Paleoslope Analysis of Slump Folds in the Devonian Flysch of Maine1

.Dwight Bradley and Lindley Hanson2

u.s. Geological Survey, 4200 University Drive, Anchorage, AK 99508. dbradly@usgs.gov

ABSTRACT

Ancient submarine slide and slump deposits in the Devonian flysch of central and northern Maine show considerablevariation in fold style, from symmetric to asymmetric to sheath geometries. Building on earlier work by Farrell andEaton, we suggest that the spectrum of fold styles reflects the degree of simple shear within each slump deposit. Wepresent a stereographic approach to paleoslope analysis that exploits fold hinge attitudes, axial surface attitudes, sheathaxes, vergence, S- and Z-asymmetry-depending on the style of slump folding. Our paleoslope determinations fromwidely scattered locations across the Devonian foreland basin in Maine show a regionally consistent pattern of west-erly to northwesterly slopes.

directions we deem reliable enough to report.Among the others, the most common problem wasthat the lower and upper contacts of a supposedslump horizon were not exposed, making it diffi -cult to rule out a tectonic origin for the deforma-tion and impossible to restore the slump folds topaleohorizontal. Slump deformation in some of thegiant outcrops was so chaotic that kinematic analy-sis was not even attempted. At small outcrops,slump horizons were found that contained only oneor two folds, which, we will argue below, is notenough to be very meaningful. All of these prob-lems eroded what might have been a much largerdata set.

The final difficulty, which we address here, isthat even among the most tractable slump deposits,there is considerable variation in structural styleand a corresponding variation in stereonet patternsof fold hinges and axial surfaces. At one extreme,fold hinges are clustered and poles to axial surfaceslie on a great circle; at the opposite extreme, hingeslie along a great circle while poles to axial surfacesare clustered. The problem is how systematicallyto treat these data, without resorting to ad hoc ap-proaches that work on some slump deposits but notothers.

Introduction

In the interval between the Taconic and Aca-dian orogenies, turbidites were deposited acrossmuch of Maine (figure 1 ). Many previous workers(Moench and Boudette 1970, p. A-1-17; Griffin andLindsley-Griffin 1974; Hall et al. 1976, p. 61; Lud-man 1977, p. 12; Pankiwskyj 1979, p. 40; Hansonand Bradley 1989; Roy et al. 1991) have inter-preted chaotic rock bodies within these Silurian-Devonian turbidites as slump deposits formed as aresult of downslope mass wasting. The slump de-posits have proven useful for paleogeographic stud-ies, because they suggest the presence of submarineslopes steep enough to fail. Although slump foldscan also reveal paleoslope directions, little efforthas been made in Maine to glean this sort of quanti-tative information. Among the papers cited above,the only one to report an actual paleoslope direc-tion is that by Hall et al. (1976), for a slump atGrand Lake Matagamon (figure 1 ).

In this paper, we present paleoslope determina-tions for six inferred slump deposits in the De-vonian flysch of Maine. We came across theseslumps during the course of a regional paleocurrentstudy, which focused on large, riverbank exposuresin structurally simple, homoclinal sections. Out ofdozens of inferred slump deposits we encountered,only the six described here have yielded paleoslope

Geologic Setting of Devonian Flysch in theAcadian Orogen of Maine

The slump deposits are from three depositionalbelts of Devonian flysch in the Maine Appalachians

1 Manuscript received July 31, 1997; accepted December 7,

1997.2 Dept. of Geological Sciences, Salem State College, Salem,

MA 01970.

[The Joumal 0£ Geology, 1998, volume 106, p. 305-3181 No copyright claimed £or this article. It remajns in the public domajn. 0022-1376/98/10603-0001$01.00

305

)"'>1.)

Figure I. Map of Maine showing major Silurian-Devonian paleogeographic belts, and locations of the main slumpsdiscussed in the text. Additional locations are as follows: 1-Gauntlet Fallsj 2- Tobey Falls, 3- Barrows Fallsj4-Coles Cornersj 5-Arnolds Landing; and 6-Greenville Junction. Large arrows show inferred paleoslope directionsbased on the present study, except at Grand Lake Matagamon, which is a generalized direction from Hall et al. ( 1976).

