Geometrical arrangement of calcite cementation within shallow marine sandstones

Abstract

Calcite cementation within shallow marine sandstones typically occurs as (1) continuously cemented layers, (2) layers of concretions, and (3) concretions scattered throughout the sandstone. This paper presents a model for the nucleation and diffusion-controlled growth of calcite cement which explains how these three types of calcite cementation arise by diagenetic redistribution of carbonate fossils originally contained within the sandstones. The model predicts that nucleation points in a system where clastic carbonate is concentrated in layers, will be confined to these layers, and a continuously cemented layer or a layer of stratabound concretions will form from the carbonate fossil-rich layer depending upon the original amount of clastic carbonate present within the layer. The semi-regular distance observed between stratabound concretions is a result of each nucleation point lowering the concentration of dissolved calcite in the surrounding sandstone and thereby inhibiting nucleation in close proximity to earlier formed nuclei. In a system with a homogeneous distribution of clastic carbonate, no preferred levels of nucleation are present, and this explains how a uniform three-dimensional distribution of concretions may arise. Flattening of concretions parallel to bedding is a natural consequence of diffusion-controlled growth in a system where the clastic carbonate is concentrated in layers, and need not have anything to do with permeability anisotropy. Formation of continuous calcite cemented layers by merging of concretions explains why closely spaced vertical geochemical profiles through a calcite cemented layer often are widely different.