Discussion on sea-level change and facies development across potential Triassic-Jurassic boundary horizons, SW Britain/Stephen Hesselbo, Stuart Robinson and Finn Surlyk reply

Journal of the Geological Society, Nov 2004 by Hallam, Tony, Wignall, Paul, Hesselbo, Stephen, Robinson, Stuart, Surlyk, Finn

Journal. Vol. 161, 2004, pp. 365-379

Tony Hallam & Paul Wignall write: we welcome the paper by Hesselbo et al. (2004) for its thorough description and careful evaluation of the environmental significance of an important group of system boundary strata, but beg to take issue on a number of key points.

(1) Most importantly, while we agree with the evidence of a regressive phase within the Cotham Member, we maintain in marked contrast to their opinion that there was also a sharp regression followed by a rapid transgression at the junction of the Langport Member (of the Lilstock Formation) and Blue Lias. It is pertinent before discussing the evidence in SW England further to take into account the broader European context (Hallam & Wignall 1999, with many relevant references cited therein). The evidence is especially clear in Germany where there was extensive shallowing in the latest Rhaetian marked by progradation of sandstone over shales. In northern Frankonia (Bavaria) fluvial sandstones infill channels incised into late Rhaetian marine strata and are overlain by marine Hettangian. The sea-level rise in the earliest Hettangian (planorbis Zone) was evidently rapid, with the limit of marginal marine sandstones in the eastern part of southern Germany being pushed back at the expense of fully marine shales to the maximum extent achieved during the whole Hettangian Stage.

Both in the north and south of Europe a similar pattern of successive sea-level fall and rise can be inferred. A clear end-Triassic regressive pulse can be recognized in the Danish Basin, while both in southern Sweden and NW Poland the upper Rhaetian is missing and there is an unconformity at the base of the Jurassic. In the Northern Calcaerous Alps of Austria, widespread emergence at the end of the Triassic is recognized, with the creation of karst surfaces on emergent reef complexes, while in the few areas of more continuous sedimentation in basinal settings the base of the Jurassic is marked by eroded limestone clasts from emergent areas, or by an exceptional red mudstone horizon interpreted as marginal marine, in the midst of blue-grey fully marine deposits.

In England north of the southern Midlands the marine Hettangian Blue Lias Formation rests with a hiatus on an eroded surface on the Rhaetian Penarth Group, with the upper Langport Member of the Lilstock Formation missing. Collectively the evidence is about as convincing as can be expected from the stratigraphie record of a notable regrcssive-lransgressive couplet at the system boundary.

Naturally, since the margins of the Bristol Channel were an area of relative subsidence at the critical time, with the most continuous marine sections known in northern Europe, any effect of sea-level fall is likely to have been minimized. Nevertheless such evidence is present. As regards the foreshore of the south Devon coast, Hallam (1988) argued for emergence at the end of deposition of the Langport Member (White Lias) on the basis of knife-sharp truncation of Diplocraterion burrows, many of which are eroded down to the base of their U-shaped burrow (Wignall 2001, fig. 8). An obvious prediction from this is that eroded limestone clasts might be expected at the base of the black shale directly overlying the Langport in adjacent areas of more continuous sedimentation, with no local indication of erosion, such as at St Audrie's Bay on the Somerset coasts. Such evidence was sought successfully by Hallam (1990) who, working in conjunction with Alastair Ruffell, found angular clasts of pale micrite, ranging in length up to 5 cm, within the bottom 2 cm of the basal Blue Lias black shale. Such clasts were not common, but about 20 were found in half-an-hour's intensive collecting. That Hesselbo et al. have failed to confirm this discovery probably relates to the much deteriorated quality of the coastal outcrop after the time the discovery was made in 1988. It is most decidedly not due to a 'mistaken identification of some other feature of this horizon such as the development of carbonate nodules' (Hesselbo et al. 2004, p. 374). It is worth adding here that the clasts occur in association with a layer of phosphatic nodules, suggesting a significant stratigraphie horizon.

Subsequent support for Hallam's claim was achieved by Wignall (2001) following examination of superb, newly revealed exposures on the Devon coast. He confirmed an episode of end-Langport Member erosion, which initially exposed a scmilithified substrate on the seafloor, with truncated Diplocraterion burrows cross-cut by sharper-margined burrows. Seafloor Unification was completed later and both Diplocraterion and firm-ground burrows are cross-cut by borings. Oysters are occasionally found cemented to the top surface. Subsequently local erosion, brecciation and redcposition of the topmost beds of the White Lias took place. In our view this represents very shallow-water deposition of the Langport Member. A nearidentical development of the Langport Member at Long Itchington on the East Midlands Shelf was similarly interpreted (Radley & Swift 2002). In total these features indicate a dramatic increase of a broad range of erosion and erosion-and-redeposition features in the topmost beds of the Langport Member suggestive of shallowing. In contrast Hesselbo et al. (p. 377) favour deposition 'in areas where the ramp was steep and the facies characterized by gravity-flow deposits' during base-level rise. Apart from the unlikelihood of such flows coming to rest on steep ramps it overlooks the observation that the larger clasts at the base of the breccia are essentially in situ having only been moved a few centimetres, with the result that bedding features can be traced laterally from undisturbed strata (Wignall 2001, fig. 4). In their review of modern carbonate depositional environments, Inden & Moore (1983) stress the frequency of early cementation and break up into clasts by storm action. Episodic emergence above sea level facilitates this early cementation, and can help to explain the frequency of limestone clasts of various sizes throughout the White Lias.