TAPHONOMY OF CONCHOSTRACAN-DOMINATED LACUSTRINE DEPOSITS IN THE KIRKPATRICK BASALT (JURASSIC, ANTARCTICA)

The Kirkpatrick Basalt (Jurassic) of South Victoria Land and the Central Transantarctic Mountains of Antarctica, includes sedimentary interbeds of lacustrine origin that vary in inferred depositional setting from shallow ephemeral ponds, some with microbial mat accumulations, to deeper permanent lakes. Fossil assemblages in these rocks are dominated by conchostracans, but insect nymphs, ostracodes, neopterygian fish, and plant material are locally abundant. The presence of a similar biota in contemporaneous deposits across several types of lacustrine deposits provides the opportunity to compare the taphonomy of these depositional settings and organisms.

Comparative taphonomic study of conchostracan, ostracode, insect, and fish specimens from the three lacustrine settings involved scanning electron microscopy, energy dispersive x-ray spectrometry, as well as thin sections. Conchostracan carapaces and the bones and scales bones of fish are preserved primarily as calcium phosphate, ostracode carapaces are preserved as calcium carbonate, and insect and plant material are preserved with calcium sulfate. Except for chitinous and cellulose material, the mineralogy of the fossils reflects the original skeletal composition of the organisms. The style of preservation of each group of organisms was relatively conserved across the different depositional settings, with the exception of those with microbial mats.

Although the primary preservation of conchostracan carapaces is phosphatic, portions of most carapaces were replaced by silica. Both fabric-replacive and fabric-destructive silicification is present. Fabric-replacive silicification, which occurred early in diagenesis, preserves a higher level of microstructural detail than phosphatization. Conchostracan carapaces preserved in microbial mat sediments exhibit a greater degree of silica replacement than do carapaces preserved in other deposits, and the presence of microbial mats may have enhanced the relative degree of silicification relative to other deposits.

This work was supported in part by the National Science Foundation (NSF OPP-0229757).