Date of Award
12-2025
Degree Type
Report
Degree Name
Master of Science (MS)
Department
Geosciences
Committee Chair(s)
Benjamin Burger
Committee
Benjamin Burger
Committee
Evey Gannaway Dalton
Committee
Joshua Lively
Abstract
The Upper Triassic Cow Branch Formation of the Dan River Basin, located in southern Virginia and northern North Carolina, has long captivated generations of geoscientists. Since the 1970s, researchers have sought to unravel the mystery surrounding the depositional environment of this ancient lake system. This world-class lagerstätte hosts a diverse assemblage of fossil fauna and flora, with a notable preservation bias toward fossil vertebrates, most prominently the archosauromorph, Tanytrachelos ahynis. The geology of the Cow Branch strata continues to evoke a persistent question: Are these sediments representative of a shallow or deep lake?
I undertook a petrographic and geochemical study of the Cow Branch Formation to examine the unique depositional environment that led to this remarkable fossil assemblage. The petrographic study was done on rock samples collected from road and railway cuts around the Cow Branch depositional center. Samples were also analyzed for total organic carbon (TOC) percentages and examined for 87Sr/86Sr isotope ratio values to test hypotheses of a shallow or depth lake depositional environment relative to the preservation of these remarkable fossils.
The rock samples collected from Cow Branch Formation contain angular clasts, possible fragile detrital muscovite grains, highly chemically weathered quartz grains, remnants of highly altered feldspar, and possible glauconite – all of which are likely indicative of an immature sediment source feeding a deep lake with steepening slopes. Geochemical analysis indicates high levels of organic carbon (14.73% – 12.23%) within the black shales of the Solite Quarry in the Cow Branch Formation, which preserves most of the fossils, while the organic carbon in underlying strata is significantly less (0.15% – 0.46%), indicating a deepening anoxic lake. Geochemical results show relatively high levels of 87Sr/86Sr isotope ratios, indicating an isolated freshwater lake without connection to marine saline waters of the proto-Atlantic Ocean, within a region influenced by continental uplift. My research supports that the Cow Branch Formation consisted of a deep lake environment, geographically isolated and situated within the steep topography of the Appalachian Mountains. This suggests a unique environment for an aquatic fauna of reptiles, indicating a possible alpine reptilian fauna in the warmer climates of the Triassic.
Recommended Citation
Workman, Gina M., "The Depositional Environment of the Triassic Cow Branch Formation, Dan River Basin of Virginia and North Carolina Through Petrographic Study and Geochemical Analysis, and in Relation to Taphonomic Processes and Vertebrate Morphology" (2025). All Graduate Reports and Creative Projects, Fall 2023 to Present. 127.
https://digitalcommons.usu.edu/gradreports2023/127
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