A New Map and Stratigraphic Model of the Manx Group Lithofacies Of

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Geological Society, London, Special Publications

Lithofacies of Lower Palaeozoic deep-marine sediments in the Isle of Man: a new map and stratigraphic model of the Manx Group

D. G. Quirk and D. J. Burnett

Geological Society, London, Special Publications 1999; v. 160; p. 69-88 doi:10.1144/GSL.SP.1999.160.01.06

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Lithofacies of Lower Palaeozoic deep-marine sediments in the Isle of Man: a new map and stratigraphic model of the Manx Group

D. G. QUIRK 1 & D. J. BURNETT Geology, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK 1Present address." Burlington Resources (Irish Sea) Limited, 1 Canada Square, Canary Wharf London El4 5AA, UK

Abstract: A classification scheme for lithofacies in the Lower Palaeozoic Manx Group has been designed based on a simple assessment of the sandstone:mudstone ratio, bed thickness and sedimentary structures. The scheme was used to acquire high-resolution, standardized field data over areas of good exposure on the Isle of Man. These data were then correlated in order to produce a lithostratigraphic framework and a simplified geological map. Overall, the succession appears to young northwestwards from early Arenig, sand-rich sediments on the southeast coast to mud-rich sediments of mid-late Arenig age inland and on the northwest coast. A number of alternative structural reconstructions have been made which fit to a greater or lesser degree with limited biostratigraphic data. The favoured model involves a compromise between inferred duplication of stratigraphic units by reverse faults and repetition of similar depositional cycles or sequences within the succession. This model implies that the Manx Group is between 5400 and 9250 m thick and contains within it both a lateral facies change (from proximal to distal character east-west) and an overall fining-upwards stratigraphic signature. The sediments are interpreted to have been rapidly deposited on the west dipping margin of a basin situated between the Isle of Man and southeast Ireland, which forms an embayment on the northwest edge of Eastern Avalonia. The main depositional processes were turbidity currents and debris flows, with evidence of possible current deflection or reworking of the upper parts of turbidites by northwards flowing contour currents. During the Caledonian orogeny, the basin was inverted with the development of northeast-southwest trending thrusts and east-west dextral faults. Tentative evidence also exists for the presence of an older north-northeast-south-southwest normal fault trend, although whether it was active during the Arenig is uncertain. It is recommended that future stratigraphic work should concentrate on testing possible correlations between and within the mixed sand-mud Lonan, Port Erin, Injebreck and Creggan Mooar Formations.

During recent years it has become clear, mainly Group, rather than the more subjective approach of from micropalaeontological evidence (e.g. assigning the rocks to formations and mapping their Molyneux 1979), that the stratigraphy of the Isle of boundaries. However, although it appears that all Man proposed by Simpson (1963) is flawed. Little the sediments are marine (and probably deep explanation is given by Simpson (1963) of the marine) in origin (Orr & Howe 1999), it was found criteria used to identify and classify his units, that existing classification schemes for these sort of making it difficult to use his stratigraphic sediments provide insufficient resolution to framework in the field and casting doubt on his subdivide the Manx Group into more than a few structural model (cf. Fitches et al. 1999). For distinctive rock types. For example, the lithofacies example, Quirk & Kimbell (1997) and Quirk et al. scheme of Picketing et al. (1989) is useful for (1999b) have shown that the Isle of Man is sedimentological analysis but not ideal for mapping traversed by major northeast-southwest and (cf. Woodcock & Barnes 1999; Woodcock & east-west faults, unrecognized by Simpson (1963), Morris 1999). Instead, a new lithofacies scheme making the great lateral persistence of some of his was developed specifically for the Manx Group stratigraphic units seem untenable (see Ford et al. based on simple sedimentological characteristics 1999). A solution to these problems was to resurvey such as sandstone type, mudstone content, the distribution of different rock types in the Manx sedimentary structures and bed thicknesses. The

From: WOODCOCK, N. H., QUIRK, D. G., FITCHES, W. R. & BARNES, R. R (eds) 1999. 69 In Sight of the Suture: the Palaeozoic geology of the Isle of Man in its lapetus Ocean context. Geological Society, London, Special Publications, 160, 69-88. 1-86239-046-0/99/$15.00 ©The Geological Society of London 1999. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

70 D.G. QUIRK & D. J. BURNETT

Ballure k ] Port-e-Vullen . ] Mauahold s~by Glen Glen TraieFarkan d"Port Mooar WiIl's Port Stran( Cornaa Peel Group, PeelCastle

Glen Maye

Round 1'1 "Onchan Head Ta.ble ~ DouglasHead Lag-ny-Keeilley The Sttx - Marine Drive Fleshwic~

The,~~GanseSOund,,,,,~ O~x'~" I y~~i~ Cass-ny-Hawin Port St. Mary ' Langness The Chasms Castletown ! Calf of Man Group 0 km 10

Fig. 1. Map of the Isle of Man showing coastal regions and localities referred to in the text.

scheme is summarized in Table 1, illustrated in Fig. pebbly mudstone and mudstones with iron- 2 and described in detail in a Supplementary manganese carbonate bands, respectively (Table 1). Publication. 1 Sandstone-bearing lithofacies are divided into three The lithofacies nomenclature that has been field-based classes, and further subdivided in a adopted reflects the sandstone:mudstone ratio of similar way to the mudstones. Sandstones (S L, S I the sediments: e.g. a lower limit of 55% mudstone and SH) comprise quartz arenites containing defines lithofacies class M (mudstone) suffixed by moderate to low amounts of interbedded mudstone. the letters V, H, I and L to indicate very high Quartzites (QH and Qv) are coarser grained, (95-100%), high (90-95%), intermediate (70- recrystallized quartz arenites with no visible 90%) and low (55-70%), respectively. In addition matrix. Wackes (W H and W u) are lithofacies to these standard mudstone lithofacies, lithofacies containing varying amounts of lithic fragments and M r, and MIC are two distinctive types that comprise feldspar grains. After a pilot study, the scheme was applied in mapping outcrops in detail along coastal exposures lSupplementary Publication No SUP 18134 (12 pages) contains detailed descriptions of each lithofacies. It is and inland transects. The results were then used to available from the Society Library or the British Library correlate the succession and to determine the gross Document Supply Centre, Boston Spa, Wetherby, West lithostratigraphic and structural relationships, based Yorkshire LS23 7BQ, UK. on large-scale trends in the dominant lithofacies Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

Table 1. Main field characteristics of lithofacies in the Manx Group, described in detail in the Supplementary Publication No. SUP 18134

Lithofacies Type section Sand- Sandstone bed Distinguishing characteristics Bouma Pickering Lithostratigraphic unit in Field term used by Associated stone (%) thickness (cm) units class* which commonly found Lamplugh (1903) lithofacies

M v Round Table < 5 < 0.1 Dark grey, massive Te El.l, El.2 Barrule Fm, Glen Rushen Fm Barrule slate M H, Mp (, MI) M u Maughold Head 5-10 < 0.5 (0.1 av) Laminated Te E2.2, D2.3 Baxrule Fm Barrule slate M v (, Mr,, M I) M I Sulby Glen 10-30 < 1 (0.2 av) Laminated, slumping common Tde C2.3, D2.3 Injebreck Fm (lower unit) Unassigned M L (, Mp, M H, QH) MIC Niarbyl south 5-20 < 2 (0.15 av) Contains bands of Fe-Mn carbonate Tde C2.3, D2.3 Creggan Mooar Fm Unassigned Mp, M v (, MI) M L Port Erin 30-45 < 3 (1 av) Planar (-ripple) laminated Tde C2.3 Port Erin Fm Mostly unassigned MI, SL, QH, Qv Mp Ballanayre Strand < 30 Conglomeratic (matrix supported) A1.3 Fleshwick Unit, Lady Port Fm Crush conglomerate Mv (, MI, Wu, QH) S L Traie Farkan (S) 45-60 < 5 (l av) Ripple-planar laminated Tcde? C2.3 Lonan Fm Flags SI, ML (, QH) S l Port Mooar 60-80 < 0.1-15 (7.5 av) Quartzose, ripple-planar laminated Tcde? C2.3, C2.2 Lonan Fm Flags SH, SL (' WH' QH) S n Port Cornaa 80-100 1-50 (10 av) Quartzose, convolute lam. common Tabcd BI.1 Ny Garvain Fm (lower unit) Flags SI QH Traie Farkan (N) 80-90 3-50 (12.5 av) Quartzite, commonly planar laminated Tabcde B1.2 Creg Agneash Fm Grit M L (, Mp, MI, SL) Qv The Chasms 80-100 10-100 (40 av) Quartzite, massive (-planar laminated) Tabcd BI.1 Mull Hill Fm, Keristal Mbr Grit M L, S I (, Mp) S v Purt Veg 95-100 50-300 (125 av) Arkosic, planar-massive, coarse-gran. Tab A2.8 Santon Formation (rare) Grit S H (, M~ ?) W H Marine Drive 80-90 3-50 (12.5 av) Wacke, commonly planar laminated Tbcde B1.2 Santon Fm (Marine Drive) Flags SI (, SH) WHp Lag ny Keeilley 80-90 2-100 (50 av) Wacke, conglomeratic, planar laminated Ta A2.5 Eary Cushlin Unit (very rare) Unassigned Mp, SL, W E W U Not defined 85-100 1-300 Commonly wacke, undifferentiated Tabc BI.1, C2.1 Glion Cam Unit Grit Mp, M v MTc, M H

