Subdivision and subsurface correlation of the Ellenburger group in north-central Texas




Hendricks, Charles Leo, 1907-

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A series of rocks known as the Ellenburger group of Texas was deposited during Upper Cambrian and Lower Ordovician time. The Ellenburger group is composed of a succession of limestones and dolomites reaching a maximum thickness of over 2000 feet, from which the group thins and disappears in certain underground areas. A series of faunal zones have been recognized within the group, which serve to identify the age of various sections found on its outcrop. The group is marked by a large unconformity at the top throughout most of its extent, but over a portion of its occurrence it is overlain by sediments of Middle Ordovician age, in apparent conformity. Interpretation of faunal evidence has indicated that an unconformity exists at the base of the group. The structural attitude of the Ellenburger is obscured by the unconformity at its top, since contours drawn on the old erosion surface actually present a topographic picture rather than an unmodified structural picture. The sedimentary history of the Ellenburger is not known, hence there is uncertainty as to what part of the change in thickness exhibited by the group is due to non-deposition, and what part is due to erosion subsequent to deposition. Further, a close correlation between the outcropping beds and those in the subsurface extension of the group has not been established. Fossils are never found in well cuttings from the Ellenburger, and only very rarely in cores, hence there has been no age correlation of the subsurface sections. This paper is concerned with the discovery and application of a means of solving the Ellenburger problems outlined above. The recognition of correlative units within the group which are evident from both surface and subsurface data is essential to the solution. Such units must have a lithologic basis, since fossils are not recoverable from well cuttings. The Ellenburger group exhibits a variable alternation of limestones and dolomites of varying textures, which do not offer means of correlation. The insoluble residues from these limestones and dolomites, however, show characteristics peculiar to certain units within the group. The plan of the present paper is to describe these units, show their relation to the faunal units recognized on the outcrop of the group, and illustrate their occurrence in a portion of the subsurface. From the extent and relationships of these units conclusions may be drawn as to the structural and stratigraphic relationships of the Ellenburger group, and the age of the subsurface sections may be determined. A study of samples from the subsurface extent of the Ellenburger has been carried on by the writer over a period of several years. During the years 1940 and 1941 a rapid reconnaisance survey was made of the entire Ellenburger outcrop area, with selected sections studied in detail. In the summer of 1940 a detailed map was made of an area in southeastern San Saba County, showing the outcrop of the recognized subdivisions. The results obtained from those studies form the basis for this report