Micro-characteristics of sustained, fine-grained lacustrine turbidites in the Cretaceous Hwangsan Tuff, SW Korea

Abstract

A distinction between the classic Bouma turbidite and a hyperpycnite is needed because hyperpycnal flows provide a direct link between the terrestrial source and the depositional sink. These deposits are a potential record of river floods and hence contain tectonic and climatic information. Unlike the sandy turbidite or hyperpycnite, the muddy counterpart is generally poorly documented. Based on detailed sedimentological analysis of a deltaic mudstone unit within the Cretaceous Hwangsan Tuff, SW Korea, this paper addresses the characteristics and depositional processes of the lacustrine muddy turbidite or hyperpycnite. The Hwangsan Tuff is composed of reworked volcaniclastics up to 300 m thick, deposited in a lake margin, on a delta front to the basin plain. The thin-bedded mudstones constitute a diverse type of turbidites. Most mudstone beds generally show normal grading and are wavy in form, but internally demonstrate the existence of a depositional break such as an erosional surface and the repetition of rippled units. Thin-bedded mudstone with a pronounced normal grading is interpreted to have been deposited by small, dilute turbidity currents. However, a composite bed of lower massive siltstone overlain by a structureless claystone suggests deposition from slow-moving, high-density flows with high clay content. The hyperpycnal flow deposits in muds are characterized by a thickness change in the horizontal laminae, internal erosion scour, and the recurrence of rippled units, all of which are features indicating deposition from flow fluctuations in long-lived hyperpycnal flows. Abundant plant debris and a lack of bioturbation also support this hypothesis. The hyperpycnal flows form the mouth bars to delta front bodies of the lake margin.