Sedimentary environments and palaeogeography of the late ...csmgeo.csm.jmu.edu/Geollab/Whitmeyer/IrelandField... · gradually shallowing sedimentary sequence, described recently by

Sedimentary environments and palaeogeography of the late Llandovery and earliest Wenlock

of North Connemara, Ireland

D A V I D J O H N W I L L I A M P I P E R

8 References.

C O N T E N T S Introduction • . . . •

The lithostratigraphic classification and age of the rocks . Lough Mask Formation Kilbride Formation

(A) General . (B) First Division (c) Second Division (D) Third Division (p) Fourth Division (F) Environmental interpretation

Tonalee Formation Lettergesh Formation . Conclusions

(h) Palaeogeographic development (B) Fault control of sedimentation

3 3

• 3 5

37 3 8

38 38 39 40 4 t

4 I 42 43 46

• 4 6

• 48 49

S U M M A R Y

The upper Llandovery rocks of North Conne- mara record a gradual marine transgression at the northern margin of the Lower Palaeozoic geosyncline. The lowest rocks are red sandstones deposited from mainly southward flowing rivers• These are overlain by lagoon and coastal barrier sandstones and siltstones, and deeper water shelf siltstones. Red marine siltstones and mudstones, deposited in Ioo to 2o0 m depth of water, are found near the top

of the Llandovery sequence. Deeper water turbidite conglomerates and sandstones, with some thick mudstones, both overlie and laterally interfinger with the sequence of non-marine and shelf sediments. Basement faults controlled rates of subsidence during shelf sedimentation, and topographically defined the margins of turbidite accumulating basins. A palaeogeographic synthesis of the area is presented.

I. In troduc t ion

THE UPPER LLANDOVERY and basal Wenlock rocks of Nor th Connemara lie on the nor thern margin of the British Lower Palaeozoic geosyncline, a few tens ofkilometres south of the Highland Boundary Faul t (Fig. I). Non-marine, shallow water marine and deeper water turbidi te sediments are found in close association, with progressive deepening of the depositional environment. The middle and upper Wenlock (possibly extending to lower Ludlow) rocks of the area have a

Jl geol. So¢. vol. xa8~ 1972 , pp. 33-5 I, 8 figs. Printed in Northern Ireland•

8

34 D. Jr. W. Piper

gradually shallowing sedimentary sequence, described recently by Laird and McKerrow (1970).

The Silurian rocks rest unconformably on the Connemara Schists and Ordovi- dan rocks of the South Mayo Trough. They were subsequently block faulted, tilted and gently folded during late Silurian or Devonian times, accompanied by dyke and sill intrusion (McKerrow & Campbell i96o ). In the west of the region, Ordovician rocks are thrust over the Silurian, but at Lough Nafooey, the thrust fault passes entirely into the Ordovician, and the Ordovician-Silurian junction is unconformable. The amount of movement on this fault is unknown. McKerrow and Campbell's (i 96o) map of the region is generally in accord with the author's data, except in the Glen Trague-Bunnacunneen area, where they recognise a series of east-west anticlines and synclines. The author's interpretation, based on a more detailed stratigraphic subdivision of the rocks, is of a block faulted structure, with a repetition of stratigraphic units from one block to another (Fig. 2). There has been a little post-Carboniferous block faulting throughout the region (McManus i967) and a few Tertiary dykes are found (McKerrow & Campbell I96O ). The area is cut by the large dextral strike-slip Maam Faults, which have a long history of movement dating back to the Ordovician (Stanton x96o ). The total post-Silurian displacement is about 5 km.

At the present day, the region is mountainous, with a Carboniferous summit peneplane at around 600 m above sea level (Dewey & McKerrow 1963). Exposure is good on steep glacial valley sides, but many of the flatter areas are bog covered.

The first systematic work on the region was in the i87os by the Geological Survey of Ireland. Gardiner & Reynolds (i 912, 1914) mapped the north-eastern part of the region, and McKerrow & Campbell (i96o) have recently mapped the rest of the region. Piper (i967) reviews previous work in the area.

L e t l e ~ K i t o y Horbour

'~ ~ ° g ° ' g " ~ ~ " m ~ ' , , , !' i i / ' , BENCHOONA . ~.""~?'--~ q, i!li '. FAULT Lettershonbolly-J~

ORDOVICIAN ~ o u g h

' ~ Lough Nofooey ~ - - ~ M o s k

" ~ C ONNEMARA SCHISTS " ~ " ' ~ ~ Z~'~

o.oo

FIo. i . T h e lower Silurian rocks of Nor th Connemara . Inset shows locat ion of area (HBF = High land Boundary Fault) . Based on Piper, I967, Fig. x.

Sedimentary environments and palaeogeography of North Connemara, Ireland 35

T A B L E I : Silurian stratigraphy of North Connemara (in part based on Laird & McKerrow z97o )

Culfin Supergroup: metres

Killary Harbour Group Salrock Formation 8x 7 Lough Muck Formation 2oo-28o

Upper Owenduff Group Glencraff Formation 65 Lettergesh Formation x52o

[includes Gowlaun Member at base 3o-37 o] Lower Owenduff Group Tonalee Formation o-75

Kilbride Formation o-34o Lough Mask Formation o--z 70

2. T h e l i t hos t r a t i g r aph ic classification a n d age of the rocks

Laird and McKerrow (i97o) have named the complete Silurian sequence of North Connemara the Culfin Supergroup. This they divide into three groups (Table i). The uppermost, the Killary Harbour Group, is about I km thick and comprises shallow water marine and estuarine sediments. Below this, the Upper Owenduff Group comprises 1.5 km of dominantly turbidite sediments. The Lower Owenduff group, up to 0.5 km thick, comprises fluviatile and shallow water marine deposits. In this paper, the Lough Mask, Kilbride and Tonalee Formations, comprising the Lower Owenduff group, and the Gowlaun Member of the Letter- gesh Formation, at the base of the Upper Owenduff Group, are described.

