GEOLOGICAL JOURNAL, VOL. 30, 165-182 (1995)

Stratigraphy and structure of Devonian f luvial sediments, western Beara Peninsula, south-west Ireland

ANDREA JAMES BadleyAshton, Winceby House, Winceby, Horncastle, Lincolnshire LN9 6PB, UK

AND

JOHN R. GRAHAM Department of Geology, Trinity College, Dublin, Ireland

A stratigraphic and structural summary of 5 km of Upper Devonian strata of south-west Beara, part of the Munster Basin, south-west Cork is presented. Five formations are recognized on the basis of lithofacies geometries and associations. The lowest Caherkeen Formation comprises rapidly alternating sheet-like, plane-bedded, low-angle, cross- bedded sandstones and laminated, rippled or desiccated mudrocks. The incoming of thin lenticular sandstone packages, commonly with high-angle cross-bedding, distinguishes the overlying Eagle Hill Formation. The succeeding Reen Point Formation comprises thicker bedsets of plane-bedded, low-angle and high-angle cross-bedded sandstones, separated by thick, massive mudrocks. Intraformational breccias and calcareous nodular siltrocks are distinctive features. The Tholane Formation is characterized by thick massive red-green mudrocks and a general absence of coarser grained lithologies. This sequence was deposited in a terminal fluvial fan in a basin characterized by decreasing subsidence rates and gradient with time. The overlying Toe Head Formation represents the deposits of a fluvial coastal plain. It has siltrocks that are predominantly green, often with preserved fish and plant debris. There is a relative absence of high- angled cross-bedded sandstones and an abundance of flat-laminated and inclined parallel-laminated sandstones.

The structure of the Beara Peninsula comprises a WSW plunging anticlinorium with a single cleavage formed during the Variscan orogeny. Evidence from locally transecting cleavages and from mapping indicates dextral transpression. Development of the three main fault trends was contemporaneous with the folding. The well-known copper mineralization of the Allihies area is associated with east-west fault trends, a pattern observed elsewhere in the western Munster Basin. Basement involvement during both basin development and deformation is likely, but is difficult to test.

KEY WORDS Old Red Sandstone; fluvial sedimentology; dextral faults; copper mineralization

1. INTRODUCTION

The rocks described in this paper are part of a large extensional basin, the Munster Basin (Capewell 1965), developed in southern Ireland during the late Devonian and filled with predominantly non-marine ‘Old Red Sandstone’ facies rocks (Naylor and Jones 1967; Graham 1983; Graham and Clayton 1989). The northern and eastern margins of this basin can be defined by changes in thickness and facies of the Old Red Sandstone sediments, but the southern and western margins are not exposed. Transport of sediment was consistently from the north along the fault-bounded northern margin and from the east along the eastern margin (Graham 1983). Rocks exposed along the western coastal areas of Iveragh, Beara and Mizen (Figure 1) consistently show a west to north-west origin for the palaeocurrents. In general sediments become finer away from the basin margins and also with time and a terminal fan model of deposition has been proposed (Graham 1983). The central parts of this basin, dominated by the finer grained portions of the fill, are still poorly described and it is difficult, given the present database, to correlate over any distance, particularly in an east-west direction, thus hindering refinement of the terminal fan model.

CCCOO72-1050/95/020165-18 0 1995 by John Wiley & Sons, Ltd

Received 6 April 1994 Accepted 30 January 1995

166 A. JAMES A N D J . R GRAHAM

Kikalherine pt

EALLYCROVANE HARBOUR

LEGEND

TO8 H88d Form8lion

Thol8n8 Formation

Reen Point Form8llon Tholans Break

E8gh Hill Formation

b8sic igneous Intrusion8

EALLYDONEGAN BA m8lOI f8UitS

Crow Head

5km 1 J

Figure 1 . Geological sketch map of the k d r d Peninsula and its regional geological setting. Shading indicates the area studied in detail. Faults shown to the east of this are based on interpretation of aerial photographs

There has also been considerable discussion of the style of tectonic deformation. This has centred on the relative importance of thin-skinned ‘thrust’-style tectonics compared with thick-skinned tectonics with dextral transpression (Cooper et a[. 1984, 1986; Sanderson 1984; Price and Todd 1988). As with the sedimentology and stratigraphy, valued judgements of the various arguments are limited by a lack of detailed data on the surface structure over significant areas of the basin.

This paper concentrates on rocks exposed in the western part of the Beara Peninsula (Figure l), a rugged, treeless south-west promontory with superb coastal and sporadically good inland exposure of Upper Devonian continental and Lower Carboniferous marine clastic sediments. The marine sediments flank the northern and southern sides of the peninsula and form the limbs of a broad south-west plunging anticlinal structure, with the older continental deposits in the core forming the central part of the peninsula. The broad Variscan anticlinal structure of the peninsula became apparent during mapping towards the middle of the last century (Jukes 1860, 1864). Coe and Selwood (1968) presented a regional stratigraphic scheme for this south-western part of the Munster Basin and had earlier (Coe and Selwood 1963) provided a study of folding across the area. Observations of cleavage, veins and fractures were interpreted on a more regional scale by Sanderson (1984). Detailed structural research in the area has been limited, except for that relating to the large copper mine at Allihies (Sheridan 1964; Viswathaniah 1959; Matthews 1964; Fletcher 1969; Reilly 1986).

