Ribbon rocks revisited: the upper Cambrian (Furongian) Hwajeol Formation, Taebaek Group, Korea

Alternations of thin-bedded limestone and shale, or ribbon rock, commonly occur throughout lower Palaeozoic carbonate successions; however, their formative processes are still unclear. In this study, we discuss the origin of the ribbon rocks of the upper Cambrian Hwajeol Formation, Korea, based on detailed microfacies analysis of a similar to 2-m-thick interval. Five sedimentary microfacies were identified: normally graded calcarenite to shale; parallel-laminated shale; lime mudstone; wackestone-to-packstone; and bioclastic-intraclastic packstone-to-conglomerate. Shale facies were most likely formed by frequent storm-induced bottom currents, whereas, lime mudstone facies were deposited in situ by suspension settling of micrite, mudflows, or growth of keratose sponges on the seafloor, and/or formed by early diagenetic growth. Conglomerate/packstone/wackestone indicate infrequent, larger-scale events, e.g., mega-storms, tsunamis, or earthquakes. We propose a new formative model for tempestite-type ribbon rock based on the Hwajeol example, and suggest that this model can be differentiated from the other types of ribbon rocks-tidalite, turbidite, and diagenetic types. Formation of the tempestite-type ribbon rocks would have been promoted by the characteristic environmental conditions of the early Palaeozoic, in particular sea-water chemistry that promoted calcite precipitation and the paucity of burrowers. Detailed microscopic observations can thus provide clues to elucidate previously unknown sedimentary processes in the deeper parts of carbonate platforms.

Automated summary

The origin of ribbon rocks in the upper Cambrian Hwajeol Formation in Korea is discussed based on detailed microfacies analysis. Different sedimentary microfacies indicate varying formative processes, such as storm-induced bottom currents for shale facies and suspension settling or early diagenetic growth for lime mudstone facies. The proposed formative model for tempestite-type ribbon rock suggests that environmental conditions in the early Palaeozoic promoted their formation, characterized by sea-water chemistry that promoted calcite precipitation and a scarcity of burrowers.