SEDIMENT COMPOSITION OF BURGESS SHALE TYPE LAGERSTÄTTEN: IMPLICATIONS FOR SOFT-TISSUE PRESERVATION

Sedimentary rocks that yield not only fossilized skeletons but also evidence of soft-tissues that normally decay away are a vital source of paleobiological information. Their utility has been demonstrated most strikingly in investigations of the Cambrian explosion of animal diversity – one of the most significant events in the history of life on Earth. Cambrian Burgess Shale-Type (BST) fossil assemblages preserve carbonaceous remains of soft tissues and entirely soft-bodied organisms in fine-grained siliciclastic rocks.

There has been a substantial effort to understand the taphonomy of BST fossils, since it is vital to interpretations of their biology. Sediment composition of BST localities, and the abundance of certain clay minerals in particular, has been invoked as a significant factor in the preservation of soft tissues. Clays have been posited to slow decay via adsorption and consequent deactivation of autolytic enzymes, and through their involvement in the polymerization of organic tissues. However, geological evidence for associations between the abundance and mineralogy of clays, and occurrences of exceptionally preserved BST fossils is scant.

We present preliminary X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy data for stratigraphic suites of samples from Cambrian localities with BST preservation including the Marjum and Wheeler shales of the western USA and the Kaili Formation of south China. XRD was performed using a cobalt-source diffractometer on randomly oriented bulk powders, and quantitative clay mineralogy obtained using the methods of Środoń et al. (2001, Clays and Clay Minerals) focusing on the region 69-75° 2theta. Iron-rich clay minerals such as berthierine, glauconite, and iron-rich illite constitute a major component of the clay mineral composition of BST sediments, with kaolinite also common. Our preliminary data permit hypotheses that link enhanced soft-tissue preservation to the role of diagenetic clay minerals. However, XRD data acquired from a limited number of Cambrian shales that preserve only hard parts such as trilobites suggest enrichments in iron-rich clay minerals may not be unique to sedimentary rocks hosting BST preservation.