Abstract
Qiangtang Basin is a large Mesozoic marine sedimentary basin located on the northern Qinghai-Tibet plateau. Multiple sets of evaporites were observed in the Middle Jurassic Buqu Formation through field outcrop description and thin sections analysis, so as to form the sets of evaporite–carbonate sedimentary sequences. The evaporites in the Buqu Formation are represented by gypsum and anhydrite rocks that contain minor secondary diagenetic features such as massive, rosette, chicken-wire, and satin spar gypsum. Two lithofacies association types were identified in the Buqu Formation, including sparry oolitic limestone–dolomite–gypsum/anhydrite and micrite–carbonaceous mudstone–gypsum/anhydrite. The former was deposited in an intertidal–supratidal (tidal flat) setting and the latter was deposited in a subtidal lagoon setting. Two depositional sequences were distinguished, including Buqu sequence 1 and Buqu sequence 2. Gypsum of the intertidal setting is mainly developed in a lowstand system tract in both Buqu sequences 1 and 2, while gypsum of the subtidal setting is mainly developed in a highstand system tract in Buqu sequence 2. Combining the measured evaporites thickness in the field with sedimentary facies analysis, the distribution of the Middle Jurassic Buqu Formation evaporites is clear in the Qiangtang Basin, which may serve as a local seal for oil and gas plays in this region.
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