The
electrical cross section figure
,
illustrates the two-dimensional electrical resistivity cross-section near the
Selma plantation. The section view is towards the north, with the western margin
on the left and the eastern margin towards the right. The total depth extent--from
mountain highland to deepest penetration--is approximately 1000 m. Catoctin
mountain lies on the left top of the figure, and the western border fault is
coincident with a low resistivity tongue (~300 to ~85 Ohm-m; blue-greens-to-medium
green) that dips steeply towards the east. The surface trace of this fault has
mapped control, and it appears at the eastern foot of Catoctin mountain on the
presentation of Lee and Froelich (1989). There
is some suggestion that we have sufficient resolution in this survey to determine
some subtle non-planar geometry in the fault plane. This is interpreted as fault
kinking (or en-echelon arranged offsets in the dip direction) with depth. Beyond
the single instance of a non-planar fault kink, we do not see further evidence
of listric fault development at depth. Deeper penetration (derived from lower
frequency surveys, with closer station spacing) are among the required parameters
for yet higher fault geometry resolution). Further to the east in the
above figure ,
the relatively low resistivities in the range ~55 to ~450 Ohm-m (deep blues
to medium greens) are interpreted to be the mountain wash fans, weathered regolith,
and soils derived from the Leesburg member conglomerate, whereas the resistivities
in the range ~700 to ~4000 Ohm-m (yellows to deep oranges) are interpreted to
be coincident with the relatively unweathered conglomerate itself. The Leesburg
member conglomerate appears in isolated outcrops that are scattered across the
landscape north of the city of Leesburg. Seismic reflection profiling commissioned
by Loudoun County near their land fills has revealed that such outcroppings
are the tops of conglomerate "spires" or "pinnacles", that
are rooted at 250 to 500 m depth, and are thus laterally contiguous. The rise
to the surface of the orange ~3000 to 5000 Ohm-m near station 004 and the region
in the eastern portion of the above figure
are interpreted to be spires.
|| Culpeper Basin AMT || USGS - Eastern Earth Surface Processes Team || USGS - Geology || USGS ||
Contact: Herbert A. Pierce
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