ORGANIC REEFS OF PENNSYLVANIAN AGE IN HASKELL COUNTY, TEXAS

The reflection seismograph was used to map a prominent organic reef in the Pennsylvanian system near Stamford in central northern Texas, and the results were checked by drilling. Since seismic wave velocity in the limestone comprising the reef is the nearly twice that in the enclosing shale, distinctive reflections from beds below the base of the reef were mapped above their true position by an amount equal to almost half the reef limestone thickness. The false structure so mapped has been interpreted as showing the topography of the upper surface of the reef. As such, it is a mound 3 miles long and half as wide, with a corrected height of 1,000 feet and a central depression 300 feet deep, resembling a present‐day atoll. Numerous wells confirm the presence of this reef, but cast serious doubt upon the simple topographic interpretation of the seismic map. The atoll‐like central depression appears to be an illusion, caused by the reef having a relatively high‐velocity shell and a lower velocity center. The Stamford reef is an exceptional development along a seemingly continuous reef belt mapped similarly with refection seismograph for 25 miles. Where reef development is less extreme along this belt, and along two others of a closely related type, the reef trends show as long, as gently curving, somewhat irregular seismic “highs” with many small contours closures. Detailed seismic interpretation of these reefs, and of the structure underlying horizons which they obscure, must await improved geologic understanding of the reefs and of the subtle changes in lithology that modify seismic wave velocity. Stamford reef illustrates also what a difference there can be between seismic closure, an obvious trap by subsurface mapping, and the location of a good oil field.