Abstract
The graded sandstones in Shiiya and Nishiyama formations (Miocene and Pliocene) developed in Higashiyama oil belt, Niigata prefecture, seem to be transported and deposited under the influence of both so-called turbidity currents and bottom currents.
The graded sandstones in these formations can be divided into three types (A, B and C) by mineral compositions. A-type sandstone may rather be classified as sandy tuff, for it includes the detritus from Tertiary andesitic rocks in 80% or more of all sand particles.
By separating a hand specimen according to its sizes, we find that the contents of quartz decrease in B-type and increase in C-type, as the sand grains become finer. Those of feldspars (mostly plagioclase) increase in both B-and C-types. These tendencies of the compositional changes with the grain size are also observed in the consecutive changes of composition from the base to the top of a graded sandstone.
The origin of quartz and feldspars of B-type sandstones is mostly from Tertiary volcanic rocks, and that of C-type sandstones is from the basement rocks of pre-Tertiary ages.
Now if it is permitted to suppose the 'paleo-Higashiyama High' in the middle part of this oil belt, A-, B-and C-type sandstones may be said to be developed in western-, northern-and southern-area surrounding the high, respectively.
