Cortical fabrics of Upper Jurassic ooids, Arab Formation, Saudi Arabia: Implications for original carbonate mineralogy

Abstract

This study reports the occurrence of partly leached ooids, interpreted to have been originally aragonitic, that still preserve intra-cortical fabric details in the Upper Jurassic of Saudi Arabia. A thin dark layer of insoluble material (probably organic matter) separates adjacent carbonate lamellae. Selective leaching of carbonate occurs in each lamella, yielding intra-cortical moldic porosity. It is the presence of the thin dark layers that allows fabric details to be preserved during and after leaching of the ooids. Locally, the intra-cortical moldic pores may be infilled by finely crystalline equant calcite cement that is interpreted to be of meteoric origin. In all cases, this equant calcite cement has nucleated on top of the underlying dark layer of insoluble material and has grown upward and outward until reaching the next layer. If allowed to go to completion, crystal growth in this fashion will produce a radial fabric, similar to or even mimicking that seen in many ancient ooids. This is an important observation because well-preserved radial fabrics seen in ancient ooids have previously been interpreted to represent original calcite mineralogy. Results from this study suggest that a well-preserved radial calcite fabric may be indicative of either originally aragonite or calcite mineralogy.