NEW STUDIES ON THE MIDDLE MIOCENE HUMBOLDT FORMATION IN NORTHEASTERN NEVADA

Middle Miocene lacustrine and fluvial sedimentary rocks of the Humboldt Formation (including the Carlin Formation of Regnier, 1960) in northeastern Nevada were deposited in several basins separated by low-relief highlands. Sediments in the Elko basin, between the Adobe Range and the Ruby-East Humboldt Mountains (REH), include fluvial deposits sourced in the Adobe Range and farther west, coarse alluvial fan deposits along the REH, and lacustrine deposits in the middle of the basin; the lacustrine environment expanded northward through time. To the west, sediments in the Carlin basin include basal ash-rich lacustrine sediments and overlying alluvial and pedogenic deposits. Thickening sedimentary sequences in both basins lapped extensively onto and partially buried many of the adjacent highlands, including a low Adobe Range upland that incompletely separated the early stages of the two basins. Syn-sedimentation faulting in the Elko Basin was relatively minor except along the west side of the REH. High-angle faulting along the west side of the Adobe Range induced alluvial and debris-flow sedimentation in the Carlin basin and created a more pronounced highland between the Elko and Carlin basins. Miocene sediments in the Ivanhoe and Santa Renia Fields (ISR) area west of the Tuscarora Mountains include lacustrine deposits with some overlying fluvial deposits. Sedimentary facies and clast sources indicate that the Tuscarora Mountains were a middle Miocene highland that shed clastic sediments to both the west and east into the ISR area and northwestern Carlin basin. Limited published geochronologic data indicate that lacustrine sedimentation in the various basins began at about 16±1 Ma, suggesting that drainage through the basins was unimpeded until that time. Extensive post-sedimentation faulting disrupted the Carlin basin and ISR area, but it apparently was less significant in the Elko basin except along the REH. Pliocene and younger regional erosion has removed much of the original basin fill in all areas and resurrected some of the pre-sedimentation topography. Thus, the extents and thicknesses of sediments were greater in the middle Miocene than they are now, and modern highlands are the products of pre-sedimentation topography, syn- and post-sedimentation faulting, and (or) late Cenozoic erosion.