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

contours in red are over Jurassic dikes.
anomalies that clearly outline the locations of some of the dikes
(Fig. 13).
Typical examples of magnetic lows over exposed early Meso-
zoic basins are the Danville basin in Virginia (Fig. 13) and parts
of the Newark and Gettysburg basins in Pennsylvania and New
Jersey. The Dunbarton basin in South Carolina-Georgia is an
example of a magnetic low over a basin buried beneath Coastal
Plain sediments (Marine and Siple, 1974).
Ground magnetic surveys across early Mesozoic basins are
rare. However, many detailed profiles across Mesozoic diabase
dikes and sheets provide resolution not attainable by aeromag-
netic surveys (see Daniels, 1980, for examples).
GRAVITY
The range of measured densities of sedimentary rocks in
early Mesozoic basins is generally lower than that of most enclos-
ing Precambrian crystalline or lower Paleozoic sedimentary and
crystalline rocks. Thus, moderate negative gravity anomalies
would be expected where the preserved strata in the basins are
thick; however, the geometry of the basins is commonly not
Post-Paleozoic activity
I
0
20 Km
readily apparent from Bouguer anomaly maps because of the
dominance of large-amplitude regional gravitational fields. After
regional gradients are removed, the margins of the basins are
usually discriminated, especially where there are major faults and
thick sedimentary sections. Interpretation of basin geometry using
gravity maps is further complicated by the presence in some areas
of large volumes of intrusive diabase sheets, dikes, and basalt
flows; however, the large density contrast between diabase (den-
sity  3.0 gm/cm3) and early Mesozoic sedimentary rocks (den-
sity  2.5 to 2.7 gm/cm3) facilitates meaningful analysis of
diabase sheets (Daniels, 1980).
Bouguer gravity fields over parts of the Culpeper and
Newark-Gettysburg basins are shown in Figures 14 and 15, re-
spectively (Wise and Johnson, 1980). Trends in Bouguer gravity
anomalies are not deflected at the eastern boundary of the Cul-
peper basin because the foliated basement rocks trend nearly
parallel to the basin along much of the eastern margin. Also, the
density contrast between the early Mesozoic sedimentary rocks
and the adjacent phyllites is small, the preserved Triassic section
at the eastern basin margin is thin, and the Bouguer gravity is
dominated by a regional field, the source of which is deeper in the