Abstract

Facies models for modern nearshore marine bar sedimentation formed under wave-influenced conditions (Clifton et al., 1971; Davidson-Arnott and Greenwood, 1976) have been applied to the Upper Ordovician-Lower Silurian Elmina Sandstone in the Elmina area of the central coast of Ghana. Six analogous sedimentary facies of nearshore marine bar origin are present:l) an offshore facies characterized by very fine-grained sandstone and dominated by plane bedding and small-scale ripple cross-lamination and in some places by ripple drift cross-lamination, showing landward-dipping lee-side laminae (north-northwest); 2) a bar crest facies composed of fine-grained sandstone with common slump structures and dominantly characterized by sets of landward-dipping (from northeast to northwest) trough cross-stratification and units of subhorizontal plane beds; 3) a trough facies of very fine-grained sandstone with some mud laminae characterized by small-scale tipple and ripple drift cross-lamination, generally striking almost at right angle to the shoreline (north-northeast); 4) a rip channel facies composed of fine-grained, seawarddipping (south-southeast), trough cross-stratified sandstone, making an erosional contact with the underlying facies; 5) a surf facies of fine-grained sandstone dominated by sets of plane beds and units of seawarddipping trough cross-stratification (south-southeast); 6) a beach face facies composed of fine-grained sandstone showing interbedded sets of subhorizontal plane beds. The interpretation of these facies as units belonging to a marine sand bar is confirmed by mineralogic, textural, and biogenic evidence. The sandstone is composed of several depositional regressive sequences. Sand deposition probably occurred in a rapidly subsiding area with a high rate of sediment supply. The abundant sediment supply possibly caused a general progradation of the shoreline to the south-southeast. The absence of mudstone layers within the entire sandstone unit indicates that deposition of sand occurred in a very shallow basin which maintained enough turbulence to prevent mud deposition.

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