Abstract

Eocene siliceous rocks exposed on Barbados contain both opal-A (biogenic silcia) and opal-CT, and are interpreted as uplifted abyssal-plain or trench deposits, probably correlative to Horizon A c beneath the Atlantic Ocean floor. Rocks of hemipelagic origin on Barbados are well layered to massive and consist of radiolarite layers (T layers), matrix-supported radiolarian mudstone (F layers), and layers with an intermediate abundance of radiolarian tests (N layers). Large d-spacings of opal-CT (4.08-4.11 A) and low height to width peak ratios indicate poor crystallinity. Opal-CT is most common in silica-rich T layers, and opal-A is best preserved in mud-rich F layers. D-spacing doesn't correlate with Al 2 O 3 weight percent or layer type as defined by relative abundance of radiolarians versus mud matrix. We infer that the harder, more indurated T layers had better initial permeability prior to induration and that diagenesis was probably influenced by both porosity and permeability differences and to a lesser extent by chemical and mineralogic differences between F and T layers. Comparable diagenetic states between Horizon A c siliceous sediments below the Atlantic Ocean floor and accreted sediments on Barbados indicate that accretion of siliceous sediments was not accompanied by deep burial and/or elevated temperatures.

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