MODELING THE MECHANISMS CONTROLLING THE DEVELOPMENT OF KEYHOLE PASSAGE MORPHOLOGY

Cave passage morphology is an important indicator of the paleohydrological conditions under which a cave passage was formed. Keyhole passages are a commonly occurring type of passage morphology that consists of a round, wide top with a smaller canyon incised in the bottom of the passage. These passages typically indicate a shift from phreatic to vadose conditions. However, there are multiple mechanisms by which such a transition can occur, and little is understood about which of these mechanisms is most common, or whether aspects of keyhole morphology may indicate the primary formation mechanism. CaveCHAMP is a physically based cave evolution model that can simulate the evolution of passage shape along an extended conduit, including effects of ventilation and CO2 dynamics in the air-filled portion of the conduit. We have found several ways to model keyhole passage formation using CaveCHAMP, including manipulating discharge, chemistry, and tectonic forcing. We have also simulated the changes to keyhole morphology along a conduit and shown that the morphology maintains shape upstream when created by a sudden decrease in discharge. In contrast, the keyhole widens and becomes less distinct upstream when created by base-level fall. These differences indicate that field observations of keyholes may allow us to constrain the conditions under which they formed.