Stratigraphy and Paleogeography Maps of the Rio Grande Rift in Northern New Mexico
The diagrams listed and discussed below are in press for inclusion in:
"Middle to Late Cenozoic Development of the Rio Grande Rift and Adjacent Regions in Northern New Mexico," by Gary A. Smith
in Geology of New Mexico
Edited by Greg Mack, Katherine Giles, and Virgil Lueth
New Mexico Geological Society Special Publication, 2003
Geologic Map of the Rio Grande Rift in Northern New Mexico
Stratigraphy of the Rio Grande Rift in Northern New Mexico
Paleogeographic Maps
The paleogeographic maps represent generalized volcanic, tectonic-upland, and depositional elements in the landscape averaged around five time slices at 30 Ma, 25 Ma, 15 Ma, 10 Ma, and 5 Ma. The maps cover the same area as a geologic map of the northern New Mexico part of the Rio Grande rift. The locations of the cities of Los Alamos (LA), Santa Fe (SF), and Taos (T) are shown in purple on each paleogeographic map for reference.
Brief text explanations are provided along with links to each color paleogeographic map. Complete explanations of the described phenomena, and citations of appropriate published work, are provided in the Geology of New Mexico manuscript (publication expected in late 2003).
Paleogeography ~30 Ma
At the close of the Laramide orogeny, in late Eocene, the area of the present Rio Grande rift was part of a broad uplift, more than 150 km across, which was largely denuded of Phanerozoic strata. Oligocene deposition occurred across large areas of northern New Mexico, and resulting strata rest upon Precambrian rocks in many places, suggesting inversion of Laramide structures. Oligocene strata include volcaniclastic sediment shed southward from the San Juan volcanic field, and derived from the Cerrillos Hills south of Santa Fe. Precambrian highlands in the Tusas and Sangre de Cristo Mountains, and Sierra Nacimiento were also sediment sources. The Tusas Mountains stood sufficiently high to divert drainages southeastward from the San Juan volcanic field. Precambrian-derived detritus eroded from the Tusas Mountains (Ritito Cgl; lower member of the Abiquiu Fm.) is interbedded with volcaniclastic debris on the north side of the range (Conejos Fm.), and extends for tens of kilometers southward where eastward thinning across faults along the west margin of the rift near Abiquiu implies early motion on rift-bounding faults. Precambrian clasts are also found along the southern margin of the Picuris Range, and constitute the Vallejos Fm. (of uncertain age) near the Colorado border. North of Santa Fe, volcaniclastic debris from the Cerrillos Hills (Espinaso Fm.; Bishop’s Lodge Mbr. of Tesuque Fm.) is interbedded with and overlies coarse gravel derived from Precambrian and Paleozoic rocks. These strata (lowermost Nambé Mbr of Tesuque Fm) mark the initiation of Santa Fe Group deposition in a nascent Española basin, superposed on the crest of the Laramide Pajarito uplift. The early basin may have been bounded by inverted fault motion on the Pajarito and Picuris-Pecos faults. Paleozoic clasts in the lowermost Nambé Member may be derived from Paleozoic outcrops east of the Picuris-Pecos fault.
Paleogeography ~25 Ma
An extensive apron of reworked pyroclastic debris extending south and southwest from the Latir volcanic field dominates latest Oligocene and early Miocene deposits. Abiquiu Formation strata, comprising part of this volcaniclastic apron, clearly thin westward across intrabasinal faults and the rift-bounding Cañones fault, indicating basin subsidence at this time. The Tusas Mountain topographic barrier was overtopped at about the time of eruption of the Amalia Tuff (25 Ma) to form the Questa caldera. Miocene uplift of the Santa Fe Range may have begun at this time by rupture of the Picuris-Pecos fault in the distal hanging wall of the Española basin. Basaltic, and locally basanitic, volcanism was widespread in northern New Mexico and southern Colorado.
Paleogeography ~ 15 Ma
The middle Miocene geography reflects the mature development of the currently recognized rift basins. Sediment flooded the basins from the north, largely eroded from the extinct volcanic San Juan and Latir volcanic fields with locally important contributions from Precambrian-rock uplands. Quartzite detritus from the Truchas Peaks area was dispersed into the Española basin around the southern and northern ends of the slowly rising Santa Fe Range. The Dixon Member of the Tesuque Formation, composed of admixed Precambrian- and Paleozoic-clast gravel derived from farther east, marks the course of the ancestral Rio Embudo. Volcanism began in the Jemez Mountains volcanic field, in the western Española basin at about 16 Ma. Tephra is widespread in the basin-fill strata, but is almost entirely related to distant volcanism in Nevada and Idaho, rather than in the nearby Jemez Mountains. Widespread eolianite of the Ojo Caliente Sandstone provides a prominent middle Miocene marker in the regional stratigraphy. Middle Miocene basin-fill sediment and basalt are currently preserved at high elevations in the Red River-Valle Vidal area; these rocks apparently represent part of the eastern step-faulted margin of the San Luis basin that was later raised in the Taos Range footwall uplift.
Paleogeography ~10 Ma
By 10 Ma, strong tilting in the San Luis and Española basins restricted deposition to areas adjacent to the footwall uplifts. Basin-floor tilting was enhanced by hypothesized hanging-wall hinge zone rupture that uplifted the Tusas Mountains and Santa Fe Range. The Peñasco basin was delineated as a part of the eastern Española basin that was not involved in Santa Fe Range uplift, and bounded on the north by the Picuris Range, actively uplifted along the Embudo fault. The Rio Embudo is marked by the distribution of the Cejita Member of the Tesuque Formation. Jemez Mountains volcanism was constructing substantial topography along the western margin of the Española basin, deflecting drainage eastward along the current Rio Chama valley. Aggradation ceased in the Abiquiu embayment at about 10 Ma. A coarse-gravel train marking the ancestral Rio Chama and an early Rio Grande in the west-central Española basin is clearly recognized in the southeastern Jemez Mountains prior to 7 Ma, and may have been established by the time of the pictured reconstruction.
Paleogeography ~ 5 Ma
Upper Miocene and Pliocene rocks delineate geography very similar to the present one. Deposition was strongly asymmetrical within both the Española and San Luis basins, and especially within graben near the basin master faults. The Jemez Mountains were constructed to their highest elevations prior to latest Pliocene and Pleistocene caldera-forming eruptions. Pliocene basalt volcanism formed the Taos Plateau, Ocate, and Cerros del Rio volcanic fields. At least one Ocate lava flowed along the course of the ancestral Rio Embudo into the Vadito area, indicating a headwaters for that drainage far to the east of the modern divide. Quartzite clasts near Santa Fe, suggest that the ancestral Pecos River flowed around the rising south nose of the Santa Fe Range and into the Española basin until piracy to its current course in late Pliocene or Pleistocene. Ancestral Rio Grande gravel, mostly fed from the Rio Chama watershed, interfingers with Jemez-derived volcaniclastic fans near Los Alamos, and continues southward into the Santo Domingo basin. Pliocene deposition in the Moreno and Mora valleys is related to extensional inversion of Laramide thrust faults.
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Updated December 2002
