Petrographic and Diagenetic Analysis of Sidi Assid Formation (Ain Tobi Member)
أ. اسامة الغول
المؤتمر الدولي للتقنيات الجيومكانية - ليبيا1
International Conference on Geospatial Technologies -Libya1
9-6/ 12 /2015
Osama Salem Alghoul
Biruni remote sensing center
1. General Introduction
The Jabal Nafusah escarpment extend along 400 km, from the coast in the east (near the Alkums) to the Tunisian border in the west, where it continues westward before turning sharply northward within Tunisia (Figure.1.1). in Libya it rises up to 300 m from the Jefarah plain that lies to the north to the foot of the mountain, reaches up to 800 m peak in the south.
This study investigates the sedimentology and diagenesis of the Cretaceous outcrops in the jabal nafusah escarpement, NW libya.
Sidi Assid Formation (Upper Cretaceous) is outcropped along Jabal Nafusah in northwestern Libya (Figure1.1). Jabal Nafusah Escarpment extends from the coast of the Mediterranean Sea near Al Khums in a broad arc 350 km south and west to the Libyan-Tunisian border where it is 130 km from the coast. Jabal Nafusah continues into Tunisia, turns north, and dies out near Gabes, and it is known in that area as the Jifarah Escarpment.
Figure 1.1. Location map of NW Libya showing the position of the measuremed section along Jabal Nafusah (Fatmi 1978).
The Upper Cretaceous carbonate of the Sidi Asside Formation (Ain Tobi Member) in NW Libya are an example of bedded dolomite, which have a complex diagenetic history (Gregg and Shelton, 1990). Ain Tobi dolomite has a gradational contact with the overlying Yifran Marl member which is difficult to distinguish in places and an unconformable contact with the underlying Kiklah Formation.
1.1. Aims of Study
The Ain Tobi Member is composed mainly of dolomite, which was deposited in the Cenomainian described as dolomite of cenomainian age (Desio et al.,1963).
It is exposed in the Jabal Nafusha escarpment.
1. To investigation the primary facies development and architecture of the Ain Tobi Member.
2. Assessing the diagenetic characteristics of the various carbonate facies in the Ain Tobi Member.
3. Detecting the diagenetic factors that influence permeability and porosity. 4. investigation the Original and mechanism of Ain Tobi dolomitization in the studied Section.
1.2.Data base and Methodology:
1.2.1 Field procedures
One outcrop locality Was selected in NW Libya to provide a representative suite of upper Cretaceous sediments in the area. Standard field procedures were employed (e.g. tucker, 1982), accomparid by detailed analysis of one stratigraphic Section (see Appendix 1).
The rock samples have been studied to identify of grain size, fossils and texture according to dunham (1962). Fresh surfaces where chosen for sampling wherever possiple.
1.2.2 Laboratory Procedures:
Investigation included description and logging of approximately 70 m by using a hand lens and binocular microscope. Samples were collected from levels of lithologic changes and features of intrest.
Approximately 15 unpolished thin-section were prepared in the Libyan petroleum Institute lab (LPI) in Tripoli. These were examined petrographically For rock fabric and to determine the presence or abundance of constituent grains, matrix and cements
Part of each studied level was stained with alizarin red S and potassium ferricyanide. Description of fabrics is based on Friedman (1965), and carbonate classification, in principle, follow Dunham (1962).
Some thin-section were impregnated with blue–dyed resin in order to determine the relative porosity of the rock. Representative rough surface of limestone samples were coated with gold and examined under the SEM.
2.4 Nafusah Uplift
2.4.1 Geomorphology of nafusah uplift
The Nafusah Uplift is a major east-west ridge which separates the Ghadamis Basin from the Jifarah Basin. It extends for 400km from Misratah to the Tunisian border, and continues in Tunisia as the Dahar Uplift and in Algeria as the Talemzane Arch. It is bounded to the north by the Jifarah Fault and the Jifarah Basin and to the south by the Ghadamis Basin (Figure 2.3). The southern margin is also partially faulted. During the early Palaeozoic the area of the Nafusah Uplift formed part of the Ghadamis Basin and a thick sequence of Palaeozoic rocks was deposited.
2.4.2 Geological Sitting
During the Permian the Nafusah Uplift formed a barrier between the marine sequences of the Tethys Ocean to the north and the Continental Post Tassilien rocks to the south. By the Triassic times the arch had been greatly reduced in elevation and Triassic, Jurassic and Early Cretaceous sediments were deposited over the arch and into the Hamadah Basin to the south. The arch was covered by the Cenomanian transgression and deposition of marine rocks continued until Eocene times.
The Nafusah Arch was reactivated during the mid-Tertiary in response to the closing of the Tethys. It was subjected to uplift accompanied by wrench faulting along the Jafarah Fault (Figures 2.3 , 2.4).
The region has been the site of deposition for marine and non-marine sequence, divided into specific formation (Fig 2.4).
Fig 2.3 Generalized structural cross-section illustrate the tectonic evolution of the offshore Pelagian area, the Jafara Basin, Jabal Nafusa escarpment. (after Boot et.el.1989).
Figure 2.4. The time stratigraphic succession of the Mesozoic and Tertiary sedimentary sequence of the Jabal Nafusah and Mizda (Fatmi et., 1978)
2.5.4. Upper Cretaceous
2.5.4.1 Sidi Assid Formation
Christie (1955) introduced the terms Ain Tobi Limestone and Yafrin Marl, as two successive rock units between Nalut Formation at top and the Kiklah Formation at base in Gharyan area.
The Ain Tobi Limestone as designated by Christie (1955) is different in the lithology and the stratigraphic position from the Yafrin Marl.
The lithological characteristics as well as the geographical distribution of the formation are given within the description of its two members.
