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Four major acoustic facies units, mapped from high-resolution seismic profiles (3.5 and 4.5 kHz), are defined for the modern fan lobe of the Mississippi Fan: transparent, multiple reflector, diffuse prolonged, and hummocky. These acoustic units are compared with the general lithologic character and specific lithologic boundaries within sediment recovered in the upper 50 m from hydraulic piston cores at nine Deep Sea Drilling Project (DSDP) sites from the middle and lower Mississippi Fan. Previous studies comparing short piston and gravity cores with acoustic facies in submarine fan areas indicate that specific lithologies can be expected to occur with specific acoustic facies. These earlier studies suggest that a good correlation exists between acoustic units and lithologies as follows: transparent and debris flow deposits, multiple reflectors and laminated sediments, diffuse prolonged and sand beds greater than 1 m thick, and hummocky and either debris flow or sediment-wave deposits. In our study, correlation between lithologies and acoustic units (both within and between units) is inconsistent, ranging from excellent to poor. Excellent correlation is observed, at Sites 614 and 615 on the lower fan, between the multiplereflector unit and bedded sediments, and the diffuse-prolonged reflector unit and thick sand beds. Another good correlation occurs between dipping silt beds and laminations, cored at Site 616, and the hummocky unit. At both Sites 621 and 622, we drilled into the transparent unit in the midfan channel and recovered visually homogeneous mud with poorly developed grading detectable in x-radiographs. These sediment textures suggest deposition from a very sluggish masstransport process, such as might be expected in the transition between a debris flow and a high-density turbidity current. Correlation between lithologies and acoustic facies at Sites 617, 623, and 624 is less clear. At all three sites, we recovered laminated and thinly bedded silts in a mud matrix, with the percentage of silt increasing downsection. It appears that at these sites the diffuse-prolonged return is caused by acoustic interference from many small lithologic changes rather than the presence of thicker sand beds.

Acoustic Facies and Sediment Composition of the Mississippi Fan Drill Sites, Deep Sea Drilling Project Leg 96