Cyclic patterns in the Lower Ordovician Dumugol Formation, Korea: Influence of compaction on sequence-stratigraphic interpretation in mixed carbonate–shale successions

Abstract

The Lower Ordovician mixed carbonate–shale Dumugol Formation of Korea was analysed to compare the stratal thickness and facies proportion of measured and decompacted successions and to assess the effect of compaction on the interpretation of cyclicity. Metre-scale cyclic units (MSUs) in the formation consist in part of outer-shelf shale, distal to intermediate mid-ramp limestone–shale couplets, and proximal mid-ramp lime mudstone facies. A total of 119 MSUs were recognised in the studied section. Thickness reduction was estimated based on petrographic evidence, including compressed to uncompressed burrows, firm- to hardground surfaces and differentially compacted laminae, and supplementary values of decompaction factors taken from the literature. Five oscillatory units in the apparent Fischer plots consist of thickening- to thinning-upward packages composed of shallowing- to deepening-upward MSUs. Similar thickness–facies relationships were observed in the lower two oscillatory units of the decompacted Fischer plots; these units are rich in shale. In contrast, a transition to an antithetic thickness–facies pattern occurs in the following units, with common occurrences of limestone facies. The different thickness–facies relationships in the measured and decompacted Dumugol successions indicate that a large portion of the major controls on the formation of sedimentary cyclicity changed from sediment accumulation rate to relative sea-level fluctuations. Such modification by compaction of the original sediment column appears to have been caused by early cementation in calcareous sediments, which resulted in differential thinning between limestone and shaly deposits. These findings reveal that diagenetic influences on sequence-stratigraphic and cyclostratigraphic interpretations have been underestimated, with consequent potential effects on associated local to regional geodynamic models, particularly for mixed carbonate–siliciclastic successions.