IEG 환경지질연구정보센터

10월 25일(목)․퇴적학

2007 추계지질과학연합학술대회 초록집 217

Soft-Sediment Deformation Structures in the Cretaceous non-marine deposits of the Southeastern Gyeongsang basin,

Korea: Occurences and origin

Hee Cheol Kang․In Sung Paik

․Ho Il Lee․Sook Ju Kim

Dept. of Environmental Geosciences, Pukyong National University, Busan 608-737, Korea [email protected]

A variety of soft-sediment deformation structures formed during or shortly after deposition occur in the Cretaceous Seongpori, Goseong, and Dadaepo formations of the southeastern Gyeongsang Basin exposed along coastal areas for 0.5~2 km long. These are mostly present in a fluvial plain facies. In this study we provide the features of different kinds of soft-sediment deformation structures that occur within a fluvial succession and focus on analysis of the timing and mechanism of deformation and the interpretation of trigger mechanisms. The soft-sedi- ment deformation structures in the study area occur in fine- to coarse-grained sandstones, mudstones and tuffaceous sandstones, and they can be classified into four groups: load struc- tures (flame structures, ball-and-pillow structures, boudinage structures), water-escape structures (cusps, pillar structures), soft-sediment intrusions (clastic dykes and sills) and other structures (deformed laminations, bed separations, dislocation breccias, slumps, and synde- positional faulting). These structures seem to be associated with different mechanisms of deformation and formed at different times. Load structures form in response to gravitational instabilities related to density differences or uneven loading and deformation starts after abrupt reduction in shear strength of the sediments. Water-escape structures are related to the deformation associated with local fluidization and dewatering of lower unconsolidated sediments. Water and fluidized particles move upward, cutting and deforming overlying low-permeability sediments. Small and mesoscopic soft-sediment intrusions are interpreted to be derived from the injection of liquidized sands into overlying strata. Deformed laminations seem to be induced by liquefaction and fluidization processes localized in coarser-grained sediments isolated in finer-grained sediments, which act as barriers to permeability. Bed sepa- rations are generally linked to progressive disintegration of finer-grained thin beds by ductile extension, dislocation breccias to brittle failure of more lithified layers during extension, and slumps to gravitational downslope movements. Syndepositional faulting is a brittle to semi-brittle deformation that takes place when the pore pressure is not sufficient to liquefy the sediments. The timing of described soft-sediment deformation can be grouped into three types based on field evidence: load structures and syndepositional faulting formed during depositional process (syndepositional deformation); water-escape structures, deformed lam- ination, bed separations, dislocation breccias, and slumps formed shortly after deposition (penecontemporaneous deformation); clastic dykes and sills formed after a bed has been covered by younger layers (postdepositional deformation), but nearly seem to be similar to penecontemporaneous deformation in the timing. The most probable triggering mechanisms for these deformations are deemed as seismic shocks, based on following field observations:

(1) lateral continuity of the deformation structures over long distances; (2) the restriction of the structures to discrete stratigraphic intervals being correlatable over large areas; (3) similarity to the structures formed experimentally; (4) confinement between undeformed layer and deformed layer distinctly different in origin; (5) occurrences of numerous soft-sedi- ment deformation structures in single level; and (6) absence of slope to indicate gravity sliding or slumping; The soft-sediment deformation structures in the study areas are thus interpreted to be seismites, which represent a intermittent record of the active tectonic and sedimentary processes during the development and evolution of three formations from the upper Early Cretaceous to the Late Cretaceous.

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