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.
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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.