Evolution of Pliocene to Pleistocene sedimentary environments in an arc-continent collision zone: evidence from the analyses of lithofacies and ichnofacies in the southwestern foothills of Taiwan

Abstract

Lithofacies and ichnofacies analyses reveal an excellent example of gradually subsiding and rapid infilling and uplifting of the sedimentary basin in the southwestern foothills of Taiwan, along the strike of the active Plio-Pleistocene orogeny. Ten well-exposed sections, roughly evenly distributed along a southwestern transect, were studied to construct the three-dimensional distribution of lithofacies and ichnofacies. Basin evolution is discussed in terms of temporal and spatial facies variations. In early Pliocene time, the shallow marine facies of the northern part of the study area were deposited on the passive Eurasian continental shelf. The sedimentary basin was deeper in the south, with a lower slope and/or basin floor setting. The ongoing arc-continent collision caused flexural bending of the crust, inducing gradual subsidence of the basin during Pliocene and early Pleistocene time. Around 0.98 Ma, the onset of more rapid orogenic movements accelerated the basin subsidence, as indicated by rapid deepening of the facies northward along the basin axis, as well as by the widespread development of mass-gravity transportation deposits and channel cutting structures. However, the basin was soon infilled and uplifted as part of the orogen within 0.8 my. Basin subsidence is much greater than previously indicated.