*Pickering et al. (1989). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

300

2(X)

!Sv 100, I I

50" 40-

~-~ lO Su r~

....~~3 4 3,

r.~ 1,

0.5 0,4 ME 0.3

0,2 MI

M[-

0.05 0 410 ~0 60 7~ 80 ~0 1;0 Fig. 2. (a) Graph comparing sandstone percentage with bed thickness for mud-rich and sand-rich lithofacies; (b) Photograph showing samples of lithofacies ranging from low mudstone to high (a) sandstone percentages. The labels are 1.5 cm high. See Table 1 and Supplementary Publication No. % sandstone SUP 18134 for further details. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 73 and the observed or inferred positions of bound- interval contains an estimated 25-75% of another aries between markedly different groups of lithofacies, then it is given a shared name (shown lithofacies. The interpretation was also improved with horizontal stripes in Fig. 4). Where another by integration with the pioneering geological lithofacies makes up only 5-25% of the interval, it survey work of Lamplugh (1903) who mapped the is regarded as a minor constituent (shown with entire island in detail (cf. Fig. 3). coloured dots in Fig. 4). Where a lithofacies forms The results are shown in Fig. 4 as a set of litho- < 5% of an interval it is generally not depicted, logical columns. Each column represents a more or except where pebbly mudstone or beds of quartzite less continuously exposed section separated from help to characterize the interval. adjacent sections by observed or inferred faults, intrusive bodies or gaps in exposure. Each lithofacies has been assigned a colour in logical pro- Correlations and vertical succession of the gression with the next most similar lithofacies. An Manx Group interval is shown as containing a single lithofacies if at least 75% of the interval conforms with the There is a virtual absence of marker beds in the description of that lithofacies (Table 1). If the Lower Palaeozoic rocks of the Isle of Man and

Legend

D Non-Manx Group

Major intrusions

Barrule Slates (equivalent to @ lithofacies Mv and occasionally MH) Agneash and other Grits @ (equivalent to lithofacies QH, Qv, Sv and Wu with background • , ,,-. • • , "•" lithofacies of SL, MH, MI and ML) Lonan Flags (equivalent to ® lithofacies SH, SI, SM, SL, ML, WH and occasionally MI) Not separated (equivalent to 1 • " 'i '~ Vqlithofacies MH, MI, MIC, ML, WU, WHp, SI and SL) ..... Fault

X,t'-" Z/ 10km

Fig. 3. Simplified map of lithotypes surveyed by Lamplugh (1903). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

74 D.G. QUIRK • D. J. BURNETT currently only a poor biostratigraphic framework East coast on which to tie the stratigraphy (cf. Molyneux Access along the coast between Onchan and 1999; Orr & Howe 1999). However, certain Maughold is generally good and has allowed lithofacies clearly occur grouped together in areas confident correlations to be established within the or tracts with a specific lithofacies signature (Fig. quartzose sand-rich lithofacies (S L, S I and S H) 4). Within each tract, correlations can be made with which dominate this part of the succession. Only a fair degree of confidence but correlations between where a probably sand-rich lithofacies (?S n) tracts are more difficult to establish. Some exposed in Dhoon Bay has been strongly flexibility has been used during correlation because recrystallized due to contact metamorphism by the of inferred lateral facies variations and some Dhoon Granite is there any uncertainty in the imprecision in classifying certain outcrops, classification. In all, a total stratigraphic thickness particularly inland where weathering and limited of c. 1800 m is exposed but this estimate is exposure can be a problem. Within tracts, corre- complicated by the presence of some large folds lations across faulted boundaries have been made and strike-slip faults (Fig. 4). using the minimum offset required to produce a The key to correlating this tract is the recognition good match between columns, unless there is of a distinctive 500-550 m thick section (interval geological evidence to suggest otherwise. Alter- 20-25) of highly sand-rich lithofacies (S H) forming natively, the data in each column can be used the lower unit of the Ny Garvain Formation. Below independently of the lithostratigraphic interpre- and above this section the sediments are more tation shown on Fig. 4. Thus, in the future, the muddy, in the Lonan Formation (interval 10-20, columns can be rearranged and added to as new lithofacies ML-S I) and the upper unit of the Ny field and laboratory data are acquired that help Garvain Formation (interval 25-30, mostly improve the correlations. lithofacies SL-S 0, respectively. Woodcock et al. Probable and possible correlation surfaces are (1999) have assigned the coastal section from numbered upwards from oldest to youngest (Fig. Braggan Point to Onchan Head, north of Douglas, 4), making the general assumptions that: to the Santon Formation lying within the axis of the • in an overall sense, lithofacies units get younger Douglas syncline. However, the lithofacies are to the northwest, except where the contrary was identical to the lower unit of the Ny Garvain observed in the field; Formation north of the Dhoon Anticline (Fig. 4). • there are no missing sections and therefore the The type Santon Formation (south of Douglas) is full stratigraphy can be reconstructed. more muddy but almost certainly correlates with the lower unit of the Ny Garvain Formation. The These assumptions are unlikely to be completely axial trace of the Douglas Syncline appears to cut valid and therefore the correlation surfaces set a down-section from Douglas to Port Groudle (Fig. lithostratigraphic rather than chronostratigraphic 4), suggesting that the fold axis plunges to the framework. None the less, they have allowed units southwest. The top of the Lonan Formation to be defined which, on the whole, conform with (correlation surface 20) is marked in places by a the formal lithostratigraphic scheme of Woodcock thin package of quartzites (lithofacies Qv), the et al. (1999). However, where there is a difference Keristal Member of Woodcock et al. (1999). A in interpretation, or where new subdivisions have faulted contact between the top of the Ny Garvain been made, the term 'unit' has been adopted here to Formation and the overlying Creg Agneash distinguish it from the formations and members Formation (correlation surface 30) is present at defined by Woodcock et aI. (1999). Maughold Lighthouse. The main observations are briefly synthesized in separate sections below, for which Fig. 1 can be used as a location map, and are summarized Northeast coast diagrammatically in Fig. 5. This framework is then A thickness of 1200 m of highly mud-rich litho- used in later sections to interpret the lateral and facies (Mv-M H) occurs between Maughold Head vertical relationships in the Manx Group with and Port e Vullen. By contrast withWoodcock et al. which structural cross-sections and a basin model (1999), these rocks are assigned to the Barrule are developed. Formation (interval 40-50) due to their close similarity in lithofacies and thickness with rocks

Fig. 4. Compilation of lithofacies mapped by authors between 1995 and 1997 presented in the form of correlation panels and simplified map. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

;tone with 40-55% mudstone. sandstone with 20-40% mudstone. ~adstone with 20-40% mudstone. ded sandstone with 0-5% mudstone. d quartz arenite with 10-20% mudstone. i I ~ i; bedded quartz arenite with 10-20% mudstone. ~acke with 0-20% mudstone. :led wacke with O- 15% mudstone. • pebbly mudstone. ly wacke.

CRONK NY ARREY LAA

u5 Z ......

