New Mexico Geological Society Annual Spring Meeting — Abstracts

Lowermost Mississippian rocks in the San Juan basin of northwest New Mexico are Kinderhookian(?) and earliest Osagean in age and are restricted to the western margins of the basin. They overlie rocks of Late Devonian age and were laid down as carbonate sediments during a regional transgression in northern New Mexico. The Osagean marine transgression moved east and south from the Four Corners region and north from south-central New Mexico, flooding a terrane of Precambrian igneous and metamorphic rocks. The Precambrian surface on which the transgression occurred was irregular, and islands above the late Osagean sea include the Zuni-Defiance highlands, the Uncompahgre highlands, and the Pedernal highlands. The Osagean rocks that formed adjacent to the highlands include supratidal and intertidal lime mudstone, anhydrite, gypsum, dolomite, quartz sandstone, and shale. In more open marine environments, calcareous sand shoals were composed of pellets, bioclasts of crinoids, brachiopods and bryozoans, and ooids and, oolites. The end of Osagean time was marked by regional marine regression and uplift and erosion of the carbonate platform.

A regional marine transgression during Meramecian time is documented in the upper part of the Redwall Limestone in the subsurface of the Black Mesa basin in northeast Arizona, in the upper part of the Leadville Limestone in the subsurface of the western part of the San Juan basin and equivalent rocks in southeast Utah, and in outcrops of the Tererro Formation of the Arroyo Penasco Group in the Nacimiento and San Pedro Mountains on the eastern side of the San Juan basin. These marine bioclastic carbonate rocks are composed of dolomite, lime mudstone, oolites, crinoiqs, foraminifera, algae, brachiopods, and pellets.

In Late Mississippian time, the region was differentially uplifted and large areas of Mississippian rocks were removed. The remaining carbonate rocks were subjected to solution and a thick regolith developed. On the east flank of the San Juan basin, in the San Pedro and Nacimiento Mountains, Mississippian carbonate rocks of the Arroyo Penasco Group are unconformably overlain by continental red beds of the uppermost Mississippian (Chesterian) Log Springs Formation. Reworking of this continental regolith by the transgressive Pennsylvanian sea to form the Molas Formation is documented in the San Juan Mountains and in the subsurface. Throughout the region, Mississippian sedimentary rocks are truncated by Pennsylvanian sedimentary rocks.