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L <br />8 <br /> lt <br />A <br />d <br />d fl <br />i <br />t <br />d <br />l <br />f <br />d <br />• <br /> re <br />swamps <br />e <br />as, an <br />ns. <br />oo <br />p <br />a <br />reat o <br />the sea pro <br />uced erosion of <br />i the Purgatoire before the next incursion, still in early Cretaceous time. <br /> A broad, shallow sea invaded the area depositing the massive quartz sand- <br /> stones and thin interbedded shales of the Dakota formation. As the sea ad- <br /> vanced and water depth increased, mud, silt, fine sand and calcium carbonate <br /> were deposited and lithified to form the Graneros Shale, Greenhorn Lime- <br /> stone, Carlile Shale, Pliobrara formation and Pierre Shale. Apparently <br /> epeirogenic movements that signalled the beginning of the Laramide Revolution <br />J are recorded by the fine sands in the upper Pierre. The sea withdrew to the <br /> east with the Trinidad formation being deposited in the high energy coastal <br /> environment. As the sea continued to retreat, mud, silt, sand, and carbona- <br />~ ceous material of the Vermejo formation were deposited in floodplain, delta <br /> and swamp environments. <br /> In latest Cretaceous time source areas to the west were being uplifted <br />' and, relatively coarse sediments being shed to the study area as the Raton <br /> Basin began to form. These sediments are preserved as the upper Vermejo and <br />( Raton formations and include the Cretaceous-Tertiary boundary. A second . <br />l pulse of tectonism, in the Paleocene resulted in the shedding of coarse <br /> elastics from the west to form the lower beds of the Poison Canyon formation. <br /> Following deposition of the Poison Canyon, tectonic forces uplifted <br /> the;Sangre de Cristo Range and created several open synclinal folds to <br /> the east. Sediments of the Cuchara formation were deposited in environ- <br /> ments similar to the Poison Canyon. The sediments were folded before being <br /> eroded from the study area. The Eocene Huerfano formation was deposited <br /> on piedmonts and floodplains during a time of thrust faulting that estab- <br /> lished the present structure of the Sangre de Cristos. The Laramide <br /> Revolution culminated with deepening of the Raton Basin, uplifting of the <br /> Sierra Grande uplift, nornal faulting and the intrusion of acidic to basic <br /> igneous rocks. Later in Tertiary time periods of erosion and deposition <br /> occurred followed by erosion to present levels. <br />rI Geomorphology <br />The geomorphic history of the region is complex and replete with • <br />numerous controversial aspects. Levings, 1951, provides a comprehensive <br />