Abstract

Resedimented conglomerates and associated turbidites occur in the Miocene late orogenic basin of northwestern Hokkaido, Japan. The Kotanbetsu Formation, which mainly contains these deposits, occurs at the front of the Hidaka uplift which resulted from a major collision in Miocene time. The sediments below and above the Kotanbetsu Formation represent shallow water deposition. From the facies changes and micropalaeontological data, an abrupt subsidence of the order of 1000 m is inferred for the beginning of the Kotanbetsu deposition. Due to this rapid subsidence of the basin floor, at least 3000 m of resedimented conglomerates and turbidites were deposited in the basin. The conglomerates comprise the following facies: (a) massive (i.e., unstratified) conglomerate; (b) massive conglomerate with major zones of disrupted bedding; (c) conglomerate with clast imbrication, associated with cross-bedded sandstone; (d) inverse- to normally-graded conglomerate; (e) graded-stratified conglomerate; and (f) pebbly mudstone.

These sediments were deposited by a variety of processes, including gravity flows in proximal reaches, slumps, channelised mass flows and debris flows and turbidity currents. Compositional, textural and fabric data from the conglomerates correlate well with the directional data obtained from the associated turbidites.