Architecture and sedimentary evolution of the southwestern Gissar carbonate platform (Uzbekistan) during the Middle‒Late Jurassic

Abstract

Abstract Sedimentological investigations have been carried out in the southwestern part of the Gissar Range (Uzbekistan) in order to document the stratigraphic evolution of a vast Middle–Upper Jurassic carbonate platform developed on the northern margin of the Amu Darya Basin. Two major sequences are distinguished, based on their contrasting facies pattern, platform configuration and carbon isotope signature: (1) a Callovian Sequence and (2) an Upper Jurassic Sequence, potentially Oxfordian to Kimmeridgian in age. A carbonate ramp with a double proximal-distal polarity developed during the Callovian Sequence. Oo-bioclastic grainstones were produced in two permanently agitated shallow water areas developing to the north and to the south. Patch and pinnacle reefs mainly built up close to the storm-wave base, passing distally to pellet- and foraminiferal-oncoid-rich mid-to outer-ramp deposits. The top of the Callovian Sequence is a regional discontinuity that could have formed by subaerial exposure of the platform. This probable hiatus could record a major climatic change associated to the Middle‒Late Jurassic transition. The Upper Jurassic Sequence consists of metre-thick alternations of pellet/algal-pack- to grainstones, microbial oncoid floatstones, ooid grainstones, and mudstones, which have likely accumulated in a vast lagoon protected by a large-scale barrier reef observed south of the study area. Gypsum and anhydrite content gradually increased towards the top of the Upper Jurassic Sequence, ending up with a thick chicken-wire gypsum accumulation recording the progradation of a large-scale sabkha over the lagoon. This phenomenon is related to a progressive aridification and restriction of the Amu Darya Basin that could have begun during the Middle/Late Oxfordian and reached its climax during the Tithonian with massive halite deposition. Comparable events occurred in Jurassic platforms located along the Tethyan margins, pointing to regional and global controls on sedimentation.