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Palaeoenvironmental Interpretation of The PALAEOENVIRONMENTAL INTERPRETATION OF THE TRIASSIC SANDSTONES OF SCRABO, COUNTY DOWN, NORTHERN IRELAND: ICHNOLOGICAL AND SEDIMENTOLOGICAL STUDIES INDICATING A MIXED FLUVIATILE-AEOLIAN SUCCESSION JAMES O. BUCKMAN, PHILIPS. DOUGHTY,MICHAEL J. BENTON and ANDREW J. JERAM (Received 19 May 1997) Abstract The Sherwood Sandstone Group at Scrabo, Co. Down, NorthernIreland, has been variously interpretedas aeolian or fluviatile. The sedimentological and ichnological data show that these sandstone-dominatedfacies were deposited within a mixed fluviatile-aeolian regime, in which fluviatile processes were responsible for the majorityof the preservedsedimentary sequence. The sequence contains a moderately diverse non-marine ichnofauna comprising the invertebratetrace fossils Biformites, Cruziana/Rusophycus(Isopodichnus), Herpystezoum(Unisulcus), Planolites, 'small arthropodtrackways', 'large arthropodtrackways' (cf. in part Paleohelcura), and the vertebrate trace fossil Chirotherium? (as well as a number of other, presently unnamed, vertebrate footprints). The recorded ichnofauna greatly increases that previously known from the Irish Triassic, comparesfavourably with that from the rest of Europe, and representsthe only known Irish locality for reptiliantrackways. Introduction north-east, are exposed at Scrabo Hill (Charlesworth 1963). Several Palaeogene The Sherwood Sandstone Group (Triassic) dolerite dykes and sills are exposed at the in north-eastIreland comprises part of a thick southern end of the hill, and these form a sequence of non-marine sediments, with a resistant cap that has shielded the sandstones maximum thickness of 1850m (Parnell et al. against removal by ice during the last 1992), which are rarely exposed and known glaciations. mostly from borehole data. At Scrabo (Fig. 1) Various environmental interpretationshave the Sherwood SandstoneGroup occurs within a been made of the Scrabo succession. The deep trough of Carboniferousto Triassic age succession has been interpretedas representing (Parnell et al. 1992), which is developed on a aeolian conditions (Charlesworth1953; 1963), structuralhigh (the Longford-Down Massif). with the caveat that some parts might have had Approximately 37m of strata, which dip at 50 a waterlain origin (Charlesworth 1953). Irish Journal of Earth Sciences 16 (1997/8), 85-102. 85 This content downloaded from 137.222.248.202 on Sat, 23 Jan 2016 22:40:54 UTC All use subject to JSTOR Terms and Conditions 86 Buckman- Ir. J. Earth Sci. (1997/8) SScrabo CountryPark Quarry STUDY sAREA ScraboTower Road WPermo-Triassic outcrop GOLF 11 CLUBCLU IRISH SEA BLSNEWTOWNARDS BELFAST Fig. 1-Location maps: A. map of Ireland with approximatelocation of the study area indicated; B. map of Northern Ireland, showing the location of Scrabo, south-west of the town of Newtownards, and outcrop of Permo-Triassic sediments;C. detailed map, showing the positions of localities I and 2 respectively. Opinions of other authors include: a shallow trace fossils are commonly overlooked, or waterlain origin of deposition, although not mentionedonly in a cursoryfashion. We believe elaboratingon whetherthis is marine,fluviatile that it is importantto documentand considerthe or lacustrine (Wilson 1972); a river-channel taxonomicand ethologicalposition of ichnotaxa environment, with no evidence for aeolian from less ideal situations,in order to maximise deposition (Parnell et al. 1992); and a shallow- theirpotential in environmentalinterpretation. water depositional environment of variable This paper documents and describes the energy, with an additional aeolian influence ichnofauna from the succession at Scrabo, increasing towards the top of the Sherwood places it within its sedimentologicalsetting and Sandstone Group(Smith et al. 1991). discusses the environmentalimplications that Trace fossils recorded in this investigation lead to the present interpretation. either occur in small quantities or are poorly preserved. However, they form an important Systematic ichnology component of the environmentalinterpretation. Although an increasing number of well Previously recorded trace fossils from the preserved non-marinetrace fossil assemblages Scrabo succession include vertebratefootprints have been described (Bromley and Asgaard (Young 1883; Ashley 1946; Swinton 1960a; 1979; Pollard 1981; Pickerill 1992), such is not 1960b; Sarjeant 1974), worm-burrows(Ashley usual for non-marine environments, and their 1946; Charlesworth 1953; 1963) and shrimp- This content downloaded from 137.222.248.202 on Sat, 23 Jan 2016 22:40:54 UTC All use subject to JSTOR Terms and Conditions Ir. J. Earth Sci. (1997/8) - Buckman 87 burrows (Smith et al. 1991). New collections Material and occurrence. K7997, K12527, indicate a