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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K Manspeizer and Others<br />
[Wj = p9-j<br />
. .. . . . . . . pz4<br />
ALLUVIAL FAN LACUSTRINE FLUVIAL-DELTAIC FLUV IAL-PLAYA<br />
Figure 11. Geologic cross section of the Newark basin, drawn along the Delaware River, where the<br />
border fault is depicted as a listric normal fault merging with low-angle detachment faults that are<br />
inclined beneath the continental margin and where the distribution of major lithofacies is largely a<br />
function of the half-graben geometry. From Manspeizer, 1988.<br />
whereas fluvial-deltaic deposition of the Stockton Formation<br />
(Fig. 4) occurred in small, actively growing intrabasinal grabens,<br />
subsequent deposition of the predominantly lacustrine Lockatong<br />
Formation (Fig. 4) occurred in a very large, asymmetric half<br />
graben measuring about 7,000 km2 (Fig. 11). The change in<br />
sedimentation and basin geometry reflects a change in the locus<br />
and style of faulting, which shifted westward from the intra-<br />
basinal axis with its many small faults to the basin margin. There<br />
an earlier thrust fault may have been activated as a listric normal<br />
fault, becoming the major border fault throughout the history of<br />
the basin, ai evidenced by a thick sequence of time-transgressive<br />
border conglomerates. These changes may represent a change<br />
from normal faulting to primarily strike-slip faulting, which ex-<br />
tended into (and perhaps through) the Early Jurassic, resulting in<br />
syndepositional deformation, e.g., en echelon folding and faulting<br />
(Fig. 6).<br />
TRANSITION FROM RIFTING TO DRIFTING<br />
The transition from rifting to drifting, which was accompa-<br />
nied by sea-floor spreading, is recorded in the offshore basins by<br />
the postrift unconformity (Figs. 5, 7 to 10). Throughout parts<br />
of the margin the younger rocks are disconformable on older<br />
rocks; where they overlie deformed synrift strata, the contact is<br />
viewed in seismic profiles as an angular unconformity (Figs. 8 to<br />
10). The postrift unconformity is one of the most important da-<br />
tums on the shelf, marking a fundamental change in the tectonic<br />
evolution of the Appalachian orogen. Whereas the rift stage<br />
probably involved thermal doming, uplift, and stretching of the<br />
crust and was accompanied by faulting, igneous <strong>activity</strong>, and<br />
rapid filling of fault-governed troughs, the drift stage involved<br />
slow cooling and subsidence only of the eastern edge of the<br />
orogen and hence is marked by stratigraphic overlap on a broad<br />
scale (Manspeizer, 1988).<br />
The unconformity is diachronous, appearing to be Triassic<br />
on the Scotian Shelf and Early Jurassic on Georges Bank (Mans-<br />
peizer and Cousminer, 1988; see also Klitgord and others, 1988).<br />
The age of the unconformity is of major importance, because it<br />
affects interpretation of: (1) the ages of the underlying synrift<br />
strata; (2) the time of uplift, breakup, and Tethyan or perhaps<br />
Arctic transgression; and (3) the rates of subsidence and thermal<br />
maturation (for details see Manspeizer and Cousimer, 1988). De-<br />
termination of the age is controversial and centers on the interpre-<br />
tation of the COST G-2 well of Georges Bank basin (see Fig. 5;<br />
Poag, 1982; Manspeizer and Cousminer, 1988). This well, drilled<br />
to a depth of 6,667 m, is the deepest and most important strati-<br />
graphic test well on the U.S. margin (Figs. 4, 5). Poag (1982),<br />
primarily from seismic data, asserts that the well bottomed in<br />
Late Jurassic evaporites. However, based on age-diagnostic paly-<br />
nomorphs from cores, Cousminer (1983) and Cousminer and<br />
Steinkraus (1988) have determined that the well has penetrated a<br />
thick Upper Triassic evaporite section of dolomite with limestone<br />
and anhydrite, bottoming in Upper Triassic salt (Fig. 4). They<br />
report that Middle Jurassic palynomorphs are found at the 4,040-<br />
m level and Late Triassic dinoflagellates at about the 4,440-m<br />
level, and that the postrift unconformity occurs within an attenu-<br />
ated Early Jurassic (Liassic) section (less than 300 m thick) of<br />
carbonates and evaporites at about the 4,150-m level. Thus, the<br />
COST G-2 cores document that this outboard basin records a<br />
marine transgression in the Late Triassic, uplift and erosion in the<br />
Early Jurassic, and subsidence with the onset of sea-floor spread-<br />
ing and marine carbonate deposition in the Middle Jurassic<br />
(Manspeizer and Cousminer, 1988). The source of the Late Trias-