Two AMT transects have been performed above the diabase lopolith outcropping in the vicinity of Mitchells, Virginia. This large intrusive is here referred to as the "Rapidan diabase lopolith", deriving it's place name from the largest nearby community. It has been mapped as a great sheet-like body (Lee and Froelich, 1989; Smoot and Sutphin, personal communications, 2002). In detail, the body is a concave-up intrusive that cradles in it's interior a massive inlier of Balls Bluff siltstone, grading into it's contact metamorphosed equivalent hornfels. The geometry of the body is thus lopolithic.
The AMT traverse paths for our survey crossed each
other at nearly right angles (Location Maps),
making possible some limited three-dimensional inferences. Figures 
and 
illustrate the two two-dimensional electrical resistivity cross-sections through
the lopolith to depths below grade of approximately 500 meters. The shorter
of the two traverses (line 1) is oriented NW by SE and is given in figure .
The longest traverse (line 2) is oriented SW by NE, and is illustrated in figure
. In the NW-SE cross-section, the diabase body is inferred to
be coincident with the relatively high resistivity yellow-orange to deep orange
regions (~ 1,500 to 6,500 Ohm-m), overlain by a hornfels veneer of (maximally)
some 10-20 meters thickness (blues-to-greens, green-to-yellow; ~200 to 600 Ohm-m,
overall). It is significant, we think, that at depth the diabase body appears
to be divided into two resistivity anomalies. We interpret this division as
being mainly induced by undulations in the contact zone topography between the
diabase and the soft and unconsolidated, or semi-plastic and semi-consolidated
Balls Bluff siltstone at the time of intrusion. Undulations in contact topography
are relatively common for basic intrusions invading sediments, particularly
if the sediments are not fully lithified, as is appropriate for the Culpeper
basin. Basaltic intrusions into unconsolidated or poorly
consolidated sediments, such as in the Salton Sea geothermal region of California,
commonly produce undulatory contacts. In figure ,
the SW by NE transect illustrates a 2-D electrical resistivity cross-section
with two prominent high resistivity lobes (~2,000 to 10,000 Ohm-m; yellow-orange
to deep orange). We interpret these lobes as irregularities (high amplitude
undulations) in the contact region between the diabase intrusion and the Balls
Bluff siltstone developed during intrusion. Multiple intrusion episodes also produce this type of complex contact structure, and frequently
(routinely) incorporate lenses or screens of country rock between them. Multiple
intrusion episodes are now known to have widely occurred
in the Gettysburg basin (M. Ryan and D. Daniels, unpublished data), and
would be expected for virtually all Mesozoic basins invaded by diabase during
the Lower Jurassic. Indeed, both multiple intrusion pulses and upper contact surface
undulations in intrusion-host rock topography are to be expected for the Rapidan
lopolith as well as other Culpeper basin lopoliths. Screens of Balls Bluff siltstone
caught up between diabase intrusion pulses would thus remain today as hornfels
screens, which are characteristically more intensely fractured, more susceptible
to weathering, and would be much lower in bulk electrical resistivity than their
diabase neighbors. The blue tongue
of rocks extending to depth beneath stations Cul_008 and Cul_009, for example,
is interpreted to be one such hornfels screen. Overall, the Rapidan diabase
lopolith has a thickness of at least ~550 meters in the vicinity of Michells,
Virginia. The body is believed not to be monolithic but, however, to have significant
three-dimensional character in it's upper contact surface and in it's interrelationships
with the hornfels above it.
|| Culpeper Basin AMT || USGS - Eastern Earth Surface Processes Team || USGS - Geology || USGS ||
Contact: Herbert A. Pierce
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