RECOGNITION AND INTERPRETATION OF SILICIFIED TRACE FOSSILS IN PERMIAN STRATA OF NORTHERN UTAH

Shallow-water accumulations of alternating silica-dominated and carbonate-dominated deposits occur in the Middle-Late Permian Phosphoria and Park City Formations in northern Utah. This occurrence is associated with the widespread Permian Chert Event, a large tract of high silica production across the western coast of early Pangea. These beds represent a very different ecosystem relative to modern shallow-marine environments where the production of carbonate dominates. Due to the increased silica present in the Permian depositional system, silicification of both body and trace fossils was common. Silicification likely was facilitated by the presence of organic material, which acted as nucleation points for the accumulation of chert. However, silica demonstrates the ability to grow outside the constraints of its original template, especially in the case of trace fossils. Because of this, the original size and shape of the trace may be obscured by diagenetic growth of the chert nodule, sometimes making identification difficult. Observation of silicified trace fossils in northern Utah has allowed a compilation of macroscopic criteria useful for identification of traces preserved in a siliceous medium. These include (a) measuring for uniformity of dimensions (diameter); (b) complexity of pattern (presence, number, and angle of branching); (c) textural differences preserved in chert versus the surrounding material; and (d) lack of correlation with other sedimentological or structural controls (presence of current alignment or strong relationship with fracturing). Petrographic observations of pyrite and clay stylolites within chert nodules may provide another avenue for identifying trace fossils in chert nodules. A better understanding of how trace fossils are preserved in a cherty medium will allow for a fuller assessment of the environmental conditions that led to the production of silica-enriched systems, like those present during the Permian.