Description: The flow of deep-sea turbidity currents in meandering channels has been of considerable recent interest. Here we focus on the secondary flow associated with a subaqueous bottom current in a meandering channel. For simplicity, a saline bottom current can be used as a surrogate for a turbidity current driven by a dilute suspension of fine-grained sediment that does not easily settle out. In the case of open-channel flow, i.e., rivers, the classical Rozovskiian paradigm is often invoked to explain secondary flow in meandering channels. This paradigm indicates that the near-bottom secondary flow in a bend is directed inward, i.e., toward the inner bank. It has recently been suggested based on experimental and theoretical considerations, however, that this pattern is reversed in the case of subaqueous bottom flows in meandering channels, so that the near-bottom secondary flow is directed outward (reversed secondary flow), towards the outer bank. Experimental results presented here, on the other hand, indicate near-bottom secondary flows that have the same direction as observed in a river (normal secondary flow). The implication is an apparent contradiction between experimental results. We use theory, experiments, and reconstructions of case studies from field-scale flows to resolve this apparent contradiction based on the densimetric Froude number of the flow. We find three ranges of densimetric Froude number, such that a) in an upper regime, secondary flow is reversed, b) in a middle regime, it is normal, and c) in a lower regime, it is reversed. We apply our results at field scale to previous studies on channel-forming turbidity currents in the Amazon submarine canyon fan system (Amazon Channel) and the Monterey Canyon and a saline underflow in the Black Sea flowing from the Bosphorus. Our analysis indicates that secondary flow should be normal throughout most of the Amazon submarine fan reach, lower-regime reversed in the case of the Black Sea underflow, and upper-regime reversed in the case of the Monterey canyon. The theoretical analysis predicts both normal and reversed regimes in the Amazon submarine canyon reach.

Description: The formative mechanisms of many geologic features attributed to the passage of turbidity currents are still obscure due to the difficulties in documenting the dynamics of turbidity currents in the field. Submarine channels and lobes have been extensively documented using seismic tools and outcrops. The actual morphodynamics of their emplacement remain, however, only poorly understood. In this paper we present experimental research that documents the morphodynamic and stratigraphic evolution of self-channelized subaqueous fans emplaced by turbidity currents. A large-scale facility is used to document the fan structure and internal stratigraphy that developed after repeated flow events. The fan surface tended to prograde and aggrade from event to event. Within this general pattern, the experiments exhibited the formation, migration, and abandonment of well-defined depocenters that tended to stack compensationally. These depocenters were typically characterized by one or more channels bounded by levees. The deposits of successive runs showed a tendency to cyclically expand-contract in the strike direction, and extend-backstep in the dip direction. The overall pattern of grain-size variation consisted of downstream fining. The sediment in the channels, however, tended to be coarser than in the levees bounding it. As the fan evolved from flow event to flow event, successively more of the sediment fed in bypassed the platform on which it developed. The evolution of morphologic and stratigraphic patterns observed in the experiments shows several key similarities with channelized fans at field scale.

Description: Detailed biostratigraphy in Site 1006 based on planktonic foraminifers and nannofossils shows large-scale sedimentation rate variability in the Florida Strait west of the Great Bahama Bank. A 'floating' cyclostratigraphy based mainly on resistivity logs and magnetic susceptibility data has been fixed to the biostratigraphy in the absence of magnetostratigraphy. The strongest orbital cycle present is the precessional beat, which is present in the borehole logs throughout the record. Counting the cycles resulted in an accurate time scale and thus a sedimentation rate time series. Spectral analysis of the sedimentation rate time series shows that the short-term cycle of eccentricity (~125 k.y.) and the long term cycle of eccentricity (~400 k.y.) are pervasive throughout the Miocene record, together with the long-term ~2-m.y. eccentricity cycle. The Great Bahama Bank produced pulses of shallow carbonate input once every precessional (sea level) cycle during the Miocene and perhaps two pulses per cycle in the early Pliocene. The amount of sediment exported in these pulses appears to be controlled by eccentricity modulation of the precessional amplitude and therefore the amplitude of the sea-level rise. Finally, an increase in sedimentation rate just after the Miocene/Pliocene boundary is attributed to a change in the location and strength of sediment drift currents in the Florida Strait due to reorganization of the currents following the closure of the Panama Isthmus.