We investigated the geological structure and hydrogeological properties of the Omagari Fault in Neogene siliceous sedimentary rocks of the Horonobe area, northern Hokkaido, by geological mapping, borehole investigations, reflection seismic survey, and audio-frequency magnetotelluric survey. As a result of the investigation, the 3-D distribution of the Fault was clarified and the following were shown. In addition, the magnetotelluric survey in particular was effective for mapping of the 3-D distribution and hydrogeological properties of the fault. The Omagari Fault has a fault zone, about 120 m wide, that consists mainly of the damage zone, and has a permeable structure. The magnetotelluric survey shows several high-resistivity zones, one of which corresponds to the Omagari Fault inferred from the reflection seismic surveys. The high resistivity zones are correlative with the concentration zones of low-saline water, which suggests infiltration of ground water through the permeable Omagari Fault zone. The Omagari Fault over-steps at the earth surface and at the same time is converging in subsurface zones.
The Complex Response model originated by Schumm and Parker (1973) claims that multiple paired stream terraces can form as a result of “complex” mutual feedbacks between the downstream and upstream reaches of the river system in response to a single drop in base level whereby the river initially becomes ungraded and then recovers a graded state. To critically test this model, a series of analog flume experiments was conducted, during each of which it was observed how the alluvial lower reaches of a river responded to base level fall under the conditions of constant sediment discharge, constant upstream water discharge to convey the sediment downslope, and a uniform flume slope. The model river was graded using a downstream weir; sudden removal of the weir created a discrete drop in base level. The results of the experiment suggest that (1) even in a graded system, there can form “incipient” stream terraces owing to autogenic or self-organizing geomorphic process of the river, (2) base level fall functions simply to amplify the relief of pre-existing incipient terraces and is not necessarily responsible for the formation of stream terraces, thus, (3) the complex response of river systems is regarded as a manifestation of the alluvial autogenic responses, and (4) the autogenic responses observed in the runs are in the form of fluvial-diffusive processes. The upscaling of a model alluvial river implies that in a prototype, small-scale, high-gradient, gravel-dominated natural system, the diffusive alluvial response will completely regain its graded state in a very short time after becoming ungraded by a discrete drop in base level.
The Samondake Unit in the Neo-Izumi area, central Japan, represents a sandstone-dominant accretionary complex of the Mino Terrane. It is divided into two fault-bounded subunits, the Kuzawadani Subunit on the northeast and the Kouchidani Subunit on the southwest. Although these subunits show different features in deformation grade and lithologic assemblages, they consist largely of contemporaneous deep-marine siliciclastic succession assigned to middle Jurassic. Owing to the penetrative shears oblique to bedding plane at low-angle and lack of available key beds, it was difficult to reconstruct the original lithostratigraphy accurately. The facies study having been carried out on well-exposed and less-sheared sections discriminated seventeen facies and five facies associations (Fa.A-E). Among them, the facies associations A and B are comparable to the channel-fills and lobe-like deposits, respectively. The facies associations C is defined as a marginal facies of deep-marine fan, and includes interlobe and lobe fringe deposits accompanied with sheeted sandstones and mud drapes. The facies associations D and E are assigned to hemipelagic and pelagic sediments on the basin-plain and ocean-floor, respectively. Among them, the facies association B is the most dominant one in the Samondake Unit. This suggests the trench fan as a depositional system of the Samondake Unit.
In this paper, we discuss depositional process and sequence stratigraphy of the Pleistocene Kioroshi Formation, Shimosa Group beneath the Omiya Upland, central Kanto Plain, central Japan. On the basis of sedimentary facies and fossil data such as molluscan and diatom assemblages, the Kioroshi Formation can be subdivided into the lower and upper parts which were formed by a incised-valley system and a barrier-island system, respectively. Each part represents successive upward-fining and upward-coarsening units composed mainly of mud and/or thin-interbedded sand and mud. Based on the correlation between pollen zones, tephrochronology, and the MIS curve, the lower part is considered to have deposited during the sea-level rise of MIS 6 to early MIS 5e and most of the upper part during the gentle sea-level fall of late MIS 5e. The sea levels suggest that the incised-valley system is interpreted as lowstand and transgressive systems tracts and the barrier island system as a highstand systems tract.