Journal of Geology ANALYSIS OF SLUMP FOLDS 307

tact is depositional, showing that the chaotic struc-ture originated on the seafloor rather than at depthdue to tectonism. Elsewhere in the region, manyoutcrops of Carrabassett !Formation consist entirelyof disrupted, fragment~d thin-bedded turbidites;these rocks typically display a fragment foliation,and the relative importance of soft-sediment versustectonic deformation is unknown. Coherent strataof the Carrabassett Formation include thin- andthick-bedded turbidites and massive sandstones,which we believe represent slope-basin and chan-nel subenvironments, tiespectively (Hanson andBradley 1989). Paleocurrent analysis at 16 sites inthe Carrabassett Formation reveals that the domi-nant flow direction was toward the north, with asecondary component toward the east (Hanson etal. 1993, p. CC-14). The paleocurrents record a newpaleogeographic regime; pre-Carrabassett strata inthe Central Maine basin were deposited by paleo-currents flowing either toward the southeast orsouthwest (Hanson and Bradley 1994).

Seboomook Group. Devonian turbidites alongthe Moose River- Travel~r Synclinorium (figure 1 )are assigned to the Seboomook Group. Detailedstudies at Grand Lake Matagamon (Hall et al. 1976;Pollock et al. 1988) have established that the Seboo-mook is a deep-water, base-of-slope.turbidite suc-cession associated with a west-prograding deltaiccomplex. Paleocurrent a~alysis of the SeboomookGroup at 10 sites indicates flow toward the west(Hanson and Bradley 1994). Inferred slump depositsare present but are a re~atively minor part of theSeboomook Group in this belt (Hall 1973; Hall etal. 1976).

St. John River Formation. In the CbnnecticutValley-Gaspe deep-water basin of northernmostMaine, most of the Seboomook Group has been as-signed to the informally named St. John River For-mation (of Roy et al. 1991). About two-thirds of thestrata belong to turbidite facies B, C, D, or mud tur-bidites, preserved in coherent, homoclinal stratalsuccessions. Most of the remainder of the forma-tion consists of siltstone-rich chaotically deformedstrata inferred to represent slump deposits. Sheathfolds are common (figuJe 2b). Paleocurrent datafrom three sites in the Connecticut Valley-Gaspebasin in Maine indicate flow toward the west (Han-son and Bradley 1994).

(figure 1 ): the Carrabassett Formation of the cen-tral Maine basin, an undifferentiated part of theSeboomook Group along the Moose River- TravelerSynclinorium, and the St. John River Formation (ofRoy et al. 1991) in the Connecticut Valley-Gaspebasin. These units contain variable proportions ofslate and graywacke, and stratigraphic thicknessesare typically > 1 km. The Devonian flysch of Mainehas been interpreted as a foredeep succession, thedeformed precursor to the classic Catskill delta ofthe Appalachian Basin (e.g., Bradley 1987; Hansonet al. 1993; Bradley 1997). In the areas of interest,the main affects of the Acadian orogeny were re-gional open- to tight-folding, development of a NE-striking cleavage, chlorite- and sub-chlorite-graderegional metamorphism, gabbroic and granitic plu-tonism, and cordierite- and andalusite-grade con-tact metamorphism in surrounding aureoles (e.g.,Osberg et al. 1989).

Carrabassett Formation. The Carrabassett For-mation, of probable Early Devonian age (e.g.,Moench and Pankiwskyj 1988; Osberg et al. 1985),is the youngest regionally extensive rock unit inthe Central Maine basin (Hanson and Bradley1989). Although it has not yielded diagnostic fos-sils, its age can be inferred from: ( 1) its stratigraphicposition about 1.5 km above an early Ludloviangraptolite in the Smalls Falls Formation; (2) litho-logic similarity with the Lockhovian and PragianSeboomook Group of the Moose River Synclino-rium; and (3) the fact that it was deformed, region-ally metamorphosed, and intruded by a number ofplutons with early Emsian concordant U /Pb zirconages of 404-408 Ma (Bradley et al. 1996). We havesuggested that the Carrabassett Formation was de-posited on the N-facing frontal slope of an advanc-ing Acadian orogenic wedge (Hanson and Bradley1989). This interpretation is based on its immedi-ately pre-Acadian age, our recognition of slope fa-cies, including slump deposits, and on the regionalpaleocurrent pattern.

