Tectonics and Depositional Environments in Early Pennsylvanian of South-Central New Mexico
The Pennsylvanian Magdalena Group unconformably overlies Precambrian to Mississippian strata throughout south-central New Mexico. Regional southward tilting, warping, and erosion resulted in progressively older pre-Pennsylvanian subcrop relations from south to north. Normal marine shelf sedimentation was represented by upward shoaling cycles containing mixed siliciclastics and carbonates. Channels cut into Ordovician strata in the southern Caballo Mountain area suggest that this area was a relatively deep valley or trough during Late Mississippian-Early Pennsylvanian erosion. Marine transgression from the south backfilled the preexisting stream valleys with sequences of packstone, wackestone, shale, and coarse quartzarenite. Topographically higher areas were blanketed wit thin beds of laminated bioclastic packstone, grainstone, lime mudstone, and shale. Semirestricted tidal to nearshore environments are reflected in local sections by finely laminated hematitic dolomites, oncolitic wackestones, limestone conglomerates containing reworked Mississippian chert, and terrigenous shales supporting petrified wood float. Depositional onlap farther north resulted in a thinner and younger sequence.
The cyclic nature of these deposits reflects a proximity to the shoreline and suggests an abruptly shifting base level, possibly controlled by Early Pennsylvanian glacial eustasy. The time-stratigraphic position of the valley-fill depositional unit may be broadly correlated to other Late Mississippian-Early Pennsylvanian valley-fill sequences recognized in the Delaware basin and Grand Canyon.
AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.