Hempel, Peter and Bohrmann, Gerhard (1990) Carbonate-Free Sediment Components and Aspects of Silica Diagenesis at Sites 707, 709, and 711 (Leg 115, Western Indian Ocean). Proceedings of the Ocean Drilling Program: Scientific Results, 115 . pp. 677-698. DOI 10.2973/odp.proc.sr.115.181.1990.

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Abstract

From the equatorial Indian Ocean, carbonate-free portions of sediment samples of Paleocene to Miocene calcareous
oozes and chalks from Sites 707, 709, and 711 were studied using X-ray diffraction measurements and the scanning electron
microscope. Downhole variations in biogenic opal, quartz, barite, and clinoptilolite were investigated. The abundance
patterns of these major mineral phases show several similarities and may be used for additional lithologic correlations.
Variations in biogenic opal contents reflect biogenic silica productivity. Beside the general pattern, a succession in
biogenic silica decrease through time is generally recorded since the Oligocene. This succession started earliest at northernmost
Site 711 and latest at southernmost Site 707, including Site 709 within these two. Opal-A variations as well as
the barite distribution may be influenced by the paleoposition of the sites in relation to the high-productivity zone,
which today lies south of the equator. Authigenic clinoptilolite apparently formed in two different modes. In deeper
sediment intervals, clinoptilolite was the last mineral phase formed associated with enhanced silica diagenesis. In late
Oligocene to middle Miocene sediments, clinoptilolite was the only authigenic silica phase encountered where otherwise
strong opal dissolution was observed. The sponge spicules showed special dissolution features probably related to microbiological
activity. Silica concretions mainly composed of opal-CT and authigenic quartz occur in carbonate-rich environments
and are formed during later diagenesis when burial depth causes the sediments to reach higher temperatures.
Opal-CT concretions in carbonate-free siliceous oozes were found at Site 711 and are probably formed during an
early stage of silica diagenesis.

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