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Response of the Marine Infauna to Triassic–Jurassic Environmental Change: Ichnological Data from Southern England ⁎ Colin G

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Response of the Marine Infauna to Triassic–Jurassic Environmental Change: Ichnological Data from Southern England ⁎ Colin G Palaeogeography, Palaeoclimatology, Palaeoecology 244 (2007) 223–241 www.elsevier.com/locate/palaeo Response of the marine infauna to Triassic–Jurassic environmental change: Ichnological data from southern England ⁎ Colin G. Barras a,b, Richard J. Twitchett c, a Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK b Department of Earth Sciences, School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK c School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK Received 15 February 2006; accepted 20 June 2006 Abstract The trace fossil record through the Triassic–Jurassic boundary interval is examined at three sites in central and southern England (St. Audrie's Bay, Somerset; Pinhay Bay, Devon; and Long Itchington, Warwickshire). The lower ‘Pre-Planorbis Beds’ of the Blue Lias Formation record low ichnotaxonomic diversity, low bioturbation intensity, small burrow diameters, and an absence of deep tier bioturbation. The stepwise reappearance of ichnotaxa following this interval is similar at the three sites, suggesting similar rates of recovery in the benthic marine ecosystem and highlighting the potential contribution of these trace fossils to stratigraphic correlation. Mass extinction in the Late Triassic is increasingly linked to the emplacement of the Central Atlantic Magmatic Province (CAMP). Timing of the onset of CAMP volcanism in the UK is currently imperfectly known, but potentially occurred within either the Westbury Formation or the Lilstock Formation. If the onset of CAMP was within the Lilstock Formation, the modestly diverse trace fossil assemblage of the Langport Member of the Lilstock Formation suggests that emplacement of CAMP had little lasting effect on the marine benthos in central and southern England. The decline in benthic activity recorded in the ‘Pre-Planorbis Beds’ appears to be unrelated to the onset of CAMP and any associated environmental change. It is more likely related to an episode of marine anoxia. An episode of marine anoxia also provides a suitable causal mechanism for the reduction of infaunal tiering. © 2006 Elsevier B.V. All rights reserved. Keywords: Trace fossil; Extinction; Ichnofauna; Anoxia; Recovery 1. Introduction marine diversity loss, or the rate of generic extinction, it was the fourth most severe event of the Phanerozoic, but It is generally accepted that there was a global mass in terms of ecological impact on the biosphere it is extinction event at, or near, the end of the Triassic (e.g. ranked third (McGhee et al., 2004). Despite this Newell, 1967; Benton, 1995; Hallam and Wignall, importance, the Late Triassic event remains poorly un- 1997), although a few authors have raised some doubts derstood, and has only recently become the focus of concerning the severity of the event (e.g. Cuny, 1995; sustained research. It now seems likely that the extinc- Hallam, 2002). In terms of the apparent magnitude of tion was due, at least in part, to the effects of formation of the Central Atlantic Magmatic Province (CAMP). ⁎ Corresponding author. Earth, Ocean and Environmental Sciences, CAMP lavas have been isotopically dated to between University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK. 199 and 201 Ma (Marzoli et al., 1999; Hames et al., E-mail address: [email protected] (R.J. Twitchett). 2000; Knight et al., 2004). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.06.040 224 C.G. Barras, R.J. Twitchett / Palaeogeography, Palaeoclimatology, Palaeoecology 244 (2007) 223–241 There are a number of potential kill mechanisms p. 26), and were presumably similarly dominant relating to the formation of large igneous provinces (e.g., throughout the Phanerozoic. The trace fossils that they Coffin and Eldholm, 1994; Wignall, 2001a). The large- produce do not suffer from taphonomic effects such as scale release of CO2 and associated climate change is transport, reworking or dissolution that plague the body frequently cited to link CAMP and mass extinction. High fossil record, especially during mass extinction intervals concentrations of atmospheric CO2 would lead to the when the quality of the shelly fossil record is notably under-saturation of carbonates within the world's oceans, reduced (c.f. Twitchett, 2001), and they also provide greatly affecting the large numbers of calcite and information on environmental change. aragonite secreting organisms (Hautmann, 2004). Evi- dence from the study of goethite (Yapp and Poths, 1996) 1.1. Study area and the stomatal diversity of fossil leaves (McElwain et al., 1999; Retallack, 