Journal of the Graduate Research Center


Methods for establishing precise correlations between isolated surface outcrops in flat-lying formations which are exposed poorly in areas of low relief are few in number if they exist at all. This point is well illustrated by the Upper Cretaceous Austin Chalk in north-central Texas. It is a relatively easy matter to correlate generally on the basis of lithology and stratigraphic or map position between the sparsely distributed outcrops and to recognize that one is within the lower, middle or upper Austin. However, acceptable correlations of a more specific nature (e.g., bed-for-bed or the exact relationships between two sequences which contain no apparent diagnostic criteria) have been difficult to achieve. Various techniques have been employed. C. I. Smith (1955) used a method whereby the vertical profile of a weathered outcrop was drawn to scale to show thickness of resistant and non-resistant beds and their resulting horizontal relief. Thus, by comparing and "fitting" the profiles of various outcrops, he believed that correlation could be achieved. The validity of Smith's method has never been conclusively evaluated. Some flaws are apparent, however. The degree of weathering of an outcrop is of paramount importance to his method. The present writers have observed the striking difference in horizontal profile that exists between deep road cuts in the Austin Chalk seen within about a year after they were made, as compared with stratigraphically and lithologically similar cuts which are five to ten years old. In addition to this principal factor of time, the rate of weathering is affected by such variables as orientation of exposure with respect to sunlight, drainage, degree of slope, and cover on adjacent areas. None of these is easily evaluated in terms of its effect on specific outcrops. Williams (1957a) employed a method in which the horizontal outcrop profile, the percent insoluble residue and the color (using the National Research Council Standard Color Chart) were used as criteria for correlation. The added criteria of composition and color have added appreciably to the degree of validity of Smith's method. Williams was able to make detailed correlations with apparent success between outcrops separated by as much as three miles. Other geologists have employed fossils of very limited vertical range (e.g., Parapuzosia, Clark, 1960) as guides to the specific beds which contain them, assuming that over the short distances involved little disparity between time and lithologic surfaces would exist. In view of the apparent success of correlations based on thermoluminescent glow-curves in other areas (Saunders, 1953, and Parks, 1953) we have applied this technique to selected outcrops of the Austin Chalk in Dallas County, Texas, with the hope that it might provide an additional tool with which to make detailed correlations in this formation and other similarly poorly-exposed, flatly-dipping rock sequences. With this in mind, representative samples were carefully selected from the collection made by Williams (ibid.) from the lower and middle Austin in southern and central Dallas County and were subjected to thermoluminescent ("glow-curve") studies. Samples were chosen from beds which Williams considered to be correlative at various stratigraphic horizons and over varying distances between outcrops. These samples also represented compositions ranging from about 88% carbonate and 12 ¼ insoluble (presumably terrigenous material) to 63% carbonate and 37% insoluble.

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