Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the
Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January, 1978
Description:
Many of the small-scale topographic features (dimensions
of centimeters to kilometers) found on the Blake-Bahama Outer
Ridge (western North Atiantic, water depth greater than 4000 m)
and in the Rockall Trough (northeastern North Atlantic, water
depth greater than 2000 m) have been formed as bed forms of
deep currents. These bed forms, all developed in cohesive sediments,
include current ripples (spacings of tens of centimeters,
formed transverse to the flow), longitudinal triangular ripples
(spacings of meters, formed in sandy muds and parallel to the
flow), furrows (spacings of tens to 100's of meters, formed
parallel to the flow and presently either erosional or depositional),
and regular sediment waves (spacings of a few kilometers,
now found oblique to the flow and migrating either
upstream or downstream). The local distribution of any given
bed form is influenced by the presence of larger features. Bed
forms are often found in zones which strike parallel to the
regional contours.
Debris flows, affecting areas of 1000's to 10,000's of
square kilometers, are also present in these areas. A debris
flow studied in the Rockall Trough is erosional at its shallowest
depths and depositional at greater depths. Gravitational flows strike perpendicular to the contours. Pockmarks (tens
of meters in diameter, marking fluid seeps) are also found on
the Blake-Bahama Outer Ridge.
The larger topographic features (greater than several
meters) with steep slopes (greater than about 20°) can be
observed on surface echo-sounding profiles either as fields of
regular hyperbolic echoes (e.g., echoes from regularly spaced
furrows), fields of irregularly spaced, dissimilar hyperbolae
(e.g., echoes from blocks, ridges, and folds in debris flows),
or as regular features whose structure is often obscured by
side echoes (e.g., echoes from sediment waves). Although near-bottom investigations are required to describe the features,
the nature of the sea floor can often be inferred from the
character of the echo-sounding profile. Similar echo-sounding
records in different areas of the ocean indicate the presence
of similar sea-floor features.
The morphology of the bed forms studied and the current
and temperature structure of the overlying water column lead
to conclusions about bed form origin and present-day interactions
with deep currents.
Furrows form as erosional bed forms during high-velocity
(〉20? cm/sec) current events by large, helical secondary
circulations in the bottom boundary layer. Once formed, furrows
may develop into depositional features, or they may continue
as erosional ones, depending on the local currents and the
sediment supply.
Large, regular sediment waves may be formed at current
speeds of 5 to 10 cm/sec by lee waves generated by topographic
irregularities on the sea floor, such as submarine canyons,
or by instabilities in the flow of deep, contour-following
currents. Sediment waves develop where there is an abundant
supply of sediment and steady mean currents. Waves appear to
migrate upstream where tidal current fluctuations are smaller
than the mean velocity, and downstream where they are larger.
Near-bottom currents appear to be faster on the downstream side
of upstream-migrating sediment waves than on their upstream
side. The resulting variations in bed shear stress lead to
higher sedimentation rates on the upstream side and bed form
migration in that direction.
Description:
This research was made possible by National Science Foundation grants DES 73-06657 and OCE 76-22152, and Office
of Naval Research contract N00014-74-C-0262; NR083-004 to Woods Hole Oceanographic Institution, NSF grant OCE 74-01671
to Lamont-Doherty Geological Observatory, and numerous NSF
grants and ONR contracts to Scripps Institution of Oceanography.
Keywords:
Marine sediments
;
Submarine topography
;
Ocean circulation
;
Knorr (Ship : 1970-) Cruise KN31
;
Knorr (Ship : 1970-) Cruise KN51
;
Robert D. Conrad (Ship) Cruise RC18
;
Point Loma (Ship) Cruise
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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