High Relief Topography on the Jurassic Unconformity (Alabama)
J.R. English, J.S. Oslund, 1983. "High Relief Topography on the Jurassic Unconformity (Alabama)", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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Along the edge of the interior salt basin in the southeastern United States there is a high relief unconformity. This unconformity was developed during deposition of the Jurassic Louann Salt and was buried by Jurassic sediments during the subsequent marine transgression related to the divergence of the South and North American continents and subsidence of the basin. The morphology of the unconformity may be characteristic of early stages of continental plate divergence. A seismic line near the edge of the salt basin in south Alabama demonstrates the acoustic response along the unconformity.
The area described in this paper is located in the southeastern United States along the edge of the interior salt basin (Figure 1). The unconformity is Jurassic in age and can be identified from Florida to Texas. Because it is regionally extensive, it truncates a wide variety of older rocks. The major units it truncates are Paleozoic sediments of the Appalachian and Ouachita orogenic belts, metamorphic rocks associated with these orogenic events, and Mesozoic continental sediments of the Neward group which were deposited during the early stages of divergence. There were four Jurassic units younger than the unconformity in the area shown on the cross section (Figure 2) and interpreted record section (Figure 3). These are the Norphlet, Smackover, Haynesville, and Cotton Valley. In Alabama, the Norphlet is a shallow marine and eolian sandstone; the Smackover is a shallow marine carbonate; the Haynesviller is a prograding sabkha and continental sequence; and the Cotton alley is a continental sandstone sequence. There are unconformities at top of Haynesville and Cotton Valley which modified the topography of the older unconformity where is was not buried. The seismic section (Figures 3, 4, 5) is located in south Alabama just north of the updip edge of the salt. It is a 24-fold, common-depth-point (CDP) dynamite line shot in 1980. The line parallels regional dip and is 5 mi (8 km) in length. Well control indicates that the rocks beneath the unconformity are granulites and mica-rich metamorphic rocks. The area is interpreted to be the subsurface equivalent to the Brevard zone of the Appalachians.