Counting marmots…
ABSTRACT In Pennsylvanian and early Permian time north-central New Mexico and the adjacent part of Colorado were the site of the Rowe-Mora geosyncline—a narrow, north-trending depositional basin bounded on the west by the ancestral San Luis-Uncompahgre highland and on the east by the ancestral Sierra Grande uplift. The southern part of the geosyncline was divided by the Pedernal uplift, the main trough being at the west, and the Las Vegas sub-basin at the east. Sediments of the Magdalena group of Pennsylvanian age were deposited as a predominantly marine suite during formation of the geosyncline, and sediments of the overlying Sangre de Cristo formation of Pennsylvanian and Permian age were deposited as a predominantly non-marine suite which finally filled the geosyncline. During Laramide orogeny the troughal part of the geosyncline was elevated to form the Sangre de Cristo uplift and the shallower eastern part was downfolded to form the Raton basin. Rocks of the Magdalena group exhibit two facies. Thick limestone and sandstone and interbedded shale were deposited on a shelf in the southern part of the geosyncline. These rocks are as much as 2,500 feet thick. The shelf facies thickens northward and grades rapidly into a geosynclinal facies. The gradation occurs along a belt trending northeastward across the mountains and into the subsurface of the southern part of the Raton basin. The geosynclinal facies is dominantly black shale containing thick beds of sandstone and thin beds of limestone. This facies originally was more than 10,000 feet thick in the deeper part of the trough and is more than 5,000 feet thick in the subsurface of the western part of the Raton basin in Mora County. In the southern part of the geosyncline on the Pedernal uplift, the shelf facies of the Magdalena group is truncated locally by the Sangre de Cristo formation. At the east side of the Las Vegas sub-basin the shelf facies is truncated by the Sangre de Cristo formation, which rests on Precambrian rocks of the ancestral Sierra Grande uplift. At the north in Colfax County, on much of the eastern limb of the Raton basin, the Sangre de Cristo formation or Triassic rocks rest on Precambrian rocks of the ancestral Sierra Grande uplift. A thick sequence of the geosynclinal facies of the Magdalena group is present in the mountains at the west. These rocks wedge out beneath the Sangre de Cristo formation in the subsurface of the western part of the Raton basin. Thick black shales of the geosynclinal facies might be sources of petroleum; however, the interbedded sandstones and limestones are “tight.” Sandstones of the shelf facies generally are more porous and permeable. Limestones of the shelf facies could be the best reservoirs, and shows of oil and gas have been reported from these rocks in the Las Vegas sub-basin and near the southern end of the mountains. Small bioherms and associated calcareous clastic rocks occur at places in the shelf facies, particularly near the western margin of the Pedernal uplift, and may be present also in the subsurface along the eastern margin of the uplift and along the northeast-trending belt of transition from shelf to geosynclinal facies. Combination stratigraphic and structural traps may occur along the belts of wedge-out of the Magdalena on the Pedernal uplift and the west flank of the ancestral Sierra Grande uplift. Whether suitable reservoir rocks are present in these areas of wedge-out is not known. A thin but widespread sequence of Devonian(?) and Mississippian age contains cavernous limestone and breccia, but these rocks lie beneath possible source beds and rest on Precambrian crystalline rocks.
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