PALEOGENE HINTERLAND TO FORELAND TOPOGRAPHY: EOCENE DEPOSITS OF NORTHWEST UTAH

In northwest Utah, Eocene plutons (~42-34 Ma) and ductile shear zones coincide with hinterland metamorphic core complexes and other extended metamorphic terranes, based on outcrops in the Albion and Grouse Creek Mountains and the Pilot Range. Thermochronology indicates Eocene cooling of metamorphic rocks, requiring tectonic and/or erosional unroofing. Eocene volcanic rocks (~43-36 Ma) are present in basins adjacent to the metamorphic terranes as well as in sparse, scattered outcrops east to the Wasatch Front. We add two key sites of volcanic rocks and their stratigraphic association—the northern Hogup Mountains and the southern West Hills north of Tremonton--to improve knowledge of the paleogeography of the area, which is described below from west to east:

At the metamorphic complexes near the Nevada border, Eocene rhyolite flows and tuff lie in ~1-km-thick sedimentary basin sections of lacustrine and fluvial deposits. Andesite flows are present locally. Farther east at the Hogup site, Eocene rhyolite flows lie under poorly sorted boulder gravels containing far-travelled granite and chert. The nearest Eocene granite outcrops are 40 km distant, and lithologically unlike the boulders. The Hogup outcrop distributions are consistent with a broad, SE-trending paleochannel. In the West Hills, an Eocene tuff lies on beach gravel, and is topped by a thick conglomerate of locally derived debris. The conglomerate is similar to Eocene conglomerate that is widely exposed farther to the east in the foreland, and which has been attributed to both thrust faulting and extensional basin origins. Eocene depositional systems thus appear to exhibit patterns tied to late Mesozoic and Paleocene tectonic belts. The former metamorphic hinterland was topographically high but with low local relief, with spatially coincident extensional(?) basins that contain sedimentary rock similar to rocks of the much later Miocene half-graben basins. River valleys may have flanked this high area, flowing eastward toward areas of thick proximal clastic accumulations near recently active thrust-fault mountain fronts and in incipient Eocene extensional basins.