60-20 Ancient Shells Tell Tales: Microfossils Indicate Provenance of Carbonate Gravels in the Ogallala Formation, Texas

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

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Abstract:

The Miocene-Pliocene Ogallala Formation gravels at Caprock Canyon and Janes Quarry contain diagnostic microfossil assemblages that provide critical insights into their Paleozoic source rock origins and test whether carbonate clasts derive from northerly sources in eastern New Mexico versus more distant southerly sources in the Trans-Pecos region. Through detailed microfossil analysis, I identified brachiopods, crinoids, echinoderms, and bryozoans that are characteristic of ancient marine shelf environments. These fossils occur as disarticulated skeletal elements rather than intact reef-building organisms, indicating transport from continental shelf deposits rather than reef complexes.

The microfossil assemblages strongly indicate derivation from eastern New Mexico Pennsylvanian carbonate formations, particularly the Sandia, Gray Mesa, and Atrasado formations. These formations contain diverse crinoid genera and species along with abundant brachiopod assemblages that formed in shallow marine continental shelf environments during the Pennsylvanian Period. The high diversity of fossil types observed in the Ogallala gravels matches the known fossil content of these Pennsylvanian units, providing strong evidence for a northerly source region.

The specific combination of fossil types observed—dominated by disarticulated skeletal elements and lacking massive corals or sponges—supports derivation from shelf environments rather than reef complexes. The fragmented nature of the fossils indicates mechanical transport and reworking during fluvial transport from the source region to the depositional site.

Paleodrainage reconstruction suggests that Miocene-Pliocene streams eroded Pennsylvanian outcrop belts in eastern New Mexico and transported carbonate debris eastward into the developing Southern High Plains. This eastward transport pattern supports a northerly source interpretation rather than derivation from Trans-Pecos carbonate formations. The geography of fossil transport—from New Mexico sources to Texas deposition sites—reflects the large-scale drainage reorganization that occurred during Ogallala Formation deposition.

This study demonstrates that microfossil analysis of detrital carbonate fragments can effectively determine provenance of complex polymictic gravels and provides insights into ancient river systems, erosion patterns, and Miocene-Pliocene paleodrainage evolution across the southwestern United States.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10855

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