2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 10
Presentation Time: 4:15 PM


WALLACE, Alan R., US Geol Survey, MS 176, Univ. Nevada, Reno, Reno, NV 89557-0047, alan@usgs.gov

Many basins and ranges in NE Nevada originally formed in the late Oligocene/early Miocene, were partially buried by middle Miocene sediments, and were exhumed by late Miocene and younger erosion. Miocene basin sedimentation began at ~16 Ma behind fault and volcanic dams, and sediments partially buried the highlands. Most basin margins were atectonic except for the detachment-controlled west flank of the Ruby-East Humboldt Ranges. Most dams were breached between 15 and 13 Ma; the Ruby-East Humboldt detachment sustained sedimentation until ~10 Ma. Post-sedimentation faults cut basins and highlands alike. After 10 Ma, streams reintegrated and carried newly formed sediments out of the region. In the Humboldt River basin upstream from Beowawe, as many as 16 strath terraces have been cut into Miocene sediments and some older rocks. Terraces in all drainages are consistent in number and relative elevations, and terrace knickpoints young downstream, indicating response to a basin-wide base-level drop. Erosion has preferentially removed >400 m of poorly consolidated Miocene sediments, exhumed buried basement-cored highlands, and produced a drainage system that mimics that in the middle Miocene. One of the lower terraces formed at ~2.3 Ma (Reheis et al., 2003), indicating substantial earlier erosion. The Humboldt River was regionally extensive before 2-3 Ma and then became restricted to northern Nevada; post-2 Ma sediments eroded from northeastern Nevada were deposited in fault-controlled basins in northwestern Nevada.

These post-Oligocene processes affected both mineral deposits and ground water. Many epithermal deposits formed near Miocene lakes, and the lake distributions provide exploration guides for these deposits. Supergene alunite dates at Eocene gold deposits reflect the pre- and post-sedimentation stages of exposure and erosion, each of which weathered the deposits. Mineral deposits in NE Nevada are being exhumed or eroded, whereas those in NW Nevada are being concealed, producing two regionally different exploration concepts. Miocene basins are important sources of ground water, and the complex basin facies, post-sedimentation faulting, and variable erosion and burial have produced compartmentalized aquifers that strongly affect the properties and recharge of these reservoirs.