Swift
Stephen A.
Search Results
Site synthesis report of DSPP sites 417 and 418
Cenozoic geology of the continental slope and rise off western Nova Scotia
Swift
Stephen A.
No Thumbnail Available
1 - 2 of 2
Technical Report
This document summarizes information relevent to planning, execution, and interpretation of results from a study of the interaction of
sound in the 2-30Hz band with deep ocean seafloor using sea-surface sources, seafloor receivers, and borehole seismometers emplaced
by wireline re-entry at Deep Sea Drilling Project sites 417 and 418 in the western North Atlantic. We summarize published scientific
results from borehole sampling of water, sediment, and rock, from wire line logging, and from borehole seismic experiments. We present
new results from analysis of total power recorded by receivers clamped in basement during the borehole seismic experiment on DSDP
Leg 102. We document non-drilling investigations of the site and the nature and location of re-entry cones and transponders. We describe
the physical oceanography of the region and the speed of sound in water. We provide an extensive bibliography on published results from
scientific investigations at 417/418. This document was completed prior to 1989 surveys of sites 417 and 418.
Thesis
The outer continental margin of Nova Scotia is divided by a diapir
province, 40-110 km wide and ~1000km long, that trends subparallel to the
shelf edge along the upper continental rise and slope. The growth pattern
for a small region of this margin (61°-64°W) during the Late Cretaceous and
Cenozoic was studied using seismic stratigraphy and well data. Structure
maps show that a steep continental slope existed landward of the diapir
province (~2200-3800 m water depth) from Early Cretaceous until Miocene
time when onlapping upper rise sediments reduced the gradient. Shelf edge
canyons were cut during the late Maestrichtian-early Paleocene,
Eocene-Oligocene, and Pleistocene. Extensions of Tertiary canyons onto the
slope are poorly defined, but small Paleocene fans of interbedded chalk and
mudstone on the upper rise indicate that slope canyons existed at that
time. Abyssal currents eroded the upper rise and smoothed relief on the
continental slope in the Oligocene and middle(?) Miocene. In the Miocene,
turbidites may have ponded on the upper rise landward of seafloor highs
uplifted by salt ridges or pillows. Pliocene-Pleistocene sediments drape
over pre-existing topography. At the beginning and end of the Pleistocene,
turbidity currents, caused by delivery of large sediment loads to the shelf
edge by glaciers, eroded the present canyon morphology.
The late Cenozoic section of the lower continental rise thins seaward
from ~2 km near the diapir province and rests on Horizon Au, a prominent
unconformity eroded during the Oligocene by abyssal currents. The
morphology of the lower rise is largely due to construction by down-slope
deposits shed in the Miocene-Pliocene from uplift of the diapir province.
Abyssal currents episodically eroded sediment, but current controlled
deposition formed only a thin (<300 m) deposit in the Pliocene(?). Uplift
in the diapir province accelerated during the Pleistocene and olistostromes
up to 300 m thick were shed onto the lower rise. In the latest Pleistocene,
sediments transported down-slope by near-bottom processes accumulated west
of a sharp boundary running near 62°30'W from 500 m seaward to the abyssal
plain. To the east, hemipelagic sediments accumulated above 4300 m, while
turbidity currents, originating in deep canyons to the east, and abyssal
currents reworked sediments below 4300 m. A glacial sediment source and
relict shelf morphology controlled sedimentation processes and, thus, the
location of depocenters on the slope and rise.