Origin and timing of siderite cementation in Upper Ordovician glaciogenic sandstones from the Murzuq basin, SW Libya M.A.K. El-ghali a,b, * , K.G. Tajori b , H. Mansurbeg a , N. Ogle c , R.M. Kalin c a Department of Earth Science, Uppsala University, Villava ¨gen 16, SE 75236 Uppsala, Sweden b Department of Earth Science, Faculty of Science, Al-Fateh University, P.O. Box 13696, Tripoli, Libya c School of Civil Engineering, Environmental Engineering Research Centre, The Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, Northern Ireland Received 15 July 2005; received in revised form 8 February 2006; accepted 10 February 2006 Abstract The origin and timing of siderite cementation have been constrained in relation to depositional facies and sequence stratigraphy of Upper Ordovician glaciogenic sandstones from the Murzuq basin, SW Libya. Optical microscope, backscattered electron imagery, and carbon and oxygen stable isotope analysis have revealed that siderite is of eo- and mesogenetic origin. Eogenetic siderite is Mg-poor with a mean composition of (Fe 91.7 Mg 1.5 Ca 0.3 Mn 6.5 )CO 3 , and occurs in paraglacial, tide-dominated deltaic highstand systems tract (HST) sandstones, in paraglacial, foreshore to shoreface HST sandstones and in postglacial, Gilbert-type deltaic lowstand systems tract (LST) sandstones. This siderite is typically of meteoric water origin that influxed into the LST and HST sandstones during relative sea level fall and basinward shift of the strandline. Mesogenetic siderite, which engulfs and thus postdates quartz overgrowths and illite, is Mg-rich with a mean composition of (Fe 72.2 Mg 21.7 Ca 0.8 Mn 5.3 )CO 3 and occurs in the paraglacial, tide-dominated deltaic HST sandstones, in paraglacial foreshore to shoreface HST sandstones, in glacial, tide-dominated estuarine transgressive systems tract (TST) sandstones, in postglacial, Gilbert-type deltaic LST sandstones, and in postglacial, shoreface TST sandstones. d 18 O V-PDB values of this siderite, which range between K22.6 and K13.8‰, suggest that precipitation has occurred from evolved formation waters with d 18 O values between K14.0 and C1.0‰ and was either meteoric, mixed marine– meteoric and/or marine in origin by assuming postdating quartz overgrowths and illite temperature between 80 and 130 8C. q 2006 Elsevier Ltd. All rights reserved. Keywords: Siderite; Glaciogenic sandstone diagenesis; Sequence stratigraphy; Depositional facies; Upper Ordovician; the Murzuq basin; SW Libya 1. Introduction The origin, elemental and isotopic composition, and distribution patterns of siderite cement in sandstones from a wide variety of depositional environments and diagenetic regimes have been the focus of numerous studies (Matsumoto and Iijima, 1981; Curtis et al., 1986; Mozley, 1989; Pye et al., 1990; Morad et al., 1994; Huggett et al., 2000). Siderite typically precipitates from reducing, non-sulphidic pore waters that have evolved in suboxic, methanogenic geochemical conditions (Garrels and Christ, 1965; Froelich et al., 1979; Berner, 1981; Hem, 1985; Morad, 1998). Siderite chemistry has been used to unravel the origin of pore waters which can be either marine, mixed marine–meteoric or meteoric in compo- sition (Curtis and Coleman, 1986; Bahrig, 1989; Mozley, 1989; Mozley and Wersin, 1992; Baker et al., 1996) and all can be influenced by either transgression or regression events (Morad et al., 2000). Defining the geochemistry and distribution of siderite in a sequence stratigraphic context, which is adopted in this study, allows a better understanding of the parameters that control its chemical composition and formation. The depositional facies and sequence stratigraphic framework of the Upper Ordovician glaciogenic sandstones (i.e. glacial, paraglacial, and post- glacial), outlined by El-ghali (2005), made this study feasible. The diagenetic regimes used in this study are: (i) eodiagenesis (0–2 km depth and at less 70 8C), which includes alterations that have occurred where the pore water chemistry was influenced by surface conditions (depositional waters and climate), (ii) mesodiagenesis (depths over 2 km and at Marine and Petroleum Geology 23 (2006) 459–471 www.elsevier.com/locate/marpetgeo 0264-8172/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2006.02.002 * Corresponding author. Present address: Department of Earth Science, Uppsala University, Villava ¨gen 16, SE 752 36, Uppsala, Sweden. Tel.: C46 18 4712552; fax: C46 18 4712591. E-mail addresses: mohamed.kalefa@geo.uu.se, elghali_geo@yahoo.co.uk (M.A.K. El-ghali).