Sequence stratigraphic framework of the upper San Andres Formation and equivalent basinal strata in the Brokeoff Mountains, Otero County, New Mexico
William M. Fitchen
The upper San Andres Formation, upper Cutoff Formation and Cherry Canyon Tongue (Permian, Guadalupian) in the southern Brokeoff Mountains comprise two mixed siliciclastic-carbonate, intermediateorder (high-frequency) sequences on the basis of observed seismic-scale stratal geometries. integrated with detailed facies and high-order cycle stacking pattern analysis. These sequences are correlative to the two upper San Andres high-frequency sequences (GI2 and G13) of Kerans et al. (this volume) and can be traced throughout the Brokeoff Mountains and along the Algerita Escarpment. Each sequence contains a largely basinally restricted, siliciclastic-dominated lowstand to transgressive systems tract, the updip part of which can be correlated to one or more sandstone-based carbonate cycles on its respective platform. The transgressive systems tracts are downlapped along the slope by clinoformal carbonate and mixed carbonate-siliciclastic strata of their overlying progradational highstand systems tracts. The G I2 lowstand to transgressive systems tract onlaps the slope of the underlying G4 highstand and is provisionally correlated to the uppermost Brushy Canyon Formation on the basis of its stratigraphic position in the basin, i.e., within meters of the top of the Cutoff Formation, because its basal platform facies tracts are offset approximately 8 km basinward relative to similar facies tracts in the underlying sequence and due to the presence of a widespread karst profile that underlies the unit on the platform. The GI2 highstand systems tract is composed of sigmoidal carbonate clinoform cycles that can be traced from the peritidal ramp crest facies tract to the subtidal toe-of-slope facies tract. These cycles prograded the platform margin about 0.5 km. The sequence boundary at the top of GI2 is marked by minor erosional scour and stratal truncation along the outer shelf and slope. The G13 lowstand to transgressive systems tract onlaps the G12 highstand slope. It is definitively correlated to the lower two-thirds of the Cherry Canyon Tongue and provisionally correlated to the Lovington sandstones of the subsurface. The GI3 highstand is composed of sigmoidprogradational carbonate cycles that prograded the platform margin about I km and sigmoid- to obliqueprogradational, mixed carbonate-siliciclastic cycles that prograded the margin an additional 5 to 6 km. The G I3 highstand is capped by a sequence boundary with minor erosion and stratal toplap along the outer 1-2 km of the platform and local karst development further landward. Paleocavems and grikes within the karst profile extend downward as much as 30 m below the sequence boundary, which suggests a minimum relative sea level fall of similar magnitude.
- Fitchen, William M., 1993, Sequence stratigraphic framework of the upper San Andres Formation and equivalent basinal strata in the Brokeoff Mountains, Otero County, New Mexico, in: Carlsbad Region, New Mexico and West Texas, Love, David W.; Hawley, John W.; Kues, Barry S.; Adams, Jim W.; Austin, George S.; Barker, James M., New Mexico Geological Society, Guidebook, 44th Field Conference, pp. 185-193.