VARIATIONS IN TEXTURAL BANDING OF OOIDS OVER STRATIGRAPHIC HEIGHT OF THE JOHNNIE OOLITE AND THEIR GEOCHEMICAL CORRELATIONS: IMPLICATIONS FOR EDIACARAN PALEOCEAN CHEMISTRY

We examined a stratigraphic section of the Johnnie Oolite containing a large Ediacaran negative δ¹³C excursion (Bergmann et al., 2011). The section was also studied for Carbonate Associated Nitrate (CAN) concentrations, which may serve as a proxy for the degree of ocean oxygenation as well as for the concentration of fixed nitrogen in the environment of deposition.

Here we conducted a comparative study between stratigraphic variability of ooid textures, a set of redox-sensitive trace metals, and CAN concentrations in order to place some initial constraints on the extent of diagenetic processes and to gain a greater understanding of the depositional environment. Petrographic examination of the textural properties of ooids that form the rock revealed potential indications of primary formational environment changes across the Johnnie Oolite. Two types of ooid textures were observed, defined here as “banded” and “unbanded”. The degree of banding, which is recognized and defined by sharp terminations of the carbonate crystals in clear spherical patterns around the nucleus, was observed to change upwards through the Johnnie Oolite. The cause of banding textures is not well known, but among considered possibilities are: 1) a cessation and then resumption of crystal growth, 2) a chemical change in the environment of growth, or 3) the termination and genesis of a new layer of crystals due to physical or chemical processes.

Focusing on a single ~3 m stratigraphic section, the North Springs Mountains area, we observe a clear shift from heavily banded ooids near the base to unbanded grains near the top, with a distinct change occurring between 2-2.5 m. The lack of any marked changes in sedimentary sorting point away from a physical change across the section and towards a chemical or biological influence. Increased Fe concentrations within banded ooids and not within unbanded ones relative to their matrix, which are taken to be primary given their deviation relative to Mn concentrations, point to a shift in the local redox environment. Observed correlations between ooid formation texture and δ¹³C and CAN values support this conclusion, and further point towards some distinct shift in the local paleocean chemistry recorded within the Johnnie oolite.