A highstand oolitic sequence and associated facies from a Late Triassic lake basin, south‐west England

Abstract

High energy, lake‐shoreline carbonate sequences are rarely documented in the geological record. However, one example occurs in the Upper Triassic Mercia Mudstone Group (MMG) of southern Britain. The MMG is one of a number of thick, non‐fossiliferous mudstone deposits associated with North Atlantic Mesozoic rift basins. The origin of the MMG mudstones is the subject of current debate, with marine, playa‐lacustrine and alluvial–aeolian models having been proposed. Shoreline features have been documented from the northern margin of the basin, but the rarity of such features elsewhere in the MMG has led many workers to doubt a lacustrine origin for the mudstones. Wave‐dominated, lake‐shoreline deposits have been recognized in several sections from the southern basin margin in the Clevedon area of the Bristol Channel in south‐west England. These deposits provide evidence for the development of a sizeable perennial to semi‐perennial hypersaline lake in which the MMG mudstones accumulated. Shoreline sediments overlie alluvial stream and sheet‐flood deposits, and pass from transgressive gravel–conglomerate beach units with bioclasts, influenced by shore‐normal waves (deposited under semi‐humid conditions), to lower gradient, highstand oolitic sands affected by more varied wave approach (deposited under progressively more arid conditions), which culminated in lowstand, oolitic strand‐plain deposits overlain by a playa‐mudflat unit. Shoreline deposits record a simple shallowing‐upward transgressive–highstand–lowstand sequence. However, a change from a reflective (transgressive) to dissipative (highstand) shoreline is believed to represent a climatically induced change in prevailing wind direction. Shoreline features recognized in the MMG are similar to those of recent playa‐lacustrine basins of the western United States. Ooids display a variety of size, fracture and dissolution features in addition to beachrock fabrics, suggesting that they were originally composed of radial aragonite, similar to modern ooids from the Great Salt Lake, Utah.

Fabrics, facies control and diagenesis of lacustrine ooids and associated grains from the Upper Triassic, southwest England

“…These deposits are succeeded by well-sorted, trough cross-strati®ed grainstones, representing shoreface shoal deposits, which are interbedded with, and overlain by, moderate to poorly sorted packstones and wackestones, and subhorizontally strati®ed grainstones and packstones, representing intershoal and lower foreshore deposits respectively. Features recognized in this highstand succession (Figure 2, unit 4) are characteristic of a dissipative, ramp-type shoreline (Milroy 1998;Milroy and Wright 2000), and are succeeded by intraclast conglomerates (Figure 2, unit 5) representing foreshore reworked beachrock deposits. The highstand unit is conformably overlain by a lowstand unit consisting of sandstones (Figure 2, unit 6) representing littoral strandplain deposits, which marked shoreline regression and a return to more arid conditions (Milroy 1998;Milroy and Wright 2000).…”

“…Features recognized in this highstand succession (Figure 2, unit 4) are characteristic of a dissipative, ramp-type shoreline (Milroy 1998;Milroy and Wright 2000), and are succeeded by intraclast conglomerates (Figure 2, unit 5) representing foreshore reworked beachrock deposits. The highstand unit is conformably overlain by a lowstand unit consisting of sandstones (Figure 2, unit 6) representing littoral strandplain deposits, which marked shoreline regression and a return to more arid conditions (Milroy 1998;Milroy and Wright 2000). The uppermost part of the succession consists of dolomitic silty mudstones ( Figure 2, unit 7), interpreted as playa mud¯at deposits which probably represent a separate depositional unit.…”

“…Understanding the primary composition and structure of ancient ooids is often problematical due to diagenetic changes which, to a greater or lesser extent, act to destroy or obscure such features (Sherman et al 1970;Bathurst 1975;Simone 1981;Medwedeff and Wilkinson 1983;Richter 1983). Although the vast majority of ooids are marine in origin, non-marine ooids have been recognized in a variety of modern lake systems (Eardley 1938;Halley 1977; Copyright # 2002 John Wiley & Sons, Ltd. Milroy and Wright (2000), reproduced by permission of Blackwell publishing (based on BGS 1:250 000 sheet ST47 (1968)). …”

“…Composite sedimentary log of the Clevedon dolomite succession with summarized descriptions and interpretations of main facies units (transgressive, highstand and lowstand units) together with palaeocurrent trends and inferred lake-level changes referred to in the text. modi®ed from Milroy and Wright (2000), reproduced by permission of Blackwell Publishing.…”

“…38 p. g. milroy and v. p. wright Wright 2000). The transgressive succession gives way to planar and cross-strati®ed grainstones and packstones representing a highstand shoreline unit ( Figure 2, unit 4).…”