SILICIFIED TRACE FOSSILS AND ICHNOFABRICS IN THE PERMIAN OF NORTHWEST UTAH

Silicification is a diagenetic process that affects both trace and body fossils found in the abundant chert-bearing Permian strata of the northwest desert of Utah. A limited trace fossil assemblage of mid- to deep- tier infaunal burrowers are present in the Trapper Creek Formation and Gerster Limestone found at the Lemay Island field area. Nodule like growths of microcrystalline quartz are found in close association with bioturbated horizons. The combination of compaction and diagenetic overprinting due to the growth of chert in the mixed carbonate/biogenic silica rich-sediments (e.g. sponge spicules) can alter and erase the sedimentary fabric created by the original fossorial excavators. Silicification is typically found in close association with organic-rich deposits. Notable examples include the brachiopod fauna in the Permian strata of west Texas, and the petrified flora in the Triassic strata of northern Arizona. Microfacies analysis indicates this alteration occurs early in the unit’s diagenetic history.

Preferential diagenesis via silicification leads to an incomplete preservation of the infaunal community that was likely once present in the sediments of Lemay Island. Identification of the limited trace assemblage preserved by silicification is difficult. Burrows well preserved in these conditions include the ichnogenera Thalassinoides and Rhizocorallium. This preferential preservation is proposed to occur because of these traces proclivity to passively fill with overlying sediment, and the deep-tier rank of this type of bioturbation. Paleoenvironmental analysis indicates that these units represent a shallowing upward series from alternating calm-water and storm deposits in a subtidal inner ramp setting, to shallow brachiopod dominate subtidal deposits. Common techniques used in the identification of trace fossils, such as uniformity of dimension, textural differences, and complexity of pattern, require new interpretations to grapple with the challenges presented by silicification.