Depositional processes of uniform mud deposits (unifites), Hedberg Basin, northwest Gulf of Mexico: New perspectives

Abstract

Structureless mud deposits (unifites) are a common depositional phenomenon in the intraslope basins of the continental slope of the northwest Gulf of Mexico. High-resolution acoustic data and long sediment cores from Hedberg Basin (middle slope) were used to provide a better insight into the origin of these enigmatic depositional units. The unifite, occurring in Hedberg Basin, is structureless under visual observations, but x-ray radiographs reveal that it consists of three divisions: (1) U1 division, representing waning sequences of fine-grained (mud) turbidites, (2) U2 division, characterized by faint, normally graded mud layers with occasionally wispy, silty interlaminae, and (3) U3 division, characterized by a uniform appearance with no obvious sedimentological structures other than a faint banding of slightly siltier and slightly more clayey layers. High-resolution geophysical data reveal that there is no connection between intrabasinal sediment failures and the occurrence of unifites. We suggest that unifites have resulted from the deposition of a long-lasting (1.53 months), pulsating (showing internal surges), fine-grained turbidity current (dated at 28.629 ka) that originated from the flow transformation of a series of massive sediment failures on the shelf edge and/or upper continental slope. The fine-grained nature of the turbidity current is probably caused by the depositional segregation of the flow by depositing its coarsest material in intraslope basins in more proximal locations. We propose that the uniform texture of the unifite is attributed to the development of a stratification interface (lower stratified layer) generated by the continuous introduction of a turbulent sediment cloud in the intraslope basins. The lowest two unifite divisions are considered to develop prior to and during the development of the stratified layer.