Chaotic deposits comprise about 75% of the for-mation (Hanson and Bradley 1989). Coherent tur-bidites are interlayered with the chaotic deposits,and in several key places it is clear that the two areinterbedded rather than tectonically juxtaposed.The most revealing location is in the contact aure-ole of the Moxie pluton at Gauntlet Falls (figure 1;Hanson 1994!, where gently dipping turbiditeswere laid down over the irregular surface of a > 10-m-thick zone of disharmonically folded and dis-membered thin-bedded turbidites (figure 2A). cha-otic and coherent strata alike were contact-meta-morphosed before acquiring the prevasive regionalcleavage ubiquitous outside the aureole. The con-

Analytical Techniques

Identification of Slump Deposits. The regionalstructure of our study area is simple and straight-forward; in typical exposures one finds homoclin-ally dipping beds cut by a regional cleavage. The

308 D. BRADLEY AND L. HANSON

A

Figure 2. A. Slump deposit at Gauntlet Falls. Outcropis within the contact aureole of the Moxie Pluton, in an-dalusite-grade rocks. The strata were contact-metamor-phosed prior to Acadian regional deformation, which leftno imprint aside from the gentle bedding dip. Olistos-trome below the bedded turbidites consists of steeplydippling, dismembered siltstone bed segments and a fewrootless folds. Interbedded chaotic and coherent strataare also present in the Carrabassett Formation at LittleWilson Stream, Tobey Falls, Barrows Falls, and ColesCorners (figure 1!. B. Sheath fold from a well-exposedslump horizon in the Temiscouata Formation (Seboo-mook equivalent), northwestern New Brunswick, a fewtens of kilometers along strike from Big Rapids (figure1 ). The movement was roughly toward or away from theobserver. The three slump deposits from Big Rapids, ana-lyzed here, display comparable folds, but are not as pho-togenic.

B

folded zones we identify as slump deposits are visu-ally striking: anomalous, disharmonically foldedlayers enclosed in strata affected only by the re-gional folding (figure 3 ). The strata below and aboveface in the same direction. The six folded zones forwhich we quote paleoslope directions all includedismembered bed segments. Despite the intensityof deformation, there is no evidence of syn-defor-mational veining, for example, at fold hinges on theconvex sides of competent beds. A slump shouldhave a detachment at its base and a depositionalcontact at its top (Elliott and Williams 1988). Ofthe six folded zones for which we quote paleoslopedirections, the upper contact is exposed at four andthe lower contact is exposed at four. In all cases,the outcrop evidence is consistent with these crite-

ria. In four instances, the folds demonstrably pre-date. regional cleavage, and in no case is the reversetrue (Hanson 1994; Kusky et al. 1994). In all si:xcases, the weight of evidence supports a slump ori-gin. Alternatively, a tectonic explanation for anyof the disharmonically folded zones would requirespecial pleading.

Reference Frames and Retrodeformation. Two ref -erence frames are normally used in analyzingslump folds in tectonically deformed rocks: ( 1) thepresent-day reference frame, in which folds are ac-tually measured, and (2) the stratigraphic referenceframe, in which the original attitudes of slumpstructures are interpreted. The stratigraphic refer-ence frame is found by restoring to horizontal thehost bedding that stratigraphically overlies or un-

309ANAL YSIS OF SLUMP FOLDSJournal of Geology

Figure 3. A. Tape-and-compass map of the stratabound slump horizon at Harrington Lake, showing upright, symmet-ric folds. B. Stratabound slump horizon showing asymmetric folds, from the Seboomook Group (St. John River Forma-tion of Roy et al. 1991! near Three Mile Island, Allagash River. Sketch from a photograph. c. Tape-and-compass mapof a stratabound slump horizon showing synthetic faults and folds, and antithetic folds, from Little Wilson Stream.Tb and Tc are the B and C divisions of the Bouma turbidite sequence.

derlies a slump. Ideally, this procedure should ex-actly reverse any changes in attitude due to de-formation, just as in paleocurrent analysis. Thepossibility that strata hosting a slump may haveoriginated on a slope with some initial dip is nor-mally glossed over in studies of ancient rocks (fig-ure 4). This is not an unreasonable simplification,since most submarine slopes dip less than lO°, butit does complicate the understanding S- and Z-foldasymmetry.

All but one of the slump deposits were restoredto paleohorizontal by the standard single-tilt cor-rection: host bedding and slump structures wererotated about an axis parallel to bedding strikethrough an angle equal to the bedding dip. This isappropriate for the Devonian flysch in central andnorthern Maine, which is affected by a single gener-ation of upright, NE-trending, open-to-tight foldswith a single axial-planar cleavage. (There are partsof the Acadian Orogen-for example, southwesternMaine, New Hampshire, and Massachusetts-

where deformation was polyphase, and the single-tilt correction would be wholly inappropriate.) Atwo-step rotation was used for one slump (LittleWilson Stream; see below), where regional beddingstrike was deflected during emplacement of anearby pluton. For want of mesoscopic strain mark-ers, we did not account for the minor effects of pen-etrative tectonic strain in the paleoslope analysisthat follows. Where we were able to measure andcorrect for strain in our regional paleocurrent stud-ies, it turned out to have a negligible impact. Spe-cifically, in the Madrid Formation at Amolds Land-ing lfigure 1 ), 26 cross laminae in subvertical bedsyielded a mean paleocurrent direction of 254°, sub-ject to single-tilt correction only, compared to 249°,subject also to orthographic strain correction (Ran-son et al. 1993!.

