Marine Isotope Stage 6 Canyon and Spillover Deposits of the Bryant and Eastern Canyon Systems, Northwest Gulf of Mexico: Importance of Fine-Grained Turbidites on a Delta-Fed Prograding Slope

Abstract

Abstract A large set of long sediment cores and high-resolution seismic-reflection data were used to study the sedimentology of the delta-fed prograding slope of the northwest Gulf of Mexico, during Marine Isotope Stage (MIS) 6 (125–180 ka). Bryant and Eastern canyon systems were the main pathways during this period through which Mississippi River sediment was delivered down the continental slope. Two MIS 6 sedimentary environments have been defined on the slope of the northwest Gulf of Mexico: intracanyon and overbank environments. The erosional action of turbidity currents in Bryant and Eastern canyons was focused mainly on the thalweg, whereas, on the rest of the canyon floors, the flows acquired a mainly depositional character, resulting in the formation of inner levees and terraces. MIS 6 deposits in overbank environments consist entirely of thick (> 50 m), widespread (> 15–20 km), and continuous successions of mud turbidites, which indicate that spillover processes were dominant along Bryant and Eastern canyons. The total absence of bioturbation structures and hemipelagic sediments in MIS 6 overbank deposits indicate high sedimentation rates exceeding 200–600 cm/ky. The majority of the overbank deposits are normally graded, indicating that most of the turbidity currents during MIS 6 resulted from sediment failures on the outer shelf and upper continental slope. Sedimentological observations indicate that a complete mud turbidite sequence (T0–8 sequence in the nomenclature of Stow and Shanmugam 1980) results from a waning low-density turbidity current consisting of four successive flow-regime stages: (1) an initial erosional stage (≥ 30 cm/s), (2) deposition of the coarsest, noncohesive fraction (silt) of the suspended sediment in the flows (30–100 cm/s), (3) cyclic deposition of cohesive (mud) and noncohesive (silt) laminae due to fluctuations in the shear stress in the boundary layer of the flow (12–30 cm/s), and (4) deposition from a silt-depleted and slowly moving (≤ 15 cm/s) sediment cloud.