

strata of the miogeocline are all intrUded by·the Cassiar Batholith

(Figure 1), one of a number of mid-Cretaceous (90-110 Ma) granites of

6

the Omineca Belt (Gabrielse and Reesor, 1974). The youngest unit known

to be included in the Sylvester Allochthon is a Triassic (Carnian)

carbonate (see Fossiliferous Limestone unit). From these relationships,

emplacement of the allochthon on the basal Sylvester fault must be

bracketed between latest Triassic and mid-Cretaceous.

Closer timing of emplacement of the Sylvester has been attempted

by correlation with other parts of the Slide Mountain Terrane and by

analogy with constraints known to apply to outboard accreted terranes.

Regionally, in southern British Columbia, large-scale west-vergent

defo~ation, responsible for the present map pattern, is late syn­

metamorphic to post-metamorphic (Ross et aI, 1985; Archibald et aI,

1983; Parrish, 1979). Peak metamorphism there has been dated as 165-155

Ma (Parrish, 1979; Archibald et aI, 1983). Some allochthons of the

Slide Mountain Terrane south of the Sylvester are believed to be

involved in the west-vergent deformation (the Crooked Amphibolite and

the Antler Group, Ross et al., 1985; Struik, 1982) or effected by late

syn-metamorphic plutonism (the Milford and Kaslo formations, Archibald

et al., 1983). This has led to the association of emplacement of the

Slide Mountain Terrane with or just prior to the episode of west-vergent

defo~ation and so to timing of obduction as mid to late Jurassic (Brown

and Gabrielse, 1983; Archibald et al., 1983; Ross et al., 1985).

However, the author querries the strength of the correlation of the

effected units with the Slide Mountain Terrane (discussed below) and so

the rest of the Slide Mountain, the Sylvester Allochthon included, may

7

be only tenuously associated with timed deformation and metamorphic

events.

In an even broader mode, the emplacement of the Slide Mountain •

Terrane has been timed by the arrival of the more outboard Stikinia,

Cache Creek and Quesnellia terranes. Monger, Price and Templeman-Kluit

(1982) suggest that these three terranes were amalgamated by the end of

the Triassic into composite Terrane I. Minimum ages of Upper Jurassic

and of Lower Cretaceous for first amalgamation then collision

respectively are derived from clast provenance relations in the terrane-

specific Bowser Basin on Stikinia (Monger, Price and Templeman-Kluit,

1982). However, because the suture zone between composite Terrane I and

North America coincides with the metamorphic welt of the Omineca Belt,

Monger, Price and Templeman-Kluit propose that deformation, burial, and

metamorphism in the Omineca Belt correlates with collision ~nd accretion

of Terrane I. Thus, emplacement of the Slide Mountain Terrane, because

it is sandwiched inboard of Terrane I, is once again timed by inference

with mid to late Jurassic metamorphism in southern British Columbia.

Howev~r, little to no evidence ties the Slide Mountain Terrane with

amalgamation of Terrane I, and none of the clast-provenance evidence

used to directly time arrival of Terrane I bears specifically on the

Slide Mountain Terrane.

In summary, whatever the weaknesses of each individual

arguement! none have been mustered for an emplacement age for the Slide

Mountain Terrane other than mid to late Jurassic, and this lies well

within the latest Triassic to mid-Cretaceous bracket determined locally

for the Sylvester Allochthon.

8

Structural ~g Tectonic Ana1ysi~ of th~ Syl~ster Allochthon

The results of new, detailed (1:50,000) mapping, structural

analysis,and paleontologic sampling in the Sylvester Allochthon from a

study area in Cry Lake map area (Figure 1) expand the present

understanding of the nature and history of the Sylvester Allochthon and

readdress paleogeographic interpretations of the Slide Mountain Terrane.

The work reported here will show that the Sylvester assemblage comprises

oceanic rock types ranging in age from Late Devonian to Late Triassic.

As mapped, complex lithologic juxtapositions demonstrate that the

Sylvester "stratigraphy" is tectonic; the internal structural style of

the allochthon can be characterized as a nested stack of interleaved

lithotectonic slices. The Sylvester Allochthon will be correlated not

only with the Slide Mountain Terrane, but also with similar terranes in

the Brooks Range in Alaska and with the Golconda Allochthon in Nevada,

to show that together these terranes represent the telescoping ofa vast

amount of Paleozoic ocean floor.

LITHOLOGIC COMPOSITION OF THE SYLVESTER ALLLOCHTHON

The remarkable"and distinctive juxtaposition of units in the

Sylvester Allochthon forms the basis of structural conclusions and

tectonic interpretations. The lithologic and structural fabric of this

unusual assemblage is map-scale in nature: a 1:50,000 scale map of the

detailed Sylvester study area is a primary element of the data base (see

Figure 3, in pocket). Map units used in the study area are

lithotectonic rather than conventional stratigraphic formations. They

may include one or more rock type, they range in thickness and lateral

extent from hundreds of meters up to kilometers, and each lithotectonic

unit is distinguished by more-or-less layer-parallel bounding faults.

All rocks of the Sylvester Allochthon comprise the Sylvester Group as

defined by Gabrielse (1963). However, in order to emphasize the

tectonostratigraphic nature of the analysis presented here, the

lithotectonic map units of the Sylvester Allochthon will informatly be

referred to as the Sylvester assemblage.

Because the characteristics of the mapped units are critical to

interpretation, brief descriptions are included in the following

section. Also described briefly are important and distinctive units

outside the mapped area. Not included are units of the Sylvester mapped

and described by Gordey (Gordey, Gabrielse, and Orchard, 1982).

Reported here also are new microfossil ages from sampling

conducted in the course of this program and of fundamental importance to

the Sylvester stratigraphic and structural study (Table 1). Radiolarian

9

TABLE l. Age Assignments from Microfossil Collections

Stmple Age Micro- Lithology Unit Stmple Age Micro- Lithology Unit Site FOllllil Site Fossil

1 not Devonian conodont Fetid Dol_ite NAIl 22 L Devonian- radiolarian black banded chert DMlld Mississippian

2 M-L Devonian conodont Fetid Dol_ite NAIl 23 L Devonian radiolarian black banded chert IlMUd

3 8 Frasnian conodont Fetid Dol_i te NAIl 24 M Devonian (?) radiolarian black banded chert IlMlld

4 8. Frasnian conodont Platy Li_stone NAIl 25 8 (?) Pel"llian radiolarian black chert PUa

5 L Tournasian conodont Black Clastic NAIo .

26 L Pel"llian radiolarian green chert PlIa 6 M Devonian- conodont Black Clastic NAIo

B Misllillllippian 27 Morrowan- conodont Pennsylvanian Is IPU. Atokan

7 8-M Tournaaian conodont Black Clastic NAIl 28 Morrowan conodont Pennsylvanian 1. IPIl.

8 8-M Tournasian conodont Black Clastic NAIl Atokan

9 Pennsylvanian conodont banded chert IPIb 29 8 Pennsylvanian conodont Pennsylvanian la IPU.

10 L Tournasian conodont Is in argillite MIa 30 8 (?) conodont Pennsylvanian Is IPIl. Pennsylvanian

11 TournB8ian- conodont Is in argillite MIa Viaean 31 Pennsylvanian- conodont Pennsylvanian Is IPIl.

basal Pel"llian 12 M Misllissippian radiolarian black cherl. HlIld

32 Pennaylvanian radiolE.rian white chert MPlIlc 13 L Mississippian radiolarian white chert HPlllc

33 L Pennllylvanian radiolllrian black arg with MPIIlb 14 M Miallisllippian radiolarian grey-green chert MPIlIc (?) - Pel"llian chert pods

15 Mississippian- radiolarian ,rey-green chert MPIllc 34 Pennaylvanian- radioillrian olive nodular chert MPIIlb Pennsylvanian boundary 8 Pel"llian

16 Miaaissippian- radiolarian grey-green chert MPlllc 35 Missillsippian radioillrian black arg with MPIllb Pennsylvanian boundary chert beds

17 8-L Pel"llian radiolarian red chert MPIIlc Outside Detailed Study Area boundary

Tournaaian conodonl carbonate pod Caa~iar

18 Missi .. ippian- radiolarian white chert MPlllc Greenlltvne Pennllylvanian boundary

Per.ian conodont green chert Cassiar

19 Pel"llian radiolarian red chert MPIlIc (-Pennllylvanian?) Greenstone

20 Misllillsippian radiolarian white chert MPIIlc L Tournasian conodont black 1i_lItone Midway Liaaestone

21 Millllillllippian rlldiolarian white cherl HPlllc ..... Q

11

separation and identification was conducted by the author with co­

operation from D. Jones and B. Murchey of the u.s. Geological Survey in

Menlo Park. All conodont identification was made by M. Orchard of the

Geological Survey of Canada (GSC), Cordilleran Division, Vancouver;

references to conodont ages in the following and succeding sections are

sourced from Orchard.

a joint contribution.

Package I

Microfossil fauna will be described elsewhere in

Ynit MIa. Predominately black argillite and siliceous argillite.

It includes interbedded light grey or more commonly black chert members

which are in most places thin and discontinuous but become thicker to

the south. All are non-fossiliferous. Unit MIa also includes vesicular

basalt with a few poor pillow forms. North of the Major Hart River the

unit includes a thick carbonate member which yielded Mississippian

conodonts. Argillite is ubiquitously cleaved, with common pencil

cleavage, so that original bedding is lost. Evidence of shearing is

common and the variety of rock types present in the unit may reflect

tectonism rather than an initial depositional relationship. Unit MIa is

discontinuous and variable in thickness, ranging up to 200-250 m.

Unii Ia~. Massive fine-grained green gabbro. This unit

generally caps unit MIa, and in places clearly intruded the argillite.

Elsewhere contacts with gabbro appear faulted. Unit Iaa is 300-400 m

12

thick, massive, and continuous, but thin (1-2 em) discrete ductile shear

zones were observed within it.

Yni1 !pI~. Banded, orange and green chert. In a few locations

the chert is massive, but generally it is well bedded in 5-10 em beds

with 1-2 em cleaved argillite interbeds. The chert can be pea green or

grey, but commonly it is a distinctive orange color with pea green

argillite interbeds. In some places it grades downward into a basal

member of black banded chert. Unit IPIb is commonly cleaved or

mullioned and ubiquitously folded into open to tight and isoclinal,

cascading, outcrop-scale (2-5 m) fold trains (see Figure 6). Both

similar and parallel fold forms occur. The unit has been fairly

uniformly recrystallized to a fine, sugary quartizitic texture whic~ may

include pyrite grains. No radiolarians survive, however conodonts were

recovered which yield a Pennsylvanian age. Unit IPIb is variable in

thickness and discontinuous along strike. It is thickest (200+ m) at

its occurrance south of the Major Hart River, however it is with

certainty tectonically thickened.

Y~!1 !£. Dark brown and black massive and vesicular basalt.

Unit Ic includes only minor black chert and argillite at its base. Unit

Ic may be related to unit MIa, however it is considerably thicker (300-

400 m) and is dominately volcanic rather than argillitic.

