Definition of the Pliocene Guge and Toling formations and sequence stratigraphic division in the Zanda basin, Tibet, China

Based on the contact relationships, sedimentary cycles, sedimentary systems and lithofacies and lithologic characteristics of the stratigraphic section, the Pliocene fluvio-lacustrine sediments in Zanda basin are divided into 2 formations, 5 sedimentary facies and 8 lithologic members. The first lithologic member of the Guge Formation consists of conglomerate, belonging to fluvial deposits; the second, third and fifth members are dominated by fine clastic rocks, belonging to shallow lacustrine deposits; and the fourth member consists predominantly of clayey fine clastic rocks, belonging to deep lacustrine sediments. The first, second and third members of the Toling Formation consist of a sequence of clastic rocks including conglomerate, pebbly and coarse- and medium-grained, calcareous feldspathic litharenite and medium and fine sandstone, belonging to typical alluvial (diluvial) fan deposits. ESR and paleomagnetic dating indicate that the age of the fluvio-lacustrine sediments of the Guge Formation is late Pliocene (4.40-2.47 Ma) and the age of the Toling Formation is early Pliocene (5.44-4.40 Ma) in the Zanda basin. The Zanda basin is a syngenetic downfaulted basin. Its evolution progressed through three stages: the transgressive systems tract, lake expansion systems tract and lake shrinkage systems tract during the Pliocene tectonic movement.