Hypogenic Origin of Robber Baron Cave: Implications on the Evolution and Management of the Edwards Aquifer, Central Texas, USA

Robber Baron Cave is formed the Late Cretaceous Austin Chalk, in Bexar County, Texas. The cave exhibits numerous features that demonstrate a hypogenic origin, including a 1.5-km-long network maze pattern, fissure-floored passages, passage ceilings laterally enlarged adjacent to a contact with an upper confining unit, and authigenic sediments. The Edwards Aquifer provides the only source of water that could create the conditions necessary to form the cave, and provides modern analogs through artesian flows from the nearby San Antonio and San Pedro Park Springs. Anecdotal reports from the early 20th Century describe flowing streams and pools in sections of the cave no longer accessible.

The Edwards Aquifer enlarged westward by stream incision along the Balcones Fault Zone, exposing down-faulted permeable units to allow groundwater discharge from lower elevation locations. Stream incision rates indicate that the hypogenic conditions necessary to form Robber Baron Cave occurred at least 1.05 Ma, and thus set a minimum age for accretion of the Bexar County portion of the aquifer. The presence of this and other hypogenic caves and artesian springs in the Austin Chalk, above the upper confining unit of the Edwards Aquifer, demonstrate areas of significant localized upward flow into the Austin and the paleo land surface. Identification of these areas is important in establishing areas of more stringent land use regulations to prevent aquifer degradation through these high permeability features situated outside of the recognized aquifer recharge zone.