Koji Yagishita

Antidunes and Traction-Carpet Deposits in Deep-Water Channel Sandstones, Cretaceous, British Columbia, Canada

ABSTRACT Sandstone beds intercalated with thick shale of the upper member of the Haida Formation (Albian-Turonian) in the Queen Charlotte Islands, British Columbia, Canada, show thinly laminated graded bedding. The sandstones generally are present as successions of more than a few tens of inversely to normally graded thin beds. The beds at the base of each succession contain much coarser detritus than the beds at the top. Each succession of graded beds shows a lens-shaped channel geometry. Wavy bed forms similar in shape to the hummock-and-swale topography that is thought to form hummocky cross-stratification (HCS) characterize the uppermost part of the succession. However, grain fabric and spacing of the bed forms indicate that they are of antidune origin. It is concluded that the successions of multiple laminated sandstone beds were produced by high-density turbidity currents that were probably associated with a gravity transformation, in which a successive freezing of traction carpets took place. Lack of shale in the succession together with the common presence of antidunes suggest that each succession was the product of one sedimentation event without decrease in stream power.

Paleocurrent and fabric analyses of fluvial conglomerates of the Paleogene Noda Group, northeast Japan

Abstract Clast-supported fluvial conglomerates of the Paleogene Noda Group, northeast Japan, consist of two contrasting formations; the lower Minato Formation comprising dominantly acidic volcanic gravels, and the conformably overlying Kuki Formation of Mesozoic basement gravels. Crudely horizontally stratified or massive sheets of conglomerates, facies Gm, comprise most of the sediments of both the formations, and show no abrupt lateral changes of facies. Debris flow deposits are absent from the sediments of the group. Paleocurrent analysis from gravel imbrication of facies Gm suggests that sediments of the Minato Formation were deposited by eastward paleoflow and had multiple sources in the western volcanic range. Such characteristics of the formation seem to show contrasting sedimentological aspects when compared with a typical alluvial fan or a fan delta. Sediments of the lower part of the group were formed as a coalescing braid delta. Elsewhere, denudation of Lower Mesozoic basement rocks in the northern source area provided sources for the uppermost part of the group (Kuki Formation). Fabric and structure of the deposits give some clues to understand the complexities of small-scale fluvial topographies. Symmetrical or asymmetrical concave-up gravel alignments of the conglomerate (facies Gt) suggest a small-scale channel dissecting obliquely into a broad longitudinal bar of facies Gm. Elsewhere, dip orientations of imbricated gravels of facies Gm coincide well with the maximum dip direction of cross-bedding within the overlying sand bar (facies Sp). However, the facies change from Gm to Sp is remarkable. Features of gravel imbrication provide useful information to understand bounding surface problems.

Grain fabric of hummocky and swaley cross-stratification

Abstract Bed-form and grain fabric of two types of hummocky and swaley cross-stratification from the Pleistocene formations in the Kanto Plain, cental Japan, display clearly distinctive flow conditions of storm (typhoon)-generated combined flow. The grain alignment in the vertical profile of a domal structure of hummocky cross-set, together with variable strike directions of a lamina, suggest that the cross-stratification was produced under the waning stage of oscillatory-dominant combined flow. Elsewhere, the preferred grain orientation along the maximum dip direction of a large swaley structure shows a small angle of upcurrent imbrication both on the stoss and lee sides of the laminae. The stratification within the swale is asymmetric, but the strike directions of a lamina are persistently oriented in a preferred direction. The fabric and the structure of the swale indicate that the cross-stratification was formed as a very low-angle mega ripple under unidirectional-dominant combined flow. The grain fabric of both cross-sets clearly shows that in spite of a superficial resemblance to antidunes, neither of the two structures was formed in flow conditions for antidunes.

