Age Dating of Past Slope Failures of the Sigsbee Escarpment within Atlantis and Mad Dog Developments
Authors: Young Alan G. , Bryant William R. , Slowey Niall C. , Brand John R. , Gartner Stefan 
Published: October 2008

Abstract
A large number of slope failures (slumps or submarine slides) have occurred in the geologic past along the Sigsbee Escarpment between the Atlantis and Mad Dog Developments. A multidisciplinary study was conducted to define the risk of submarine slide activity in the context of the past historical activity that created 35 large scale slumps. Large diameter piston cores were obtained at 95 sites and deep soil borings were drilled at 8 locations to define the physical and engineering properties of the sediments and to determine the estimated time that these mass movements occurred along the Escarpment.
Age dating (14C and d18O) on the cores and biostratigraphic analyses on borehole samples provided stratigraphic age control required to understand the time scales associated with the structural evolution of the Sigsbee Escarpment, the sequence and timing of the major slump events, and the relative activity and occurrence interval of the smaller debris and turbidity flows.
The steep slope angles and upward and lateral movement of the underlying salt mass has resulted in slope instabilities producing gravity-driven slides. This study confirmed that each major "slump feature" is typically the product of a large number of individual soil mass failure events. A total of 79 age dates were measured by C-14 techniques on 95 cores taken within and around the slumps to give an historical account of the level of activity. The age dating and core results were correlated with the geophysical data to assess:the state of seafloor activities and processes,historical account of the slide events, andground truth the geologic/geotechnical properties within each slump area.
A significant level of submarine slide activity has occurred along the Sigsbee Escarpment in this region over the last 25,000 years. The soil mass failures have reduced in number, magnitude, and level of activity in the last 8,000 years. The rates of sedimentation accumulation were higher during the sea level low stand and are now lower during the sea level high stands. Thus, the large slide events and the attendant debris flows creating the slumps occurred primarily during the periods of rapid lowering of the sea level. The most recently active slump event occurred in the Atlantis area about 2,970 years ago. Some slumps have not been active in the last 10,000 to 16,000 years.

Introduction
The dynamic behavior of the Sigsbee Salt Nappe as described in the companion papers (1,2) is the single most important influence in the geologic and geomorphic development within the shallow section (upper 3,000 ft) in the area of the Atlantis and Mad Dog Developments. The Sigsbee Escarpment represents the topographic expression of a thin-skinned sediment section overlying a series of enechelon, coalescing, Pleistocene emplaced allochthonous salt nappes (3, 4, 5, and 6). The upward and lateral movement of the underlying salt mass has resulted in the seaward movement of sediments and subsequent over-steepening of the slope. Thus, the movement of the salt, over-steepening of the slope, and resulting deformation of the overlying sediment are the processes that formed this complex topographic and geologic feature encompassing steep slope, faults, and slumps as illustrated in Figure 1.