Sediment Coarse Fractions from the Western Indian Ocean and the Gulf of Aden (Deep Sea Drilling Project, Leg 24)

Abstract

Hemipelagic sediments in the Gulf of Aden, pelagic deep-water deposits in the Somali Basin and pelagic shallow-water sequences in the central Indian Ocean were penetrated and recovered during Leg 24 of the Deep Sea Drilling Project. The coarse fractions from these sediments have been studied. They have provided evidence that the Gulf of Aden has existed at least since middle Miocene as an ocean basin with typical hemipelagic sediments (high total sedimentation rates, high terrigenous input, high contents of organic carbon, high sedimentation rates of calcareous nannoplankton and planktonic foraminifera, high proportion of benthic fossils, abundant fish remains) suggesting its water masses to have been very fertile. The region had an arid climate during Miocene and Pleistocene times, while it seems to have been less dry during Pliocene (plant debris). An elevated ridge crest not accessible to coarse sediment components from both sides of the gulf had already been developed in the Miocene. The region now occupied by the >5000-meter-deep Alula-Fartak Trench which offsets Sheba Ridge seems to have been passable for sediment transport from the Arabian side of the gulf to the southeast during Pliocene. Upper Cretaceous deposits above the Somali Basin basement east of Chain Ridge are already true pelagic sediments. The preservation of calcareous sediment components of both deep Somali Basin sites suggests that a large portion has been replaced from areas of shallower water depths. Pliocene sediments east of Chain Ridge contain shallow-water-derived components which may have come either from the Gulf of Aden or from an area northeast of Carlsberg Ridge. The central Indian Ocean sites contain early Tertiary to Quaternary pelagic oozes and chalks. The sediments of all three sites are pertinent to the equatorial high production belt thus precluding major geographic readjustments of that piece of oceanic crust in a north-south direction since early Tertiary. The degree of fragmentation of planktonic foraminifera offers a useful tool for determining paleodepth although these sites were nearly always well above the calcium carbonate compensation depth. This is important for the interpretation of vertical sea-floor movements in connection with shifts of the spreading centers. The pattern of preservation of planktonic foraminifera also shows that a high proportion of the recovered sediments are displaced. This is substantiated by the joint occurrence of well-preserved planktonic foraminifera and true shallow-water-derived components in coarse-grained horizons in a matrix of poorly preserved calcareous oozes.

Initiation of subduction zones: implications for arc evolution and ophiolite development

“…This terrigenous input may be expressed as very rapidly deposited, landward thickening clastic wedges (Tucholke & Mountain 1979;Klitgord & Grow 1980), at times mixed with basalt (Moore 1973), or by more subtle dispersion of the terrigenous component in hemipelagic deposits (Whitmarsh, Weser, Ross et al 1974;Thiede 1974). There is adequate evidence that the basal strata are not typically oceanic in character.…”

Microfacies of Some Sediments from the Western North Atlantic: Paleoceanographic Implications (Leg 44 DSDP)

“…About 300 to 800 grains per sample were counted and separated in several classes using a stereomicroscope with incident Iight. This method was developed by Sarnthein ( 197 1), and Thiede (1974). The various components are given in Table 2.…”