Abyssal Sediment Waves
G.S. Mountain, B.E. Tucholke, 1983. "Abyssal Sediment Waves", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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Abyssal currents strong enough to erode the sea floor and carry suspended sediment great distances have been detected in many of the world's oceans. In contrast to episodic turbidity currents that flow down-slope and deposit entrained sediments in expansive and level turbidites, these currents are the expression of continual thermohaline circulation that typically flows along bathymetric contours and deposits transported sediments as cryptically-bedded "contourites" (Heezen and Hollister, 1964). The Western Boundery Undercurrent is an example of such a current. Most of its water mass is derived from sources in the Norwegian-Greenland Sea, and in its equator-ward, anti-clockwise flow along the margin of the North Atlantic Basin, it controls the growth of numerous "drift" deposits such as the Feni, Hatton, Gardar and Erik ridges in the north and the Hatteras, Blake-Bahama, Caicos and Greater Antilles outer ridges in the south. Seismic profiles across these drifts show that some contourite deposits have grown in a complex and irregular manner, while in others the conditions of erosion, transport and deposition have remained surprisingly uniform for millions of years. The accompanying profile is an example of the latter.
The profile is a portion of multichannel line 77 collected and processed by Lamont-Doherty Geological Observatory. Asingle-channel analog version of this same profile has been published elsewhere (Tucholke and Mountain, 1979; Tucholke and Laine, in press). The profile is located on the lower continental rise 300 km (186 mi) east of North Carolina where it crosses the crest of the Hatteras Outer Ridge (see accompanying location map). This sediment drift was first identified near its southern end at 330N where it has clear topographic expression (Rona, 1969). The ridge is roughly 550 km (341 mi) long by 150 km (93 mi) wide, and is oriented parallel to regional bathymetric contours. The ridge's western flank is buried by Pleistocene turbidites that form the lower continental rise terrace. The undulating swells of the exposed seaward flank of the ridge have been termed the "lower continental rise hills," and their origin has been variously attributed to gravity slides, abyssal current deposition, and erosion by turbidity currents (see Tucholke and Laine, in press). However, as can be deduced from this profile alone, the hills are the modern expression of large sediment waves that have been accreting for a significant length of time. They are oriented nearly east-to-west in the area of this profile (Asquith, 1979); thus their trend is inclined 300 to the direction of the prevailing current, much like sediment waves in other areas (Flood and Hollister, 1974; Embley et al, 1980).
This profile was collected 24-fold and was recorded at a 4 msec sampling rate. The processing applied after stacking included spherical divergence correction, predictive deconvolution, time-varying filtering from 15 to 80 Hz, time-varying gain and trace equalization. These parameters were balanced to show the structures at moderate depth with maximum clarity; consequently the resolution in the uppermost 150 msecs of the sediment column is less than ideal. However, Asquith (1979) showed with 3.5 kHz echosounder records that the migrating sediment waves visible on the accompanying profile continue upward to the sea floor.