Quazi Taufiqul Islam, Jim Crump
University of Texas at Dallas
Richardson, Tex.
The Middle Ordovician Simpson group in Oklahoma is segregated into five formations. In ascending order these are the Joins, Oil Creek, Mclish, Tulip Creek, and Bromide formations (Fig. 1). The Simpson group is important from a sedimentological viewpoint because the sequence records the first influx of clastic sediments over a region which had previously been the site of a vast amount of carbonate accumulation, namely the Arbuckle group. From an economical standpoint the Simpson group has had a great significance to the oil industry as it has been, and still is, a prolific producer of oil and gas in Oklahoma.
The purpose of this paper is to study thickness variations of the Simpson group on a somewhat smaller scale: the Arbuckle Mountain region of south central Oklahoma (Fig. 2).
Such a study is of interest in that it will give an indication of both the geometry of the depositional basin as well as the nature of the shelf to basin (known as the Southern Oklahoma aulacogen) transition during Simpson time (Fig. 3).
Thickness data, both surface and subsurface, were used to produce an isopach map for the Simpson group in the study area (Fig. 4). The subsurface data were obtained from both electric logs and well completion cards. Nearly all of the well data utilized penetrated the entire Simpson group. In the few exceptions where wells did not penetrate the entire thickness of the Simpson, they did go at least down into the basal Oil Creek sand unit. In such a case, the base of the Simpson was inferred by using conservative estimates based on thickness from adjacent wells where a complete section was recorded. Surface thicknesses for the Simpson group were calculated from Ham and McKinley's (1954) geological map of the Arbuckle Mountains in areas where the Simpson group is exposed in its entirety, unaffected by faulting and where the dip angles are fairly uniform.
For purposes of continuity, the areas in the Arbuckle Mountains where uplift has been successful in eroding Simpson sediments have had thickness restored by contouring across the now eroded areas. This aids in visual examination of the geometry of the Simpson deposits. Some of the wells drilled close to major fault traces show repeated lithologic sections on electric logs indicating a faulted sequence. Lack of data close to these major fault zones does not permit sufficient control to determine the effects of displacement. We believe that the study area is large enough to allow contouring across the faulted areas without losing credibility to the resulting isopach map.
Two cross-sections have been constructed from subsurface data in order to show thickness variations across the study area (Figs. 5, 6). Positions of these cross-sections were selected to show maximum thickness variation from shelf to basin.
STRATIGRAPHIC SUBDIVISIONS
Due to the problems discussed below, the logs used in construction of the cross-sections for this study have been subdivided into three formational divisions (Figs. 5, 6). These are, in ascending order: (1) Joins and Oil Creek, (2) Mclish, and (3) Bromide. The total thickness of the Simpson group, the main interest in this study, is unaffected by this breakdown.
The Tulip Creek is recognizable on outcrop as a discrete formation. However, the Tulip Creek is gradational into the overlying Bromide formation, creating difficulty in distinguishing the two formations at the surface, especially in the southern part of the Arbuckle Mountains. There is a great deal of controversy over the recognition of the Tulip Creek in the subsurface. The difficulty in distinguishing this formation in the subsurface apparently arises from the fact that the basal Tulip Creek sand is often poorly developed, making it extremely difficult to distinguish from sand units within the Mclish formation. Because of this discrepancy, we have chosen to consider the Tulip Creek as a basal Bromide sand.
The Joins formation is recognizable in most of the logs utilized in this study. However, its generally thin nature, relative to other Simpson formations, has led to inclusion of the Joins with the overlying Oil Creek formations.
STRUCTURAL ASSOCIATIONS
Three distinct Paleozoic geotectonic provinces are recognized in the region of southern Oklahoma. In the southeast, the Ouachita Mountains show a record of ocean development and subsequent orogeny. Immediately to the west, in south-central Oklahoma, are the Arbuckle Mountains characterized by a folded and faulted sequence of Paleozoic and Precambrian rocks. Further west, the Wichita Mountains are composed of outcrops which are primarily igneous rocks of Cambrian age. Both the Wichita and Arbuckle Mountains are a part of a now deformed basin which was characterized by rapid subsidence. This narrow, elongate feature is the Southern Oklahoma aulacogen (Fig. 3).
Development of the Southern Oklahoma aulacogen involved three evolutionary stages. These are: (1) rifting stage, (2) subsiding stage, and (3) deformational stage. The characteristics of each of these successive stages are as follows: The rifting stage was represented by uplift and graben formation followed by Middle Cambrian igneous activity of both intrusive and extrusive character. The subsiding stage of the aulacogen was dominated by Late Cambrian through Early Mississippian sedimentation. The resulting deposits were predominantly shallow-water carbonates with a subordinate amount of relatively deeper-water limestones, sands, and shales. It was during this stage that the sediments of the Simpson group were laid down. Due to differences in the subsiding rate, sediments within the aulacogen are about seven times thicker than equivalent strata on the adjacent craton. The deformation stage, which began in Late Devonian and continued through to earliest Permian time, is represented by the present configuration of paired basins and uplifts across southern Oklahoma (Fig. 3). Deformation within the Arbuckle Mountains during the Pennsylvanian was dominated by displacement along major high-angle fault zones, many of which may have originated as normal faults as early as Cambrian during the rifting stage. Several models have been proposed to explain the structural complexities within the aulacogen. Some of these models are: (1) gravity slide, (2) vertical uplift, (3) horizontal compression, (4) oblique slip, and (5) wrench faulting. In the Arbuckle region the relationship between fold axes and major fault traces as well as offset facies boundaries strongly supports a left-lateral wrench fault hypothesis. However, it is hard to explain all the structural complexities within the aulacogen by one single mode, making it reasonable that a combination of processes may have independently played a dominant role during successive deformational stages.
