The disappearance of the Late Cretaceous Bangong-Nujiang residual seaway constrained by youngest marine strata in Geji area, Lhasa Terrane

Abstract

For a long time, the key scientific issues regarding to when and how did the Bangong-Nujiang residual seaway disappear are still controversia. This paper focuses on the Late Cretaceous sedimentary strata in the Geji area by studying the stratigraphy, lithofacies and sedimentary facies. This stratigraphic unit, named here Tangza Formation was overlied unconformablely by the Jingzhushan Formation of continental conglomerates. The Tangza Formation represents the youngest marine strata both in the Lhasa terrane and Bangong-Nujiang suture zone. According to the sedimentary facies, the Tangza Formation can be divided into three members, the lower member comprises predominantly mudstone, grayishgreen to fuchsia siltstone, lenticular conglomerate beds and sandstone in which parallel bedding and massive structure are commonly developed whereas cross-bedding is not well developed. The middle member mainly comprises grayish-green orbitolina limestone, marlstone, hybrid beds, grey-green shale with small laminated aggregates sandstone and siltstone. The upper member comprises purple-red pebble coarse sandstone with laminated aggregates conglomerate and sandstone in which massive structure and parallel bedding are developed. The sedimentary facies analysis of the Tangza Formation shows a transition from fan delta plain subfacies to front fan delta subfacies in the early stage, which indicates the deepening of palaeo-water depth, followed by fan-delta subfacies, which indicates shallowing of palaeo-water depth. The foraminifera fossils constrained an age of Cenomanian stage (101-94 Ma) for the depositional age of the Tangza Formation. The detrital composition shows that the lower member of the Tangza Formation is dominated by volcanic lithic fragments. The content of feldspar in the upper member decreased to 7%, and lithic fragments increased to 53%, and the proportion of sedimentary rocks increased significantly. Detrital zircons from the lower member of the Tangza Formation yield a primarily age population of 101-163 Ma (peaking at ~110 Ma), with εHf(t) values (-7.5 to +15) and additional age ranges of 424-525, 713-980, 1812-2113 and 2425-2550 Ma. The upper member of the Tangza Formation yield a primarily age population of 101-127 Ma (peaking at 110 Ma), with εHf(t) values (-6 to +10) and additional age ranges of 147-166, 211-271, 509-680, 795-1056, 1816-1879 and 2360-2513 Ma. Detrital composition, detrital zircon geochronology and Hf isotopes altogether show a significant provenance change within the Tangza Formation, from the north Lhasa Terrane and Bangong-Nujiang Suture Zone to Langshan Formation and middle Lhasa Terrane. Combined with regional paleogeography analysis, the disappearance of the Bangong-Nujiang residual seaway in the Geji area where the Tangza Formation was found occurred ca. 94 Ma. Furthmore, the residual seaway was not retreating from the east to the west, but contemporaneously retreats from the north-central Lhasa Terrane.