Environment of Deposition, Geochemistry, and Diagenesis of the Beda Formation, SW Sirte Basin, Libya

Garea, Bashir Bashir (1991) Environment of Deposition, Geochemistry, and Diagenesis of the Beda Formation, SW Sirte Basin, Libya. MSc(R) thesis, University of Glasgow.

Abstract

The study of the Middle Paleocene Beda Formation, in block NC74F of the southwestern Sirte Basin, was undertaken to investigate the environment of deposition, the geochemistry, the subsequent diagenetic history, and the resultant porosity of part of the lower member of this formation. Six lithofacies (F1-F6), characterized by shallow-marine sediments (mudstones and wackestones), dolomite, sulfates, and argillaceous carbonate with thin shale interbeds, were recognized in these sediments showing contrasting lateral and vertical distribution. These facies represent three small-scale shallowing-upward cycles. The lower two cycles of the succession begin in the lagoonal part and accrete through intertidal to supratidal sediments, whereas, the third cycle at the top of the succession begins with a new marine transgression, and is terminated by a laterally widespread evaporite sulfate unit. Geochemical results show strikingly high values of strontium (Sr), while other trace elements (reported in their oxide forms), such as iron (Fe), manganese (Mn) etc., are generally low. The high Sr values are due to the precipitation of minor amounts of celestite (detected by XRD) in the sediments. The Isotopic study shows that the Beda dolomites are depleted in delta18O with quite negative delta18O values ranging between -1.5 to -5.7 per mil (PDB). The delta13C values of these dolomites tend to show a more marine delta13C isotopic signature, with an average of +3.27 per mil (PDB). The sulfur delta34S isotope data measured from the evaporite facies ranges between 19.8 and 22.28 per mil (CDT), and these are generally within the range of delta34S isotope values characteristic of marine evaporites of Tertiary age. Diagenesis of the Beda sediments studied has resulted in a number of diagenetic changes. These are dominated by cementation, dissolution, dolomitization, and compaction. Two generations of calcite cements were recognized. The first generation of cement occupied the original intergranular and intragranular porosity, while the second filled residual primary pores and molds and vugs resulting from the first stage of dissolution. Dolomitization followed at least the first phase of calcite cement, and 60-70% of the studied intervals have been dolomitized. On the basis of the sedimentological, petrographic, and geochemical evidence, the dolomite is interpreted as of early, near-surface origin, associated with hypersaline conditions. Evidence of a second stage of dissolution is found as secondary voids and molds filled with calcium sulfates (gypsum-anhydrite). This postdates calcite cementation and dolomitization. The late (or burial) diagenetic events recognized are development locally, of baroque dolomite cement, and Pb-mineralization. Examination of porosity data from porosity logs shows a wide range of variation in total porosity and shows a general increase in porosity in an eastward direction. This is because the porosity has been less affected by sulfates cementation relative to the western part of the study area.

• Item Type: Thesis (MSc(R))
• Qualification Level: Masters
• Keywords: Geology, Geochemistry, Sedimentary geology
• Date of Award: 1991
• Depositing User: Enlighten Team
• Unique ID: glathesis:1991-78288
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 28 Feb 2020 12:09
• Last Modified: 28 Feb 2020 12:09
• URI: https://theses.gla.ac.uk/id/eprint/78288