~!ii~,.~ii!~ iP~i!i i!!i ii!i Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 75

exposed around Clagh Ouyr in the central-north This interval is assigned to the Maughold area (Fig. 4). However, the possibility remains that Formation by Woodcock et al. (1999) on the basis the eastern (lower) part of the section represents a of correlations with a highly muddy section on the lateral variant of the upper part of the Creg northeast coast, between Maughold Head and Port Agneash Formation. The base of the Maughold e Vullen (equated with the Barrule Formation here), Head-Port e Vullen section is exposed at [SC 497 and a pebbly mudstone-bearing interval on the 916], where a significant part of the Creg Agneash south coast, between Bradda Head and Lhiattee Formation (interval 30-40) has been faulted out by Beinee (the Fleshwick Unit, Fig. 5). On top of the a large east-west dextral fault (Quirk et al. 1999b). Creg Agneash Formation is the very highly mud- The top of the section is interrupted by several thick rich lithofacies (My) of the Barrule Formation, felsitic intrusions and associated shear zones estimated to be c. 1200 m thick in the vicinity of around Port e Vullen and Port Lewaigue (Fig. 4). A Clagh Ouyl, assuming that there is no structural short muddy section (lithofacies My, MIC and Mp) repetition. The Barrule Formation appears to thin at Ballure is thought to be faulted in from higher up rapidly between Snaefell and Beinn y Pott ([SC 397 in the succession, probably the middle part of the 879]-[SC 381 860]) and on the west side of North Injebreck Formation (interval 60-65). Barrule (e.g. [SC 414 900]). This thinning corresponds with the apparent truncation of units within the underlying Creg Agneash Formation and the Central-north area overlying lower unit of the Injebreck Formation. Figure 4 shows a composite section made up of the This relationship, and the fact that linear best exposed sections along traverses between aeromagnetic anomalies are observed at these Laxey and the northern escarpment. It is estimated boundaries, suggest that the Barrule Formation is that the stratigraphic thickness of this part of the partly fault bounded, e.g. by the North Barrule succession is at least 8000 m (assuming no fault lineament at the base of the Barrule Formation repetition), dipping on average 45 ° to the north- (Quirk et al. 1999b). The North Barrule lineament west. An overall change is seen from quartzose coincides with the position of a trial mine adit at sand-rich lithofacies in the lower (eastern) part of [SC 387 872], which intersected a mineralized fault the succession to mud-rich lithofacies in the upper of unknown orientation. (western) part. The interval between the Barrule and the Glen Probably the oldest rocks in the area, in Glen Rushen Formations (interval 50-70) is defined as Roy, comprise low to moderately sand-rich the Injebreck Formation (Woodcock et al. 1999), lithofacies (SIzS 0 correlating with the Lonan and is here divided into three units. However, the Formation (interval 10-20) on the eastern coast. stratigraphic picture is complicated by the inferred Above this is the lower unit of the Ny Garvain presence of at least one major fault (Quirk et al. Formation (interval 20-25), the distinctive highly 1999b). The lowest unit (interval 50-60) occurs in sand-rich lithofacies (SH) seen, for example, in Glen Auldyn and Lhergyrhenny and comprises Glen Agneash. The more muddy upper part of the moderately mud-rich and quartzite-bearing Ny Garvain Formation (interval 25-30) is best seen lithofacies (MI-QH) with similarities to parts of the at Creg ny Baa. Creg Agneash Formation. A pebbly, highly mud- The Laxey Valley exposes a 1700 m thick unit rich lithofacies (Mp-M v) in Ballakerka forms the above the Ny Garvain Formation known as the middle part of the Injebreck Formation (interval Creg Agneash Formation (interval 30-40) con- 60-65). This Slieau Managh Unit pinches out sisting predominantly of moderately mud-rich and southwards (Fig. 4). Its thickness is hard to quartzite-bearing lithofacies (MI-QH). It appears to estimate, due to probable faulting and lateral facies thin to less than half its original thickness and variations. A large east-northeast-west-southwest become slightly less sandy in a northeast direction. trending fault is interpreted to separate it from the However, at least some of the change in thickness lower unit of the Injebreck Formation. However, is due to faulting at Maughold Head (see preceding the extreme scarcity of pebbly mudstone (Mp) section). The Creg Agneash Formation has been between the Barrule and Glen Rushen Formations divided into a lower, middle and upper unit on the (interval 50-70) further south suggests that some basis of three apparent fining-upwards cycles (Fig. lateral facies variation may also exist, possibly due 4). At the base of the lower and middle units, the to syn-sedimentary movement on the same fault quartzites are particularly thick and well developed (Quirk et al. 1999b). The Slieau Managh Unit around Creg ny Baa and Windy Corner, reminiscent probably correlates to the east with a faulted section of the Mull Hill Formation in the south of the island exposed on the coast at Balhire (Fig. 4). The base of (lithofacies Qv). The upper unit, which is the the upper unit of the Injebreck Formation is marked thickest, is distinctly less sandy, comprising mostly by a thin package of quartzites (lithofacies QH) and lithofacies M I with the occasional quartzite bed. the rest of the interval (interval 65-70) consists of a Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

76 D.G. QUIRK ~; D. J. BURNETT

Glen Mooar

1000m

Niarbyl | 93~ u 'GLIONCAM UNIT" 90~--~ J .... ~ 1~I,c"...... Qua=Bends f-- CREGGAN MOOAR [2_ -- ~[ M [ Will'sStrand N ~ g FORMATION ~--~_ ,c ~1 ~ N ~ I" . " I . ~-~ ;I ~ GlenHelen ">.. S?..... I-- " q OO Fh..... 1N~., 780~-~'~ ..... q I I -- 80 ~ GLENRUSHEN ~Nx~ ? [L ~q

Gob yn Ushtey~ 770~ ~: M~?N~N { .... dhoo ~--q 7FO~-~IO~--~NN9 ? G~o "~

'EARYCUSHLIN UNIT' P--'E~M+11 L~ l+ P ~ ~. ~-:~ .... N ~¢-%~ w~ ' I Gob yn Ushtey ~ ~ F~--~

La-}v K e'll~, (( ~ L~_~"%..,.-J.... 'SLIEAU f... Bradda Hill e.. e, ~. ~ ~ [:~__~ ...... MANAGH ,O"~ ~ GlenAuldyn ~" ¢-~'.~ UNIT' ~--~Mp/ ~ .... _-9_~. __ ~'~%~ -- ¢e ~---" "~l "~"- Ballakerka \~'~ ~ ~- = ~ MI+QH Port Lewaigue 7-'~ 5O -- |--~ 50~ MH+M~vlMIMH/Mv BARRULEFORMATION ~ My --+-- ---+-t ___N 40 -- . ---i ,,=,~ SL--Z~. / I b ~ Qq MULL HILL FMTN~---/ "_ 2] M,+& X---- N Maugho]d ?38 ~ |~/-~TraTen~'-Sl~a-- N 38 _'7- "A .--or --°4 Lu CREG & 7 -~ MCQ. AGNEASH ~+zl~.lQH Ga~r |~/k-:2. L+ o H '- -~ FORMATION The Whing S Douglas 35 -T• °" PORT i o ° SL/QH ~--" -1. ~:1 ERIN J I~---~M/s ----a'----TU ff'-~-~ ~-- 3o _, .[ ..... -- • SI+SL I.- NY GARVAIN 8 Ald 'ck Perwick Bay I ~ : .4 ..... -- ~ -. cc FORMATION ~ Ix ~o ~.1~v" MULLHILL FMTN T, ?20 ", / M~7 ...... -- -~ Sl+Sk LONAN ~:3~ Gansey ~ Z~M~÷QH -- ~Port Soderick Fiddlers Green ~ +ML FORMATION La×ey N Douglas ?m ~--"r~-- -- PORX -- ~01"-'~"~~ -- --- ~_~-~]/~ ERIN I7-_ *_ ~ML/SL ?sTheS~e~tmd~ -- FMTN --?8~f+ 5 --

b S Langness

Fig. 5. Schematic lithostratigraphy of the Manx Group based on Fig. 4. The principal lithofacies in each unit are shown on the fight-hand side of most columns. Note that the lithostratigraphic position of the Mull Hill and Port Erin Formations (bottom left-hand corner) is uncertain and two alternatives are given. Table 2 gives further information.