L o u g h Nofooey

0 0 0 0 0 IT I'-ILEI 3"~ ,-, 0 _ ,-,'r n r l l

o o( Bunnocunneen 0

. . . , . . . . . . - . .

. . . , . . . . . . • . . . . • , .

:.:---Z ! . . . . . . " "

• . . - . . . . . . - - ~ . . , . • - . ~ , . . . • / i - : . ! °

• . . . . . . .

• " ' - ' ' . ' 2 " . "

Upper Lettergesh Fm.

- ~ Lower Owenduff Gp.

~ . ' . ' . ~ . . .

n Trogue . _ . . . ~ , , -7"~.

"~ r , ~ . d 30 • . i uemug m°re:

0 ~ km Ii " / / ' '~

O c ~ 9

Gowlaun Mbr. conglomerote ~ Benbeg Mudstone

,S Connernoro Schists 0 Ordovicion

FzO. 2. Geological map of the area around Glen Trague.

36 D. J. W. Piper

The contact of the Upper and Lower Owenduff Groups is strongly diachronous with interfingering in places, and is older in the west than in the east (Piper 1967). Figures 3 and 4 summarise the stratigraphy of the area. The Lough Mask Formation comprises unfossiliferous fluviatile sandstones, and its age is unknown. It passes conformably up into the shallow water marine Kilbride Formation, which is not older than Upper Llandovery C5, and is probably restricted to that horizon (McKerrow & Campbell I96O; Laird & McKerrow in press; L. R. M. Cocks pers. comm.), on the basis of the brachiopod faunas. The overlying Tonalee Formation is almost unfossiliferous. One brachiopod fauna, dominated by Eocoelia, was dated by Cocks as most probably around the C 5 to C 6 boundary;

WEST EAST Lough Fee Lettershonbolly Kilbride

U PP ER L E__.T TJ R 2 E 2 H F 2 R._M A__T I 2N

GOWLAUN MEMBER ~ TONALEE FORMATION - -

S ~ .. KILBRIDE FORMATION

FIG. 3" Schematic representation of the stratigraphy of the lower Silurian of North Connemara.

Maximum thickness of beds below the upper Lettergesh Formation is about half a kilometre. Modified from Piper, x967, Fig. 2.

Lettershanbatly A [ ~ B

Tonolee I a' T O N A L E E

L. Nafooey Kilbride c i c'

! l ,

,~,~ i ~ l L ' "! i ' i

KILBRIDE ! i i l i ! i ! i ' l I ! jl,==

' , ' " . " " ' i

L°u° '

0 km 2 = . . . . . J

FZG. 4" East-west cross section through the Lower Owenduff Group of North Connemara.

Inset (cf. Fig. I.) shows outcrop of Kilbride Formation and location of sections. I = First Division; 2 = Second Division; 3 = Third Division; 4 = Fourth Division.


it is probable, however, that this fauna was derived by penecontemporaneous erosion of the immediately underlying topmost Kilbride Formation. The Tonalee Formation comprises marine red mudstones, which accumulated in deeper water than the Kilbride Formation: similar red mudstones are widespread in Britain around C 6 times.

Rickards & Smyth (i968), in a review of the few graptolites known from the Lettergesh Formation, recognized the lower Wenlock murchisoni and riccartonensis zones from the Clonbur area. Recently, Rickards (pets. comm.) has found a crenulatus zone fauna in the Gowlaun Member of Kilbride. In the west of the region, the base of the Lettergesh Formation must be C 5 Upper Llandovery or older, from the interfingering relationships with the Lower Owenduff Group.

3. Loch Mask Formation

In its type area (Piper i967) , the Lough Mask Formation has a basal extrusive keratophyre (up to I 15 m thick) overlain by a sequence of massive and cross bedded red sandstones. At the top of the Formation, red mudstones and siltstones interbed with the sandstones.

The keratophyre is restricted to the east of the area (Fig. 4). Its petrology was described by Gardiner & Reynolds (i9I 2). Its extrusive nature is clearly demon- strated by the occurrence of large detrital clasts in overlying rocks. In many outcrops, the keratophyre has a characteristic spheroidal appearance, with spher- oids 3 ° to IOO cm diameter. McKerrow & Campbell (i96o) interpreted these as pillow lavas, but since the spheroidal form is independent of the internal structure of the keratophyre, in some cases even truncating amygdales, it is more probable that they result from weathering.

Massive and cross bedded medium red sandstones are the dominant lithology of the Lough Mask Formation. Especially in the lower parts of the succession, some of the sandstones are coarse or even pebbly. Cross bedding is usually grouped, and of Allen's (i963) omikron andpi types, in sets io to IOO cm thick. A few channels have been recognised in places where the filling and the surrounding sandstone are easily distinguished in the field. Cross bedding directions are generally to the south (Fig. 5)- Petrologically, most of the sandstones are arkoses and subgrey- wackes (terminology of Pettijohn 1957).

The unimodal cross bedding directions, red colour, lack of fossils, and presence of channels and rare pebbles all suggest deposition in a fluviatile environment. The lack of fine lithologies interbedding with the sandstones suggests the rivers were of a braided type (Allen i965).

In the southern and western parts of the area, where the Lough Mask Formation is unconformable on Connemara Schists, irregularities in the unconformity are filled with quartz-schist breccia up to 2o m thick. This is probably also offluviatile origin; it passes gradually up into cross bedded sandstones, the clasts are well sorted, the schist clasts appear fresh, not chemically weathered, and the proportion of quartz to schist increases upwards.

