Depositional origin of subtidal meter-scale cyclic successions in the Cambrian Hwajeol Formation

Abstract

Meter-scale cyclic successions of limestone and marl or mudstone commonly formed in shallow marine subtidal environments during the early Paleozoic. Such alternations have been believed to represent primary processes and/or post-depositional. The cyclic facies changes in association with primary processes have been interpreted as allocyclicity and autocyclicity. The Furongian Hwajeol Formation of the Taebaek Group in mid-eastern Korea is host to such early Paleozoic subtidal cyclic successions, showing limestone- mudstone alternations. Sedimentological and geochemical analyses including diagenetically inert element ratio analyses suggest that the cyclic successions were originated from sedimentary origin rather than differential diagenesis. Furthermore, trace elements analysis reveals that the mud fractions formed under suboxic or anoxic conditions, while the limestone facies accumulated under oxic conditions. The repetitions of oxic and oxygen-depleted conditions can be interpreted to be a result of the repeated deisolation and isolation of the water mass in the semi-closed epicontinental sea partly restricted from the open ocean. Repetitive opening and closing of the sea caused vitalization and deactivation of the carbonate factory in the platforms. Accordingly, this restriction of water-mass can deactivate the carbonate platform on the silled basin. This hypothesis provides a plausible explanation for the formative mechanism of the subtidal cyclic successions with meter-scale alternations of subtidal limestone and gradually upward thickening mudstone beds. The gradually thickening of mudstone beds would reflect an increasingly lengthened period of closure and deoxygenation of the semi-closed epicontinental sea due to gradual decrease in rising rate during low-order transgression and gradually extended duration of higher order relative sea-level stillstand or fall superimposed low-order transgressions.