CREATION AND BURIAL OF A MAJOR MESOZOIC LANDFORM: NEW MICROFOSSIL EVIDENCE BEARING ON THE AGE OF THE J-2 UNCONFORMITY (GRAND STAIRCASE - ESCALANTE NATIONAL MONUMENT, UTAH)

The first palynomorph assemblages reported from the Carmel Formation (Jurassic), southern Utah, indicate a middle to late Bajocian age for the lower part of the Judd Hollow Member, and establish a maximum age for onset of deposition of the Carmel Formation (San Rafael Group) over the J-2 regional unconformity. These microfossil assemblages therefore constrain the minimum age for the J-2 regional unconformity exposed in southern Utah, and provide constraining evidence for the minimum age of the Temple Cap Sandstone (Sinawa and White Throne Members, San Rafael Group) below that unconformity. The age of the Judd Hollow Member was established independently on the basis of two types of microfossil evidence: pollen and spore assemblages and dinoflagellate assemblages recovered from the same samples collected from a locality near the Cockscomb, ~60 km east of Kanab, Utah. The presence of a moderately diverse dinoflagellate assemblage in these samples suggests that the Judd Hollow Member was deposited under fully marine conditions; the abundance and diversity of pollen and the lack of ammonites in this marine unit suggest deposition in a low energy shallow marine environment. The composite paleontological evidence from the oldest rocks above the J-2 unconformity, from sites in Montana and southern Utah, suggests that the north-to-south incursion of the Carmel seaway was a relatively rapid event, transgressing an area approximately 1200 km long in no more than 2-3 million years (an average net transgression rate of ~4-6 km/10 ky). The presence of datable extrusives in the upper part of the Carmel Formation, which bracket the age of these microfossil assemblages, indicates that the development of the J-2 unconformity and the southward transgression over its erosional surface were nearly concurrent with Middle Jurassic felsic volcanic activity to the west and/or southwest of southcentral Utah. The rapid formation and inundation of the J-2 erosional surface probably resulted from the uplift and/or tilting, then downdropping of the western edge of the North American Plate as it overrode or attempted to override the Pacific Plate during the Middle Jurassic.