Abstract

Crystalline rocks buried beneath Atlantic Coastal Plain sediments in the Washington, D.C., area commonly have weathering profiles that resemble exposed saprolite of the Piedmont. Cuts along the Washington Metropolitan Area Transit Authority (WMATA) route in northern Virginia show a bedrock weathering profile beneath Cretaceous and younger sediments.

Three lines of evidence indicate that this buried weathering profile (saprolite) has formed in the subsurface and is post-Miocene in age:

  1. The thickness of the profile, ranging from 2 to 15 m, is thinner beneath clays and thicker beneath sands, apparently a function of the permeability of the overlying material.

  2. The buried profile shows no evidence of soil formation at its upper surface and, therefore, no evidence of ever having been sub-aerially exposed.

  3. Consolidation tests on the cohesive clayey silts in the samples of the weathering profile indicate mechanical equilibrium with the present overburden, whereas consolidation tests on superjacent Cretaceous clays indicate over-consolidation resulting from a thicker pre-upper Miocene overburden.

The post-Miocene age for the subsurface saprolite profile shows that not all saprolite beneath Cretaceous sediment is pre-Cretaceous in age. These observations have important implications for the interpretation of age of the exposed Piedmont saprolite.

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