TAPHONOMY OF SOME EXCEPTIONALLY PRESERVED ORGANISMS FROM THE LOWER CAMBRIAN OF LAURENTIA

Fossils of both biomineralized and non-biomineralized tissues from three lower Cambrian deposits suggest that preservation occurred along several taphonomic pathways, as indicated by a small variety of mineral compositions. Analytical studies, including microstructural analyses, energy dispersive x-ray [EDX] analyses, and elemental mapping, were performed on material from the Sirius Passet deposit of North Greenland, the Indian Springs deposit of Nevada, and the Kinzers deposit of Pennsylvania.

Non-biomineralized tissues have been preserved as organic carbon, Fe-sulfides, Fe-oxides, clay minerals, and phosphates. Organic carbon is rather common in Kinzers and Sirius Passet material, principally in algal and bacterial remains, but was not detected in specimens from Indian Springs. In all three deposits, nonmineralizing organisms are commonly preserved, in whole or part, by thin films of minerals represented today by Fe-sulfides, Fe-oxides, and Fe-rich clay minerals. The Fe-oxides are probably alteration products of pyrite. For fossils composed of clay minerals, it is unclear whether the clays formed during early-stage diagenesis or late-stage alteration. A combination of processes leading eventually to clay mineral preservation is possible. Exceptional preservation of organisms by phosphate minerals is relatively rare in the studied material.

Biomineralized parts of organisms in the studied deposits are commonly leached. Leaching of calcitic, aragonitic, and phosphatic hard parts has lead to moldic preservation in shale. Decalcification is more widespread than is dephosphatization.

Exceptional preservation in the lower Cambrian deposits of Laurentia implies a synergistic relationship between biological, chemical, and physical conditions at the sites of deposition. Aqueous conditions must have been sufficiently inimical, at least temporarily, to exclude scavengers, bioturbators, and certain microbial biodegraders. However, development of early diagenetic mineral films on organismic remains was probably mediated in most circumstances by microbial activity. Dysoxic sediment, or reducing microenvironments within the sediment column, and relatively rapid burial rates, also must have played key roles in exceptional preservation.