The sediments of the Cretaceous Gyeokpori Formation in south-western South Korea accumulated in a lake in which mainly siliciclastic rocks were deposited, with some interbedded volcaniclastics. The nearby volcanic activity resulted in unstable lake margins inducing a dominance of gravity-flow deposits. The high sedimentation rate facilitated soft-sediment deformation on the sloping margin. The deposition of numerous gravity-flow deposits resulted in a vertically heterolithic stratification. The slumps are composed of different lithologies, which is expressed in different types of deformation due to the difference in cohesion between sandy and mussy layers within the slumps. Coarser-grained (cohesionless) slumps tend to show more chaotic deformation of their lamination or layering. The difference in slumping behaviour of the cohesive and non-cohesive examples is explained and modelled.
A unique soft-sediment deformation structure is recognized. This structure has not been described before, and we call it ‘envelope structure’. It consists of a conglomerate mass that has become entirely embedded in fine-grained sediment because slope failure took place and the fine-grained material slumped down with the conglomerate ‘at its back’. The cohesive laminated mudstone formed locally slump folds that embedded the non-cohesive overlying conglomerate unit, possibly partly due to the bulldozing effect of the latter. This structure presumably can develop when the density contrast with the underlying and overlying deposits is exceptionally high. The envelope structure should be regarded as a special – and rare – type of a slumping-induced deformation structure.
Intrastratal flow is a pro cess that is still poorly un der stood, rarely de scribed and dif fi cult to in ter pret in an cient rocks. Sed iments in the Cre ta ceous lac us trine Gyeokpori For ma tion of south west ern South Ko rea con tain some cha ot i cally de formed sand stone lay ers with de formed mudstone clasts that are as cribed to this pro cess. The in ter pre ta tion is based on the fact that these lay ers can not be ex plained as a re sult of sub aque ous de bris flows or mass trans port, whereas the sed i men tary context, in clud ing the pres ence of other soft-sed i ment de for ma tion struc tures, in di cates that intrastratal flow must have been phys i cally pos si ble. The sed i men tary set ting was a lake in which mainly siliciclastic rocks were de pos ited, with some interbedded volcaniclastics. The nearby vol ca nic ac tiv ity caused seis mic shocks that af fected the un sta ble lake mar gins result ing in the dom i nance of grav ity-flow de pos its, but also in a high sed i men ta tion rate that fa cil i tated soft-sed i ment de for mation partly caused by intrastratal flow. This must have hap pened fairly fre quently dur ing a prob a bly lim ited time-span, as sev eral lay ers show ing traces of intrastratal flow are pres ent within a suc ces sion of only <1 m thick. The com bined data on the geo log i cal set ting and our find ings re gard ing the or i gin of the var i ous soft-sed i ment de for ma tion struc tures may help to rec og nize the traces left by intrastratal flow else where in the geo log i cal re cord. Key words: intrastratal flow, Gyeokpori For ma tion, soft-sed i ment de for ma tion struc tures, lac us trine en vi ron ment, Cre taceous.
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