EARLY LOAD-INDUCED FRACTURING IN PROGRADING CARBONATES

The description of non-tectonic fracturing of lithified carbonates is rare, likely due to formation of such fractures outside easily accessible near-surface settings. In older rocks, burial and diagenesis often precludes the determination of the exact timing of (early) fracturing. The Bahamas Drilling Project core Clino recovered the upper part of a young prograding platform margin and has enabled the examination of a set of fractures interpreted to be non-tectonic— formed by rapid loading of a clinothem package. Three intervals of open and partially infilled fractures were recovered in Clino and are closely tied to the lithologic variation in the slope sediments. Fracturing has occurred specifically within the lowstand, coarse, well-cemented early Pliocene skeletal deposits some 45-m thick. The overlying and underlying finer-grained hemipelagic sediments are not commonly fractured. Our model suggests that early cementation of the lowstand skeletal sands, followed by rapid margin progradation, provided significant load to generate brittle fracturing (shear and extensional type) in the cemented slope deposits. Laboratory deformation experiments, magnetic fracture-plane orientation, and the timing of load emplacement, are consistent with load-induced fracturing. The interpretation of the Clino data supports a local, non-tectonic mechanism for the formation of macroscopic fractures. This previously unrecognized early fracture porosity could act as major fluid-flow pathway for diagenesis of the platform and slope.