GCAGS 2016: Gulf Coast Association of Geological Societies with the Gulf Coast Section SEPM 66 th Annual Convention - September 18-20, 2016, American Bank Center in Corpus Christi, Texas Accepted Abstract GCAGS Spet-2016, Corpus Christi, Texas Silicification in the Cyrenaican Miocene Carbonate-Evaporite Sequence, NE Libya: origin, occurrence, facies and sea level relationship Khaled S. Amrouni 1,2&4 , Michael C. Pope 1 , Ahmed S. El-Hawat 2 , Aimen Amer 2&3 , Adel A. Obeidi 2&7 , Essa A. Elbileikia 2&5 , Hassan S. El-Bargathi 2&6 , Osama Rahil Shaltami 2 , , Matthew P. Wehner 1 , Ahmed M. A. Al-Alwani 2 amrouni@neo.tamu.edu /abcde_909@yahoo.com , mcpope@geos.tamu.edu , ashawat@lttnet.net , aamer@slb.com , adelobeidi@yahoo.com , essap2005@yahoo.com , Hasbargati@yahoo.co.uk , osama.rahil@yahoo.com , nwgeologist@tamu.edu , ahmedalalwani87@gmail.com /a.al3alwani@yahoo.com , 1 Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843 2 Department of Earth Sciences, Garyounis University, Benghazi, Cyrenaica, Libya 3 Principal Geologist-Fracture and Petroleum Systems Specialist, Schlumberger, Houston, Texas, and Ahmadi, Kuwait 4 Wireline Engineer Schlumberger, Local Mobile, Libya 5 Departments of Geology and Geophysics, and Geological Engineering, Missouri University of Science and Technology, Rolla, MO 65401 6 Senior Exploration Geologist at TOTAL E&P Libya 7 Senior Petrophysicist, Schlumberger, Total E&P Libya, Woodside Energy, ADCO, and Qatar Petroleum Abstract This work investigates the distribution, occurrence, and origin of silica in the Cyrenaica Miocene shallow marine carbonates, along a 135 km strike section, and its relation to the depositional facies and the sequence stratigraphic framework. Twenty-nine detailed measured stratigraphic field sections, 14 gamma-ray profiles, and 4 carbon stable isotope curves were used to define the Cyrenaican Miocene detailed facies relationships and their sequence stratigraphic context. The Ar-Rajmah Group Cyrenaican Miocene facies consists of red algal reefs, bioclastic packstones, oolitic grainstones, and microbialites that are associated with evaporites and siliciclastics. These facies are arranged within in two 2nd order supersequences that comprise six 3rd order sequences. 503 rock samples were collected for thin section petrographic analysis and XRF geochemical analysis. As observed in the field, silica is very common in the ramp crest oolitic grainstone facies and peritidal microbialite facies, but rare in the subtidal red algal and bioclastic packstone facies. The silica commonly occurs as chert nodules of reddish-bluish light gray color in the ramp-crest and peritidal facies and whitish light gray color in the subtidal facies. In addition, the silica forms up to 20 cm thick, discontinuous layers in the porous mixed microbial-oolitic grainstone facies. In thin section, the silica forms as disseminated silica, microquartz, and chalcedonic quartz. It replaces matrix, grains, cements, and even forms authigenic fan-shaped chalcedonic cement that filled up pore spaces. In the paragenetic sequence of the Cyrenaican Miocene, silicification always comes as the last replacement process after dolomitization, dedolomitization, and gypsum replacement. All studied silica samples are length slow while the gypsum plate is inserted in the XPL position. A study of all 503 samples did not reveal any evidence for biogenic origin for the silica. However, the XRF analysis of the same samples did reveal a linear relationship between aluminium and silicon (R² = 0.8143). The relationship is Al (wt. %) = 0.1646 Si (wt. %) + 0.11405 as empirically determined. In Cyrenaican Miocene carbonate-evaporite sequence the diagenetic silica occurrence and distribution is strongly facies controlled and has no correlation with the sequence stratigraphic surfaces or systems tracts. Also the silica origin is continental weathering rather than being biological as proposed by the strong direct proportional geochemical relationship between the Si and Al, as well as the petrographic analysis.