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9 <br />7 <br />i <br />and Raton formations and are prominently displayed in the Picketwire <br />• ~ Valley. They range in thickness from a few inches to more than 20 feet <br />~ and are, for the most part, basaltic in composition. Quarternary deposits <br />will be described in the section dealing with the geomorphology of the <br />area. <br />Structure and History <br />The principal structural feature of the region is the Raton Basin, a <br />broad, asymmetric syncline whose axis trends northward. The eastern limb, <br />which includes the study area, is gently dipping relative to the steep <br />~ dips of the western limb. The Raton Basin was formed during the latter <br />phases of the Laramide Revolution when the Sangre de Cristo Range was <br />thrust eastward. Several steep, normal faults, trending east to northeast <br />and with displacements rarely exceeding 25 feet have been mapped between <br />~ Weston and Starkville (Wood, et al., 1957). <br />The geologic history of the region can 6e summarized in four phases <br />• ~ (Precambrian to late Paleozic, ~+tesozoic, Tertiary and Quarternary). During <br />the Precambrian many thousands of feet of sediments accumulated, were de- <br />farmed, intruded by large plutons and metamorphosed to primarily gneiss, <br />J schist and quartzite. A prolonged period of dominant erosion persisted to <br />early Pennsylvanian time. If early Paleozoic sediments were deposited, <br />they were subsequently removed by erosion. In early Pennsylvanian time a <br />transgression of the sea concurrent with uplift of the Ancestral Rocky <br />Mountains resulted in the deposition of detritus. By the middle of the <br />Pennsylvanian, regression of the sea commenced. Middle Pennsylvanian through <br />early Permian is represented by shallow marine and terrestrial sediments. <br />j <br />The Permian and Triassic are not represented by rocks in or near the study <br />area. <br />Early Mesozoic events included a transgression of the sea. The Ocate, <br />Wanakah(?), and Morrison formations in the study area appear to represent <br />coastal (beach), freshwater lake and floodplain environments. In early <br />Cretaceous time a major transgression of the sea from the south occurred. <br />Sandstones of the Purgatoire formation represent beach and offshore bar <br />• - deposits and shales resulting from low energy environments such as coastal <br />