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

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