Early Cretaceous slumps and turbidites with peculiar soft-sediment deformation structures on Lingshan Island (Qingdao, China) indicating a tensional tectonic regime

Abstract

Some sections on Lingshan Island, in the Yellow Sea, close to the city of Qingdao, show the Early Cretaceous Lingshandao Formation. The exposed succession consists mainly of turbidites, some of which show uncommon soft-sediment deformation structures, and contains well-developed, large-scale slumps that consist of sets of turbidites. The sediments were deposited as part of lobes of turbidites and hyperpycnites (deposits of flood-generated sediment-laden bottom currents derived from an estuary) in a deep-sea environment that formed part of an inter-arc basin with a tensional tectonic regime.Some of the slumps have dimensions of the order of metres to tens of metres; they consist of the turbidites that build most of the succession. The soft-sediment deformation structures in the turbidites represent a wide variety of types, most of which are common in subaqueous environments with a high accumulation rate. The most interesting are, however, structures that indicate active tectonic tension. These structures are boudins, which are present with two subtypes. The most common are pinch-and-swell structures, sometimes leading to isolated parts of the affected sandy layer, enveloped by shales of the under- and overlying shale layers. The second type consists of domino boudins, which have, as far as we are aware of, not earlier been described from sediments that were still unlithified during their formation.The presence of the boudins and associated structures, which indicate synsedimentary stretching related to an extensional tectonic setting, form an additional argument in the long-lasting debate about the question of whether a compressional or an extensional tectonic regime dominated in the area during the Early Cretaceous.