South of Lough Fee, thick lenses of cobble conglomerate in the Lough Mask Formation give it such a distinctive character that it has been mapped as a separate

38 D. J. W. Piper

member--the Lecknavarna Member. The cobbles are well rounded and well packed, and appear to fill channels. The interbedding cross bedded sandstones are similar to those elsewhere in the Lough Mask Formation, and the conglomerates are interpreted as channel deposits brought down in exceptional floods.

At the top of the Lough Mask Formation, laminae and thin beds of mudstone, siltstone and poorly sorted fine sandstone interbed with thicker units of cross bedded medium sandstone. Some of the sandstones are calcareous, and nodules are found in places. No body fossils have been found, but simple vertical and hori- zontal linear tubular structures, most probably worm burrows, are common. The interpretation of this lithology depends on the interpretation of the Kilbride Formation, and is discussed below.

4. Kilbride Formation

(A) Q NERA The base of the Kilbride Formation is defined by the appearance of thick green and grey beds above the red Lough Mask Formation. The Formation comprises fossiliferous sandstones and siltstones, with a maximum thickness of 34 ° m in the type locality (Piper I967). Four main lithological divisions can be distinguished, and these are described individually below. Many of the fossil assemblages correspond to the depth correlated faunal communities proposed by Ziegler (I965) and Ziegler, Cocks and Bambach (1968).

(B) FIRST DIVISION

This comprises poorly sorted almost unfossiliferous sandstones, often passing gradually up from the sandstones and mudstones at the top of the Lough Mask Formation. The lowest sandstones are medium or fine grained, in beds 5o to t 5o cm

¢ JX

0 km 5 '1'

/ ~ e e ~ ~f~

F I o. 5. Palaeocurrents in the Lough Mask Formation. Each arrow represents the mean of at least ten directions of high angle large scale

cross bedding. Base map has movements along strike-slip faults restored to Silurian positions (cf. Fig. I).

Sedimentary environments and palaeogeography of North Gonnemara, Ireland 39

thick, which are often graded and very poorly sorted. The sorting of the sandstones gradually improves stratigraphically upwards, until well sorted medium sandstones, some of which are cross bedded, are found. The muddy laminae so characteristic of the top of the Lough Mask Formation are absent.

(C) SECOND DIVISION

The Division usually has an erosive base on the First Division. I t is dominated by well sorted sandstones, but is lithologically very variable. Six main facies can be recognised.

(i) Microconglomerate facies, characterized by small rounded pebbles mainly of vein quartz, of modal size around 6 mm, forming beds or laminae interbedding with quartzose sandstone. Cross bedding is common. Detailed correlation of microconglomerate beds over a distance of IOO m at Benlugmore (Fig. 6) shows that

. . :... ". :.'. :!.ii ( . ' . : . : . : . . . . ' . . .

s ...... I , / -

I

. . . . . . . ,Jjl I

~ 7

I a I ° o

D MASSIVE SANDSTONE ~,THIRD DIVISION

J SANDSTONE & SILTSTONE

~ TONALEE FORMATION

FOURTH DIVISION

FIRST DIVISION

LOUGH MASK FORMATION

SECOND DIVISION

I o. 6. Sections showing the stratigraphy of Kilbride Formation around Benlugmore. 'Walls' indicate principal faults. Inset map shows position of sections relative to

these faults, which are also shown on Fig. 2.

4 ° D . J . W . Piper

the base of the facies here is strongly channelled, and that individual beds lens out rapidly. The axis of the channel persisted throughout the deposition of the microconglomerate. Gastropods and siltstone clasts are found in some microconglomerates.

(ii) Eocoelia sandstone, comprising massive medium sandstone, with impersistent bands and lenses of fossils at some horizons. The fossils are dominated by Eocoelia. Fine quartz pebbles similar to those in the microconglomerate are occasionally found with the fossils. Cross bedding is absent, but there is some scour and fill structure.

(iii) Pebbly sandstones, similar to the Eocoelia sandstone, but with some pebble bands. The pebbles are mainly of Ordovician lithologies, and up to 8 cm diameter.

(iv) Quartzite facies, of medium to very fine quartzites. Some are thick bedded or massive, others have very regular thin bedding, with possible ripple marks and diapirs. Pi cross bedding is found in a few sets, and some channels are known. Much of the facies has abundant vertical Skolithos burrows: a trace fossil widespread in lower Palaeozoic quartzites (Hallam and Swett x966 ).

(v) Lenticular red sandstones, cross bedded and petrologically indistinguishable from the Lough Mask Formation sandstones, are found in several localities in the Second Division. Local field relations suggest they are channel filling.

(vi) Laminated fine sandstones and siltstones are known from only one area, around Benlugmore.

The poor exposure allows only quantitative statements on the relationships between facies. The microconglomerates and pebbly sandstones are the thinnest and most variable facies, show the most evidence of channelling, and are found near the base of the Division. The red sandstones are only found associated with these two facies, and they have not been found in the First Division. The quartzite and Eocoelia sandstone facies are the thickest and most persistent facies, and are generally found near the top of the Division. They appear to be mutually exclu- sive, the quartzites occurring in the north east, and the Eocoelia sandstones in the south and west. The pebbly sandstones are found only in the west of the area. South-east of Lough Nafooey, the quartzite facies disappears very rapidly, suggesting an original channelled relationship.

D) THIRD DIVISION

This comprises a number of interfingering sandstone and siltstone facies. The facies are much more persistent than those of the Second Division, and they grade into one another. The 'end member' facies are:

(i) thick bedded quartzose medium sandstones, with rare lenses of bioclastic material, and much low angle scour and fill, but no cross bedding.

(ii) alternating laminated fine sandstones and bioturbated siltstones. The sandstones are fine or very fine, often sharp based, internally laminated, with bioturbated tops passing up into siltstone. Convolutions are common in some sandstones. The relative proportions of sandstone to siltstone, and the grain size and sorting of the sandstones, are both very variable.


