NEW BIOSTRATIGRAPHIC AND RADIOMETRIC AGES FOR ALBIAN-TURONIAN DAKOTA FORMATION AND TROPIC SHALE AT GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT AND IRON SPRINGS FORMATION NEAR CEDAR CITY, PAROWAN, AND GUNLOCK IN SW UTAH

New biostratigraphic and geochronologic data from predominantly marine strata of Albian-Turonian age at Grand Staircase-Escalante National Monument (GS-ENM) with equivalent strata farther west near Parowan and Gunlock, Utah reveals new information about the age of marine facies of the Dakota Formation and Tropic Shale, and nonmarine facies of the Iron Springs Formation. At GS-ENM, the oldest marine fossils occur in the upper beds of the Dakota Formation, which are of late Cenomanian age. Recent collecting has yielded the ammonites Dunveganoceras problematicum natronense, Dunveganoceras cf. D. albertense, and Metoicoceras cf. M. frontierense from a channel fill sandstone bed in the upper part. Inoceramus cf. I. prefragilis, indicating the zone of Calycoceras canitaurinum, has also been collected from the lowermost marine shale, and the highest sandstones of the Dakota have yielded a Vascoceras diartianum assemblage, indicating that a nearly complete late Cenomanian biostratigraphic sequence exists. A new age for the middle part of the Dakota 2 miles south of Tropic was obtained by 40Ar/39Ar laser fusion dating of sanidine crystals from a bentonite bed. The age is 96.06 +/- 0.30 Ma and identifies the sample as late early Cenomanian. Farther west, near Parowan, the oldest marine fauna associated with the Tropic transgression are part of the early Turonian Mytiloides kossmati biozone. Even farther west, near Gunlock, Utah, a new age for the base of the equivalent nonmarine Iron Springs Formation was obtained by 40Ar/39Ar laser fusion dating of sanidine crystals from a bentonite bed. The age is 101.7 +/- 0.42 Ma and is latest Albian. The lower part of the Iron Springs at Gunlock is older than the Dakota at GS-ENM. Underlying conglomerate referred to as the Dakota Formation near Gunlock by previous workers is probably time equivalent to the Cedar Mountain Formation of central Utah.