Package II

Un11 ~!!~. Green interbedded chert, tuff and basalt. This unit

is extremely well layered, with 15-60 em beds of basalt, radiolarian

chert, siliceous argillite, tuff, and volcaniclastics (1-5 mm grain

size). All are sea green to olive to dark green. The volcanic rocks

are more massive near the base of the unit and include rare pillow

forms. The green cherts yielded abundant Late Permian radiolaria.

13

South of the Rapid River a black banded chert sequence occurs at the top

of unit PlIa which also yielded Early(?) Permian radiolaria. Unit PIIa

is one of the largest units in the Sylvester; it is as much as 2000 m

thick and 25-30 km long.

YBi1 lIb. Black argillite. This thin (20 m) argillite unit

contains minor grey very fine-grained sandstone horizons. Both can be

hornfelsed where in contact with unit Pllt tonalite.

Unit 11£. Undated interlayered chert, sandstone and carbonate.

Interlayered in beds 5-10 m thick, this unit contains black chert, black

argillite, micritic carbonate, one lens of clean white quartzite and one

of milky quartz grit in an olive argillaceous matrix. No fossils were

recovered. The unit may be structurally inter layered on a very fine

scale but no positive evidence was found to document this. Black chert

of the unit may correlate with chert of Unit DMlld.

YBi1 DMllg. Devono-Mississippian black banded chert. A thick

(200-300 m) sequence composed entirely of well banded (5-10 em) chert

with little to no argillite interbeds. Only minor mesoscopic folding

was observed but the unit may, nevertheless, be tectonically thickened.

Samples from throughout the unit gave a radiolarian fauna which includes

ages of not younger than Mid to Late Devonian and not older than Late

Devonian.

14

Unit lIe. Siliceous tectonite. This unit is a strongly

foliated and weakly lineated siliceous tectonite. It includes steel-

grey shale and light green phyllite but is predominated by finely

laminated (2-5 mm) green chloritic siliceous tectonite. Small (2-15 em

amplitude) tight folds are common. In a few places the tectonite is

intruded by unfoliated aphanitic diabase.

Unit Pllt. Light grey-green megacrystic tonalite. The tonalite

includes quartz and plagioclase phenocrysts (2-4 em) in a very fine-

grained light grey to green matrix. Phenocryst composition is 40%

quartz, 50% plagioclase and 10% biotite and amphibole. The tonalite has

been dated on both zircon and sphene separates by J. Mortensen and gives

a U-Pb age of 276 ± 6 Ma (see Figure 10). Some contacts of the tonalite

are intrusive and others are faults. South of the Rapid River the

tonalite forms a large sill, over 1000 m thick; elsewhere it is a much

more irregularly shaped body. Unit Pllt is continuous for approximately o

20 km along strike. Near latitude 59 the tonalite includes two other

lithologies, entirely embedded within the intrusive mass as large

country rock remnants. These are; unit IIf, an undated grey chert, and

unit IIg, maroon and white thin rhyolite interlayered with red and buff

colored argillite and minor carbonate.

Unit MIln. Nizi Formation. The Nizi is at least 500 m thick,

and consists mainly of massive bedded carbonate. It has a distinctive

basal member which is a carbonate cemented, bioclastic, matrix supported

sandstone gradational to a carbonate cemented volcanic sandstone. Its

upper layers are characterized by the presence of large (5 cm diameter)

crinoid columnals. A maroon shale and siltstone member about 3 m thick

occurs in the upper part of the fonmation. The Nizi Fonmation has a

type section which has been well described by Mamet and Gabrielse

(1969). The Nizi has yielded both Chesterian foraminifera (Mamet and

Gabrielse, 1969) and Penmo-Carboniferous conodonts.

15

Ynit jPllm. Pennsylvanian limestone. Unit IPllm is

predominately a crystalline grey carbonate with abundant irregular

silica replacement bands, and includes a distinctive 1-2 m thick sea

green spiculitic chert member. It is in places well bedded and in

other places quite' massive. It includes rare, poorly preserved crinoid

columnals, and a Morrowan to Atokan conodont fauna. Unit IPIlm is

approximately 100 m thick.

Ynii IIp. Pillow basalt. This unit is unmapped and undated.

Reconnaissance shows it has many pillow forms. It mayor may not be

part of adjacent Unit Plla.

Package III

Unii !!!~. Dark brown to black argillite. Unit IlIa is

unmapped and undated. It may correlate with units i, ii, and iii of

Gordey, Gabrielse and Orchard (1982) along strike to the NW.

Unit MPIllb. Mississippian to Permian black chert sequence.

This approximately 800 m thick unit has a sequence of members, each less

than 50 m thick, from bottom to top as follows: light grey ribbon

~hert; black arenite, black chert, argillite and lithic wacke; black

argillite, siliceous argillite and finely laminated black chert beds 10

em thick (Mississippian); olive green nodular chert (Pennsylvanian or

16

early Penaian); dark grey blocky quartzite; blocky, irregularly cleaved

black argillite with discontinuous layers, pods, and nodules of black

chert (late Pennsylvanian(?) to early Permian). The sequence may be

internally faulted as it contains several strike-parallel serpentinite

slivers. The entire succession is pervasively intruded by a very fine­

grained green diorite.

Uni1 MfIII£. Mississippian to Permian red, green and white

chert. Unit MPlllc may have black argillite and siliceous argillite at

its base but it is predominately a continuous chert.sequence. Chert at

the base of the unit is white to grey with a Mississippian fauna visable

to the eye; white chert grades upward into grey and grey-green chert

with a Mississippian-Pennsylvanian boundary fauna;-the unit is capped by

a poorly bedded, brick red clay-rich chert of Early-Late Permian

boundary age. Like unit MPlllb, unit MPlllc is extensively intruded by

bodies of dark green diorite or gabbro which range in size from

hypabyssal seams in chert, to including chert as septa in large bodies,

to peak-capping massifs of gabbro. Unit MPlllc is also internally

folded and faulted; it can lie at steep dips and includes slivers to

mappable sized bodies of serpentinite. Thus, the white-gre~n-red chert

sequence may occur more than once across the strike of unit MPlllc.

Therefore, with the intrusive diorite component, the thickness of unit

MPlllc (approximately 3000 m) is significantly greater than the

thickness of the Mississippian-Permian chert sequence (150-200 m) alone.

Unit MPlllc is at least 30 km long.

Yni! Mlllg. Basalt with minor interlayered chert.

Predominately a massive basalt unit with fairly common poor pillow

17

shapes and possible pillow breccia piles. Intermittent grey and black

chert occurs, some with baked contacts with the basalt. One chert

sample from unit Mllld contained spicules only, another contained a poor

mid-Mississippian fauna. The unit includes common slivers of dark green

serpentinite.

Yni~ IIIe. Serpentinite. Many small discontinuous slivers

(less than 10 m wide) of reticulated antigorite serpentinite occur

within and between various units of package III. Those which are large

enough to map separately are included in unit IIIe. All of the

serpentinite appears to occur as pods and lenses along faults.

Yni~ IIIl. Massive gabbro. This very fine grained, dark green

to black massive gabbro mayor may not be in fault contact with other

units.

Yni~ ll!g. Black clastic. Unit IIIg is an undated black

argillite that includes black siliciclastic arenite and a member of

volcaniclastic conglomerate. The unit also includes some

unfossiliferous carbonate lenses.

Package IV

Package IV is largely unmapped and is only known from spot

observations.

Yni~ IV~. Foliated hornblende diorite.

Yni~ IYQ. Altered ultramafic and serpentinite.

Yni~ IVc. Equigranular, isotropic, green, biotite-bearing

quartz diorite.

18

Units Outside the Detailed Study Area

Q~!iar g~~to~ ~ng Qh~~. Highly altered and commonly

sheared greenstone with inter layered green and grey siliceous argillite

and chert, and minor carbonate pods. The unit occurs in the mine at the

Cassiar property of Brinco Mining Resources Ltd, at Cassiar, B.C. It

has been well described by Gabrielse (1963). A green chert within the

unit yielded Permian (-Pennsylvanian?) conodonts and a small carbonate

pod Tournasian conodonts.

~idw!I Lim~!tone. Thin (20 m) black crystalline limestone with

pervasive black replacement chert. The unit occurs at the north end of o

the Sylvester Allochthon, along latitude 60 , on Si1vertip Mountain.

Locally, it occurs near the base of the Sylvester, and is interleaved

with thin chert and argillite units. Late Tournasian conodonts were

recovered from the unit.

Fo!!ilif~QY! ~imestQn~. Described by Gabrielse (1963) this

unit occurs north of Cassiar, near the Blue River. Gabrie1se (1963 and

personal communication) reports the occurrance of ~~!Qbi~ and

ichthyosaur vertebrae of Late Triassic, possibly Carnian age.

f~r~fysu!in! ~i~stQn~. Described by Gabrielse (1963) the unit o

occurs in the northern Sylvester at approximately latitude 59 30', The

limestone contains abundant Permian (lower Guadalupian) fusulinids of

the genus Par~!usulin~, described by Ross (1969), which are known to

occur elsewhere only in northern California and near Kettle Falls,

Washington, at El Antimonio, Sonora, and in the Word Formation, west

Texas (Ross, 1969; Ross and Ross, 1983), all but the last also in

19

suspect terranes.

Blu~ River Ultr~afic. Described by Wolfe (1965) and Pinsent

and Hirst (1977), it is a complex body of mainly dunite and peridotite

but also including talc, serpentinite, diorite, gabbro and rodingite.

The Blue River Ultramafic may be as much as 500 m thick and is several

kilometers in length. Gabrielse (1963) and Pinsent and Hirst (1977)

also briefly describe the Zus Mountain ultramafic body which is similar

in size and composition. The Blue River and Zus Mountain ultramafics

are the largest ultramafic units in the Sylvester Allochthon.

The Sylvester Allochthon comprises a limited suite of rock

types: argillite, bedded chert, massive and pillow basalt, and gabbro,

with less common carbonate, serpentinite and ultramafic, somewhat more

leucocratic plutonic rocks, and volcaniclastic and siliciclastic

arenites.

This assemblage clearly derives from oceanic, non-continental

sources. Pelagic micritic non-fossiliferous· carbonate, banded chert and

thick accumulations of monotonous hemipelagic argillite are usually

interpreted as deep ocean basin deposits. Pillow basalt a are submarine

extrusives. Major element chemistry on a few representative samples of

massive Sylvester basalt units mostly from outside the detailed study

area (Gabrielse, personal communication) is consistent with tholeiitic

composition, but no more specific genetic characterization can be made.

Although the Sylvester is oceanic in nature, it is not, however,

ophiolitic. The association of units in the Sylvester is not a shuffled

or dismembered ophiolite because virtually all units of the Sylvester

originated in only the sedimentary and extrusive veneer of oceanic

crust, in and above ophiolite layer 1 (Coleman, 1977). Pillow basalt

piles (unit 2 of Gordey, Gabrielse and Orchard, 1982) occur with

interpillow carbonate pods and interflow carbonate beds; massive basalts

occur with chert at their base (unit Mllld). Where their contacts are

not faults, Sylvester gabbros everywhere intrude a sedimentary pile

(unit Iaa, and units MPlllb and c). Thus Sylvester basalt and gabbro

cannot represent ophiolite layers 2 and 3, as their contact

relationships with sedimentary units suggest all may have orjginated in

volcanic edifaces built above the ocean floor. (Similarly, a seamount

origin has been proposed for basalts and gabbros of the Angayucham

terrane by Jones, Silberling and Coney, 1983; Murchey and Harris, 1985.)

