ABSTRACT

The Jurassic Smackover Formation within the Manila Embayment of southwest Alabama was deposited on a carbonate ramp modified by paleotopographic highs. The Smackover Formation consists of limestone, dolostone and silici-clastic units. Although low energy lithofacies were dominant within the embayment, moderate to high energy lithofacies are associated with shallow water across paleotopographic highs. These formed locally permeable lithofacies which became sites of preferential dolomitization. Also, during burial diagenesis, authigenic clays formed from ions within pore fluids of the rock. Authigenic clays in large quantities may have had an effect on the formation of dolomite within the embayment. Authigenic clay occurrence within the Smackover Formation of the Manila Embayment has largely been ignored in the past.

Scanning electron imagery (SEI) and energy dispersive spectral (EDS) analytical techniques identified small quantities of illite and/or mixed-layered illite clay within the carbonates and clastic units of the Smackover Formation. The mixed-layered illite may be illite-smectite, -vermiculite, or -mica. The clays form pore-lining and filling morphologies with crenulated platy habits and occasional fibrous or ribbon-like projections which sometimes coalesce to form plates. X-ray diffractometry (XRD) could not be used for identification because of small quantities of clay minerals present.

The clays seem to have an irregular distribution throughout the embayment, although they were found to be most common within the dolostone lithologies, probably due to these rocks high porosities. However, authigenic clay occurrence was found to be sparse and their growth was a late, deep burial diagenetic event, occurring after dolomitization and therefore having no effect on the formation of dolomite.