TAPHONOMY OF SOME UNUSUAL LATE CRETACEOUS AMMONOID FOSSILS FROM CENTRAL UTAH

Unusual samples of the Late Cretaceous ammonoid Prionocyclus macombi have been collected from the Juana Lopez Member of the Mancos Shale at the Hadden Hills locality near Huntington, Utah. These ammonites are found in anomalous concretions wherein one side of the fossil remains exposed while a concretionary mass has formed on and around the other side of the shell. This study was conducted in an attempt to understand the paleoceanographic and/or diagenetic factors which allowed for such peculiar concretionary growth to take place.

Several meters up section from the target zone, normal concretions with P. wyomingensis were collected for comparison. Stratigraphic analysis of the collection site revealed numerous intermittent layers of ash throughout the interval. These ash layers appear to surround the anomalous concretions, and in some cases lie directly above or below the exposed side of the fossil ammonites. The visible side of the ammonites (i.e., not inside the concretion) appear to be encrusted in a thin layer of gypsum. Thin sections from both the Prionocylcus macombi and the P. wyomingensis samples were obtained and imaged in BSE and CL in order to locate any areas of anomalous mineral growth. XRF analysis was conducted on several samples from within the P. macombi zone, as well as on the control samples from the P. wyomingensis zone. In the anomalous P. macombi samples, the iron to calcium ratio within the space originally occupied by the organism decreased by nearly a factor of ten when compared to the average ratios within the matrix of the concretion. Within the P. wyomingensis samples, this same iron to calcium ratio nearly doubled within the space originally occupied by the organism. We hypothesize that the chemistry and orientation of the numerous ash layers within the Prionocylcus macombi zone created both physical and chemical constraints on the diagenesis responsible for the anomalous concretions that enveloped the ammonites there. Further stratigraphic and chemical analysis of the samples may confirm that the ash layers which lie in direct contact with these unusual concretions acted as a physical/chemical constraint on standard diagenetic processes.