Oriented carbonate concretions in a paleoaquifer: Insights into geologic controls on fluid flow

Abstract

Oriented carbonate concretions appear to provide a powerful new tool in the evaluation of flow through heterogeneous aquifers. Calcite cement forms elongate bodies of indurated rock that have distinct and mappable spatial orientations. We have mapped ~200 of these concretion orientations as well as the hydrogeologic characteristics of the host sediments in outcrops of the Pliocene-Pleistocene Sierra Ladrones Formation in central New Mexico. The average azimuthal concretion orientations is 150°, which is consistent with the southeasterly groundwater flow suggested by past and present basin geometry. Using a first-order approximation, the dimensionless transverse velocity is estimated from the orientation data. The spatial correlation structure of the dimensionless transverse velocity is distinctly different in the two units investigated. However, both are consistent with existing stochastic models. The differences between the units and the similarities with theoretical models suggest that concretion orientations can be used to study how fluids flow through heterogeneous systems over scales ranging from centimeters to tens of kilometers.