Fondren Science Series


A review of the literature indicates that clastic dikes, generally considered curiosities, are common features of many marine shales and marls. Excepting the Eagle Ford shale, dikes have been found by the writer in all of the Upper Cretaceous groups of northeastern Texas. Studies of twenty-one Woodbine dikes in McLennan County indicate that most of these were formed by the deposition of marine sediment in tension cracks opened along the sea floor during the time that an upper sand member of the Pepper formation was deposited. This hypothesis is based on the presence in the dikes of bedding, as indicated by horizontally oriented pelecypod valves, shale fragments, and pieces of lignite. The cracks in which the sand was deposited were formed by slumping within the Pepper shale, which shows intrastratal crumplings, slip planes, spiral rolls, isolated rolls, and small elongate blebs of silt within the claystone above and below beds of siltstone. After the cracks were filled, slumping continued, dikes were offset by slip planes, and by compression and shear, were contorted in varying degrees. As a result of this contortion some dikes contain pelecypod valves oriented parallel to the containing walls. This continued slumping caused the water-saturated sand to flow into cracks created thereby and thus to form intrusive dikelets.

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