Previous reports on the complex Madill-Aylesworth area of Marshall and Bryan Counties, Oklahoma, have emphasized the role of faulting in explaining relationships between the Goddard Shale (Lower Springer) and older stratigraphic units. The writers find a systematic pattern at the base of the Goddard such as would be formed by deep erosion of a pre-Springeran anticline. Thick sections of Goddard Shale are found in off-structure positions and in a belt presumed to occupy a deep valley along the axis of the old anticline. Detailed cross sections and reconstructions to an early Pennsylvanian datum indicate an axial valley over 2500 feet deep (cut largely in Simpson and Arbuckle) between strike ridges formed by the limbs of the anticline.

According to the writers' hypothesis the area had a very considerable erosional relief at the time of onlap of Goddard sedimentation and, in fact, had a strong resemblance to the "sheepherder structures" exposed today in the Rocky Mountain province. Sedimentary reflections of the buried topography are not obvious. However, there are isolated instances of conglomerate with pebbles of older units at the base of the Goddard. Further, productive sands within the Goddard form a trend along the southerly limb of the assumed anticline as though reflecting the influence of a buried topographic ridge. The absence of a similar sand

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build-up on the northern flank of the old structure is partially explained by the depth of pre-Dornick Hills erosion.

Major high-angle faulting is a prominent feature of the present-day structure, but the extent of faulting associated with the pre-Springeran structure is difficult to estimate. If there was active fault movement with the older structural development, the numerous Simpson oil accumulations are primarily fault traps. However, if faulting was unimportant in pre-Springeran time the Simpson pools represent truncation traps, presumably filled by younger migration.

W. A. BECKMAN, JR.

W. A. Beckman, Jr., has been a geologist with the Evanston Exploration Corp. since graduation from Northwestern University in 1958. In addition to work in southern Oklahoma, he has been actively engaged in the geology of the Michigan Basin and with the crystalline rocks of central Wisconsin.

L. L. SLOSS

L. L. Sloss has been on the faculty of Northwestern University since 1947, combining an interest in the geology of southern Oklahoma with other more strictly academic pursuits.

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