Early diagenesis of selected Middle Ordovician organic buildup deposits and oolite shoals from the Southern Appalachians

Author

Kenneth J. Tobin

Date of Award

12-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Kenneth R. Walker

Committee Members

T. Broadhead, S. Driese, C. Mora, E. Wehry

Abstract

This dissertation addresses four major problems, which involve: (1) the stable isotopic variation of Ordovician meteoric calcite precipitated under a landscape lacking vascular land plants, (2) the role of non-oxic, marine diagenesis in ancient platform-marginal settings, (3) the potential for the preservation of ancient marine δ¹⁸O compositions, and (4) Ordovician seawater temperature. Meteoric cement from the buildup fades of the "Chickamauga" Formation at Red Mountain Expressway (RME) and Tidwell Hollow (TWH) sections in Alabama was examined. This unit has a complex early diagenetic history consisting of multiple periods of marine and meteoric cementation. Equant (drusy) calcite is interpreted as meteoric because its stable isotopic composition (δ¹³C = -0.1 to -1.3 %o PDB, δ¹⁸O = -4.0 to -7.9 %o PDB) is too variable to be accounted for by marine or mixing zone precipitation. Meteoric diagenesis is unrelated to overlying exposure surfaces. Equant (drusy) calcite at RME has positive δ¹³C and δ¹⁸O values and has Mg values. The above geochemical signatures indicate precipitation at a site that was distal from the recharge zone. Formation of buildup deposits occurred during a period of rising relative sea level. Consequently, flow of meteoric fluids under a submerged platform is the only mechanism that can reconcile available stratigraphic, petrologic, and geochemical data at RME. Ordovician, pre-vascular land plant, meteoric δ¹³C values (from TWH and other formations that precipitated from more proximal sites to recharge) are slightly depleted relative to coeval marine carbonate by 1 to 4 %o, especially when compared with post-Silurian meteoric carbonate that can document up to -11 %o depletion of ¹³C relative to coeval marine calcite. The oolite member of the Ottosee Formation (Middle Ordovician) has bladed-to-fibrous calcite, which formed under oxygen depleted conditions atypical of most abiotic marine precipitates. Specifically, the sequence of early diagenetic events was: (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) diagenetic stabilization of metastable phases, such as ooids and turbid marine cements. Stabilization mainly post-dated bladed-to-fibrous calcite formation but some stabilization preceded and was coeval with bladed-to-fibrous cementation. Cathodoluminescent and minor element data indicate that ooid and initial marine cement precipitated from oxic fluids. Minor dissolution of ooids and initial marine cement was associated with aerobic degradation of organic material decreasing alkalinity through generation of CO₂. Translucent, bladed-to-fibrous calcite precipitated from suboxic to anoxic (post-oxic), marine fluids that were modified by bacterially mediated Mn⁴⁺, Fe³⁺, and SO₄^2- reduction. Translucent, bladed-to-fibrous cement has a primary low-magnesium calcite mineralogy and was unaffected by diagenetic stabilization as indicated by δ¹³C and δ¹⁸O compositions near the Ordovician marine value. This is unlike ooids and turbid marine calcite with a metastable magnesium calcite mineralogy that were dramatically altered by stabilization as reflected by negative δ¹³C and δ¹⁸O ooid values. Additionally, this study documents a locally variable cathodoluminescent zonation patterns in post-oxic cement, which is at variance to the cement stratigraphy paradigm of regionally correlatable CL zones. Fibrous calcite from organic buildups in the Holston and Effna formations was examined as a potential proxy for the stable isotopic composition of Ordovician seawater. On the basis of minor element and stable isotopic compositions three fibrous calcite end members can be delineated. (1) Mn-rich, ¹³C and ¹⁸O depleted fibrous calcite was altered by Mn-rich fluid early in the diagenetic history of the cement. (2) Fe-rich, ¹⁸O depleted fibrous calcite is interpreted as reflecting post-depositional alteration by burial fluids. (3) Fe- and Mn-poor, ¹³C and ¹⁸O enriched fibrous calcite that represents least-altered fibrous calcite. Least-altered marine calcite and other pre-Carboniferous marine calcite have δ¹⁸O = -4 to -5 %o PDB consistent with precipitation from marine water having a δ¹⁸O composition of -1 %o SMOW at a temperature of 30 to 33 °C. This temperature estimate is high because least-altered marine material probably has had some light ¹⁶O added to it during stabilization.

Recommended Citation

Tobin, Kenneth J., "Early diagenesis of selected Middle Ordovician organic buildup deposits and oolite shoals from the Southern Appalachians. " PhD diss., University of Tennessee, 1994.
https://trace.tennessee.edu/utk_graddiss/10601