Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater

Elongate calcite concretions are thought to form parallel to the direction of groundwater flow at the time of their precipitation, and are potentially very useful in determining the nature of paleogroundwater flow and geological variables that influence flow. This study focuses on the mode of growth of such concretions, which has not been previously investigated in detail. Most of the concretions examined are from the Oligocene to Pliocene-Pleistocene, Santa Fe Group of New Mexico, which is the synriffing basin fill of the Rio Grande rift. Based upon their macroscopic characteristics, elongate concretions can be classified into three main types: uniform, composite, and zoned. The uniform and zoned concretions have similar, relatively smooth exteriors, but differ internally, with the zoned variety displaying internal concentric zonation on broken surfaces. Uniform concretions consist of mixtures of early micrite/microspar and later sparite that have different elemental chemistry (mainly Mg variation) and are present throughout the concretion. Petrographic examination reveals significant porosity (up to 15%) in the interiors of some uniform concretions. Zoned concretions consist mainly of sparite, which commonly varies both texturally and in elemental composition in a concentric manner. The composite variety has a rough warty exterior and consists of an amalgamation of pea-sized, spherical poikilotopic crystals of calcite. Uniform and composite concretions appear to have formed from the simultaneous precipitation of calcite throughout the volume of the concretions (i.e., early and later stage cements are pres-ent throughout). Zoned concretions appear to have formed from concentric growth in which early calcite precipitated at several interior sites (core zones) and was covered by successive layers of younger calcite. Most of these sites of inferred initial precipitation occur preferentially near the up-gradient ends of the concretions, which suggests that the zoned concretions grew in part through preferential precipitation in the direction of flow. The observed petrographic and geochemical characteristics indicate that the elongation is the result of growth within elongate zones of favorable pore-water chemistry (e.g., zones of elevated carbonate alkalinity). Such zones probably formed down-gradient of fragments of organic matter undergoing microbial decay