Style of Slump Folding and Stereographic Meth-ods. Farrell and Eaton (1987,1988) presented an el-egant model that accounts for much of the varia-tion in structural style of slump folds (figure 5!. In

310 D. BRADLEY AND L. HANSON

Figure 4. Vertically exaggerated block diagram showing idealized slump folds on a slope and at its base. correspond-ing stereonets illustrate orientations of poles to axial surfaces (filled circles) and fold axes (open circles). The paleoslopedirection could be determined by the separation arc method from either fold, but note that the right-hand limb of theslump would be transformed from an S to a Z were the slump to travel from the slope out onto the basin plain. Thisproperty of slump-fold asymmetry has not been mentioned in previous discussions of the separation-arc method, butfortunately, it does not make much practical difference because ancient slump deposits are restored to an inferredpaleohorizontal.

deposits, fold axes cluster in the paleoslope direc-tion, rather than normal to it. Here, the downslopemean-axis method is useful. In the model shownin figure 6, slump folds originate with their axesparallel to the slope; with moderate downslopemovement, fold axes begin to spread along a girdle,and with continued movement they begin to clus-ter in the transport direction. In the extreme (figure6C ), fold axes cluster in the downslope direction,parallel to sheath axes, and the girdle disappears.Fold axes are reoriented as a result of simple shearduring downslope movement; the only folds to es-cape reorientation are those with initial orienta-tions exactly parallel or exactly perpendicular tothe strike of the slope.

The separation-arc method of Hansen (1971)also accounts for downslope rotation of fold axes,but it is better suited for slump folds with axes dis-tributed along great circles rather than in clusters.Natural folds similar to the one in figure 4A havebeen described from a tundra slide by Hansen( 1971 ), and from a snow slide by Lajoie ( 1972). Han-sen (1971) showed that the downslope direction bi-sects the angle (Hansen's "separation angle") be-

their model, slump deposits that have undergoneonly short translations tend to display upright, par-allel folds, the product of bedding-parallel contrac-tion. Slumps translated further experience a super-imposed simple shear, which produces inclinedasymmetric folds, then recumbent similar folds,and in the extreme, sheath folds.

Since the pioneering work of Jones (e.g., 1940),many paleoslope determinations from slump foldshave relied heavily, or even exclusively, on the ori-entation of fold axes. The oldest and most widelyapplied method is based on the supposition that thetrend of fold axes formed by downslope movementshould be normal to the paleoslope (e.g., Jones1940). Woodcock (1979) called this the alongslopemean-axis method. The idealized slump folds infigure 6A are amenable to the alongslope mean-axismethod. Stereoplots of fold axes alone, however,yield two equally viable choices of paleoslope di-rection, i.e., either east or west in figure 6A. Thecorrect choice can be made from a combinationof other paleogeographic information (e.g., Jones1940), and (or) the vergence of slump folds.

Woodcock (1979) has shown that in many slump

312 D. BRADLEY AND L. HANSON

Figure 6. Three structural styles ofslump folds stacked from the topdown in order of increasing simpleshear. Arrows in stereonets showslope direction. A. Initially, fold axesshow tight clustering normal toslope. Alongslope mean-axismethod gives the paleoslope direc-tion. B. With moderate downsloperotation, fold axes acquire S and Zasymmetry and define two fieldsalong a great circle corresponding tothe slope. SA is the separation arc,which gives the paleoslope direc-tion. c. With continued downsloperotation, the separation arc narrowsand then disappears completely, asfold axes are reoriented toward thedownslope direction. Downslopemean-axis method (or better yet,sheath axes) gives the paleoslope di-rection.

".

~

-5.

.~

~

the case of a tectonically undeformed slope like theone in figure 4. In the present-day reference frame,asymmetric folds at the base of a slope have axialsurfaces that dip up slope (figure 4B). Axial surfacesof folds on a slope may either dip gently downslope(figure 4A), or gently up slope. In the stratigraphicreference frame, however, axial surfaces of syn-thetic folds dip up slope. Consequently, in analyz-ing slump folds from dipping stratal which will besubject to structural tilt-correction, it doesn't mat-ter whether the folds came to rest on a slape, or atthe base of a slope: the relationship between foldaxial surfaces and the paleoslope direction is thesame.

