NEW MERAMECIAN UNIT ON MISSISSIPPIAN GRANITE MOUNTAIN ISLAND, MIDDLE RANGE, WEST-CENTRAL UTAH

Granite Mountain Island (GMI), an emergent area, is located astride an Early Mississippian Antler orogenic forebulge. Regionally, normal Mississippian stratigraphy comprises, in ascending order: the Kinderhookian upper member of the Pilot Shale, Kinderhookian-Osagean Joana Limestone, and Osagean Delle Phosphatic and Osagean-Meramecian Needle Siltstone Members of the Chainman Shale. In contrast, at Granite Mountain, the thin Delle Member rests disconformably on the Upper Devonian lower Pilot Shale and is separated by 24 m of siltstone from an overlying 50-cm-thick lag bed containing small early Meramecian rugose horn corals and Zone 13-14 calcareous foraminiferans.

In the Middle Range, 5 km east of Granite Mountain, we discovered a previously unmapped outcrop belt of Joana Limestone, ~10 m thick. The thin Joana section is stratigraphically complete, containing all its normal units, as do similar thin nearby sections in the Deep Creek Mountains to the NW and Dugway Range to the NE. At the Middle Range locality, the Joana is overlain by a previously unrecognized, 5- to 10-m-thick, olive-gray, cherty grainstone containing siltstone chips, very fine quartz sand grains, and abundant silicified tiny to large rugose horn corals and rugose and tabulate colonial corals. We correlate this unit with the thin coralline lag bed at Granite Mountain. However, it contains not only <1-cm Cyathaxonia; colonial corals, as much as 1 m across; and narrow rugose corals, up to 20 cm long; but also caninoid corals, 6.5 cm in diameter. Caninoids this large generally do not occur below the upper Chesterian in the western U.S. The new unit extends at least 200 m along strike, appears to overlie the Joana Limestone on an irregular depositional surface, and is overlain by the Needle Siltstone Member. We tentatively interpret the unit to represent an offshore biostrome on the east side of GMI separated by a bay or lagoon from the coralline bed at Granite Mountain, which represents a beach deposit. Work is in progress to document the biostratigraphy, paleoecology, and depositional setting of the new unit.