Abstract

The Oligocene strata of northern Taiwan originally formed part of the sedimentary cover of the China continental margin prior to being deformed and exposed by the collision orogeny in the Pliocene-Quaternary. These strata consist of interbedded sandstones and mudstones deposited in a storm and wave dominated shallow marine environment. On the basis of lithology, sedimentary structures, and paleontological data, five depositional facies, namely the outer-shelf, middle-shelf, inner-shelf, lower shoreface, and upper shoreface, are distinguished. These facies are usually stacked in coarsening and shoaling-upward successions and can be grouped into five parasequence types. Each parasequence reflects the sedimentary response to the high-frequency sea-level changes in various parts of a ramp-type continental shelf. In terms of the sequential changes in parasequence type and thickness, the parasequences can be further stacked into progradational, retrogradational and aggradational parasequence sets. Associated with other characteristic features, such as the bioturbation intensity, fossil abundance, and the presence of calcareous concretions and glauconite pellets, the parasequence sets help delineate the sequence boundaries, transgressive surfaces, and maximum-flooding surfaces in each studied section. These boundaries and surfaces are correlated among different sections and used to break the Oligocene strata into six sequences. This sequence stratigraphic scheme provides an integrated model of the facies architecture of the Oligocene strata and clearly defines the nature of each systems tract.

By comparing the Oligocene sequence stratigraphy of northern Taiwan with the global sea-level chart, it is interesting to note that the Oligocene sea-level fluctuations of northern Taiwan are generally consistent with the third-order changes on the Vail curve. This consistency suggests that the Oligocene deposition in northern Taiwan was highly affected by eustatic sea-level changes. The major difference lies on the amplitude of the fluctuations in the lower and middle Oligocene. The inconsistency is most likely related to tectonic basin subsidence associated with rifting of the China continental margin during Early Oligocene time.