Few determinable body fossils are found in the division; the few fossils in the lenses of bioclastic material in the massive sandstones are always very comminuted, with corals and crinoid ossicles the commonest recognizable fragments.

Around Benlugmore, the general fining upward trend of the Kilbride Formation is broken by an extensive development of massive sandstones of the Third Division at the top of the Kilbride Formation (Fig. 6).

(E) FOURTH DIVISION

This division is found at the top of the Kilbride Formation throughout most of the region (Figs. 4 and 6). It comprises siltstones, usually fossiliferous and bioturbated. In its lower part corals are often abundant, whereas brachiopods (mainly of Clorinda Community--L. R. M. Cocks and A. M. Ziegler, pets. comms.) are more common in the upper parts. At Lettershanbally, there is a gradual passage up from Clorinda Community faunas, through siltstones with small trilobites, into almost unfossiliferous siltstones and mudstones with rare graptolites. A similar faunal sequence has been proposed by Ziegler, Cocks & Bambach (I968) for the transition from shelly to graptolitic facies. A benthos with hard skeletons may have been present only in the photic zone.

(F) ENVIRONMENTAL INTERPRETATION

The Second Division holds the key to the environmental interpretation of the Kilbride Formation. In the gradually transgressive sequence of the Llandovery of North Connemara, it is the first horizon at which fully marine fossils are found. Of all the Kilbride Formation, it has the coarsest sediments, and the greatest indi- cation of high energy conditions, including well sorted sediments, abundant channels and cross bedding, and often rests with an erosional base on the underlying strata. On these grounds it is interpreted as a nearshore marine environment, influenced by powerful wave and current action.

The underlying First Division in places shows a continuous gradation in petrology and bedding characteristics between the Lough Mask Formation and the Sec- ond Division. This suggests a mixing of sediment from two sources, fluviatile and shallow water marine. Trace fossils are common. The First Division can be com- pared with the deposits of modern coastal lagoons. Although muds are the dominant surface lithology in many lagoons, the scouring associated with gradually shifting passes in the seaward barriers results in tidal delta sands associated with these passes being the dominant lithology in three dimensions. Such an interpretation is supported by the cross bedding in the First Division being in the same direction as in the Lough Mask Formation; in tidal lagoons, ebb currents are usually dominant. The fine facies at the top of the Lough Mask Formation may represent muddy tidal flat deposits around the margins of the lagoon.

A corollary of the postulate of tidal deltas in the First Division is that much of the Second Division was associated with passes in the barrier, rather than with shore face deposition. This is supported by the frequent evidence of deposition in channels, on a variety of scales, and the presence of red sandstones filling some channels. The more extensive Eocoelia sandstone and quartzite facies, and the

42 D. J. W. Piper

Third Division, suggest more uniform offshore conditions. The sharp based sandstones with convolutions and bioturbated tops in the Third Division suggest episodic powerful sedimentation of sand; this was probably associated with storm activity, as described by Reineck, Gutmann & Hertweck (I967) from the Helgo- land Bight, possibly augmented by exceptional outflow through tidal passes. Local shoal conditions probably account for the more massive sandstones of the Third Division. The Fourth Division is interpreted as deposited in deeper water beyond the range of significant wave or current activity.

Superficial examination of the faunas suggests Ziegler's Pentamerus and Costis- tricklandia Communities are poorly developed or absent. In the top of the Third Division and base of the Fourth Division, in Kilbride and elsewhere, Cocks (pers. comm.) has recognized an assemblage dominated by Dolerorthis psygma, which is unknown in such numbers elsewhere in the Llandovery, and may be equivalent to the Costistricklandia Community.

In gross terms, the North Connemara region fits in with the depth correlated faunal communities proposed by Ziegler (i965) and Ziegler, Cocks & Bambach (I968). Only two of the communities--the Eocoelia and Clorinda--can be firmly identified. The Eocoelia Community is found in sediments of the Second and Third Divisions which, on independent sedimentological grounds, were deposited in shallower water than those of the Clorinda Community in the Fourth Division. A detailed palaeontological investigation, including study of the favositid-coral and Dolerorthis assemblages, is desirable.

5. Tonalee Formation

The base of the Tonalee Formation is defined by the first appearance of red mudstones above the Kilbride Formation, which in most areas is sudden, but in some is gradual. The red colour is due to small haematite grains in the mudstone. The maximum thickness of the Formation, 75 m, is found in the type section at Tonalee. The Formation is almost unfossiliferous; Mr David Smith collected a small brachiopod fauna from Kilbride, which has been identified by Dr Cocks as belonging to the Marginal Clorinda Community (Cocks & Rickards I969). Parts of the Formation appear bioturbated; many others preserve depositional lamination undisturbed.

Green feldspathic tufts, usually in beds about one cm thick, are common in the Tonalee Formation, and a few beds of tuff of medium sandstone grade tens of cm thick are known. The tufts are commonest in the upper part of the Formation, but inadequate exposure prevents correlation between sections. The tufts contain small amounts of biotite and quartz, but feldspar is the dominant mineral.

At Trean, unusually coarse lithologies of the Tonalee Formation are exposed along the shore of Lough Mask. Fissile coarse siltstone, usually laminated and sometimes cross laminated, interbeds with structureless siltstones and mudstone. The lithology is reminiscent of parts of the Third Division of the Kilbride Forma- tion.

