Mississippian carbonate ramp-to-basin transitions in south-central New Mexico; sequence stratigraphic response to progressively steepening outer-ramp profiles

The sequence stratigraphic architecture of Mississippian strata from the Sacramento and San Andres Mountains, south-central New Mexico, illustrates the combined effects of relative sea-level change and progressively increasing depositional gradients on sedimentary processes and resultant stratal geometries along the outer parts of a carbonate ramp-to-basin transition. Mississippian strata can be subdivided into three progradational sequences (Sequences 1-3) and a fourth, basin-restricted, onlapping sequence (Sequence 4). Sequences 1-3 contain wackestone- to mudstone-dominated transgressive systems tracts that are overlain by packstone-to grainstone-dominated progradational systems tracts. Progradational systems tracts created distinct ramp-to-basin transitions (outer ramp margin) that are characterized by basinward stratal thinning and a concomitant change to muddy lithofacies. Sequence 4 consists of onlapping, fining-upward stratal packages of peloidal-skeletal grainstone that grade upward to laminated, spiculitic lime mudstone. The outer-ramp-margin facies of Sequences 1 through 3 gradually steepened the outer ramp profile until the cumulative effects of the steepening caused the depositional gradient to surpass an "equilibrium grade" (i.e., the gradient at which deposition and erosion/bypass are about equal). Once the depositional gradient exceeded the equilibrium grade for the caliber of sediment supplied to the outer ramp margin, widespread submarine erosion and sediment bypass on the outer ramp was initiated, resulting in deposition of basin-restricted strata of Sequence 4. Similar relationships and inferred processes have been recognized in progradational siliciclastic shelf deposits and are collectively described by previous workers as "slope readjustment". The Mississippian ramp-to-basin transition in south-central New Mexico illustrates how sediment deposition, dispersal, and erosion responded to evolving depositional gradients and in turn controlled resultant, large-scale stratal geometries.