A Study on the Sedimentation of Coal in the Joban Coal Field (I) Environs of Isohara-machi, Ibaraki Prefecture

Abstract

To develop a new colliery, we have restudied the geological structure of the Joban Coal-field in detail, employing for this purpose a topographical map exceeding 1/10000 in scale., The previously accepted formations have been stubdivided for the purpose., The Ishiki bed of the late Dr., S., Tokunaga (including the Shiramizu coal bearing bed and Iwaki sandstone bed of tlae late Dr., Watanabe) was subdivided into several Cycles, each representing cyclical sedimentation., Each Cycle (Rinneso) is treated here as a formational unit because of being mapable in our geological map., Each unit is named according to stratigraphic terminology., The coal bearing or terrigenous formation is composed of many thin cyclical beds, while pure marine formations form large scaled cyclical- beds (Megacycle) as shown by the Asagai and Shirasaka formations of authors, for the latter case the lithological characteristics were the basis for treating it as a formation unit., Each cyclical formation is composed of the following rocks; from base to top, Conglomerate or coarse sandstone, Medium or fine sandstone, Sandy shale or shale sometimes with coal seams., Where the cyclical formation is too thin for mapping, two or three such cycles are combined and mapped in the same colour for conveniense., The stratigraphical sequence of the formation in the present studied area is as follows; Quaternary Alluvial deposits unconformity Terrace deposits unconformity Plateau deposits unconformity Tertiary Pliocene Miocene Oligocene Pre-Tertiary Taga Group Yungagaya Group Shiramizu Group Hanakawa formation Tempizan formation Isohara formation Futatsuzima formation unconformity Kozawa formation Ashigaya formation Kunugidaira formation unconformity Shirasaka formation Upper Asagai formation Lower Asagai formation Sekiya cycle Nemoto cycle Kisara cycle Nakahira cycle Maehanzo cycle Upper Yamashita cycle Lower Yamashita cycle Hikitorizawa cycle Kurumaoki cycle Azuhata cycle unconformity Basement complex In the course of the study we have recognized the following two important facts: (1) the number of cycles in the Hanakawa formation (Igarashi, 1945, M., S., ) differs in each district in the Joban coal field, in spite of the fact that the above mentioned megacycle can be trace throughout., In other wards 1 megacycle can be trace in all of the following districts while only 21 cycles occur in the Isohara districts, 24 cycles in the Yumoto district and only 8 cycles in the Futaba district., (2) In general, the number of cycle in the coal sedimentation has been father traced successively over this region except incomplete one, though the sedimentary facies in the Hanakawa formation is variable laterally., The development of different cycles together with the prersisting of the megacycle in the mentioned districts affors us reliable data for coal exploitation and exploration in the Joban coal field., Employing the relation between coal beds and cyclic sedimentation, we have been able to bring forth data which would not have been gained by previous methods.,