Bottom Deposits of the Submarine Valley of Toyama Bay

Abstract

Of the Submarine valleys known to exist on the continental shelf of Toyama Bay, two types are found; one has its head existing in the recent river Zintugawa, and the other which is near Yokata, seems to have no direct relation with now existing rivers., Both valleys cut into the continental shelf, which is 30m in depth., Each of these submarine valleys is provided with a bottom inclining more steeply than the continental shelf, and also with very steep flanks., From studies on the bottom deposits of the submarine valleys and of the continental shelf, the following features were recognized: 1., No effect of current could be found within the submarine valley, and also the bottom sediments seem to have been not disturbed., 2., The bottom sediments taken in present survey differ from those plotted in the chart issued by the Hydrographic Department of Imperial Japanese Navy, in the following points, a: bed rocks were found exposed in the valley slope off Yokata, while a part was found to be covered with mud., The bed rock consisted of tuffaceous sandstone quite similar to the foundation rocks of the Tertiary of Kurehayama in the west of Toyama., b: an exposure of peat was found in the middle part of the continental shelf of the studied area., c: gravels are widely distributed over the continental shelf as well as in the submarine valley, and covered with mud., The gravels collected from the sea bottom are found to comprise the same components as those collected from the river bed of the Zintugawa., d: amechanical analysis of the size grade of sand and mud, shows that from their relation to both depth and distribution, they may be recent sediments., No direct relation could be found between the size grade of the sand and mud collected together with the gravels of the same locality., 3., No evidence was found to suggest that the submarine valleys were founded as a result of submarine erosion or of submarine fault., The age of the submarine valleys is probably very young.,