PALEOCURRENT AND FABRIC ANALYSES OF FLUVIAL CONGLOMERATES OF THE PALEOGENE NODA GROUP, NORTHEAST JAPAN

Clast-supported fluvial conglomerates of the Paleogene Noda Group, nor theast Japan, consist of two contrasting formations; the lower Minato Formation comprising dominantly acidic volcanic gravels, and the confo rmably 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 ab rupt lateral changes of facies. Debris flow deposits are absent from t he sediments of the group. Paleocurrent analysis from gravel imbricati on of facies Gm suggests that sediments of the Minato Formation were d eposited by eastward paleoflow and had multiple sources in the western volcanic range. Such characteristics of the formation seem to show co ntrasting sedimentological aspects when compared with a typical alluvi al fan or a fan delta. Sediments of the lower part of the group were f ormed as a coalescing braid delta. Elsewhere, denudation of Lower Meso zoic basement rocks in the northern source area provided sources for t he uppermost part of the group (Kuki Formation). Fabric and structure of the deposits give some clues to understand the complexities of smal l-scale fluvial topographies. Symmetrical or asymmetrical concave-up g ravel 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 coi ncide 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 inform ation to understand bounding surface problems.