Control exerted by rock anisotropy on chemical compaction localization in Bahamian-type peritidal limestones of the Apulian Platform (Italy)

In this contribution we present the results of a work aimed at assessing the geological, petrophysical and mechanical factors affecting chemical compaction localization (overburden-inducedpressure solution) in tight, Bahamian-type platform limestones. The products of such compaction include bed-parallel stylolites, which represent structural features that may affect the permeability of the rock due to the presence of clayish insoluble residue along them. By combining field and laboratory studies (sedimentological, structural, petrographic and statistical analyses), we have investigated Upper Jurassic and Cretaceous peritidal limestones originally pertaining to the Apulian Platform realm and now exposed in three distinct Italian locations: Maiella Mountain, Gargano Promontory and Murge Plateau. All these limestones are crosscut by bed-parallel stylolites that are more abundant (up to 10 times longer and less spaced from each other) in the laminated microbial bindstones than in the other, more isotropic carbonate facies (mostly structureless lime mudstones to grainstones). A first-order control of bed thickness on spacing and, hence, localization of bed parallel stylolites is excluded. By contrast, lithological factors clearly played a major role on localize stylolites within the microbial bindstones. One prominent factor is likely the ubiquitous occurrence in bindstones of bed-parallel laminae. Accordingly, petrographic thin sections display stylolites localized at laminae boundaries. However, bedparallel laminae are ubiquitous also in some grainstones where stylolites are only a little more abundant than in the nonlaminated counterparts. So, other factors peculiar of microbial bindstones may have dramatically influenced stylolite localization as well. A key factor is probably the ubiquitous occurrence of former fenestral voids that, to date, are mostly filled by coarsely sparry calcite cement. Similar distributions of stylolites in structureless limestones (mudstones to grainstones) are consistent with rock grain size having probably played only a minor role in their localization (stylolites slightly enhanced in muddy limestones). By contrast, clay amount of the rock (0-4% by volume) seem have not played any significant role in enhancing stylolite localization. To sum up, our results point to a major role played by the lithological characteristics of limestones on the development and localization of bed-parallel stylolites. Additional ongoing laboratory experiments including petrophysical and geomechanical analyses may help to improve these results and to better understand the role played by stylolite localization on the overall rock permeability.