Abstract

Sedimentary furrows are longitudinal bed forms which form in fine-grained, cohesive sediments of the deep-sea floor as well as in similar sediments in estuaries and in large lakes. Deep-sea furrows have been observed where bottom currents often flow in one direction at 5 to 20+ cm/sec. Currents are much stronger (greater than 50 cm/sec) in furrowed estuarine environments, but sediment accumulation rates are higher. A review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes. Those developed in muddy, cohesive sediments tend to have steep walls and flat floors, while those developed in carbonate-rich cohesive sediments are broad and have gently sloping walls. At abyssal depths, furrows can often be detected by their distinctive echo patterns on 3.5-kHz echo sounders—a series of similar, evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor. However, different furrow cross sections may generate distinctive hyperbolic echo patterns. A model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring, directionally stable, and episodically strong currents. Furrows are initiated when secondary circulations in the boundary layer align coarse, generally light debris in sand ribbon-like bed forms. As the coarse, light sediments move along the sea floor, they abrade small longitudinal troughs into the muds. The grooves may fill in during intervals between strong current events if sedimentation rates are too high. Once a furrow is established on the sea floor, further development may be controlled by the balance between sediment deposition and erosion, where deposition occurs continuously, and erosion occurs only episodically during brief periods of increased current strength. “Narrow” furrows (width 1/5 to 1/15 of spacing) appear to develop where deposition exceeds erosion, whereas “wide” furrows (width 1/2 of spacing) develop where erosion exceeds or equals deposition. Dissolution may play an important role in the formation and evolution of furrows in carbonate-rich cohesive sediments. Secondary circulation patterns over established furrows may be strongly controlled by bed-form topography.

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