Genesis of Carbonate Concretions in the Upper Ludlowville, Middle Devonian of Erie County, New York

Abstract

<p> Concretions in a zone about 4 m. below the Tichenor Limestone formed just below the sediment water interface. Their growth was initiated about an organic-rich fossil cluster and was probably completed before they were more than 5 to 8 m below the sediment surface. Chemical products of organic decay, notably bicarbonate ions and ammonia, diffused outward, raising the pH and precipitating calcite from connate waters already nearly saturated with respect to calcium carbonate. These conclusions derive from the shape of the concretions, from their relation to the enclosing shales, and from their overall structure , particularly the position of pyritic fossil clusters. The relative volumes of soluble material (calcite) in the concretions are consistent with relative pore volumes through the upper 5 m. of recent, fine-grained, clayey sediments. The postulated genetic sequence agrees with recent work on carbonate diagenesis. Restriction of the concretions to discrete layers was most probably due to a widespread comnbination of high rates of organic productivity, high rates ·of sedimentation, and low rates of water circulation, that lasted a short period of time and resulted in the rapid burial.of much undecomposed organic material. </p>