Variations in Textural Banding of Oӧids Across the Johnnie Oӧlite: Valuable Petrographic Indicators of Depositional Conditions or Vestiges of Diagenetic Activity?
Abstract
Stratigraphic sections of an oölite bed from the Rainstorm member of the Johnnie Formation (the Johnnie Oӧlite) in Death Valley, CA which have been shown to preserve a record of the large negative δ13C excursion commonly referred to as the "Shuram Excursion" were examined chemically and petrographically to constrain their depositional environments and diagenetic history. Two types of oöid cortical textures had previously been noted across sections. These textures, defined here as "banded" and "unbanded", were classified by the number of sharp terminations of the radial carbonate crystals in clear spherical bands around the nucleus. Quantifying spatial relationships of the two textures revealed that trends in cortical texture across stratigraphic section were observable at only one of the six studied sections, that at the Northern Springs Mountains (NSM). This section exhibits a clear linear trend from approximately 99% heavily banded (3+) oöids at the base to only 22% near the top. Parallel and adjacent sections that were deposited coevally or time-transgressively show no similar trend, but rather are composed of almost 100% heavily banded oöids. Textures are taken to be primary, with little diagenetic influence upon their petrographic preservation, possibly due to early mimetic dolomitization. While there is no consensus on the exact mechanisms that promote oöid banding, an attempt was made to determine which of many proposed causal mechanisms was most likely to have prompted the observed stratigraphic trend in texture. Correlations between textural and geochemical variability point to a depositional environment in the NSM area that was incrementally physically separated from the oceanic basin, where the other sections were deposited. While chemical signature of the NSM section may not be representative of the global ocean conditions, the fact that none of the other studied sections exhibit a stratigraphic trend in cortical texture, as well as their largely similar trends in physical sedimentological characteristics and geochemical signatures points to similarities in their depositional environments, lending further credence to their interpretation as a single continuous transgressive system that may record changes in Ediacaran global seawater chemistry.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP11B2024Z
- Keywords:
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- 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 4914 Continental climate records;
- PALEOCEANOGRAPHYDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHY