Observations from high resolution bathymetry surveys, sidescan images and direct observations from submersibles and ROVs have shown that there are an impressive set of nearly parallel troughs (or furrows) running in a band several kilometers wide along the base of the Sigsbee Escarpment. In the area of the Atlantis and Mad Dog prospects individual furrows are 20 to 30 m wide and 3 m to 8 m deep. We have both measured and directly observed strong bottom currents flowing parallel to these furrows.
Neither the reason for the strong currents nor their association with the zone of furrowed seafloor is fully understood at this time. However, the high-resolution geophysical data collected from an AUV as part of the Atlantis area survey show that the pattern of furrows is interrupted in various places along the base of the escarpment. In several places the interruption corresponds to location of a debris flow deposit. Sub-bottom profiles across these debris flows show that they have armored the underlying sediments and protected them from erosion. This indicates that the strong bottom currents are presently eroding the furrowed area. It also suggest that, at an earlier time the currents were not strong and in this time the now-missing sediment (up to 10 m) had been deposited. These observations, when combined with detailed knowledge of the shallow stratigraphy of the area, data concerning the erodibility of the bottom sediments, and time-series measurements of currents provides a method for evaluating the age of the debris flows.
This analysis has been carried out for a number of individual debris flow deposits along the base of the escarpment. It is shown that, although there has been considerable activity of these mass gravity flows over the past 20,000 years, the activity has decreased significantly over the last few millennia. These techniques have also proved useful as a screening tool to evaluate the relative ages of various features across the area so the ancient and more recent features can be differentiated.
In the course of evaluating the geohazards on and below the Sigsbee Escarpment at the Atlantis and Mad Dog deep-water prospects a number of unique features were discovered. The work was conducted by a team of investigators. A portion of the work was directed to evaluating the relative hazards posed by mass gravity flows that can cascade down steep subsea slopes such as those found along the Sigsbee Escarpment. A description of the overall project is given in Jeanjean et al. (2003 - this volume). A comprehensive discussion of the evaluation of the mass gravity flow hazards is presented in Niedoroda et al. (2003 this volume). In this paper we describe the results of a portion of the overall study that was directed at evaluating the reoccurrence rate of major debris flows in the project area. Two independent methods of analyses were applied to estimate the age of a number of debris flow deposits. One relied on nuclear age dating and detailed analyses of the shallow stratigraphy. The other relied on a novel application of sediment dynamics based on measured erosion rate parameters and measured currents.