Sedimentological, petrographic and geochemical constraints on the origin of extensive dolomites of the Cretaceous Sidi as Sid Formation (Ain Tobi Member) in NW Libya.

The Cretaceous Sidi as Sid Formation outcrops in the Jabal Nafusah Escarpment, northwest Libya. It is composed of two members; the lower Ain Tobi Member and the upper Yifran Member. The Yifran Member consists of marl, dolomitic limestone and gypsum. The Ain Tobi Member is composed entirely of dolomite and in this study is named the "Ain Tobi Dolostone Member". The Sidi as Sid Formation is part of an inner ramp sequence which graded from restricted ramp in the south and southwest into an open to semi-restricted ramp in the northeast. The entire succession is without biostratigraphic markers. Sedimentological, petrographic, geochemical and isotopic studies have led to the definition of five diagenetic dolomite types within the Ain Tobi dolostones; these reflect more than single diagenetic events which were involved in the formation of these dolomites. Type-1 dolomite is laminated, fine, subhedral, stoichiometric to nearly stoichiometric and poorly ordered. It is characterized by homogeneous luminescence, relatively high Sr and Na contents and low Fe and Mn, positive carbon isotope values and negative oxygen isotopic composition. It is suggested that Type-1 dolomite is formed near-surface by reflux dolomitization. Type-2 dolomite is calcian-rich, poorly to moderately ordered and very fine. It replaces packstone/grainstone facies and shows bright yellow luminescence. Type-2 dolomite is Sr-enriched and Fe, Mn, Na-depleted and possesses a similar isotopic composition as dolomite Type-1. Therefore, it is interpreted as a replacive dolomite formed at the near-surface under the influence of mixed water. Type-3 dolomite is a fine to medium dolomite, interbedded with quartz. It is calcian to nearly stoichiometric and moderately ordered. Type-3 dolomite is characterized by dirty orange luminescent cores and very thin bright luminescent rims, high Fe and Mn, but low Sr and Na contents. It has negative carbon and oxygen isotopic composition and is interpreted as dolomite formed in an intermediate stage probably post-compaction under the influence of mixed marine-meteoric water in shallow burial. Type-4 dolomites are calcian to nearly stoichiometric, well ordered, medium to coarse and have dirty orange luminescent cloudy cores and dull clean rims. Type-4 dolomites possess low trace element contents due to recrystallization from precursor dolomites. Type-4 dolomites have normal marine oxygen and carbon isotopic compositions. The Type-4 dolomites are interpreted as being formed by continuous dolomitization processes involving marine-meteoric pore-water mixing in shallow burial situations. Type-5 dolomite (saddle dolomite) is coarse to very coarse with orange luminescent cloudy cores and dull clean rims. It is calcian-rich, well ordered and posses low Fe, Mn and Sr contents and a relatively high Na concentration. Its isotopic composition is the lightest in oxygen and has a similar carbon composition compared to other dolomite types. Type-5 dolomite is interpreted as being formed at a late stage in deeper burial and elevated temperature environment. Two types of replacive calcite (dedolomite) are found to replace dolomite in the Ain Tobi sequence. The first, is found close to the unconformity surface. These calcites show replaced cores and thin unreplaced dolomite zones. These dedolomite crystals are characterized by complex luminescent zonation and are interpreted as an early replacement under the influence of marine-meteoric mixing water. The second type is observed in the upper part of the Ain Tobi sequence and is characterized by very coarse or blocky crystals with replaced rims and/or whole dolomite rhombs. This calcite is interpreted as a dolomite replacement formed at a late stage under the influence of freshwater as indicated by very light delta18O values.