broadly mud-rich lithofacies association (M v, M H, the succession in the central-north area (interval M I, M L and SL). It shows a higher illite crystallinity 70-80). Approximately 600 m of the formation is grade than the middle and lower parts of the exposed in Glen Helen, where it is exclusively Injebreck Formation, supporting the idea that it is composed of very highly mud-rich lithofacies bounded on its southeast side by a reverse fault (My). However, a variable, but generally moderate (Quirk et al. 1999b). It is difficult to trace the to highly sand-rich lithofacies (SuSH), similar to Injebreck Formation towards the Central Valley and the Ny Garvain Formation, is present around Glen further south because of poor exposure. However, Dhoo and Carrick. The stratigraphic relationship of the Barrule Formation appears to be offset dextrally this Glen Dhoo Unit to the Glen Rushen Formation by a large east-west fault known as the Greeba and the underlying mixed lithofacies of the Lineament, evidence for which is also seen on Injebreck Formation is uncertain (Fig. 5). Acritarch aeromagnetic data (Quirk et al. 1999b). data presented by Molyneux (1999) suggest that the The Glen Rushen Formation occurs at the top of Glen Dhoo Unit is early Arenig in age whereas the Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 77

Glen Rushen Formation, which occurs along-strike example, around Port Jack (lithofacies SL-SI) from it, is mid-Arenig in age (Table 2). Field further north. In this interpretation, the amount of mapping and aeromagnetic data (Quirk et al. stratigraphic offset across the fault at Purt Veg may 1999b) indicate that they are in faulted contact (Fig. be as much as 700 m in a dextral sense, although 4), suggesting a number of possible structural this estimate depends on assumptions made in models which are discussed later. reconstructing the rather faulted succession south of Purt Veg (Fig. 4). The alternative correlation is with the Creg Agneash Formation (interval 30--40) Southeast coast, including Marine Drive which shows a generally similar mud-rich back- Most of the succession exposed from Santon Head ground lithofacies (MI) but contains significantly to Loch Promenade in Douglas is thought to greater amounts of quartzite (lithofacies Qn)- correlate with the Ny Garvain Formation (interval Based on a tentative match with the Laxey Valley 20-30) which is present further north (Fig. 4). (Fig. 4), the likely stratigraphic offset in this However, differences in the lithofacies led interpretation is c. 3000 m by sinistral movement. Woodcock et al. (1999) to instead assign this There is a marked similarity in the mud-rich succession to the Santon Formation, albeit the lithofacies between Purt Veg and Langness and the lateral equivalent of the Ny Garvain Formation. Port Erin Formation, exposed further south The area has been divided into three separate (lithofacies MuML; Fig. 5), to which the section is sections (Santon, Marine Drive and south Douglas) therefore assigned. Whether the Port Erin with uncertain mutual relationships. Formation is the lateral equivalent of the Lonan The south Douglas section comprises c. 1200 m Formation or the Creg Agneash Formation is yet to of moderate to highly sand-rich lithofacies (SI-Sn), be established. Detailed correlations in this area are the base of which, on Douglas Head, may represent difficult to make because of structural complexity the top of the Lonan Formation (correlation surface and the relationships shown on Fig. 4 remain 20). Marine Drive contains a highly wacke-rich speculative. lithofacies (Wn) unique to that section, which may represent an immature lateral equivalent of the South coast (Port Erin-Gansey) highly sand-rich lithofacies (Sn) in south Douglas (Fig. 4). However, the presence of a ?northwest- In the area around Port St Mary, The Sound and southeast sinistral fault offsetting the Douglas Port Erin, the Manx Group is affected by some Syncline by 1500 m at Keristal and an east-west large faults leading to uncertainties in correlation, ?dextral fault juxtaposing different lithofacies at particularly between The Sound and Aldrick and Fiddlers Green allows for the possibility that the between Port Erin and Cregneash. Almost 2000 m Marine Drive section has been faulted in from of low to moderately mud-rich sediments belonging lower in the succession (Quirk et al. 1999b). to the Port Erin Formation are overlain by the Mull Approximately 450 m of section is exposed Hill Formation comprising c. 300 m of quartzites between Keristal and Santon Head. As well as and mudstones (mostly lithofacies Qv and ME). moderate to highly sand-rich lithofacies (SI-Sn), it The quartzites are concentrated towards the base of includes an unusually thick-bedded, very highly the Mull Hill Formation. The contact with the Port sand-rich lithofacies (Sv), interpreted as a turbidite Erin Formation (correlation surface 20-38) is best channel. It appears to correlate with a moderately seen at The Chasms, at Port St Mary, where it is mud-rich lithofacies containing thin bedded overturned, and at Aldrick, truncated by a major quartzites (M I + QI-I) (Fig. 4) which may represent a fault (Fig. 4). Assuming that the Port Erin levee deposit. Formation is not equivalent to the highly sandy parts of the Santon-Ny Garvain Formations, the Mull Hill Formation may correlate to the northeast South coast (Langness-Purt Veg) in one of two ways; either with the upper part of the A vertical northwest-southeast trending fault at Creg Agneash Formation (interval 38-40), which Purt Veg [SC 324 703] marks an important change contains few quartzites in the Laxey River, or with in lithofacies (Quirk et al. 1999b). The direction the lower parts of the Santon and Ny Garvain and amount of offset on this fault is uncertain and Formations (interval 20-25) (Fig. 5), which contain two possibilities exist, depending on how the low to equivalent amounts of quartzose sand but in a moderately mud-rich lithofacies (ML-MI) between different type of lithofacies (SH). A second thinner Purt Veg and Langness correlates with the package of quartzites occurs within the middle of succession to the northeast (Fig. 5). The correlation the underlying Port Erin Formation at the north end favoured by Woodcock & Barnes (1999) assigns of Port Erin. This package could correlate with the the section to the Lonan Formation (interval 5-20) base of the middle unit of the Creg Agneash implying that it is much muddier here than, for Formation (correlation surface 35) or else marks Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

Table 2, Estimated thickness, sequence stratigraphic interpretation and possible correlatives for each of the main lithostratigraphic units shown in Fig. 4, north of Cronk Ny A rrey Laa

Lithostratigraphic Estimated Sequence stratigraphic Biostrati- Established Possible Potential subdivision thickness (m) interpretation graphic age correlative correlatives correlative

Lady Port Fm c. 2100 m Tectonically complex Late Arenig Creggan Mooar (_) Glion Cam Unit ?500-1000 m Late highstand ?Early Arenig? Glen Dhoo (3,4) Ny Garvain (4) Upper Injebreck (2) Creggan Mooar Fm >900 m Early highstand Lower Injebreck (2, 3, 4) Upper Injebreck (3, 4) Lonan (4) Glen Rushen Fm >600 m Transgression Mid-Arenig Barrule (2, 3, 4) Slieau Managh (3, 4) Glen Dhoo Unit c. 1000 m Lowstand Early Arenig Ny Garvain (2, 4) Glion Cam (3, 4) Upper Injebreck Unit 950 m (?Late) highstand Eary Cushlin a (S) Lower Injebreck (3, 4) Creggan Mooar (3, 4) Lonan (4) Slieau Managh Unit >1000 m Slope instability ?Trem(-Aren)? Upper Fleshwick b (S) Barrule/Glen Rushen (3, 4) Lower Injebreck Unit c. 1200 m (?Early) highstand Lower Fleshwickc (S) Creggan Mooar (2, 3, 4) Upper Injebreck (3, 4) Lonan (4) Barrule Fm 1200 m Transgression Glen Rushen (2,3,4) Slieau Managh (3, 4) Creg Agneash Fm 1700 m Late lowstand- Port ErirdMull Hill (S) early transgression Ny Garvain Fm 1050 m Early lowstand Early Arenig* Santon d (S) Mull Hill e (S) Glen Dhoo (2, 4) Glion Cam (4) Lonan Fm 550 m Late highstand Port Erin f (S) Injebreck/Creggan Mooar (4)

