An Ancient Estuary Deposit in Gemudi Guizhuo Province, China

Abstract: Estuarme depositional environments have been seldom recognized from antient deposits. The deposits of Changxing Age, Late Permian in Gemudi were deposited in estuarine-tidal depositional environments. 325 core holes and several outcrops, in an area of 325 square kilometre, were studied in this paper. The ancient estuarine depositional environments are reconstructed through the following aspects. Is It is explained by microscopic study that the clastic material is transported from western Kongdian Rise. The water body at Gemudi are desalinated by river discharges, which are confirmed by lower contents of boron in mudstones and the brackish-water fauna that is characterized by small body and thinner shall, such as chonetessp. lingulasp. and some fragments of marine fauna. 2) Tidal sedimentation dominate the main depositional prosesses. The paticuiar combination of sedimentary structures which often occur in modern tidal environments, such as the flaser bedding, lenticular bedding, the double mudbeds and the chevron cross-bedding. 3) According to the regional setting, Gemudi is sited in the mouth of an ancient river that empted into the sea from the west. The evidences of land runoff can be found in deposits, such as the large trunks. 4) The vertical tidal flat deposits, bay fine-grained clastic and carbonate deposits, are similar to the modern estuarine sequences. 5) From the depositional dip direction, the sandstone contents decrease at first, then rise again, and the marine deposits especially the limestone increase. 6) It is found from the isopach maps of sandstones that the tidal channel are developed in the western and the tidal ridges and bay deposits are dominated in the eastern part. That the tidal energy is stronger than the fluvial energy is the major cause of estuary developing, in Gemudi. The relative strength between basin reworking and fluvial discharge is prerequisite for estuary or delta environment developing where an river empted into sea. Three subfacies can be recognized in Geniudi estuary, they are marginal tidal flat (including tidal channels and tidal flats), bay and tidal ridge. In a word, ancient esluarine deposits can be recognized by the analysis of paleosaiinity, evidences of tidal sedimentation, land runoff and regional settings.