THE CHONDRITES ICHNOFOSSIL: A COMPARATIVE STUDY

The Chondrites ichnofossil is a fodichnial burrow with branches of equal diameter that do not cross or anastamose, and was most likely produced by nematodes. The presence of these burrows in a stratigraphic section often serves as a proxy for low-oxygenation and typically occurs along bedding planes in turbidite series, shallow marine shales/silts, and storm sands. Chondrites has a stratigraphic range of occurrence of Cambrian to Tertiary, with newer discoveries of Chondrites in deep sea muds. Some ichnologists advocate a narrower range for the ichnogenus, with a first appearance datum in the lower Ordovician. This study will compare the occurrences of Chondrites in two stratigraphic sections of different age. A series of densely bioturbated Chondrites-bearing beds is found within a section of the Middle Cambrian Gros Ventre Formation in the Burgess Junction region of the Big Horn Mountains in Wyoming. This distinct unit, which is almost four meters thick, serves as a marker bed for correlation between exposures with its green siltstones, mudstones, and abundant Chondrites ichnofossils. The other interval examined in this study is the Pioneer Valley Member of the Kope Formation (type Cincinnatian, Upper Ordovician). These intervals of cyclical carbonates and siliciclastics yield abundant Chondrites in siltstone horizons. Both the Gros Ventre and Pioneer Valley exhibit Chondrites in silty horizons nearly void of body fossils, which may have implications for the paleoenvironmental and paleoecological setting of Chondrites. Additionally, as some researchers choose to designate Cambrian Chondrites as a different ichnogenus than Ordovician Chondrites, this study will address this potential discrepancy by evaluating some of the earliest examples of this ubiquitous ichnogenus by comparing their overall structure, morphology, and frequency of occurrence.