Silica Diagenesis in Accreted Eocene Siliceous Rocks (Horizon Ac) on Barbados

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 Ac 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 Al2O3 weight percent or layer type as defined by relati e 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 Ac 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.