Detailed stratal tilt information for radioisotopically dated Tertiary rocks in the Lemitar Mountains of the central Rio Grande rift allows discrimination of two episodes of extension in the late Oligocene (largely 28.6 to 27.4 Ma) and middle to late Miocene (16 to 10 Ma). Rapid deformation in the late Oligocene (at least 25% extension) produced narrow, wedge-shaped accumulations of ash-flow tuffs and mafic lavas within half grabens defined by closely spaced (2 to 5 km) planar-rotational normal faults. Volcanic accumulation rates during this episode were sufficient to bury the developing fault-block topography, resulting in preservation of only minor amounts of intercalated sedimentary rocks. Following a period of weak extension in the early Miocene, rates of extensional strain again began to increase in the early middle Miocene (about 16 Ma) and continued to increase until about 10 Ma, when a more moderate rate ensued. During the time interval of 16 to 10 Ma, the Lemitar Mountains area was extended about 30% along widely spaced (5 to 15 km) planar-normal faults. Increased stratal-tilt rates in the middle Miocene caused the development of accommodation space to locally exceed sediment supply on the down-thrown side of a major intrabasinal fault (Silver Creek fault), resulting in topographic closure and the development of playa deposits within the lower Santa Fe Group (Popotosa Formation). As stratal-tilt rates further increased in the late Miocene, fluvial-lacustrine sedimentation offlapped toward narrow zones of maximum subsidence along active faults, causing the carving of widespread unconformities on older sediments on hanging-wall dip slopes. As tectonism waned in the latest Miocene-early Pliocene, sediment supply exceeded the development of accommodation space, resulting in onlap and aggradation of the upper Santa Fe Group (Sierra Ladrones Formation) that enabled the eventual integration of an axial-fluvial system (ancestral Rio Grande) between the longitudinally arrayed half grabens to the north and south of the Socorro Basin. The late Oligocene episode of rapid extension appears to be related to strain focusing by local thermal weakening due to voluminous concurrent magmatism in the area. In contrast, the late Miocene extensional event may be of regional scale, as shown by evidence for contemporaneous tectonism in other areas and the development of widespread unconformities between Miocene and Pliocene deposits of the Santa Fe Group in many basins of the Rio Grande rift.

Abstract Each of the sedimentary basins of the Rocky Mountain province is geologically unique. All have been structurally controlled but by different episodes and styles of tectonic activity. At least some of the structures have been reactivated one or more times during the Phanerozoic. Because of the great geologic diversity, no one worker could be authoritative about the geology of the province. Consequently, each basin in this chapter has its own specialist authors. They are identified with their appropriate texts and listed at the beginning of this chapter. Although an attempt has been made to standardize the format of the basin subchapters, some differences in style and geological focus appropriately reflect the differences among the various authors. Little attempt was made to force uniformity in details of approach to the basin analyses. Organization of the chapter is geographic. Basins west of the Rio Grande Rift system are stratigraphically more closely related to the Cordilleran miogeocline than to the craton and are treated first. These basins are described in order by age, and geographically from south to north. Those basins lying east of the Rio Grande Rift and more closely related to the craton are presented in order from south to north. Prior to Pennsylvanian time, the site of the modern Rocky Mountains was largely that of a broad, shallow marine shelf at the inner margin of the Cordilleran miogeocline. Late Paleozoic basins developed concurrently with the uplift of the Ancestral Rocky Mountains as the Precambrian basement structural fabric was strongly reactivated