Two types of plane beds under upper-flow-regime in flume experiments: evidence from grain fabric

Abstract Laboratory experiments that aimed to produce plane beds under upper-flow-regime were conducted using a flume of 15×700 cm with quartz sands of mean size of 0.72 mm (0.47 φ ). Two types of plane beds were formed: (1) laminae formed and buried by the migration of low-relief asymmetric ripples near- or subcritical flow (Froude Number, Fr =0.94, Run 5), and (2) laminae formed without producing any apparent low-relief bedform but under supercritical flow ( Fr =1.98, Run 12). The mean flow velocity (83 cm/s) and the mean depth (8 cm) in Run 5 are larger than those in Run 12 (62 cm/s and 1 cm, respectively). Upcurrent grain imbrications in Run 12, however, are better developed than those in Run 5. Such an imbrication pattern may be useful to recognize the formational processes of plane beds under upper-flow-regime.

Planar cross-bedding associated with rip currents of Upper Cretaceous formations, northeast Japan

Abstract The sedimentary structure produced by rip currents is mainly characterized by seaward-dipping planar cross-bedding. Field observations from the Upper Cretaceous (Santonian) Kunitan and Taneichi formations in northeast Japan show that the occurrence of the cross-bedded units extends laterally from the ancient beachlines but is restricted in vertical extent. The planar cross-set bed of the Kunitan Formation overlies very coarse-grained, parallel-laminated and trough cross-bedded sandstone beds of longshore current origin and underlies a pebbly conglomerated bed that was probably formed by a storm-generated bottom current. Above is a zone of alternated beds of bioturbated and parallel-laminated or hummocky, cross-stratified, fine-grained sandstones, representing either lower shoreface or transitional depositional environments. The planar cross-bedding of the Taneichi Formation is intercalated with the underlying trough cross-bedding and with the overlying bioturbated or parallel-laminated, very fine-grained sandstone bed which also includes hummocky cross-stratification. Both planar cross-set beds of the Taneichi and Kunitan formations are present in the succession which fines upward, suggesting an increasing depth of deposition. Such field occurrences indicate that bedforms produced within deeply incised rip channels which extend towards the sea can be untouched by ravinement and remain well preserved during transgression.

Basin evolution within the Kitakami Massif, northeast Japan: relationship between sedimentation, tectonics and volcanism in an incipient Neogene continental back-arc basin

Abstract The Ichinohe basin in the western margin of the Kitakami Massif, northeast Japan, consists of the Early to Middle Miocene Yotsuyaku and Keiseitoge formations and represents the incipient stage of Neogene back-arc sedimentation that extends toward the north and northwest. Megaclast breccias derived from Mesozoic basement rocks of the Kitakami Massif were transported from southern areas as debris flows. The debris-flow deposits of the Yotsuyaku Formation grade northwards along the basin axis into clast-supported, braided-stream deposits, where floodplains developed. In the center of the basin voluminous volcanic eruptives of the Keiseitoge Formation were derived from eastern and southeastern volcanic ranges and formed transversal distributaries. The deposition of both clastic and pyroclastic sediments suggests that volcanism and tectonic uplift took place simultaneously during basin evolution. Parallel-laminated, fine-grained sediments intercalated with lignite beds suggest lacustrine deposition, and large cross-beds of volcanic breccias indicate Gilbert-type progradation into the lakes. A number of small lakes in the center of the basin was probably formed as the result of damming tributary mouths by the main drainage from eastern and southeastern volcanoes or the reciprocal blocking by lahars and non-volcanic debris flows derived from the southern uplift. Damming episodes produced by thick lava flows were not recognized in this study. The evolution of the N–S trending, longitudinal Ichinohe Basin indicates that E–W extensional tectonism related to the opening of the Sea of Japan affected not only the Green Tuff Region, to the west of the studied basin, but also even the Mesozoic basement during the Miocene. Although the mingling of volcanic and basement-derived deposits indicates the complexities of sedimentary facies and provenances, our view of the stratigraphic architecture has relevance to the interpretation of the evolution of other extensional basin systems.