GEOLOGY OF THE ALAMOSA 30’ X 60’ QUADRANGLE, COLORADO — AN EAST-WEST TRANSECT ACROSS THE SAN LUIS BASIN OF THE NORTHERN RIO GRANDE RIFT

The geologic map of the Alamosa 30’ x 60’ quadrangle reveals a geologic transect across the central San Luis Basin of southern Colorado, the largest of the northern Rio Grande rift basins. The basin geometry reflects an east-dipping structural half-graben, bound on the east by numerous, dominantly down-to-west normal faults of the Sangre de Cristo fault zone. East of the fault zone, the structurally uplifted, Precambrian-cored, Sangre de Cristo Mountains rise to an elevation of nearly 4,300 meters, almost 2,000 m above the valley floor of the actively subsiding basin. Superposed on the half-graben are fault-bound structural highs and intervening lows, forming closed sub-basins. From west to east the north-trending Las Mesitas, Sunshine Valley-Basaltic Hills, and Sanchez sub-basins are separated by structural highs that are flat-topped mesas at the San Luis Hills and San Pedro Mesa; the sub-basins record an eastward migration and younging of active depocenters. The western flanks of the San Luis Hills and San Pedro Mesa form the footwall of the down-to-west Manassa fault and western Sangre de Cristo fault zone respectively. The youngest deposits offset by the Manassa fault are late Oligocene, whereas the Sangre de Cristo fault in the San Pedro Mesa area offsets deposits as old as Miocene and as young as middle Pleistocene. Holocene and late Pleistocene surficial deposits are offset adjacent to the Sangre de Cristo Range front. The San Luis Hills preserve Conejos Fm. volcanic and 28-30 Ma intrusive deposits of the Southern Rocky Mountain volcanic field, more typically associated with the San Juan Mountains to the west, and are unconformably overlain by ~26 Ma basaltic lavas of the Hinsdale Formation. Basaltic lavas of the Hinsdale Fm. locally exhibit fanning dips adjacent to mapped normal faults suggesting syntectonic, extensional faulting related to early rifting. However, erosion of the Conejos Fm. volcanic deposits to their plutonic underpinnings suggests rapid unroofing of the San Luis Hills area prior to eruption of Hinsdale Fm. basalts. Consequently, rift initiation was not entirely coincident with the onset of basaltic volcanism but was preceded by development of at least local early-rift depocenters, the westernmost of which were later abandoned.