Depositional Facies and Model for Mixed Siliciclastics and Carbonates of the Yates Formation, Permian Basin
J. M. Borer, P. M. Harris, 1991. "Depositional Facies and Model for Mixed Siliciclastics and Carbonates of the Yates Formation, Permian Basin", Mixed Carbonate-Siliciclastic Sequences, Anthony J. Lomando, Paul M. Harris
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Subsurface and outcrop data show the distribution of siliciclastics, carbonates, and evaporites across the inner, middle, and outer portions of the shelf for the Yates Formation (Permian, Late Guadalupian) of the Permian Basin.
The evaporitic inner shelf consists of thick intervals of anhydrite and minor halite interbedded with anhydrite-cemented, Red Argillaceous Siltstone/Sandstone. The sheet-like geometry of the beds and lack of evidence for channels, as shown by log correlations, suggest that sheetflood and eolian processes may have been the dominant modes of sediment transport.
The middle shelf was dominated by siliciclastic deposits and separated the evaporitic inner shelf from the carbonate-rich shelf margin. Siliciclastic facies in the middle shelf consist of alternating Light Brown Arkosic Sandstones (shoreline deposits), Dark Gray Argillaceous Siltstones (wet mud flats transitional to shallow lagoons), and Red Argillaceous Siltstones similar to that of the inner shelf. Carbonate lithologies consist predominantly of structureless to algal-laminated, peloidal dolomudstones that locally show signs of erosion (ripped-up clasts). Green-Gray Dolomitic Subarkosic Siltstones/Sandstones occur in the middle- to outer-shelf region and are associated with algal-laminated dolomudstones and minor, thin-bedded, pisolitic packstones (tidal flat to shallow lagoon deposits).
The shelf margin consists of Red Anhydritic Siltstone/Sandstone that passes downdip to Gray Bioturbated, Kaolinitic Dolomitic Quartz Sandstone. These siliciclastics occur with dolomites of the shelf margin pisolite shoal complex, with the Gray Sandstones deposited in a more shallow marine environment than the subaerially deposited Red Siltstones.
Vertical stacking of the various facies is interpreted to be the result of cyclic sea-level variation and is thus a shelf-wide phenomenon. Silts and sands were transported across the shelf and to the shelf margin during lowstands of sea level, and outer-shelf clastics were reworked during subsequent sea-level rises. Cyclostratigraphic analyses suggest that the siliciclastic deposition and stacking of facies occurred during three orders of relatively low-amplitude, sea-level fluctuations. A depositional model summarizes the effect of these sea-level fluctuations on facies distribution for shelf areas with different slopes and/or subsidence (i.e., Central Basin Platform vs Northwest Shelf). An understanding of the sea-level fluctuations and the variable shelf profiles enhances facies correlations in a mixed siliciclastic and carbonate system like the Yates Formation.
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Mixed Carbonate-Siliciclastic Sequences
The study of carbonate-siliciclastic mixed sequences has seen an increase in the number of investigations that focus on mixed settings as part of the continuum between the carbonate and clastic end members. Cyclic deposition in mixed basins by reciprocal sedimentation has become one of the foundation blocks for sequence stratigraphy. In addition, these mixed sequences have a variety of distinctive petroleum reservoir characteristics, important for both exploration and development programs. The emphasis now is on reevaluating ancient sequences in the light of a more dynamic understanding of spatial and temporal variations and controls on these sequences. Examples in this volume are subdivided under the following headings: Shelf Wide, Coastal and Inner Shelf, Middle to Outer Shelf, Slope to Basin and Paleokarst. Many mixed sequences have been described in the literature, but understanding the controls of these sequences from a process approach in now in an adolescent stage.