Abstract: Sand Petrology and Bioclastic Components of "Black Shell" Turbidite, Hatteras Abyssal Plain
H. A. Curran, R. D. Elmore, W. J. Cleary, O. H. Pilkey
A single large turbidite has been traced over a 44,000 sq-km area of the Hatteras abyssal plain using samples from 56 piston cores. Entering the basin from the Hatteras Canyon system, the flow traveled at least 500 km in a southerly direction and resulted in a tongue-shaped turbidite with a width of 100 to 140 km. The turbidite is characterized by its high percentage (up to 40%) of blackened molluscan shell fragments and by a coarser grain size than other turbidites in the cores. It has been correlated between cores on the basis of grain size and mineralogy, its distinctive molluscan and foraminiferal assemblages, relative thickness, and similarity of stratigraphic sequence in the cores.
In basal samples, maximum grain size decreases toward the margins of the flow, ranging from 1.0^phgr near the basin entry point to 1.75^phgr at the distal edges. Definite lateral and vertical sorting of sands is shown by decreasing mean-grain size laterally, distally, and upward. As mean-grain size decreases, the percentage of rock and molluscan fragments also decreases and the percentage of quartz and feldspar increases. Monocrystalline quartz, often highly strained, and several types of polycrystalline quartz are the dominant (54 to 68%) sand-grain components. Feldspar content is generally high (10 to 32%) with microcline particularly prevalent in coarser samples and plagioclase increasing in finer grained samples. Terrigenous lithic fragments (Tr-8%) are composed principally of fel spathic gneisses and sandstones. Nonskeletal carbonate rocks, phosphorite, glauconite, and heavy minerals are in trace to small percentages.
Skeletal components consist chiefly of bivalve and gastropod shell fragments and foraminiferal tests. Blackened shell fragments are largest and most common at the proximal end of the turbidite with shells of the clam Mulinia lateralis and the snail Nassarius trivittatus being the dominant identifiable species. A benthic-foram assemblage dominated by Elphidium clavatum comprises up to 60% of the total foraminiferal fauna from basal samples taken along the axis of the flow.
Radiocarbon dates from shells show that the flow occurred in late Pleistocene time, and the faunal assemblages indicate that most of the material of the turbidite was derived from shallow environments of the Pleistocene continental shelf.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC