Anatomy of offlap: Upper San Andres Formation (Permian, Guadalupian), Last Chance Canyon, Guadalupe Mountains, New Mexico
— Mark D. Sonnenfeld
San Andres Formation outcrops in Last Chance Canyon are interpreted to contain two large-scale depositional sequences, upper San Andres sequence 3 and upper San Andres sequence 4. Facies associations, volumetric proportions of carbonate and siliciclastic strata and depositional topography change progressively through the course of the upper San Andres sequence 4. These aspects change in a predictable fashion and correspond to position within the lowstand, transgressive and highstand systems tracts. Units within the lowstand to transgressive systems tracts record a progressive decrease in sedimentation rate, depositional energy and siliciclastic content. This reflects a long-term transition from detrital, siliciclastic-dominated, point-sourced slope sedimentation to increasingly autochthonous, carbonate-dominated, line-sourced slope sedimentation. Within carbonate strata of the transgressive systems tract, long-term relative sea level rise is interpreted from thick outer-shelf deposits, highly aggradational to mounded fusulinid shoals and net stratigraphic rise of the fusulinid facies tract. The most carbonate-rich and bioherm-bearing interval of the entire sequence overlies a distinctive maximum flooding surface capping the transgressive systems tract. Ensuing high-frequency sequences of the middle to late highstand systems tract show pronounced progradational offlap and record a progressive increase in the volume of siliciclasts accumulated on the outer shelf. A karsted toplap surface represents a subaerial unconformity and sequence boundary capping upper San Andres sequence 4. Embedded within upper San Andres sequence 4 are numerous high-frequency sequences that show the following similarities to larger, seismic-scale depositional sequences: (1) stratal geometries; (2) the nature of bounding surfaces; (3) the timing of bioherm development; (4) siliciclastic to carbonate facies evolution; and possibly (5) point-source to linesource evolution. One of the most important aspects of stratigraphic self-similarity, best documented within the highstand systems tract of upper San Andres 4, involves the seaward transition from asymmetric shallowingupward hemicycles at the toe-of-slope and seaward. Each symmetric cycle includes an inferred deepeningupward phase or "transgressive hemicycle," characterized by a waning siliciclastic influence, succeeded by a shallowing-upward phase, or "regressive hemicycle," characterized by flourishing, prograding carbonates. The concept of qualified stratigraphic self-similarity does not diminish the importance of facies variation or "differentiation" within and among sequences; it merely emphasizes that the physical stratigraphy of a sequence reflects spatial and temporal variations in accommodation/sediment supply ratios but is relatively independent of absolute duration.
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- Sonnenfeld, Mark D., 1993, Anatomy of offlap: Upper San Andres Formation (Permian, Guadalupian), Last Chance Canyon, Guadalupe Mountains, 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. 195-203.