Sedimentation in the Vicinity of the Sigsbee Escarpment during the Last 25,000 yrs
Authors: Slowey Niall , Bryant William R. , Bean Daniel A. , Young Alan G. , Gartner Stefan 
Published: October 2008

A thick section of sediments was deposited along the lower continental slope in the vicinity of the Atlantis and Mad Dog Developments in the northwest Gulf of Mexico during the time period spanning the last glaciation to the present. Sediments were collected using large diameter piston cores and borings, and studied to understand how variations in sedimentation rate and sediment properties relate to changes in climate and sea level. Sediment bulk density and compressional-wave velocity were measured using a multisensor core-logging device; afterward, the cores were split and the sediment lithologies were described. Based on these data and the interpretation of high-resolution seismic data collected at the core sites, samples were taken from the cores to determine the ages of the sediments using the following methods: radiocarbon dating, downcore changes in the oxygen isotopic composition of foraminifera shells, and nannofossil occurrence.

A detailed chronology for the sediments deposited during the period of time spanning the last glaciation to the present was determined and compared to other sediment characteristics. The establishment of this chronology was an essential step in the assessment of potential geohazards because it provides a fundamental framework for understanding how environmental processes affect the geologic and geotechnical characteristics of the seafloor. For example, the ages of regionally persistent high-resolution seismic reflectors were established and related to changes in sediment properties, providing an important temporal framework for the interpretation of the seismic, lithologic and geotechnical data. A significant decrease in the rate of sediment accumulation occurred as sea level rose from the lastglacial maximum to the late Holocene. The data supports the suggestion that the sediment accumulation rate is also influenced by changes in the flux of glacial meltwater into the Gulf of Mexico.

Introduction

The lower continental slope of the northwest Gulf of Mexico in the vicinity of the Mad Dog and Atlantis Developments is draped by a thick section of late Quaternary-aged hemipelagic sediments. The geological and geotechnical characteristics of the sediments are determined by the combined influences of several environmental processes. Important processes include the structural evolution of the Sigsbee Escarpment, sediment deposition and erosion, the sequence and timing of major slump events, and the relative activity and occurrence interval of smaller debris and turbidity flows. To fully understand the nature of each process and its impact on sediment characteristics, it is essential to understand the ages of the sediments and relationships between temporal changes in sedimentation, sediment properties, and variations of both climate and sea level.

Toward this end, a high-resolution stratigraphic framework was developed for sediments deposited at the Mad Dog and Atlantis Developments during the late Quaternary. The framework includes the following principal elements:a precise chronology and sea level/climate history derived from the oxygen isotope compositions and radiocarbon ages of foraminifera, and nannofossil biostratigraphy;detailed measurements of sediment bulk density, hemipelagic velocity, shear strength, and lithostratigraphic characteristics; andhigh-resolution seismic profiles and side-scan sonar imagery.