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Major sedimentary basins in this, the eastern part of the Basin and Range province, are the Orogrande and Pedregosa basins of Mississippian, Pennsylvanian, and Wolfcampian age, the San Mateo-Lucero and Estancia basins of Pennsylvanian age, the Carrizozo and Quemado-Cuchillo (Foster, 1957) evaporite basins of Leonardian age, the Early Cretaceous basin near the Hatchet Mountains, and the continental basins of vols canic piles of Late Cretaceous age near Elephant Butte and Steeple Rock. Numerous Cenozoic intermontane graben basins dot the region, with the southern part of a long north-south string of interconnected grabens now followed by the Rio Grande and called the Rio Grande structural depression. Sediments filling the Cenozoic basins are mainly of Late Miocene. Pliocene and Pleistocene in age.
Pre-Mississippian Paleozoic rocks remain only south of about 33°45^prime N. Lat. The basal Paleozoic unit, the Cambrian-Ordovician Bliss Sandstone, thickens depositionally southward and southwestward. The Early Ordovician El Paso Limestone thins northward due to intra-Ordovician erosion whereas the Middle and Late Ordovician Montoya Dolomite is relatively uniform in thickness where overlain by Silurian rocks. The Silurian Fusselman Dolomite thins northward partly due to erosion during Late Silurian and Early Devonian time. The Devonian shaly strata are relatively uniform in thickness, although marking the first large scale influx of clay and silt; as with all older Paleozoic rocks they appear to have been deposited in shallow epicontinental seas.
The Pedregosa basin was autogeosynclinal, receiving thick deposits of Middle Mississippian crinoidal limestones, Late Mississippian arenaceous calcarenites, Pennsylvanian limestones, and Wolfcampian interbeds of limestone, black shale, and redbeds. The Orogrande basin began as a poorly defined autogeosyncline in which siliceous Middle Mississippian limestones and Late Mississippian arenaceous calcarenites were deposited, then became zeugogeosynclinal during Late Pennsylvanian time as detritus was swept westward from the Pedernal landmass, and during Wolfcampian time was filled by limestones and shales that grade northward into redbeds. The Estancia basin was a small Pennsylvanian zeugogeosyncline, and the San Mateo-Lucero basins were autogeosynclines connecting the Pennsylvanian seas orthward toward the San Juan and Paradox basins.
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