Lateral Variations in Core, Log, and Seismic Attributes of a Mass Transport Complex in the Ursa Region, IODP Expedition 308, Northern Gulf of Mexico

Abstract

Abstract Integrated Ocean Drilling Program (IODP) Expedition 308 in the northern Gulf of Mexico collected geotechnical and petrophysical properties of several mass transport complexes (MTCs) in the upper 600 meters below seafloor. We present a core-log-seismic integration of the largest MTC in the area. The MTC is identified in seismic data by semi-transparent to chaotic seismic facies. Close inspection of this facies in the area of Site U1324 reveals intact blocks of undeformed material. The base of the MTC is recorded in logging-whiledrilling (LWD) data as a characteristic sharp offset from highto- low values in the LWD resistivity, LWD bulk density, and sonic velocity, which is imaged as a strong reflector in seismic. The internal deposits are characterized by high LWD resistivity, LWD bulk density, and sonic velocity. At the core scale the internal deposits are composed of faulted, folded, and tilted mud with rare silt lamina. At Site U1322, core through this MTC was much more deformed than at Site U1324. This observation may record different deformation intensity and/or transport distance despite being in the same MTC unit and ~10 km apart. The characteristic signature in the resistivity log and seismic data of this MTC allows for correlation to nearby industry wells where shallow logs exist. Introduction Mass transport complexes (MTCs) are ubiquitous on continental margins where they re-shape the seafloor, transport large volumes of sediment, threaten offshore facilities, and produce tsunamis that threaten coastal communities. MTCs are generally defined as the deposits of seismically resolvable mass movement events that include deposits of slumps, slides, and debris flows1,2. MTCs are potential drilling hazards and can act as seals for hydrocarbon reservoirs. Several recent studies have focused on the 3-D seismic geomorphology of MTCs3,1,4,5,6,7. These studies and others have shown that MTCs are a major component of deepwater environments. A comprehensive study of MTCs in the Amazon Fan showed that MTCs have a high degree of consolidation relative to their burial depth8. This property of MTCs has been related to the slow penetration time of jetted conductors and suction anchor piles9. Integrated Ocean Drilling Program (IODP) Expedition 308 in the Ursa Region, northern Gulf of Mexico, cored, logged, and sampled, several prominent MTCs in the upper 600 meters (~2000 feet) below seafloor. A high-resolution 3-D seismic volume shot specifically for hazard analysis provides detailed images of the MTCs. The goal of this paper is a detailed analysis of the top, base, and internal deposits of the largest MTC by integrating core, log, and seismic data at Sites U1324 and U1322. We document lateral differences in this MTC from site-to-site. We then close by correlating this unit to two nearby industry wells. Geologic Setting The Ursa Basin lies 210 km (~125 miles) southeast of New Orleans, Louisiana (USA) on the continental slope in water depths ranging from 800-1500 meters (2600-4900 ft.) (Figure 1). IODP Expedition 308 Sites U1324, U1323, and U1322 are located in Mississippi Canyon blocks 897, 898, and 855, respectively. Sedimentation rates were very high in this area during the Late Pleistocene, averaging 10mm/yr at Site U1324 and 3.8 mm/yr at Site U132210. The Blue Unit is a ponded sand-prone unit infamous for causing shallow-water flow problems in this area11,12.