Serpentinite" occurs scattered throughout the Sylvester as thin (commonly

only 1-10 m), discontinuous slivers localized along faults. The

Sylvester assemblage also includes several ultramafic bodies (e.g. the

Blue River, Zus Mountain, and Cry Lake bodies, outlined by Gabrielse,

1963, 1979; and Wolfe, 1965), each fault bounded and similar in size to

most other Sylvester units. These ultramafic units originated in the

lower layers of ophiolitic crust (although Saleeby, 1984, suggests

mechanisms for their emplacement on the ocean floor) but they do not

occur in volume commensurate with having floored the other upper crust

components of the Sylvester assemblage. Furthermore, sheeted dikes, and

cumulates and layered gabbros are conspicuously absent. Therefore the

21

Sylvester assemblage is best characterized as originating in layer 1 at

the top of oceanic crust. and any basement to these units that could

properly be called ophiolitic is nowhere included in the allochthon.

The Sylvester assemblage also contains a small volume and small

number of units which suggest a somewhat more complex environment of

deposition. Siliclastics. both fairly clean quartz sandstone and cherty

argillite arenite. occur as small tectonic slices. and as lenses and

layers within argillite units (units IIIg and IIc; and units 1 and III

of Gordey. Gabrielse and Orchard. 1982). The quartz sandstone in

particular requires a fairly evolved or truely continental source, which

suggests that the siliciclastic units derive from an ocean basin

environment characterized by sediment transport. Volicaniclastic or

tuffaceous units (unit PIla) may be related to some kind of island

magmatic activity. Thus. in the Sylvester assem~lage a broad range of

ocean floor paleoenvironments are represented. including these

transitional from oceanic-to-continental and oceanic-to-magmatic island.

The Sylvester was first interpreted as bracketed within the very

restricted age range between the Chesterian Nizi Formation believed to

be at or near the top of the Sylvester and the youngest underlying unit.

the Middle and Upper Devonian McDame Group and Black Clastic (Mamet and

Gabrielse, 1969; Monger, 1977). Gordey, Gabrielse and Orchard (1982)

presented the first evidence of a much more extensive age range,

Mississippian to Permian, for units of the Sylvester Allochthon. With

extensive additional sampling and microfossil dating reported here

(Table 1), the age range of units in the Sylvester is well documented

and now known to span Late Devonian to Triassic.

Figure 4 is a summary of all dated Sylvester assemblage units.

22

It is strikingly evident from Figure 4 that virtually all rock types

included in the Sylvester assemblage are represented throughout the

known age range of the Sylvester as a whole. Chert of Late Devonian age

and several cherts each of Mississippian, Pennsylvanian and Permian age

have been found. Several Mississippian, and.a Pennsylvanian, Permian,

and a Triassic carbonate are present. Neither Triassic chert nor

Devonian carbonate are as yet known from the Sylvester. Pillow and

massive basalt, and argillite have been dated only where interbedded

with carbonate or chert: they include Mississippian, Pennsylvanian and

Permian units of the former, and Mississippian and Permian units of the

latter. It should be noted that ultramafics, gabbro, and arenite from

the Sylvester are essentially undated. Nevertheless the assemblage

clearly consists of a small suite of oceanic rock types most or which

are represented in different units of ages throughout late Paleozoic to

Triassic time.

It is clear that some Sylvester lithotectonic units have had

quite independant tectonic histories. In its tectonite fabric, unit lIe

shows a style and degree of deformation not seen elsewhere in the

Sylvester. Banded chert of unit IPIb is strongly folded and mullioned,

and sufficiently recrystallized to have destroyed all radiolaria; in

contrast most other chert units are unrecrystallized and radiolarian­

bearing. Conodont coloration, however, is fairly consistent throughout

208

~CJ C:JC:}

~,.

'" w 245 ~ <

<' ...J

258 ;$

rf > .J

~ a: < w

286 ~

~"t" ~"t"

4." #'

f<,~ ~

t-

ffi :z: t,)

320 ~ t,) ~

"'" ~

~~ :S.

~Cj ~

~(;j

CQ

"0 t:S ::E 0

I-Z ::J

360 w ~ <

<' ...J

374 $ e

! ~ 387

~ > ..J a: < w

408

PACKAGE II

UNIT PDt TONALITE

:E ~

5 := c: t:I ::E ~

Z ;:)

1~

1 'J CI e, I-Z ::J

'" "'" ~ :!: IU Q.

~ ot- =:; 1UQ: ::! Q:IU 0 IU:Z: Q: >-u ~ :s. j..:. :!:u ""'t-IU"'" "'tQ: IUQ "'IU Q::::- "'t:z: 0 CQu

~~ !IS Qu CI ""''''t ~ ::!"'" Q.CQ I-Z "C

PACKAGE III

EXTENSIVE DIORITE /NTRUS/I ~ -,-r .> -:-" ;- - ... -

r .l '" JI.... ... "" J'\ • ~ '01 _ ~

~ ... I.. ~ r ',..

~.l'" v ..... >, ..)L ,.. ,.\ f" :- ..l ---'- ....

.......

•••• .I

::::- .., .... ,)

1 ~ ~ I I I

I

r.=:

~~ ~~ I I I

I······· .......

" < .::

< •.••

FIGURE 4. Chronostratigraphic SUImSl"Y Chart. Lithology and age range of all dated units from sch~tically indicate structural juxtsposition: in the detailed study area; units from Gordey I

-

I I I

1 I ....... " .. - ~

I I

g ....... <" .0.

I-CI) e:u Lui:: :tCl) u..q: ~ .... Uu ..q:~ .... -112:t

.D

a a. :E I-Z ::J

PACKAGE I

CI) .... ~ i ~ ~ ::! a e: ..q:

~ ~I-

I- Lull: Z LuLu e::t :::;) au

• Lu a ~ ..q: e: 0

f !:: Z ::>

FROM: GORDEY. GABRIELSE AND ORCHARD

( 1982)

l­II:

~

II •···~···1 ........

:::::::: I

y -c-

/ ~ [ill]

HI the Sylvester Allochthon. Lines with arrows )Os. Vertical lines separate units which are Dot r, Gabrielse and Orchard, (1982) are included.

NORTHERN SYLVESTER UNITS

vvvv~ vvvvv vvvvv vvvvv

-:::-:-:-:-

",,'vvvv vvvv

I-0: LU :I: o

CCocs ~LU enz eno <I­OCl)

Z LU LU 0: (!)

II CI) ~

en :::;)

o a: w u. :::; Ci) en o u. en

~

< z :::; ::J en ::J u. < a: < c.

en ...I

> < ;: c ~

I

23

the Sylvester Allochthon (M. Orchard, personal communication), which

suggests a shared allochthon-wide heating at some point.

24

The present ordering of units within the Sylvester assemblage is

complex and variable. The Sylvester assemblage is not chaotic but it is

non-systematic. Groups of units in close spatial association (packages,

described below) have no obvious genetic relation with each other, no

more so than with any other unit anywhere in the allochthon. Direct

juxtaposition of depositionally incompatible lithologies, such as

radiolarian chert and carbonate, exemplify this. Monger's (1977)

initial characterization of the Slide Mountain Terrane as dominately

sedimentary at its base and dominately extrusive at its top does not

bear out in the Sylvester assemblage. Although resistant basalt caps

many of the tallest peaks in the allochthon, chert, argillite,

carbonate, basalt and gabbro are all equally dispersed and juxtaposed

throughout the Sylvester sequence. Finally, whereas depositional

contacts are preserved within units, the contacts between all

lithotectonic units are layer-parallel faults. Older-over-younger as

well as younger-over-older sequences are common. The relative sequence

of units can be variable along strike; for example unit Mlln in places

directly overlies unit Plla, but unit IPllm discontinuously intervenes.

Anyone lithotectonic unit may occur in a series of discontinuous

slivers. On the basis of these observations, it is clear that the

"stratigraphy" of the Sylvester assemblage varies widely from place to

place and is of entirely tectonic origin.

Within the Sylvester aSsemblage there is no indication that any

two lithotectonic units started out in lateral facies continuity or in

vertical aggradational succession. As stated, many different rock types

are present in all ages. Thus there is no way to order Sylvester

lithotectonic units into one sequence. No real stratigraphic evolution

results simply by reshuffling Sylvester slices into an arbitrary or

artificial chronologic sequence (compare to Miller et al., 1984; Snyder

and Brueckner, 1983). No conventional chronostratigraphic

interpretation can be made; no basin history is determinable.

In summary, the Sylvester assemblage is composed of units

derived from a deep ocean floor, from volcanic and intrusive edifaces

constructed on that ocean floor and, to a lesser extent, from

transitional oceanic settings. Ophiolitic basement originally

associated with these ocean lithologies is not now included in the

allochthon. Most Sylvester rock types are present throughout the

allochthon's Late Devonian to Late Triassic age range. The different

units of the Sylvester Allochthon are now associated in an assemblage

which, while not chaotic, is characterized by a tectonic stratigraphy.

INTERNAL STRUCTURE OF THE SYLVESTER ALLOCHTHON

The Sylvester Allochthon comprises a stack of fault-bounded

lithotectonic slices (Harms, 1984; Gordey. Gabrielse and Orchard. 1982).

This structural style is a fundamental distinguishing characteristic of

the Sylvester; it pertains throughout the terrane as a consistent very

large scale structural fabric.

The lithotectonic slices which make up the Sylvester Allochthon

are generally tabular; they taper and pinch-out laterally. both along

and across strike. and so are grossly lensoidal. Each of these

structural units is an order of magnitude smaller than the terrane

itself. They can be remarkably thin. commonly consisting of only one.

or a few related or repeated lithologies suqh as inter layered

basalt/argillite and chert/argillite sequences. Sylvester slices range

in size from one of the largest. a chert/basalt/tuff unit which is in

places 2000 m thick and is continuous along strike for almost 20 km

(unit Plla). to a distinctively colored chert (unit IPlb) nowhere more

than 50-150 m thick but occurring discontinuously along strike for

approximately 10 km. It is clear that the lateral extent of the

structural units far exceeds their thicknesses.

Sylvester lithotectonic slices are bounded by layer-parallel.

subhorizontal. planar faults. They are not disposed like shingles or

fish-scales and so are not truely imbricate. Rather they are a nested

and interleaved pancake stack. with overlapping lateral pinch-outs

(Figure 5. particularly section C"-C"'). Mutually overlapping and

26

C

IYb

CJJ •• '. &: -, •• .-....... Ill.

E ............. .......... ,. DMnd ............ ............

C' c ..

ALTERED ULTRAMAFIC ~ BLACK CHERT, LIMESTONE AND SERPENTINITE

Dc AND SANDSTONE ---

FOLIA TED HORNBLENDE PDt DIORITE EARLY PERMIAN TONALITE

DEVONO-MISSISSIPPIAN BLACK BANDED CHERT tm MDn NIZI FM CARBONATE

-Utttttti lIb BLACK ARGILLITE

FIGURE 5. Sylvester Allochthon Structure Cross Sect Line of coaposite section C-C'-C"-C'" shown on Fi~ for jagged Unit PIlt tonalite contacts.

