Sequence Stratigraphy of Carbonate Ramp-to-Basin Profiles in the Mississippian of South-Central New Mexico: Influence of Antecedent Topography on Ramp Evolution and Stratigraphic Architecture

S. L. Bachtel, S. L. Dorobek

Mississippian strata in the San Andres and Sacramento Mountains of south-central New Mexico represent a 100-km wide, erosional remnant of a carbonate ramp that bordered the southern terminus of the Transcontinental Arch. Preserved strata consist mainly of outer ramp, slope, and basinal facies; inner and mid-ramp strata that presumably were deposited to the north of the study area were truncated by extensive post-Mississippian erosion.

These strata comprise four, transgressive-regressive stratigraphic sequences that progressively prograded basinward during successive cycles of relative sea-level. We define transgressive-regressive sequences in these strata based on vertical lithofacies changes along dip-oriented transects and by detailed documentation of local stratal geometries. The first two sequences record the evolution of a low-gradient, homoclinal ramp (Sequence 1: Caballero Formation-Andrecito Member) to a slightly-steepened ramp (Sequence 2: Alamogordo-Tierra Blanca Members). The maximum depositional gradient produced by the progradational systems tract (PST) of Sequence 2 (Tierra Blanca Member) was about 5°, as measured at the maximum slope that developed during terminal stages of the PST. The transgre sive systems tract (TST) of Sequence 3 (Arcente Member) passively draped/infilled the depositional topography produced by the Tierra Blanca Member. Large-scale slumping in the Arcente Member is most common down-dip of the terminal ramp-slope crest of the Tierra Blanca Member, indicating the strong influence of antecedent topography on subsequent deposition and stratigraphic development. Extensive slope erosion and sediment bypass occurred during the late PST of Sequence 3 (Dona Ana Member), and was further enhanced during the subsequent relative sea-level fall. The geographic coincidence of major slope erosion/failure in the Dona Ana Member just basinward of the terminal ramp-slope crest of the Tierra Blanca Member further demonstrates the effects of antecedent topography. Sequence 4 (Ra cheria Formation) is an package of fine-grained, basinal facies that bypassed the ramp-slope crest of Sequence 3 and onlapped the erosional upper boundary of Sequence 3.

In strike-oriented exposures of PST's, overlapping lobes of carbonate sand are obvious and indicate that sand was supplied to the outer ramp from discrete points. This resulted in "reciprocal lobate geometries" (i.e., younger carbonate sand lobes filled-in depositional lows adjacent to older sand lobes). The combined effects of slope failure and antecedent topography on subsequent deposition make sequence stratigraphic interpretations for these strata difficult to construct. Sequence stratigraphic interpretations for other, poorly exposed or structurally complex carbonate ramps may be limited if the relative effects of antecedent topography and syndepositional failure can not be observed.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995