Tectonic and Sedimentary History of Raton Basin and Notes on San Luis Basin: ABSTRACT

The Raton basin of northeastern New Mexico and southeastern Colorado is a Laramide structural basin bounded on the west by the Sangre de Cristo uplift, on the north by the Wet Mountains uplift and Apishapa arch and on the east by the Sierra Grande and Las Animas arches. The basin is strongly asymmetrical and the northerly-trending axis is near the Sangre de Cristo uplift. The intrabasinal Cimarron arch separates the structurally deeper northern part of the Raton basin from the shallower, southern, Las Vegas subbasin. During most of Paleozoic time the Raton basin and its bounding uplifts were part of the relatively stable "Continental backbone." The oldest known sedimentary rocks in the basin are Devonian (?). In Early Pennsylvanian the Rowe-Mora basin was formed in the area of the present Sangre de Cristo uplift and the western half of the present Raton basin. The Rowe-Mora basin was bounded on the west by the intermittently rising San Luis uplift and on the east and north by the ancestral Sierra Grande Apishapa and Wet Mountains uplifts. An unstable-shelf facies of the Magdalena Group of Pennsylvanian age in the southern part of the Rowe-Mora basin is 1,500-2,500 feet thick. These rocks grade abruptly northward into a geosynclinal facies which is as much as 6,000 feet thick in the Las Vegas subba in. The Magdalena Group is absent from the Cimarron arch, but it probably is present in the western half of the northern part of the Raton basin where it may be 4,000 feet thick. Orogenic debris of the Sangre de Cristo Formation of Pennsylvanian and Early Permian age was derived mainly from the San Luis uplift, filled the Rowe-Mora basin, and lapped into Precambrian rocks of the other bounding uplifts. The Sangre de Cristo Formation is 700-3,500 feet thick at the south, and 6,000-10,000 feet thick at the north. Higher Permian rocks and Upper Triassic and Upper Jurassic rocks have average aggregate thicknesses ranging from 2,300 feet at the south to 1,100 feet at the north. These deposits blanketed the entire region and buried most of the late Paleozoic uplifts. Cretaceous shales interbedded with some sandstones also blanketed the region. These rocks are about 4,500 feet thick at the north, and remnants in the Las Vegas subbasin are 900-1,000 feet thick. The latest Cretaceous and early Tertiary rocks, which are about 12,000 feet in aggregate maximum thickness in the northern part of the Raton basin, were derived mainly from the rejuvenated San Luis uplift. During early and middle Tertiary the western part of the Paleozoic Rowe-Mora basin was elevated to form the Sangre de Cristo uplift, and the present Wet Mountains uplift and the Apishapa, Las Animas, and Sierra Grande arches were formed. In late Tertiary the San Luis uplift was tilted eastward and its eastern part foundered to form the northern part of the complex Rio Grande trough. The San Luis basin in southcentral Colorado is the northeastern part of this tilted and faulted block. The western part of the basin merges into the eastern flank of the San Juan dome. The eastern and northern boundaries are a complex fault zone along the western margin of the Sangre de Cristo uplift which merges, around the northern end of the basin, with the Sawatch and Gunnison uplifts. Much of the San Luis basin is filled with upper Tertiary and Quaternary sediments and interbedded andesites and basalts that are at least 2,000 feet thick locally. This basin fill rests on lower and middle Tertiary volcanics that are related to the volcanics of the San Juan Mountains. Because the region of the San Luis basin was a part of the Paleozoic and Laramide San Luis uplift, it is doubtful that extensive areas of Paleozoic and Mesozoic rocks are preserved beneath the volcanics. End_of_Article - Last_Page 1875------------