AAPG ACE 2018

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Rock Magnetic Cyclostratigraphy of the Pliocene and Quaternary Sediments of the Fisher Valley Basin, Utah

Abstract

At ~140 meters thick, the Pliocene and Quaternary basin-fill sediments of Fisher Valley, Utah are likely the best and largest record of latest Cenozoic/Quaternary conditions in the entire Colorado Plateau. These sediments are second-cycle red bed deposits derived from Quaternary erosion of upper Paleozoic and Mesozoic sedimentary bedrock atop the rising Onion Creek Salt Diapir. During deposition of these sediments, cyclic changes in climate in conjunction with periodic pulses of diapiric uplift resulted in the development of crude cycles within these deposits. Cycle bases are coarser sand and gravel, and these grade upward into a massive deposit of finer eolian sand, terminating with the development of a paleosol. Here we attempt to extract a rock magnetic cyclostratigraphy from these sediments, which has the potential to better define the timing and duration of sedimentation and the Quaternary uplift of the Onion Creek Salt Diapir.

Samples were collected at 25 cm intervals throughout the entire section, for a total of ~550 samples. These samples were then prepared by manually removing any organic material, gently crushing the sediment, and then packing sediment into nonmagnetic plastic 7cc cubes, which serve as the primary specimens for collecting rock magnetic cyclostratigraphy data. An AGICO MFK1-A kappabridge was used to measure the bulk magnetic susceptibility (χ) for each of these specimens. After data collection, spectral analysis was used to filter the continuous cyclic signals, attributed to climate change, from the more sporadic noise, attributed to diapir uplift. Cyclic variations became evident once the χ-data was smoothed and plotted in relation to depth. These cycles are potentially the result of astronomical variations; however, the actual Milankovitch forcing is unclear at this time.

Currently, available data show that a rock magnetic cyclostratigraphy can be obtained from the upper Cenozoic Fisher Valley deposits, which can later be used to conjunction with other rock magnetic parameters and dating methods (magnetostratigraphy) to prepare a high-resolution age model for these deposits. This age model will then be used to more accurately date the timing and duration of sedimentation and various pulses of movement the Onion Creek diapir underwent during the Quaternary.