Fluvial-Shallow Marine Depositional Systems of the Futaba Group (Upper Cretaceous)

Depositional facies and sequences

Abstract

The Futaba Group is the Lower Coniacian to Lower Santonian, fluvial to shallow-marine sedimentary succession distributed in the southern Abukuma Belt in north Honshu. Fifteen depositional facies were discriminated. They include alluvial fan, meandering river, braided river, transgressive lag and upper shoreface to inner shelf sediments. It is divided into three formations, the Ashizawa (the Asamigawa and Obisagawa Members), Kasamatsu and Tamayama Formations in upward sequence. Each formation consists of a third-order depositional sequence (DS).
The first depositional sequence (DS 1) has a lowstand systems tract (LST) as the lower main part of the Asamigawa Member distributed in the northern half of the area. It is lithologically characterized by upper alluvial fan conglomerate and mid to lower fan sandstone and siltstone nonconformably overlying lower Cretaceous granite (sequence boundary 1 : SB 1). In the southern periphery of the distributed area the member contains talus or upper fan angular conglomerate. The upper part of the member distributed over the area is interpreted to be transgressive conglomerate with a sharp base called a ravinement surface (RS 1) which was formed by transgressive shoreface erosion. It may represent transgressive systems tract (TST). The following highstand systems tract (HST) is represented by the upper shoreface to inner shelf, medium to fine sandstone of the Obisagawa Member.
The Kasamatsu Formation (DS 2) unconformably (SB 2) overlying the preceding depositional sequence (DS 1), is substantially characterized by fluvial upward-fining successions composed of channel sandstone and flood plain siltstone. The third sequence (DS 3) is characterized by thick fluvial (braided river) sandstone of the lower to middle part of the Tamayama Formation interpreted as LST and the upper shoreface to inner shelf sandstone of the upper Tamayama interpreted as HST. The sudden lithofacies change between the Kasamatsu and Tamayama Formations implies some large-scale changes in fluvial depositional systems by sea-level fall (SB 3). The transgressive conglomerate just above RS 2 in the basal part of the upper Tamayama Formation represents a thin TST. The upper sequence boundary (SB 4) of DS 3 shows the conspicuous clinounconformity between the Futaba Group and the Paleogene Shiramizu Group.