--> Carbon and Oxygen Isotope Variations on an Ancient Carbonate Platform: A Case Study From the Middle–Late Triassic Al Aziziyah Formation, Northwest Libya

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Carbon and Oxygen Isotope Variations on an Ancient Carbonate Platform: A Case Study From the Middle–Late Triassic Al Aziziyah Formation, Northwest Libya

Abstract

This study presents the first δ13C and δ18O stratigraphies of the Middle-Late Triassic Al Aziziyah Formation. The Al Aziziyah Formation was deposited on a gently sloping carbonate ramp within the Jifarah Basin of northwest Libya and consists of gray limestone, dolomite, and dolomitic limestone interbedded with shale. Facies of the Al Aziziyah are: tidal sandstone, peritidal, barrier, shallow subtidal, deep subtidal and basinal facies. The Ghryan Dome and Kaf Bates sections were sampled and analyzed for carbon and oxygen isotopes (1) to integrate high-resolution carbon isotope stratigraphy with an outcrop-based sequence stratigraphic framework to build the stratigraphic correlation and provide better age control of the Al Aziziyah Formation and (2) to evaluate the influence of depositional environments on the isotopic values. Carbon isotope data identify seven sequences within the Ghryan Dome. Correlations between the sections indicate five sequences of δ13C depletion and enrichment. Because Carnian ocean circulation was restricted in the some parts of the basin, the Al Aziziyah Formation δ13C curve only partially correlates with the proposed Triassic global δ13C curve. This correlation indicates that the Al Aziziyah Formation was deposited during the Ladinian and part of the Carnian stage. The enrichment of δ13C values (-7.8 to +3.9‰) in certain intervals relative to brachiopod values (-0.5 to +3.0‰) likely reflects local withdrawal of 12C from the ocean due to increased productivity, and/or whole rock sediment composed of calcite admixed with aragonite. The depletion of δ13C is clearly associated with exposure surfaces and with shallow carbonate facies. However, heavier δ18O values (-9.3 to +0.5‰) relative to Triassic brachiopods (−3.9 to −0.6‰) are related to evaporatic enrichment of 18O, as indicated by the abundance of evaporite within the peritidal facies. In contrast, the depletion of δ18O is related to diagenesis due to freshwater input.