“BORED” INTRACLASTS REVISITED: MACROBORINGS IN THE LATE CAMBRIAN (FURONGIAN) FLAT-PEBBLE CONGLOMERATES REVEAL THEIR PALEOECOLOGICAL IMPLICATIONS

Macroborings are known to be rare to absent in the middle to late Cambrian successions, the time between the end-early Cambrian extinction of archaeocyath reefs and advent of new organisms during the Ordovician (Ordovician Bioerosion Revolution). In this study, we report bioerosion structures from the lime mudstone clasts of flat-pebble conglomerates in the Hwajeol Formation (Furongian), Taebaeksan Basin, Korea. Two types of macroborings (tube-shaped Trypanites-like and clavate shaped Gastrochaenolites-like structures) are recognized, in which both penetrate micritic clasts and filled with bioclastic grainstone matrix. Although it is not certain at current point whether the Hwajeol macroborings were formed when the substrates were firm or hard, evidences including sharp boundaries of macroborings, unaltered shapes of macroborings, and occurrence of macroboring on hematite-coated micritic clast penetrating the coating suggest that they likely formed after cementation of micritic substrates. The Hwajeol macroborings would have formed on the micritic clasts that are rounded during the transportation and/or on the micritic hardgrounds which are eroded and formed flat pebbles. Occurrence of the bioerosion structures within clasts of the Hwajeol flat-pebble conglomerates supports previous idea that macroborers would have survived in hardgrounds during the middle to late Cambrian after extinction of archaeocyath reefs. It further implies possibilities to find similar structures in the middle to late Cambrian successions, which has been much overlooked.