Hypogene Processes in the Balcones Fault Zone Segment of the Edwards Aquifer of South-Central Texas

Abstract

The Balcones Fault Zone segment of the Edwards Aquifer of south-central Texas is one of the most important and prolific karst aquifers in the United States. It is formed within the lower Cretaceous Edwards Group Limestone. Since deposition, it has undergone subaerial exposure, burial in the upper Cretaceous, faulting, igneous intrusion, uplift in the Miocene, and weathering processes. The Balcones Fault Zone consists of mostly normal en-echelon faults with as much as 300 m of displacement striking northeast–southwest and dipping down toward the Gulf of Mexico. It forms the Contributing, Recharge, and Artesian zones of the aquifer. Karst characteristics of the Edwards Aquifer are the result of both epigene and hypogene processes, which continue today. The subaerial exposure of the Edwards Limestone (Recharge Zone) contains epigene karst features typical of a karst landscape that contains remnants of relict hypogene processes. Many relict caves in the Edwards Limestone outcrop show evidence of being formed by ascending water. Some appear to be associated with paleo-springs that were abandoned as water levels in the aquifer declined. Evidence of current hypogene processes is found in the saline water zone, which is part of the Artesian Zone. Extremely high permeabilities have been developed by dissolution at depth and driven by a number of processes including artesian hydraulic heads, mixing corrosion, and biogenic acids. As a result, well production in the Artesian Zone is commonly limited only by the size of the pump.