UNDERSTANDING SOFT-SEDIMENT DEFORMATION IN A MIDDLE/LATE ORDOVICIAN CALCARENITIC SHOAL COMPLEX

Soft-sediment deformation is universally common in the geologic record, and much of our understanding about it comes from fine- and medium-grained siliciclastics in various terrestrial systems of Holocene age. Based on these largely modern analogs, such deformation is related to increases in pore-fluid pressure about a critical threshold level, most commonly due to slope-induced movement, sediment loading, oscillatory movement by storm waves or seismicity. Such deformation, however, is also locally abundant in ancient, marine carbonates, for there are currently no modern analogs. Because deformation intensity is related to the density of the deforming substance, and carbonates are denser than siliciclastics, threshold pressure levels and triggering mechanisms could be greater for carbonates. In the Middle/Late Ordovician Lexington Limestone of central Kentucky, deformation is abundant but occurs largely in fine-grained (calcisiltites or fine-grained calcarenites), argillaceous units, in which fine grain size and/or argillaceous content help to confine and enhance pore-fluid pressure beyond the critical level, most likely due to cyclic, seismic loading. Here, in contrast, we report major deformation in coarse-grained calcarenites and calcirudites, in which the abundance of interconnected porosity should not permit such deformation. These units represent shoal complexes, which are commonly bound by hardgrounds or shales that may have confined or enhanced pore-fluid pressure, thereby facilitating deformation during cyclic loading. Early cementation, which could compartmentalize the complex, or extremely large triggering events may have also contributed to deformation in these coarse-grained carbonates. Regardless of the trigger, deformation in these rocks extends the range of occurrence in carbonates and suggests that extenuating circumstances, like bounding hardgrounds or shales and early cementation, are necessary to deform such coarse-grained, carbonate rocks.