Precambrian and Cambrian rocks are often separated by the “Great Unconformity”, a global contact that can represent a gap spanning hundreds of millions of years of time. A coeval unconformity in the Sino-Korean Block has recently been documented from several different areas of China; however, the stratigraphic position of this boundary in the eastern margin of the Sino-Korean Block in the Taebaek Group, central-eastern Korea, is still elusive. We present an integrated approach incorporating detailed field observations, sedimentary petrography, and detrital zircon geochronology on the basal stratigraphic units of the Taebaek Group (Jangsan, Myeonsan, and Myobong formations) to resolve this issue. In contrast to the traditional view that all three formations are of early Cambrian age, that the Myeonsan Formation is a lateral equivalent of the Jangsan Formation, and that the Myobong Formation conformably overlies the other two formations, our data indicate that the Myeonsan Formation can be a lateral equivalent of the basal Myobong Formation. We propose that the Great Unconformity possibly lies between the Jangsan and Myeonsan/Myobong formations: the Jangsan Formation is probably of Precambrian age, and the Myeonsan and Myobong formations are early Cambrian in age. This result suggests that the Great Unconformity on the Sino-Korean Block extends to the eastern margin of the block in the Korean Peninsula. The recognition of the Great Unconformity in the Taebaek Group will help to clarify the stratigraphic correlation, tectonic history, and paleogeographic reconstruction of the eastern Sino-Korean Block during the Cambrian.