Submarine Diagenesis (Aragonite Dissolution, Cementation by Calcite, and Dolomitization) in Ordovician Galena Group, Upper Mississippi Valley: ABSTRACT

Abstract

The Galena Group is a fossiliferous, dominantly carbonate unit about 85 m thick, which was deposited in a broad epeiric sea mostly below wave base. Submarine dissolution of aragonite and cementation by calcite appear to have proceeded simultaneously in Galena sediments. Commonly, the enclosing sediment (mostly carbonate mud) lithified before shell aragonite dissolved, resulting in moldic voids. Most of these voids were later cemented with block calcite spar; some were filled by bioturbation. Some burrow fills became lithified but the enclosing matrix remained soft. Where this occurred, sparry replacements of aragonitic bioclasts now exist only within these burrow fills; lithification preserved bioclast outlines. Where scouring later exhumed lithified burrow fills, they produced topographic highs on scoured surfaces or intraclasts with meniscus-fill fabric. Hardgrounds very commonly occur in most exposures of the Galena; many are bored. Sparry calcite fills cracks in some hardgrounds, and is transected by borings or by the overlying bed. Spar-filled voids suggestive of former aragonitic clasts are preserved in the upper centimeters beneath hardgrounds in some strata which otherwise lack these fossils. Fine to very fine crystalline dolomite fills burrows extending downward from many hardgrounds. Individual dolomite crystals are abraded at scoured surfaces and at margins of intraclasts. These features suggest that dolomitization occurred on the seafloor. End_of_Article - Last_Page 697------------