Notes on Mode of Occurrence and Origin of the Cherts in the Southern Tamba District

Cherts found in plaeozoic formations of the southern Tamba district are both think and areally extensive. Cherts bearing formations of the palaeozoic are found to be of middle to early permain age on the basis of palaeontological evidaences. There are bedded cherts and massive ones in the district. They occur as highly irregular shaped lenses in shale, schalstein, and rarely sandstone and limestone. The bedded cherts are rhythmically layered cherts. The layers, three centimeters to six thick, are separated by partings of black shale or red tufaceous shale. The intraformational folding is generally strong in such bedded cherts. The filed evidences show that the cherts of Tamba are mainly of direct inorganic origin and contempaneous with the associated clastic sediments. Although the silica of some cherts was supplied by the associated submarine voｌcanic flows (schalstein), many cherts were normally precipitated in sea water which contains silica in solution or the from of colloid. Predominance of fine terrigenous sediments to which much schalstein and small amount of limestone are added indicates that chert bearing formations are never neritic but must be a sediment of central part of geosyncline. Furthermore, all the chert apparently was deposited when relatively little clastic material was being supplied to the Chichibu Geosyncline