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GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 68 (2018), Pages 35-50

Geochemical Analysis of the Carbonate-Evaporite Miocene Outcrops in the Cyrenaica Region of Libya: Cyrenaica Platform and Sirt Basin

Khaled S. Amrouni, Osama Rahil Shaltami, Ahmed S. El-Hawat, Michael C. Pope, Aimen Amer, Essa A. Elbileikia, Hassan S. El-Bargathi, Adel A. Obeidi, Khalid A. M. Mustafa, Ahmed M. A. Al-Alwani, Mohamed SH. Abdalla El-Jahmi, Salah S. Abdelsalam El-Ekhfifi, Matthew P. Wehner

Abstract

This geochemical study investigates the dominant Miocene paleoclimate, weathering and hinterland rocks nature in the Cyrenaica region of Libya as well as the oxygen level of the Miocene Tethys Ocean in which the Cyrenaican carbonate-evaporite sequences were deposited. Sixty seven detailed measured stratigraphic field sections along 500 km were used to define the Cyrenaican Miocene detailed facies relationships and their sequence stratigraphic context. The studied shallow marine Miocene carbonate rocks in Cyrenaica include the Ar-Rajmah Group in Cyrenaica and the Wadi Yunis members in Sirt Basin. The Cyrenaican Miocene facies consists of red algal reefs, bioclastic packstones, bioclastic wackestones, oolitic grainstones, and microbialites that are associated with coarsely crystalline selenite gypsum and siliciclastics tongues of green shale and sandstone. The red algae reefal facies is restricted to the Cyrenaican Platform.

A total of 543 rock samples were collected for X–ray fluorescence (XRF) analysis from five selected measured sections. The XRF data include 8 major elements (Ca, Mg, Si, Al, Fe, Ti, K, and C1) and 11 trace elements (Rb, Ba, Sr, Cu, Zn, Cr, V, Zr, Nb, Th, and U). The XRF geochemical analysis of the Miocene carbonate-evaporite rocks of Cyrenaica reveals important and fundamental relationships between these major and trace elements.

The concentration of the continental element Al shows positive correlations with Si, Ti, Fe, Rb, Ba, K, Cu, Zn, Cr, and V and confirms the source of these elements are aluminosilicates that were chemically weathered and transported from the hinterland through rivers to the Miocene Tethys Ocean. The K2O/Al2O3 ratio (0.03 to 0.9) indicates that clay minerals and feldspars are the source of the Al. This chemical weathering processes requires a humid climate to occur.

The Ca vs. Si plot shows a reverse relationship, confirming that these two elements came from different sources: marine for the Ca and continental for the Si. Ca is positively correlated with the Sr and Mg, indicating the marine origin of the Ca. The carbonate minerals are the sole carrier of the Mg and Sr elements. The Mg/Ca ratio (0.01 to 0.4) shows that calcite and dolomite are the dominant carbonate minerals in the Cyrenaican Miocene rocks.

The wide range of Ti/Zr ratio (3.3 to 132.9) indicates that these deposits are chemically mature. The studied sections show dominant low to medium U and authigenic U concentrations as well as dominant low to medium U/Th, Cu/Zn, and V/Cr ratios, suggesting that these sediments formed under suboxic to oxic conditions.

Major and trace elements in the carbonate-evaporite Miocene rocks of Cyrenaica were very useful tools to explain the paleoclimate, weathering nature, hinterland rocks, and the redox status of the shallow marine depositional basin water. Cyrenaica during the Miocene was humid, chemical weathering was dominant, the hinterland rocks were rich in clay minerals and feldspars, and the Cyrenaican shallow part of the Tethys Ocean water was suboxic to oxic.


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