THE IMPORTANCE OF LITHOLOGIC VARIABILITY AND STRATIGRAPHIC ARCHITECTURE IN THE DEVELOPMENT OF EOGENETIC KARST SYSTEMS

Integrative characterizations of complete karst systems on low-lying eognetic carbonate platforms are rare and often limited to areas of direct observation where caves can be entered and explored. Because the hydraulic properties of eogenetic limestone have been implicitly assumed to be homogeneous, classical models of carbonate island karst development stressed the importance of geochemical interfaces in controlling cave and vug development. explaining the largest cavern systems as results of either 1) mixing dissolution at platform margin, or 2) phreatic/vadose zone dissolution.

New data from core descriptions and wireline logs obtained in 16 boreholes drilled in the Sandy Point region of San Salvador Island suggest that sharp contrasts in facies’ permeability play at least an important role in the development and localization of non-matrix porosity. Results of this analysis include a detailed stratigraphic framework that recognizes 7 units: Pleistocene 1, Mid Pleistocene 1-2, Early Pleistocene 1-3, and the Pliocene Dolomite. Maps of cave and vug distributions reveal a distinct spatial control by several of the stratigraphic boundaries. The most extensive cave network occurred just above the contact between Mio-Pliocene dolomites and Pleistocene limestones. Secondary, less extensive horizons of cave development were found perched above low-permeability caliche crusts and paleosol exposure surfaces.

We interpret the association of cavernous and vuggy horizons with exposure surfaces to indicate that they formed during lower sea levels when vadose infiltration encountered laterally extensive, low-permeability bedrock, which redirected the flow of water laterally. Cave formation by transient perching of vadose infiltration would allow more reasonable timescales for cave development than mixing models, which require anomalously high bedrock dissolution rates. These results assist in understanding the complex development and distribution of non-matrix pore systems on low-lying eogenetic platforms and their fundamental relationships to platform geology.