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

crust. A pronounced north-northeast-trending gravity high oc-
curs just west of Leesburg (Fig. 14). The high is approximately
coincident with the normal western boundary fault of the basin
from north of Aldie to the Virginia-Maryland border, a distance
of about 30 km. The high is interpreted as due to the high density
of Paleozoic metabasalt that underlies the basin. The steep re-
gional Apalachian gravity gradient just east of the towns of Mid-
dieburg and The Plains and just west of Warrenton is caused by
thinning of the crust (Pratt and others, 1988); thus the regional
gradient must be evaluated and removed for reliable determina-
tions of basin thickness.
One of the best published examples of a gravity analysis of a
large basin, the Gettysburg and Newark basins of Pennsylvania
and New Jersey (Sumner, 1977), is shown in Figure 15. The
TRIASSIC - JURASSIC ROCKS
Diabase and Basalt
Sedimentary Rocks
PRE -TRIASSIC ROCKS
Undifferentiated Paleozoic Sedimentary
a n d Crystalline Rocks
~ ' O J
rnilligals
Post-Paleozoic activity
residual gravity map (Fig. 15) has a total relief of about 20 mgal,
with closed gravity highs over major diabase bodies and lows
over the sedimentary rocks exposed in the southern parts of the
basin. The coincidence of negative residual anomalies with thick
sedimentary areas and positive residual gravity anomalies with
shallow basement or diabase bodies is well displayed, and a close
correlation between major geologic features and gravity data is
apparent.
RADIOACTIVITY
The aeroradiometric map of part of the Culpeper basin and
vicinity (Fig. 16) shows variations in intensity of gamma-ray
radiation from geologic sources that, for this map, were analyzed
Figure 15. Gettysburg and Newark basins, Pennsylvania and New Jersey. Comparison between Bou-
guer gravity (A) and residual (B) derived by removing the regional gravity gradient (after Sumner,
1977).