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The Tithonian-Hauterivian section of Site 416 contains about 10 per cent limestone and marlstone in well-defined beds between shale and sandstone. Depositional structures, grain composition, and diagenetic fabrics of the limestone beds as well as their association with graded sandstone and brown shale suggest their deposition by turbidity currents below calcite-compensation depth. Carbonate material was initially shed from a shallow-water platform with ooid shoals and peloidal sands. Soft clasts of deep-water carbonate muds were ripped up during downslope sediment transport. Shallow-water mud of metastable aragonite and magnesium calcite was deposited on top of the graded sands, forming the fine tail of the turbidite; it has been subsequently altered to tight micritic limestone that is harder and less porous than Coccolith sediment under similar overburden. Abnormally high strontium contents are attributed to high aragonite contents in the original sediment. This suggests that the limestone beds behaved virtually as closed systems during diagenesis. Drowning of the platform during the Valanginian is inferred from the disappearance of the micritic limestones, the increase in abundance of phosphorite, of ooids with quartz nuclei, and of the quartz content in the calciturbidites. In Hauterivian time, the carbonate supply disappeared completely. The last limestone layers are lithoclastic breccias which indicate that erosion has cut into well-lithified and dolomitized deeper parts of the platform rather than sweeping off loose sediment from its top. Of all Tithonian sections recovered from the Atlantic, only the one at Site 416 contains abundant shallow-water debris and was deposited below the calcite-compensation depth. It is probably located on mid-Jurassic or older crust that had already deeply subsided and was close to the continental slope by Tithonian time. The other sites, located on younger crust, stood high on the flank of the mid-ocean ridge, and were consequently above the compensation level and beyond reach of shallow-water detritus shed down the continental slope.

Mesozoic Calciturbidites in Deep Sea Drilling Project Hole 416ARecognition of a Drowned Carbonate Platform

Mesozoic Calciturbidites in Deep Sea Drilling Project Hole 416ARecognition of a Drowned Carbonate Platform