Abstract <p>For the first time, a fragment of an early Paleozoic residual basin has been identified in the southern folded framing of the Siberian Platform. This basin was responsible for the erosion of clastic rocks that gave rise to both the Siberian Platform and the terranes of the Central Asian Fold Belt. The obtained isotope-geochronological and geochemical data on the composition and age of metasedimentary rocks are presented in a review of the junction of two major structures of the fold belt—the Tuva-Mongolian microcontinent and the Khamsara Terrane. It has been shown that the studied metasedimentary sequence consists of graywacke that formed in Late Cambrian time, asynchronously with the Vendian–Cambrian sedimentary cover of the microcontinent, due to Neoproterozoic island-arc sources and the disintegration of rocks from an ancient continental block. The metasedimentary sequences are part of a residual basin located between the island-arc tectonic blocks and the Siberian Platform, independent of the sedimentary basins of the Tuva-Mongolian microcontinent.</p>