경상남도 하동군 금남면 대송리 부근의 하산동층에서 산출되는 패각화석층

Abstract

The Hasandong Formation, the middle part of the Sindong Group of the Gyeongsang Supergroup, consists of mostly alternating channel sandstones and interchannel fine deposits. In these fluvial deposits, lacustrine deposits (a few tens of meters thick) are intercalated at some localities. Calcic and vertic paleosols are common in the floodplain deposits composed of mostly purple to red or greenish grey beds. In study area lacustrine deposits without red beds occur between fluvial deposits including red beds. In the lacustrine deposits, gastropod and pelecypod shells are intermittently concentrated, and some vertebrate bone and carbonized wood fragments are contained. These lacustrine deposits are classified into 7 sedimentary facies and their depositional environments are interpreted as follows: (1) Dunoid medium sandstone facies- sandflat; (2) Graded fine sandstone facies- sandflat; (3) Planar- to cross-laminated fine sandstone facies- sandflat; (4) Disrupted silty mudstone facies- mudflat; (5) Calcareous sandy to silty mudstone facies- mudflat; (6) Interlaminated to very thinly interbedded fine sandstone to siltstone and mudstone facies- sandflat to mudflat; (7) Calcareous shaley silty mudstone- mudflat to shallow lake. The shell deposits in this study area are divided into three types based on lithology, mode of shell occurrence and associated fossils. Type 1 shell deposits occur in interlaminated to very thinly interbedded fine sandstone to siltstone and mudstone facies. In Type 1 shell deposits, mostly gastropod shells and some pelecypod shells occur in fine sandstone to siltstone with sharply erosive lower contacts as beds or lenses in thickness of 0.5 to 10 cm . The gastropod shells are Brotiopsis wakinoensis. They occur as whole ones, breakage ones, and some fragments. These concentrations are composed of moderately to highly abraded shells aligned with bedding without bioerosion. This indicates that the fauna is thanatocoenosis due to subaerial exposure and drying, and then shells were transported from the inhabitation (parautochthonous) when sheet flooding occurred during wet period. Type 2 shell deposits occur in the graded fine sandstone with erosive lower bedding contacts, and gastropod and pelecypod shells occur together forming lenses in thickness of 5 to 10 cm. The gastropod shells are Brotiopsis wakinoensis in mostly breakage ones, fragments and some whole ones. Pelecypod shells are articulate or disarticulate, and randomly oriented. They occur as highly abraded shells without bioerosion. This indicates that shells were transported from the inhabitation(parautochthonous) when sheet flooding occurred during wet period. Hydraulic energy for type 2 shell concentration is deemed to have been higher than that of Type 1 shell deposits. Type 3 shell deposits occur in the calcareous shaley silty mudstone without sharp erosional bases, and contain gastropd shells. The gastropod shells occur as beds or patches and totally about 40 layers of the shell concentrations in thickness of 0.5 to 2 cm are observed in the 3 m thick sequence. The shells are exclusively Brotiopsis wakinoensis in whole ones, breakage ones and some fragments. Some shells show preferred orientations and minimal damage by bioerosion and abrasion. This indicates that shells were preserved in life position (autochthonous). The study area might have been mudflat and sandflat which was located in floodplain. The floodplain lake is deemed to have been shallow and temporary one that was affected by sheet flooding. Gastropods and pelecypods inhabited floodplain lake. The taphonpmic types of the shell deposits might have been influenced by the paleoclimatic condition. The paleoclimate during the taphonomic processes is interpreted that wetting and drying periods were alternated under generally warm and arid climate.