The aims of this paper are to provide a modern lithostratigraphy and sedimentological interpretation for the Old Red Sandstone rocks and a geological map of western Beara which demonstrates the local structure. These observations are then placed in their regional context.

DEVONIAN FLUVIAL SEDIMENTS. SW IRELAND 167

Transilion Series

r Transilion Series

Carbonilerous Slate 1 Coomhola

Grits

'Old Red Sandstone'

BOLDV 1955 Cast la lown- E w a Island

Carboniferous Slates

Coomhola Grits

'Old Red Sandstone'

A l l l h l a 8 - CDSllOlOWn

Ardgroom- Eyar laa

Upper Cwmhola

Middle Cwmhola Formation

Reentrusk Group Purple Sandstone

Group

I

Curnmem Slats a Sandstono Grwp

Allihies Sandstone Grwp

Ballydonepan c Slate Group

Lower Slate Group

COE h SELWOOD

1 9 6 3 Was1 Baara

Black Slates

Cmmhola Beds

'Old Red Sandstone'

DUCHARME

N o r t h B a l l y c r o v a n o

1 9 6 8 I W I L L I A M S 1 p ~ ~ ~ 4 l 1 Of . a/. 1 9 8 9

Black Slates

Upper Cwmhola Formation

Middle Cwmhola Formation

Lower Coomhola Formation

Formation Formation

Tholane Formation Formation

Gun Pant Reen Point Formation Formation

+ L L

Eagle Hill Formation

- Caherkeen Formation

Figure 2. Comparison of the IJpper Devonian stratigraphic nomenclature used for the Beara Peninsula. Correlation lines are approximate only, as different workers have used different criteria in defining their units

2. LITHOSTRATIGRAPHY AND SEDIMENTOLOGY

The c. 4000 m of continental sedimentary rocks exposed in this area are predominantly fine grained. A lack of obvious marker horizons and biostratigraphic controls, along with some structural complexity and only subtle changes in lithology have hindered the erection of a workable lithostratigraphy (Figure 2) . Earlier subdivisions of the succession were based principally on colour and grain size variations (Husain 1957; Viswathaniah 1959).

The present work has used lithofacies geometries, sedimentary structures and facies associations as well as grain size and colour to delimit five formations. Local names, where formations are best exemplified, are used because of the present correlative uncertainty within the Old Red Sandstone of the Munster Basin. However, the Toe Head Formation, with its type section on the south coast (Figure I ) , is accepted here because a suitable regional correlation exists (Graham 1972, 1975; Cotter and Graham 1991). Boundaries are readily mapped along coastal sections where the exposure is continuous but, because they are extremely gradational, precision is difficult inland where exposures are sporadic. A summary of the stratigraphy is given in Figure 3 and the formations are described in ascending stratigraphy below.

20. Caherkeen Formation 11950 m+rhick)

The base of the Caherkeen Formation is not exposed in the area studied, but may occur further ENE (i.e. up-plunge), unless there is structural repetition of the sequence.

The exposed formation generally comprises randomly interbedded sheet-like (100 m+), very fine- to fine-grained sandstones and siltrocks. The proportion of siltrocks increases up through the formation, resulting in an overall large-scale fining-upward trend. The sandstones are typically planar laminated or

168

5000m

4000m

3000m

2W0m

1 WOm

Grain Size Sid 1 m

1 I1 I 1 I1 I 1 I I U

sharp transition1

Toe Head Fm. (400m)

distributary fan deposits

Tholane Fm.

flwdbasin deposits

(344m)

Reen Point Fm. (325m)

'fixed channel' and overbank deposits

Eagle Hill Fm. (1 900m)

'mobile channel' and overbank

deposits

A . JAMES A N D J. R. G R A H A M

Figure 3. Summary lithostratigraphic column for the Beam Peninsula

Caherkeen Fm ( 1 %Om+)

high energy proximal and

distal sheelflood deDOSltS

(for key see fig. 4)

DEVONIAN FLUVIAL SEDIMENTS. SW I R E L A N D 169

a) b) Caherkeon Fm. Eaglo Hill Fm.

Grain Size Grain Size Sivi I rn

association 3

association 1

H association 2 t3

overbank deposits

minor channel deposits

overban k deposits

major channel deposits

overbank deposits

c) Roon Point Fm.

Grain Size Sivi I rn

overbank

channel

overbank

channel

d) Tholano Fm.

