Reflux stratabound dolostone and hydrothermal volcanism-associated dolostone: a two-stage dolomitization model (Jurassic, Lebanon)

The Kesrouane Formation, which is characterized by pervasive dolomitization, has a stratigraphic thickness that exceeds 1000 m. It is part of a broad carbonate platform deposited in the Levant region and represents 60% of the Lebanese Jurassic rocks. Two genetically distinct dolostones are recognized within this unit: (1) fine-to-medium crystalline non-planar grey dolostone; and (2) coarse-crystalline planar beige dolostone. The former is stratabound and of Early Jurassic age (Sr-87/Sr-86 = 0.707455). This dolostone locally exhi-bits pseudomorphs of evaporite nodules, pointing towards seepage-reflux dolomitization by hypersaline- to marine-related fluids. Exposures of the coarse-crystalline dolostone are associated with regional pre-Cretaceous faults, along which Late Jurassic volcanics also occur. Sedimentological and diagenetic considerations coupled with microthermometry support a hydrothermal origin for this dolostone, with T-H values of primary inclusions between 50 and 80 degreesC. The related dolomitizing fluids are mesosaline (3.5-12.0 eq. wt% NaCl), and are believed to result from the mixing of evaporative brines and sea water. Dolomitization is thus believed to have occurred in two stages, whereby fluids invaded the host rocks first by seepage-reflux, explaining the resulting Early Jurassic stratabound dolostone, and later through fracture flow along the faults associated with the Late Jurassic volcanism, explaining the coarse-crystalline hydrothemal dolostone.