Toru Nishikawa¹

(1) Chiba University, Chiba, Japan

ABSTRACT: Tectonic control of paleogeography associated with a barrier-island system documented in Late Pleistocene paleo-Tokyo Bay, Japan

Paleo-Tokyo Bay (0.45 - 0.07 Ma) is a relict of Plio-Pleistocene Kazusa-forearc basin and is characterized by a barrier-island system. The tectonic movements in paleo-Tokyo Bay is still active and have affected deformation of the sedimentary infill of the bay.

The Upper Pleistocene Kioroshi Formation is the upper sequence of the sedimentary infill of paleo-Tokyo Bay developed in response to transgression and following normal-regression during rise and highstand stages of a glacioeustatic sea-level cycle from the oxygen isotope stages 6 to 5e. In the southern part of paleo-Tokyo Bay, a volcanic-ash bed dated about 120 ka (coded "SIP") is intercalated in the uppermost part of the Kioroshi Formation. This volcanic-ash bed overlies lagoon (and/or overlying meandering-river), flood-tidal-delta (and/or overlying tidal-flat), foreshore, and backshore deposits. The spatial variation in lithofacies assemblages below the volcanic-ash bed is interpreted to show contemporaneous spatial variation in paleogeography during a regressive stage: Foreshore and backshore developed in northwest-southeast directed elongated area composing a spit, and lagoon and flood-tidal delta developed behind the spit. This paleogeography corresponds with local tectonic frameworks represented by the present-day altitude of Upper Pleistocene marine terrace and is interpreted to have been controlled by tectonic movements in paleo-Tokyo Bay. Lagoonal deposits developed around synclinal axes, whereas sandy deposits of foreshore, backshore, and flood-tidal-delta environments developed around anticlinal axes. The result show that the formative processes of paleo-Tokyo Bay have been controlled not only by glacioeustatic sea-level changes but also by local tectonic movement in the bay.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado