Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Deposition and Diagenesis of the Ediacaran Khufai Formation, Huqf Supergroup, Oman: Preservation Potential for the Shuram Carbon Isotopic Excursion
(1) Division of Geological and Planetary Sciences, California Institute of Techology, Pasadena, CA.
(2) Department of Earth Sciences, University of California, Riverside, Edmonton, AB, Canada.
The carbon isotope ratios of carbonate rocks can record meaningful perturbations to Earth’s biogeochemical cycles. However, these ratios can also be subject to diagenetic resetting or overprinting. We seek to clarify the diagenetic history of the Shuram carbon isotopic excursion by examining the onset in the upper Khufai Formation using organic geochemistry, stratigraphic analysis, and detailed petrography. This anomaly is the largest known deviation in δ13C in Earth history dropping from 6‰ to -12‰ near the boundary between the Khufai and Shuram formations and recovering slowly into the overlying Buah Formation. While this perturbation has been well documented, it has been variably interpreted as a primary environmental signal, the result of deep burial diagenesis, or a product of meteoric diagenesis.
To address the burial history of the Khufai, we conducted preliminary biomarker analyses. The Khufai as a whole has very low TOC although OC is enriched in the lowermost deep-water facies compared to the upper microbial laminites, stromatolites, tepees, and grainstone facies. The bitumens appear to be indigenous to the Nafun Group based on overall characteristics in accordance with the biomarker parameters of Grosjean et al. (2009), including low 2-Methyl hopane indices (1-3%) compared to the Ara or Athel intra-salt rocks (usually >7%) and C29 sterane dominance in the for C27-C30 steranes. Hopane- and sterane-based thermal maturity parameters place these extracts as immature to early-oil window maturity. This is supported by RockEval that produced Tmax values < 440 °C, which are inconsistent with deep, high temperature burial (>100°C).
In the Huqf area of central Oman the upper Khufai records net deepening, capped by a transgressive oolite prior to deposition of Shuram formation siltstones. The isotopic anomaly begins in the upper 20 m of the Khufai, dropping gradually from positive values in peritidal facies and then sharply from 0‰ to -4‰ in the oolite bed. Thin sections of the uppermost grainstones show multiple phases of cementation, but also reveal primary radial fibrous fabrics preserved in oolitic facies. Our observations are inconsistent with massive recrystalization of the unit during exposure or meteoric diagenesis. Micro-drilled ooids initially used for chemostratigraphic reconstruction further support a primary origin. More fine-scale isotopic work is currently being done to constrain the cementation history.