Paleoslope Determinations

In the foregoing discussion, the various idealizedstereographic patterns were based on many mea-surements around a single fold. In all the ste-reoplots that follow, however, the structural dataare from different folds-generally one hinge lineand one axial surface per fold. By necessity, we use

measurements from several folds to build a com-posite picture of the fold geometry .

Slump Deposits with Upright, Symmetric Folds. Wereport a single example of a slump dominated bypure-shear deformation, a spectacular horizon thatresembles a rumpled rug (figure 3A). The foldedzone crops out along the shore of Harrington Lake(figure 1; for details see stop 3 of Hanson et al. 1993,or stop 9 Kusky et al. 1994). The folded zone over-lies a moderately dipping (average dip is 48°), homo-clinal section of thin-bedded turbidites; a mean pa-leocurrent direction of 265° was obtained from 55cross laminae (Hanson et al. 1993, p. CC-13). Thebasal contact of the folded zone is a detachmentthat parallels the underlying bedding; disruptedbeds just above the detachment are discordant toit. The upper contact is not exposed, but coherentturbidites occur a few meters farther upsection.The exposed part of the slump is about 8.5 m thick.One large fold is overprinted by, and hence is olderthan, the regional cleavage that makes a -60° anglewith the axial surface (figure 3A).

The largest folds are upright and symmetric.

Journal of Geology 313ANAL YSIS OF SLUMP FOLDS

Figure 7. Two folds with opposite vergence (V arrowsj,each having portions that have S asymmetry, and por-tions having Z asymmetry. A minor change in theamount of plunge can change the sense of asymmetry ofthese folds, but not their vergence. Vergence is thereforea much more robust and useful property than S or Zasymmetry.

Near the base, some of the small folds are asym-metric. Fold axes form a loose cluster on the ste-reoplot (figure 8A) but are nondiagnostic becausethe alongslope mean-axis method yields a paleo-slope of either 070° or 250°, whereas the downslopemean-axis method yields a paleoslope of either160° or 340°. About half the folds are asymmetricand can be described as S's or Z's. The S and Z fieldsoverlap, however, so the separation-arc methoddoesn't work. In contrast with the other slumpfolds in figure 8, poles to axial surfaces are distrib-uted about a vertical great circle, which strikesabout 250°. The folds show no preferential verg-ence. However, given the style of the large folds(figure 3A) and the tight clustering of axial planestrikes, the best solution is a slope of either 070° or250°. In light of the paleocurrent data and regionalrelations, which indicate that the Seboomook is the

deep-water facies of a deltaic sequence farther east(Hall et al. 1976), the best choice is 250°. In hind-sight, then, the alongslope mean-axis methodwould have yielded the correct result.

Slump Deposits with Asymmetric Folds. Twoslump deposits in our study display asymmetricfolds with inclined axial surfaces. These are broadlysimilar to the slump illustrated by Farrell ( 1984, hisfigure 2B). The first is in the St. John River Forma-tion (of Roy et al. 1991) near Three Mile Island onthe Allagash River (figure 1 ). The folded zone is 20-30 cm thick and sandwiched between thin-bedded,silt turbidites (figure 3B). Upright bedding dips71° E. The younging direction in underlying bedsis evident from subtle grading. The base of thefolded zone is marked by a throughgoing bed of silt-stone deformed by two small folds that are recum-bent in the stratigraphic reference frame (figure 3B).The folded zone itself consists mostly of dark, chlo-ritic slate, but within it are dismembered beds ofsiltstone, which outline tight, asymmetric, nearlyrecumbent folds with thickened hinges. The imme-diately overlying silts tone bed, which youngs in thesame direction as beds below the folded zone,shows no evidence of bedding disruption or out-crop-scale folding. This bed everywhere overliesslate, in which bedding cannot be seen, but it trun-cates the projections of axial surfaces defined by thefolded silts tones a few centimeters stratigraph-ically below. A well-developed regional slaty cleav-age affects both the folded zone and beds above andbelow. Dihedral angles between fold axial surfacesand cleavage range from nearly parallel to 14°. Thefolded zone is most readily interpreted as a slumpaccumulation, although a tectonic origin cannot beruled out. The upper contact appears to be deposi-tional; the lower contact sheared. Quartz veins arenotably absent around the fold hinge areas. In con-trast, at a nearby stratabound duplex of undoubtedtectonic origin, such veins are abundant and clearlyindicate that duplexing post-dated lithification(Bradley and Bradley 1994). After single-tilt restora-tion of enclosing beds, a well-defined separation arcof s- and Z-fold axes indicates a paleoslope to thenorthwest (figure 8B). Poles to fold axial surfacesalso cluster tightly and indicate the same slope di-rection (310°, combined visual mean). This is simi-lar to the westerly paleocurrent directions men-tioned above. .

