New Mexico Geological Society Annual Spring Meeting — Abstracts

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 pre-0xisting stream valleys with sequences of quartz arenite, bioclastic grainstone, pelloidal-algal wackestone and shales. Topographically higher areas were blanketed with thin discontinuous beds of bioclastic grainstone, laminated packstone and shales. Semi-restricted tidal to nearshore environments are reflected in local sections by finely laminated hematitic dolomites, nodullar shales, ripple-laminated silty lime mudstones-wackestones, pelloidal and oncolitic grainstones, sandy limestone conglomerates and black carbonaceous shales supporting petrified wood float. Depositional onlap farther to the north resulted in younger and more terrigenousI sequence.

The cyclic nature of these deposits reflects a proximity to the shoreline and suggests a rapidly shifting base level, possibly controlled by Early Pennsylvanian glacial eustasy and/or tectonic loading. The time-stratigraphic position of the valleyfill depositional unit may be broadly correlated to other Late Mississippian-Early Pennsylvanian valley-fill sequences recognized in the Delaware Basin and Grand Canyon.