At Benlugmore, the Formation thins out against massive sandstones of the Kilbride Formation Third Division (Fig. 6). There is a transition lithology about

Sedimentary env#onments and palaeogeography of North Connemara, Ireland 43

I m thick of bioturbated coarse siltstone, containing a few corals and brachiopods, indicating a Clorinda Community. Westwards, the Tonalee Formation thickens rapidly to more than 25 m over a distince of a few hundred metres (crossing a small fault). 250 m west of the point where the Tonalee Formation finally pinches out, there is a highly calcareous sandstone lens (about 4 ° cm in diameter) packed with fossils, of the Eocoelia Community. This is identical with some of the fossil lenses in the immediately underlying Third Division sandstones. Most of the Ton- alee Formation in this locality consists of mudstone with conspicuous siltstone laminae, sometimes with syn-sedimentary recumbent folds. Beds of rolled corals and other bioclastic debris are found in the immediately overlying Benbeg mudstone in this area. Because there is no evidence of current activity sufficient to transport sand, the whole lens is interpreted as a single rolled-in clast, derived by penecontemporaneous erosion of the Third Division sandstones.

Although red mudstones of similar age are widespread in Britain, the cause of the red colour is unknown. It certainly cannot be ascribed to deposition in a ter- restrial oxidizing environment; the Tonalee Formation appears to be a deep water shelf sediment deposited near the depth limit of intense bioturbation and of ben- thic fauna with hard skeletons. The frequent occurrence of red beds in marine sequence of the Lower Palaeozoic and Pre-Cambrian suggests environmental conditions in which little change was necessary to tip the balance in favour of sufficiently oxidizing conditions to produce red sediment. The lesser abundance of such red beds in later geological times may be the result of geochemical changes attendant on the colonisation of the land by plants, or the great increase in infaunal burrowing benthos in deeper water. Whatever change was necessary to initiate red sedimentation seems to have had no marked effect on faunas (e.g. Squirrel & Tucker I96O , p. i42; Whittard 1928 , p. 78i, I932, pp. 868, 877 ). The apparent synchroneity of the Ce red mudstones could have resulted from a slightly restricted sea (such as the present day Mediterranean or Arctic Ocean) covering the north west European area, and permitting the development of the suitable geochemical environment. The association with tufts may well be of no significance: tufts and other evidence of volcanic activity are widespread at various times through the Silurian (Ziegler, McKerrow, Burne and Baker 1969; Laird and McKerrow i97o), and the occurrence of tufts does not correlate with the occurrence of red beds (Butler x937; Dr D. Palmer pers. comm.).

6. Lettergesh Formation

The upper part of the Lettergesh Formation, consisting mainly of bedded turbidites, has been studied in detail by Laird (Laird & McKerrow 197 o) and the present author has only worked on the Gowlaun Member at the base of the Formation. The stratigraphy of the Member is discussed below, but the sedimentology is mentioned only briefly, since it has been presented in detail elsewhere (Piper I97ob).

The Gowlaun Member consists primarily of a mudstone facies, with two extensive lenticular developments of conglomerates and coarse sandstones, one in the

44 D. J. W. Piper

east around the Maam Fault zone (Figs. I and 2) and one in the west from Letter- shanbally to Lettergesh (Fig. 7). The conglomerates and coarse sandstones form massive impersistent beds. The conglomerates comprise well rounded and sorted pebbles of Ordovician volcanic rocks and Dalradian quartzites, with a northern provenance (Piper i97oa ). The sandstones are petrologically greywackes (Petti- john i957) ; they are mostly massive, but some are graded and resemble 'fluxo- turbidites'. In several localities the coarse sediments die out rapidly, with interfingering or channelling into the mudstone facies. The detail of these transitions (Piper I97ob ) indicates that the coarse sediments were deposited in well-defined bathymetric lows. The mudstone facies comprises true mudstones, with rare graptolites, with interbedding laminated siltstone and very fine sandstone beds.

The stratigraphy of the western coarse sediment lens is shown in Fig. 7. Strati- graphic data on the interfingering of the Upper and Lower Owenduff Groups are given in Piper (i967) , and on the Maam Valley coarse sediment lens in Piper (I97ob).

The area east of the Maam Faults (Fig. 2) is tectonically more complex, and marker horizons are rare. Measuring sections up from the top of the Tonalee Formation, where possible, has shown that there is considerable lateral variation in the lower parts of the Lettergesh Formation in this region. Except around Kil- milkin and in Central Glen Trague, a thick mudstone, siltstone and fine sandstone sequence, designated the Benbeg mudstone unit, is found at the base of the For- mation. It is at least 240 m thick at Benlugmore, and the North Glen Trague succession is probably equally thick, but the base is not exposed. In Kilbride, it is only about 4 ° m thick, and it is little thicker south of Lough Nafooey. The Benbeg unit in these northern areas has only thin very fine laminae of sandstone; the

Lettergesh Lough Fee Lettershonbolly

" i - ~ ~ . . ~ . . ' . " . - - ~

LO RATE B. SANDSTONE

[ ~ ] MUDSTONE

MUDSTONE 8 SANDSTONE

km 2 ~SCHIST CLAST CONGLOMERATE

o;

°t 8

ALEE Fro.

~ SANDSTONE 8= FINE CONGLOMERATE

FI(2. 7" Detailed stratigraphy of the Gowlaun Member west of Lettershanbally. Localities are indicated on Fig. I.


thicker succession in the south includes many thick beds of very fine almost structureless sandstone.

The Benbeg Mudstone is succeeded suddenly by a sequence of graded medium and coarse sandstones, often with laminated, cross laminated or convoluted fine sandstones and siltstones at the tops of beds. In the more westerly sections (south of Lough Nafooey, North and South Glen Trague) there is some microconglomerate near the base of the section, but further east in Kilbride this is absent.

This distinctive change in sediment is tentatively regarded as a correlatable horizon. This correlation is confirmed between North and South Glen Trague and Benlugmore by the stratigraphy of the immediately underlying Benbeg Mudstone. The thinness of the Benbeg Mudstone south ofLough Nafooey and in Kilbride may result from either:

(i) the sandstone overlying the Benbeg Mudstone south of Lough Nafooey correlating with some coarser horizon within the Benbeg mudstone or

(ii) the sandstones overlying the Benbeg Mudstone in the two areas being equivalent, with the mudstones varying laterally.

