Abstract

More than 140 m of upper Cenozoic basin-fill sediments were deposited and then deformed in Fisher Valley between about 2.5 and 0.25 m.y. ago, in response to uplift of the adjacent Onion Creek salt diapir. In addition to these basin-fill sediments, minor amounts of eolian and fluvial sand were deposited in Holocene time. The sediments, whose relative ages are known from the stratigraphy, are predominantly sandy, second-cycle red beds derived from nearby Mesozoic rocks; most were deposited in a vertical sequence, filling a sedimentary basin now exposed by fluvial dissection. We have applied a variety of established and experimental dating methods to the sediments in Fisher Valley to establish their age and to provide time control for the recent history of the Onion Creek salt diapir.

Volcanic ash beds provide time datums at 610,000 yr (Lava Creek Ash) and 730,000 yr (Bishop Ash); the related eruptions have been dated by potassium-argon (K-Ar) and fission-track methods. Paleo-magnetic stratigraphy also provides time markers at the Brunhes-Matuyama boundary (730,000–790,000 yr) and at the Matuyama-Gauss boundary (2.48 m.y.); normal events within the Matuyama chronozone are missing or are obscured by secondary remagnetization. Uranium-trend analyses yield ages of 530,000 yr for the Lava Creek Ash horizon, 210,000–240,000 yr for the top of the basin-fill sediments, and 9,000 yr for Holocene eolian sand. Average accumulation rates of secondary carbonate and clay in the ten paleosols in the section were calculated from the amounts of these materials above the Lava Creek Ash and the Brunhes-Matuyama boundary; these rates yield estimates of 260,000 yr (carbonate) and 320,000 yr (clay) for the top of the basin fill. Amino-acid analyses of bulk soil materials are complex, but some data appear to change systematically with relative age. Thermoluminescence (TL) analyses proved inapplicable because of residual TL at the time of deposition and because the age range of the method was probably exceeded. Holocene deposits yield radiocarbon dates between modern and 9,300 yr B.P.

These age estimates provide a time framework for the history of late Cenozoic deposition and geomorphic change in Fisher Valley, and they constrain the late history of deformation of the subjacent Onion Creek salt diapir. The use of multiple dating methods in a well-established stratigraphy lends confidence to several critical age estimates and provides needed tests of experimental dating methods.

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