A slump in the Carrabassett Formation (figure3C I illustrates some complications. The 30 cmthick folded zone occurs within a homoclinal, well-bedded succession along Little Wilson Stream (fig-ure 1 ). Sedimentary structures are remarkably wellpreserved; tectonic strain is negligible. The folded

314 D. BRADLEY AND L. HANSON

c PSB

310

I These two becQInel

'S: jo/dsdurjng i, ttlt-correchQn ,

ASYMMETRIC FOLDS

PRESENT-DAYREFERENCE FRAME

Pole to bedding andretrodeformation path

Figure 8. Lower hemisphere equal-area projections of structural data from slump horizons in Maine. Each plot showsthe pole to bedding, as measured, and the tilt-correction path along which bedding was restored to paleohorizontal.To facilitate comparisons between different slump deposits and with hypothetical models, all other data have beenplotted only in the stratigraphic reference frame, subject to the same rotation(s) as bedding. Poles to cleavage areplotted in the stratigraphic reference frame to aid comparison between cleavage and fold axial surfaces. PC is paleocur-rent direction as determined from nearby strata (see text for references). PS is the inferred paleoslope direction, withazimuth in degrees.

zone is about 40 cm thick and is sandwiched be-tween medium-bedded sandstone turbidites dis-playing the Bouma B and C divisions. Bedding dips79° N; graded bedding in both underlying and over-lying beds shows that the section is upright. Thebase of the folded zone is a detachment surface(figure 3C ). The folded zone consists of interlayereddark metapelite, cross-laminated siltstone, andsandstone. The largest folds in the outcrop have

sheared lower limbs recumbent in the stratigraphicreference frame; these repeat the section within thefolded zone and are labeled as "main syntheticfolds" in figure 3C. The smaller folds are both anti-thetic and synthetic. The Little Wilson Stream ex-posure lies in the contact aureole of the Onawa Plu-ton where the strike of bedding is nearly E- W, about33° clockwise of the regional bedding strike; the de-flection of bedding strike either came during or

STRATIGRAPHIC REFERENCE FRAME

.Pole to fold axial surface ..Pole to cleavage

("'I Fold axis (S and Z .Sheath fold axis

Journal of Geology 315ANALYSIS OF SLUMP FOLDS

head of Big Rapids. Its lower contact is not exposed;it is overlain along what appears to be a deposi-tional contact by thin-bedded silt turbidites, whichin turn are overlain by thick-bedded graywacke tur-bidites. Upright bedding dips 75°£. All of the foldswithin the slump horizon appear to be rootless. Theaxial surfaces of some folds are cut obliquely bythe regional cleavage. Some bed fragments are inthe shape of asymmetric S- or Z-folds; others haveeye-like patterns in plan view. Fold axes clusterfairly tightly (figure 8E) and, given the sheath-styleof folding, the downslope mean-axis method isdeemed appropriate, yielding a paleoslope of either314° or 134°. In light of the paleocurrent data, 314°is the best choice. Poles to axial surfaces cluster inthe northwestern quadrant, supporting the north-westerly paleoslope. Because S and Z fold axes over-lap, a separation arc cannot be defined.

A third slump, several tens of meters thick, wasmeasured in the middle of Big Rapids (figures 1 and9C ). Its lower contact is not exposed; it is deposi-tionally overlain by thick-bedded graywacke tur-bidites, which in turn are overlain by a fragmentedolistostromal horizon. Upright bedding dips 65°£.The slump folds are defined by rootless sandstonebed segments set in a slate matrix; the axial sur-faces of some of the folds are cut obliquely by theregional cleavage. None of the measured fold axeshave a clear sense of asymmetry; hence the separa-tion-arc method is not applicable. Furthermore, incontrast with the other sheath-folded slump depos-its, fold axes here display a 180° spread along a greatcircle (figure 8F). Neither the normal mean-axis,downslope mean-axis, nor separation-arc methodsare applicable. Poles to axial surfaces tightly clusterabout a westerly trend. A single sheath axis trendsNW-S£. Together, these data suggest a paleoslopetoward about 286°.

Many other large outcrops in the St. John RiverFormation (of Roy et al. 1991), and in the correla-tive Temiscouata Formation in nearby New Bruns-wick (figure 2B), consist entirely of chaotically de-formed, silt-rich turbidites that resemble the threeslump deposits at Big Rapids. Ductile-lookingsheath folds are abundant, despite the very lowgrade of regional metamorphism (prehnite-pumpel-lyite; Osberg et al. 1985). Unfortunately, becauseoverlying or underlying beds are not exposed, the

, stratigraphic reference frame is unknown for theseprobable slump deposits.