The second correlation is much preferred, since: (a) the ripple drift current indicators in the graded sandstones south of Lough

Nafooey are consistently to the south or south-west, and the sediment is coarse, yet shows the characteristics of mature turbidites. A sudden barrier a few hundred metres away in North Glen Trague, with fine sandstone and mudstone accumulating beyond (presumably on higher ground), is difficult to imagine;

(b) the Benbeg Mudstone south of Lough Nafooey and in Kilbride has almost no sandstones, whereas the much thicker succession in Glen Trague has some parts composed almost entirely of bedded fine structureless sandstone. This is consistent with Benbeg mudstone deposition being synchronous through this area, and thicker deposits forming in the (deeper water) area of Glen Trague than in the area south of Lough Nafooey.

At Maam, there is a similar sudden appearance of graded sandstones; this overlies a sequence including not only mudstones, but also microbreccias and structureless coarse and medium sandstones. The base of the graded sandstones is believed to correlate with Glen Trague.

These graded sandstones are taken as marking the base of the upper Lettergesh Formation (the top of the Gowlaun Member). Thus throughout most of the region east of the Maam Faults, the Gowlaun Member comprises only the Benbeg mudstone unit, but in the immediate vicinity of the Maam Fault zone (at Maam, Kilmilkin and Bunnacunneen) coarse sediments are also found.

In the stream bed of Glen Trague (Fig. 2) several metres of typical Gowlaun Member conglomerate are exposed. There is no other solid exposure in the area, with the exception of some steeply dipping green bedded fine sandstones (too small to appear on Fig. 2) downstream from the conglomerate. These appear from their attitude to be possible correlatives of part of the Benbeg mudstone unit at Benlug- more. In the drift around the eastern end of the conglomerate outcrops, and further downstream, there are abundant large blocks of Tonalee Formation red mudstone, a lithology otherwise extremely rare in Glen Trague. This suggests that the Tonalee Formation probably outcrops in the vicinity, which is consistent with the structure

4 6 D . J . W . Piper

of the area to the south-east. Conglomerates are found elsewhere only in the lower part of the Gowlaun Member, and the occurrence of Tonalee Formation in Glen Trague suggests that this locality is no exception.

The Gowlaun Member is interpreted (Piper i97ob ) as a deposit of a slope area between a shallow continental shelf and a deep basinal area accumulating turbidites. The principal evidence is from detailed stratigraphic relations (e.g. at Lettershanabally, Piper 1967), which show that coarse current deposited Gowlaun Member sediments accumulated in bathymetric lows, while muds accumulated on intervening highs. The stratigraphic position of the Gowlaun Member in a transgressive sequence between shelf sediments and turbidites suggests that the lows acted as funnels of shallow water sediment to deep water, and were inter- mediate in depth between the shelf sediments and the turbidites. Such a situation is accurately parallelled by modern basin slopes or continental margins, but by no other environment. There is no direct evidence in the Gowlaun Member of the development of a fan topography, but a submarine canyon has been found. It is situated south of Lough Fee, and is cut into Connemara Schists (Fig. 7). It is about ioo m deep and about I km wide on its upper lip. The extreme base of the feature is not exposed, but in size and steepness it corresponds closely to small present day submarine canyons, such as the La Jolla Canyon, off southern Cali- fornia (Shepard and Dill i966 ). It is filled with coarse, often poorly sorted, Gowlaun Member sediments.

7. Conclusions

(A) PALAEOGEOGRAPHIC DEVELOPMENT

I. The nature of the sub-Silurian unconformity has been described by Piper (I97oa). To the north of the North Glen Trague Fault (Fig. 8b), Ordovician rocks, mainly of the lowermost Ordovician Lough Nafooey Group, outcropped in lowest Silurian times. South of the Fault, Connemara Schists of quite high meta- morphic grade outcropped. The topography was irregular, with a relief of hundreds of metres. The absence of Ordovician clasts in the lowest Lough Mask Formation fluviatile sediments resting on Connemara Schists, and vice versa, suggests that river valleys were along strike, and that topography was strongly controlled by faults parallel to the North Glen Trague Fault. The palaeocurrent measurements from these lowest sediments are too few in number to provide independent support for this hypothesis, but in no way contradict it. The extrusion ofkeratophyre lavas also dates from this earliest part of the Lough Mask Formation.

2. During Lough Mask Formation deposition, a southward flowing river system established itself over the former east-west system (Fig. 8c). In some sections, the first incoming of Ordovician clasts from the north forms a prominent marker horizon in the field. Some of the sediment was derived from the Dalradian north of the Highland Boundary Fault. Over most of the area, an extensive piedmont plain developed, but in the west, coarse sediment was carried right to the shoreline, suggesting a steep hinterland. The greater steepness on land was mirrored by steeper submarine topography, and coarse sediment was carried by turbidity

Sedimentary environments and paIaeogeography of North Connemara, Ireland 47

currents down submarine canyons cut into steep basin slopes. Conglomerates and coarse sandstones were deposited on submarine fans at the mouths of canyons (Piper x97ob), while fine sands, silts and clay may have been carried much greater distances, or ponded up if the basin was closed. Such fine sediment deposits were preserved on higher areas, and in between distributary fans. There was a gradual marine transgression, which soon spread over the outcrop of the Connemara

.o h. ? 1,

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B

: : . ' / '~4~" [~rI'MXSlV[ ~.UV~"xP~R ~ sm crams S I .uoov I , s , . . ' /~ ~ o o - \

DE[P SN[LF SEA CUT mr lUOIIIAEIXI[ ¢ANYOllS lOOm--" . _ . . / . /

F

F I o 8. Palaeogeography of North Connemara in the lower Silurian. A. Base map with movements along strike-slip faults restored to Silurian positions. B. Basal Lough Mask Formation times. C. Middle Lough Mask Formation times. D. Lower Kilbride Formation times. E. Upper Kilbride/basal Tonalee Formation times. F. Immediate post-Tonalee Formation times (Gowlaun Member).

