Abstract:
The results of a detailed geophysical investigation
conducted by the Woods Hole Oceanographic Institution in
1975 have been used in conjunction with other available
information to reconstruct the geologic history of the
passive continental margin off New England. Rifting between
northeastern North America and Morocco during the
Middle-Late Triassic produced a complex series of horsts
and grabens in Precambrian/Paleozoic crust. Intra-rift
sediments consist of clastics, evaporites, and volcanics.
Continental separation occurred and sea-floor spreading
began 195-190 my B.P. The boundary between "normal"
continental crust and crust radically altered by fracturing
and intrusion may be represented by a pronounced basement
"hinge zone". Prior to margin subsidence, extensive sub-
aerial erosion carved a "break-up unconformity"-reflector
"K" which truncated pre-existing rift structures and which
must be approximately the same age as the oldest oceanic
crust. Within the overlying "drift" sediments, six acoustic
horizons have been regionally traced and correlated with
strata sampled by a well drilled on the western Scotian
Shelf. The total sediment thickness of both rift and drift
sequences beneath Georges Bank may be 13 km, of which more
than 80% is Jurassic in age. A Mesozoic reef/carbonate
platform complex situated on the outer shelf-upper slope
was an effective sediment barrier until the early Late
Cretaceous, when prograding clastics buried the complex.
This carbonate build-up and its basement foundation of
altered continental or oceanic crust are responsible for
the geographic position and steepness (5-8°) of the present
continental slope south of Georges Bank.
