South-central New Mexico has been well known for its excellent exposures of Mississippian strata. The Sacramento Mountains have become famous for their fortuitous exposures of the Lake Valley shelf margin. A transect of measured sections there exposes the transition from shallow carbonate shelf with beautifully developed reefs to a starved basin with isolated pinnacle reefs to the south. A new transect, to the west in the San Andres Mountains, reveals a complete shelf-to-basin transect, from shoreline to shelf-margin to starved basin. There, however, no reefs are present in time-equivalent strata. Studies there, integrating physical stratigraphy, depositional systems, microfacies analysis, and conodont biostratigraphy and biofacies, allow formulation of a leeward shelf ec facies depositional model for the Lake Valley formation. This is combined with the Sacramento Mountains transect to delineate a model of an Early Mississippian Orogrande basin in the same position as the Pennsylvanian-Permian Orogrande basin and the present Tularosa basin. A series of such basins may have existed along the southern flank of the ancient Transcontinental arch in the southwestern United States.

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