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