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The objective of this project is to identify and characterize depositional processes by chemically imaging patterns of heavy mineral sorting in siltstones and sandstones of the Middle Permian Brushy Canyon Formation. To date, we have conducted two field trips to measure sections and collect samples in outcrops of the Salt Flat Bench. We have scanned outcrop and core samples. This work is contributing to thesis projects for a PhD student and an MS student. Three major lines of investigation have emerged from our initial results.

Windblown Dust

Windblown dust is a potentially important but difficult-to-quantify source of siliciclastics for sedimentary basins worldwide. Positively identifying windblown deposits requires distinguishing them from other low density suspension transport deposits. For instance, laminated very fine grained sandstones and siltstones of the Upper Permian Brushy Canyon Formation have been variously interpreted as 1) the deposits of slow-moving, low-density turbidity currents, 2) distal overbank deposits of turbidity currents, 3) the deposits of turbulent suspensions transported across a pycnocline (interflows), and 4) windblown dust. This facies forms the bulk of Brushy Canyon Formation slope deposits, so understanding its origin is critical to understanding the evolution of the basin as a whole.

Our geochemical mapping shows that these rocks are up to two times enriched in very fine sand sized zircon grains relative to Bouma A divisions of associated turbidites with similar grain sizes, suggesting substantial turbulence during transport that prevented substantial grain interaction with bed surfaces. However, in contrast with the A divisions, the laminated sandstones and siltstones never show evidence of scour or amalgamation, implying that flow turbulence did not interact with underlying beds. These observations are most consistent with windblown interpretations for Brushy Canyon Formation slope sediments. Proximal loess deposits are often characterized by elevated Zr/Al, supporting this interpretation. If correct, this implies that evolution of this early deepwater slope system was controlled largely by short-distance aeolian transport of very fine sand and silt from the coast.

Heavy mineral incorporation into Brushy Canyon Formation slope deposits as reflected in laminae-scale bulk Zr and Ti abundances may preserve a long-term record of local wind intensity during the Middle Permian. If so, then relative Zr and Ti abundances should be correlatable between different vertical sections. A PhD student, Ms. Kannipa Motanated, is currently measuring heavy mineral abundances in siltstones collected from several sections below and above the Salt Flat Bench sandstones to test this prediction.

Turbidite A Divisions

Zircon and rutile grains in turbiditic structureless sandstones (Bouma A divisions) of the Brushy Canyon Formation have calculated terminal fall velocities significantly less than associated feldspar grains, despite the fact that all three grain populations are very well sorted. Indeed, standard deviations of grain sizes for these populations are statistically indistinguishable, suggesting that they all had identical fall velocity distributions. The most likely explanation for the discrepancy in calculated fall velocities is that all grains settled through a dense suspension of sediment, and that the lighter and larger feldspar grains were more hindered by grain interactions than the denser and smaller zircon and rutile grains. Preliminary calculations based on effective medium theory suggest that Brushy Canyon Formation turbidity currents were hyperconcentrated, with suspensions more than 20% sediment by volume. Ms. Motanated is currently leading a team of undergraduate researchers in refining these calculations using settling experiments.

Sedimentary Structures and Sorting in Siltstones

Geochemical scans in slope siltstones and channel facies associated with bypassing turbidity currents have revealed heavy mineral lags and stranded ripples not visible in outcrop or slab. An MS student, Mr. Spencer Gunderson, is currently comparing channel siltstones and slope siltstones on the basis of heavy mineral structures in order to develop a model of currents and sediment transport during times when sand was not being deposited on the Brushy Canyon Formation slope.

Impact on PI’s Career

This grant has allowed the PI to branch into a new area of sedimentology with application to petroleum geology, and has already spurred contacts with industry that have led to access to new core for other research projects and potential future sources of funding. The project has also spun off an interesting avenue of research into the causes of very early diagenesis in organic-rich fine-grained sediments that is now being pursued by a new MS student in an active gas shale play. It has indirectly led to collaborations with researchers at other universities in areas well outside of the PI’s previous area of expertise.

Impact on Students’ Careers

Mr. Gunderson reports that this grant has given him credible experience in a petroleum-productive basin that has allowed him to successfully pursue an internship and a long-term job offer from a petroleum company. Ms. Motanated has received direct financial support from this grant. Her career goal is to become a professor in petroleum and sedimentary geology. Her research experience as part of this project is therefore directly relevant to her long-term career.