RECURRENT EVENTS ON A QUATERNARY FAULT RECORDED IN THE CLAY MINERALOGY OF RAISED MARINE TERRACE SEDIMENTS

Landward reverse faults cut through unconsolidated sediments on raised marine terraces, composed of beach gravels overlain by clay-silt deposits. The faulting history, critical to the safety of the public and infrastructure in the southeastern Korea, has not been yet clarified due to the complexity of repeated faulting and changing site environments. In this study, clay minerals sensitive to environmental change were investigated to gain insight into the faulting history. The clay infillings of footwall Unit I gravels were characterized by smectite and smectite-kaolin mixed-layer (S/SKML) minerals, whereas the infillings of hanging wall Unit I were composed mostly of kaolin. The clay-silt Unit II overlying footwall Unit I was marsh sediments characterized by S/SKML-rich clays, and Unit II was absent in the hanging wall. Footwall Unit II changed into eolian clay-silt Units IV and V including illite, chlorite, and vermiculite via transient Unit III, whereas in the hanging wall, Unit I was directly overlain by Unit V. Smectite, the major clay mineral in the Neogene tuff underlying terrace sediments, were weathered into SKML in the nearby outcrops. S/SKLM-rich mud was washed from the soils, and filled the small marsh basin on footwall Unit I formed by the scarp of the uplifted hanging wall. The deterioration of marsh due to subsequent faulting and uplift results in a transition from marsh to eolian deposition. Three faulting events were identified after Unit I deposition. Fine clay particles are a useful indicator to reconstruct the recurrent history of a Quaternary fault disturbing unconsolidated sediments.