Refer to Fig. 5 for an overview of the principal lithofacies present within each lithostratigraphic unit. Biostratigraphic ages are based on data in Molyneux (1999) and Orr & Howe (1999). *based on correlation with Santon Formation; (___)and/or Glen Rushen Formation and/or Glion Cam Unit; (2), (3), (4), in structural Models 2, 3 and 4, respectively (see Fig. 7); (S) in south of Isle of Man; ac. 800 m thick; 6c. 440 m thick; Cc. 800 m thick; dc. 1100 m thick; ec. 300 m thick; fc. 2400 m thick. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 79 the top of the oldest part of the Manx Group account for c. 3400 m of stratigraphic offset (Quirk (correlation surface 10). et al. 1999b). The section exposed on the coast north of the thrust, from Lag ny Keeilley to Gob yn Ushtey, forms part of the Eary Cushlin Unit (Fig. Southwest coast 5). It contains a varied lithofacies association, A marked change in lithofacies occurs at the north different from that on Cronk ny Arrey Laa, end of Port Erin across a major east-west trending including a unique conglomeratic wacke (Wx_ir,). fault at [SC 193 697] (Quirk et al. 1999b). Due to a number of faults and shear zones bounding Depending on whether the Port Erin Formation on and cutting this short section, as well as its the south side of this fault is correlated with the relatively poor exposure, correlation is problematic Creg Agneash Formation or with the Lonan (cf. Fitches et al. 1999). However, the nearest Formation (Fig. 5), the fault accounts for either c. similar lithofacies association (Mv-SL) occurs 2800 or 5400 m of stratigraphic offset, respectively, within the upper part of the Injebreck Formation by apparent dextral movement. In contrast to the (interval 65-70) around, for example, the Blaber south and east coasts, pebbly mudstones River, which is the correlation tentatively suggested (lithofacies Mr,) make up an important part of the here and supported by Woodcock et al. (1999). succession north of the fault, in addition to quartzites and very highly mud-rich lithofacies Niarbyl south (QH-Mv). In total, c. 2300 m of succession is interpreted in this southwest area. The succession is The section between Gob yn Ushtey and Niarbyl here called the Fleshwick Unit rather than the consists of a relatively poorly exposed lower part, Maughold Formation (cf. Woodcock et al. 1999), to south of Fheustal, forming the upper part of the avoid structural inconsistencies and problems in Eary Cushlin Unit, and a well-exposed upper part, equating the lithofacies. Roberts et al. (1990) show north of Fheustal (Fig. 5). On the basis of that the lower part of the Fleshwick Unit has a correlations with the central-north area (Fig. 4), significantly lower illite crystallinity grade than the most of the very highly mud-rich Glen Rushen Port Erin Formation, supporting the idea that it is Formation (interval 70-80) appears to have been from higher in the succession. The lithofacies in the faulted out at Fheustal (Quirk et al. 1999b). Bradda Head-Lhiattee Beinee section are most Consequently, the section south of here, comprising similar to those of the lower and middle units of the moderately mud-rich and pebbly mudstone Injebreck Formation (interval 50-65). However, lithofacies (Mr-Mp), is correlated with the upper there are no pebbly mudstones in the lower part of unit of the Injebreck Formation (interval 65-70). the Injebreck Formation (interval 50-60) around The Creggan Mooar Formation (interval 80-90) Lhergyrhenny and Glen Auldyn (Fig. 4), implying lies north of Fheustal and consists mostly of that moderately mud-rich lithofacies (MI) may pass moderately mud-rich lithofacies with characteristic laterally into pebbly mudstones (lithofacies Mr,). red-brown iron-manganese carbonate bands (MIc) The very highly mud-rich and pebbly mudstone (Kennan & Morris 1999). Inland this lithofacies is lithofacies (Mv-Mp) on Cronk ny Arrey Laa are rarely exposed, but it reappears on the northwest correlated with the underlying Barrule Formation coast within the Lady Port Formation (interval (interval 40-50) but, similarly, there are no pebbly 90-100). It may, however, be indistinguishable mudstones in the Barrule Formation in the north- from lithofacies M I inland in the Injebreck east, e.g. around Clagh Ouyr (Fig. 4). However, as Formation, as the suspicion is that the iron- explained below, the pebbly Slieau Managh Unit manganese bands only become obvious on wave- may also correlate with the Barrule Formation washed outcrops. In fact, the only other place because of possible fault repetition, in which case where lithofacies MI¢ has been observed is on the debris flows are a common feature at this level coast near Ramsey at [SC 460 934], probably (?mid-Arenig). As in the northeast, where the base within the Injebreck Formation (Fig. 4), with which and top of the Barrule Formation are probably the Creggan Mooar Formation may correlate (see fault-bounded, the Cronk ny Arrey Laa section is below and Table 2). Approximately 900 m of separated from the overlying Eary Cushlin Unit and Creggan Mooar Formation is estimated to be probably also from the Lhiattee Beinee section by present and it is bound to the north by a shear zone approximately northeast-southwest trending faults and fault separating it from the graded wackes of (Quirk et al. 1999b). the Silurian Dalby Group (Morris et al. 1999). Although the boundary is not well exposed, the Glion Cam Unit is thought to overlie the Creggan Lag ny Keeilley Mooar Formation (Woodcock et al. 1999). Limited The northwest side of Cronk ny Arrey Laa is outcrop suggests that it is 500-1000 m thick and marked by a southeast dipping thrust estimated to consists mostly of lithofacies W U. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

80 D.G. QUIRK • D. J. BURNETT

Northwest coast of the Maughold Formation of Woodcock et al. (1999), between Maughold Head and Port e Vullen, A highly faulted section containing diverse comprises lithofacies very similar to the Barrule lithofacies (My, Wu, M v and Mic ) is exposed Formation and appears to connect with it (Fig. 4). between Will's Strand and Glen Mooar. This The Barrule Formation itself is cut out on the represents the Lady Port Formation (interval southwest side of Snaefell by the North Barrule 90-100) which is tentatively estimated to be c. Lineament (Fig. 6). A similarity in lithofacies 2200 m thick, excluding several thick felsitic between the lower and upper units of the Injebreck igneous bodies (Fig. 4). It is thought to represent Formation supports the idea of possible fault the highest part of the succession, as supported by repetition (see below). The rest of the area around a late Arenig acritarch age (Molyneux 1999) and Glen Dhoo, Cronk Sumark and Sulby Glen, a low illite crystallinity values (Roberts et al. 1990). region left uninterpreted by Woodcock et al. The presence of lithofacies W U, Mic and M v, often (1999), has been mapped and the lithostratigraphy in fault-bounded packets, as well as thick intervals informally defined by the current authors (Fig. 4). of pebbly mudstone (lithofacies My), indicate that Instead, however, a large area in the centre of the the unit may contain slivers of Glion Cam Unit, and island, south of the Central Valley, has been left Creggan Mooar and Glen Rushen Formations. It is uninterpreted because of poor exposure. Unlike also worth noting that lithofacies Mi¢ at Gob y Woodcock et al. (1999), the present authors assign Deigan [SC 283 873] is remarkably similar in the rocks between Langness and Purt Veg [SC 324 appearance to an interval on the coast near Ramsey 703] to the Port Erin Formation rather than the at [SC 460 934] assigned to the Injebreck Lonan Formation, again on lithofacies grounds. Formation (Fig. 4). The Lady Port Formation can Likewise, the Ny Garvain Formation is correlated only be traced for a limited distance inland where a with the Santon Formation rather than with the Port faulted boundary is inferred with the poorly Erin and Lonan Formations, which contain far less exposed Glion Cam Unit (Woodcock & Morris sandstone. 1999) (Fig. 4).