2001; Beerling, 2002)certainly Late Triassic and Early Jurassic strata are well suggests a significant rise in CO2 levels across the exposed in southern England, especially along the coast, Triassic–Jurassic (T–J) boundary. Negative δ13C excur- have been studied for more than 200 years, and are of sions across the T–J boundary have been reported in the global significance. The study area includes the Global Queen Charlotte Islands, British Columbia (Ward et al., Stratotype Section and Point (GSSP) for the base of the 2001), Astartekløft in Jameson Land, East Greenland, and Sinemurian Stage (Bloos and Page, 2002) and part of the St. Audrie's Bay in Somerset, England (Hesselbo et al., area falls within a recently designated World Heritage 2002), further suggesting some perturbation of the global Site. Here, the Triassic–Jurassic succession was exam- carbon cycle. Analysis of palaeosols shows little evidence ined around St. Audrie's Bay, Somerset and Pinhay Bay, of any such perturbation (Tanner et al., 2001; Tanner, Devon. At St. Audrie's Bay the succession is exten- 2002), although the temporal resolution of this study may sively faulted, and was studied from outcrops between have been inadequate to record any transient rise in CO2 Blue Ben (ST 110440) and Helwell Bay (ST 083433). (Beerling, 2002). The succession at Pinhay Bay is more condensed, and a Examining the biotic patterns recorded through the complete succession was exposed to the immediate east Triassic–Jurassic interval may help to assess the validity of Pinhay Bay (SY 320908). Supplementary data were of these competing theories. In the present study, added from a third study site: the Southam Cement changes in the benthic marine ecosystem are assessed Works at Long Itchington, Warwickshire (Fig. 1). by examination of the Triassic–Jurassic trace fossil record of southern England. Trace fossils provide the 1.2. Stratigraphy and depositional environments only source of useful information on the responses of the soft-bodied benthos to past extinction events (Twitchett In southern England, the sediments of the Penarth and Barras, 2004). Unmineralised taxa are dominant in Group give way to the Lias Group across the T–J modern marine ecosystems (Allison and Briggs, 1991, boundary (Fig. 2). Although the limestones, shales and Fig. 1. Location of the studied sections in England. C.G. Barras, R.J. Twitchett / Palaeogeography, Palaeoclimatology, Palaeoecology 244 (2007) 223–241 225 Fig. 2. Lithostratigraphic summary of the Triassic–Jurassic succession in the study area. The Triassic–Jurassic boundary interval lies between the last occurrence of conodonts and the base of the Psiloceras planorbis Chronozone (cf. Hounslow et al., 2004). calcareous mudstones of the Penarth Group are of port Member boundary (Hesselbo et al., 2004). The top marine origin, ammonites and many other typically of the Langport Member at Pinhay Bay is conglomeratic, stenohaline organisms are very rare (Donovan et al., following local (and probably submarine) erosion 1989). This has led to the suggestion that the palaeoen- (Wignall, 2001b; Hesselbo et al., 2004). Shale-filled vironmental conditions were atypical, possibly reflect- fissures and hollows reported from the upper surface of ing abnormal salinity in a restricted lagoonal setting the conglomerate are interpreted as evidence of disso- (Hallam and El Shaarawy, 1982). However, Hesselbo lution following brief subaerial exposure (Wignall, et al. (2004) noted that debris from stenohaline echi- 2001b). However, Hesselbo et al. (2004) found no evi- noids is often relatively common (e.g. in the Lilstock dence for subaerial exposure at this time, and concluded Formation) and so, even though extensive transport of that the shales are entirely marine. these hardy bioclasts cannot be ruled out, they imply a The overlying Blue Lias Formation of the Lias Group substrate rather than salinity control on faunal diversity. typically comprises a rhythmical alternation of laminated The Westbury Formation of the Penarth Group shale, marl, and limestone and is well exposed at all sites. consists predominantly of dark grey mudstones with The fine laminations coupled with the presence of pyrite rare limestone horizons, deposited in a quiet offshore framboids in the basal Blue Lias Formation at Pinhay setting, with evidence of shallowing towards the top Bay have been cited as evidence of anaerobic deposition (Hesselbo et al., 2004). These are overlain by the in water depths of a few tens of metres or greater Lilstock Formation, which is commonly divided into two (Wignall, 2001b). Although each laminated shale pack- members: the Cotham Member below and the Langport age within the Blue Lias Formation represents anoxic Member above. The Cotham Member comprises a deposition, there is an upward increase in the Th/U ratio in sequence
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