Grain Size Sivl l r n

KEY TO SEDIMENTARY STRUCTURES

Trough cross-bedding Planar bedding

Undulatory cross-bodding Current r ipplo lamination

Planar cross-bedding ~r;rl B i o p r b a t i o n (moderate to high angle) Desiccation cracks

Calcareous modules Intraformat ional c l a t t s

Planar cross-bedding H (low angle)

Log seal. motros

6

0 )

Too Hoad Fm.

Grain Size S i v l l r n

Grain Sizo Si - s i l t vf - very f ino sand f - fino sand m - medium sand

Figure 4. Selected sedimentological logs to illustrate the typical, distinguishing features of each formation. (a) Caherkeen Formation (V630495). (b) Eagle Hill Formation (V638509), (c) Reen Point Formation (V630527), (d) Tholane Formation (V548473) and (e) Toe

Head Formation (V629531)

display low-angle cross-bedding. High-angle cross-bedding is relatively absent compared with the overlying formations. Three broad associations are recognized within the Caherkeen Formation (Figure 4a).

170 A . JAMES A N D J. R. G R A H A M

AssocYation 1 Thin ( <0.6 m), sheet-like (100 m+), very fine- to fine-grained sandstones bounded by gently scoured or flat surfaces compromise 95% of this association. Scouring is only of the order of a few centimetres and basal concentrations of ‘rip-up’ clasts are rare. The sandstones may be stacked or separated by thin (20-100 mm) lenticular or sheet-like siltrock drapes. They are predominantly planar-laminated, low-angle cross-bedded and massive (Figure 5a and 6a) and are interpreted as the deposits or upper flow regime to transitional dune -plane bed conditions (Turner 1981; Turner and Monro 1987). The siltrock drapes and relative absence of lower flow regime bedforms may suggest extremely rapid decreases in discharge. High-angle cross-bedding is rarely observed.

Association 2 This association comprises rapid, random alternations of thin (< 0.4 m), tabular (100 m+), fine to coarse siltrocks and very fine-grained sandstones bounded by flat, laterally continuous surfaces. The siltrocks commonly show planar lamination, normally graded bedding (individual beds < 50 mm thick) or unidirectional climbing ripple lamination (Figure 5b). Primary sedimentary structures are locally disturbed by penecontemporaneous dewatering structures, suggesting rapid deposition, desiccation cracks indicating subaerial exposure (Figure 6b) and bioturbation.

Association 3 Purple-grey siltrocks, with normally graded bedding, form more than 90% of this association. The units are typically sheet-like and individual beds (< 50 mm thick) are laterally continuous for 30 mf (limit of high-quality exposures). Irregular discontinuous internal erosion surfaces and spoon-shaped scours ( < 2 m wide; < 0.6 m deep; Figure 6c) and cross-bedding (Figure 6d) are evident in places, e.g. Allihies Point, Crow Head. Sheet-like rippled siltrocks and plane-bedded sandstones are uncommon forming less than 7% of this association.

Summary The lowest exposed 500 m of the Caherkeen Formation are characterized by the random alternations of lithofacies associations 1 (1.5-10 m thick) and 2 ( 2 4 m thick). These are interpreted as high-energy proximal and distal ephemeral sheet flood deposits, respectively, based on the preponderance of sheet-like, rapidly deposited, upper flow regime sediments, the random arrangement of lithotypes, evidence of rapidly fluctuating hydrodynamic conditions and episodic exposure (James 1989). The deposits are analogous to flash flood sediments in the middle reach of the Finke River, Australia (Williams, 1971) and classical flood deposits in Bijou Creek, Colorado (McKee et al. 1967). This formation shows an overall fining upwards trend with an upward increase in lithofacies associations 2 and 3, reflecting a change through to more distal sheet flood conditions. The upper part of this formation is locally dominated by lithofacies association 3, e.g. around Ballydonegan Bay, which represents deposition on mudflats marginal to the locus of sheet flood influx. Normal grading, the lateral continuity of beds and the relative absence of features indicative of traction indicate that deposition occurred primarily from suspension in ponded water, rather than from sheet washing. These ponds were subject, however, to periodic exposure and channelization by residual flood waters (Figure 6c; James 1989). Such mud-rich sections, with evidence for sheet flooding, have been reported from distal terminal fan areas in both modern (Sneh 1983; Abdullatif 1989) and ancient sediments (Hubert and Hyde 1982; Stear 1983; Olsen 1989).

The Caherkeen Formation is approximately equivalent to the Lower Slate Group of Husain (1957), the Ballydonegan Bay Group of Viswathaniah (1959) and the Caha Mountain Formation of Coe and Selwood (1968; Figure 2) and it characterizes the sheet flood dominated stage 1 of the basin’s evolution described by Graham et al. (1992).

