Sequence stratigraphy of the upper San Andres Formation and Cherry Creek Tonque (Permian, Guadalupian), southern Brokeoff Mountains, New Mexico
Fitchen, William M.
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Outcrop exposures of the upper San Andres Formation and Cherry Canyon Tongue (Permian, Guadalupian) in the Brokeoff Mountains, New Mexico provide a seismic-scale cross-section through the margin of the Northwest Shelf and adjacent Delaware Basin. Stratal patterns such as onlap, offlap, downlap, and toplap, which are commonly used by seismic interpreters to identify sequences and systems tracts in the subsurface, can be observed directly in the area and integrated with facies distributions to generate a high-resolution sequence stratigraphic framework. The upper San Andres Formation and Cherry Canyon Tongue comprise a third-or fourth-order sequence (85 to 145 m-thick) bounded by unconformities and their correlative conformities (sequence boundaries). Lowstand/shelf margin, transgressive, and highstand systems tracts within the sequence were recognized on the basis of bounding surfaces and cycle stacking patterns. Cycle stacking patterns were analyzed with respect to geometry, component facies distribution, thickness, and the nature of lateral termination (e.g. onlap). The basal sequence boundary of the upper San Andres-Cherry Canyon Tongue sequence overlies the middle San Andres highstand platform. A basinward shift in facies tracts across this sequence boundary is evidenced by the vertical progression from offlapping ramp margin and slope strata of the middle San Andres highstand systems tract to onlapping, ramp crest strata of the upper San Andres lowstand/shelf margin systems tract. Erosion of underlying slope and toe-of-slope strata is evident along the boundary; carbonate megabreccias locally overlie the sequence boundary at the toe-of-slope. As a result of the strongly offlapping and toplapping character of the upper San Andres sequence, the upper sequence boundary intersects early highstand systems tract strata in the platform interior and late highstand systems tract strata closer to the terminal ramp margin. The sequence boundary is marked locally by a karst horizon; the best development of karst occurs in ramp margin strata of the early highstand systems tract. The karst is characterized by sandstone-filled rundkarren, grikes, and caverns that extend up to 30 m downward into San Andres strata. Toplap and minor stratal truncation mark the sequence boundary along the top of the late highstand systems tract. Subtidal to peritidal cycles of the Grayburg Formation (lowstand/shelf margin systems tract and transgressive systems tract) onlap the upper sequence boundary. The lowstand/shelf margin systems tract of the upper San Andres sequence is composed of a thin (6 to 25 m-thick) aggradational to slightly progradational cycle set that onlaps the basal sequence boundary along the platform and downlaps the sequence boundary for a distance of 100 m along the slope. Peritidal sandstone and carbonate facies of the systems tract pass basinward within a few hundred meters into a distinctive bryozoan-sponge-crinoid shelf margin buildup, which in turn passes basinward into toe-of-slope allodapic carbonates and discontinuous carbonate megabreccias derived from the underlying sequence. Relatively thin ramp crest cycles of the lowstand/shelf margin systems tract are capped by a transgressive surface that is locally erosional. The stacking pattern of cycles in the lowstand/shelf margin systems tract reflects relatively low rates of accommodation and sediment production. Siliciclastic sediment bypass across the shelf and slope may have been active during deposition of this systems tract. The transgressive systems tract of the upper San Andres sequence comprises 1) an aggradational to weakly progradational cycle set of ramp crest/ramp margin carbonates and siliciclastics (40-45 m-thick), 2) a proximal, onlapping slope apron complex of carbonates and siliciclastics (25-30 m-thick), and 3) a distal, marine onlap-wedge of basinal siliciclastics (the lower Cherry Canyon Tongue, 30-80 m-thick). Ramp margin/upper slope carbonates of the transgressive systems tract downlap the transgressive surface along the platform. On the platform, the transgressive systems tract is composed of relatively thick ramp crest cycles containing a high ratio of subtidal vs. supratidal facies. The systems tract is capped by three to four sandstone-based cycles, the upper two of which can be traced downslope into the slope apron complex. The top of the slope apron complex is downlapped by strongly progradational cycles of the overlying highstand systems tract. The downlap or maximum-flooding surface marks the top of the transgressive systems tract. Towards the basin, allodapic carbonates of the slope apron complex thin and pinch out into the body of the lower Cherry Canyon Tongue. These relationships indicate that lower Cherry Canyon Tongue sandstones were bypassed to the basin during high-frequency lowstand intervals superimposed on a longer term relative sea level rise. In contrast, allodapic carbonates within the slope apron were supplied to the slope during high-frequency highstand intervals. The highstand systems tract of the upper San Andres sequence (15-85 m-thick) comprises strongly progradational cycle sets composed of ramp crest through slope carbonates and sandstones. Clinoform cycles downlap onto the slope and basinal segments of the transgressive systems tract (lower Cherry Cherry Canyon Tongue). The highstand can be subdivided into two major cycle sets, termed the "early highstand" and "late highstand" respectively. The early highstand is characterized by carbonate-dominated sigmoid progradational clinoform cycles that can be traced from the ramp crest to the toe-of-slope. In the platform interior, early highstand cycles are thin and contain a higher ratio of supratidal facies relative to cycles of the transgressive systems tract. Early highstand cycles aggraded the ramp crest some 15-20 m and prograded 1-2 km into the basin. The late highstand is characterized by mixed carbonate-siliciclastic, sigmoid to oblique progradational clinoform cycles that toplap shelfward into the upper sequence boundary at low (1-2°) to high (10-15°) angles. Late highstand cycles did not aggrade the platform, rather, the former ramp crest area was a subaerial sediment bypass zone during deposition of these cycles. Late highstand cycles prograded 5-6 km into the basin. A progressive decrease in accomodation rates through the highstand systems tract is inferred from a decrease in the ratio of aggradation to progradation, a concommitant change from sigmoid to oblique progradational style, an increase in clinoform slope angle, and an increase in siliciclastic sediment flux. The basal sequence boundary of the upper San Andres sequence can be correlated to the unconformity between the Cherry Canyon Tongue/Brushy Canyon Formation and Cutoff Formation along the Western Escarpment (southern Guadalupe Mountains). The Brushy Canyon Formation, which is restricted to the basin, is interpreted as a "lowstand fan" equivalent in time to the basal sequence boundary and perhaps to part of the lowstand/shelf margin systems tract in the Brokeoff Mountains. The basal sequence boundary correlates to the top of the lower-middle San Andres sequence on the Algerita Escarpment (central Guadalupe Mountains). This sequence boundary can also be correlated to the base of the Cherry Canyon Tongue in Last Chance Canyon (central Guadalupe Mountains), although correlation between the platform sections of the Brokeoff Mountains and Last Chance Canyon is somewhat uncertain. The upper sequence boundary correlates to the San Andres-Grayburg contact on the Algerita Escarpment and in Last Chance Canyon. The correlation of this boundary to the Western Escarpment remains uncertain. The progressive decrease in accommodation exhibited by upper San Andres late highstand strata in the Brokeoff Mountains, and the inferred correlation of lower Grayburg shelf sandstones into the upper third of the basinal Cherry Canyon Tongue, supports the hypothesis that the terminal ramp margin of the upper San Andres sequence lies 8-10 km north of the Western Escarpment near the New Mexico-Texas state line. This entails that the upper sequence boundary lies within the Cherry Canyon Tongue on the Western Escarpment.