Distinctive lithofacies architecture of sand ridges built by storm and ocean currents: An example from a Middle Pleistocene shallow‐marine succession on the Boso Peninsula, Japan

Abstract

Sand ridges are present in modern storm-dominated and tide-dominated shallow-marine environments worldwide, but the number of published ancient examples is limited. The role of ocean currents in the formation of sand ridges is also poorly understood. This study investigated the lithofacies architecture of a lenticular sandstone body (ca 70 m thick and ca 30 km long) in the upper Mandano Formation (ca 0.6 Ma) on the Boso Peninsula, Japan, to establish criteria for identifying sand ridge deposits built by storm and ocean currents. The sandstone body is unconformably underlain by coastal and valley-fill deposits and fines upward to outer-shelf muddy deposits. It consists mainly of medium-grained to very coarse-grained sandstones and pebbly sandstones that are moderately to intensely bioturbated and are represented by three-dimensional compound dune deposits. The body consists of six 10 to 20 m thick units (units 1 to 6 in ascending order), which are each defined by an erosional base and locally capped by storminduced fluid mud deposits. Each unit also contains low-angle (ca 5°) large stratification inclined obliquely to palaeocurrents, indicating lateral accretion surfaces. Units 1 and 2 are characterized by south-eastward migration (locally north-westward), smaller dune deposits, storm-induced sedimentary structures and cold-water molluscs indicating a palaeowater depth of up to 50 m. The other units are represented by east/north-eastward migration, larger dune deposits, warm-water molluscs indicating a palaeowater depth of up to 200 m, and a general lack of storm-induced sedimentary structures. Temporal changes in the migration directions and lithofacies features of the units indicate that sand ridge formation was initially controlled by storminduced currents that were subsequently replaced by the northward or north-eastward intruding palaeo-Kuroshio Current in response to the expansion of the shelf during an overall transgression. The initiation and development of the sand ridges may have been supported by the supply of clastic sediments eroded from the underlying coastal and valley-fill deposits.