Palaeonenvironmental implications of plant, insect and other organic-walled microfossils in the Weald Clay Formation (Lower Cretaceous) of southeast England

Abstract

A remarkable variety of plant and other organic-walled microfossils that are between 0.1 and 5.0 mm in maximum diameter have been recovered from two Weald Clay sections in Surrey, southeast England. The associations of these ‘mesofossils’ with each other, and with lithofacies, invertebrate macro- and microfossil occurrences and palynofacies require further investigation, but among a number of preliminary conclusions that have been drawn based on the data accumulated so far are the following. The amount of particulate organic matter of mesofossil size in a bed can usually be estimated from the general aspect of the lithology, but its composition is much harder to predict. As for older Wealden (Hastings Beds) occurrences, deposits in which megaspores are abundant often contain scattered small wood and cuticle fragments that are visible to the naked eye. Those displaying remains of insects of similar or larger size tend, not surprisingly, to yield the richest assemblages of mesofossil-sized fragments of this group. However, such zooclasts occur more widely than is apparent from the horizons that are obviously insect-bearing. This suggests that, more often than has generally been appreciated, minute black particles in standard palynological preparations of nonmarine sedimentary successions may include arthropod fragments. The common occurrence of charred remains ofWeichselia reticulataand other plants in association with disarticulated bits of insects may partly reflect both a type of vegetation that was prone to desiccation and some of the insects that lived within it, lightning-induced fire during droughts having been responsible for their death. A tendency towards an inverse relationship between this plant-insect association and large numbers of megaspores and/or seeds indicates that other habitats are also represented. Several of the megaspore genera are probably the products of aquatic or waterside plants. Delicate leafy shoots of uncertain affinity occur in several beds, but the general scarcity of larger pieces of wood and foliage suggests that the conifers and other gymnosperms may have been confined mainly to better drained parts of the Wealden lowland, and to the higher ground bordering it. Indicators of fresh to low salinity water were recovered from more than half of the samples examined. This is consistent with the general environment of sediment deposition inferred from the associated invertebrate faunas and dinoflagellate cyst assemblages.