ABSTRACT Exceptional exposures of progradational platform margin slopes of Albian age were mapped along a 5-km, near-continuous, dip-oriented transect in northern Mexico with the goal of producing an outcrop model for analogous Cretaceous reservoirs. The study area at the El Cedral is located along the western margin of the Devils River grainstone belt flanking the muddy, intrashelf Maverick Basin. Four relatively complete clinothems and portions of two adjacent clinothems are recognized in the Las Pilas Formation in the study area. The Las Pilas clinothems exhibit complex sigmoid-oblique geometries and have dimensions of 40–60 m of vertical relief, depositional dip angles of foreset beds ranging from 8 to 15°, and dip-oriented lengths of 1–2 km. Internally, in foresets of clinothems, thick- to massively bedded, sigmoid- to oblique-shaped lenses exhibit overlapping, seaward-stepping patterns toward the southeast. The primary sediment fabrics of topset, foreset, and bottomset beds of the Las Pilas clinothems are grainstones and grain-rich packstones. Coated grains are the predominant grain type, with subordinate amounts of micritized mollusk fragments and intraclasts. Porosity is predominantly moldic, but primary interparticle porosity is common. These lime sands were winnowed, coated, and micritized along the high-energy topsets of clinoforms before being exported seaward toward the breakpoint, where they cascaded onto the steep frontal slopes as sporadic grain flows or mass flows. The master bounding surfaces that define the clinothems likely represent episodes of nondeposition and abandonment of the surface, perhaps occurring during shifts in the locus of deposition laterally along the shoal margin. The architecture and composition of the Las Pilas clinoforms provide observable data for comparison with analogous subsurface reservoirs in the Aptian Shuaiba Formation (Bu Hasa field) and Cenomanian Mishrif Formation (e.g., Umm Adalkh field) of the Arabian Gulf.

Abstract Dip-oriented profiles of the Yates-Capitan shelf margin were independently constructed from exceptional outcrops in McKittrick and Slaughter Canyons in the Guadalupe Mountains of west Texas and southern New Mexico. Comparison of the two profiles reveals significant similarities in the position and character of high-frequency sequence boundaries, the internal architecture of facies tracts and cycle-stacking patterns, and offlap angles of both the shelf and reef. The evolution of the Yates-Capitan shelf margin is recorded by systematic long-term trends in key depositional variables measured on individual high-frequency sequences in each canyon. This comparison of the two profiles, located along strike 25 km apart, provides a three-dimensional model of the extent and variability of genetic components of the Late Permian margin of the Northwest shelf of the Delaware basin. Comparison of the sequence stratigraphic models for the Yates Formation in McKittrick and Slaughter Canyons indicates that four complete high-frequency sequences can be confidently correlated along strike. Fundamental architectural characteristics of the Yates-Capitan shelf margin are evident in the comparative profiles from each canyon. (1) The Yates Formation in both canyons exhibits initial aggradational geometries followed by strong progradational patterns. The volume of the shelf-crest pisolite complex in the Yates progressively expands through time in concert with a reciprocal contraction of the outer-shelf facies tract. The time-equivalent Capitan reef in both canyons exhibits remarkably similar patterns of stepwise alternations of aggradational and progradational growth that relate to changes in high-frequency sequence architecture through time. (2) The thickness of the Yates varies considerably both across the dip of the shelf margin and along strike between the two canyons. The 240%—600% increases in downdip thickness primarily reflect the inherited depositional topography. The thickness variation along the 25-km strike distance may indicate significant lateral variability in subsidence and/or accumulation rate along the Yates-Capitan shelf margin. (3) Several individual siliciclastic beds can be correlated between each canyon, and the distribution of siliciclastics within individual high-frequency sequences exhibits similar patterns of retrogradation, aggradation, and progradation. Correlative facies-stacking patterns integrated with long-term variations in progradation:aggradation ratio and derived offlap angle act as sensitive indicators of relative changes in base level. Comparison of our integrated field observations with subsurface data from the northern Northwest shelf (65–90 km away), Central Basin platform (approximately 150 km away), and Eastern shelf (>300 km away) reveals a consistent, basin-wide pattern of the internal architecture of the high-frequency sequences that compose the Yates Formation. These widespread similarities provide compelling evidence for regional sea-level control on sequence development around the Permian basin.

The Bonanza King Formation forms imposing mountainside exposures in several tilted fault block ranges in the southern Great Basin of Nevada and eastern California (Fig. 1). The shallow-marine carbonates that characterize the Bonanza King were deposited during the early drift phase of development of the Cordilleran passive margin, which originated in response to breakup of a Late Proterozoic supercontinent around 600 to 550 Ma (Stewart and Suczek, 1977; Bond and others, 1984; Levy and Christie-Blick, 1991). The broad continental margin was oriented essentially east-west within tropical latitudes (10 to 15° N; Scotese and McKerrow, 1990), conducive for the development of thick accumulations of carbonates.

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ABSTRACT The Bonanza King Formation of the southern Great Basin is composed of 150 to 250 carbonate parasequences (0.5 to 7 m thick) that provide a remarkable "strip chart" of Middle to Late Cambrian third-order accommodation history. Six superb exposures of the Banded Mountain Member of the Bonanza King Formation were logged on a decimeter scale and correlated to generate a detailed platform-to-basin transect. These cyclic carbonates were deposited on a flat-topped, fully-aggraded platform that extended approximately 250 to 300 km across the early Paleozoic passive margin of southern Nevada and eastern California. The Banded Mountain Member ranges from 400 to 1330 m in thickness and forms a westward-thickening wedge from craton margin facies in the eastern Mojave to shelf-edge and base-of-slope facies in the Last Chance Range. Thick successions of meter-scale carbonate parasequences show systematic changes in parasequence type, dominant lithofacies, and thickness vertically through the cyclic interval that were governed by long-term, third-order changes in accommodation. Fischer plots provide a graphic illustration of changes in accommodation space through time, and appear to be a valuable tool for correlating sequence boundaries and systems tracts. Similar patterns of positive and negative slopes on Fischer plots of the five platform sections of the Banded Mountain Member define four major sets of long-term increases in accommodation followed by long-term decreases in accommodation that correspond to four distinct depositional sequences. Translating sequence stratigraphic principles originally defined for seismic-scale siliciclastic systems to outcrops of cyclic carbonates requires a deemphasis of stratal geometries and an increased awareness of correlative vertical changes in stacking patterns of component parasequences. Sequence boundaries and transitions between systems tracts on flat-topped platforms, such as the Bonanza King, are believed to be zones rather than distinct horizons because of the effect of the higher frequency sea-level signal superimposed on the long-term, third-order event. Sequence boundary zones bracketing the four depositional sequences within the Banded Mountain Member are characterized by thin, tidal-flat dominated parasequences exhibiting abundant evidence for repeated episodes of exposure. Transgressive systems tracts in the Banded Mountain Member are characterized by a lower succession of parasequences composed of subequal amounts of tidal-flat and subtidal lithofacies passing upward into thicker parasequences with higher percentages of deeper subtidal facies. Highstand systems tracts in the Banded Mountain Member are characterized by thinning-upward stacks of parasequences showing progressively shallower peritidal conditions within the component lithofacies.