Comparison with the lithostratigraphy of Faults Woodcock et al. (1999) Interpretation of tectonic lineaments on the Isle of Man (Quirk & Kimbell 1997; Quirk et al. 1999b) The results of the present study have mostly been has indicated that the Manx Group is compart- incorporated into the formal lithostratigraphy mentalized into a number of fault-bound slices. The proposed by Woodcock et al. (1999), but there are Windy Comer Fault of Woodcock & Barnes (1999) specific differences which are briefly discussed is not recognized. However, several larger faults are below. identified which, despite affecting the stratigraphy, have not been included in the map of Woodcock et Lithostratigraphy al. (1999). These include an east-west mineralized fault traversing Maughold Head (e.g. [SC 497 A number of the formations defined by Woodcock 915]), an east-northeast-west-southwest shear zone et al. (1999) have been subdivided here on the basis which cuts off the northern end of the Douglas of obvious lithofacies trends. However, the present Syncline (e.g. [SC 442 808]) and a number of authors are more conservative in extrapolating north-south and east-northeast-west-southwest lithostratigraphic units through areas of poor lineaments in the central-north area (Fig. 6). exposure and across large faults, so that few units are shown to continue uninterrupted across the island. For example the Injebreck Formation is Structural interpretations shown confined to the north (Fig. 4) whereas The lithostratigraphic units of the Manx Group Woodcock et al. (1999) continue it to the west coast appear generally to dip and young to the northwest. (the Eary Cushlin Unit in this paper). The Some stratigraphic repetition is likely to occur Maughold Formation of Woodcock et al. (1999) across large northwest dipping reverse faults (Quirk has been dropped due to perceived differences et al. 1999b), but its magnitude is uncertain without between the lithofacies at the southern and northern better biostratigraphic control. Seismic evidence ends of the island where it is best exposed. Instead, suggests that at least some of the northwest dip in the Maughold Formation in the south is named the Manx Group is due to post-Caledonian tilting in informally here the Fleshwick Unit, which is the footwall of the Eubonia-Lagman Faults (Quirk thought to overlie the Barrule Formation. Hence, it et al. 1999b). These offshore faults lie close to the is tentatively correlated with the lower part of the east coast of the Isle of Man, along pre-existing Injebreck Formation (Fig. 5). The northern outcrop Caledonian weaknesses (Quirk et al. 1999a). A Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 81

,~*" J & ~

Fig. 6. Simplified lithostratigraphic map of the central-north area showing principal geological boundaries and line of section used in Fig. 7. BKF, Ballakaighin Fault; BWL, Baldwin Lineament; GAL, Glen Auldyn Lineament; GDL, Glen Dhoo Lineament, GHL, Glen Helen Lineament; MHL, Maughold Head Lineament; MKL, Mount Karrin Lineament; NBL, North Barrule Lineament; SGL, Sulby Glen Lineament. total of 1-4 km of normal movement is recorded controlled by a number of observed or inferred following extensional events in the early northeast-southwest reverse or normal faults, Carboniferous, early Permian, late Jurassic and east-west dextral faults and north-south sinistral early Tertiary (Quirk & Kimbell 1997). faults (Fig. 6; Quirk et al. 1999b); On the basis of the geological boundaries • little of the succession is missing (Fig. 5); mapped in Fig. 6, four alternative structural cross- • early Arenig or possible Tremadoc acritarch sections have been constructed for the north of the dates from near the Peel Harbour Fault are not island (Fig. 7). These illustrate possible strati- representative of the age of the Glion Cam Unit graphic-structural scenarios ranging from a mini- (cf. Molyneux 1999), except possibly in Model 4 mum of fault repetition (Fig. 7a) to a maximum of (Fig. 7d). fault repetition (Fig. 7d). The direction of dip of the The total thicknesses quoted below are based on the main faults is generally inferred rather than succession north of Lag ny Keeilley (Figs 1 and 6) observed. Lettering is used to order the succession and do not take into account the possible lateral in each model (A being the oldest, N being the equivalence of the Lonan Formation (550 m thick) youngest) and to indicate proposed correlations, with the apparently much thicker Port Erin such as the Ny Garvain Formation equating with Formation (c. 2400 m thick) on the south coast the Glen Dhoo Unit in Model 2 (C 2 in Fig. 7b). (Table 2). In general, thicknesses may be Several assumptions have been made during their overestimated due to the difficulty in recognizing construction, in particular: contractional structures in certain intervals, • different lithostratigraphic units with similar particularly those that are predominantly muddy lithofacies associations may correlate (Table 2; such as the Barrule Formation. Figs 7b-d); • lateral facies variations are limited except in Model 1 some cases where lithofacies QI4, Qv, Mp, M v and MIC are present (Fig. 4); Figure 7a assumes that a continuous succession • the overall structure is not complex and is exists from the Lonan Formation to the Lady Port Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

Lady Port Fmtn "n~ Lady Port Fmtn Glion Cam Unit CregganIvlooar Fmtn S Glen RushenFmtn Q ~Glen RushenFmtn Glen Dhoo Unit ~ ~) ~/.'X~ Glen DhooUnit ~. MountKarrin Upp Inje~eckFmtn = ~" L X'.~ ~ Upp ,njebreckFmtn T Slieau ManaghUnit ~~Siieau ManaghUnit / = -.~ e ...... Lwr fnjebr~;kFmtn ~ (~ ~ LwrInjebmck Fmtn ~---Barrule Fmtn t 1' Creg AgneashFmtn m~ ..., ~-,~ Ng "~ NyGarvain Fmtn ~_=~ ~.

LonanFmtn LonanFmtn m

~,.t~ ¢3 ~.~ ~.~

~. 3~, ~ ~;" -.~ ~ < ,¢-. c ~-= ~ Lady Port Fmtn ~ .,t... ~.~ x.- L Lady PortFmtn ~'" 5" ~,~ ~r./~ ~3 .~ O i .X .~i.~CregganMooarFmt n ~ - CregganMooar Fmtn ~ ;~~ E - ~ ~Glen RushenFmtn Glen RushenFrntn

L\ Upp ,njebreckFmtn ~ ~ Upp InjebreckFmtn

~" / "X,~---- Lwr lnjebreckFmtn u~ = Lwr tnjebreckFmtn

~-, k~O ~ CregAgneash Fmtn h Fmtn

~'~ I--> 'F NyGarvainFmtn I ~~ NyGarvaJnFmtn

.~ ~ LonanFmtn m I I~ Lo.~ F~tn Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 83

Formation with minimal fault repetition. The Glen supported by Woodcock & Barnes (1999) and Auldyn, Glen Dhoo and Ballakaighin Faults throw Woodcock & Morris (1999). down to the west. This interpretation implies that the Manx Group is c. 12 750 m thick (or 10 650 m Mo&13 if the tectonically complex Lady Port Formation is excluded). Based on two similar acritarch dates Figure 7c shows a continuous succession from the from the Glen Dhoo Unit and from the lateral Lonan Formation to the Injebreck Formation. The correlative of the Ny Garvain Formation (the Glen Rushen Formation and Slieau Managh Unit Santon Formation) (Table 2; Molyneux 1999), the are equivalent to the Barrule Formation, due to model suggests that a probably unreasonable stratigraphic repetition caused by reverse offset on 6500 m of sediment was deposited during the early the Glen Dboo and Glen Auldyn Faults. The Slieau Arenig (c. 5 Ma). The youngest acritarch date is Managh debris flows may have been sourced from late Arenig from the Lady Port Formation at the top a normal syn-sedimentary precursor to the Glen of the succession (Table 2). Auldyn Fault. Later reverse offset would be due to Caledonian reactivation. The Creggan Mooar Formation and upper Injebreck Unit are thinner, Model 2 more distal correlatives of the lower Injebreck Unit, Figure 7b provides the best fit with the biostrati- with the Glion Cam and more proximal Glen Dhoo graphy of Molyneux (1999), with the geophysical Units interpreted to overlie these. However, interpretation of Quirk et al. (1999b) and with biostratigraphic evidence (Molyneux 1999) does geologically reasonable sedimentation rates. It not support this interpretation, as the Glen Dhoo assumes that a continuous succession exists from Unit is older than the Glen Rushen Formation. The the Lonan Formation to the upper unit of the estimated thickness of the Manx Group in this Injebreck Formation, but that the early Arenig Glen model is 7750 m (or 5650 m excluding the Lady Dhoo Unit is equivalent to the Ny Garvain Port Formation). However, to conform with the Formation by virtue of a large normal fault (the biostratigraphy, an alternative is also suggested on Mount Karrin Lineament) that throws down to the Fig. 7c where the younger Glen Rushen Formation east. The Slieau Managh Unit may represent a set is juxtaposed against the Glen Dhoo Unit by normal of debris flows sourced from the fault. This unit faulting along the Glen Helen Lineament. In this may either be confined to the area between the case, the Glen Rushen Formation is not the lateral Mount Karrin and Glen Auldyn Faults or it may equivalent of the Barrule Formation. This correlate with the Barrule Formation, implying that alternative interpretation adds another 2000 m to the lower and upper units of the Injebreck the estimated thickness of the Manx Group in Formation are lateral equivalents (Fig. 7b). Model 3. Alternatively, the normal fault may be entirely post- depositional. The mid-Arenig Glen Rushen For- Mo~14 mation is a thinner correlative of the Barrule Formation and the Creggan Mooar Formation is a Figure 7d shows the most stratigraphic repetition thinner, more distal equivalent of the Injebreck due to reverse movement on the Glen Auldyn, Glen Formation, a correlation supported by the presence Dhoo and Ballakaighin Faults, and an additional of the distinctive lithofacies Mic at a coastal expo- fault, known as the North Barrule Lineament, sure near Ramsey (Fig. 4). The Glen Auldyn, Glen which cuts out part of the Barrule Formation. The Dhoo and Ballakaighin Faults throw down to the complete succession is represented by the Barrule- west. South of the section, the Glen Helen Slieau Managh-Glen Rushen lateral correlatives at Lineament throws down to the east. It either the base, followed by the Lonan-Injebreck- represents a west dipping reverse fault reactivating Creggan Mooar correlatives, in turn overlain by the the Glen Dhoo Fault or, more likely, is an extension Ny Garvain-Glen Dhoo-Glion Cam correlatives of the east dipping Mount Karrin normal fault, and, finally, by the Creg Agneash Formation. The implying that the Glen Dhoo Fault is also normal. lateral correlatives tend to thin and fine westwards. In Model 2, the Manx Group reaches a maximum The structural configuration shown on Fig. 7d is thickness of 9250 m if the Slieau Managh Unit does reminiscent of northwest dipping structures imaged not correlate with the Barrule Formation (or on offshore seismic sections along-strike from the 7150 m if the Lady Port Formation is excluded). If Glen Dhoo, Glen Auldyn and North Barrule this does correlate, the Manx Group is c. 7500 m Lineaments (Quirk et al. 1999b). None the less, as thick (or 5400 m if the Lady Port Formation is in the previous model, this stratigraphic order is not excluded). Possible syn-sedimentary fault activity supported by acritarch dates. These indicate that the during deposition of the Slieau Managh Unit is Glen Dhoo Unit is older than the underlying Glen consistent with a model of Arenig tectonism Rushen Formation. An alternative interpretation, Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

