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Journal of the Geological Society; 1999; v. 156; issue.6; p. 1113-1124;
DOI: 10.1144/gsjgs.156.6.1113
© 1999 Geological Society of London

Article

Regular terrestrially derived palynofacies cycles in irregular marine sedimentary cycles, Lower Lias, Dorset, UK

HELEN K. WATERHOUSE1,2

1 Department of Geology, University of Southampton, Southampton Oceanography Centre, Empress Dock, European Way, Southampton SO14 3ZH, UK

The Lower Lias of Dorset, England comprises prominent limestone-marl-shale alternations considered to be obliquity orbitally controlled, probably enhanced by diagenesis. Palynofacies analysis, the study of microscopic particulate organic matter in a sediment, identifies six superimposed palaeo-environmental variations through 12 m of this sequence: (1) 90 cm cycles in abundance of terrestrially derived organic particles, (2) 45 cm cycles in abrasion of wood particles, (3) 1.2–1.8 m cycles in shape of wood particles; (4) irregular cycles (20 cm to 5.2 m) in limestone deposition, (5) irregular alternations (50 cm to 2.5 m) in shale-marl deposition and (6) irregular variation in marine phytoplankton abundance. These are interpreted as: (a) obliquity forcing of marine currents, bottom water oxygenation and organic preservation, (b) precessional forcing of rainfall, position of storm wave base and limestone deposition, (c) eccentricity driven shallow marine energy changes preventing limestone deposition, (d) irregular marine processes affecting shale-marl deposition and marine plankton productivity. Thus, three orbital cycles acted simultaneously with different effects on different environments. Palynofacies analysis is a sensitive tool for extracting palaeoenvironmental information, particularly terrestrial variations recorded in marine sedimentary rocks. In addition, it is demonstrated that regular proxy-climatic orbital cycles are preserved within irregular sedimentary sequences.


Keywords: Lower Lias, Dorset, palynofacies, orbital forcing, palaeoenvironments.




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