SIMPSON DEPOSITION
Disconformably overlying the Arbuckle group in Oklahoma is the Middle Ordovician Simpson group, a sedimentary sequence consisting of interbedded limestone, sandstone, arid shale. Deposition during Simpson time was fairly continuous in southern Oklahoma, while more-sporadic sedimentation to the north resulted in thinning of the sedimentary units. The greater thickness of the Simpson sediments in southern Oklahoma is related to syndepositional subsidence within the Southern Oklahoma aulacogen; the adjacent craton to the north was comparatively stable.
Four pulses of subsidence submerged the Southern Oklahoma aulacogen during Simpson depositon; the Oil Creek, Mclish, and Tulip Creek formations were deposited during the first three pulses, while the fourth pulse, which coincided with a rise in sea level, led to deposition of the Bromide formation.
Decker was the first to recognize that each of the Simpson formations reflected a more or less complete sedimentary cycle with a sandstone unit at the base of each of the upper four formations and an intraformational conglomerate at the base of the lowermost Joins formation. Positioned above each of the basal sandstones, and in effect completing the sedimentary cycle, is a general sequence of interbedded limestones, shales, and sands. It has been pointed out that because of lateral facies changes the cyclicity exhibited by Simpson sediments is highly imperfect.
The clean, well-sorted sandstones which mark the base of the formations of the Simpson group are interpreted as reflecting progradational sands units deposited in upper and lower shoreface environments during periods of relatively little subsidence. The ultimate source of sand was from the Canadian Shield, and the sand probably entered Oklahoma from the west and northeast. As each period of subsidence began, marine shales with interbedded limestones transgressed out of the basin on top of the well-sorted sand units.
DISCUSSION AND CONCLUSION
The study area can be subdivided into four distinct tectonic zones. These are: (1) an area of thin, fairly uniform Simpson deposits in the northern portion of the study area, (2) a northwest-southeast trending narrow zone of abrupt thickening of Simpson sediments in the southwest portion of the study area, (3) a thick accumulation of Simpson sediments in the southwest portion of the study area following the same trend as the abrupt zone of thickening previously mentioned, and (4) a relatively small, isolated area of thick Simpson sediments in the southeast portion of the study area (Fig. 7).
The relatively thin, uniform nature of Simpson sediments in zone 1 suggests that the area reflects deposition across a stable cratonic shelf. Moving in a southwesterly direction across the study area, an abrupt zone of thickening of Simpson sediments is encountered which marks a transition to much more tectonically unstable area during Simpson (and probably through most of Cambro-Ordovician) time. The transition across this zone of thickening is nicely exhibited in the cross-section A-A' (Fig. 5). The 1 Horner well, which records a relatively thin Simpson sequence, was deposited on the stable shelf. The 1 Freeman Heirs well, which shows drastic thickening of the entire Simpson package over a short horizontal distance, reflects deposition within the "hinge line." This zone probably coincides with the northern edge of the Southern Oklahoma aulacogen. Moving further south, the Simpson sediments continue to thicken but to a lesser degree. The maximum thickness of the Simpson sediments in the map area probably lies in close proximity to the 1 Reuel W. Little, thus defining the depocenter of the depositional basin. The depocenter is located in an area now known as the Ardmore basin.
The configuration of the isopach map defines a basinal axis which follows the same trend as the zone of rapid thickening. The thickness gradient of the basin is much more subtle in the southwest portion relative to the northeast, indicating that the basin has an asymmetrical nature.
In general, the cross-section A-A' shows thickening of the Simpson formations toward the south across the basin with the most pronounced thickness change in the region of the "hinge line." The Bromide formation, like other Simpson formations, drastically thickens in the "hinge line," but unlike the other Simpson formations the Bromide thins slightly toward the depocenter of the basin. This relationship can be explained by a shelf edge buildup confined to the region of the "hinge line" creating an anomalous Bromide thickness in the area.
Another important feature recognizable on the isopach maps (Fig. 4) is a small isolated basin roughly trending northwest-southeast in the southeast portion of the study area. The cross-section B-B' (Fig. 6) shows a significant thickening of Simpson sediments from 5 Patric to 1 Race, illustrating the presence of a basin in the area. Due to sparse data, the precise configuration of this small basin is not well-understood; however, it is apparent that this is a much smaller feature than the basin previously described.
Paleostress studies using calcite twin lamellae along the southeastern portion of the Sulphur fault within the smaller basin reveal an earlier east-west compression followed by a later north-south compression (Fig. 8). Surface fold formed by the east-west compression is highly faulted and overturned, whereas the fold formed by the north-south compression (Wapanucka syncline) is open, slightly asymmetric, and doubly plunging with rounded hinge and limbs. It trends east-west. Similar east-west trending structures in the subsurface could be favorable targets for hydrocarbon exploration in this area.
In conclusion, both the basins defined in the study area are probably the result of syndepositional subsidence adjacent to zones of weakness. These zones may be related to the initial rifting stage of aulacogen development. These two basins are most likely genetically related, and their isolation is probably due to the presence of a large Precambrian basement block (Tishomingo granite) positioned between them.
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