IDI1illill ................ ................. ................... ..................... .................... ...............

D D

ection. Figure 3 aap.

PIle PERMIAN GREEN LAYERED BASALT. TUFF AND CHERT

MISSISSIPPIAN MASSIVE AND Mmd

PILLOW BASALT WITH CHERT

me SERPENTINITE

MPmc MISS-PERM RED AND WHITE CHERT WITH DIORITE INTRUSIONS

All contacts are tectonic except

mg

lei

I?Ib

e'"

NORTH AMERICAN

MIOGEOCLINE

20 k ..

BLACK ARGILLITE AND CHERT ARENITE

MASSIVE GABBRO

BANDED PENNSYLVANIAN ORANGE AND GREEN CHERT

2000

~ • 0; E

1000

28

interleaved contacts pertain both along strike of structural trend and

can be projected up and down dip, and are responsible for the

distinctive and characteristic map pattern (Figure 3) of the Sylvester.

In such a stacking, a unit may be structurally highest locally, but no

one unit can be said to overlie all others. The Nizi Formation, thus,

does not cap the Sylvester assemblage. Locally, sfices and their

bounding faults may have domains of consistent dip; westerly dips

predominate in the map area and very shallow east dips are common around

Cassiar. However, regionally a sub-horizontal interleaved style

predominates.

As noted, Sylvester structural units are generally tabular and

their bounding faults are planar; significant deformation of either is

unrecognized. Most lithologies within the slices are also massive and

undeformed, however some lithologies show significant and characteristic

within-unit deformation. In argillite, bedding is completely transposed

by ubiquitous cleavage; pencil cleavage is common. Banded chert

displays the most striking deformation (Figure 6) and is in places

folded into close to tight outcrop-scale fold trains, with amplitudes of

2-10 m. Similar, parallel and chevron folds are equally common. Chert

beds can also be strongly mullioned (Figure 6). In unit lIe, a strongly

foliated and weakly lineated siliceous tectonite, deformation is

significantly more severe, more penetrative, and more ductile than in

any other unit. Disharmony between the structures of these variously

deformed lithologies and other massive units helps to define slice-

FIGURE 6. Characteristic Chert DeforaatioD. a) Mullionsj b) si.ilar folding and c) parallel folding in unit IPlb. Note lack of clear sense of shear in the fold train in b.

I\) CD

bounding fault discontinuities where determinative fossil evidence is

lacking.

Although different lithologies show different deformation

styles, the Sylvester has a consistent mesoscopic structural fabric.

Pencil cleavage, fold axes, mullions, and other lineations define a

consistant WNW-ESE linear fabric (Figure 7). Although very minor

numerical predominance of west-verging folds exists, no clear sense of

vergence about this lineation can be determined. Northeast-verging,

southwest-verging, and upright folds all occur in the Sylvester, even

together in single units.

30

Lithotectonic slices of the Sylvester can be grouped into

natural, second-order tectonic units which will here be termed

"packages". This is an entirely observational conclusion; small numbers

of lithotectonic units occur in mutual juxtaposition for considerable

distances along strike, and generally do not occur elsewhere in the

Sylvester outside of that domain. Across strike, packages are bounded

by no more profound structures than the sliver-bounding faults of the

slices that constitute them. And like the first ~rder slices, second

order packages are lensoidal, pinching out laterally, with dimensions

along strike much greater than thicknesses. By age and lithology units

are as unsystematically grouped within packages as they are juxtaposed

throughout the allochthon. In the study area (Figure 3) four such

second-order packages occur (see Figure 4).

It appears that there is a hierarchy of structural units and a

parallelism of structural style on several scales within the Sylvester

Allochthon. Whereas the present juxtaposition of rock types is

• • • +

• • ~ • • • . ~ • • • • • • ,.Q

• ~ f.· • • • . "' .... • .. • •

FIGURE 7. Synoptic Stereonet of Sylvester Allochthon Lineations. Lower heaisphere projection of lineations (fold axes. aullions. pencil cleavage. and tectonite stretching lineations) from all Sylvester units. Sense of rotation on folds shown where determinable.

31

32

exceedingly complex, the structures within the Sylvester are well

ordered and predictable in style. The structural fabric of the

allochthon is consistent from the scale of lithologies in slices, and

nested slices in second-order packages, to lensoidal packages within the

sheet-like Sylvester klippe itself (Figure 8).

Pre-Emplacemen! Thrus!ing

It is difficult to constrain the timing of the faulting

responsible for the remarkable internal structure of the Sylvester

Allochthon. Indeed, most slice-bounding faults remain ~datedj

imbrication occurred anytime after the late Paleozoic age of the units

involved and before or during emplacement of the allochthon along its

basal fault. The faults which bound Upper Triassic units in the

Sylvester assemblage are at least limited to post-Triassic and so to the

same loose bracket of time as emplacement of the allochthon. If it

could be assumed that all internal faults of the Sylvester had developed

synchronously, it might be concluded that intra-Sylvester faulting had

resulted from obduction-related compression. However, in two places the

identification of cross-cutting intrusions has allowed much more precise

dating of imbrication and indicates a process which was at least

episodic, and in places significantly predated emplacement of the

allochthon as a whole.

Within package II, the Nizi Formation (unit Mlln), the only

unit originally broken out of the Sylvester Group as a formation

(Gabrielse, 1963), has a well measured and dated type section (Marnet and

<40: ......... , • ...,. "'O ............ ~.~. "

