RECOGNITION OF FOX MOUNTAIN FORMATION SOLVES ENIGMATIC DEVONIAN STRATIGRAPHY IN CENTRAL AND NORTHERN UTAH

Heretofore, the upper Middle Devonian (Givetian) Fox Mountain Formation, which is widespread in Nevada east of the Roberts Mountains thrust, has been identified only as far eastward as the Confusion Range and Burbank Hills in westernmost Utah. The formation comprises a thick lower member composed of restricted-marine evaporite-solution breccia and pelmicrite and a thin upper member composed of fossiliferous open-marine limestone that represents the continent-wide Taghanic onlap. In the Black Rock Hills, west of the Thomas Range, west-central Utah, the Fox Mountain and overlying yellow slope-forming (YSF) unit of the basal Guilmette Formation are now identified and dated by conodonts at the base of a previously unidentified Devonian sequence. In the Star Range, central Utah, the Fox Mountain and YSF are now recognized <50 m below the basal Famennian unconformity. Thus, the overlying Guilmette is largely truncated there and is not 137 m thick as previously reported. In northern Utah, the Lower Devonian Water Canyon Formation was previously interpreted to be directly overlain by the Guilmette-equivalent, mainly lower Upper Devonian (Frasnian) Hyrum Formation. However, an intervening breccia was included either in the Water Canyon or Hyrum. This breccia is now identified as the lower Fox Mountain. Thus, at Portage Canyon, Samaria Mountain, the sequence above the Water Canyon is identified as Oxyoke Canyon(?) Sandstone, Simonson Dolostone, lower and upper Fox Mountain Formation, and YSF. At Logan Canyon, Bear River Range, the identical sequence, except for the YSF, is also present. However, the Water Canyon-Hyrum contact was previously placed between the lower and upper members of the Fox Mountain. Our study thus demonstrates that the bases of the Devils Gate Limestone of Nevada and the Guilmette and Hyrum Formations represent a virtual time line marking the onset of a late Middle to early Late Devonian carbonate platform that fringed western North America.