IDENTIFICATION OF SCAPHITES HIPPOCREPIS III IN THE TOP ABERDEEN MEMBER (CAMPANIAN BLACKHAWK FORMATION) LAG DEPOSIT, BOOK CLIFFS, EASTERN UTAH: IMPLICATIONS FOR CORRELATION IN THE CRETACEOUS WESTERN INTERIOR

Fossiliferous coarse-grained sandstones and oolitic ironstone bodies occur as isolated patches along the desert floor in the Book Cliffs region, eastern Utah. These bodies pass laterally into iron-rich siltstones and form an areally persistent marker horizon that can be correlated from Green River to east of Thompson. The index fossil Scaphites hippocrepis III has been collected from multiple localities within two separate regions. The Browns Wash collection consists of 20 specimens (11 male, 9 female) while the smaller Floy Wash collection comprises 6 specimens (all male). Males have 7-10 rounded to clavate ventrolateral nodes (av. 8.1), 2-4 rounded umbilical nodes (av. 2.4), 2-8 bullate to rounded midflank elevations (av. 4.8), 30-57 ventral ribs on the body chamber (av. 41.6), and are 17-45 mm in length (av. 31.4 mm). In contrast, female specimens have 6-10 bullate to rounded ventrolateral nodes (av. 7.5), 2-3 rounded umbilical nodes (av. 2.5), 3-5 bullate midflank elevations (av. 4.0), 42-55 ventral ribs on the body chamber (av. 48.1), and are 19-59 mm in length (av. 39.4 mm).

Previous studies have correlated the coarse-grained sandstone and oolitic ironstone horizon to a variety of intervals ranging from late Santonian (Emery Sandstone) to early Campanian. The recognition of index fossil Scaphites hippocrepis III clearly indicates a Campanian age. This is corroborated by the high resolution sequence stratigraphic framework which independently revealed a Campanian age (i.e. top Aberdeen Member, Blackhawk Formation).

Implications for correlation include: (1) member-scale boundaries in the Blackhawk Formation can be correlated with confidence into the Prairie Canyon Member of the Mancos Shale, providing a chronostratigraphic link between the nearshore and offshore deposits, (2) the coarse-grained sandstone, oolitic ironstone and iron-rich siltstone marker horizon is overlain by turbidite-rich shelf deposits of the lower Kenilworth Member at Middle Mountain, Gunnison Butte, Tusher Canyon, Coal Canyon, Browns Wash, Hatch Mesa, and Floy Wash, thus confirming the time equivalency of the Hatch Mesa succession to the lower Kenilworth Member, and (3) member- and parasequence-scale rock packages show a gradual basinward thinning not the abrupt clinoform-style pinch-outs popularized in previous studies.