Determining the source of the Ogallala Formation: Sedimentary petrology of the Ogallala Formation, Southern High Plains, West Texas and Eastern New Mexico



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Samples of sand and gravel from the Ogallala Formation at 40 sites around the Southern High Plains allow for an assessment of the bulk composition of Ogallala sediment, and an analysis of regional variation in composition. Ogallala sand and gravel are rich in lithic grains predominantly derived from sedimentary rocks, with lesser amounts derived from metamorphic and volcanic rocks. Feldspar content is very low. The basic composition of Ogallala sand and gravel is relatively uniform over the Southern High Plains study area, but with gradational variation between northern, central, and southern sample sites. The relative abundance of feldspar, volcanic lithic grains, and metamorphic lithic grains diminishes from north to south. In contrast, the abundance of sedimentary lithic grains, and in particular, carbonate rock fragments increases to the south.
Samples of pebble and cobble sized lithic grains collected from Ogallala gravel outcrops around the periphery of the High Plains provide examples of the actual Ogallala metamorphic, volcanic, and sedimentary source rocks. Of the plutonic and metamorphic rock types recognized in Ogallala pebbles, most or all appear to have been derived from the Sangre de Cristo Mountains and nearby areas in northern New Mexico. Pebbles comprised of volcanic and shallow intrusive rocks also appear to be mostly or entirely derived from source areas in northern New Mexico, in particular the Latir volcanic field, Cimarron Pluton, Sierra Grande, the Clayton-Raton and Ocate volcanic fields. Most of the sedimentary rock types recognized in Ogallala pebbles were derived from the Permian, Triassic, and Cretaceous sedimentary rocks exposed directly below and adjacent to the basal Ogallala contact and transported only a short distance from their points of origin. Pebbles of more distant origin include arkosic and lithic sandstones derived from the Pennsylvanian Sandia and/or Sangre de Cristo formations, and fossiliferous Pennsylvanian limestone pebbles likely derived from the Madera Formation, all of which are exposed along the eastern sides of the Sangre de Cristo, Sandia, and Manzano mountains. Some of the sandstones from the Jurassic Exeter Sandstone and Morrison Formation, the Cretaceous Dakota and Mesa Rica Sandstone, limestones from the Cretaceous Greenhorn and Niobrara formations, and gryphaeid coquina from the lower Glencairn Formation are also represented on Ogallala gravel. All these units are exposed along the front range of the Sangre de Cristo Mountains, in the Cimarron valley, and along the Canadian breaks in northeastern New Mexico. These general trends are compatible with derivation of Ogallala sediment from a single primary source area north/northwest of the High Plains, and transport of the sediment via a major trunk stream to the south/southeast with increasing addition of sedimentary lithic grains from tributary drainages in transit. Alternative depositional models for the Ogallala would require that sediment was contributed from three or more source terrains by separate stream systems with headwaters in different ranges extending to central and southern New Mexico. The petrology of Ogallala sand and gravel, and the distribution of Ogallala pebble lithotypes are generally incompatible with these alternative hypotheses.



Provenance, Ogallala, Southern Rocky Mountains