DIAGENESIS OF PLIOCENE TUNGHO AND KANGKOU LIMESTONES IN THE COASTAL RANGE, EASTERN TAIWAN

The Coastal Range of eastern Taiwan is the northern extension of the Luzon Arc system, which collided with and hence was partly accreted to the Asian Continental margin in the Pliocene. Two limestone units (Kangkou and Tungho Formations) were deposited atop the volcaniclastic Tuluanshan Formation when volcanism gradually declined, resulting from the arc-continent collision.

The limestone ranges from 5 to 50 m in thickness and are composed of coral fragments, planktonic and larger benthic foraminifers, algae, and rhodoliths. Overlying the limestone is a thick mudstone unit containing deep-marine fossils, indicating a rapid arc collapse (also related to the arc-continent collision) after the limestone sedimentation. Thus the Tungho and Kangkou limestones have experienced shallow marine, deep marine, deep burial, and meteoric environments.

The limestones are commonly bored, with boring passing through the bioclasts and internal sediments, which were subsequently filled with cement or calcareous mud. Fibrous and bladed cements characteristic of marine condition is common. Pressure solutions are developed mostly along boundaries between the rhodoliths and the matrix, coral and internal sediments, and along the burrow tracks. In addition, molds and voids, created by differential solution within the meteoric environment, were later filled with equigranular cements.

Dolomite of replacement origin is found in the limestones. Dolomitization took place after the limestone experienced shallow-marine cementation and meteoric diagenesis, but before deep-burial. Isotope analysis indicates that the liquid responsible for the dolomitization is normal seawater, or a mixture with small amount (<25%) freshwater.

The Tungho Limestone was deposited mostly in fore-reef, back-reef, and reef environments, whereas some of the Kangkou Limestone was formed in deeper water, such as fore-reef slope and shelf rim conditions. Dolomitization is found only in the Tungho Limestone and the shallow-water facies of the Kangkou Limestone, indicating that fluctuation of global sea level (caused exposure of the limestones) might play a role in the dolomitization.