SURFACE AND VADOSE IMPLICATIONS OF KARSTIFICATION IN EOGENETIC CARBONATES

Eogenetic karst has gained much attention over the past decade with research being done in young, diagenetically immature carbonates in Florida, Yucatan, and carbonate islands world-wide. These projects have focused on phreatic fresh-water flow, mixing effects, speleogenesis, aquifer response and storage, and contaminant transport. Less well studied are the outcomes of surface karstification and subsequent vadose flow. Exposed eogenetic carbonates develop an epikarst quite different from that found on telogenetic carbonates. Eogenetic properties (retention of primary porosity, maintenance of allochem heterogeneity, and differential cementation) are essential to the development the unique karren landforms found on young carbonate surfaces, especially in coastal areas, which are not solely caused by biological action as commonly believed and expressed by the term “phytokarst”. As eogenetic carbonates mature and approach telogenetic properties, their karren forms become more like classical examples from continental interiors. These results indicate that it is the tropically controlled depositional environment of eogenetic carbonates, rather than the tropical denudational climate, that makes eogenetic karren unique. The relative spatially uniform surficial denudation of eogenetic carbonates has lead researchers to underestimate denudation rates, producing incorrect sea-level positions from measurements of Pleistocene fossil coral terraces. The ~30% matrix porosity of young eogenetic carbonates leads to rapid (~64 mm/ka), but unfocussed, denudation that creates a uniform surface lowering (lowering exceeds observed pinnacle relief), as demonstrated by tropical Karrentisch on Guam. Below the surface, vadose flow is a mix of diffuse flow that on Guam can take up to 20 months to traverse a 100-150 m vertical section (70%), and pipe flow down vadose fast-flow routes, or pit caves, that arrives at the water table within a few hours or perhaps even minutes of the meteoric event (30%). In very young eogenetic carbonates, the high amount of depositional aragonite creates vadose flow that contains levels of Sr an order of magnitude greater than from older rocks (>2 Ma), potentially leading to incorrect trace element interpretations compared to speleothems in caves formed in older carbonates.