STRUCTURAL AND CHRONOLOGIC CONSTRAINTS ON THE NATURE OF TERTIARY EXTENSION AND BASIN FORMATION IN THE KNOLL MOUNTAIN REGION, NORTHEASTERN NEVADA

Knoll Mountain in Nevada is a horst along the southern fringe of the Snake River plain. The range is bounded on the west by the Knoll Mountain and synthetic Hice-Valder faults and on the east by the Bell Canyon fault. These faults bound half-graben basins on the west (Knoll basin) and east sides of the range. New tephrochronologic, stratigraphic, and structural data provide constraints on timing of basin development, intra-basin unconformities, and the initiation of a fault-propagation syncline.

Sedimentation in the Knoll basin began with deposition of sediment of the Blanchard mbr. of the Humboldt Fm. Tephra in the Blanchard mbr. range from 15.6 to 10.4 Ma and include the feather edge of Cougar Point Welded Tuff unit XIII (10.94 Ma). The overlying Knoll mbr. consists of primary and reworked ash of the 9.8 ± 0.2 Ma Hazen tephra. In the Knoll Creek area an angular unconformity marks the Knoll mbr.-Blanchard mbr. contact and an incised-valley filled with reworked ash from the Hazen tephra (the Cave mbr.) is cut into the Knoll mbr. along the axis of a fault-propagation syncline. The Bloody Gulch mbr. overlies aforementioned units and has an unnamed, 9.6 ± 0.3 Ma tephra at its base. The youngest tephra identified in the Bloody Gulch mbr. is the 8.4 ± 0.2 Ma Rush Valley tephra. Three tephra collected east of the Bell Canyon fault include the 10.7 ± 0.1 Ma Ibex Peak 19 tephra and the 11.3 ± 0.1 Ma and 11.6 ± 0.1 Ma Cougar Point IX and XI tephras, respectively.

The data indicate the Knoll Mountain fault was active by 9.8 ± 0.2 Ma, the Bell Canyon fault by 11.3 ± 0.1 Ma, and that adjacent basins were developed by these times. As the basins filled, an angular unconformity developed in the Knoll Creek area, possibly related to movement along the Knoll Mountain fault. This was followed by the development of the fault-propagation syncline above a blind Hice-Valder fault and fluvial incision and backfilling along the syncline axis by 9.6 ± 0.3 Ma. Incision and filling of the valley took place in less than 200,000 years prior to 9.6 ± 0.3 Ma. The timing of cessation of slip along the range bounding faults on the west occurred sometime after 8.4 Ma, and on the east, after 10.7 Ma. Overall, our data indicate that extension along Knoll Mountain is at least in part coeval with passage of the Yellowstone hot spot to the north and with extension in the Windermere Hills to the south.