48 D. J. W. Piper

Schists south of the Ordovician rocks, so that the Schists were no longer a source of detritus.

3- The marine transgression progressed northwards in the east of the area, and fluviatile sediments gave way to shallow water marine deposits (Fig. 8d). A typical transgressive shoreline, with tidal lagoons, barrier beaches and deeply scoured tidal inlets was formed. Differential subsidence was controlled by a series of major east-west faults, and to a lesser extent by north-south faults. They may have resulted in the shoreline becoming stabilized for a time along the Glen Trague-Kilbride Bay line, where the deepest and most abundant tidal channel deposits are found. At the same time in the west of the region there was a cessation of coarse deposition in the deep basin, possibly because of a change in the position of the river system, or the trapping of coarse sediment on a broad shallow shelf resulting from the transgression.

4. Coarse sediment deposition in the western basin was renewed, the basin becoming greater in extent as a result of the subsidence of a former shallow shelf area west of Lettershanbally. A similar subsidence further north may have been the cause of the renewed influx of coarse sediment, by bringing deep water close to the shoreline again. There was also a general deepening of the sea to the east as well, although the Glen Trague-Kilbride Bay line appears to have remained a high area on which sandy shoals accumulated.

5. During Tonalee Formation times, the same general pattern persisted (Fig. 8e). There was a deep basin in the west accumulating coarse sediment. In the east was a deep shelf sea (perhaps ioo to 200 m deep), shallowing to the north, and with a marked zone of shallows associated with part of the North Glen Trague Fault zone. These shallows were probably associated with penecontemporaneous fault movements, erosion of unconsolidated sediment, and the slumping of some sediment down steep slopes into deeper water. The tufts indicate volcanicity at this time.

6. The tectonic instability of Tonalee Formation times led to the establishment of deep water conditions (greater than 20o m) throughout the whole area in lower Lettergesh Formation times (Fig. 8f). In the west, a deep central basin accumulated coarse sediment; mudstones were deposited west of Lettergesh and east of Lettershanbally. In the east, a steep slope to the south formed along the line of the North Glen Trague Fault. Mudstones accumulated on this slope area, with some fine turbidite sandstones in the deeper parts to the south. The Maam Fault zone was an axis of coarse sedimentation, from which conglomerates and sandstones spread at intervals over adjacent areas.

7- At the top of the Gowlaun Member there may have been a sudden change in submarine topography, since turbidite sandstones appear to extend over the whole area; earlier topographic differences were mainly smoothed out.

(B) FAULT CONTROL OF SEDIMENTATION

Throughout the Llandovery, sedimentation in the North Connemara region was strongly controlled by topographic features and differential subsidence along major faults. We have already seen how differential subsidence and penecontemporaneous movement along the North Glen Trague Fault (separating the

Sedimentary environments and palaeogeography o f North Gonnemara, Ireland 49

Ordovician from the Connemara Schists) was important in the Benlugmore area. I t resulted in an unusually thin Kilbride Formation, shoal conditions in uppermost Kilbride Formation times, and penecontemporaneous erosion along a southward facing scarp supplying Kilbride Formation lithologies to the Tonalee Formation, and corals to the Gowlaun Member. During the deposition of the Gowlaun Member, the line of the North Glen Trague Fault separated a deep basin to the south, in which hundreds of metres of Benbeg mudstone unit accumulated, from the very thin (c. 4 ° m) mudstone succession in the area around Lough Nafooey and Kilbride. I t thus formed a steep basin margin; the southward facing scarp of Tonalee Formation times persisted in a somewhat degraded condition.

In lowest Lough Mask Formation times, the whole grain of the country was dominated by an east-west fault system parallel to the North Glen Trague Fault.

The other major fault in the region, the strike-slip Maam Fault, has been shown to have been active in Ordovician times by Stanton (I96o) and McKerrow & Campbell (i 960). In the lower Silurian, the fault zone formed a topographic low, in which thick deeper water facies of the Kilbride Formation accumulated. In Gowlaun Member times, the Maam Fault zone was one of the two areas accumulating coarse sediment.

The abrupt and persistent basin margins in the western basin east of Letter- gesh, south of Lough Fee and at Lettershanbally are also probably indicative of fault controlled topography, although in no case is there compelling evidence of differential subsidence.

ACKNOWLEDGEIVIENTS. This work has benefitted from discussion in the field with Drs M. G. Laird and H. G. Reading. I gratefully acknowledge the guidance of my research supervisors, Drs J. F. Dewey, F. P. Shepard and P. F. Friend, and the receipt of a NERC Research Studentship.

8. References ALLEN, J. R. L. 1963. The classification of cross stratified units, with notes on their origins.

Sedimentology, 2, 93-I 14. 1965. A review of the origin and characteristics of Recent alluvial sediments. Sedimentology,

5, 89-I9 x- BUTLER, A.J. I937. On the Silurian and Cambrian rocks encountered in a deep boring at Walsall,

South Staffordshire. Geol. Mag., 74, 241-257. CocKs, L. R. M. & RXCga~RDS, R. B. I969. Five boreholes in Shropshire and the relationship of

shelly and graptolitic facies in the lower Silurian. Q. Jl. geol. Soc. Lond., 124, 213-238. DEWEY, J. F. & MCKERROW, W. S. 1963 . An outline of the geomorphology of Murrisk and North-

west Galway. Geol. Mag., xoo, 260-275. GARDINER, C. I. & REYNOLDS, S. H. 19 ~2. The Ordovician and Silurian rocks of the Kilbride

Peninsula (Mayo). Q. Jl. geol. Soc. Lond., 68, 75-Ioi. I914. The Ordovician and Silurian rocks of the Lough Nafooey area, County Galway.

