MODE OF GROWTH OF ELONGATE CONCRETIONS

Various lines of evidence indicate that elongate concretions record the orientation of groundwater flow at the time of their precipitation. They are, thus, potentially very useful in determining the nature of paleogroundwater flow and understanding geological variables that influence flow. Although several studies have examined concretion orientations and other macroscopic characteristics, none have addressed the question of how they grow in any detail. Based upon their macroscopic characteristics, it is possible to classify elongate concretions into three main types: solid, solid-zoned, and amalgamated. The solid and solid-zoned concretions have a relatively smooth exterior and appear to be completely cemented. The solid-zoned concretions display internal concentric zonation on broken and weathered surfaces. The amalgamated variety is commonly incompletely cemented and consists of an amalgamation of pea-sized poikilotopic crystals of calcite. I have formulated three hypotheses for how elongate concretions grow: (1) Preferential up-gradient growth. By this mechanism concretions would grow in the direction of preferential solute delivery. (2) Preferential down-gradient growth. This might occur due to fluid pressure differences resulting from flow around a low permeability object, such as the growing concretion. (3) Precipitation at multiple nucleation sites throughout the concretion body. By this mechanism calcite must precipitate in an elongate pattern at a variety of sites. Solid and amalgamated concretions examined thus far appear to have behaved as open systems during their precipitation, in which both early and late stage cements precipitated at multiple sites throughout the concretion body. Evidence for this includes incomplete cementation of the concretion body and petrographic and geochemical evidence for small-scale zonation. Thus, the solid and elongate types likely did not form in a simple concentric manner (e.g., simple up-gradient or down-gradient growth). The solid-zoned concretions are characterized by variations in intergranular volume, which produces the apparent concentric zonation. They may have formed by up-gradient or down-gradient concentric growth, but with cyclical variations in the amount of displacive (or replacive) growth versus passive pore filling.