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Origin and Paleoecology of Maastrichtian Rockground and Chalk Facies in Southcentral Alabama

Jonathan R. Bryan
PALAIOS
Vol. 7, No. 1 (Feb., 1992), pp. 67-76
DOI: 10.2307/3514796
Stable URL: http://www.jstor.org/stable/3514796
Page Count: 10
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Origin and Paleoecology of Maastrichtian Rockground and Chalk Facies in Southcentral Alabama
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Abstract

A phosphatic rocky substrate (rockground) of Maastrichtian age has been recognized at the top of the Ripley Formation in southcentral Alabama, near the town of Braggs. The rockground is developed in a coarse-grained sandstone and is associated with a regional unconformity and probable subaerial exposure (68 Ma sequence boundary). The sandstone was subject to high-energy conditions which produced large, well-rounded cobbles. Thus, at some point, the surface was presumably a rocky shore. The rockground later hosted a low diversity epibiont fauna of bivalves, bryozoans, and serpulid worms. Regular echinoids were common. There is no evident biotic zonation or succession on the rockground or the cobbles. The epibiont molluscan fauna of the rockground, as well as the molluscs of the overlying, lower Prairie Bluff Chalk, are indicative of comparatively deep or offshore conditions. Marine vertebrate remains are common in the Prairie Bluff and include a partial skeleton of the mosasaur Mosasaurus maximus. The stratigraphic association of rockground, chalk, and mosasaur may not be coincidental. During the time of the Ripley-Prairie Bluff unconformity, the widespread Ripley Formation was most deeply eroded in southcentral and southwestern Alabama, and it is apparently in this region only that the rockground developed. As sea level rose, this distally-located, truncated surface accommodated greater water depths and hosted the chalk molluscan community. This relatively undisturbed, offshore environment was especially favorable for the preservation of an intact mosasaur skeleton.

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