Abstract

EXTENSION OF FAULT BELT INTO ARKANSAS

For many years interpreters of Ouachita structure have struggled with the problem presented by the presence of a fault belt north of the Windingstair Fault on Oklahoma geologic maps and the absence of its eastward extension on Arkansas geologic maps. The recent work of Reinemund and Danilchik, the writer, and graduate students working under the direction of L. M. Cline indicates that the belt does continue into Arkansas and contains several major thrusts. It is probable that the belt will eventually be found to be continuous along the Ouachita front from near Atoka, Oklahoma to Little Rock, Arkansas. The possibility of the belt's extension into Arkansas was recognized by some early workers according to H. D. Miser. Much of the geology along its course in Arkansas was originally mapped by reconnaissance methods for inclusion on the Geologic Map of Arkansas (1929).

LLANORIA

In 1921 H. D. Miser published a paper summarizing the evidence that had been accumulated by various workers which pointed to a southern source ("Llanoria") for Ouachita sediments. This evidence included a southward and southeastward thickening of the Jackfork, Stanley, Blaylock, and Blakely, a southward increase in the sand-shale ratio of the Stanley and Jackfork, the presence of small (1/4 inch) quartz pebbles in the lower Jackfork on the southern border of the Ouachitas, and a southern derivation of the Johns Valley erratics from a southeastward extension of the Arbuckle Uplift. Work completed since 1921 has improved our understanding of Ouachita stratigraphy in Oklahoma, and some of this earlier evidence is now questionable.

Jackfork Thickness Variation in Oklahoma

According to L. M. Cline, thicknesses of the Jackfork are 5650 feet in the Tuskahoma syncline; 5600 feet and 5800 feet in the Lynn Mountain syncline. O. B. Shelborne gives a thickness range of 5400 feet to 6500 feet in Boktukola syncline with the greater thickness to the east. The writer's work in Rich Mountain syncline shows a Jackfork thickness of about 5900 feet. These measurements do not indicate a southward or southeastward thickening of the Jackfork within the region containing these synclines, although thinning does occur to the north and west of this region.

Stanley Thickness Variation in Oklahoma

Thicknesses of the Stanley are difficult to determine. By piecing together several incomplete sections, Hendricks, et. al. , and R. B. Laudon have determined a composite thickness of 12,000 feet. Comparison of Stanley sub-unit thicknesses near the Tuskahoma syncline and the Oklahoma core area lead Laudon to the opinion that the Stanley maintains a uniform thickness over that distance. Thus, it now appears that the Stanley-Jackfork sequence does not thicken southward or southeastward within the interior Ouachitas of Oklahoma.

Sand/Shale Ratios and Maximum Grain Size Variation of the Jackfork in Oklahoma

No quantitative work has yet been published on sand/shale-ratio variations within the Ouachitas. The writer has compared the Jackfork stratigraphy of Rich Mountain syncline with descriptions published by Cline and by Shelborne of Lynn Mountain and Boktukola synclines. No obvious difference in Jackfork sand/shale ratios was noted. Certainly in Oklahoma the Jackfork does not change from almost entirely sandstone to the south to almost entirely shale to the north as was indicated by Miser (who was probably referring to Arkansas , although this was not specified). Small quartz pebbles and granules are present in the Jackfork of Rich Mountain syncline and in many other Jackfork localities. They are not restricted to the southern border of the Ouachitas in Oklahoma.

Distribution of Johns Valley Erratics

Johns Valley erratics have not yet been found in Boktukola syncline, in the east end of Lynn Mountain syncline or in Rich Mountain syncline. According to Cline the size of the erratics decreases in a southeasterly direction within the area in which they are found. These factors argue strongly against a southern or southeastern source for the erratics.

Paleocurrent Studies

Recent paleocurrent studies of Jackfork-Johns Valley-Atoka "turbidities" exposed near the Oklahoma-Arkansas border have shown a prevailing east-to-west current direction. It may be suggested that these currents originated at a southern source and were turned sharply westward; however, observational evidence of this is yet to be found.

Summary

A southern or southeastern source seems highly improbable for erratics in the Johns Valley Shale. Much of the original evidence used in support of a southern or southeastern source of Ouachita sediments is not supported by subsequent work in Oklahoma.

Although a southern source can not be ruled out for the Stanley-Atoka sequence of Oklahoma, present data suggest that any eastern azimuth should be considered as a possible source direction. It has already been postulated by Scull, et. al. , that Atoka beds of the Arkoma Basin had an eastern source.

OUACHITA STRUCTURE

Some hypotheses to account for Ouachita structure are: (1) simple gravitational sliding (or spreading) with no basement shortening; (2) gravitational sliding (or spreading) accompanied by basement shortening; (3) basement shortening with little or no gravitational sliding; (4) two deformational episodes involving any one or two of the first three mechanisms. Basement shortening here is used in the sense that two points in the basement adjacent to the deformed belt and across its strike, are closer together after deformation of the belt than before. In the absence of such shortening there can still be vertical movement of the basement. Initial vertical movement of the basement can also be followed by a horizontal component of movement due to gravitational effects. Likewise, the presence of shortening does not preclude gravitational sliding or spreading.

In (1) and (2) above structures of the sedimentary cover are primarily the result of gravitational "pulling", whereas in (3) above these structures are caused by a "pushing" of basement blocks overriding one another and carrying their sedimentary cover with them. The concentricity of fault-fold trends about the core area in Oklahoma appears to continue into northeastern Texas in the subsurface. In the Rich Mountain area these trends apparently formed perpendicularly to the deforming forces. If this relationship is true elsewhere in the western Ouachitas, then one may conclude that deforming forces were directed radially northward, northwestward, and westward from the exposed core area and its subsurface extension. The increase in deformational intensity of younger Paleozoics toward the outer margin of the salient is compatible with the hypothesis of gravitational sliding down a surface sloping outward from the core and having its foot near the periphery of the salient (hypotheses (1) and (2) above). Both the outward increase in deformational intensity and the concentric pattern of faults and folds about the Oklahoma core are incompatible with (3) above. Hypothesis (4) above can be set aside as being improbable because of the lack of known angular unconformities in the stratigraphic section. Hypotheses (1) and (2) are both handicapped by the absence of a tectonic denudation exposing a sizeable area of beds below a decollement in the core area. The amount of denudation necessary, however; would be dependent upon such things as the amount of shortening in the sedimentary cover and the relative roles of gravitational sliding and gravitational spreading.