Abstract:

The core issue of taphonomy is to study the sedimentary facies of fossilbearing rocks, reconstruct their depositional environments, and understand the paleogeographical background before and after the burying of fossils at different scales. One of current taphonomy interests focuses on the mass extinction or bonebeds of dinosaurs, including their paleogeography, paleoenvironment and preservation. Numerous Jurassic and Cretaceous dinosaur fossil sites have been discovered in China, making it number 1 in the world in terms of richness of dinosaur genera and species. The terrestrial Cretaceous Laiyang, Qingshan and Wangshi groups (13065 Ma), in ascending order, are widely distributed in Jiaolai Basin of eastern Shandong Province, China. Lithologically, the Early Cretaceous Laiyang Group consists of fluvial and lacustrine sediments and the Early Cretaceous Qingshan Group is mainly composed of medium or acid volcanic rocks and pyroclastic rocks interbeded with sedimentary rocks. The Late Cretaceous Wangshi Group is mainly deposits of alluvial fan, mud flow and braidedchannel facies in the lower part; shallow lacustrine deposits and rhythmic fluvial sediments of mudstone, siltstone, sandstone or soils (palesoil) in the middle part; and rhythmic depositions of siltymuddy conglomerate, sandstone and siltstone of mud flow, braidedchannel and flooding plain facies, locally interbeded with basalt in the upper part. Sedimentary successions and facies associations of the Cretaceous Jiaolai Basin indicate an evolution of basin and sedimentary paleogeography, i.e., from alluviallacustrine environment in humid and warm climate in the Early Cretaceous to an alluvial environment in hot and drought climate towards the Late Cretaceous. Biota of the Early Cretaceous in Jiaolai Basin is identical to the Jehol Biota in the northern areas of North China. In the Laiyang Group, a plentiful of dinosaur (theropod, sauropod and ornithopod) footprints were excellently preserved, as well as a few small dinosaurs (Psittacosaurus) or single bones of pterosaurs. A number of dinosaur bonebeds or mass extinction appeared in the middle or upper part of the Late Cretaceous Wangshi Group in Zhucheng. Until now, over ten genera and species of dinosaurs, including three species of horned dinosaurs, three species of tyrannosaurs, two species of hadrosaurs, and some specimens of ankylosaur and coelurosaurs have been unearthed. Among them, hadrosaurs account for the overwhelming majority (at least more than 95%). Most of dinosaur bones in mud flow deposits are disarticulated, only with a few articulated. Varioussized bones were mixed in and rare sorting. Bone condition variation is believed to represent deposits of mixture of allochthonous faunal elements by geological events. Three types of taphonomic sedimentary microfacies are recognized: mud flow, flooding plain and braidedchannel. Mud flow deposits are major taphonomic occurrence among these three taphonomic microfacies, indicating a preferential preservation of dinosaur material, which are believed to represent the end product of mass burying of dinosaurs and dynamic mechanism through which bones were gathered and accumulated in the Late Cretaceous. The top Wangshi Group in Laiyang mainly consists of the rhythmic depositions of purple red conglomerate, sandstone, siltstone and soils interpreted as flooding plain and braidedchannel microfacies respectively, indicating a prevailing hot and drought climate at the end of the Late Cretaceous. All bones are, here, disarticulated and rare articulated, isolated and single, highly abraded and preserved within flooding plain microfacies under large scale braided channel sandstones. Bone condition variation is believed to represent redeposited after a longer transportation by braidedchannels and alterations before or after burying. Furthermore, evolution of Jiaolai Basin and its paleogeographic environment, paleoecology and dinosaur fauna in the Cretaceous are briefly discussed as well.

Key words: taphonomy, Cretaceous, Jiaolai Basin, dinosaur, sedimentary facies, paleogeography