Volumetric Partitioning and Facies Differentiation within the Permian Upper San Andres Formation of Last Chance Canyon, Guadalupe Mountains, New Mexico

Abstract

San Andres Formation (Permian) outcrops in Last Chance Canyon are interpreted to contain two large-scale, fourth-order depositional sequences, upper San Andres sequence 3 and upper San Andres sequence 4. Embedded within each of these sequences are numerous higher frequency sequences. Reciprocal siliciclastic/carbonate sedimentation patterns enhance the recognition of cyclicity within both large-scale sequences and within many of the higher frequency sequences. Upper San Andres sequence 4 contains at least 12 smaller scale sequences that occur in landward-stepping, vertically stacked, stratigraphically rising seaward-stepping, and stratigraphically falling seaward-stepping geometric patterns. In outer-ramp/slope environments, landward-stepping, high-frequency sequences of the lowstand to transgressive systems tracts record a progressive decrease in sediment accumulation rate, depositional energy, and siliciclastic content. This reflects a long-term transition from siliciclastic-dominated slope sedimentation to increasingly carbonate-dominated slope sedimentation. Thick outer-shelf carbonate strata, highly aggradational to mounded fusulinid-rich shelf-margin deposits, and pronounced stratigraphic rise of the fusulinid facies tract all reflect long-term increases in accommodation. Concurrent with the highly aggradational and moderately progradational mode of outer-shelf to slope carbonates, siliciclastic shelf-feeder systems are inferred to have stepped progressively landward. These contrasting reflections of increasing accommodation during lowstand to transgressive systems tract deposition accentuate the dissimilar nature of carbonate versus siliciclastic sediment supply. The most carbonate-rich and bioherm-bearing interval of the entire sequence overlies a distinctive maximum flooding surface capping the transgressive systems tract. Ensuing seaward-stepping End_Page 435------------------------ high-frequency sequences of the middle to late highstand systems tract show pronounced progradational offlap and record a progressive increase in the volume of siliciclastics accumulated in the basin with a concomitant decrease in the volume of siliciclastics accumulated on the outer shelf. A karsted toplap surface represents a subaerial unconformity and sequence boundary capping upper San Andres sequence 4. Peritidal cycles within the overlying Grayburg Formation exhibit a significant seaward shift in coastal onlap across this sequence boundary. Depositional topography and time-significant surfaces within upper San Andres sequence 4 were traced across multiple geomorphic environments that extend from shelf to basin. As a result, the evolution of seismic-scale stratal geometries, depositional topography, facies associations, and volumetric proportions of carbonate and siliciclastic strata can be related with a high degree of temporal precision. By calibrating these changes to position within a stacking pattern of high-frequency sequences, we observe ordered, predictable changes in the volumetric partitioning of siliciclastics, grain-supported carbonates, and mud-supported carbonates. This volumetric partitioning is accompanied by differentiation of carbonate facies that accumulated in similar paleobathymetric settings, but in ifferent positions of the large-scale stacking pattern. For example, siliciclastic-free, mud-supported, bioherm-prone slope carbonates attain their greatest seaward extent during the latest transgressive to earliest highstand systems tract period of long-term maximum shelfal accommodation. By contrast, grain-rich carbonate and mixed carbonate-siliciclastic facies predominate in lower-slope settings during periods of reduced shelfal accommodation such as the lowstand, early transgressive, and middle-late highstand systems tracts. If facies models are to become predictive at the scale of reservoirs, they must be calibrated to the systematically changing sediment supply and accommodation conditions that appear to drive volumetric partitioning and facies differentiation.