Well-exposed Oligocene strata of the Hsuehshan Range in Taiwan were deposited in the Hsuehshan Trough along the eastern Eurasia continental margin. The Hsuehshan Trough was a restricted epeiric basin confined by a series of horsts. The Oligocene succession records a deepening-upward to shallowing-upward sequence of deltaic deposystems associated with fluvial and shallow marine environments. These environments ranged from wavedominated delta-plain through storm-dominated delta-slope to regressive tidedominated delta-front environments. The fining-upward to coarsening-upward sequence can be divided into five lithofacies and five ichnocoenoses. Ichnocoenoses characterized by Ophiomorpha, Cylindrichnus, Teichichnus, Scolicia, and Zoophycos were defined on the basis of assemblages, degree of bioturbation, diversity, and abundance.
The Lower Oligocene deposits exhibit a fining-upward trend and upward increases trend in bioturbation and abundance of deposit-feeding structures. Accompanying these changes is a transition from Ophiomorpha, Cylindrichnus, Teichichnus, Scolicia to Zoophycos ichnocoenoses, which also suggests a transgressive system from delta-plain to prodelta-slope environment. The sequential variation in lithofacies and ichnofossil assemblages coincides with changes in Early Oligocene sea level. The Upper Oligocene deposits show a shallowing-upward succession that suggests an evolution from a storm-dominated prodelta slope to a tide-dominated delta plain. The ichnofossil assemblages show a transition from Scolicia, Teichichnus to Ophiomorpha ichnocoenoses that are suggestive of regressive systems. The sequential trends documented in the Late Oligocene deposits of northeastern Taiwan are not consistent with global sea-level fluctuations and therefore, tectonic processes are invoked to have controlled local basin conditions. In general, the sequential variations documented in the Oligocene strata probably were controlled by paleoceanographic conditions that were closely linked to tectonic processes and longer-term, first -order sea-level fluctuations.