26. Eagle Hill Formntion (1900 m thick)

The Eagle Hill Formation mainly comprises thin (2-8 m) lenticular and sheet-like, very fine- to fine-grained sandstone bedsets, separated by thick (5-25 m), amalgamated units of rippled and bedded siltrocks and

DEVONIAN FLUVIAL SEDIMENTS. SW IRELAND 171

Figure 5. Field photographs of the Beara Old Red Sandstone. (a) Sheet-like, very fine sandstone from Caherkeen Formation (association 1) displaying parallel lamination in the lower sandstone and the lower part of the upper sandstone and climbing ripples in the upper part (V630495). (b) Siltrocks with parallel lamination and ripple cross-lamination from the Caherkeen Formation (association 2) (V630495). (c) Sheet-like geometries of very fine sandstones and siltrocks from the Eagle Hill Formation, Reenmore

Point (V566488). (Continued on next page)

172 A. JAMES A N D J . R . GRAHAM

(d) Extensive exposure of Eagle Hill Formation, Cod’s Head illustrating sheet-like nature of the beds. (e) Thick lens of intreformational conglomerate (behind map board) within sandstone bedset, Reen Point Formation, Cod’s Head (V545478). Beds below this show low-angle trough cross-bedding and scattered intraformational clasts. (0 Intraformational conglomeraie (on which hammer rests) within thick mudrock sequence, Tholan Formation, Reen Point (V630536). Prominent vertical fabric

is cleavage

DEVONIAN FLUVIAL SEDIMENTS. SW IRELAND 173

‘7 , 1m , bioturbated

banded silt rwk

3 ~~. plane bedded structure -

sandstone

desiccation cracks desiccati cracks , lOcm ,

Y

d) olane banded sills (bands

asympt

rippled silt rwk plane-bedded - low

. angle cross bedded very-fine sandstone

. convolute bedding

. ripple lamination plane bedded

. very-fine sandstone

. truncated topset

tabular crass bedded fine sandstone

- subvertical cleavage , 30cm ,

Figure 6. Field sketches illustrating: (a) plane-bedded and low angle cross-bedded sandstone bedsets, association I , Caherkeen Formation; (b) disruption by desiccation cracks; ( c ) late stage scour and fills within association 3, Caherkeen Formation; (d) cross- bedded siltrocks, association 3, Caherkeen Formation; (e) channelized, trough cross-bedded sandstones, Eagle Hill Formation; (0 ripple and laminated overbank deposits, Eagle Hill Formation; (8 ) intraformational breccia lags, at the base of a cross-bedded

sandstone, Reen Point Formation; and (h) inclined parallel-laminated sandstones, Toe Head Formation

174 A. JAMES A N D J . R . GRAHAM

sandstones (figures 4b, 5c and 5d). The ratio of sandstone bedsets to siltrock-dominated sections tends to increase upward through the formation, producing a broadly coarsening upward motif.

The sandstone bedsets comprise two to three sheet-like or lenticular units (0.3-0.8 m thick; 6-10 m to limit of exposure in length), internally characterized by plane bedding and high-angle cross-bedding. Preserved topsets and water escape structures are rare. Discontinuous intraformational conglomerates associated with rare inclined heterolithic stratification (IHS; Thomas et al. 1987) are restricted to the lower parts of the formation. These and the incoming of high-angled cross-bedded sandstones mark the base of this formation (e.g. south of Rahis Point, at Dooneen Mine and the eastern part of Dursey Island (Figures 1 and 6e).

The siltrock-dominated sections comprise either lenticular beds with irregular scoured basal and upper surfaces, or sheet-like beds of siltrock and very fine-grained sandstones (similar to lithofacies association 2 of the Caherkeen Formation). Plane bedding, normally graded bedding and current ripple lamination are predominant, and beds are commonly disturbed by water escape structures (Figure 60, desiccation cracks and, rarely, by bioturbation.

The sandstones are interpreted as ephemeral ‘mobile channel’ deposits (cf. Friend 1983) and the siltrocks, with sandstone incursions, their associated overbank sediments. Shallow mobile channels are suggested by the lenticular, typically unstacked character of the sandstones and the predominance of shallow, upper flow regime plane beds. The abundance of interbedded non-cohesive sand-rich beds within adjacent siltrock (overbank) deposits would support rapid lateral migration (Smith 1976) and the paucity of intraforma- tional lags indirectly reflects gentle channel margins with limited undercutting. The rare, steep IHS sets towards the base of the formation are unusual and localized and reflect initial channelization into more cohesive Caherkeen mudflat deposits.

Possible modern analogues for the deposits of the Eagle Hill Formation are the ephemeral channels and interdistributary areas of the Markarda terminal fan, India (Parkash et al. 1983) and the Gash terminal fan delta in Sudan (Abdullatif 1989). Similar sequences are described from ancient sediments by Beer and Jordan (1989), Olsen (1989) and Stear (1983).

