60-50 Fresh vs. Weathered: Basaltic Clast Alteration as a Key to Miocene Paleodrainage in the Ogallala Formation

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

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Abstract:

Basaltic clasts within the Miocene-Pliocene Ogallala Formation at Caprock Canyon and Janes Quarry exhibit distinct petrographic variations that provide insights into their volcanic source regions and transport history. This study tests whether these basaltic gravels derive from northerly sources in New Mexico and/or southern Colorado versus southerly sources in the Trans-Pecos region. A basalt pebble sampled from Caprock Canyons State Park (sample 25OGCC7) contains at least one olivine glomerocryst. Two samples from Janes Quarry (samples JA-16 and JA-17) both contain iddingsite, with the former having one olivine phenocryst remaining.

Petrographic analysis reveals a spectrum of basalt compositions and alteration states, ranging from fresh olivine-bearing varieties to highly weathered samples with complete iddingsite replacement. Sample 25OGCC7's olivine glomerocryst is fresh, indicating minimal alteration. Sample JA-16 is less weathered than JA-17 due to its possession of an olivine phenocryst; iddingsite forms when olivine comes into contact with water, which removes the iron atoms in the olivine structure. The variety of basalt types suggests derivation from multiple volcanic sources with different eruption ages and post-emplacement alteration histories.

The olivine glomerocryst of 25OGCC7 indicates that the sample is young, or that it was rapidly transported and buried. The two Janes Quarry samples' iddingsite indicates that those samples are older and/or were exposed to prolonged weathering. The mixing of variably altered basaltic clasts within Ogallala gravels indicates integration of volcanic debris from temporally and spatially distinct source terranes during Miocene-Pliocene drainage development.

Based on distance from potential sources and weathering characteristics of the samples, 25OGCC7 is thought to have originated from the Raton lava flows (northerly source), while JA-16 and JA-17 are thought to have originated from the Trans-Pecos lava flows (southerly source). This petrographic diversity of basaltic clasts provides a record of regional Cenozoic volcanism and demonstrates the utility of detrital volcanic analysis for understanding paleodrainage evolution. Evidence of water-weathered olivine indicates that paleorivers transported igneous rock from both the Raton and Trans-Pecos regions during the Miocene, building a more thorough understanding of southwestern United States geology during this period.

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

doi: 10.1130/abs/2025AM-8979

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