Early Cretaceous sedimentary records of the early-stage continental rifting in the Songliao Basin, NE China

Abstract

Well SK-2, a continental scientific drilling well in the Xujiaweizi fault depression of the Songliao Basin, has drilled through ca 2359 m of the Early Cretaceous Shahezi Formation. Herein, we focus on the sequence groups SSI and SSII in the lower part of the Shahezi Formation (interval 4357.38–5695.02 m) from well SK-2. There are fourteen different lithofacies and seven lithofacies associations. Syn-eruptive pyroclastic deposits and their proportions in successions indicate that SSI varies from dominantly pyroclastic fill to dominantly weathering-derived clastic fill, reflecting the development of volcanism in the early syn-rift stage. SSII exhibits another, similar evolutionary cycle. Whole-rock geochemistry of rocks from the Shahezi Formation reveals that the source rocks are felsic, andesite, and sedimentary rocks from the upper crust, and that the intermediate-acid rocks are originated from partial melting crustal materials and are hybridized with input from the mantle. In the seismic profile that intersects well SK-2, the thinning direction switches back and forth during the formation of the Shahezi Formation’s four sequence groups; that is, there is turnover superimposition in vertical stratigraphic subsidence, demonstrating strike-slipping during deposition of the Shahezi Formation.The major rift stage, especially during the deposition of the lower part of the Shahezi Formation, involved strike-slipping dominated extension. This conclusion is supported by the presence of abundant dark mudstone and coarse clastic progradation in the transgressive stage, as well as by the periodicity of volcanism combined with stratigraphic turnover superimposition.