Petrography and stable oxygen and carbon isotopic composition of the Antalo Limestone, Mekelle Basin, Northern Ethiopia: implications for marine environment and deep-burial diagenesis

Abstract

The depositional environment of the Antalo Limestone was previously interpreted from regional field-based studies and microfacies. However, stable oxygen and carbon isotopes are also invaluable proxies of paleo-environmental conditions, depositional, and diagenetic environments. Hence, the analyses of stable oxygen and carbon isotopic composition of this unit were examined from bulk limestone samples and integrated with a detailed petrographic study to interpret depositional environments and diagenetic evolution. The samples were collected from three outcrop sections, which together represent the full thickness of the unit. The microfacies identified from these samples indicate shallow–deep marine depositional as well as meteoric phreatic, marine phreatic, and deep burial diagenetic environments. The limits of variations of the oxygen isotopic ratios range from − 10.46 to − 3.56‰, with − 6.07‰ mean value. The limits of variations of the carbon isotopic ratios range from − 0.02 to 2.56‰, with 1.4‰ mean value. The calculated z values range from 123.08 to129.7 and the paleo-temperatures are estimated to be 32.4–73.52 °C, with an average temperature of 46.5 °C. The carbon isotope ratios indicate marine-water sources. This was confirmed from z values (> 120), indicating that the samples are marine carbonates. The depleted oxygen isotope ratios and high-temperature values, on the other hand, represent deep-burial diagenesis, which indicates the isotopic composition and the temperature conditions of the cement precipitating diagenetic fluids. The compositional fields of the δ18O–δ13C cross plots, also indicate marine carbonates subjected to mixing zone–deep-burial diagenesis. Therefore, both the petrographic and stable oxygen and carbon isotopic evidences are mutually in agreement, indicating related depositional and diagenetic environments.