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

for relative contributions from isotopes of the elements K, U, and
Th. Reconnaissance ground-based gamma-ray spectrometry in
areas containing specific lithologies was done to provide a cali-
bration of the aeroradiometric map. Although the ground survey
disclosed areas where human activity may have altered the natu-
ral radioactivity pattern, the airborne survey shows that the re-
gional variations in the Culpeper basin and vicinity are mainly
controlled by the natural distribution of bedrock.
The aureole of metasedimentary rocks (baked zone, hom-
fels) adjacent to diabase intrusive rocks in the Culpeper basin
forms either a steep gradient or local radiometric highs. Daniels
(1980) reports that sedimentary areas immediately adjoining
baked zones in Fairfax County have higher radiometric values
than the areas of baked and intrusive rocks. He suggests that the
effects of the diabase intrusion may have extended beyond the
mapped thermal zone and that the highs may possibly be due to
enrichment of radionuclides mobilized from rocks closer to the
hot diabase.
Most of the prominent high anomalies are in red-brown
siltstone. These anomalies are characterized by tightly closed con-
tours with steep gradients, especially in areas where fresh siltstone
is exposed in quarries or pits (Froelich and Leavy, 1982) or in
homfels formed by metamorphism of the siltstones. Some radio-
metric highs are underlain by either limestone conglomerate or by
quartz and schist-pebble conglomerate. Some pre-Mesozoic rocks
that border the basin, notably acidic pelitic schists, metasiltstones,
metagranitic rocks, and quartzites, produce local highs.
SEISMIC REFLECTION
Reflection seismic data over offshore early Mesozoic basins
are readily available (Grow, 198 1 and references therein; NOAA,
1978). Considerably less data from exposed basins are in the
public domain. In general, the signal-to-noise (S/N) ratios of
reflection data obtained over exposed basins is not as good as that
over basins beneath a cover of Coastal Plain sediments or off-
shore. Excellent reflections, however, can be recorded from Trias-
sic coal, from Jurassic basalt flows and diabase sheets, and from
some layered lacustrine sequences.
Reflections from faults bounding early Mesozoic basins are
often not well recorded, and their presence is generally inferred
from the abrupt termination at the basin boundary of nearly
continuous reflections from the sedimentary rocks in the basin.
The velocities of Triassic and Jurassic sedimentary rocks in
the eastern United States are commonly in the range of 4,500 to
5.500 m/sec. Triassic and Jurassic velocities are usually lower
than those of the enclosing country rock, which are commonly
equal to or greater than 6,000 m/sec.
Reflection Seismic Signature of an Early Mesozoic Basin
Offshore near Virginia
As a reference standard for the quality of reflection seismic
data that would be desirable over any Mesozoic basin in eastern
Post-Paleozoic activity 339
North America, it is useful to examine offshore USGS Line 28
(Fig. 17) near the eastern shore of Virginia (NOAA, 1978; Dysart
and others, 1983). Although these data should be migrated (Wa-
ters, 1978) before an interpretation is made, the general outline of
the basin is clear. The thickness of the basin is about 4 km (2.5 to
4.2 sec beneath SP 800). Westward-dipping reflections from the
Triassic rocks terminate at crystalline basement. The angular un-
conformity between the westward-dipping rocks and the overly-
ing flat-lying rocks above the "breakup unconformity" (Falvey,
1974) suggests that the basinal rocks were rotated counterclock-
wise along a shallow-dipping, concave-upward deformation zone
while undergoing eastward translation as a result of crustal exten-
sion and thinning.
Reflection Seismic Signature of an Onshore Exposed Early
Mesozoic Basin, Culpeper Basin, Virginia
Seismic data have been obtained at locations designated
Line 1 and Line 2 (Costain and others, 1982; Schorr and others,
1986) in Figure 14. Line 1 is located over a saddle in a northeast-
trending gravity high evident on Figure 14. The source of the
gravity high is interpreted to be a thrusted anticlinal fold in the
crystalline rocks that underlie the basin, as shown by the seismic
data below 0.6 sec at station 80 in Figure 18.
Nearly continuous reflections (in places interfered with by
diffractions and reflected refractions) from largely phyllitic base-
ment rocks beneath the basin are believed to be from stacked
thrust sheets and folds from metamorphosed rocks that crop out
at the faulted basin margin 2.6 km to the east. Interval velocities
of basement rocks computed from the stacking velocities are in
the range of 4,000 to 5,000 m/sec, somewhat low for crystalline
rocks. Beneath Line 1, the thickness of prominently faulted Trias-
sic strata varies from less than 950 m (0.3 sec) at the eastern end
of the line to about 1,700 m (0.7 sec) at the western end.
Reflections from within the early Mesozoic section and from
the Mesozoic/basement rock interface beneath Culpeper Line 2
in Figure 19 are not as distinct as on Line 1. Along Line 2 the
early Mesozoic rocks dip steeply to the west (26O to more than
60') and consist mainly of sandstone interbedded with basalt
flows. Furthermore, the basement is probably predominantly
stacked thrust sheets of lower Paleozoic sedimentary rocks and
Proterozoic volcanic rocks; thus the contrast in velocity and den-
sity with the Jurassic sedimentary rocks and basalts is lower.
West-dipping shallow reflections tied to surface outcrops are
truncated at depth against the sharp sub-basement reflector. The
thickness of Jurassic sedimentary rocks beneath Line 2 is varia-
ble, about 1,500 m (0.4 sec) on the west, thickening to a maxi-
mum of 3,500 m (1.25 sec) at the east end of the line.
Reflection Seismic Signature of an Onshore Buried Early
Mesozoic Basin, Summerville, South Carolina
Few seismic signatures of early Mesozoic onshore basins
concealed beneath the sediments of the Atlantic Coastal Plain are
available. One of the best examples, shown in Figure 20, is from