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Evidence for the early Toarcian Carbon Isotope Excursion (T-CIE) from the shallow marine siliciclastic red beds of Arabia - PubMed

  • ️Sat Jan 01 2022

Evidence for the early Toarcian Carbon Isotope Excursion (T-CIE) from the shallow marine siliciclastic red beds of Arabia

Mahmoud Alnazghah et al. Sci Rep. 2022.

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE) and its corresponding Carbon Isotope Excursion (CIE) have been reported widely across the Tethyan region and globally. In Arabia, and based on ammonite dating, the time window of the T-OAE coincided with the deposition of the reddish siliciclastic unit of the Marrat Formation. However, no evidence of the T-OAE/CIE was ever reported from Arabia because these red beds were previously interpreted as continental deposits. Recently, these red beds have been recognized as shallow marine deposits which opened an opportunity to assess the occurrence and expression of T-OAE-CIE in Arabia. In this study, a multiproxy geochemical characterization was performed on the Toarcian Marrat Formation to infer the chemistry of the paleowater column and identify intervals of possible T-OAE/CIE in Arabia. While the low concentrations of redox-sensitive elements (Mo, U, V, Cr) may indicate a shallow oxic marine settings, the coupled negative δ13Corganic excursion and apparent increase in the chemical weathering suggests that the deposition of Marrat red beds coincided with the development of T-CIE and possibly time-equivalent to the T-OAE globally. The origin of reddening is interpreted to have occurred during the middle Marrat deposition due to the stabilization of unstable hydrous iron oxides to hematite under oxic marine conditions. The proposed model further indicates the possible development of source rocks in the deep, anoxic environment counterpart where the T-OAE may be expressed. Since our study documents the first record of the T-CIE and discuss the origin of shallow marine siliciclastic red beds in the Arabian Plate, this will have significant implications for the overall understanding of the T-CIE globally and for hydrocarbon exploration through realizations of potential new source rocks associated with the OAEs in the Toarcian and other time intervals.

© 2022. The Author(s).

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1

(A) Geological map showing the location of the Khashm adh Dhibi outcrop where the stratigraphic section was measured and sampled. (B) Generalized stratigraphic column of the Late Triassic—Early Jurassic in Saudi Arabia.

Figure 2
Figure 2

The measured section of the Marrat Formation in Khashm adh Dhibi with sample locations (red arrows) with plots of the calculated enrichment factors (relative to average shale) of some redox sensitive elements (Cr, Cu, Ni, U, V, Mo), Chemical Index of Alteration (CIA), δ13Corg, and TOC. It is notable to highlight that the lowermost siliciclastic parts of the lower Marrat is not included in this measured section.

Figure 3
Figure 3

(A) Toarcian paleogeographic map of the world. (B) Close-up paleogeographic map of the Western Tethyan realm showing the geographic location of Arabia relative to some localities where T-CIE was reported. (C) Correlation between the δ13Corg obtained from the Marrat Formation and the negative excursions in δ13Corg reported from different basins within the Tethys,,–,,.

Figure 4
Figure 4

Cross plot of the enrichment factors of Mo and U showing significant enrichment in Mo with no coeval enrichment in U. This trend for the middle Marrat suggests that Mo is likely being scavenged by other phases in the sediment, .

Figure 5
Figure 5

Schematic diagram explaining the proposed model for the middle Marrat red beds, where the redding was possibly taking place in well, oxygenated shallow marine settings ,while the Toarcian oceanic anoxic conditions were dominating the deeper settings.

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