2003 Volume 109 Issue 7 Pages 363-382
This paper is an attempt to provide a precise geological time scale for paleobiologists conceraed with the Upper Cretaceous fossils of the Haboro area and its neighborhood in northwestern Hokkaido, Japan. The Upper Cretaceous deposits exposed in the upper reaches of the Haboro-gawa (river) consist of six formations within the groups in ascending order as follows: (1) Tenkaritoge Formation, mainly composed of mudstone; (2) Haborodake Formation, conglomerate; (3) Shirochi Formation, alternating beds of sandstone and mudstone; (4) Lower Haborogawa Formation, predominantly claystone and siltstone (5) Upper Haborogawa Formation, upward-coarsening sequence of siltstone to coarse-grained sandstone; (6) Nagareya Formation, upward-coarsening sequence of claystone to silty sandstone. The former three formations belong to the Middle Yezo Group, and the latter three to the Upper Yezo Group. In order to enable direct correlation among the adjacent areas, subdivision into a total of eleven lithostratigrafic units within the sequences from Tenkaritoge to Nagareya Formations is newly proposed for this area. Borders between the units do not cross "chronological planes" so far as we trace three key-marker beds of acidic tuff. The adequacy of this newly proposed framework in this area was also tested by occurrences of macro fossils. The distribution of both ammonites useful for international stage correlation and several inoceramids zonal index species is quite concordant with previous knowledge established in other areas. Several other ammonites, which previously have been regarded as relatively long-ranging species, are found here to occur within quite restricted ranges.