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Sedimentology and Chemostratigraphy of the Ediacaran Shuram Formation, Nafun Group, Oman

Abstract : A large portion of the Neoproterozoic Ediacaran period, extending from the end of the Marinoan glaciation (c. 635 Ma) to the Precambrian-Cambrian boundary (c. 542 Ma), is occupied by large negative carbon isotope excursions that are closely linked in time with the first appearance of animals in the fossil record. These carbon isotopic signals include the Marinoan 'cap sequence' excursion, the Shuram-Wonoka and Precambrian-Cambrian boundary excursions (Fig. 0.1).
The Neoproterozoic of Oman displays an essentially complete succession from the Marinoan upward, resting on top of a 822-825 Ma basement. The Huqf Supergroup is well exposed in the core of the Jabal Akhdar of northern Oman. It contains the glacigenic Abu Mahara Group, which has yielded a U-Pb zircon date of 723+16/-10 Ma from a tuffaceous sediment sampled around its base. The glacial sequence of the Fiq Member ends with the Hadash 'cap carbonate' Formation, believed to be Marinoan-aged (c. 635 Ma). The basalts of the Saqlah Member mark the start of an episode of crustal extension.
The Nafun Group above the Marinoan cap carbonate (Hadash Formation) is well exposed in both the Jabal Akhdar and the Huqf region of east-central Oman. It comprises two siliciclastic to carbonate 'grand cycles', both initiated by significant transgressions: these cycles comprise the Masirah Bay/Khufai formations and the Shuram/Buah formations. The Khufai Formation displays a pattern of deposition consistent with a carbonate ramp setting and represents a shallowing-upward carbonate cycle (HST), from outer-ramp facies to cross-stratified grainstones and back-shoal mid-ramp deposits, to inner-ramp shallowing upward cycles. The end of Khufai highstand deposition is marked, basinward, by small incised channels, followed by a flooding into transgressive monotonous shales deposited below storm wave base. The Shuram Formation in the Huqf area (proximal part of the basin) records progressive shoaling through a stack of shallowing upward, storm-dominated parasequences. Eventually the siliciclastic Shuram Formation is gradationally overlain by the progradation of the Buah carbonate ramp.
The Shuram (Nafun Group, Huqf Supergroup) excursion of Oman is characterised by an exceptional amplitude (+5‰ to -12‰ d13C; Fig. 0.1) and long stratigraphic record (~800 m). This carbon isotopic trend is reproducible throughout Oman, from outcrops to the subsurface, and irrespective of sedimentary facies. The entire excursion is essentially in phase with longer term relative sea level change, with the nadir in d13C occurring at the level of the maximum flooding zone of the lower Shuram, and the return to positive values occurring within the overlying Buah highstand.
The Shuram Formation is extremely well exposed for over 40 km in a roughly N-S oriented escarpment in the north of the Huqf area, where it displays a stack of shallowingupward storm-dominated parasequences. At this parasequence scale, carbon isotopic values are shown to reflect stratigraphic position within the parasequence stack, and each individual parasequence shows a trend in d13C values in the direction of sediment progradation. These combined stratigraphic-carbon isotopic observations and the fact that the trend is reproducible throughout Oman support a primary, oceanographic origin for the carbon isotopic ratios.
Radiometric ages combined with thermal subsidence modelling constrain the excursion in time and indicate an onset at ~600 Ma, and duration of approximately 50 Myrs.
The excursion is widely recognised in Oman and has potential correlatives in Ediacaran strata elsewhere. It may thus represent a characteristic feature of the middle Ediacaran period. However, these possible correlatives have a limited d13C data set and are dissected by unconformities, so that the full excursion cannot be recognized. The Doushantuo Formation of China probably records the end of the excursion at around c. 551 Ma.
The Ediacaran period is also marked by the non-global, short-lived Gaskiers glaciation around 580 Ma and possible more loosely dated coeval events. If the proposed chronology is correct, the Gaskiers-aged glaciation is embedded within the large-amplitude, long-term Shuram anomaly and appears to have had no effect on the chemostratigraphic records of Oman and other sections worldwide.
The fact that a carbon isotope excursion of this magnitude can be recognized in marine Ediacaran rocks from several continents indicates that it was a very widespread oceanographic phenomenon, reflecting the composition of seawater from which carbonate minerals were precipitated. However, the Shuram evidence demonstrates that the negative carbon isotopic excursion is unrelated to glaciation per se and that the marine carbon isotopic record cannot be used as a direct recorder of Neoproterozoic ice ages.
Any explanation of the Shuram shift must therefore imply an extreme disruption of biological systems within the oceans and a cessation of photosynthesis/biological productivity for a prolonged time (compared to Phanerozoic examples of perturbations of the carbon cycle; Fig. 0.1), challenging our understanding of the global carbon cycle. The preservation of the mass balance suggests an involvement of a sufficiently large reservoir of 13C-depleted material (e.g. organic carbon).
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Contributor : Erwan Le Guerroué <>
Submitted on : Thursday, October 16, 2008 - 1:40:37 PM
Last modification on : Tuesday, March 5, 2019 - 9:30:11 AM
Long-term archiving on: : Monday, June 7, 2010 - 6:48:51 PM

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Erwan Le Guerroué. Sedimentology and Chemostratigraphy of the Ediacaran Shuram Formation, Nafun Group, Oman. Sciences de la Terre. Eidgenössische Technische Hochschule Zürich (ETHZ), 2006. Français. ⟨tel-00331341⟩

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