HIGH PRODUCTIVITY IN THE EARLIEST TRIASSIC OCEAN - BLACK SHALES, SOUTHWEST JAPAN

Bedded cherts containing the Permian/Triassic (P/T) boundary exposed i n the Tanba and Chichibu Belts, Southwest Japan, contain interbedded o rganic-rich black shales. The organic carbon concentration of black sh ales before thermal maturation was evaluated to be about 3% (average), considering their maturity levels. Kerogens do not contain any debris of higher-plant origins, but are mostly composed of structureless amo rphous kerogen. Black shales comprise organic matter derived mainly fr om plankton with additional organic matter derived from bacterial acti vity, showing that the high primary production took place intermittent ly in the earliest Triassic, A stratified ocean with a well-developed oxygen-minimum layer rich in nutrients was intermittently disturbed du e to tectonically and/or climatically induced active ocean current sys tems in the earliest Triassic, resulting in upwelling, causing remarka bly high primary production, to form organic-rich black shales. (C) 19 98 Elsevier Science B.V. All rights reserved.