Keiji Horikawa1, Makoto Ito1
(1) Chiba University, Chiba-city, Japan
ABSTRACT: Nonuniform proximal-to-distal changes in thickness, frequency, and paleospeeds of sandstone beds in a transgressive outer-shelf in the middle Pleistocene (ca. 0.7 Ma) Kazusa forearc basin, Boso Peninsula, Japan
Proximal-to-distal changes in thickness, frequency, and paleospeeds of sandstone beds in a transgressive outer-shelf environment were investigated from the Middle Pleistocene Kakinokidai Formation in the Boso Peninsula, Japan. The Kakinokidai Formation is characterized by silty sandstones and sandy siltstones intercalated with turbidite sandstones (Beds 1 to 50 cm thick). Most of the formation was interpreted to have developed in responsed to the rise in glacio-eustatic sea-level from oxygen isotope stages 18-17 and are intercalated with many volcanic ash beds. Mapping of volcanic ash beds from the proximal-to-distal areas permits detailed bed-by-bed correlation of sandstone beds in the outer-shelf succession.
In general, sandstone beds do not show any simple trends of progressive fining and thinning to the offshore-direction; frequency and thickness of sandstone beds first increase then decrease, followed by the increase again to the further offshore-direction. In response to such lateral variations, paleospeeds of a sandstone bed show depletive and accumulative conditions, reflecting local topographic irregularity represented by slump scars. Locally developed accumulative flows appear to be more erosive and resulted in bypassing of turbidity currents. In contrast, local decrease in outer-shelf gradient permits the development of depletive flows that resulted in more frequent deposition of sandstone beds in a more distal area. The finding indicates that the transgressive outer-shelf is characterized by higher gradient of sea floors and locally developed topographic irregularity that do not permit progressive fining, thinnig, and waning of paleospeeds of shelf sandstone beds, compared with the generally accepted proximality model from modern highstand shelf environments.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado