Soft-sediment deformation structures in alkaline lake deposits of Lower Permian Fengcheng Formation, Junggar Basin, NW China: Implications for syn-sedimentary tectonic activity

Abstract

Soft-sediment deformation structures (SSDS) are an important tool for interpreting paleoseismicity and depositional environments in ancient basin successions. The northwestern margin of Junggar Basin is a key region for understanding the paleogeography and basin attribute of the Junggar Basin during the early Permian. In the Lower Permian (Kungurian) Fengcheng Formation, nine lithofacies are grouped into fan delta, fluvial delta, and shallow, deep and alkaline lacustrine facies associations. Vertical transitions of the sedimentary environments show that the lake basin experienced expansion during deposition of the Lower (P1f1) and Middle (P1f2) members and the shrinkage during the Upper Member (P1f3), which is consistent with the paleogeographic distribution of lithofacies in different members. Conventional cores show a wide variety of SSDS, including ductile (folds, convolute lamination, and load structures), hybrid ductile-brittle, and brittle deformation structures (microfaults, breccias, boudinage and sedimentary dikes). SSDS mainly occurred in a still and deep lacustrine environment. These deformation structures were not formed in any specific lacustrine environment, as there is no difference with the undeformed sedimentary units of the same sedimentary facies. Thus, the triggering mechanism is interpreted as intermittent earthquakes. Lake expansion under arid condition and SSDS intervals with vertical repetition and possible lateral continuity indicate that most seismicity was during the Middle Member (P1f2), corresponding with the main activity on basin-bounding normal faults. The enhanced faulting at this time generated accommodation, creating lacustrine conditions in the basin depocenter. It is thus clear that the early Permian Mahu sag represents an active rift basin with bounding faults, and is not a foreland basin.