Use of Rock-magnetic Methods in Determining the Effects of Paleoclimatic Change and Halokinetic Deformation on Terrestrial Sedimentary Deposits: Examples From the Quaternary Blackwater Draw Formation (Texas), Quaternary Harpole Mesa Formation (Utah), and Permian Undifferentiated Cutler Group (Utah)
Stine, Jonathan Michael George
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This dissertation consists of three separate projects investigating how changing environmental and tectonic conditions effect the rock-magnetic properties of sedimentary redbeds in Texas and Utah. The first project, discussed in Chapters 2-3, is an environmental magnetic study of the Pleistocene-aged Blackwater Draw Formation, located in the Southern High Plains (SHP) region of Texas. I constructed a three-phased paleoenvironmental model, by correlating the bulk magnetic susceptibility (ꭓ), bulk magnetic susceptibility of Anhysteretic Remanent Magnetization (ꭓARM), Isothermal Remanent Magnetization (IRM) values vs. depth curves to various textural and geochemical datasets. In this model, the basal-most section correlates to a more arid environment where eolian magnetic particles sourced from the southern Pecos River experienced relatively little weathering, thus preserving higher magnetic values. The second phase correlates to a more humid time period where a mixture of southerly Pecos River sands and northerly derived glacial loess were deposited and then weathered at intensities substantial to result in depletion of the magnetic signal. Finally, the third phase correlates to a semi- humid/semi-arid time period where the eolian sediments (sourced from both the North and the South) were subjected to precipitation levels greater than Phase 1 but less than Phase 2, which allowed for the neoformation of superparamagnetic ultra-fine-grained magnetite during pedogenesis, resulting in a magnetically enhanced signal. The second project, discussed in Chapter 4, involves an environmental magnetic study of the Harpole Mesa Formation, a sequence of Pliocene to Upper Pleistocene-aged basin-fill sediments, which were deposited directly adjacent to the Onion Creek Salt diapir (OCSD), located in Fisher Valley, Utah. The ꭓ– and ꭓARM – values vary cyclically with depth, and thus are tentatively interpreted to correlate to astronomically forced 100-ka eccentricity cycles. Moreover, the percent frequency dependence of magnetic susceptibility %ꭓFD show a negative trend from the Bishop Ash (~0.774 Ma) to the Lava Creek B ash layer (~0.631 Ma). However, this negative trend is interpreted to reflect a faster rate of basin-subsidence due to faster rise of the OCSD, which in turn caused the migration of the medial portion of the alluvial-fluvial system in a process analogous to the retrogradation of shorelines along continental margins. This change in sediment sorting rates is the primary cause for the relative increase in %ꭓFD for those sediments coeval with this halokinetic event. This indicates that although the paleoclimate signal is preserved, it is mixed with the halokinetic signal, therefore care should be taken when inferring a paleoclimate record from this important Quaternary sedimentary sequence. The final project investigates the anisotropy of magnetic susceptibility (AMS) of the Permian aged undivided Cutler Group, which are exposed immediately north of the OCSD. These late Paleozoic sediments form a halokinetic structure known as a salt shoulder, characterized as the low-angle segment of the salt-host rock sediment interface where the margin of a passive salt- diapir, -stock, -wall, or -glacier steps abruptly inboard (i.e., towards the center of the diapir). To better understand how this structure forms, independently oriented samples were collected from 83 sites, and the magnetic petrofabric investigated via AMS. Overall, the AMS results indicate that sediments of the undivided Cutler Group within the OCSD-salt shoulder were likely syndepositionally deformed. Evidence for this conclusion include: the primarily oblate fabric, relatively low corrected anisotropy degree values, and the fact that the majority of specimens yield a magnetic foliation that is parallel to bedding. Nevertheless, on a larger scale the specimens display a mangetic lineation that are largely oriented NW-SE, which is oblique to paleoflow directions (~NE-S), while being parallel to the elongate shape of the OCD. Moreover, on a more local scale the magnetic lineations are roughly parallel to smaller scale halokinetic folds adjacent to the salt-sediment interface. This indicates that the magnetic lineations are recording halokinetic deformation due to late Paleozoic salt flow of the OCSD. Overall, the results of this dissertation show the power in using a holistic approach when conducting rock-magnetic studies. Therefore, to more robustly support the conclusions implied from the rock-magnetic data, it is highly recommended that one has a clear understanding of the geologic setting of the research area while using independent data to support one’s paleoclimate or structural models.