The Compartmentalization of
the Culpeper Basin by Intrusive Diabase Complexes:
Diabase Septa and their Function
The Culpeper basin is approximately 128 km. in length, and is about 32 km. in maximum width. It is cored along its central axis with a great complex of diabase dikes, sheets, and irregularly-shaped bodies. The relatively low permeability, K (in meters2), of diabase is typically in the range 10-17 < K < 10-21 m2, and this low permeability is broadly correlated with relatively high electrical resistivities. Permeability values of 10-17 m2 and below are considered to be virtually impermeable. For reference, a permeability of 1 Darcy is equal to 10-12 m2. Since the permeability of the surrounding sandstones, siltstones, conglomerate sand their thermally altered equivalents is typically in the range 10-12 < K < 10-14 m2 (with correlative lower electrical resistivities), there are at least three orders of magnitude--and potentially upwards of nine orders of magnitude--difference in permeability between the diabase and its surroundings. This means that bodies of diabase are expected to deflect, impede, impound, and otherwise strongly interfere with the regional flow of ground water in the basin. When complexly arranged in vertical stacks of sheets, and/or horizontal boxworks of intrusives, these bodies compartmentalize the basin flow--both horizontally and vertically. Implications of such compartmentalization include the potential for: (a) somewhat stagnant flow at depth within a compartment, and relatively freely-flowing fluids outside; (b) isotopically old ground water within a compartment, and younger waters outside; and (c) higher levels of total dissolved solids in compartmentalized waters, and lower values outside. We can thus think of the basin as a great 'supertanker', and the diabase bodies as bulkheads within this vessel. Ryan and Yang (1999), and Ryan et al (1997, 2000, 2002) discuss compartmentalization in low- and high- temperature systems, including the roles of fractures on hydrothermal convection, and Laczniak and Zenone (1985) outline the overall hydrology of the Culpeper basin.
Paradoxically, the diabase bodies that that form compartment boundaries ("septa") are typically massively fractured. The key to understanding why fractured diabase is rendered relatively impermeable in the bulk, is the recognition that the fractures are virtually all subparallel, and lack 3-D interconnectivity. When exposed in highway road cuts or in quarries, the fractures are vertically oriented, strike N-S ± 15-20º, have interfracture spacings on the order of about 20-30 cm, and lack significant cross (connecting) fractures. Moreover, where fractures may be traced along their lengths, they are frequently short (5-10 meters), and die out within the rock mass. Thus fractures of limited length and little 3-D interconnectivity render the diabase rock mass relatively impermeable and low in electrical conductivity (i.e., high resistivity). These attributes contrast sharply with the adjacent hornfels and/or the neighboring sandstones and siltstones.
|| Culpeper Basin AMT || USGS - Eastern Earth Surface Processes Team || USGS - Geology || USGS ||
Contact: Herbert A. Pierce
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