Consideration on the Paleogene Existing Underground and Distributing on the Surface in Northern Hokkaido, Based on Pollen Analysis

Abstract

About 10 years ago, two wells were drilled in northern Hokkaido: MITI Wakkanai well in 1971 and Masuporo SK-1D well in 1973. The informations gotten from these wells strikingly changed the common idea on the stratigraphy of this region by that time. That is, thick Paleogene sediments composed of coal-bearing formation in lower part and argillaceous formation with marine fauna in upper part were encountered in these wells. These were correlated to the Ishikari and Poronai groups respectively based on lithology and fossil evidences. As these Paleogene sediments had not been found on the surface of this region, it had been considered by that time that Paleogene does not exist in this region. However, it is regrettable that the detailed informations of the Paleogene have not been published.
Recently the Tertiary in Hokkaido has been studied from many points of view and many different opinions have been proposed mainly based on microfossil evidences. It may be necessary to reexamine the Tertiary. In such a situation the above-noted unpublished informations gotten from the wells become very significant. This is just the reason that the author dares to report this paper now.
The author fortunately had a chance to carry out the palynological analytical research on the abovenoted two wells. The results of the research are reported in the present paper, mainly on the Paleogene. The Paleogene is divided into two parts: an upper marine part and a lower shallow marine to non-marine coal-bearing. They are correlated to the Poronai and Ishikari groups respectively also based on the result of pollen analysis. Especially, a pollen floral sequence found in the Paleogene in the Masuporo well is valuable for the Tertiary climatic history as a typical climatic change at the end of the Paleogene time well represented.
Sediments called the Magaribuchi or Kiyohama Formation are distributed on the surface in the area for the present study. In 1970 the author reported that the sediments are correlated to the Momi jiyama Formation in the Ishikari region, Paleogene. The sediments are also correlated to the upper part of the Paleogene found in the wells after the result of pollen analysis.