ABSTRACT: Volumetric Partitioning and Facies Differentiation within the Upper-Upper San Andres Sequence (Permian, Guadalupian), Last Chance Canyon, Guadalupe Mountains, New Mexico

SONNENFELD, MARK D., Colorado School of Mines, Golden, CO

Current sequence stratigraphic models emphasize bounding surfaces and stacking patterns while portraying relatively static or unchanging facies associations. Last Chance Canyon provides an example of dynamic outer-shelf to slope facies associations that vary over base-level transit cycles of several temporal scales (approximately third to fifth order). The following facies association attributes consistently vary in response to accommodation trends: (1) volumetric partitioning of siliciclastics on the shelf versus the slope and basin, (2) topset sandstone continuity, (3) timing of point- versus line-sourced slope feeder systems, (4) progradation:aggradation ratio and width of the upper-slope fusulinid wackestone/packstone facies tract, and (5) timing of bioherm development.

Slope strata within the transgressive (TST) and early highstand (HST) systems tracts of the upper-upper San Andres depositional sequence record a progressive decrease in sedimentation rate, depositional energy, and siliciclastic content. This transition reflects the transition from point-sourced sand bypass off the shelf to sand entrapment on the shelf. This transition is concurrent with an expansion of the fusulinid facies tract from 500 m to 1 km in width. Highly aggradational, mounded fusulinid shoals of the TST and earliest HST form low-angle sigmoidal clinoforms with a relatively low progradation:aggradation ratio of 58:1. The early HST is the most carbonate-rich interval of the entire upper-upper San Andres and contains slope crinoid-bryozoan bioherms and brachiopod-sponge reefs A net progradation: aggradation ratio of 200:1 for the middle HST reflects pronounced offlap and the transition from sigmoidal to complex sigmoid-oblique clinoforms. These geometrical changes are coincident with the progressive contraction of the fusulinid facies tract from 300 m to 100 m. The initial three genetic sequences of the middle HST mark the resumption of siliciclastic input to the outer-shelf and slope and contain laterally continuous 2-5 m outer-shelf sheet sandstones and 20+ m slope sandstone wedges that rapidly taper basinward. Subsequently, siliciclastics are increasingly partitioned into slope settings, resulting in discontinuous outer-shelf sandstones and volumetrically expanded slope sandstone wedges. During the late HST sands completely bypassed the shelf through larg r, point-sourced conduits and the fusulinid upper slope facies tract was replaced by obliquely prograding peloid grainstones.

Facies associations also vary in response to even higher-frequency base-level transit cycles that define individual genetic sequences within the upper-upper San Andres. This is expressed most clearly by siliciclastic/carbonate facies partitioning, timing of bioherm development, and point- to line-source evolution analogous to the entire upper-upper San Andres. These scale-independent similarities probably result from the fact that siliciclastic transport and accumulation, clastic poisoning effects on authochthonous carbonate generation, and basin hydrographic criteria critical to carbonate production (water depth, temperature, and nutrient flux) were controlled similarly by third-, fourth-, and fifth-order relative sea level changes. The limits of self-similarity may indicate the uppe and lower bounds at which allocyclic mechanisms control stratal geometry and facies; that is, they may indicate a lower level at which autogenic factors determine facies variation and an upper level at which extrinsic, noncyclic factors such as faunal evolution, pre-existing basin configuration, or detrital sediment supply dictate the evolution of stratal geometries and facies associations.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)