84 D.G. QUIRK & D. J. BURNETT also shown on Fig. 7d, overcomes this problem by Therefore, a change from low to high relative sea making the Glen Helen Lineament a normal fault level is likely to be reflected in the deep-marine juxtaposing the younger Glen Rushen Formation environment as a change from clean sandstones against the Glen Dhoo Unit. If, instead, the Glen within a turbidite fan (lowstand systems tract) to Rushen data are ignored, then the poorly mudstones (transgressive systems tract) to mixed constrained biostratigraphic data from near Peel sandstone-mudstone turbidites with possible debris may indicate that the base of the Glion Cam Unit is flows (highstand systems tract). This corresponds pre-Arenig in age (Molyneux 1999). This with the change seen from the base of the Santon- interpretation implies that the Manx Group may be Ny Garvain Formations (early Arenig) to the top of as little as 4500 m thick. the Injebreck Formation (?mid-Arenig) (Table 2). The Creg Agneash Formation, above the Ny Summary Garvain Formation, displays a fining-upwards signature (Fig. 4) typical of the lower part of the Although Model 2 (Fig. 7b) is favoured here, all transgressive systems tract. The Barrule Formation four models have aspects to recommend them. is thought to represent the upper part of the Resulting estimates of the thickness of the Manx transgressive systems tract. There are insufficient Group vary from 4500 to 10650m, with an biostratigraphic data to be sure whether there is average of 7000 m. These estimates do not include another, younger relative sea-level cycle recorded the highly faulted Lady Port Formation (c. 2100 m in the Manx Group, from the base of the Glen Dhoo thick) at the top of the succession, nor the Port Erin Unit to the top of the Glion Cam Unit (Table 2). Formation (c. 2400 m thick) in the south of the Consequently, this part of the succession could be a island, with which the upper part of the Lonan partial or complete repetition of the early-?mid Formation (550 m thick) may correlate (Table 2; Arenig cycle (Fig. 7b-d). Fig. 5). Woodcock & Barnes (1999) record a divergence between palaeoflows recorded from flute casts Sequence stratigraphic interpretation and (west directed) at the base of turbidite beds basin model (typically lithofacies QH, Qv, WH and MI) and ripple crests (north directed) at the top of other beds No matter what structural interpretation is put on (typically lithofacies S L, S I and SH). They propose the Manx Group, important lithological variations a model for the deposition of formations exposed are apparent within the succession (Fig. 5). On the on the southern and eastern coast of the Isle of Man east side of the island, the 550 m thick lower unit of involving 2-10 km wide turbidite lobes along an the Ny Garvain Formation comprises > 80% thin- actively faulted margin. The high concentration thick-bedded quartz arenites (lithofacies S I and SH), parts of the turbidity currents are shown running whereas the 1200m thick Barrule Formation west, subparallel to the continental margin, due to consists almost entirely of dark grey mudstone deflection at east-northeast-west-southwest (lithofacies My). A similar contrast is seen between trending fault scarps, whereas the higher, low other formations, such as the Glen Dhoo Unit and concentration parts of the currents flow northwards, the Glen Rushen Formation. Alternations on this undeflected by the scarps, towards the deeper basin sort of scale in deep-marine sediments are typically (Woodcock & Barnes 1999). The present authors caused by second- (10-80Ma) or third-order regard this model as unlikely for the following (1-10 Ma) variations in the relative height of sea re ason s: level, the different units within each cycle or • evidence for lobe geometries is minimal, with sequence being assigned to systems tracts (Vail et sand-dominated intervals easily correlated over al. 1977). Sand bypasses the shelf during periods of distances of at least 10 kin; falling and low relative sea level (lowstands), when • bimodal palaeocurrent data are not usual in it is carried down submarine canyons into the faulted areas where, typically, the wide variety of deeper parts of the basin by turbidity cun'ents, slopes produces a polymodal flow distribution depositing thick submarine fans in an otherwise (e.g. Boote & Gustav 1987; Prosser 1993); distal environment (Galloway 1998). A subsequent • turbidites usually flow down the continental rise in relative sea level causes transgression, when slope, not along it, whether there are faults sands are trapped on the shelf and muds are mostly present or not (e.g. Galloway 1998); deposited further out in the basin. Thereafter, a • evidence for thickness variations or other syn- subsequent highstand in relative sea level is sedimentary features indicative of active faults in associated with deltaic progradation on the shelf the Manx Group is circumstantial. which may lead to oversteepening and slumping at the front of the delta causing mixed sand-mud An alternative model is developed here building on turbidites to flow basinwards. geophysical interpretations by Kimbell & Quirk Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 85

100 km 1 .~

i / i ? ...'k~3v~ IAPETUS OCEAN .. -" "" ...,-~kiddaw"" " , ,,..Lz

--" " ~ ~.~ Group ~

"" ~ t~ EASTERN .~ " .,. --" o* ,~...... ,. AVALONIA '~ ,i. X ., .." ".~I{ Group ......

MANANNAN BASIN / • o /

Ribband / Group ,~ " ~

?Welsh Basin ? ~o • ? • • " ?

° " " Armorica

...... 1 Fig. 8. Tentative palaeogeographic reconstruction of the northwest margin of Eastern Avalonia during the early Arenig showing conceptual current patterns. The relative positions of areas indicated, particularly the Welsh Basin, are speculative and differing amounts of anticlockwise rotation during the late Caledonian have not been taken into account (see Piper & Crowley 1999; Piper et al. 1999). See text for discussion.