Recommended Procedures

Slump folds are so variable in geometry that a flex-ible approach is required in paleoslope analysis: no

after the pluton was emplaced. Structural correc-tion involved ( 1) a 33° counterclockwise vertical-axis rotation to correct for the deflection aroundthe pluton, and (2) single-tilt correction of beddingto horizontal. Fold axes within the slump horizonare fairly tightly clustered (figure 8C ), but it is notobvious, from the fold axes alone, whether theytrend in the paleoslope direction, or normal to it.The separation-arc method is useless because S andZ folds do not form distinct groupings. (Two of thefold axes were classified in the field as Z folds, butduring tilt-correction, they crossed the primitivecircle and thus had to be reclassified to S folds.)Axial surfaces of the synthetic folds dip south, theantithetic ones dip north. The axial surfaces sug-gest a paleoslope of about 336°, corrected forOnawa pluton deflection. The fold axes, therefore,trend roughly parallel to the inferred slope direc-tion. For comparison, a mean paleocurrent direc-tion of 037°, corrected for Onawa pluton deflection,was determined from 56 directional indicatorsalong Little Wilson Stream (Hanson and Bradley1994).

Slump Deposits with Sheath Folds. We analyzedthree slump deposits characterized by sheath foldsalong a 2-km section of the St. John River knownas Big Rapids, near the town of Allagash (figure 1 ).All three slump deposits are in the St. John RiverFormation (of Roy et al. 1991). The best-developedsheath folds are in an outcrop at the foot of Big Rap-ids. The folded zone is several tens of meters thickand consists of folded bed segments of sandstone(figure 9A and B), enclosed in a black slate matrix.An intact, throughgoing upright bed which dips89° E underlies the folded zonej the upper contact isnot exposed. Folding clearly predated cleavage, asindicated ( 1) by the trace of folded bedding shownin figure 9A, exposed on a planar cleavage surface,(2) by a 180° distribution of bedding-cleavage inter-section lineations along a girdle (figure 9A), and(3) by several folds whose axial surfaces are stronglyoblique to the regional cleavage. Fold axes show afairly tight clustering along a NW-SE trend (figure8D), and given the sheath style of folding, thedownslope mean-axis method is applicable. Twosheath axes also trend NW -SE. Together, these datasuggest a slope toward either 318° or 138°j in lightof the regional paleocurrent direction, 318° is theclear choice. The poles to fold axial surfaces clusterin the northwestern quadrant, which would tendto confirm the 318° paleoslope. The separation-arcmethod does not work for this slump deposit be-cause none of the folds have a clear sense of asym-metry .

A second zone with sheath folds is located at the

316 D. BRADLEY AND L. HANSON

Figure 9. A. Sketch of a pre-cleav-age slump fold at the foot of BigRapids, exposed on a cleavage face.Stereonet shows a great-circle dis-tribution of bedding-cleavage inter-sections from this outcrop, whichconfirms that folding predated theregional cleavage. B. Fold fragmentwhose axial surface is transected bythe regional cleavage, from the footof Big Rapids. c. Dismembered bedsand rootless folds from the slumphorizon in the middle of Big Rapids.

B.cross-sectional view ofcleavage surfaceA. map view NE

sw NE sw50cm

5cm

c. oblique cross section

Slate withcleavage trace

Sandstone,siltstone

50cm

could be made around a fold like that in figure 6C,but the results would only lead in a roundaboutway to the same conclusion as could be drawn froma single measurement of the sheath axis. Slumpfaults and any associated slip lineations, if present,should also be measured.

The attitudes of any tectonic folds (axis and axialsurface axis), cleavage, and mesoscopic strainmarkers are needed to retrace the deformation pathand thus restore the slump to paleohorizontal. Ste-reographic techniques for retrodeformation are thesame as for paleocurrent analysisj the pitfalls arelikewise the same. During retrodeformation, if aslump-fold axis crosses the primitive circle, itssense of asymmetry will flip from S to Z, and vice-versa.

