Middle Valley
Bent Hill Area
Sedimentology
The sediments recovered in the Bent Hill area are characterized by Holocene and Pleistocene hemipelagic turbiditic deposits. The sedimentary column is predominantly composed of hemipelagic and terrigenous turbiditic beds but it also contains intrabasinal autochthonous and locally resedimented sulfide sands and breccias. Two major units are identified. Unit I consists of fine-grained hemipelagic sediment of Holocene and late Pleistocene age. Unit II is characterized by interbedded hemipelagic sediment of Pleistocene age. As these sediments are hosts for the various types of sulfide mineralization and hydrothermal alteration, several subunits are distinguished based on the dominant alteration mineral: Subunit IIa (unaltered), Subunit IIb (carbonate), Subunit IIc (anhydrite), and Subunit IId (silicates). Unit I is about 25 m thick; the occurrence of clastic sulfide and locally derived sands makes it difficult to identify this unit at Site 1035. Any sediment that overlies the shallow clastic sulfide was assigned to Unit I. In Subunit IIa the uppermost 7 to 17 m is interbedded by a 3- to 4-m-thick slump and mud-clast breccia, and turbidites beds are less than 1 decimeter thick. Subunit IIb is characterized by calcite and dolomite occurring as concretions, disseminated crystals, and thin layers. Subunit IIc is characterized by moderately altered clay to silty clay that contains authigenic anhydrite occurring as nodules, irregular veins, coarse crystalline concretions, and disseminated fine crystals. Local dissolution of anhydrite is correlated with high sulfate content of pore water from the same interval.
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