Provenance interpretation of Tertiary sandstones from the Cheju Basin (NE East China Sea): a comparison of conventional petrographic and scanning cathodoluminescence techniques

Abstract

Cheju Basin is a Tertiary intracratonic basin filled with largely nonmarine sediments and is located SW of the Korean Peninsula beneath the NE East China Sea. Commercial petroleum companies drilled 10 deep wells in this basin between the late 1970s and the 1990s. We obtained samples from four representative wells for provenance analysis, using both conventional petrographic techniques and scanning cathodoluminescence (SEM-CL) imaging of quartz. This paper examines differences in provenance interpretation arising from the use of two different analytical techniques. On the basis of framework mineralogy, quartz undulosity and polycrystallinity, and feldspar compositions, we interpret that Cheju Basin sediments were derived from a mixed source rock terrane. Petrographic data suggest that plutonic source rocks were predominant, metamorphic rocks less abundant, and volcanic and sedimentary least important. SEM-CL imaging of quartz from nine of the same samples examined petrographically yielded a different provenance interpretation. SEM-CL results indicate that metamorphic quartz greatly predominates over plutonic quartz in all samples. This discrepancy in interpretation probably stems from the difficulty in distinguishing between plutonic and metamorphic quartz by petrographic methods. SEM-CL analysis shows that volcanic quartz is absent in many samples, even in samples that contain modest amounts of volcanic rock fragments, probably reflecting the generally low content of quartz in many volcanic source rocks. The SEM-CL method cannot directly detect sedimentary (recycled) quartz. Thus, neither petrographic analysis of sandstones nor SEM-CL analysis of quartz may provide completely accurate interpretation of provenance, implying that an integrated approach to provenance analysis is most reliable. Our data suggest that the Cheju Basin areas may have been connected by land to a sediment source on the Korean Peninsula during Oligocene to Miocene time. Alternatively, or additionally, a land source may have existed to the northwest along a narrow massif or suture belt now inundated by the East China Sea. Regional subsidence in Pliocene time initiated the onset of marine conditions in the Cheju Basin; however, one or more of the same land sources continued to furnish sediments to the basin.