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Sedimentation in low-gradient desert margin systems; a comparison of the Late Triassic of Northwest Somerset (England) and the late Quaternary of east-central Australia

M. R. Talbot, Kristine Holm and M. A. J. Williams
Sedimentation in low-gradient desert margin systems; a comparison of the Late Triassic of Northwest Somerset (England) and the late Quaternary of east-central Australia (in Paleoclimate and basin evolution of playa systems, Michael R. Rosen (editor))
Special Paper - Geological Society of America (1994) 289: 97-117

Abstract

The Norian Mercia Mudstone Group of west Somerset (England) is dominated by red dolomitic mudstones and siltstones with thin beds of limestone, sandstone, and nodular or discoidal gypsum or celestite. Primary sedimentary structures are poorly preserved in the clastic units, but evidence of bioturbation and pedogenic features, including rootlet traces, is abundant. In contrast, many limestones contain parallel or ripple lamination. At the time of deposition, peloids seem to have been relatively common in the mudstone and carbonate facies. The sedimentary structures, plus the presence of ooids,Botryococcus, and charophyte fragments indicate that the carbonates are of subaqueous origin, but they may also show evidence of gullying, exposure, and desiccation. Accumulation of the west Somerset Mercia Mudstone Group took place in a low-relief continental basin that had many features in common with the present arid-semiarid interior of Australia. The mudstones and siltstones are of mixed flood-plain and piaya origin. Some of the sediment was deposited from sheet floods, but material of distant, aeolian origin also made a significant contribution. Rivers entering the basin seem to have carried some of their load as peloidal mud aggregates. The carbonate units represent relatively wet climatic intervals when fresh to brackish lakes occupied shallow depressions on the alluvial/playa plain. Lakes were chiefly maintained by surface runoff, but the presence of spring-related carbonate grains implies that groundwaters may also have been of importance locally. Peloidal micritic carbonate grains in the lacustrine deposits are probably of local aeolian origin and are analogous to the clay pellets found in and around many Australian playas. The evaporites are early diagenetic, in part pedogenic precipitates that formed from saline groundwaters during arid periods. Cyclic alternation of carbonate and mudstone indicate that climatic change may have exerted an important control upon sedimentation during Norian time in southwest England.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 289
Title: Sedimentation in low-gradient desert margin systems; a comparison of the Late Triassic of Northwest Somerset (England) and the late Quaternary of east-central Australia
Title: Paleoclimate and basin evolution of playa systems
Author(s): Talbot, M. R.Holm, KristineWilliams, M. A. J.
Author(s): Rosen, Michael R.editor
Affiliation: Universitetet i Bergen, Geologisk Institutt, Bergen, Norway
Affiliation: University of Minnesota, Limnological Research Center, Minneapolis, MN, United States
Pages: 97-117
Published: 1994
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 111
Accession Number: 1994-026176
Categories: Stratigraphy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. cols., 3 plates, sketch maps
S44°00'00" - S10°00'00", E113°00'00" - E154°00'00"
Secondary Affiliation: Elf Aquitaine Norge, NOR, NorwayMonash University, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199412
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