MUDSTONE LITHOFACIES IN THE KIMMERIDGE CLAY FORMATION, WESSEX BASIN, SOUTHERN ENGLAND - IMPLICATIONS FOR THE ORIGIN AND CONTROLS OF THE DISTRIBUTION OF MUDSTONES

Citation
Jhs. Macquaker et Rl. Gawthorpe, MUDSTONE LITHOFACIES IN THE KIMMERIDGE CLAY FORMATION, WESSEX BASIN, SOUTHERN ENGLAND - IMPLICATIONS FOR THE ORIGIN AND CONTROLS OF THE DISTRIBUTION OF MUDSTONES, Journal of sedimentary petrology, 63(6), 1993, pp. 1129-1143
Citations number
72
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geology
Journal title
ISSN journal
0022-4472
Volume
63
Issue
6
Year of publication
1993
Pages
1129 - 1143
Database
ISI
SICI code
0022-4472(1993)63:6<1129:MLITKC>2.0.ZU;2-J
Abstract
Lithofacies analysis has been undertaken on a suite of apparently mono tonous organic-rich mudstones from the Upper Jurassic Kimmeridge Clay Formation (KCF) in the Wessex Basin, southern England. Using a combina tion of hand-specimen description, whole-rock geochemistry, optical pe trography, and electron optical petrography, five lithofacies have bee n identified. These are: clay-rich mudstones, silt-rich mudstones, nan noplankton-rich mudstones, laminated mudstones, and concretionary carb onates. These facies can be distinguished from one another on the basi s of physical sedimentary structures and the relative proportions of a llochthonous, autochthonous, and diagenetic components in each. All fi ve of the KCF lithofacies studied were deposited in a marine shelf env ironment below fair-weather wave base but in relatively shallow water close to storm wave base. Deposition occurred in regions where sand an d other coarse elastic allochthonous debris were not being supplied an d the bottom waters varied from oxic to anoxic. The main factors influ encing the development of the five facies were: distance from source a rea, primary productivity, bottom-water anoxia, absolute bathymetry, e lastic dilution, and local sediment accumulation rates. Specifically, the clay-rich mudstones were deposited in regions farthest down the se diment-transport paths, commonly in areas by-passed by the main sedime nt supply and beneath surface waters of relatively low primary product ivity. In contrast, the silt-rich mudstones were deposited in more pro ximal areas, closer to the source of the sediment, in regions where su rface productivity and sediment accumulation rates were much higher. T he nannoplankton-rich mudstones accumulated beneath regions where prod uctivity in the surface water layers was fairly high (dominated by coc colithophoroid production) and the supply of elastic sediment was insu fficient to significantly dilute the autochthonous fraction of the sed iment. Unlike the other facies, laminated mudstones were deposited in areas where the bottom waters were anoxic, surface productivity was en hanced, and local sediment accumulation rates were high. The concretio nary carbonates were precipitated by diagenetic processes in areas whe re sediment accumulation rates were very low. These facies description s have enabled a detailed sedimentological study of the Kimmeridge Cla y Formation to relate depositional and diagenetic processes directly t o the facies present, thereby allowing classical facies analyses to be undertaken on apparently homogeneous mudstone successions.