T15 Lakes Past, Present, and Future—Archives of Climate Variability, Paleoenvironment, Geohazards, and Economic Resources I

Sunday, 9 October 2022
8:00 AM - 12:00 PM
607

PALEOENVIRONMENTAL CHANGE IN A PALEOGENE-AGED HIGH-ALTITUDE LAKE BASIN; SEDIMENTOLOGY AND FACIES CHARACTERIZATION OF THE SHEEP PASS FORMATION, NEVADA

Paper number: T15-3
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

The Sheep Pass Formation (SPF) type section, located in the southern Egan Range of east-central Nevada, is a >1 km thick package of non-marine, alluvial, fluvial, and lacustrine carbonate strata spanning the uppermost Cretaceous (app. 70 Ma) through the middle Eocene (app. 45 Ma) and is interpreted to have been deposited in a high elevation basin. Given its age and interpreted depositional setting, the SPF provides a unique record for terrestrial paleoenvironmental change during a known global greenhouse interval. Previous research characterized six sedimentologically distinct members (A-F) in the SPF, which include alluvial conglomerates, bioclastic and microbial lacustrine carbonates, fluvial carbonate-cemented sandstones, poorly lithified palustrine carbonates, and rare paleosols. However, no micro-scale sedimentological characterization has been completed to describe facies and sub-facies variations within members or laterally between multiple measured sections. We combined outcrop, hand sample, and thin section microscopy observations, as well as X-ray diffraction mineral compositional analysis from >150 samples collected across two measured sections in the SPF to classify sedimentological change at the micrometer-scale up to 100s of meters. We created a new facies characterization framework that accounts for features like porosity, siliciclastics, crystalline spar abundance, bioclasts, and microbialite fabrics to expand SPF sedimentology beyond previous member-wide facies generalizations. Our observations indicate that SPF facies are more varied than previously defined, particularly within the lacustrine and microbial carbonates in Member B. For example, we expanded the Dunham classification scheme to include sparry packstones and muddy grainstones, and defined over three unique thrombolite morphologies based on clot size, matrix abundance, and texture. Overall, we observe a marked difference in facies trends and inferred environmental conditions between the two measured sections, suggesting that proximity to the paleo-lake basin margin controlled differences in water depth, siliciclastic input, and biological activity.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379715

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