LATE CENOMANIAN (LATE CRETACEOUS SCIPONOCERAS GRACILE BIOZONE) PALEOGEOGRAPHIC EVOLUTION OF THE GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT REGION: IMPLICATIONS OF RECENT ADVANCES IN HIGH-RESOLUTION AMMONOID BIOSTRATIGRAPHY

Ammonoid biostratigraphic analysis of the Dakota Formation-Tropic Shale boundary interval gives new insight into local paleogeography. New data shows the contact is diachronous, but not in a way that has been suggested previously. West of the East Kaibab Monocline the contact coincides closely with the Metoicoceras mosbyense-Sciponoceras gracile biozone boundary and four distinct, laterally persistant concretionary zones are present between the top of the Dakota and the first appearance of the genus Burroceras. At Trail Canyon, about 70 km west of the East Kaibab structure, the base of the Tropic still coincides with the base of the S. gracile Biozone and all four concretionary zones are present. East of the Echo Monocline the contact lies at the same chronostratigraphic level and this relationship persists eastward to Navajo Point. However, between these two structures, the base of the S. gracile Biozone lies up to 2-3 meters below the contact, and the highest of the four concretionary zones is essentially resting on the Dakota. Inferred depositional environments for the upper Dakota between the two structures range from estuarine and foreshore to supratidal, indicating the area wasn’t deeply inundated by marine waters until after the middle of S. gracile time (Eumophaloceras septemseriatum subzone). Taken together, these relationships demonstrate the presence of a paleotopographic high, partly of structural origin, between the Kaibab and Echo structures. The presence of early diagenetic siderite in the basal Tropic west of the high contrasts strongly with its complete absence to the east, suggesting the high also demarcates a boundary between significant freshwater influence on lowermost Tropic depositional systems during S. gracile time. Qualitative observations also show that west of the high, Tropic ammonite taxa such as Euomphaloceras and Worthoceras are relatively uncommon, while east of the high these taxa are much more common. If born out by more rigourous analysis the ammonite distribution indicates the feature may have continued to act as a microbiogeographic barrier even after its complete submergence.