Petrogenesis of carbonate lithofacies along an evolving Paleozoic shelf, lower Chickamauga Group and equivalents, Middle Ordovician, northeastern Tennessee

Petrography, cathodoluminescence and elemental geochemistry offer insights concerning the petrogenesis of Middle Ordovician shelf limestones (Chickamauga Group and equivalents).

Shelf micrites and slope calcisiltites have textures similar to Holocene calcite-dominated matrix precursors. Fine grained carbonates are interpreted to be the breakdown products of Mg-calcite skeletons. Trace element compositions of micrites form a trend connecting Mg-calcite lime mud and micrites lithified during meteoric diagenesis. Slope calcisiltites have elemental concentrations indicate Sr retention during lithification. which A calcite-dominated micrite precursor offers an additional mechanism for matrix dolomitization in shelf sequences.

Skeletal components exhibit four distinct styles of preservation: 1) skeletal molds occluded by calcite spar; 2) skeletons preserved as replacement spar or replacement-cement mosaics; 3) skeletons with excellent microstructure retention as ferroan or nonferroan calcite; 4) microcrystalline components preserved as calcite. Comparison with characteristics nonferroan of carbonates indicates Middle Ordovician particles in (1) and (2) were originally aragonite, and those in other groups groups (3) and (4) were calcite/Mg-calcite. Marine cements occur as fibrous calcite rims, microdolomite-bearing syntaxial overgrowths and microcrystalline calcite, and are interpreted as originally Mg-calcite. Constituent particle analysis indicates the original shelf sediments consisted primarily of magnesian calcite.

Samples were grouped into six geochemical facies based on lithologic and elemental data. Peritidal limestones and outer shelf limestones are depleted in whole-rock Sr, Fe and Mn relative to argillaceous limestones and deep-water limestones; drowned limestones comprise a separate group substantially enriched in Fe and Mn. The regional and stratigraphic distribution of geochemical facies reflect the complex interaction of depositional and diagenetic factors.

Basinal shales appear to have influenced the diagenesis of equivalent limestones. Diagenetic reactions in shales are interpreted to have added Mn and Fe, and removed Mg from interbedded slope 1limestones. Drowned carbonates encountered these metal-rich fluids migrating in response to fluid potential gradients. Anoxic marine waters were the source of metals in syndepositional oxide deposits in shelf-margin limestones. Deep burial diagenesis of outer shelf facies included the introduction of basin-derived saline brines. Burial history analysis indicates movement of waters through porous zones during the Middle Paleozoic.