The Eagle Hill Formation appears to equate with the Allihies Sandstone and Cumeen Slate Formations of Viswathaniah (1959; Figure 2), although his subdivision of this unit, based on very subtle gradational changes in colour and grain sizes, could not be upheld during the present fieldwork. The broad coarsening upward motif reflects an upward increase in the ratio of mobile channel to sheet flood deposition and is characteristic of stage 2 in the basin’s evolution (Graham et al. 1992).

2c. Reen Point Formation (325 m thick)

The incoming of thick sheet-like and lenticular intraformational breccias (0.1-1 m thick) (Figure 5e), containing mudclasts and more rarely reworked calcareous nodules, interpreted as pedogenic in origin, marks the base of the Reen Point Formation (at Cod’s Head and Dursey Head, Figures 1 and 6g) and distinguish it from the underlying sections. The Reen Point Formation is characterized by thick, lenticular and sheet-like fine- to very fine-grained sandstone bedsets (2-10 m), separated by thick (2-20 m) purple- grey mudrocks (Figure 4c).

The sandstone bedsets predominantly comprise plane bedded and low-angle cross-bedded units (0.2- 1 m thick; > 100 m length) deposited under shallow upper flow regime to transitional dune-plane bed conditions. High-angle tabular and trough cross-bedding are subordinate, but more abundant than in the Eagle Hill Formation and imply a greater frequency of deeper flows and lower flow regime conditions. The abundance of mud clasts further suggests steeper channel margins with bank collapse and undercutting. Structures classifying as hummocky cross-stratification (see Toe Head Formation, below) are rare.

The siltrocks are mainly massive with fewer ripple and laminated siltrock/sandstone incursions compared with underlying formations. Patchy green colouration is evident, especially towards the top of the formation (e.g. south-east of Reen Point, Figure l), and zones of disseminated calcareous nodules (1-2 m thick), interpreted as poorly developed palaeosols, are common.

DEVONIAN FLUVIAL SEDIMENTS. SW IRELAND 175

The relatively thicker sandstone bedsets are greater frequency of stacked, cross-bedded sandstones suggests greater lateral restriction of channel areas, i.e. fixed channel systems (Friend 1983), compared with the Eagle Hill Formation. This may be a function of the increased cohesiveness of the associated, relatively sand-poor, overbank deposits. Dynamic channel switching by avulsion is interpreted from the sharp vertical changes between channel sandstones and overbank mudrocks. This increased segregation of channel-floodplain deposits is typical of stage 3 in the basin’s evolution described by Graham et al. (1992). The Reen Point Formation deposits show similarities with modern ephemeral anastomosed systems, e.g. Coopers’ Creek, Australia (Rust 198 l), although these recent examples comprise generally coarser grained deposits.

2d. Tholane Formation (344 m thick)

This is a mud-dominant formation characterized by thick, massive, rarely rippled and bedded, red and green silt/clay rocks and crops out north of Ballycrovane Harbour and at Tholane Breaker south of Cod’s Head (Figures 1 and 4d). Rare desiccation cracks, poorly preserved fish debris and thin ( ~ 0 . 1 m) sheet- like intraformational conglomerates, rich in reworked calcareous (pedogenic) concretions, also occur (Figure 5f) and imply episodic flooding, temporary pond development and subaerial exposure characteristic of flood basin settings. Mud-dominant flood basins are not well documented in modern environments with channel systems similar to those described for this succession. However, they are well documented in many ancient sequences (Allen and Williams 1978, 1979; Smith 1980; Stear 1983; Turner 1978) where intraformational conglomerates with mainly pedogenic clasts are common. Part of the river system supplying fines to this basin crops out further east, implied by the eastward disappearance of the Tholane Formation and its lateral replacement by Reen Point type sandstones east of Lough Fadda. Ducharme (1968) gave this unit member status because of this lateral discontinuity in strike, but its mappability over a wide area to the south (> 10 km) allows formation status. The Eagle Hill, Reen Point and Tholane formations are considered to be approximately equivalent to the poorly defined West Cork Sandstone Formation of Coe and Selwood (1963, 1968), the top of which they marked at the last red bed (Figure 2).

2e. Toe Head Formation (400 m i thick)

The Toe Head Formation is now recognized to be a regionally mappable unit at the top of the Old Red Sandstone succession (Graham 1975; Williams et al. 1989; Cotter and Graham 1991). Its base is marked by the incoming of grey-green sandstones ( > 2 m), which approximately coincides with the disappearance of red beds. Its top is marked by the incoming of marine, grey heterolithic beds (Graham, 1975). The formation broadly comprises alternations of thick fine-grained sandstone bedsets up to 12 m thick and siltrock units of similar scale.