•• " .-.....:..1' N ......... ,

~~~.--(~ ........ ~~ .. ------~

SYLVESTER LITHOTECTONIC

UNIT

,. , , ,",0,,1" ........ .........

...........

NORTH AMERICA

FIGURE 8. Schematic Sylvester Allochthon Structural Style.

FAULT

AMERICA

:->chemulie bLoek dingra .. of lhe internal structural style and hierarchy of' structural units within the SyLvester ALlochthon. Although schematic, the geometries of units shown are hU5erl on t.rue relationships observed in the Sylvester and are represented at an apPl'oximllte but realistic relative scale. w

w

34

Gabrielse, 1969) which includes a distinctive basal, coarse, sandy and

bioclastic limestone. Along strike in package II the Nizi is variously

thrust over either Pennsylvanian carbonate (unit IPIIm) or Late Permian

tuff, basalt and chert of unit PIla. Notably, along the line of section

in Figure 9a, above unit PIla, only the basal Nizi unit occurs; the rest

of·the known upper Nizi stratigraphy has been truncated by the thrust

emplacement of Late Devonian chert and associated undated strata. Along

and across this contact, tonalite of unit PlIt occcurs in a sill-like

body, showing baked margins in the basal Nizi below and well-exposed

intrusive contacts with argillite below the Devonian chert unit above.

The tonalite, therefore, clearly intruded this sliver-bounding fault o

(Harms, 1985a). Two kilometers north of latitude 59 the northward

extension of the tonalite body intrudes both the overlying Nizi and

underlying Pennsylvanian carbonate unit where the two carbonates are

juxtaposed. The relationship of the tonalite to this thrust fault ·is

not exposed but the distribution of the three units indicates that the

tonalite intrudes this thrust fault as well.

Both zircon and sphene separates from the tonalite have been U-

Pb dated by J. Mortensen (Department of Geological Sciences, University

of British Columbia). The results (Figure 10) are only very slightly

discordant; giving the best age estimate of 276 ± 6 Ma and a maximum

possible age of approximately 285, all within the Early Permian.

Thus, thrusting within package II occurred prior to the Early

Permian intrusion of the tonalite and so significantly predates the age

of emplacement of the Sylvester Allochthon. It is important to note

that elsewhere, as for example the contact with unit lIe, or with the

2000

e • • E

x

1000 YE:::::::l========~======:C======:l========t:======J:======:l======:J

2000

. ! . E

2

y

311m

b

UNIT me • i Y'

1000 ~======~========~=======c======~========~======~======~======~ I :nlll

fIGURB 9. Pre-Emplace.ent Thrusting. Locations of sections shown in Figure 3. a) Cross Section X-X'. Section shows the relationship between thrust truncation of the Nizi, tonalite intrusive contacts and post-intrusion unit PIla thrust contact. b) Cross Section V-V'. Section includes two coeval but distinctly different chert sequences within package III. Pervasive intrusion of both units by the same fine-grained diorite is shown schematically.

35

::;) .. .., .. >-.c Q. ..

o ..

. 04

.03

.2 .3 207 Pb1 235 U

FIGURE 10. 206Pb*/23Bu_207Pb*/35U Concordia Diagram for Unit PlIt Tonalite. Age deterainalion by J. MOrtensen at the Departaent of Geosciences, University of British Colusbia. Plot shows very slight discordancy of sa.ples, and the saall range of possible ages. a) .sgoetic -200 aesh fraction, b) non­.agnetic +200 .eBh fraction, c) bulk sphene fraction, d) Pb loss in processing suspected in this fraction. It is excluded from the analysis. Errors at 2~.

36

37

younger, Late Permian volcanic sequence (unit PIIa) the tonalite is

itself truncated by sliver-bounding faults. After intrusion, package II

continued to develop tectonically, as the Early Permian tonalite and the

faulted sequence it intrudes were involved in further imbrication.

The Sylvester Allochthon includes another non-gabbroic intrusive

unit. the Four Mile. It has been mapped only in reconnaissance but

appears to consist of two or three separate and distinct units. Near o

the Four Mile River (along latitude 59 ) one of these intrusives, a

biotite-bearing granite, has yielded a Permian age (266 ± 3.5 Ma from K-

Ar analysis, H. Gabrielse, personal communication). Thorough

interpretation of all three units awaits U-Pb dating now underway. Like

unit PIIt tonalite, it may be that these units intrude the imbricate

fabric of the Sylvester and/or are bounded by it. Preliminarily, then

it appears that both pre-Permian thrusting and Permian intrusion may not

be limited only to package II.

A second, and different, pre-emplacement thrusting episode is

documented in package III just south of Sheep Mtn. in McDame map area

(Figure 9b). There, unit MPIIIc, which elsewhere comprises a

distinctive continuous chert sequence ranging from Mississippian to

Permian, locally includes only the grey Pennsylvanian and brick red

Permian chert. The chert is thrust over a second, different

Mississippian to Permian chert sequence, unit MPIllb, which consists of

Mississippian black argillite and thin bedded chert, nodular olive grey~

green Pennsylvanian to Early Permian chert, black arenite, and Late

Pennsylvanian to Permian black chert and argillite. Both units are

38

pervasively intruded by the same very fine grained, green diorite. The

age of the diorite is unknown but it must post-date the Permian cherts

and predate the package and sliver-bounding faults which truncate it and

units MPlllb and MPlllc. Therefore, thrusting of unit MPlllc chert over

unit MPlllb chert must have occurred after the Permian, and prior to

emplacement of the Sylvester.

Thus, not less than three temporally well spaced episodes of

thrusting are known to have occurred within the Sylvester; prior to

Early Permian (but post-Pennsylvanian), after the Permian, and post­

Triassic.

Referring again to the chronostratigraphic summary (Figure 4),

while pre-emplacement thrusting was occurring between some units of the

Sylvester, other units were still accumulating on the ocean floor. This

suggests that the telescoping process occurred in the oceanic domain.

Whereas several distinct episodes of thrusting are now known to

have occurred, dated, pre-emplacement sliver-bounding faults are

otherwise physically indistinguishable from each other and from all

other undated faults. The internal structural fabric of the Sylvester

Allochthon, although complex, thus shows a certain amount of regularity

which must reflect the influence of a systematic and repeatable process.

BASAL SYLVESTER FAULT

The basal Sylvester fault is a regional, subhorizontal

structural surface which separates underlying miogeoclinal strata of the

Cassiar Platform from the overlying klippe of exotic Sylvester oceanic

lithologies. Detailed mapping of the trace of the sUb-Sylvester fault

documents that it is essentially undeformedi it is continuous, planar,

and subhorizontal. Minor relief along the fault surface can be

attributed to ramping over the autochthon. No evidence exists for

infolding of autochthonous units with the basal Sylvester fault and

Sylvester assemblage, nor for faulting which cuts the basal Sylvester

fault and imbricates allochthon with autochthon.

As a major geologic discontinuity, the basal Sylvester fault is

remarkably undistinguished in its expression. The Sylvester Allochthon

lies everywhere on what is regionally the top of the ancient North

American continental margin sequence; an unnamed grey and black

argillite unit which correlates with the Devono-Mississippian Black

Clastic or Earn Group farther north on the Cassiar Platform and within

Selwyn Basin. The basal Sylvester fault follows this preferred horizon

and lies within well cleaved argillite which in places may grade without

obvious interruption upward into chert-argillite or basalt-argillite

units of the Sylvester and downward into recognizable argillite of the

continental margin stratigraphy. Because of this lack of clear

distinction between basal Sylvester and miogeoclinal argillite in McDame

map area the whole Black Clastic was originally, erroneously, included

39

40

in the Sylvester as a "Lower Sylvester" sub-unit (Gabrielse, 1963).

Gabrielse and Mansy (1980) later recognized the continental association

of the bulk of the unit: the term Lower Sylvester should be abandoned.

The indistinct expression of the basal Sylvester fault, however,

should not lend doubt to its existence. First, there are no volcanic

units within the underlying miogeocline that might have served as

feeders for in situ accumulation of the voluminous intrusive and

extrusive mafic igneous rocks of the Sylvester. Also, in contrast to

the planar, undeformed surface which separates miogeoclinal from oceanic

assemblages, both the allochthon and the autochthon are involved in

separate, complex deformation systems. Thus, considerable structural

disharmony occurs across the basal Sylvester fault. Finally, the black

argillite at the top of the North American sequence below the Sylvester

Allochthon in Cry Lake map area includes thin carbonate beds which are

now known to be Visean-Tournasian in age (see Table 1, samples 5, 6, 7,

and 8; conodont dating by M. Orchard). This agrees with ages from the

correlative Earn Group in Yukon (Gordey, Abbott and Orchard, 1982). The

Sylvester assemblage includes both Late Devonian and Visean-Tournasian

units (units DMIIIc and MIa). Therefore, while it may not pertain to

all units immediately above the basal Sylvester fault, an overall,

older-over-younger, Sylvester assemblage-over-North American sequence

relationship has been established.

Mesoscopic structures developed in upper miogeoclinal strata in

Cry Lake Map area and limited in distribution to immediately below the

Sylvester Allochthon suggest a SW to NE emplacement of the allochthon.

41

Flanking the Sylvester fault, fine streaking and rodding is commonly

observed in platy limestone of the McDame Group at the top of the

miogeocline carbonate sequence. In one locality, extreme elongation of

stringocephalus brachiopods in the fetid dolomite of the McDame Group

was observed. These features constitute a subtle but consistent NW

lineation fabric in the upper autochthon skirting the Sylvester contact

(Figure 11). On a larger scale, outcrop sized, open, slightly

overturned, NE verging folds (Harms, 1984) and moderate off-set thrust

faults ramping upsection to the NE (Figure 12) occur in the McDame Group

limited to within a kilometer of the base of the Sylvester Allochthon.

These outcrop-scale NE verging structures are superimposed on larger

scale SW verging structures described below. Because of their limited

areal distribution the NE verging structures are interpreted as

resulting from emplacement of the allochthon across the top of the

miogeoclinal sequence along the basal Sylvester fault, and so indicate a

sw to NE emplacement direction.

• • • • ~

•

• •

+ •

• • • • • 4! •

•

FIGURE 11. Autochthon Lineations. Lower hemisphere projection of lineations from upper miogeocline strata immediately below the Sylvester Allochthon.

42

FIGURE 12. HE Verging Structures in the McDa.e Group below the Sylvester Allochthon. Both views look NW. a) Overturned folds, with 25 m aaplitudes. Basal Sylvester fault is just out of view to the left. b) Minor thrust fault cutting upsection to the HE. Note man for scale.

43

SUB-SYLVESTER AUTOCHTHON STRUCTURES

The ancestral North American stratigraphic sequence which

structurally underlies the Sylvester Allochthon (Figure 13) is part of

the regional Cassiar Platform miogeoclinal assemblage described by

Gabrielse (1963, 1975, 1979). At its base is a relatively thick

Proterozoic to Silurian sequence of clastic and less abundant carbonate

strata of the Ingenika (Mansy and Gabrielse, 1978), Atan, and Kechika

Groups, the latter capped by a thin, distinctive, black shale and

graptolitic siltstone. Overlying this succession are two thinner

carbonate units, an unnamed Siluro-Devonian unit and the Mid to Upper

Devonian McDame Group. The Siluro-Devonian unit comprises a basal

carbonate-cemented, clean quartz sandstone (informally, the "Tapioca

sandstone"), overlain by a massive bedded but connnonly thinly laminated

dolomite ("Laminated dolomite"). The McDame Group includes a dark grey

to black, fetid dolomite below a thin, siliceous, platy limestone

(informally, the "Fetid dolomite" and "Platy limestone" respectively).

Between the Siluro-Devonian and the McDame Groups is a disconformity

with appreciable local relief.

At the top of this miogeocline assemblage is an unnamed,

predominately argillite unit of variable thickness. Below the southern

end of the Sylvester, it is a thin (10 m), fissile, distinctively "gun-­

steel" colored shale (described also by Gordey, 1980). To the north,

around the Major Hart River, the gun-steel shale is replaced by black

argillite, typically with characteristic iron-staining along cleavage.

44

___ DECOLLEMENT ___ ~~~E~ BLACK bIn" ... " SI'.' CLASTIC ·'Sltt",.

McDAME PLATY LS

FETID DOLO"'TE

LA"'NATED DOLO"'TE ~~ ~ii: > 3 UNNAMED

TAPIOCA SAND~TONE ~!2 ...Ie

E o II)

'i o E o :!!

E o o ..,

___ DECOLLEMENT~!"""!,!"",,,!-" ~~~~~ bl.c" SI,.ptollt/c .h.,. j:ai _ _ _ 15m

z < ii:

Zm

~~ ::II)

:::!a: fl)w IlL elL ~::I

I C :i

Z « ii: m ~ « I)

a: w :: o ....I

KECHIKA

ATAN

INGENIKA

".,/ou.', tlSlht colo,." ph''''t.

with .ttt.to". .,," I. b.".

