Sandstone/chert and laminated chert/black shale couplets, Cretaceous Uhangri Formation (southwest Korea): depositional events in alkaline lake environments

Abstract

The Cretaceous Uhangri Formation, southwest Korea, is a lacustrine sequence surrounded by acidic volcanic terrain and comprises sandstone/chert and laminated chert/black shale couplets in the upper part. In both cases, the Uhangri chert is composed mainly of SiO 2 (74–81%), Al 2O 3 (10–12%) and K 2O (6–7%). Its impure nature is largely due to the presence of feldspar grains and clays within a microcrystalline quartz matrix. The Uhangri chert is characterized by common soft-sediment deformation, in-situ brecciation, slumping and no organic traces, which collectively indicate its inorganic origin. The sandstone/chert couplets are alternations of massive, graded, crudely stratified, rippled, or ripple-cross-laminated sandstone layers and generally homogeneous chert layers. The laminated chert/black shale couplets consist of alternating light- and dark-coloured laminae of chert, and overlying black shale. These couplets were most likely deposited by episodic riverine inflows into a silica-rich alkaline lake. The freshwater input could trigger direct inorganic silica precipitation through an abrupt decrease in lake-water pH. The sandstone/chert couplets would be deposited by high-density underflows, whereas the laminated chert/black shale couplets by fine-sediment dominant overflows.