The Toe Head sandstone bedsets are distinguished from those of the Reen Point Formation by a greater percentage of flat-laminated and inclined parallel-laminated sandstones (sensu Paola et al. 1989) and a relative absence of high-angle cross-bedding. Structures which meet the published diagnostic criteria of hummocky cross-stratification are a common, distinctive feature of some Toe Head sandstone bedsets (Cotter and Graham 1991, Figure 4; James 1989; Figure 6h). This facies was described and discussed in detail by Cotter and Graham (1991), who interpreted it as the product of the transition to upper flow regime conditions in which sediment grain size and rate of sediment fallout were important controls. High water-tables (waterlogged conditions) are suggested by the green coloration and preservation of plant and fish debris in both sandstones and mudrocks. Prominent intraformational conglomerates with reworked pedogenic clasts indicating local incision are characteristic only of the lower part of the formation.

In a complete section through the Toe Head Formation at Cahermore (Figure 1) there is a decrease upward in the proportion and thickness of sandstone bedsets and an increase in the presence of grey mudrocks displaying limited grain size separation. Such vertical changes are recognized over a wide area of the Toe Head Formation outcrop (see Graham 1975; Cotter and Graham 1991). It was suggested by

I76 A . JAMES A N D J . R. G R A H A M

Williams et al. (1989) that these differences may be sufficient to define two members, although the mappability of these remains unproved.

The stratigraphic position of the Toe Head Formation below marine clastic and above continental flood basin deposits suggests a coastal fluvial plain environment of deposition (Graham 1972, 1975; Kuijpers 1975; Cotter and Graham 1991). Many of the sandstone bedsets possibly represent a series of broad distributary fans, which formed as ‘fixed’ river systems expanded on reaching flat coastal plains with relatively higher water-tables. The first definitive evidence for marine conditions roughly coincides with the top of the Toe Head Formation in terms of both sedimentary structures and palynofacies data (Cotter and Graham 1991).

3. BIOSTRATIGRAPHY

Palaeontological evidence, regarding the age of the Old Red Sandstone sequence of Beara, is sparse and unfortunately restricted to the top part of the succession. Samples collected from the Toe Head Formation at Rahis Point, Cahermore and Reen Point (Figure 1) and processed for palynological analyses indicate an upper Devonian/Strunian age (LL, LE Biozones, G. Clayton, Pers. Comm. 1989). The lower parts of the marine sequence have been dated as topmost Devonian (LN Biozone) (Higgs et al. 1988). Suitable lithologies for the preservation of miospores are not found below the Toe Head Formation and thus, despite considerable effort, the rest of the succession remains undated.

Fish debris has been found in the Tholane and Toe Head formations, but their poor preservation has hindered their specific identification and biostratigraphic potential. Fragmentary plant macrofossils comprising striated stems are rare and again restricted to the Toe Head Formation. Identifiable trace fossils (Chondritrs and Skolirhos) are scant and of environmental rather than stratigraphic use (Cotter and Graham 1991).

4. SUMMARY OF THE DEPOSITIONAL HISTORY

The limited evidence for channel incision throughout the lower two-thirds of the Beara Old Red Sandstone is typical of many other Old Red Sandstone successions (Friend 1978, 1983). The general terminal fan model of Friend (1978) applied to the Munster Basin by Graham (1983) seems appropriate. Recent terminal fans are common in semi-arid areas (Parkash et al. 1983; Abdullatif 1989), but this association with climate may not necessarily apply to Devonian sections because of the effects of varying vegetation cover through time (Schumm 1968). The extensive sandflats and mudflats interpreted for the Beara Old Red Sandstone are documented in Tertiary (Beer and Jordan 1989) and Permian (Stear 1983) successions of comparable scale.

The Old Red Sandstone of Beara demonstrates gradual changes in facies proportions through time. The apparent fining-up-coarsening-up sequence exhibited in the lowest two formations is not evident in the laterally correlative sections 10 km to the north (Graham et al. 1992). There is no obvious ordering on a small scale. The dominance of sheet flood deposits in the Caherkeen Formation gradually gives way to ephemeral mobile channel deposits in the Eagle Hill Formation. This subtle coarsening upward sequence is thought to be due to a progradation or the fluvial fans south and east across the basin centre. Palaeo- currents are from the west of north-west throughout in this area (Graham et al. 1992, figure 2). The reason for this progradation has been interpreted as a reduced rate of subsidence coupled with continued large sediment supply.

The Reen Point, Tholane and Toe Head formations provide evidence for increased segredation between channels and flood plains. The development of a thick muddy sequence in the Tholane Formation suggests that rivers, or even valleys, may have been temporarily incisec. The thick intraformational conglomerates show that rivers were cutting down into mud-rich alluvium on which palaeosols had developed. The sharp alternation between channel sandstones and flood plain muds indicates the increased importance of channel avulsions. These changes have also been interpreted as due to decreased gradients and rates of subsidence through time (Graham et al. 1992).

