Jurassic-Cretaceous Boundary Strata In Southern Tibet

The boundary between Jurassic and Cretaceous is the most problematical of all Phanerozoicsystem boundaries, have long been debated and not yet been resolved on a global scale. Themarine boundary strata outcrop in southern Tibet, with controversy on the boundary horizon.This work studies five sections from Gyangze and Nagarze areas. The Linxi Section of Nagarzethat contains the uppermost Jurassic to Lower Cretaceous strata is a new section studied bypresent work. Many samples were collected in the field. Studies on the lithostratigraphy,biostratigraphy, sequence stratigraphic and isotope chronologic stratigraphy have been done. Thepresent work is mainly focus on the paleontology and sedimentology, as well as the lithology.The biostratigraphy is mostly besed on the calcareous nannofossils, ammonites and bivalvesfrom the boundary strata. The sedimentary research includes the lithologic and sedimentaryenvironments analyses. The following results have been achieved:1. Abundant calcareous nannofossils have been found from the Jurassic-Cretaceousboundary strata. The age is from Berriasian to Valanginian of early Cretaceous. Bybiostratigraphic correlation , the Sangxiu formation in Nagarze is correspond to the lower Jabulaformation in Gyangze.2.According to the biostratigraphic correlation of calcareous nannofossils, ammonites andbivalves, as well as lithologic stratigraphy, the Jurassic-Cretaceous boundary has beenrecognized. The age of Weimei Formation in the Gyangze and Nagarze areas is Tithonian ofuppermost Jurassic;the Jiabula Formation in Gyangze and the lower Sangxiu Formation inNagarze belong to Lower Cretaceous. The Jurassic/Cretaceous boundary, therefore, is betweenthe Sangxiu and Weimei formations in Nagarze and between the Jabula and Weimei formationsin Gyangze. The boundary is marked by the appearance of Nannoconus steinmannii minor,Spiticeras, and the disappear of Virgatosphinctes, Aulacosphinctes.3.The study of the zircon SHRIMP chronology of andesite from the upper SangxiuFormation gives age of 136±3.0Ma to the volcanic rock of Kadong section, which is limited toearlymost Hauterivian. The age of the volcanic rock in Rimowa section is 133±3.0Ma andbelongs to middle Hauterivian. Therefore, the age of the volcanic rock in upper Sangxiu Formationis early-middle Hauterivian. Therefore, the shale of lower Sangxiu Formation isBerriasian-Valanginian in age. The general age of Sangxiu Formation is from Berriasian toHauterivian. The Jurassic/Cretaceous boundary is at the bottom of Sangxiu Formation. Theisotope age of the upper Sangxiu Formation is 133-136Ma, with the underneath 200m thick shalebeds of the lower Sangxiu Formation, we estimate that the age of the bottom of SangxiuFormation is correspond with 145.5Ma of the global Jurassic/Cretaceous standard boundaryin the international geological time scale.4. According to the sequence stratigraphic theory, the Late Jurassic-Early Cretaceous stratain Kadong section have been divided into 4 three-order sequences. Based on the sequencestratigraphy study, the environment of studied area changed from high water level to lower waterlevel during Late Jurassic-Early Cretaceous. The sea-level change of this area is consistent withthe globe sea-level change curve at the same period, which is useful to recognizeJurassic/Cretaceous boundary.5. The Late Jurassic-Early Cretaceous sedimentary facies and environments have beenstudied. Three sedimentary zones of near shore clastic, shallow sea clastic and hypo-deep seazones have been recognized. And nine sediment fascias have beensubdivided. TheJurassic/Cretaceous boundary is at the top of sandstone of Weimei Formation, which representsa regression during the Jurassic and Cretaceous boundary time in the Tibet-Tethys Sea, which shows samefeature as the global sea level change.