GEOLOGIC MAPPING OF THE PANGUITCH 30 X 60 QUADRANGLE, SOUTHWEST UTAH—NEW INSIGHT INTO THE AGE AND CORRELATION OF UPPER CRETACEOUS FORELAND BASIN STRATA AND THE AGE OF THE LOWER MIOCENE MARKAGUNT MEGABRECCIA

Data collected during the third year of a four-year STATEMAP- and UGS-funded effort to map the geology of the Panguitch 30´x60´ quadrangle—which straddles the Basin and Range and Colorado Plateau Provinces at the south edge of the Marysvale volcanic field—constrain the age and correlation of Upper Cretaceous foreland basin strata and the age and extent of the Miocene Markagunt Megabreccia.

The Grand Castle Formation, which consists of three distinct lithostratigraphic members, is key to mapping and correlating Upper Cretaceous strata across the Markagunt and Paunsaugunt Plateaus. Our mapping shows that the age of the lower two members, originally thought to be Paleocene, is Late Cretaceous; the subsequent discovery of a theropod track and palynomorphs of Santonian or Campanian age in the middle member of the Grand Castle confirms the revised age. All three members were traced southward from the type section into Cedar Canyon where the middle sandstone member overlies and interfingers with strata tentatively mapped as Wahweap(?) Formation. Although we lack definitive age control, the lower Grand Castle conglomerate member may be correlative with the Drip Tank Member of the Straight Cliffs Formation, and the middle sandstone member may be correlative with the capping sandstone member of the Wahweap Formation; eastward correlation of the upper Grand Castle conglomerate member remains unresolved. Alternatively, strata in Cedar Canyon may represent an unusually thick section correlative with the John Henry and Drip Tank Members of the Straight Cliffs Formation, or may correlate with an unconformity on the Kaiparowits Plateau. A similar section is present on the west flank of the Paunsaugunt Plateau that is locally overlain by strata tentatively assigned to the Kaiparowits Formation; we are as yet uncertain of the correlation of this section with the Markagunt and Kaiparowits Plateau sections.

Newly identified exposures in grabens on the west flank of the Markagunt Plateau demonstrate that Markagunt Megabreccia, an enormous lower Miocene catastrophic gravity slide deposit, locally overlies the 22.0-Ma Harmony Hills Tuff. This is strong evidence that the megabreccia postdates the 22.75-Ma Haycock Mountain Tuff, which was once thought to unconformably overlie the gravity slide and thus constrain its upper age.