Sedimentary Petrology and Tectonic Significance of Tertiary Sediments in Southern White River Valley, Nevada

William Harris Diguiseppi

Up to 20 m of undated Tertiary sediment have been exposed by incision of the White River in southern White River Valley, Nevada. The sediments, deposited in two distinct environments and time periods, range from conglomerate to mudstone. Older sediments comprise alluvial-fan and lacustrine facies deposited in a closed basin. Alluvial-fan grain size and provenance patterns indicate centripetal transport into a topographic low located on the western margin of the valley. Very fine grained sandstone to mudstone basin-center deposits contain pennate diatoms and perhaps other microfossils. The basin-fill deposits are unconformably overlain by stream-terrace conglomerate. Such terraces are found along the White River as far south as Lake Mead and relate to the present southward external drainage. Multiple terrace levels, formed during incision, reflect base-level changes in the Colorado River System.

The valley is bounded by normal faults, on the east by the White River fault and west by the Pahroc fault, in apparent contrast to the half-graben form of nearby basins. The alluvial-fan deposits are cut by the Pahroc fault but appear to postdate slip on the White River fault. Facies patterns in the basin fill thus indicate concentration of fine-grained sediment near the fault that was active during deposition. Terrace conglomerates indicate progradation of coarse clastics across the fine basinal deposits after activity on the bounding fault ceased. These relationships support the recent hypothesis that coarse clastic sedimentation in a basin may reflect post-tectonic rather than syntectonic deposition.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.