DEVONIAN FLUVIAL SEDIMENTS. SW IRELAND 177

Equal Area Stsreog rap hlc

Projectlonm tor beddlng pole^

Northam Fault Zone N

W E

N Cmlral FtulI 2ona

E

Southorn Fault Zone

E

LEGEND I Fault Trands 1 - Fold Zone bounderias (Mslor TrDnd 1 Faults)

-X- Synclina

.4- Anticline - Malot faults

_ - _ Form linas * Main mines

_ _

/ Trend 1

/ Trmd 2

/ Trend 3

NORTHERN FOLD

Kilcatherine R.

BALLYCROVANE HARBOUR

Figure 7. Simplified structural geological map, illustrating the main fold zones and principal fault trends

5. STRUCTURE

The peninsula essentially comprises a large ENE anticlinorium, which plunges to the south-west and is traversed by a single ENE trending cleavage and three basic fault sets. Cleavage may be penetrative in the finer lithologies, but is an obvious spaced differentiated cleavage in the coarser lithologies, interpreted as the product of pressure solution. In detail, gradual changes in grain size within beds are frequently emphasized by cleavage refraction. Parasitic fold axes are roughly parallel to the main structure.

5a. Structural zones

Three broad structural zones are evident from both aerial photographs and ground control: a Northern and Southern Zone, representing the northern and southern limbs of the main structure, and a Central Zone, representing its core (Figure 7).

Northern Zone Aerial photographs of this region show continuous, straight ENE trend lines and an apparent absence of fold closures. Stereographic projections of poles to bedding show no regional plunge (Figure 7). Minor folds are asymmetrical, parallel 1B type (Ramsay 1967), with axial planes inclined to the south-east (70"). They have flat-lying short southerly limbs ( <: 3 m) and long, steeply dipping northerly limbs (> 5 m, limit of exposure) indicating southward vergence to the core of the anticlinorium (Figure 8a and 8b).

178 A. JAMES A N D J. R . G R A H A M

1)

A broad anticline developed as the area was SublBCted to north-west to soulh-east compressional forces

) Secondary axial planes developed, resulting in 'sleplolds' on the northern and southern limbs and symmetrical folds in the axial part 01 the main structure

I) Continued compression and dextral displacement resulted in brinle failure and fault development The central fold zone was uplined and lilted down lo the south-west whtlsl folds in the northern and southern zone tiahlened

NW

NW SE

focm U

SE NW

I l m I

Figure 8. Interpretive diagram to explain the variation between the three fold zones (a); field sketches to illustrate folds within the northern zone (b), southern zone (c) and central zones (d, e)

Sou t hem Zone Strike lines are again straight and laterally continuous with an ENE trend. The fold style and scale is a mirror image of the Northern Zone, with axial planes dipping steeply to the north-west (70"). Similarly, no regional plunge can be deduced from aerial photographs or stereoplots (Figures 7, 8a and 8c).

Cmtrul Zone Unlike the parallel 1B folds of the Northern and Southern Zones, these are characterized by steep near- vertical axial planes and WSW plunging fold axes (plunge 15-30"; Figures 7, 8a, 8d and 8e). Vergence changes randomly across the zone, with axial planes dipping both steeply to the north and south. Wavelengths of minor folds range from 3 to 25 m. Wavelengths of the main parasitic folds, apparent from aerial photographs, decrease northwards across the Central Zone from 3-4 km to 100-200 m. This decrease in wavelength occurs on the north-facing limb and away from the major Beara Anticline axis. These structural zones are separated from each other by ENE trending lineaments interpreted as faults (Figure 8a).

Coe and Selwood's (1963) structural zones are similarly defined, but the positions of their structural zone boundaries differ slightly from those in Figure 7. They draw the southern boundary of their 'Northern Zone' south of Ballycrovane Harbour, thus incorporating broad south-west plunging, near-upright synclines ( < 500 nih) within their zone of southward verging 'step-folds'. They also define an additional 'Transition Zone' between their 'Northern' and 'Central Zones'. This zone is poorly defined as showing

DEVONIAN FLUVIAL SEDIMENTS. SW IRELAND 179

asymmetrical folds identical to those of their 'Northern Zone', but with a regular WSW plunge. Current field work suggests that no clear distinction exists between folds from their 'Transition' and 'Central Zones'.

5h. Cleavage

A single, steeply inclined pressure solution cleavage is developed with a north-east or ENE trend (040- 070"). A limited number of measurements in the present study suggests that cleavage strikes anticlockwise to minor fold axes in the Central and Southern Zones (10-30") and clockwise to fold axes in the Northern Zone (10-14'). Field evidence (Sanderson 1984; James 1989) shows no basis for Coe and Selwood's (1963) proposed two-phase cleavage development. Their evidence, based on observations of folds cross-cut by minor cleaved intrusions, can also be explained by syntectonic igneous intrusion during a prolonged phase of deformation. Transecting cleavages are consistent with the formation and moderate rotation of folds before and during cleavage development.

