Mineral composition of sedimentary formations in the North Atlantic Ocean

Abstract

Sedimentary formations established in the western North Atlantic basin byJansa et al. (1979) and their coeval counterparts from the eastern North Atlantic were studied for their mineral composition using X-ray powder diffractometry. More than thousand samples from different legs of the Deep Sea Drilling Project were analysed semiquantitatively. The results are presented as diagrams of about thirty five different mineral phases versus depth/stratigraphy in the single boreholes. Sites 105, 106, 386, 387, 391, 417, and 418 were studied from the western North Atlantic, and Sites 367, 369, 370, 415, and 416 from the eastern North Atlantic off W-Africa. Specific mineral spectra were found from most of the formations which can thus be additionally characterized. The change of mineral composition along the formation boundaries is in most cases sharp but along some of the boundaries it is transitional. Local variations of mineral spectra within the same formation can mainly be referred to distance from source areas. The sedimentary section studied from the western North Atlantic basin ranges in age from Oxfordian to Quaternary. It consists of: argillaceous limestones (Cat Gap Formation); limestones (Blake-Bahama Formation); claystones and shales (Hatteras Formation); zeolitic claystones (Plantagenet Formation); nanno-marls (Crescent Peaks Member); siliceous oozes, clays, and cherts (Bermuda Rise Formation), and hemipelagic muds (Blake Ridge Formation) with locally redeposited shallow-water carbonates (Great Abaco Member). A corresponding section in the eastern North Atlantic is only developed at Site 367 where similar mineral assemblages could be found. At Sites 369, 370, 415, and 416 the sediment sequence is dominated by turbiditic influx of terrigenous components alternating with pelagic sediments; this can clearly be depicted from the mineral suites but it is not possible to establish sedimentary formations which can be paralleled to those within the western North Atlantic. Seismically mappable reflectors which were calibrated at the sediment sequence of the drillsites, are in most cases related to lithological changes and mineral composition. These acoustic horizons are very often coincident with formation boundaries. Besides different mineral composition, changes in grain size and diagenesis but also erosional unconformities are responsible for the impedance contrasts leading to the reflectors as already pointed out byTucholke (1979).