Characteristics and sedimentary processes of lamina-controlled sand-particle imbricate structure in deposits on Lingshan Island, Qingdao, China

Abstract

Imbricate structure is a common sedimentary structure and is well developed in sandy sediments. Here, we report a lamina-controlled fine-sand-particle imbricate structure in a set of very fine grained sedimentary rocks (fine sandstone, siltstone, and mudstone) at the Dockyard and Qianceng Cliff areas of Lingshan Island, Qingdao, Shandong, China. Sets of up to 300 laminae are found in stratigraphic profiles in these areas. The laminae are generally less than 1 mm thick, with most being 0.3–0.4 mm thick and the thinnest being <0.1 mm. The dip angle of the imbricate structure varies widely, between 0° and 90°, with an average dip angle of about 40°–50°, which is higher than that of imbricate riverbed gravels (about 34°). The dip angle is a function of the shape and sorting of the particles, as well as the hydrodynamic conditions under which these fine-grained sediments were deposited. Several profiles show well-developed multiscale, soft-sediment deformation structures. Flute casts, load casts, and groove casts are also common. Fragments of carbon remains occur widely and commonly constitute stringers several millimeters thick and up to 10 cm long, together with fine clasts. Vitrain lenticles are also common. Based on the imbricate structure of the Lingshan Island deposits, it is shown that in addition to paleocurrent analysis, the imbricate structure can be used to infer information about fluid properties, transport characteristics, and sedimentary processes of the depositional environment. A near-bottom underflow, either of authigenic origin or derived from cold river water in winter in a delta interdistributary bay or delta-front environment, is inferred to have provided the hydrodynamic setting in which this imbricate structure formed. The imbricate structure, together with other sedimentary structures and features, shows that the sedimentary rocks on Lingshan Island were deposited in an inland, shallow-water environment, such as a delta, and not in a deep-water or submarine continental-slope environment.