44-5 Taphonomic and Geochemical Investigations of Variegated Bivalves in a Sabkha Paleoenvironment: The Middle Jurassic Gypsum Springs Formation, WY, USA

Session: Paleontology, Paleoecology/Taphonomy, Phylogenic Morphological Patterns (Posters)

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Abstract:

Fossiliferous horizons in the Middle Jurassic (Bajocian-Bathonian) Gypsum Springs Formation provide an opportunity to investigate the geochemical conditions and taphonomic processes of shell growth and preservation in a dynamic sabkha depositional paleoenvironment.  Throughout the Western United States, the Gypsum Springs Fm. sits unconformably over earlier Mesozoic predominantly terrestrial strata. The unit preserves cyclical flooding and evaporation in northern subtropical paleolatitudes associated with progressive transgression of the epicontinental Sundance Seaway. Modern hypersaline mud flats in restricted coastal basins in the subtropics exhibit rare chemical concentrations and low shelly invertebrate biodiversity, and may serve as potential preservational analogs to the Gypsum Springs fauna. 

We present new stratigraphic log data measured in outcrop near the Sheep Mountain Anticline in the Bighorn Basin of WY.  From these data we interpret a regional succession of arid coastal sub-environments of deposition with varying marine and continental carbonate and clastic sediment inputs. In the middle of the measured stratigraphic thickness monospecific assemblages of the bivalve Staffinella(?) are abundant in interbedded gray and pink carbonate mudstones and wackestones. Although the fossils are widely observed throughout the lateral extent of the formation, preservation is locally patchy with the best preserved specimens exhibiting variable glauconite and iron oxide replacement mineralization as green, yellow and red color differences between and within individual specimens. We analyzed new collections of these assemblages using cross sectional and elemental map SEM-EDS which reveals Si, Fe, Mg and Ti dominance associated with the variable coloration, and distinctly different chemistry from the micritic host lithology. From these data we address questions of the preserved growth histories of individual bivalves, variability across collection sites throughout the unit’s lateral and vertical extent, and replacement sequences in early and later fossil diagenesis. 

Color and chemical differences through a series of an individual bivalve’s growth lines seem to indicate sclerochronological chemical controls, while other observations of green-yellow color variegation within and between single specimens seem to indicate local and post-depositional diagenetic alteration. In addition to presenting a paleoenvironmental interpretation from a new study site of this geographically widespread formation, this study provides a new look at the environmental geochemistry of syn-growth shell accretion and early-phase taphonomic replacement in the Gypsum Springs fauna. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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