Uplift and exposure history of the Côte d'Ivoire-Ghana Transform Margin: geochemistry of pore-filling and fracture vein calcites

Abstract

Reefal and peri-platform carbonates of late Albian to Turonian‐Cenomanian age were recovered from Leg 159 Ocean Drilling Program (ODP) Sites 959, 960, and 962 which were drilled on the Cote D’Ivoire-Ghana Transform Margin. These deposits thicken and coarsen from the Deep Ivorian Basin southward toward the marginal ridge and are intercalated with pelagic micrites containing abundant planktonic foraminifers. The stratigraphic relations along with the nature of the skeletal grains and intraclasts in the carbonates suggest a depositional setting of a shallow carbonate platform that rimmed an emergent metamorphic and sedimentary terrane that served as a coeval source of clastic and carbonate components. Carbonate-rich horizons are characterized by dissolution of aragonitic bioclasts, recrystallization of micrite to microspar, and infilling of primary and secondary porosity by equant to prismatic low-Mg calcite cements. Preservation of grain morphologies as uncompacted micrite rinds suggests early dissolution and concomitant cementation at the sediment-surface or during shallow burial. Late-stage fracturing of these units during syn- and post-transform deformation is marked by extensive mineralization by quartz, barite, clay, and calcite vein fills. Based on paragenetic relations and isotopic composition, multiple stages of fracturing and mineralization are identified. All cements and fracture vein calcites have δ 13 C values significantly more negative than primary marine compositions, suggesting a dominant role of organic matter decomposition in controlling the isotopic composition of diagenetic fluids. The oxygen isotopic composition of cements and vein fills range from -1‰ to -10‰ δ 18 O. Of these, the first stage is interpreted as forming during progressive subsidence under increasing temperature conditions. A late fracturing event is recorded by calcite whose composition ranges from -8‰ to -10‰. Assuming a reasonable range of interstitial water compositions, -5‰ to +2‰ standard mean ocean water (SMOW), a maximum emplacement temperature of less than 90°C is estimated for late fracture vein calcites. This stage of fracturing and calcite mineralization, which reflects the highest temperature event of calcite formatio n, occurs in units as young as early Eocene in age and requires precipitational temperatures in excess of those expected from typi cal burial diagenesis. In light of the high temperature of emplacement, this last stage of deformation is interpreted as formin g during the passing of the spreading ridge along the Cote D’Ivoire-Ghana Transform Margin during early Eocene time, a time much later than predicted based upon previous studies.