Chert concretions and bedded cherts from the southeastern Franconian Alb, Bayerwald and Kraków-Czestochowa Upland as potential raw materials for artifact manufacturing in the Paleolithic, Mesolithic and Neolithic – a comparative petrographic study

Abstract

The Upper Jurassic carbonate deposits of microbial-sponge megafacies, which host chert concretions and bedded cherts, were deposited along the northern margin of the Tethyan Ocean. Exposures of these rocks extend from Portugal to the Caucasus Mts. Well-known localities include those in southeastern Germany and southern Poland, where these strata are similar in lithology. Petrographic studies on chert concretions and bedded cherts from Upper Jurassic successions exposed in the southeastern part of the Franconian Alb and in the Bayerwald have shown that silicification was a late-diagenetic, multistage process and that the silica presumably originated from hydrothermal solutions. The chert concretions of the Bayerwald show considerable similarity to lithologies known from the Kraków-Czestochowa Upland. In both regions, these mostly represent silicified microbial-sponge biostromes. By contrast, the chert concretions of the Franconian Alb, where younger parts of the Upper Jurassic succession are preserved, differ significantly from the concretions encountered in both the Bayerwald and the Kraków-Czestochowa Upland. The bedded cherts of the Franconian Alb are partly silicified tempestite sequences whereas those of the Kraków-Czestochowa Upland represent silicified calciturbidites. However, some fine-grained portions of these successions show macroscopic resemblance, despite their differences in development. In prehistoric times, these regions in Poland and Germany were sources of siliceous raw-materials, which might have been exported towards Bohemia, as suggested by artifacts that are believed to have originated from Poland or Germany. However, comparative studies indicate that macroscopic diagnostic features determining the origin of artifacts manufactured from either the chert concretions or the bedded cherts are of doubtful value. In particular, the “chocolate flint” distinguished by archaeologists, the origin of which is suggested to be limited exclusively to the northeastern margin of the Holy Cross Mts. or to the central part of the Kraków-Czestochowa Upland, may also represent parts of chert concretions collected in the Franconian Alb. Distinction of this variety in artifact inventories based upon individual specimens may be erroneous.