| Description |
Silicification is a diagenetic process that affects both trace and body fossils observed 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 outcropping in the Lemay Island field area. Here, nodule-like growths of microcrystalline quartz are 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. The association between silicification and the presence of biotic material is well founded in the fossil record. Notable examples include the brachiopod fauna in the Permian carbonate strata of west Texas and the petrified flora in the Triassic volcanic ash strata of northern Arizona. The effects of silicification and carbonate diagenesis are likely responsible for the incomplete preservation of the infaunal community once present in the sediments of Lemay Island. Original sedimentary structures and trace fossils in carbonate systems are especially prone to diagenetic overprinting, which commonly begins during shallow-burial or even at the sediment-water interface. Silicified burrows observed from the Lemay Island field area include the ichnogenera Thalassinoides and Rhizocorallium. Where epifaunal and shallow-tier trace fossils are all but lost to diagenesis, the preservation of these mid- to deep-tier burrow systems are evidently enhanced by the effects of early diagenesis. Several factors are likely to contribute to this preferential preservation: (1) passive filling with biosiliceous-rich carbonate; (2) impregnation of burrow walls with organic material; (3) and the ability for sea-water to more easily disseminate through highly permeable burrow systems. Common techniques used for the identification of trace fossils, such as uniformity of dimension, textural differences, and complexity of pattern, require new interpretations to grapple with the challenges inherent in parsing silicified trace fossils from chert nodules of dubious origin. |
