Bioturbation structures and paleogeography of mixed deposits of the Lower Ordovician Sandaokan Formation, Ordos basin

Abstract：For the first time trace fossils have been discovered in the Lower Ordovician Sandaokan Formation in the Zhuozishan area at the northwestern margin of the Ordos basin. Most of them are bioturbation structures whose ichnogenera and ichnospecies cannot be identified. The Sandaokan Formation comprises mixed deposits composed of alternating beds of carbonate rocks and terrigenous clastic rocks. It is divided into five members according to the lithology and rhythmicity. The distributions and characteristics of the bioturbation structures and the sedimentary environments where they were formed are analyzed and summarized in this paper. The paleoenvironments during deposition of the formation are mainly foreshore sand beaches, sandbars, dolomite flats, sandy dolomite flats, low-energy subtidal restricted platforms, coquina banks and subtidal open platforms. The characteristics of each paleogeographic unit and paleoenvironments in the depositional period of each member are expounded by the sedimentological and body fossil study, combined with the semi-quantitative analysis of bioturbation structures. It is suggested that the paleoenvironment in the early depositional period of the Sandaokan Formation was successively mainly marked by sandy dolomite flats and low-energy subtidal restricted platforms, while the subtidal open platform environment appeared repeatedly and predominated sometimes in the middle and late depositional periods of the formation, which reflects the general trend of sea-level rise. Meanwhile in the background of transgression, foreshore sand beaches, sandbars and dolomite flats, sandy dolomite flats or subtidal open platforms appeared alternately, which reflects that the Zhuozishan area was near an old land during the Middle Arenigian age. With the periodical rise of the old land or the sea level, transgression and regression and land and sea change occurred frequently; as a result, mixed deposits were developed. Study of the attitudes of the fossil orthocone nautiloids indicate that the paleo-currents were from north to south during the deposition of the Sandaokan Formation. According to analyses of the lithology and fossils, the climate was dry and hot during the deposition of the formation. The Zhuozishan area should be classed as an Early Ordovician North China subtropical dry climatic province.