SANDBERG, CHARLES A., and FORREST G. POOLE, U.S. Geological Survey, Denver, CO

ABSTRACT: Relation of Early Mississippian Outer Joana Bank to Antler Flysch Trough, Eastern Nevada

The Joana Limestone of western Utah and eastern Nevada was deposited as a more than 500-km-long, 100-km-wide, offshore, Bahamas-like bank during the late Kinderhookian (lower crenulata and isosticha-upper crenulata conodont zones) to early

Osagean (lower typicus zone). The Joana generally is 100-200 m thick, except along its margins and within a small area crossing the Utah-Nevada state line between Granite Mountain and the Antelope Range, where it is absent to less than 50 m thick. This area reflects the presence of Granite Island, which probably was situated on a submarine rise (tectonic forebulge). Initial deposits of the Joana bank were burrowed peritidal quartz sands and sandy bioclastic coquinas composed of brachiopods, fenestrate bryozoans, and syringoporoid corals. Unlike the Bahamas bank, the Joana bank deepened through time because it was being constructed on the westward-tilted, eastern slope of the Antler foreland trough. Thus, the highest beds are generally deep-water, dark, cherty micrite and crinoidal bio erms.

The thin Joana Limestone along the western bank margin in the Pancake Range, eastern Nevada, comprises low (3-15 m) light-colored crinoidal mud mounds, coarse mound-flank talus, and micritic intermound deposits. Because of the westward slope, the mud mounds accumulated in deeper (greater than 100 m) water, as indicated by a siphonodellid conodont biofacies, only during the earliest (lower crenulata zone) part of Joana bank formation. They are separated by chert-pebble hardgrounds from overlying latest Kinderhookian (isosticha-upper crenulata zone) to early Osagean (lower and upper typicus zones) debris flows and turbidites, sourced from easterly areas of the tilted Joana bank. These calciclastic deposits, constituting the Tripon Pass Limestone, were transported westward and offbank, d wn a slope inhabited by siliceous sponges and floored by the Devonian part of the Pilot Shale, as evidenced by clasts and reworked Famennian conodonts in the turbidites.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.