upp., c.,bo".t. ... " low.,

qu.,tzlt. u"lt.

fott.t." c/ •• tlc. ... " c.,bo ... t ••

---------

FIGURE 13. Cassiar Platform Miogeocline Stratigraphic Column.

E o II)

'i o g

E o o (I)

e I o o

(II

Note the relative thicknesses of the duplexed units and the more massive underlying groups, and that an alaost equal thickness of

, Ingenika Group is Dot shown.

45

46

The black argillite thickens considerably northward into McDame map

area; around Cassiar and below the northern tip of the allochthon

argillite thicknesses are substantial (80-100m) and the unit includes

barite and coarse clastic beds of chert arenite and miogeocline-derived

conglomerate and boulder beds. There, the unit is correlative with the

"Black Clastic" or Earn Group described in the Yukon (Gordey, Abbot and

Orchard, 1982; Templeman-Kluit, 1977).

Below the southwest end of the Sylvester Allochthon, along the

Turnagain River, the Paleozoic platformal sequence changes abruptly into

an off-platform shale facies in which only thinned Atan carbonate and

Devonian calcarenite persists (Gabrielse, 1979).

Throughout its outcrop length, the basal fault of the Sylvester

Allochthon lies at the top of the miogeoclinal sequence, above the gun-­

steel or Black Clastic shales. Previous reconnaissance scale mapping in

Cry Lake and McDame map areas represented the Siluro--Devonian and McDame

Group units immediately below the Sylvester as a virtually undeformed

stack (Gabrielse, 1963, 1979). However, just east of the Sylvester, in

the Four Brothers Range, the entire miogeoclinal section lies in a

large-scale, overturned southwest.-vergent fold limb (Gabrielse, 1979;

Gabrielse and Mansy, 1978). Furthermore, in the course of this study,

large scale, southwest vergent thrust faults were found in the

miogeocline immediatedly below the Sylvester Allochthon, but not cutting

the basal Sylvester Faull (Harms, 1984). There is then, the se£!ming1y

contradictory combination of a regionally eonsistent, planar

47

stratigraphic horizon for the tectonic base of the Sylvester Allochthon

and considerable deformation confined below that horizon.

The resolution of this apparent paradox lies in recognition of a

predominately southwest vergent duplex in the top of the miogeocline

section (Harms, 1985b). The duplex is developed between two decollement

horizons; one in the gun-steel/Black Clastic shales at the top of the

North American section and the base of the Sylvester Allochthon, the

second in thin black shale and graptolitic siltstone at the top of the

Kechika Group (Figure 13). The Sylvester Allochthon is the roof of the

duplex structure. In this way, the gun-steel/Black Clastic horizon

everywhere underlies the Sylvester Allochthon. Within the duplex,

horses of the Siluro-Devonian and McDame Group couplet are imbricated

along gently northeast dipping thrusts. The thrust faults are

asymptotic to the base of the Sylvester Allochthon and the basal

Sylvester Fault rather than truncated by it (in contrast to Harms,

1984) •

The duplex system is well exposed along the soulheast. flunk uf

the allochthon (section B-B'" Figure 14, and Figure 15). Several cliff

faces along or ncar this line of section expose thrust. ramps and

establish the SW vergence of the duplex (Figure 16a). To the east the

duplex system is buttressed against a thick, southwest vergent thrust

nappe of the complete, overturned Proterozoic and Paleozoic miogeocline

section. In this area, northeast-vergent "pack--thrust:ing" has occurred.

In places (see km 10 in section B-B"', Figure 14), mesC)scopic

intraformational buckle folding (Figure 16b) also results in compression

and in local unit thickening. To the extreme southwest of the duplex,

S S'

"""""""""'" """""""""'" , , , , , , " " , " " " , ,eSk, , , , , """""""""""

5

FIGURE 14. Sub-Sy 1 vester Autochthon Duplex Systell. Units: PzS=Sylvester Allochthon; n.~Da.e Group; SD=Siluro­Devonian unit; SDs=Sbale-out facies; ~Sk=Kecbika Group. a) Duplex structure section. Althougb it is defor.ed, no structures are indicated in the Kechika Group below the duplex. b) Duplex systell.ap, sbowning the location of section 8-8"', Duplex contacts shown on the .ap are predominately inferred. Map scale is 75~ of cross-section scale.

10

•• )00

, " r-\\

" ,

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

- 1

1

1

1

1

1

48

B" B"·

20km

.,'; --r ; -+---- ;'

( ('

~ =:t:: :: r:n .

. t ~ .:.'

FIGURB 15. View of the Sub-Sylvester Duplex Syata.. View is looking NW at the SI end of the Sylvester Allochthon along the I end of cross section B-B"'(~ 12-18). Kechika phyllite in i-.ediate foreground.

• CD

FIGURE 16. Duplex Systea Structures. a) Duplex thrust reap cutting upsectionto the SW. View looks SE across the Major Hart River. Basal Sylvester fault is just to the right of the picture. Cliff approxiaately 100 m high. b) Buckle folding in duplex horse responsible for local unit thickening.

50

b

along the Turnagain River, Siluro-Devonian and McDame Group imbricate

panels are thrust over a thick shale sequence of off-platform facies.

Thus, the "shale-out" at this location occurs across a thrust fault

where the facies transition is, in fact, tectonic.

The duplex system emerges below the western flank of the

Sylvester in the Cassiar area and must be continuous below the

allochthon. In the Cassiar region, the thicker Black Clastic is

included with thinner Siluro-Devonian and McDame Group strata in the

duplex.

51

Along cross section B-B'" (Figure 14) 4-5 km or approximately

25% shortening of the Siluro-Devonian to McDame Group sequence occurs as

a result of duplex telescoping. West-vergent folds and low--angle faulls

are known to occur in the Kechika, Atan and Ingenika Groups in the Four

Brothers range, immediately southeast of the Sylvester (Mansy, 1980).

This deformation, structurally below the duplex and its basal

decollement, may involve the thicker units in a second set of larger

scale structures which balance shortening in the overlying duplex

system. Indeed, a map scale, west-verging structured style is

characteristic regionally in all levels of the miogeocline assemblage in

the western Omineca Belt of northern and central British Columbia (Brown

and Gabrielse, 1983; Mansy, in press). However, immediately to the cast

of the Sylvester Allochthon and the lower miogeocline structures in the

Four Brothers Range lies the Kechika fault, one of a network of linked

and related major dextral transcurre~l faults associated _wi th Llw

Northern Rocky Mountain Trench fault 30 km farther east (Gabridse,

52

1985). These strike--slip faults were active in the late Cretaceous and

early Tertiary (Gabrielse, 1985) after emplacement of the Sylvester.

The Kechika fault truncates the eastward projection of the sub-Sylvester

duplex system, and the lower miogeocline structures below and offsets

them northward from what may have been any original eastern continuation

of the McDame - Siluro-Devonian groups duplex system.

Although exotic, the Sylvester Allochthon and the basal

Sylvester Fault are an inherent part of the upper miogeocline duplex

system. The Sylvester is the roof thrust of the duplex. As cogently

presented by Boyer and Elliot (1982) compression is achieved in a duplex

by transport along the sole fault, through each successive duplex

imbricate to the roof thrust. Displacement along the roof thrust is

therefore coeval with duplex imbrication; the emplacement of the

Sylvester onto the miogeoclinal sequence must have been synchronous with

the developement of the southwest vergent duplex in the t.op of thal.

sequence. (It is not believed, however, that transport along the basal

Sylvester fault balances with or is limited to the amount of shortening

in the underlying duplex.)

Several significant implications follow. First, recal Linl~ t.he

absence of inter-imbrication of the Sylvester assemblage with North

American strata, it seems most reasonable t.o conclude that. the Sylvester

Allochthon was emplaced across the miogeocline along the duplex roof as

one coherent sheet. The distinctive slivered internal structur:Jl style

of the Sylvester must have been acquired outboard of the ancient

cont inental margin pri or to obduct ion onto the mi op,-eoc: li n()_

Identification of pre-emplacement thrusting is in accord with this

53

conclusion. It seems likely then that the telescoped upper oceanic

Sylvester units were to some extent already detached from their basement

prior to obduction and duplexing. There is no evidence from the

Sylvester or the sub-Sylvester duplex however which might directly help

to resolve more intractible issues concerning the relationship of

cruStal-scale convergence and vergence between lower oceanic and lower

continental basement during terrane obduction to the generation of

probably coeval west-verging structures throughout the Omineca Belt (see

Price, 1986; Brown, Journeay, Lane, Murphy and Rees, 1986).

Second, although southwest verging, the Siluro-Devonian and

McDame duplex system, if cogenetic with emplacement of the Sylvester,

must be part of a general displacement of both exotic and autochthonous

assemblages toward the craton, en masse. This is compatible with the

absence of a syn-emplacement flexural basin immediately below and

flanking the Sylvester Allocht.hon. The load which would have produced

an expected flexural foredeep (Price, 197:3; Speed and Sleep, 1982;

Jordan, 1981) would have included not only t.he abducted allochthon but

also a thickening packet of the Siluro-Devonian - McDame duplex and most

likely the broader set of structures in the lower miogeocline strat.a as

well. Any syn-emplacement basin deposits would lie inboard of the

entire emplacement duplex system, and woul"d present ly be Hcross the

Kechika fault and displaced relatively to the south fr"om the" load".

FurthermQre, craton-ward displacement resulting from southwest verging

imbrication within the Siluro-Devonian and McDame duplex requires that

the whole duplex have formed by a process of wedging and delaminalion

54

(Price, 1986). Each successively active duplex horse must have driven

itself beneath the next inboard as the deformation progressed from west

to east.

Finally, it is worth reiterating that the basal Sylvester fault,

the rather unspectacular roof thrust to a duplex of miogeoclinal strata,

is a terrane boundary. Here, the obduction and emplacement of a complex

allochthon of exotic ocean-floor lithologies occurred through foreland

thrust belt-style deformation, with no evidence of a conventionally

recognized "suture".

CORRELATIVE TERRANES

The Sylvester Allochthon is one of a number of broadly

correlative suspect terranes which lie in a discontinuous belt

stretching the length of the Cordillera (Figure 17). Because of

lithologic similarities, Monger (1977) included the Sylvester Allochthon

with the Anvil allochthons of Selwyn Basin in Yukon, the Nina Creek and

Antler Groups in central British Columbia, and the Fennell, Kas10 and

Milford formations of southern British Columbia in the Slide Mountain

Terrane of Canada (Monger and Berg, 1984). In addition, the Angayucham,

Tozitna, Innoko and Seventymi1e terranes of Alaska, and the Golconda

Allochthon of Nevada can be correlated with the Sylvester and Slide

Mountain Terrane (Harms, Coney and Jones, 1984; Jones, Si1berling and

Coney, 1983). All these terranes share the same suite of oceanic rock

types in the same late Paleozoic age range, but there are other, more

distinguishing parallels as well. Each is characterized by a

distinctive internal structural style, the non-systematic stacking of

lithotectonic slices. Furthermore, these terranes have a common

regional tectonic setting, they are locally the most inboard of oceanic

suspect terrnes and are immediately juxtaposed against the ancient North

American miogeocline or similar but suspect Paleozoic continental margin

strata. Each is structurally the highest of all terranes with which it.

is in contact, lying as huge klippen on top of adjacent assemblages.

The Angayucham Terrane and its northern outliers straddle the

several continental suspect terranes of the Brooks Range and North Slope

~

SUSPECT TE~RANES • CJ Slide MOUnlain and Correlall~e ferrane, OUfboard auapeCf ferrane COllal/e

CONTINENTAL ASSEMBLAGES

E3 North "'merlcan miogeOcline

E::==I Conllnentally derived Or ~ North "'merica-relafed but sU,pect ferrane,

OVERLAP ASSEMBLAGES

o 'ntrllSlve bOdies

- Sedimentary Or a'lrUSIve unils

FIGURE 17. Suite of

SYlvester-correlative Terranes in the Cordilleran Collage.

56

ANVIL ALLOCHTHONS

FENNELL _

MILFO~OIKASLO

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

j

I j

57

as a vast rootless sheet (Coney and Jones, 1985; Coney, Silberling and

Jones, 1983). Although the Angayucham rocks have been called ophiolitic

(Patton, Tailleur, Brosge and I.anphere, 1977; Roeder and Mull, 1978), in

fact this terrane is composed of the same ocean floor lithologies found

in the Sylvester Allochthon. l1ltramafic slices may be more abundant in

southwestern exposures (Patton et al., 1977; Jones, Silberling and

Coney, 1983) but everywhere the Angayucham includes mostly gabbro,

basalt, chert, argillite, carbonate and tuff (Jones, Silberling and

Coney, 1983). As in the Sylvester, Angayucham gabbro and volcanic rocks

appear to have been derived from above ophiolitic basement; seamount

