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

Abstract

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.

Site 869

“…Sources for the igneous material presumably lay in shallow water, given the presence of shallow-water carbonate debris in the graded volcaniclastics. The shallow-water material indicates a normal-marine source area and may have derived from an off-bank environment; it is notable, for example, that the ooids are concentrically laminated and micritic and resemble the pelagic "ooids" described from a number of "deeper-water" environments in the TethyanAtlantic and Pacific Mesozoic (Jenkyns, 1972;Schlager, 1980;Haggerty and Premoli Silva, 1986). Nannofossils, planktonic foraminifers, and radiolarians were being supplied as a background sediment during this period and were diagenetically altered to produce calcareous siltstones and chert.…”

Origins, Timing, and Implications of Miocene to Pleistocene Turbidites, Debris Flows, and Slump Deposits of the Queensland Trough, Northeastern Australia (Site 823)

“…The reduced supply of terrigenous and bioclastic sediments indicates that the shelves bounding the Queensland Trough were drowned during the late Miocene (see Schlager, 1980Schlager, , 1981Robertson and Bliefnick, 1983). Debris-flow conglomerates and turbidites in the lower part of Unit VI indicate continued slope instabilities.…”

“…The late Miocene (-6.6-6.8 Ma) pulse of tectonic subsidence, together with eustatic highstands, led to the drowning of the Queensland Plateau carbonate platform (see Davies et al, 1989;Katz and Miller, this volume;Schlager, 1981). Subsequently, the predominance of pelagic foraminifers in turbidites from Site 823 indicates that the northeastern Australia shelf remained drowned until a eustatic lowstand at 2.6 Ma permitted bioclastic and quartzose sediments again to be produced on the northeastern Australia continental shelf and supplied to the Queensland Trough (see below).…”

“…Other common sand grains include glauconite and phosphate that formed as authigenic minerals on the seafloor. By examining the composition of turbidites, inferences can be made about the nature of sedimentation on the adjacent shelves, which serve as the source of turbidite sands (Rusnak and Nesteroff, 1964;Scholle, 1971;Cook and Taylor, 1977;Davies, 1977;Schlager, 1980;Thiede, 1981;Premoli Silva and Brusa, 1981;Robertson and Bliefnick, 1983;Watts and Garrison, 1986;Schlanger and Premoli Silva, 1986;Eberli, 1987Eberli, , 1991Reijmer, 1988Reijmer, , 1991Haak and Schlager, 1989;Dolan, 1989;Yose and Heller, 1989;Watts andBlome, 1990;Everts, 1991).…”

“…Platform drowning has been particularly important in determining the composition and nature of gravity deposits (see Schlager, 1980Schlager, , 1981.…”