Memoir 48—Stratigraphic framework of upper Paleozoic rocks, southeastern Sangre de Cristo Mountains, New Mexico

By E. H. Baltz and D. A. Myers, 1999, 272 pp., 89 figs., 14 plates, 3 appendices, 30 sheets, scale 1:125,000.

The Sangre de Cristo Mountains of south-central Colorado and north-central New Mexico are the physiographic expression of a southerly trending Cenozoic structural uplift that plunges gently south to die out in the Great Plains south of Santa Fe and Las Vegas, New Mexico. The uplift is bounded on the west by Neogene downfaulted and downwarped basins of the Rio Grande depression and, on the east, by broad Laramide basins that have sharply folded western limbs. The uplift was modified in Neogene time by local igneous-intrusive doming and normal faulting related to the Rio Grande rift.

Pennsylvanian and Lower Permian rocks of the Sangre de Cristo Mountains and adjacent areas in north-central New Mexico are heterogeneous marine and nonmarine deposits that accumulated in and on the flanks of structurally deep late Paleozoic basins that subsided between large uplifts of the Ancestral Rocky Mountains. The facies and thicknesses of rock-stratigraphic units change rapidly laterally, reflecting their deposition in active tectonic environments near sources of terrigenous clastic sediments. The distribution of some major facies and the northward thickening of parts of the rocks have been known generally since regional-stratigraphic studies. Biostratigraphic studies extended a general knowledge of parts of the rocks northward in the central and western part of the mountains to the vicinity of Taos, New Mexico. In the topographically and structurally high central parts of the mountains only parts of the Pennsylvanian rocks are preserved from Cenozoic erosion, and Pennsylvanian and Lower Permian rocks have been subdivided and mapped only in the southern and southeastern margins of the mountains. Therefore, many aspects of stratigraphic variations, depositional history, and paleotectonics are poorly known.

A main objective of the study was to determine, document, and paleontologically date the highly complex variations in thickness and lithofacies that occur in Lower Pennsylvanian through Lower Permian formations between the southern Pecos shelf and the deep northern Taos trough and Rainesville trough. The mapping and biostratigraphic studies for this memoir establish a more complete regional Paleozoic stratigraphic framework than existed previously. Some new hypotheses about regional stratigraphy and paleotectonics are presented in the final section.

Plate 1 is a synthesis of previously published mapping and the detailed and reconnaissance mapping done for this report. The mapping and biostratigraphic studies for this report established a more complete regional Paleozoic stratigraphic framework than existed previously. This framework is useful for interpreting general depositional environments, sources of clastic sediments, paleotectonics, and Cenozoic structure. The framework should be useful also for future detailed analyses of highly complex local late Paleozoic depositional environments that are considered only briefly in this report.

The present study also included reconnaissance of other parts of the Sangre de Cristo Mountains, study of logs and some samples from wells in the Las Vegas Basin east of the mountains, and examination of geophysical data. Regional implications of the findings, and some new hypotheses about regional stratigraphy and paleotectonics, are discussed in a final sections of the report.     All the upper Paleozoic rocks are vertically and laterally heterogeneous, and at places pronounced lithologic changes occur in relatively small distances. Therefore, this report presents much documentation and discussion of many units that can't be portrayed adequately by generalized descriptions. Also, parts of the report contain more discussion and documentation of Cenozoic structure than might be expected in a report concerned primarily with Paleozoic rocks. However, there are many places where these rocks can't be interpreted reasonably without at least a general understanding of the present structure, and vice versa.