5c. Faults and lineaments

Three basic fault trends are evident from aerial photographs (Figure 7). Trend 1 (ENE) fault traces are generally long and straight (5 km+), trending subparallel to the strike of major fold axes. Clockwise and anticlockwise bends are developed in places along the major trend 1 lineaments and are associated with high-angle trend 2 (east) and trend 3 (north-west) faults. Trends 2 and 3 faults (< 1.5 m long) rarely cross- cut trend 1 faults, but rather curve asymptotically into the.latter in plan view. The relative ages seem to vary where they do cross-cut each other, so it is most reasonable to interpret these fault sets as contemporaneous (cf. Sheridan, 1964).

This fault pattern resembles duplex patterns produced by strike-slip movements (Woodcock and Fischer 1986). The horizontal displacements of formation boundaries (< 50 m) recorded in the Eagle Hill, Cod's Head area (Figure l), the presence of anticlockwise transecting cleavages and the 's' shaped curvatures of trend 3 lineaments at Miskish Mountain (Figure 7) suggest that limited dextral movement has occurred along the major trend 1 faults. Such dextral displacement about clockwise bends has resulted in significant normal dip-slip movement (up to 2500 m) associated with east trending (trend 2) faults (e.g. Toe Head Outlier, Rahis Point). Mineralization is closely associated with these normal dip-slip, east trending faults (Viswathaniah 1959; Matthews 1964; Sanderson 1984; Reilly 1986).

Extrapolation of the dextral movement about the anticlockwise bends along the trend 1 faults (e.g. at Miskish Mountain) results theoretically in reverse faulting and the development of the associated trend 3 faults. Determination of reverse displacement has been hindered, however, by the lack of lithological contrast in this area.

Major oblique-slip displacement (up to 3000 m) is evident along the two major trend 1 faults which separate the Northern and Southern Zones (characterized by folds of negligible plunge) from the Central Zone (characterized by folds with moderate plunge to the WSW). Relative uplift and south-westward tilting of the central block along these faults could explain the variation of fold in style between these zones (Figure 8a).

As the folds in the Central Zone are transected in a manner consistent with dextral shear, and the main fault movements along the main strike faults is also dextral, it is reasonable to infer that the two may be synchronous.

6. DISCUSSION

The Old Red Sandstone succession of south-west Beara has been shown to consist of > 5 km of relatively fine-grained fluvial sediments. Throughout the bulk of this succession a terminal fan model is applicable and a general lack of channel incision in the lower parts of the sequence is characteristic. Despite a lack of

180 A . JAMES A N D J . R G R A H A M

small-scale organization, subtle changes through time record a change from sheet flood dominant via mobile ephemeral channels to more fixed channel systems. These changes can also be recognized in the more proximal sections of south-west Iveragh (Graham et af. 1992) and allow correlation between these two areas. However, this correlation is at a fairly crude level due to the very gradual nature of the changes and the lack of marker horizons. Correlation to the east along the Beara Peninsula is not possible at present owing to a lack of modern detailed information.

There is general agreement that the Munster Basin was an extensional half-graben during the late Devonian and that the Dingle Bay Fault Zone was an important synsedimentary fault zone (Price and Todd, 1988; Graham and Clayton 1989). It is most likely that several other synsedimentary normal faults are present in the basement and affect the lower parts of the Old Red Sandstone sequence. However, our present stratigraphic database is insufficient to locate the position of synsedimentary faults in the more central parts of the basin.

The style and zonation of tectonic structures does suggest some basement control (Sanderson 1984; James 1989) as does the nature and location of the tuffisite intrusions (Matthews et al. 1983). The structural style displays features which are seen throughout the western part of the Munster Basin. Folds, cleavage and major faults appear to be genetically related and to overlap in time (Sanderson 1984). Although north - south pure shear seems everywhere to be more important than east-west simple shear (Price and Todd 1988), indications of dextral shear are ubiquitous (Graham and Reilly 1976; Phillips 1985; Price and Todd 1988; Reilly and Graham 1972, 1976; Sanderson 1984). It has been noted above that many east-west faults had dip-slip plus dextral displacement and associated mineralization. These features are also seen further south on Sheep’s Head (Phillips 1985), Mizen (Reilly 1986) around Roaringwater Bay (Reilly and Graham 1976) and eastwards towards Clonakilty (Reilly and Graham 1972; Graham and Reilly 1976). The large Allihies copper mines of western Beara (Figure 1) are related to these east-west faults, but the reason for the greater concentration of copper here compared with the rest of the Carberry copper district remain unclear. The concentration of copper mines in the south-west part of the Munster Basin (Cowman and Reilly 1988) may reflect the parts of the basin where dextral transpression is best developed.

ACKNOWLEDGEMENTS

This paper summarises Ph.D. research work (AJ) undertaken at the Department of Geology, Trinity College, Dublin. Financial support from Ennex during the course of this thesis is gratefully acknowledged. A particular thanks to Dave Naylor for his help and comments throughout the work. Badley Ashton are thanked for drafting the diagrams. The paper benefited from constructive comments by Brian Turner.

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