Sequence Hierarchy and Facies Architecture of a Carbonate-Ramp System: San Andres Formation of Algerita Escarpment and Western Guadalupe Mountains, West Texas and New Mexico, by Charles Kerans and W. M. Fitchen

This study offers a high-frequency sequence stratigraphic model of carbonate-ramp strata of the San Andres Formation. It also documents, at the cycle scale,  the inner-ramp through outer-ramp portions of three high-frequency sequences and describes and maps carbonate facies within a high-frequency sequence framework. AbstractThe San Andres Formation (Leonardian-Guadalupian), as exposed along the Algerita Escarpment and in the Brokeoff Mountains, Cutoff Mountain, and the Western Escarpment of the Guadalupe Mountains, provides one of the world's most complete platform-to-basin transects of a carbonate-ramp system. More than 25,000 ft of measured sections and extensive bed tracing using oblique aerial photomosaics allowed construction of a detailed stratigraphic framework for the 22-mi dip-parallel exposures along this northwestern margin of the Delaware Basin. The hierarchy of stratigraphic cyclicity includes 2 composite sequences (long-term), 15 high-frequency sequences (HFS) (intermediate-term), and at least 100 high-frequency cycles (short-term) on the platform alone. This new high-resolution chronostratigraphic framework documents changes in the stratal architecture of a 2-m.y. period of carbonate ramp evolution. The report demonstrates the utility of a high-resolution chronostratigraphic framework in addressing hydrocarbon-exploration-scale problems and, equally important, places changing styles of reservoir-scale faciesarchitecture in a more inclusive and therefore more predictive stratigraphic framework.Facies, vertical facies successions, and facies tracts are described within the context of this chronostratigraphic hierarchy, allowing detailed assessment of changing styles of depositional models through time. Nineteen facies are arranged in 11 vertical facies successions, along with several less repetitive facies successions. These facies successions are in turn grouped into facies tracts, which are strike-parallel belts of common depth- and energy-dependent sedimentation. Linkage of multiple facies tracts from a shelf-to-basin transect within highstand and transgressive systems tracts of individual high-frequency sequences allows development of depositional models specific to each high-frequency sequence.This depositional analysis within a chronostratigraphic framework affords a better resolution of the evolution of the San Andres ramp. Major evolutionary stages include (1) a draping nonconstructional profile that mimics inherited topography (retrogradational sequence set of lower San Andres composite sequence; Leonardian 7 and 8 HFS), (2) constructional aggradational to strongly progradational carbonate ramp (progradational sequence set of lower San Andres composite sequence; Guadalupian 1-4 HFS), (3) a protracted phase of platform exposure and bypass of siliciclastic sediments to the basin center (lowstand sequence set, upper San Andres composite sequence, Guadalupian 5-1 1, Brushy Canyon Formation), (4) a ramp phase that is transitional into a rimmed shelf, with moderate to steep clinoform margin and locally developed bioherms (retrogradational sequence, upper San Andres composite sequence, Guadalupian 12 HFS), and (5) an oblique clinoform ramp with increasing biohermal boundstone at the margin (progradational sequence of upper San Andres composite sequence; Guadalupian 13 HFS San Andres and Cherry Canyon Tongue). Comparison of the facies architecture of high-frequency cycles in transgressive versus highstand portions of a single HFS, and across multiple HFS (specifically the Guadalupian 2, 3, and 4 HFS), provides qualitative and quantitative information regarding the dynamics of platform depositional models associated with changes in accommodation both within and between high-frequency sequences. Comparison of the volume of ooid-peloid grainstone across 14 cycles in 3 high-frequency sequences shows a twofold increase in grainstones in highstand versus transgressive systems tracts. A plot of thickness versus depositional-dip-width of grainstone body geometry relative to stacking pattern within the Guadalupian 2-4 HFS demonstrates both accommodation-related and basin-setting-related controls of grainstone development. Similar analyses of reservoir facies architecture within a high-frequency sequence framework will be required to maximize our understanding of geologic reservoir frameworks for three-dimensional reservoir characterization.