moderatelydiverse ichnofauna,with K13553, K13554, K13555, with a total of Biformites (the shrimp-burrowsof Smith et al. several dozen specimens, from locality 1 (see 1991), Chirotherium?, Cruziana/Rusophycus Fig. 1). (Isopodichnus), Herpystezoum (Unisulcus), Planolites, 'small arthropodtrackways', 'large Description. Horizontal endogenic full reliefs arthropod trackways' (cf. in part and convex hyporeliefs (within, and at the base Paleohelcura), as well as other unassigned of, sandstones),which are straightor curved in vertebrate footprints. All collected materials plan view, commonly cross-cut each other, run are deposited within the geological collections parallel or subparalleland occasionally appear of the Ulster Museum, Belfast (BELUM). The to originate from the same locus. Burrows are invertebrateichnogenera are typically closely 2-10mm wide by up to 75mm long and associated with intercalated fluviatile circular or ovoid in cross-section. The larger mudstone-sandstone horizons, which display burrows are thickly lined (0.5-3mm thick), features of subaerial exposure, whereas having a smooth inner surface with a pelleted vertebratetrackways occur in association with outer ornament. Smaller burrows are both fluviatile and aeolian depositional units. apparently unlined and appear pellet stuffed. Ethologically, the Scrabo ichnotaxa represent The pellets, circular or ovoid in shape and an association of fodinichnia (?domichnia), 1-3mm long, occur in a random, uniserial, or repichnia and cubichnia, in which fodinichnia biserial manner and are occasionally observed (?domichnia)are the most abundantin terms of to be composite in nature. total numberof individuals,although repichnia are more diverse in terms of ichnotaxapresent. Discussion. Although previously described as 'shrimp-burrows'(Smith et al. 1991), Triassic Ichnogenus Biformites Linck 1949 material of this style is generally referred to Biformites ichnosp. Biformites (J.E. Pollard, pers. comm.). The Figs 2 and 3b Ophiomorpha of Kennedy and MacDougall (1969), from the Weald of southern England, Fig. 2-Large block (BELUM K12527) containing a multitudeof examples of Biformites,observed in convex hyporelief; specimens occur singularly and as groups originating from a common locus, and are either straight or variably curved (scale bar = 20mm). This content downloaded from 137.222.248.202 on Sat, 23 Jan 2016 22:40:54 UTC All use subject to JSTOR Terms and Conditions 88 Buckman- Ir. J. Earth Sci. (1997/8) Fig. 4-Photograph of the vertebrate footprint Chirotherium?, indicating the presence of reptilian life within the Scrabo ecosystem; the tuberculatedappearance of the print is of a secondarynature and does not represent hide markings(scale bar = 10mm). Ichnogenus ChirotheriumKaup 1835 Chirotherium?ichnosp. Fig. 4 Material and occurrence. K12282, two 3-A. Planolites (BELUM K13556), Fig. beverleyensis consecutive from 1. hyporelief (scale bar = 10mm); B. Biformites and P. prints, locality beverleyensis (BELUM K7997), hyporelief (scale bar = 10mm). Note the composite nature of the pellets of the Description. Comprising two incomplete centralBiformites. digitgrade prints (K12282), which occur one behindthe other,overlapping. The frontand rear prints comprise at least four and three digits respectively,with the prints 55mm wide and no may be conspecific with the Irishspecimens, but longer than 39mm. The digits appearto bear the according to Pollard et al. (1993) such should impressions of claws, and the surface of each not be referredto Ophiomorpha.Additionally, printvaries from smoothto tuberculated. both Edaphichnium and Oligichnos, from Eocene palaeosols (Bown and Kraus 1983) and Discussion. Previously identified as Paleocene fluviatile-lacustrine environments Chirotherium lomasi (Swinton 1960b) but, (Melchior and Erickson 1979), are similar in given the lack of a 'thumb' impression and the terms of burrow orientationand their pelleted minimalnature of the trackway,some doubthas morphology to the Scrabo material, although been expressedas to this assignment(Delair and they lack a burrowlining and differ in the nature Sarjeant 1985, 152). This material is therefore of their pellets. Pelleted burrow forms, from a only referredto Chirotherium?ichnosp. range of different environments,would benefit from a comprehensivemonographic treatment, Ichnogenus Cruziana d'Orbigny 1842 althoughthis is beyond the scope of the present Cruzianaproblematica (Schindewolf 1921) work. Fig. 5 This content downloaded from 137.222.248.202 on Sat, 23 Jan 2016 22:40:54 UTC All use subject to JSTOR Terms and Conditions Ir. J. Earth Sci. (1997/8) - Buckman 89 valid taxonomic criterion, the Irish material is referred to Cruziana following Bromley and Asgaard (1979). For detailed discussion on the taxonomy and environmental
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