Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...

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