Article: The palaeoecology of the Actinocamax plenus Subzone (lowest Turonian) in the Anglo-Paris Basin
R. P. S. Jefferies
The plenus Subzone often conforms to a standard succession of eight beds, numbered 1 to 8 in ascending order. The distribution of aragonitic fossils in the plenus Subzone was probably controlled more by preservation than ecology. Preservation was determined firstly by temperature and secondly by turbulence.In late subglobosus times the distribution of macro- and microfauna was governed by depth of water, and the depth preferences of the various species of benthonic foraminifera were like those of their relatives in various times. There was little faunal change at the beginning of Bed 1 times, despite extensive erosion. This indicates that the pre-Bed 1 shallowing, for which there is much lithological evidence, was accompanied by only a small rise in temperature. A sudden change at the beginning of Bed 2 times to a shallower-water type of fauna was due to a considerable rise in temperature which led also to the preservation of aragonitic fossils. The peculiarities of the fauna of Beds 4-6, including the presence of Actinocamax plenus and Oxytoma seminudum, suggest a moderate decrease in temperature with the appearance of species of Central European Russian affinities. This moderate fall in temperature is reflected in the microfauna. During the deposition of Beds 7 and 8 the temperature rose again slightly.On the basis of certain simplifying assumptions the temperature of deposition of Bed 1 was probably below 5-10° C, that of Beds 4-6 about 16-9° C. and that of Bed 2 considerably higher than 16-9° C.