origins have been proposed (Murchey and Harris, 1985; Jones, SHber.L:ing

and Coney, 1983). The age range of units in the Angayucham is now known

to span from Devonian to Early Jurassic; Triassic chert "is not unconnnon

in the Angayucham in contrast to the Sylvester (Jones, Silberling, Coney

and P1afker, 19B4; Murchey and Harris, 1985). With the aid of

radiolarian dating, structures in the eastern Angayucharo, in the

Coldfoot, Alaska area, have been mapped in great detail (Jones et a1.,

in press). This work shows that although overprinted by post-"

emplacement high-angle structures, the Angayucham is characterized by an

internal fabric of lithotectonic slices stacked along layer parallel

faults with juxtapositions of disparate lithologies and older--over-­

younger sequences. Thus while subtle variat ions exist, there are no

significant differences between the character of the Angayuc:hum Terrane

and Sylvester Allochthon.

58

Closely allied with the Angayucham are the Tozitna, Innoko, and

Seventymile terranes. Their complete age ranges, internal structures

and regional settings are considerably less well known but are similar

to those of the Angayucham (Jones et al., 1984; Jones, Silberling and

Coney, 1983).

In Yukon, numerous closely-spaced klippen of rocks unrelated to

but resting on distal ancient North American margin facies of Selwyn

Basin have been identified. Templeman-Kluil (l~79) defined three

general allochthonous units within the klippen; Anvil Allochthon,

comprising greenstone, gabbro and serpentinite; Simpson Allochthon,

granitoid and mylonitized plutonic rock: and Nisutlin Allochthon,

sheared siliceous tectonite and quartz phyllite schist. Erdmer (19U5)

demonstrated that all three "allochthons" are complexley structurally

interleaved within individual klippe. Although for the most part

undated, "Anvil" rock types lithologically correlate with Sylvester and

other Slide Mountain allochthons rock types. The abundance of "Simpson"

and "Nisutlin" rock types within the Yukon allochthons however differs

markedly from the Sylvester and the rest of the suite of terranes.

Still, the "Simpson" mylonitized plutonic rocks may have counterparts in

the Four Mile and unit Pllt tonalite bodies of the Sylvester, and are

not unlike mylonites which occur in the Mosquito Terrane, a small and

perhaps related terrane exposed in a window beneath Lhe Angayucham near

Coldfoot, Alaska (Wust, Harms and Coney, 1984). And while not nearly as

abundant, unit IIe of the Sylvester is clearly analagous t.o NisuLLin

rocks. Perhaps the strongest correlation between the Yukon allochthons

and the rest of the suite of terranes is Erdmer's (1985) demonstration

of the same internal imbrication and structural juxtapositions within

them.

59

South of the Sylvester, in central British Columbia, the Antler

Group as redefined by Struik (1981) lies above the continentally derived

Barkerville and Cariboo terranes across the basal Pundata Fault (Struik,

1985a, 1982). It contains chert, siliceous argillite, diorite, and

pillow basalt. The first detailed microfossil dating from the Antler

Group, a transect across Sliding Mountain, shows a slightly more limited

age range, Mississippian to Permian, and clearly demonstrates an

internal structural style of stacked lithotectonic slices (Struik and

Orchard, 1985).

The Crooked Amphibolite, not physically contiguous to but

previously associated with Antler Group rocks and Slide Mountain Terrane

in this area, is no longer correlated with them by Struik (l985a,

1985b). Conclusions regarding timing of emplacement and associated

regional deformation styles based solely on relalionships of lhe Crooked

Amphibolite should also be abandoned for the Slide Mountain Terrane.

Rocks of the Nina Creek area, between the Sylvester and Antler

allochthons, have not been mapped or dated in detail, however rvlonger

(1973, 1977) and Gabrielse (1975) have shown they include correlative

oceanic lithologies (chert, argillite, carbonate, volcanics, gabbro and

ultramafics) of' at least Pennsylvanian and Permian age.

In southern British Columbia, argillite and volcanics of the

Mississippian Milford and Permian Kaslo formations respectively have

been considered part of the Slide Mountain Terrane (Monger, 1977; Pri.ee ,

60

Monger and Roddick, 1985; Monger and Berg, 1984). However, when

compared to the fundamental distinguishing characteristics of the larger

Cordilleran-length' suite of correlative terranes, there are reasons to

doubt the inclusion of the Milford and Kaslo. Recent mapping by

Klepacki and Wheeler (1985) represent the Milford and Kaslo as

conventional stratigraphic formations which are in turn depositionally

underlain by units of the Kootenay Terrane and overlain by rocks of

Quesnellia. Thus the Milford and Kaslo bear neither the same

distinctive internal structural style nor the same regional tectonic

setting of the suite of terranes with which they have been correlated.

In the absence of' detailed mapping in the Fennell Group rocks these

reservations may pertain there as well.

Certainly the most studied in this suite of correlative terranes

is the Golconda Allochthon in Nevada. It represents a somewhat different

balance of oceanic lithologies than the other terranes. Units in the

Golconda are composed of basalt and gabbt'o (greenstone), argillite, and

chert, but also include considerably more clastic components;

siliciclastics, volcanic-lithic greywackes and earbonate turbidites from

more transitional environments (Miller et a1., 19Hi, 1984; Silber ling

and Roberts, 1962; Stewart et a1., 1977; Snyder and Brueckner, 1983).

Nevertheless the Golconda spans the characteristic I.ale Devonian to

Permian age range (Murchey, 1982; Murchey, Jon'7s and Blome, 1983; r>liller

et a1., 1981, 1984). Recent work shows that the Haval.Lah and Sc:hoonover

Groups of the Golconda are not composed of conventional formations but

rather of a variable stack of thrust-bounded sheets (M'Lller et a1.,

1982, 1984;' Brueckner and Snyder, 1985; Snyder and Brueckner, 1983) with

61

indications of second-order groupings of lithotectonic slices within the

Golconda, similar to those in the Sylvester Allochthon (Brueckner and

Snyder, 1985). Thus, independent of its historic entanglement in

complex local issues of Nevadan Paleozoic continental margin tectonics

and the Sonoman orogeny (Gabrielse, Snyder and Stewart, ]983), the

internal characteristics of the Golconda Allochthon suggest it too

correlates with the suite of inboard, Paleozoic oceanic terranes.

More speculatively, it is possible that this belt of

correlative terranes includes others £arther south along the axis of the

Cordillera. As mentioned previously, fusulinids from the Sylvester's

Parafusulina Limestone unit are also known from El Antimonio (Hoss,

1969i Ross and Ross, 1983) in the Caborca Terrane of northwestern

Mexico. Although characterized by miogeoclinal carbonate, the Caborca

is a composite terrane which includes, along with the El Antimonio

strata, a suite of Paleozoic deep marine chert and barite (Campa and

Coney, 1983i Coney and Campa, 1984).

In summary, the chronolithologic, structural and tectonic

parallels between the Alaskan Angayuc~am, Tozitna, lnnoko and

Seventymile terranes, the various allochthons of the Canadian Slide

Mountain Terrane and the Nevadan Golconda Allochthon tie them into a

single ~elated suite. Correlation of the Sylvester with these t.erranes

suggests that Sylvester paleogeography should, at least in part,

correlate with that of the suite of terranes, and may thus have mueh

broader implications for Cor~illeran evolution •.

TECTONIC ANALYSIS: PALEOGEOGRAPHIC AND PALEOGEODYNAMIC IMPLICATIONS

In review, the Sylvester Allochthon is a tectonic assemblage of

fault-bounded slices of a variety of oceanic lithologies, almost all of

which occur in each age from Late Devonian to Late Triassic. Sylvester

assemblage units derive almost exclusively from the surface of ocean

crust. There is no evidence of original facies relationships between

the lithotectonic units of the Sylvester nor any decipherable sequence

of deposition and basin evolution. Furthermore, coeval but

depositionally incompatible lithologies; for example radiolarian chert

and carbonate, or pelagic sediments and tuff, volcaniclastic and

intrusive rock from active volcanic domains must have originated in

distinctly different oceanic paleoenvironments.

Therefore, any crude palinspastic reconstruction of the

Sylvester cannot simply reshuffle its lithotectonic slices; they cannot

be restacked into either an initial vertical ophiolite sequence or

stratigraphic column. Instead they must be restored to separate oceanic

environments distributed across an ocean floor, in all periods from Late

Devonian through Triassic. And, whereas some slices of the Sylvester,

the quartz arenites, may have originated near a continental margin, the

majority were not within reach of continental sedimentation.

In this way, the character of the Sylvester Allochthon is not

consistent with interpretation as a marginal or flanking back-arc basin.

It suggests instead derivation from a much larger expanse of Paleozoic

63

ocean. The Sylvester age range alone, Late Devonian to Late Triassic,

or not less than 150 Ma, combined with reasonable spreading rates can be

used as an indirect measure of the width of ocean it represents (sec

also Dickinson, 1977i Snyder and Brueckner, 1983). Thus, units of the

Sylvester Allochthon can be considered truely "exotic" and may be far

traveled, as the fauna of the Parafusu1inid Limestone unit would suggest

(see Ross, 1969; Ross and Ross, 1983).

In combination with other allochthons in the suite of

correlative Late Paleozoic oceanic terranes the arguement becomes more

compelling. The internal structural style and lithic assemblage

character of the Angayucham, Tozitna and lnnoko terranes, the Anvil and

Antler allochthons of Slide Mountain, and the Golconda Allochthon all

parallel that detailed from the Sylvester Allochthon. 'faken together

the amount of ocean crust they represent may be considerable. Rather

than a string of minor marginal basins along the ancient North American

continent, the Sylvester Allochthon and these correlative terranes must

instead be remnants of the same vast Paleozoic ocean, the "pl'olo­

Pacific" (Dickinson, 1977).

This conclusion is clearly in conflict wi th published

interpretations of the Sylvester Allochthon and Slide twlountain Terrane

(Monger, 1977; Price, Monger and Roddick, 1985; Klepacki and Wheeler,

1985; and by correlation in conflict with similar interpretations of the

Golconda Allochthon by Miller et a1., 1981, 1984). But. paleogeographic

interpretations which cast the Slide Mountain Terrane as a marginal

basin have been based primarily on the Milford and Kaslo in souther"n

British Columbia, where these formations appear to have facies

64

relationships with probable distal North American strata (Kootenay

Terrane), (Klepacki and Wheeler, 1985). As pointed out previously, the

Milford and Kaslo formations are not composed of telescoped

lithotectonic slices like the rest of Slide Mountain Terrane;

paleogeographic constraints on these units therefore may not be

applicable to the Sylvester and other correlative allochthons.

(Questions of paleogeography have been debated in the forum of Golconda

Allochthon geology for a number of years and with consideration of many

aspects of interpretation (Dickinson, 1977; Stewart et al., 1977; Speed,

1978, 1979; Miller et a1., 1981, 1982, 1984; Brueckner and Snyder, 1985;

Snyder and Brueckner, 1983). It is acknowledged that. discussions here

concerning Sylvester Allochthon tectonic analysis have many predecess()rs

and parallels in the Golconda literature.) Recall that the Sylvester,

and the entire suite of correlative terranes, have no recognized root

zones in the Cordilleran collage. Quesnellia is the next outboard

terrane to the Sylvester Allochthon and to the rest of the Slide

Mountain Terrane in southern British Columbia. A lale Paleozoic non­

continental basement has been recognized in Quesnellia (Monger, 1~77;

Read and Okulitch, 1977; Monger and Berg, 1984). It. is quite possjble

that this basement may prove to have some relationship to Slide Mountain

Terrane, but the possible paleogeograpic implications of' such a

correlation at this point would be highly speculative.

If the character and juxtapositions of the lithotectonic units

of the Sylvester assemblage paleogeographically represent the n~mtlallts

of a Paleozoic ocean basin, then the distinctive internal structural

65

style of the Sylvester records the telescoping of the surface layer of

that ocean. Constraints can be placed on the paleogeodynamics of the

nature of the telescoping process and the formation of the Sylvester by

the relationships within it. The age range and timing of thrusting in

the Sylvester is incompletely known, but suggests telescoping was a late

Paleozoic to earliest Mesozoic process. Parenthetically, this is the

same age as the "Sonoman" event of the western United States, however

here it is clearly not a North American orogeny. Sy.lvester telescoping

was on-going or at least episodic. And it is most reasonable to

interpret that during this broad pre-emplacement period, the process

