Study on Geologic Structure and its Development of the Area around the Ponbetsu Fault in the Central Part of the Ishikari Coal-Field, Hokkaido, Japan

Abstract

The Ponbetsu Fault extends east to west direction along the axis of the Minenobu Mountains. On the south side of the Ponbetsu Fault, the Ikushunbetsu Formation (a part of the upper Ishikari Group) directly overlies the Cretaceous Series with unconformity. In the area on the north side of the fault, rock facies of the lower Ishikari Group, from the Noborikawa to the Bibai formations, represents a marginal nature of the sedimentary basin. The Minenobu Mountains gradually uplifted in the late Cretaceous period and grew to the Minenobu Barrier during the deposition of the Ishikari Group (i.e. Paleogene).
Several derivative thrusts composing the Primary Fault Series can be observed on the south side of the Ponbetsu Fault, with E-W trend parallel to the Ponbetsu Fault. By these faults the Ikushunbetsu and Poronai formations are overlapped. Both formations are intensely disturbed, overturned or steeply inclined, forming an imbricated structure.
In the eastern part of the Poronai district, several faults with N-S or NE strike and almost vertical dip and the other faults of E-W trend with horizontal displacement separate the area into several blocks. These faults belong to the Secondary Fault Series. The underground study revealed that the E-W trending Yayoi Fault continues to the N-S trending Nakanosawa Fault.
In the area north of the Ponbetsu Fault, the Bibai Fault extends southeastwards and converges with the Makita Fault. There are a few echelon faults trending in NNW-SSE direction between the Bibai and Ponbetsu faults. On the western foot of the Minenobu Mountains, the Ishikari Group thrust up on the Pliocene Takikawa Formation along the Shimizu Fault.
Judging from the configuration and features of the faults described above, the Sorachi Coal-field was at first compressed westwards by lateral pressure in ENE direction. The Bibai Fault was developed in tensional manner when the lateral pressure was released. The echelon faults on the north side of the Ponbetsu Fault were formed by the pressure resulting in the Secondary Fault Series. The Shimizu Fault was also formed with this compression.
The structural development of the Ishikari Coal-field can be divided into two tectonic phases, Pre-Takikawa stage and Post-Takikawa stage. Structure of the Pre-Takikawa stage is characterized by the folding caused by lateral compression in ENE direction, as seen in the Sorachi Coal-field. Tensional fractures were produced at right angle to. the direction of this compression. The Bibai and other ENE-trending faults were grown as a result of later release from the compression. Because of the Minenobu barrier, a resistant body throughout the Pre-Takikawa stage, the Ponbetsu fault was formed at the southern end of the Sorachi Coal-field. Geologic movement of the Post-Takikawa stage was essentially controlled by lateral pressure from SE. The complicated structure of the Yubari Coal-field was thus formed.