DISHARMONIC FOLDS IN SOUTHERN OKLAHOMA: REVISITED

Search and Discovery Article #60004 (1999)

Editorial Note: C. W. Tomlinson (e.g., 1952), as a leading pioneer in unraveling the geologic history of Southern Oklahoma, described features he termed "rabbit-ear folds" or rootless anticline," which are disharmonic folds that formed largely because of the ductility of the Springer Group (in particular the Goddard Formation) during Pennsylvanian deformation. The shale of the Goddard might be considered analogous to the salt of pillow structures. This type of feature was rather well documented in publications largely from the 1950s.

Later, Harding (1985) showed that at least some of the disharmonic folding occurred as a relatively minor feature accompanying wrench faulting.

Presented here by means of images, with limited text from several authors, are examples of disharmonic folds in Southern Oklahoma.

Carter-Knox oil field (Reedy and Sykes, 1959) is located in the southeastern end of the Anadarko basin in southeastern Grady County and northeastern Stephens County; it covers an area of approximately 16.5 square miles and 11 miles long (Figure 1). The structure is a very steeply dipping, faulted anticline (Figure 2, Figure 3, Figure 4, Figure 5). Deformation of the Springer-Goddard and overlying Pennsylvanian strata is much more intense than Lower Paleozoic beds.

The field produces from Permian, Pennsylvanian Hoxbar, Deese, Mississippian Springer, and Ordovician Simpson reservoirs.

Milroy field (Schweers, 1959), Stephens and Carter counties, was discovered in 1917. It composes some 2650 acres in the east-central part of T.2S, R4W, and extends into T2S, R3W (Figure 6). The Milroy structure is only slightly closed on the surface, but it attains more than 2500 feet of closure in the subsurface Pre-Pennsylvanian strata (Figure 7, Figure 8, Figure 9). Its N65W axis parallels the Wichita-Criner anticlinoirium.

Production is derived from several horizons, from the Permian to the Ordovician, but the bulk of the oil has been produced from Desmoinesian and Atokan reservoirs.

In Southeast-Velma field area, Stephens and Carter counties (Parker, 1959) (Figure 6), the Springer section shows local variations and absence of beds due to interrupted or variable sedimentation and to folding (Figure 10, Figure 11, Figure 12) and faulting. The northwest-southeast-trending synclinal area between the major Milroy uplift to the southwest and the relatively minor Camp uplift to the northeast has on its southwest flank a series of serpentine folds in the Humphreys sandstone beds, which are shown to be continuous although small-scale faulting is likely present.

Fox-Graham and Caddo fields, Carter County (Tomlinson, 1952) (Figure 6), shows disharmonic, similar folding, with extensive axial thickening of major shale (Springer-Goddard) section (Figure 13, Figure 14). The former "rabbit-ears" anticline contains a lateral anticline that parallels the central arch. This secondary fold is very sharp in shallow beds that unconformably overlie truncated shales dipping away from the central fold; it is not reflected in the underlying, more competent rocks.

The crest of the Caddo anticline in ductile (Springer-Goddard) and younger beds is not displaced significantly from the crest in the older competent units.

Harding (1985) describes a positive flower structure in the Ardmore basin (Figure 15). This structure shows disharmonic folding, described three decades earlier, as noted above. A shallow antiform "is displaced by upward diverging strands of a wrench fault that has mostly reverse separations. En echelon folds are present on either side of this convergent wrench fault. . . Flower structures are identified by the unique internal fault and fold architecture, by their association with a straight and through-going zone of deformation, and by their lack of a consistent sense of separation or regional upthrown side. . .

"Sharply upturned beds form the flanks of the antiform and strike in narrow bands parallel to and on either side of the through-going wrench fault. The antiform is subparallel to the principal strike-slip zone. . . It may resemble an anticline only in its morphology. The outward-dipping flanks have been tilted independently and may never have been parts of a coherent, flexed dip reversal. The inward-dipping fault splays have profiles ranging from steep upthrust to shallow-dipping thrust and merge at depth with a central strand that is thought to be usually subvertical. The complex internal faulting of positive flower structures can unduly segment potential hydrocaron traps, and they are not as attractive hydrocarbon prospects as the less intensely deformed en echelon folds. . .

"Formation of positive flower structures is promoted by a component of convergence normal to the wrench fault, by increased strike-slip displacements, and by the presence of a thick and ductile sedimentary section."

References

Harding, T.P.,1985, Seismic characteristics and identification of negative flower structures, positive flower structures, and positive structural inversion: AAPG Bull., v. 69, p. 582-600.

Parker, E.C., 1959, Structure and lithology of the Springer in Southeast Velma-Camp area, in Petroleum Geology of Southern Oklahoma, v. II: AAPG, p. 227-248.

Reedy, H.J., and H.A. Sykes, 1959, Carter-Knox oil field, Grady and Stephens counties, Oklahoma, in Petroleum Geology of Southern Oklahoma, v. II: AAPG, p. 198-219.

Schweers, F.P., 1959, Milroy field, Stephens and Carter counties, Oklahoma, in Petroleum Geology of Southern Oklahoma, v. II: AAPG, p. 220-226.

Tomlinson, C.W., 1952, Odd geologic structures in Southern Oklahoma: AAPG Bull., v.36, p. 1820-1840.

Figures (with respective caption accompanying each)

Figure 1--Location map of Carter-Knox field, Grady and Stephen counties (from Reedy and Sykes, 1959).

Figure 2-Structure of the Pennsylvanian in Carter-Knox field, contoured on Woods sandstone (Springer) (from Reedy and Sykes, 1959).

Click here for sequence of Figures 2 and 3.

Figure 3--Structure of the pre-Pennsylvanian in Carter-Knox field, contoured on Viola Limestone (from Reedy and Sykes, 1959).

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Figure 4--Sketch of typical seismic time cross-section (SW-NE), Carter-Knox field (from Reedy and Sykes, 1959).

Figure 5--Southwest-northeast cross-section of Carter-Knox field (after Reedy and Sykes, 1959).

Figure 6--Index map showing outline of Milroy, Southeast Velma-Camp field, and Fox-Graham fields, discussed here. Southern part of Carter-Knox field is located about 10 miles north of the northwesternmost extent of the Velma complex. Caddo anticline is located approximately 15 miles east-southeast of southeastern extent of Fox-Graham field. (From Schweers, 1959).

Figure 7--Milroy field. Subcrop map showing distribution of formations beneath pre-upper Dornick Hills (pre-Atokan) unconformity; contours on unconformity (from Schweers, 1959).

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Figure 8--Structure of Milroy field, contoured on Tussy limestone (Deese—Desmoinesian) (from Schweers, 1959).

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Figure 9--Southwest-northeast cross-section of Milroy field (from Schweers, 1959).

Figure 10--Structure of Southeast Velma-Camp area, contoured on top of Humphreys sand (Springer) (from Parker, 1959).

Click here for sequence of Figures 10 and 11.

Figure 11--Structure of Southeast Velma-Camp area, contoured on top of First Sims sand (Springer, below Humphreys sand) (from Parker, 1959).

Click here for sequence of Figures 10 and 11.

Figure 12--Cross-section (AA’) of Southeast Velma-Camp area. BD=basal Deese, unconformably above Springer units; S-1=First Sims sand (after Parker, 1959).

Figure 13--Cross-section of Fox-Graham field (after Tomlinson, 1952).

Figure 14--Cross-section of Caddo anticline (after Tomlinson, 1952).

Figure 15--Positive flower structure (in seismic profile) along convergent wrench zone in Ardmore basin (after Harding, 1985). T, displacement toward viewer; A, away from viewer.