2.5.4.1.1 Ain Tobi Member .
Burollet (1963-a) described this unit as the lower member of Nefusa Group which is followed up by the Yafrin Marl and the Gharyan Dolomite. The Ain Tobi Member is unconformably underlain by the Kiklah Formation in the area west of Wadi Ghan (where the Kiklah Formation pinches out), as well as by the Abu Shaybah Formation to the east. It passes gradually into the overlying Yafrin Member, where the contact is gradional in many parts.
This member is one of the most widely distributed rock units. Along Tarhuna-Gharyan scarps it forms the topmost part of these scarps, as well as a part of the plateau surface. It also participates in the formation of the upper part of the scarp which stretches from Gharyan and westwards up to the Tunisian border. The Ain Tobi Member consists mainly of well bedded limestone (photo 5) sometimes dolomitic, yellow to light grey in colour. The basal part is characterized by the presence of abundant sand grains and quartz pebbles El Hinnawy, Cheshitev., (1975).
2.5.4.1.2 Yafrin Member.
It consists mainly of alternating beds of marls, clays and marly limestone with consistent bands of gypsum.
The Yafrin Member is well exposed in the area to the west of Wadi Ghan to beyond the limits of the investigated area. Because its soft nature, it forms yellowish slopes covered by debris lying between the Nalut Formation at top and the Ain Tobi Member at base. East of Gharyan to Tarhuna area, the Yafrin Member is highly interbedded with limestone beds which makes the differentiation of the two rock units of Sidi as Sid Formation difficult. Here the upper member of Sidi as Sid Formation is recognised by the abundant occurrence of marly bands in limestone succession El Hinnawy, Cheshitev., (1975).
3.1. Introduction
The methods used in the study is to describe the samples in the field and laboratory study using an optical microscope and electron microscope.
The nomenclature for carbonate rocks used in this work is a combination of the terminology introduced by Dunham (1962), which is implies textural aspects. The interpretation of carbonate depositional environments includes macroscopic sedimentary structures, outcrop observations and the occurrence of diagnostic facies fossils in the stratigraphic column.
This chapter first presents the results of detailed field observation, including sedimentary structures and outcrop logs (Figure 3.1).
Also It shows results and interpretations of a detailed petrographic study of fifteen samples selected from the Ain Tobi Member in Abu Ghaylan section, northwest Libya. The main objectives of this chapter are:
1- To describe and interpret the different facies of the Ain Tobi Member using petrographic Microscope observations.
2- To interpret the depositional environments of the facies .
The Ain Tobi carbonate Member is widely distributed rock unit along Jabal Nafusah Escarpment and it forms the topmost of this escarpment. Generally, the thickness of the Ain Tobi sediments increases from southwest to the northeast direction. The thickness ranges from 5 m in Wazan area to more than 110 m in the area between Mesellatah and Qasr Khiar (Figure 3.3A).
In the study area, The Ain Tobi Member is unconformably underlain by the Kiklah Formation and it grades up into the Yifran Marl Member (Elbkaai 1997).
Figure. 3.1. Measured Section, Ain Tobi Member, Abu Ghaylan section
Figure 3.2. Photograph shows the local discordance between Kiklah Formation and Ain Tobi Member caused by a fault, in Abu Ghaylan Section.
3.2 Field observations :
Sedimentary structures abserved in the study area include bioturbation, lamination and crossbedding.
3.2.1 Bioturbation Unit:
Bioturbation structures in the study area are observed in many intervals of the identified facies, exactly in the Upper 40 m.
The average thickness of a bioturbated interval is approximately 1m. The intensity of bioturbation increases from bottom to top within the same a unit expression. (Figure 3.3). Weathering resuled in a wide range of bioturbation.
The most common type of exposure is a flat to concave outward with undulated surface. This is mottled and contains of leached bioclastic.
Figure 3.3 Field photograph showing bioturbation within the Ain Tobi Member, in the Abu Ghaylan Section.
Dolomitization has obscured most of the primary depostional textures of the bioturbated beds. Most of the grains have been either leached and left as moldic porosity or they have been obscenely replaced. Bioturbation probably as result of the low energy level or slow rate of sedimentation.
There are several reasons for assuming that the Ain Tobi Member was deposited in shallow water independent. These include the transgression and deposition of marine carbonates on top of non-marine deposits, the presence of of cross-bedding, and the presence of dolomite as it is related to the most prominent models for dolomitization.
CONCLUSIONS
1) The main Cenomanian sediments encountered throughout the study area in NW Libya are dolomitic limestone to dolomite. By combining micro and macro studies of these sediments, two distinct facies can be identified, These are: (1) Mulluscan Wackstone facies and (2) Mudstone facies.
2) The environment of deposition of Ain Tobi Member is part inner ramp sequence, which ranged from open to semi-restricted ramp environments .
3) Cross-bedding and lamination, are the most common form of sedimentary structures formed in the Ain Tobi Member.
4) Dolomitazation of the Ain Tobi Member is primarily the result of seepage reflux mechanism secondary a dorag mechanism .
5) This is a strong relationship between diagenetic processes and development of porosity in rocks. Generally the porosity in the investigated area is either primary, formed during deposition or secondary, enhanced by dissolution. dolomitization and micro-fracturing of rocks. Reduction in porosity in the investigated sediments is mainly due to cementation (e.g.calcite).
6) Dolomite is characterized by brown, anhedral to euhedral crystals and fine- coars crystalline dolomite.
RECOMMENDATION
Must be selected more than three sections for Ain Tobi Member along the Nafusa Mountains to be able to study in detail inter disciplinary study, including structural, stratigraphic and sedimentological aspects This would be particularly important for detecting structural style and geometry to predict facies distribution,and can corrlelation between all sections
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