TEPHROCHRONOLOGIC AND STRUCTURAL CONSTRAINTS ON THE AGE OF THE MIOCENE-PLIOCENE (?) HUMBOLDT FORMATION AND ASSOCIATED MAMMALIAN FAUNA FROM CHALK SPRINGS NEAR KNOLL MOUNTAIN, NORTHEASTERN NEVADA AND IMPLICATIONS FOR THE TIMING OF EXTENSIONAL FAULTING

The Humboldt Formation (Fm.) west of Knoll Mountain represents synextensional basin fill in the hanging walls of the west-dipping Knoll Mountain fault and a synthetic intrabasin fault transecting the middle of the basin. The first reported ages of the Humboldt Fm. in this area are from Macdonald (1949), who documented an assemblage of mammalian fauna near Chalk Spring in the center of the basin. Macdonald found that the ranges of genera overlapped in the Clarendonian and hence he assigned a Clarendonian age (~ 13-9 Ma) to the Humboldt Fm.

New geologic mapping indicates that Macdonald’s fossil localities are in the hanging wall and footwall of the intrabasin fault. The Blanchard, Knoll, and Bloody Gulch members of Humboldt Fm. are exposed in the footwall, and were deposited during slip along the Knoll Mountain fault. These members yield 16 to 8 Ma (Barstovian, Clarendonian, and Hemphillian) tephra that provide an indirect age of slip along the Knoll Mountain fault. Furthermore, footwall fossil locations are present between ~ 12 and 9.3 Ma tephra, corroborating Macdonald’s Clarendonian age. A newly recognized member of the Humboldt Fm., the Eagle Flat member, is restricted to the hanging wall of the intrabasin fault, and represents basin fill deposited during slip on the fault. Although tephrochronologic data from the Eagle Flat member is not available, its age can be inferred on the basis of cross cutting relationships and the ranges mammalian genera. The intrabasin fault cuts 8 Ma tephra in its footwall indicating that the fault and the Eagle Flat member are younger than 8 Ma, and the ranges of several genera found in this member do not extend beyond the ~ 4.8 Ma Hemphillian-Blancan boundary. Consequently, the intrabasin fault, and the Eagle Flat member and associated fauna, are < 8 to > 4.8 Ma indicating a Hemphillian age.

In summary, the Humboldt Fm. and associated extensional faulting ranges from ~16 Ma to < 8 - > 4.8 Ma. These data indicate that 1) some of the mammal fossil localities are younger (Hemphillian) than previously recognized, 2) the intrabasin fault is younger than the main basin-bounding fault, and 3) a more protracted, younger extensional history is present in the Knoll Mountain region contrasting with other areas in NE Nevada that are characterized by major Miocene extension predating 8 Ma.