Abstract: Mississippian Carbonate Ramp-to-Basin Transitions in Southern New Mexico: Reinterpretation of Existing Lithostratigraphy and Implications for Sequence Stratigraphy of Carbonate Ramps

Steven L. Bachtel, Steven L. Dorobek

Detailed measured sections and photomosaics of Mississippian strata (Caballero through Rancheria formations) from the Sacramento and San Andres Mountains, southern New Mexico illustrate the complex sequence stratigraphic relationships that can develop along carbonate ramp-to-basin transitions.

Mississippian strata in this area consist of three stratigraphic sequences; expression of sequence boundaries is subtle because all of the strata in this interval consist of upper slope to basinal facies. The oldest sequence consists of the Caballero Formation and Tierra Blanca Member of the Lake Valley Formation. The transgressive systems tract (TST; Caballero Formation) is characterized by starved sedimentation, but the highstand systems tract (HST; Tierra Blanca Member) thickens northward and has an unique tongue-like geometry on strike-oriented cross-sectional profiles in the Sacramento Mountains. In contrast, exposures in the San Andres Mountains provide better dip-oriented views of the prograding Tierra Blanca HST. Progradation is indicated by low-angle clinoforms within slope f cies. The middle sequence (Arcente and Dona Ana Members of the Lake Valley Formation) has a volumetrically small lowstand systems tract (LST; represented by discontinuous grainstone sheets and channels), a thick TST (Arcente Member) which thins basinward to a condensed interval, and basinward thickening, progradational grainy slope facies of the HST (Dona Ana Member). Basinal facies of the Dona Ana Member represent a composite of individual fining-upward basin floor fans. Erosional truncation is observed at the base of each fan. Each individual basin floor fan is 15 to 10 meters thick and can be correlated basinwards over approximately 12 km. This new lithostratigraphic interpretation indicates that depositional relief along the post-Lake Valley unconformity is much less than previously nterpreted.

The base of the third sequence is located approximately at the Osage-Meramec boundary and is represented by a LST (Rancheria Formation) composed of calciturbidites overlain by poorly exposed shale and limestone of the TST (Helms Formation). The third sequence is truncated by the Mississippian-Pennsylvanian unconformity and only the updip portion of the sequence is exposed in the Sacramento Mountains. In the San Andres Mountains, Pennsylvanian fluvial conglomerates and estuarine(?) facies locally overlie pre-Caballero strata, indicating that significant fluvial incision of the Lake Valley-Rancheria platform occurred prior to Pennsylvanian deposition.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994