Q. Jl. geol. Soc. Lond., 7 o, IO4-118. HALLAM, A. & SWETT, K. 1966. Trace fossils from the lower Cambrian pipe rock of the north

west Highlands. Scott. J. Geol., 2, IO1-IO6. LAIRD, M. G. & MCKERROW, W. S. I97o. The Wenlock sediments of north-west Galway,

Ireland. Geol. Mag., IO7, 297-317. McKERROW, W. S. & CAMPBELL, C.J. I96o. The stratigraphy and structure of the lower Palaeo-

zoic rocks of north west Galway. Scient. Proc. R. Dubl. Soc., AI, 27-52. McMA~us, J. I967. Faulting of the sub-Carboniferous surface in eastern Murrisk, Co. Mayo.

Geol. Mag., xo4j 228-23 I.

5 ° D . J . W . Piper

PETTIJOHN, F. J. 1957. Sedimentary rocks. 2ed Harper Bros., New York. 718p. PIPER, D. J. w. i967. A new interpretation of the Llandovery sequence of North Connemara,

Eire. Geol. Mag., xo4, 253-267. I97oa. Geosyncline margin sediments in the Silurian of west Connacht, Eire. Mem. Amer.

Assoc. petrol. Geolog., x2, 289-297. I97ob. A Silurian deep sea fan deposit in western Ireland and its bearing on the nature of

turbidity currents. J. Geol., 78, 5o9-522. REINECK, H. E., GUTMANN, W. F. & HERTW'ECK, G. 1967. Das Schlickgebiet stidlich Helgoland

als Beispiel rezenter Schelfablagerungen. Senckenberg. leth., 411, 219-275. RICKARDS, R. B. & SMYTH, W. R. 1968. The Silurian graptolites of Mayo and Galway. Scient. Proc.

R. Dubl. Soc., A3, i29-i35. SHEPARD, F. P. • DILL, R. F. 1966. Submarine canyons and other sea valleys. Rand McNally, Chicago.

38I p. SQmRREL, H. C. & TUCKER, E. V. I96O. The geology of the Woolhope inlier (Herefordshire).

Q. Jl geol. Soc. Lond., xx6, i39-i85. STANTON, W. I. I96O. The lower Palaeozoic rocks of south-west Murrisk, Ireland. Q. Jl geol. Soc.

Lond., Ix6, 269-29i. WHITTARD, W. F. 1928. The stratigraphy of the Valentian rocks of Shropshire; the main outcrop.

Q. Jl geol. Soc. Lond., 83, 737-759. I932. The stratigraphy of the Valentian rocks of Shropshire; the Longmynd-Shelve and

Breidden outcrops. Q. Jl geol. Soc. Lond., I~, 859-9o2. ZmOLER, A. M. 1965. Silurian marine communities and their environmental interpretation.

Nature, Lond., 2o7, 27o-272. ZWGLFR, A. M., COCKS, L. R. M. & B.~mACH, R. K. I968. The structure and composition of

lower Silurian marine communities. Lethaia, x, x-27. ZIF.GLFR, A. M. MCKERROW, W. S., BURma, R. V. & BAKER, P. E. x969. Correlation and environ-

mental setting of the Skomer Volcanic Group, Pembrokeshire. Proc. Geol. Ass., I~, 4o9-439.

Received 2 December I969; revised manuscript received 29 October I97o; read 3o June x97 I.

D. J. W. Piper, M.A., Ph.D., F.G.S., Department of Geology, Sedgwick Museum, Cambridge CB2 3EQ.

DISCUSSION

Dr W. S. McKERROW and Dr L. R. M. CocKs congratula ted the author on his assessment of the palaeogeography of this area. They observed, however, that the maps showed a contour at a depth of I oo metres, and wondered what criteria had been used to estimate this depth ? The distribution of the brachiopod communities of Ziegler, Cocks & Bambach appears to be closely related to depth, but so far there has been no satisfactory method of determining the actual depth quant i ta- tively. The evidence of changes in communi ty above and below lava flows in Pembroke and Gloucestershire now appears to be ambiguous. I f the author 's submarine contours reflect the boundary between the Stricklandia and Clorinda Communit ies , or perhaps below the Clorinda Communi ty , this should be stated with reference to these communities, ra ther than to any par t icular depth in metres.

Dr Cocks also drew attention to the impor tance of the crenulata Zone graptolite fauna from the base of the Upper Owenduf f Group near Doon Rock. This is stratigraphicaUy above horizons with Eocoelia sulcata, and is yet another link between the sequence of graptolite zones and the evolving brachiopod lineages which have proved so useful for correlation within the Lower Silurian. Associated


with the graptolites were rare brachiopods, and one of these, Pentlandella pentlandica (Haswell 1865) , is notable since it was known previously only from its type locality in the Pentland Hills, Scotland, which may also be of crenulata Zone age.

In a written reply, the AUTHOR thanked Dr McKerrow and Dr Cocks for their remarks. The IOOm contour in Fig. 8 was highly speculative, just as the positions of rivers on the land, or the landward extent of the lagoons were also clearly speculative. The author was attempting to give a visual impression directly comparable with the bathymetry of present day environments. The I OO m contour had been placed somewhere through the middle of the Clorinda Community. Sediments containing shallower communities show evidence of considerable wave and current activity; the outer limit of the Clorinda Community, where there is also a marked decrease in the intensity of bioturbation, may well correspond to the lower limit of the photic zone. There are very few rigorous community determinations based on large samples from North Connemara; a map based on 'eyeballed' community determinations in the field was felt to be inappropriate.

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Sedimentary environments and palaeogeography of the late ...csmgeo.csm.jmu.edu/Geollab/Whitmeyer/IrelandField... · gradually shallowing sedimentary sequence, described recently by