Damenu Adefris

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

2020

The depositional environment of the Antalo Limestone was previously interpreted from regional fie... more 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 δ 18 O-δ 13 C 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.

Download

Petroleum source rock characteristics of the Mesozoic units, Mekelle Basin, northern Ethiopia

Materials and Geoenvironment

The Mekelle Basin comprises carbonate and siliciclastic rocks which could have potential for petr... more The Mekelle Basin comprises carbonate and siliciclastic rocks which could have potential for petroleum as previous studies reported without enough evidence. This research is intended to assess source rock characteristics of selected samples from the Adigrat Sandstone, Antalo Limestone and Agula Shale. Four outcrop sections were investigated and fifteen limestone and shale samples were collected. TOC was deduced by Leco SC-632 carbon analyzer, and Rock-Eval pyrolysis was determined using Rock-Eval 6. The TOC values range from 0.11 wt.% to 0.42 wt % for 80% of the samples which is less than the minimum requirements for source rocks. However, a few samples have TOC between 0.5 wt % and 1.0 wt %, indicating fair organic matter richness. The kerogen is mainly Type IV and inert. The S1 (0–0.02 mgHC/g) and S2 (0–0.07 mgHC/g) values are very low, with very high Tmax (475–527 °C), indicating very poor potential with over-mature dry gas. Therefore, the analyzed samples have low organic conten...

Download

Stratigraphy and facies analysis of the Antalo limestone (Callovian-Tithonian), Mekele Basin, northern Ethiopia

Stratigraphy and facies analysis of the Antalo Limestone (Callovian–Tithonian), Mekele Basin, northern Ethiopia, 2022

Antalo Limestone in the Mekele Basin, northern Ethiopia was studied to interpret depositional env... more Antalo Limestone in the Mekele Basin, northern Ethiopia was studied to interpret depositional environment through facies analyses and provide insights on its Jurassic sedimentological evolution and paleogeographic implications. The full thickness of the unit was investigated through outcrop description, section logging and sample collection at eight outcrop locations. Two of the sections represent the lower to middle parts of the unit with about 200 m and 270 m thickness. Five of the sections represent the middle to upper parts of the unit with thicknesses ranging from 175 to 285 m and one section represents about 72 m-thick uppermost part of the unit. The field descriptions of the unit, in terms of lithology, texture, composition, bedding and sedimentary structures, show eighteen lithofacies types made up of limestone, sandstone, shale and marl. The petrographic analyses of the samples also identify seventeen limestone microfacies ranging from mudstone to grainstones including boundstone, one dolomitic limestone and two sandstone facies (quartzwacke and quartzarenite). The facies analysis indicates that the studied unit was deposited within shallow-to deep-marine environments. There are six facies associations that were correlated with the standard microfacies types and facies zones, indicating a typical homoclinal carbonate ramp with tidal flat, lagoon, sand shoal, mid-ramp, outer-ramp and deep basin depositional systems from the shore to open sea environments, respectively. The lack of reef facies, in the studied sections, has also been considered as evidence to deduce a homoclinal carbonate ramp for the unit. These sub-environments were juxtaposed and, the lateral shifting of their position through time caused the development of vertical facies successions of different environments. It is also deduced that marine water from the Tethys Sea covered the Mekele area since the early Callovian and persisted until the Tithonian, creating favorable conditions for carbonate production and sedimentation. Therefore, the Jurassic sedimentation history of Antalo Limestone was synchronous with that of equivalent carbonate platform sequences of the Horn of Africa and other peri-Tethyan regions, confirming a related paleogeographical setting.

Download

Benthic foraminiferal assemblages and biostratigraphy of the Antalo Limestone, Mekelle Basin, Northern Ethiopia

by Damenu Adefris and Balemwal Atnafu

This study focused on the identification, detailed systematic descriptions and stratigraphical di... more This study focused on the identification, detailed systematic descriptions and stratigraphical distributions of different foraminifera and other species to make the first systematic paleontological analysis, interpret paleoenvironment and determine the age of Antalo Limestone of the Mekelle Basin. Fieldwork and section logging, with sample collection at seven outcrop locations, followed by petrography and standard micropaleontological analysis were carried out. Abundant foraminifera fossils of 17 species were identified from 42 thin sections and 5 washed samples of marl and shale. The faunal assemblage also comprised dasyclad green algae, calpionellid, brachiopods and stromatoporoids (Cladocoropsis mirabilis). The benthic foraminiferal assemblage is represented by 3 species of the order Lituolida (Freixialina planispiralis, Nautiloculina circularis and N. oolithica), 11 species of the order Loftusiida (Alveosepta jaccardi, Choffatella tingitana, Conicokurnubia orbitoliniformis, Ever...

Download

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

Carbonates and Evaporites

The depositional environment of the Antalo Limestone was previously interpreted from regional fie... more 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 δ 18 O-δ 13 C 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.

Download

Uploads

Papers by Damenu Adefris

Log In