CLARKE, PAUL R., University of Birmingham, School of Earth Sciences, Birmingham, UK

ABSTRACT: The Provenance and Facies Architecture of Upper Triassic Fluvo-Lacustrine Deposits from the UK: A Modern Analogue

Continental red beds typify much of the Upper Triassic of the UK, collectively termed the Mercia Mudstone Group and comprise a thick sequence of red siltstones, mudstones, evaporites and locally variable sandstone formations. This unit forms an important regional seal to several UK hydrocarbon provinces. Facies relationships and architecture are poorly understood because of the lack of a modern analogue for the group.

The Upper Triassic of South Wales (UK) comprises a suite of continental lacustrine to marginal-marine depositional environments with strong climatic control (seasonal mega-monsoon). These deposits include supra-tidal evaporitic flats, subject to annual lake

marine flooding and continentally derived ephemeral flood waters, similar to those in the modern Ranns of Kutch (India), which lie to the east of the Indus River valley.

The Ranns are supra-tidal evaporitic flats, which rest above normal high-tide, flanked to the south by the Arabian Sea and are annually flooded by marine waters associated with the storm tides of the Southwest Monsoon. These factors, combined with the fluvial input from the Indus River, make the area a potentially excellent modern analogue for the UK Upper Triassic. The main question to resolve in both modern and ancient examples, is the provenance of the fine-grained sediment, by floodplain accretion, marine flooding, aeolian processes or a combination of these.

This study will provide a better understanding of the depositional environments and provenance of these barren mudstone sequences. It will also provide a contribution to current reservoir models and sealing potential of Upper Triassic red beds.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid