Kansas Geological Survey, Subsurface Geology 12, p. 47-50
S. J. Mazzullo1 and A. M. Reid2
1Wichita State University 2Geological Consultant, Midland, Texas
Late Pennsylvanian and Permian strata in the Midland basin of west Texas comprise thick, dominantly progradational wedges that infilled the basin by Guadalupian time. The lower Permian section (Wolfcampian and lower Leonardian) on the north platform of the Midland basin (fig. 1A) is a complex mosaic of carbonate and siliciclastic depositional systems (fig. 1B) whose development was influenced by the interplay among recurrent sea-level fluctuations, periodic subaerial exposure, and variable subsidence rates through time (Mazzullo and Reid, 1989). The resultant sedimentary architecture of platform and basinal facies consists of three separate progradational sequences represented by the Wolfcamp and lower Leonardian Wichita and Lower Clear Fork formations (fig. 1B). Parasequences (definition following Van Wagoner et al., 1988) of variable time duration compose these sequences and record disparate constructional and destructional phases of platform development (fig. 1C). Such complexity, which is not entirely evident seismically (fig. 1D), is resolvable only by detailed regional subsurface lithologic and biostratigraphic studies.
The cumulative thickness of the Wolfcamp, Wichita, and Lower Clear Fork formations in the study area is 915-1,220 m (3,020-4,026 ft), representing a duration of approximately 22 m.y. (16.5 m.y. assigned to the Wolfcamp, 5.5 m.y. to the Wichita and Lower Clear Fork; fig. 1C). Calculated sedimentation rates are 42-55 m (139-182 ft)/ m.y. The Wolfcamp sequence is divided into seven units (e.g., "early early," "middle early," etc.; fig. 1B, C) on the basis of fusulinid biostratigraphy (Mazzullo and Reid, 1989); the late middle Wolfcamp is absent in the study area. The seven units correspond to parasequence sets that can be identified seismically throughout the Midland basin (fig. 1D). The estimated duration of each of these sets is 2.36 m.y. Individual parasequences (e.g., "A," "B," etc.; fig. 1B) within parasequence sets variously record periods of platform erosion, progradation, or backstepping (figs. 1B, C); the thinnest of these parasequences are beyond seismic resolution. The estimated duration of each of these parasequences is 0.8-1.2 m.y. Biostratigraphic subdivision of the Wichita and Lower Clear Fork is not possible because of the rarity of fusulinids. Instead, the formations were each subdivided into various parasequences ("A-D") on the basis of lithologic and mechanical log correlations; the Tubb Sandstone Member is the fifth, uppermost parasequence in the Lower Clear Fork (fig. 1B). The estimated duration of these parasequences is 0.7 m.y. in the Wichita and 0.55 m.y. in the Lower Clear Fork. Except for the Tubb Sandstone Member, these parasequences generally are beyond seismic-resolution limits (fig. 1D). Parasequences in the Wichita are progradational in lower beds of the formation and aggradational in the upper part (fig. 1B). Parasequences in the Lower Clear Fork are mostly aggradational, and those in the Tubb are aggradational to, locally, progradational (fig. 1B).
Figure 1--A) North platform Midland basin study area, west Texas, and location of NW-SE cross section in Yoakum County; B) Platform-to-basin section (subsea datum) in Lower Permian strata, illustrating subdivisions of Wolfcamp, Wichita, and Lower Clear Fork into parasequence sets composed of parasequences "A-D" (and Tubb Sand in lower Clear Fork); C) Relationships among relative sea-level and subsidence-rate changes and platform development; and D) Platform-to-basin seismic line "L" parallel to cross section (see 1 A for location). Arrows in Wichita and Lower Clear Fork point to low-stand surfaces. All illustrations modified from Mazzullo and Reid (1989).
Shallow platform and coeval deep-basinal facies are recognized within the parasequences in the Wolfcamp and lower Leonardian section (fig. 1B). Platform lithologies in Wolfcampian and lower Leonardian rocks include pervasively dolomitized lagoonal fossiliferous wackestones to packstones and local patch-reefs and platform margin reefs composed mainly of phylloid algae, Tubiphytes, sponges, and encrusting foraminifera. Varicolored shales and siliceous carbonates, interpreted as colluvium, are present throughout the section (fig. 1B). Peritidal dolomites and evaporites, platform-margin ooid shoals, and sandstones also occur in lower Leonardian rocks in the study area (fig. 1B). Basinal lithologies include dark shales, resedimented platform-carbonate debris, and in lower Leonardian strata, interbedded sandstones (fig. 1B).
Component parasequences of the Lower Permian platform sections (duration 0.55-1.2 m.y.) variously are composed of colluvial beds associated with either thin (2.5-7.0-m [8-23-ft]), stacked, shoaling-upward cycles of shallow subtidal and peritidal dolomites (and locally evaporites) or deepening-upward cycles (fig. 1B). The four sandstone beds in the Tubb parasequence are each interpreted as sea-level low-stand deposits separated by shoaling-upward carbonate cycles. Such cyclicity within component parasequences throughout the Lower Permian section reflects a basic control on facies development of short-term, relative eustatic changes through time (Mazzullo and Reid, 1989). This short-term cyclicity likely represents third and fourth-order variations within second-order cycles (the latter represented by individual parasequences in the Lower Leonardian and parasequence sets in the Wolfcampian) of 0.55-2.36 m.y. duration (figs. 1B, C). The estimated duration of these third-order cycles is 100 k.y.; these, and fourth-order cycles, likely represent Milankovitch cycles. The second-order cycles are. in turn, components of first-order cycles that are directly coincident with long-term variations in basin-subsidence rates through time (fig. 1C).
Maximum basinward progradation of carbonate platforms occurred in early and late Wolfcampian and, to a lesser extent, in earliest Leonardian (early Wichita) times. Approximately 22 km (13 mi) of progradation, during Wolfcampian time, was facilitated by the deposition of thick sections of basinal shales that served as foundations over which carbonate platforms built (Mazzullo and Reid, 1989). This progradation was coincident with a first-order high stand punctuated by relatively few second- through fourth-order low stands of long duration (figs. 1B, C). In contrast, upper Wichita and Lower Clear Fork platforms essentially accreted vertically in a regime of high basin-subsidence rates but were affected by relatively more numerous second- through fourth-order low-stand cycles of relatively shorter and magnitude than those inferred for underlying rocks (fig. 1C; Mazzullo and Reid, 1989).
Mazzullo, S. J. and Reid, A. M., 1989, Lower Permian platform and basin depositional systems, northern Midland basin, Texas; in, Controls on Carbonate Platform and Basin Development, P. D. Crevello, J. L. Wilson, J. F. Sarg, and J. F. Read (eds.): Society of Economic Paleontologists and Mineralogists, Special Publication 44, p. 305-320
Van Wagoner, J. C., Posamentier, R. M., Mitchum, P. R., Vail, J. F., Sarg, J. F., Loutit, T. S., and Hardenbol, J., 1988, An overview of the fundamentals of sequence stratigraphy and key definitions; in, Sea-level Changes--An Integrated Approach, C. K. Wilgus, B. S. Hastings, C. G. St. C. Kendall, H. M. Posamentier, C. A. Ross, and J. C. Van Wagoner (eds.): Society of Economic Paleontologists and Mineralogists, Special Publication 42, p. 39-45
Kansas Geological Survey
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Web version May 11, 2010. Original publication date 1989.