Appalachians, erosion

Harbor, D. J., 1996, Nonuniform erosion patterns in the Appalachian mountains of Virginia, Geol Soc Am Abstr Programs 28 no. 7 116. 

The 50 million years of the Devonian Period provided sufficient time for over 11,000 feet of sedimentary rocks to accumulate in the Appalachian Basin this accumulation was controlled by pulses of tectonic uplift to the east. A similar control of a northwestern source area by intermittent tectonic movement is suggested as a source for the upper wedges (B/C) in the Michigan and Illinois Basins. The lower black shales (A) of regional type represent quiescent periods of slow deposition followed by relatively more rapid sedimentation occurring in response to uplift and erosion of source areas and to depositional restrictions caused by rising arches. The total thickness of Devonian shales in either the Michigan or Illinois Basin is less than 1000 feet, so that the sediment volume involved in these two basins is far less than in the Appalachian Basin. Thus there was ample time for... 

Since the downwarping of the continental margin did not require uplift inland of the Fall Line, erosion rates on much of the land surface supplying sediment to the continental margin must have remained relatively low. Menard (1961) estimated that 7.8 x 10 km of rock must have been removed from the Appalachians over 125 Myr to account for the sediment now on the continental terrace and rise and on the abyssal plains off the east coast of North America. The mean sediment yield required to produce this material is —0.2 kg/(m yr). (For comparison, this is about the same as the sediment yield of the Missouri River drainage basin today.) Matthews (1975) has used the more extensive data on sediment thicknesses off the Atlantic Coast now available to estimate that the sediment yield of eastern North America over the past 60 Myr was 0.012 kg/(m yr) for the northern half of the coast and 0.067 for the southern half. These sediment yields can be attained with a land surface relief of a few hundred meters under temperate climate conditions and so are consistent with the hypothesis that both the elevation and relief of most of the land surface have remained moderate since the opening of the Atlantic Ocean. 

The Norphlet sands were derived primarily from erosion of the southern Appalachian Mountains located to the north (Mancini et al. 1985,1990). In the Mobile Bay area, the Norphlet is interpreted as an aeolian deposit composed of large dune complexes and associated interdune deposits (Mancini et al. 1985 Marzano et al. 1988 Dixon et al. 1989). The Jurassic-age Pine Hill Anhydrite and the Louann Salt lie directly beneath the Norphlet and the Oxfordian marine carbonates of the Smackover Formation occur directly above (Fig. 2). 

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