Suboxic to anoxic diagenesis of platform-marginal ooids and bladed-to-fibrous calcite from the Middle Ordovician Ottosee Formation (east Tennessee)
The oolite member of the Ottosee Formation (early Caradoc) has bladed-to-fibrous calcite cement that formed under oxygen-depleted conditions atypical of most abiotic marine precipitates. The sequence of early diagenetic events was as follows: (1) ooid formation, (2) precipitation of rare turbid marine cement, (3) minor dissolution of ooids and initial marine cement, (4) precipitation of translucent bladed-to-fibrous calcite, and (5) stabilization of metastable components, such as ooids and turbid marine cements. Ooids and turbid marine cements precipitated from oxic fluids. Minor dissolution of ooids and turbid marine cements was associated with aerobic degradation of organic material, which decreased alkalinity through generation of CO 2 . Translucent bladed-to-fibrous calcite precipitated from suboxic to anoxic marine fluids that were modified by bacterially mediated Mn 4+ , Fe 3+ , and SO 4 2− reduction. Translucent bladed-to-fibrous cement was originally low-magnesian calcite and has δ 13 C and δ 18 O values near other Caradoc marine values. Conversely, ooids and turbid marine calcite had a metastable magnesian calcite mineralogy. These components were dramatically altered by stabilization as reflected by negative δ 13 C and δ 18 O ooid values. Mineralogical stabilization of ooids and turbid marine cements mainly postdated bladed-to-fibrous calcite formation, but some stabilization preceded bladed-to-fibrous calcite cementation. Additionally, this study documents locally variable cathodoluminescence zonation patterns, which are at variance with the cement stratigraphy paradigm of regionally correlatable CL zones.