MISSISSIPPIAN CARBONATE RAMP-TO-BASIN TRANSITIONS IN SOUTH-CENTRAL NEW-MEXICO - SEQUENCE STRATIGRAPHIC RESPONSE TO PROGRESSIVELY STEEPENINGOUTER-RAMP PROFILES

The sequence stratigraphic architecture of Mississippian strata from t he Sacramento and San Andres Mountains, south-central New Mexico, illu strates the combined effects of relative sea-level change and progress ively increasing depositional gradients on sedimentary processes and r esultant 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-restrict ed, 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. P rogradational systems tracts created distinct ramp-to-basin transition s (outer ramp margin) that are characterized by basinward stratal thin ning and a concomitant change to muddy lithofacies. Sequence 4 consist s of onlapping, fining-upward stratal packages of peloidal-skeletal gr ainstone 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 e qual), Once the depositional gradient exceeded the equilibrium grade f or the caliber of sediment supplied to the outer ramp margin, widespre ad submarine erosion and sediment bypass on the outer ramp was initiat ed, resulting in deposition of basin-restricted strata of Sequence 4, Similar relationships and inferred processes have been recognized in p rogradational siliciclastic shelf deposits and are collectively descri bed by previous workers as ''slope readjustment'', The Mississippian r amp-to-basin transition in south-central New Mexico illustrates how se diment deposition, dispersal, and erosion responded to evolving deposi tional gradients and in turn controlled resultant, large-scale stratal geometries.