Sedimentological Studies on Paleo Lake Basins (1st Report) : On the Sedimentation of the Lake Deposit in the Onikobe Basin, Miyagi Prefecture, Japan

Abstract

From field observations of the Cenozoic strata in the Onikobe basin, Miyagi prefecture, the writer recognized lake deposits (Pleistocene ?), which be named the Onikobe formation., These are well developed along the upper course of the Arao River., The purpose of this paper is to describe the mode of sedimentation and environmental conditions under which the formation was deposited., Sedimentological problems of the Uppermost Miocene lake basins formed by the crustal deformation in the backborn ranges of Northern Honshu, Japan was described in the preceding Paper (I., KATO and I., SHIMADA, 1953)., The results are summarized as follows., (1) On the basis of lithogical characteristics the strata are classified into two parts, namely, the marginal part which is characterized by loose conglomerate and sandstone beds, and the central part by tuffaceous sandstone and mudstone beds; both parts are contemporaneous with each other., (2) The marginal part is considered to be of deltaic deposition from the distinct structural features and litho-facies., The central part from the mudstone with thin lamina which form a rhythmical facies seems to hove been deposited in a fresh water lake., (3) During almost the whole period of accumulation of the strata, dacitic volcanic activity prevailed., This may have relation with the development of the sedimentary lake basin., (4) Although the origin of this sedimentary lake basin is obscure, the writer considers it to have been formed by subsidence in the inner zone and relative upheaval in the outer zone of this area after deposition of the Uppermost Miocene lake deposits., Although there remain interesting and important problems concerning the sedimentation and tectonics of the sedimentary lake basin, further geological data is necessary to settle the problem.,