(1999) and Quirk et al. (1999b). This work suggests beds which are interbedded with thin mudstones, that the Manx Group was deposited on the west indicating that currents strong enough to move sand dipping margin of an embayment on the northwest were only intermittently active. The favoured edge of Eastern Avalonia, termed the Manannan explanation is one of episodic flow due to deflec- Basin. The thick sands of the lower part of the tion along-slope of the low-energy part of turbidity Santon Formation and the lower unit of the Ny currents by north moving bottom waters (Fig. 8). Garvain Formation, its lateral equivalent, are What is not recorded in palaeocurrent data is the therefore thought to represent part of an early swing from downslope, westwards flow to along- Arenig lowstand fan, with the basal, high energy slope, northwards flow as energy decreased, pre- parts of turbidity currents flowing westwards down sumably because this was when massive and planar the slope of the margin (Fig. 8). However, ripples laminated sands were deposited (Bouma units Tab, were produced by low-energy tractional currents e.g. in lithofacies Six, QH and Qv; Table 1). flowing to the north, apparently perpendicular to A speculative lowstand flow pattern for contour the dip of the slope, i.e. by contour currents. These currents along the northwest margin of Eastern contour currents either reworked sands initially Avalonia (Fig. 8) is one of the anticlockwise circu- deposited as turbidites or deflected the flow of lation system constrained only by the ripple cross- turbidity currents as they slowed down. Reworking lamination palaeocurrent data from the Manx may help to explain why the lower unit of the Ny Group (Woodcock & Barnes 1999). As Eastern Garvain Formation consists of a thick package of Avalonia lay in the southern hemisphere at this time unusually clean sandstones (lithofacies SH; Fig. 4) (e.g. Noblet & Lefort 1990), this direction of and also why flute casts and grading are rarely circulation may have been driven directly by the observed in lithofacies S L and S I in the Lonan Coriolis force. However, the circulation probably Formation and the upper unit of the Ny Garvain formed part of a larger, more complex flow pattern Formation (Table 1). However, the northwards as is typical in present-day oceans (Stow et al. directed ripples are usually preserved in sandstone 1996). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

86 D.G. QUIRK & D. J. BURNETT

A modern analogue for the Manannan Basin, more quartzose arenites in the Skiddaw Group were albeit in the northern hemisphere, might be the derived from a Gondwanan shelf where widespread Faeroe-Shetland channel where bottom waters shallow water and continental sandstones, known between 500 and 1700 m below sea level flow collectively as the Gr~s Armoricain, were deposited southwest from the Norwegian Basin towards the (Noblet & Lefort 1990). A similar link has been Atlantic Ocean at rates of up to 0.5 m s-1 (Stoker et suggested by Woodcock & Barnes (1999) for the al. 1998; Masson et al. 1997). The Faeroe-Shetland Manx Group, implying that a connection existed Channel, unlike the Manannan Basin, is starved of between the northwest margin of Eastem Avalonia coarse-grained turbiditic input. Nevertheless, a thin and the coastal plain deposits of Arenig age on the sandy contourite sheet is developed here at a water Armorican Massif (Fig. 8). depth of 700-850 m over an area of 60 x 10 km 2, The relationship of the Ribband, Manx and elongate-parallel to the shelf edge. Skiddaw Groups with the Welsh Basin (Fig. 8) is There are lithological similarities between the uncertain (Kokelaar 1988). It is, however, note- Ribband Group and mud-prone (distal) lithofacies worthy that Wales was the site of intracratonic in the Manx Group (McConnell et al. 1999; Morris, rifting in the late Tremadoc, then uplift followed by pers. comm.; Brtick, pers. comm.), although this is transgression in the early-mid-Arenig (Kokelaar the subject of ongoing research. In contrast to the 1988; Woodcock 1990), consistent with the struc- Manx and Ribband Groups, a good biostratigraphic tural and stratigraphic evidence at the edge of the framework is available for the Skiddaw Group, craton in the deeper-water setting of the Isle of although lithostratigraphic comparisons are not Man. straightforward. Based on acritarch data (Molyneux 1999), the quartzose Santon and Ny Garvain Conclusions Formations in the Manx Group seem to correlate with the mud-dominated Hope Beck Formation in The Manx Group dips generally northwest with the the Skiddaw Group. The sandstones in the Watch oldest sediments (quartzose sandstones) in the Hill Formation and Loweswater Formation below southeast and the youngest sediments (mud domi- and above the Hope Beck Formation are wackes nated) in the central and northwest parts of the rather than quartz arenites (Cooper et al. 1995). island. The gross structure is controlled by north- Even the slumps and debris flows in the upper east-southwest reverse faults and east-west dextral Arenig Kirk Stile Formation are different to the faults. With only limited biostratigraphic control, pebbly mudstones of the Slieau Managh Unit and several alternative lithostratigraphic correlations Lady Port Formation in that they represent larger are possible, depending on the amount of assumed scale events (olistostromes) and contain extra- fault repetition. The Manx Group is > 4500 m formational clasts (Webb & Cooper 1988; Cooper, thick, with distal units in the west juxtaposed by pers. comm.). Cooper et al. (1995) suggest a reverse faults against more proximal units in the passive margin setting for the Skiddaw Group. east. A large normal fault may define the east- Similar to most of southeast Ireland (Max et al. southeast edge of a sand-prone interval (the Glen 1990), subduction-related accretionary prisms and Dhoo Unit) in the central-north area. Whether this island arcs are not evident until the Llanvirn in the fault was active during deposition is as yet Lake District, probably when Eastern Avalonia unproven. broke from Gondwana. The lithofacies observed in the Manx Group are Kimbell & Quirk (1999) propose that the typical of a passive continental margin. A tentative Manannan Basin was initiated by rifting, probably model proposes that the succession represents the during the late Tremadoc. Felsitic igneous sheets inverted, eastern side of a basin stretching at least occur throughout the Manx Group (Lamplugh as far as Leinster, named the Manannan Basin, 1903). Many of these appear to have been intruded which formed an embayment on the northwest edge when the sediment was still soft, perhaps indicating of Eastern Avalonia. In the early Arenig, a large- that a limited amount of extension was still occur- scale turbidite fan developed in the Isle of Man area ring during deposition (Quirk & Kimbell 1997). during a lowstand in relative sea level, when the However, evidence for syn-sedimentary normal Santon and Ny Garvain Formations were deposited, faults is highly tentative (see Fig. 8 and previous and probably also the correlative Glen Dhoo Unit. discussion of Model 2, Fig. 7b). At present, the Quartzose sand bypassed the shelf and was carried authors regard the eastern margin of the Manannan by turbidity currents downslope to the west. The Basin as essentially passive and undergoing post- fan may also have been affected by north flowing rift thermal subsidence during most of the Arenig. bottom currents. The onset of a rise in relative sea This is contrary to the ideas of Quirk & Kimbell level, probably towards the end of the early Arenig, (1997). was associated with deposition of the fining- Cooper et al. (1995) propose that some of the upwards Creg Agneash Formation as the fan Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2015

LITHOFACIES OF LOWER PALAEOZOIC DEEP-MARINE SEDIMENTS IN THE ISLE OF MAN 87

became inactive. A blanket of fine-grained mud given by Ian Pope, Kate Winder, Jon Wells, Simon was deposited during the mid-Arenig (the Glen Deadman, Sean Mulligan, Roger Sims, David Rushen Formation and its probable lateral equiv- Kelly, Richard Young, Kathleen Quirk, Lisa Hill and alent, the Barrule Formation) overlain by thin- Graeme Foster. Many of the ideas in this paper have come from lively discussions with Nigel Woodcock, Rob bedded turbidites and debris flows corresponding to Barnes, John Morris, Bill Fitches, Greg Power, a relative sea-level highstand (the Creggan Mooar Padhraig Kennan, Geoff Kimbell, Tony Cooper, Peter and Injebreck Formations). Brtick, Brian McConnell, Paddy Orr, Mike Howe, Trevor Further biostratigraphic work is now required to Ford, Fred Radcliffe and Frank Cowin. The reviews of constrain the lithostratigraphic interpretations, earlier manuscripts by Dick Waters, Maxine Akhurst particularly to test whether the Glen Rushen- and Nigel Woodcock proved helpful in improving the Creggan Mooar Formations are distal equivalents paper, although they would not endorse some parts of the Barrule-Injebreck Formations, and to assess of the final version. The work was funded by NERC whether the Port Erin Formation is the correlative research grant GR9/01834, the Isle of Man Government and Oxford Brookes University. We thank Burlington of the Lonan or Creg Agneash Formations. Resources, UK for sponsoring the costs of colour The authors are grateful for essential technical support reproduction.

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