Abstract

Barite concretions were found in Eocene pelagic chalks from Israel. They are volumetrically limited and occur mainly in the upper continental slope realm of the northwestern edge of the Arabo-Nubian platform. The concretions are diagenetic, having formed after sediment slumping and initial compaction. They are absent from the equivalent platform limestones. Although the concretions are replacive in origin, minute details of the micro- and nannofossils from the replaced chalk are very well preserved. Invariably, the central areas of the concretions consist of very finely grained, randomly oriented, euhedral crystals, grading outward into larger ( nearly equal 1 mm) interlocked grains with lobate boundaries. This reflects outward growth from densly packed to relatively sparse nucleation sites, probably due to decreasing concentration of Ba (super 2+) and/or SO 4 (super 2-) in solution. Barite crystals fill foraminifera and radiolaria chambers and largely replace coccoliths and siliceous ooze, but foraminifera walls remain calcitic. The barite in the concretions is depleted in trace elements relative to the host chalk and is thus similar to other barite occurrences of sedimentary origin. Part of the associated chert concretions may have formed from displaced silica, and the liesegang-type concentric bands in local barite concretions may have resulted from the displaced iron oxide. In the absence of either a terrigenous or hydrothermal source of barium, and because of the association the barite concretions with nannofossils containing siliceous ooze and/or chert, it is concluded that in similar to deep oceanic settings, barium was derived from both particulate marine barite and the mineral plus organic phases of micro- and nannofossils. The replacing fluids originated as interstitial sea water with dissolved barium. Local bacterial reduction of sulfate is indicated by enrichment of 34 S in the barite.

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