occurred within the oceanic realm, acting in an oceanic plate. The

final stage in the tectonic history of the Sylvester Allochthon was its

obduction onto the North American continental margin. The

paleogeodynamic process which caused intra-oceanic telescoping also

resulted in telescoping between the evolving Sylvester and North

America. Pre-emplacement intra-Sylvester thrusting was a precursor for

emplacement of the exotic allochthon along a foreland style duplex

system as a culmination of the telescoping process.

However, no aspect of the Sylvester Allochthon indi.cates any

particular late Paleozoic reconstruction of arcs and subduction zones

that may have accompanied activity of the telescoping process.

Significant inconsislencies exist between the documented

structural style of the Sylvester Allochthon and the general eharacler'

of a subduction-related accretionary wedge. Structures in the Sylvester

are complex but not chaotic, the Sylvester is not a melange. The

penetrative deformation which characterizes accretionary wedge sediments

66

(Cowan, 1985; Moore, 1973; Moore and Wheeler, 1978; Karig and Sharman,

1975) is unlike the mesoscopic fabric of the Sylvester. The

predominance of map-scale, massive and continuous "bal3ement" slices of

intrusive and extrusive igneous rock, massive carbonate and thick,

banded chert piles such as characterize the Sylvester, unlike blocks and

knockers in scale and without a melange matrix, is not described in

modern accretionary wedges (Aubouin, von Huene et a1., 1982; Karig and

Sharman, 1975; McCarthy and Scholl, 1985; Fuis, Ambos, Mooney, Page and

Campbell, 1985). Furthermore, Sylvester lithotectonic slivers occur in

a subhorizontal nested stack, they do not have the consistent vergence

or imbrication direction, either landward or oceanward, which seems to

predominate in seismic sections of accretionary complexes (SeeJy, Vail

and Walton, 1974; Moore and Allwardt, 1980; Helwig and Emmet, 1981). No

blueschist facies or high-pressure metamorphic mineralogy have been

found in the Sylvester and there is no evidence to suggest the Sylvester

was ever carried below the surface of ocean crust.

It is clear that, as in the Sylvester, pervasive imbrication and

the presence of horse-like s 1 i vers have been described from accreti onary

wedges (Seely, Vail and Walton, 1974; McCarthy and Scholl, 1985; Silver,

Ellis, Breen and Shipley, 1985; Cowan, 1985). Like tho Sylvester

Allochthon and its correlative terranes, the accretionary wedge regime

is characterized by layer-parallel fault--controlled telescoping of upper

horizons and detachment from corresponding basement. However, on the

same basis a comparison has been made between accretionary wedge

tectonics and structures of the foreland thrust regime (Davis, Suppe and

67

Dahlen, 1983; Elliot, 1976). A broad brush comparison could be drawn

then between Sylvester-like structures and either a subduction-related

accretionary wedge or a foreland thrust belt; they are useful mechanical

analogues, but not necessarily direct genetic models. A spectrum of

mechanisms must exist for the thin-skinned telescoping of upper crust

and consumption of detached basement in these telescoped domains. This

was long ago recognized by Bally (1975) when he coined the terms .. A-­

type" and B-type" subduction: in foreland "A-type" systems, basement is

consumed in intra-plate, non-subduction zone deformation. In light of

the perhaps subtle but possibly significant discrepancies between the

Sylvester's structural character and the generally accepted accretionary

wedge petrotectonic signature of classic subduction (Dickinson, 1972),

and in light of emplacement of the allochthon along foreland-style

structures, it seems best to suggest no further interptetation than that

the paleogeodynamic process which shaped the Sylvest.er lay within this

spectrum between A- and B-type subduction. It must be plainly stated

that whereas geologic relationships in the Sylvester Allochthon may not

be fully accomodated without hypothesizing greater complexit ies in the

geodynamics of the classical plate tectonic paradigm, and may point. to

the existence perhaps of intraplate-like collapse and deformation in

oceanic crust, data from the Sylvester are demonstrably inadequate to

describe such a process with any greater clarity or detail.

There is a some evidence supporting t.he operation of such

complex plate tectonic processes. In the oceanic Indian plate, south of

India and west of the Ninetyeast Hidge, Stein and Oka1 -(1978), Weissel,

Anderson and Geller (1980), and Wiens et a1. (1985) document significant

68

intraplate seismicity which they interpret as thrusting, intra-oceanic

plate deformation and a diffuse, non-subduction, plate boundary. Stein

and Okal (1978) point out that this zone of deformation coincides with

the region of old, Jurassic age oceanic crust. Similarly, it must be a

critical factor in the nature of Sylv~ster paleogeodynamics that at the

time of telescoping, Permian through late Triassic, its paleo-Pacific

ocean crust was as old as or older than the oldest oceanic crust extant

today. In thickness and rigidity it may have behaved very much like the

cooled thinned or transitional continental crust below old miogeoclinal

wedges of foreland thrust belts. Certainly it suggests that de format ion

wi thin the Indian plate may be a modern analog of .Sy 1 ves ter-typ(~ oceanic

telescoping.

In a different vein, Coney (1983; Coney, Silberling and Jones,

1983; .Coney and Jones, 1985) points out that the record of accretion of

continental crust in Alaska shows it was derived largely from terranes

of oceanic affinity, which over time telescoped, collupsed and

consolidated to conti nental thicknesses wi thout classic evi LIenee 0 r arcs

or subduction complexes of the required age or scale. The duplex

between the Sylvester and the miogeoclinal strata on which it li es

(Figure 14) may be a small scale analog of the crustal scale duplexing

(Boyer and Elliot, 1982) which Coney (Coney, SilberlinJ{ and Jones, 19B~{;

Coney and Jones, 1985) has proposed to accomodate telescoping below the

Alaskan Angayucham Terrane, between the underlying terranes of the

Brooks Range.

69

Whatever paleogeodynamic process may have operated, the

combination of structural style and lithologic character of the

Sylvester ~llochthon and its correlative terranes suggests they

represent telescoped remnants of a proto-Pacific ocean. Thus from even

its most inboard terranes the Cordilleran collage can be interpreted as

the result of profound mobility and telescoping of paleo-plate tectonic

domains.

SUMMARY

1. The Sylvester assemblage ranges in age from Late Devonian to Late

Triassic. It includes chert, argillite, basalt, gabbro, carbonate and

ultramaficsj it is oceanic but not ophiolitic. Most of these rock types

occur virtually throughout the known age range.

2. Internally, the Sylvester Allochthon can be characterized as a

complex structural interleaving of fault-bounded lithotectonic slices.

There exists a natural hierarchy of these structures in packages,

showing a parallelism of structural style on several scales within the

Sylvester.

3. The juxtaposition of lithologies within the Sylvester is non­

systematic. Older-over-younger faulting is common and depositionally

incompatible rock types are juxtaposed. No basin history can be

reconstructed from the Sylvester assemblage; Sylvester "stratigraphy" is

entirely tectonic.

4. Thrusting which created the Sylvester assemblage was ongoing,

occurring in at least three episodes through a range of time

significantly "predating emplacement of the allochthon.

5. The Sylvester Allochthon was emplaced from SW to NE onto ancestral

North American margin strata after the Late Triassic and before t.he Mid-­

Cretaceous, probably in the Early to Middle Jurassic. The Sylvester

si ts on North America as a huge rootless klippf! which j s the roof of a

foreland-style duplex within the North Amer"iean units below. An older-

70

over-younger juxtaposition exists between the Sylvester as a whole and

the underlying autochthonous strata.

71

Thus, Sylvester lithotectonic slivers suggest telescoping during

a number of episodes of ocean crust deformation from originally widely

dispersed sites on the ocean floor. The Sylvester Allochthon can be

interpreted as recording consumption of vast amounts of lower ocean

crust, and tremendous thin-skinned telescoping, collapse and

consolidation of the veneer of ocean crust with no obvious evidence of

conventional arc and subduction zone-accretionary wedge paleogeography.

Continued telescoping between the Sylvester and North America t'esulted

in emplacement. of the exotic Sylvester oceanic assemblage ont.o the Nort.h

American margin across an autochthon duplex in which miogeoclinal

strata, with the allochthon, were displaced cratonward, also with no

conventional evidence of a suture. The Sylvester is only one of a

series of correlative terranes which now discontinuously stretch the

length of the Cordillera as the most inboard of accreted oceanic

terranes. Like the Sylvester, all are tectonic assemblages of fragmenl:.;

from a Paleozoic ocean; together they may represent the remnants of a

vast proto-Pacific. This suggests readdressed paleogeographic and

paleogeodynamic interpretations of these terranes as evidence of

profound mobility and telescoping wi thin even the inboard Cordilleran

collage.

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FIGURE 3

LITHOTECTONIC MAP OF THE SYLVESTER ALLOCHTHON

STUDY AREA

Scale 1:50,000

, ..... 1 ° 1 2 3 4km

Contour Interval = 20 meters

TEKLA A. HARMS, 1986

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PACKAGE I

Banded orange and green

Pennsylvanian chert

Mississippian black, siliceous, with minor chert, basalt and Ci

Mllsslve, fine grained green gil Contacts both Intrusive and te

Vesicular basalt

Basal Sylvester Fault Observed: Inferred

___ ~-1" Unit boundi'~'D fault

PACKAGE I

)r.nge and green

anlan chert

)/.n black, siliceous argillite

r chert, basalt and carbonate r.:::;;:, ~

fine grained green gabbro both Intrusille and tectonic

basslt

UNITS

PACKAGE II

Late Permian green Interbedded

chert, tuff and b'asalt

Early Permian tonalite Both Intrusille and tectonic contacts

Pennsyillanian limestone

Mississippian Nizi Fm carbonate

De IIono-Mlss/sslpplan

black banded chert

Black argillite

Interlayered black. chert, limestone

and sandstone

Siliceous chlorltlc tectonite

~""""~

/_->~---

- --_..--,.--

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PACKAGE III

Mississippian to Permian black chert

and clastics Intruded by diorite

Mississippian to Permian red and white chert Intruded by diorite

Mass/lle and pillow basalt with Interlayered chert

~ Undated brown and black argillite

o Serpentinite

o Mass/lie gabbro

r-;:;;-l Grey chert, r-;;;::J ~ occurs as large remnant within tonalite ~ Black argillite and chert arenite

Rhyolite and argillite, occurs as large remnant within tonalite

Pillow basalt

SYMBOLS

,ylvester Fault ...---- Intrusive contact 84- Lineation ~rved: Inferred

r"' Cleavage or foliation

Jndi~~ fault High-angle fault, 43

~ ~rved: \Inferred Barbs on down-thrown side ""'52 Bedding

rte

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PACKAGE IV

Foliated hornblende diorite

Altered ultramafic and serpentinite

Green quartz diorite

Microfossil sample site

Tree line

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Mississippian black. slllceou~ with minor chert, basalt and

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Vesicular basalt

Basal Sylvester Fault Observed: Interred

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.. ,..,

·ACKAGE I

Inge ' and green

"an chert

In black, siliceous argillite

UI'tII\:)

PACKAGE II

Late Permian green Interbedded

chert, tuff and b'IIsalt

Early Permian tonalite

Both Intrusive and tectonic contacts

IIPDm I Pennsylvanian limestone

Mississippian Nizi Fm carbonate

chert, basalt and carbonate ~

~ De vono-M/ss/sslpplan

black banded chert

1e grained green gabbro oth Intrusive and tectonic

asalt

'Ivester Fault ved: Inferred

Idi~~ tault ved: 'Inferred

Black argillite

Inter/ayered black chert, limestone

and sandstone

Siliceous chlorlt/c tectonite

r-;;;-l Grey chert, r-:;:;;:-l ~ occurs as large remnant within tonalite ~

Rhyolite and argillite, occurs as large remnant within tonalite

Pillow bualt

SYMBOLS

~- Intrusive contact 8a-

r"'

High-angle fault, 43

~ Barbs on down-thrown side 1"52

PACKAGE III

Mississippian to Permian black chert

and clastics Intruded by dlorlifl/

Mississippian to Permian red and white chert Intruded by diorite

Massive and pillow basalt with Interlayered chert

Undated brown and black argillite

Serpentinite

Massive gabbro

Black argillite and chert arenite

Lineation

Cleavage or foliation

Bedding

@ 5

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TEKLA ANN HARMS, Ph.D., DEPARTMENT OF GEOSCIEI

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lite

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PACKAGE IV

Foliated hornblende diorite

Altered ultramafic and serpentinite

Green quartz diorite

Microfossil sample site

Tree line

F GEOSCIENCES, UNIVERSITY OF ARIZONA, t 986

50'

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