Evolution of depositional environment of the Pliensbachian Stage (Lower Jurassic) coal‐bearing series in the Qaidam Basin, NW China

Abstract

The Qaidam Basin is one of the major coal‐bearing basins in northeastern China with the coal seams mainly preserved in the Lower–Middle Jurassic. The Middle Jurassic sedimentary environment, sequence stratigraphy, paleoclimate, and coal accumulation in the Qaidam Basin have been extensively studied, while the sedimentary characteristics and depositional evolution of the Lower Jurassic coal‐bearing series are less well known. We systematically analysed the facies association, depositional system, basin evolution, and coal accumulation of the Early Jurassic Pliensbachian‐aged coal‐bearing series from the Dameigou section in the northern Qaidam Basin. Five lithological types have been identified, ranging from conglomerates, sandstones, siltstones, and mudstones to combustible organic rocks, which were further subdivided into 12 lithofacies. These lithofacies were grouped into four facies associations, representing the braided fluvial, meandering fluvial, lacustrine, and meandering fluvial‐lacustrine transition deposits, respectively. The Pliensbachian Stage mainly developed as a fluvial‐lacustrine depositional system and an evolution history from braided fluvial to meandering fluvial, then to lacustrine, and finally back to meandering fluvial was reconstructed. We suggest that climate change is the major controlling factor for depositional evolution during the Pliensbachian. The transition of the depositional environment from fluvial to lacustrine in the study area can be attributed to the rapid climatic change induced by volcanism at this interval.

Geochemical Characteristics of the Middle Jurassic Coal-Bearing Mudstones in the Dameigou Area (Qaidam Basin, NW China): Implications for Paleoclimate, Paleoenvironment, and Organic Matter Accumulation

“…The Qaidam Basin is a prominent oil and gas exploration and production base of NW China’s Jurassic coal-bearing strata. − According to the source correlation studies of oil and gas, the hydrocarbons are derived mainly from the Lower-Middle Jurassic coal-bearing mudstone sequences. − Previous research related to the Lower-Middle Jurassic coal-bearing strata of the Qaidam Basin mainly focused on provenance, sedimentary environment, sequence stratigraphy, and palynoflora characteristics. − Although the organic geochemistry, source of organic matter, and hydrocarbon generation potential of the organic-rich shales of the Shimengou Formation (Callovian Stage) have been extensively studied in recent years, − the geochemical characteristics of the coal-bearing mudstones in the Middle Jurassic Dameigou and Shimengou formations from the Aalenian to Callovian stages in the Qaidam Basin are rarely reported. In addition, little is known about the paleoenvironmental conditions as well as their influences on the organic matter accumulation of the coal-bearing mudstones during the Middle Jurassic.…”