The interpretation of paleoslope from the struc-tural data begins with classification of the slumpaccording to the overall geometric style of foldswithin it. For slump deposits containing at leastsome upright, symmetric folds, we offer the follow-ing guidelines. ( 1) Axial surfaces are likely to havefairly constant strikes but may show scatter intheir dip and dip direction, with no preferentialvergence among the whole population. If poles toaxial surfaces show enough variation: II attitude todefine a great circle, it will strike in the paleoslope

single method will work for every slump deposit.The first order of business is to assess the evidencethat a particular folded zone is, indeed, a slump. El-liott and Williams (1988) reviewed the diagnosticvalue of various criteria for slump versus tectonicfolding; their paper, however, stresses exceptionsand caveats to the admittedly imperfect, but stillhandy, guidelines for recognizing slumps (e.g., Hel-wig 1970). Careful, detailed observations within asuspected slump are a must-especially evidencefor a depositional upper contact.

The second order of business is to measure theattitude and observe the way-up direction of coher-ent strata that overlie or underlie the slump. With-out this information, there isn't much point in con-tinuing because it will be impossible to correct fortectonic deformation.

For folds within the slump, the axis and axialsurface should be measured, and the sense of asym-metry-S, Z, or neither-observed in the presentreference frame. For asymmetric folds, the verg-ence in the stratigraphic reference frame shouldalso be assessed, and each fold classified accord-ingly as either synthetic or antithetic. For sheathfolds, the direction of most interest is the sheath'slong axis, and not the various local orientations ofthe curved fold axis. Many fold axis measurements

j

.

Journal Geology ANAL YSIS OF SLUMP FOLDS 317

pattern across a wide region (figure 1 ). This patternwould seem to assuage various concerns about po-tential pitfalls, and thus about the significance ofthe results. The first of these, already discussed atlength, is whether the disharmonically foldedzones are of slump or tectonic origin. Inappropriatestructural corrections may have been applied, per-haps leading to errors of up to a few tens of degreesin the quoted paleoslope directions. It is also plausi-ble that some or all of the slumps formed due tolocal slope failure in a submarine fan setting-forexample, the collapse of a levee into a channel-rather than on a slope of regional scale. Finally, be-cause each slump must be interpreted from a singleexposure, it is possible that the results are not rep-resentative of the whole slump sheet.

We conclude from the results summarized infigure 1 that the regional topographic grain wasroughly parallel to present structural grain, and theslopes were down toward the craton. Paleocurrentfrom the same rock units show a similar regionalpattern (Hanson and Bradley 1994). The paleoslopeand paleocurrent data together reinforce the inter-pretation of the Devonian flysch as a foredeep suc-cession that was deposited in front of a cratonward-advancing orogenic wedge (Bradley 1987, 1997).

direction. (2) In our single example at HarringtonLake, fold axes are fairly tightly clustered normalto flow direction. Hence, the alongslope mean-axismethod provides essentially the same paleoslopedirection as the axial surfaces. (3) Both axial sur-faces and fold axes are bidirectional paleoslope indi-cators in this case. Regional facies relations and (or)paleocurrent data must be relied on to choose be-tween the two possibilities.

For paleoslope analysis of asymmetric folds:( 1) Axial surfaces of the main, synthetic folds dipup slope, as do any associated contractional faults.(2) Axial surfaces of antithetic folds, as well as anyassociated contractional faults, dip in the oppositedirection. (Obviously, it is essential to decide in thefield whether a fold is synthetic, antithetic, or per-haps, neither.) (3) Fold axes are more problematic.In one of our examples, S and Z fold axes have agreat-circle distribution and define a beautiful sepa-ration arc. In the other example, however, S and Zfold axes are fairly tightly clustered in the inferredpaleoslope direction, and the fields of S and Z foldscompletely overlap. In this second case, the down-slope mean-axis method yields the same result asthe fold axial surfaces and slip lineations, andtherefore seems applicable.

For paleoslope analysis of sheath folds, we sug-gest the following guidelines: ( 1) The most usefulmeasurements are sheath axes, which, unfortu-nately, are bidirectional indicators. (2) The down-slope mean-axis method is applicable but the resultis only bidirectional. (3) The fields of any S and Zfolds are likely to overlapj S and Z folds may be rareif, as in our examples, the slump folds are root-less.

ACKNOWLEDGMENTS

The manuscript benefited from careful reviews byDykstra Eusden, Peter Haeussler, Julie Dumoulin,and an anonymous reviewer. Field work was sup-ported by NSF Grant EAR-8803233, bya series ofgrants and contracts from the Maine GeologicalSurvey, and by Dee Caldwell, head of Boston Uni-versity's Geology Field Camp. Bradley's salary dur-ing manuscript preparation was provided by aUSGS Gilbert Fellowship.

Discussion and Regional Significance

Paleoslope directions deduced from slumps in theDevonian flysch of Maine show a consistent map

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