Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
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<strong>Post</strong>-<strong>Paleozoic</strong> <strong>activity</strong><br />
CULPEPER LINE 2<br />
STATION<br />
Figure 19. Seismic section (Line 2), western Culpeper basin, Virginia. CDP data are 24-fold acquired by<br />
Virginia Tech. Display is an automatic line drawing (qoruh and others, 1987). J, Jurassic strata of the<br />
Culpeper basin. The bottom of the basin (B) is sharply defined by the contact with the underlying late<br />
Precambrian to Cambrian metamorphosed basalts and sandstones. BRMD, Blue Ridge master dkcolle-<br />
ment as interpreted from the seismic data. D, deepest regional dkcollement in <strong>Paleozoic</strong> parautochtho-<br />
nous shelf strata.<br />
the area of Summerville, South Carolina, northwest of Charleston<br />
(Coruh and others, 1981; Belcher and others, 1986). At this<br />
locality, about 770 m of Cretaceous and Cenozoic Coastal Plain<br />
sediments are underlain by about 250 m of subaerial basalt flows<br />
(Gohn, 1983). High velocities typical of Triassic-Jurassic rocks<br />
are indicated by reflections from beneath the lower velocity<br />
(2,000 m/sec) sediments of the Coastal Plain from 0.6 sec (700<br />
m) to approximately 1.6 sec (3 km) below CDP station 100.<br />
Numerous diffractions in the basin suggest the presence of diabase<br />
sills or faults.<br />
Seismic reflection surveys, especially multifold coverage<br />
with vibrator energy sources, provide some of the best geophysi-<br />
cal data, but seismic refraction surveys have also been selectively<br />
used with good results (Ackermann, 1983) to solve specific<br />
problems.<br />
DEEP ELECTRICAL RESISTIVITY<br />
The direct-current electrical resistivity method has been used<br />
to explore onshore eastern U.S. early Mesozoic basins because the<br />
Mesozoic sedimentary rocks form a prism of relatively porous,<br />
water-saturated conductive rocks that produce a good resistivity<br />
contrast with the enclosing less conductive Piedmont metamor-<br />
phic rocks (Ackermann and others, 1976). The measured resistiv-<br />
ity of Triassic rocks in the Durham-Wadesboro basin is between<br />
30 and at least 350 ohm-m (Ackermann and others, 1976). In<br />
contrast, the resistivity of the Piedmont crystalline rocks exceeds<br />
1,000 ohm-m.<br />
Vertical electrical soundings in the Durham-Sanford basins<br />
of North Carolina indicate that not only are large resistivity con-<br />
trasts present between Triassic rocks and the enclosing Piedmont