Deposition and diagenesis of the Early Permian volcanic-related alkaline playa-lake dolomitic shales, NW Junggar Basin, NW China

Abstract

This study describes and interprets the mineral and facies assemblages of lacustrine shales that occurred in an ancient volcanic-related alkaline playa-lake, considering the importance of diagenetic pathways (calcification, dolomitization, borosilicate replacement, and silicification). Three laminated lithofacies (LF) have been differentiated in the Early Permian Fengcheng Formation, NW Junggar Basin: an alginite-poor, dolomite-lean, and argillaceous-rich lithofacies deposited during high lake level stages (LF-A), an alginite-moderate, dolomite-moderate, and evaporite-present lithofacies deposited during low lake level stages (LF-B), and an alginite-dense, dolomite-rich and reedmergnerite (NaBSi3O8)-rich lithofacies deposited during periods of intense hydrothermal input (LF-C). Depositional environments influenced subsequent diagenetic pathways of the three laminated lithofacies. In the shallow-water LF-B sediments, extensive evaporite crystallization, dissolution and subsequent calcification or dolomitization occurred during eodiagenesis. The deep-water LF-A sediments underwent slight eodiagenetic modifications, but developed abundant calcite-filling sub-horizontal fractures during moderate inversion and uplift in mesodiagenetic regime. With increasing depth of burial, the previous vugs-filling calcite and dolomite were preserved well or only slightly replaced by reedmergnerite in the LF-A and LF-B sediments, but intensely replaced by reedmergnerite in the LF-C sediments during mesodiagenesis. The reedmergnerite enrichment processes in the LF-C sediments were closely related to organic matter production and thermal evolution. Algae blooms induced by spring input functioned as an important media in sinking hydrothermal boron from the not very concentrated waters. Later degradation and thermal evolution of alginite released boron and organic acids into interstitial waters, which caused carbonate dissolution and promoted reedmergnerite formation. The formation of reedmergnerite during mesodiagenesis restrained silicification process of carbonate minerals in ancient volcanic-related alkaline lake deposits.