Abstract:

The Tuchengzi Formation is the most widely distributed deposits of the Late Jurassic to the Early Cretaceous in YinshanYanshan in North China, which contains a lot of information on paleoclimate, paleogeography, basin and tectonic evolution. Vast distribution and a variety of sediments of the Tuchengzi Formation, however, restrain the interpretations of basin nature and tectonic background, and there are plenty of controversies about the Tuchengzi Formation as well. This paper has briefly reviewed those previous studies of the Tuchengzi Formation in YinshanYanshan. Furthermore, on the basis of geological survey, the paper focuses on the stratigraphy, sedimentary facies, provenances and paleocurrents of the Tuchengzi Formation. Current study indicates that the Tuchengzi Formation was involved in an evolution of sedimentary facies in spacetime, from alluvial fan to delta, lacustrine and river or alluvial fan in ascending order. Palaeo depositional centres of the Tuchengzi Formation were recognized to be located in BaotouHuhehaote of Inner Monggolia, Yanqing of Beijing and Lingyuan of western Liaoning Province respectively, which are related to the distribution of paleocurrents and prove the existence of a giant lacustrine basin in YinshanYanshan, extending from Daqingshan, Northern Hebei and Western Liaoning Province in the Late JurassicEarly Cretaceous. Provenances analysis of conglomerates in the Tuchengzi Formation shows obvious zonations. Pebbles of volcanic and sedimentary rocks are mainly in Western Liaoning Province; pebbles of volcanic rocks and some granite and limestone dominate Northern Hebei Province; and pebbles of metamorphic rocks are chiefly in Daqingshan. In term of regional distribution of basements of the Tuchengzi Formation, provenances of this formation display a good concordance with diverse basements of the Tuchengzi Formation in YinshanYanshan, and furthermore suggest that the Tuchengzi Formation were deposited by an erosion of basements in descending order. The widely distributed large scale platetrough crossbeddings within the top part of the Tuchegnzi Formation in Western Liaoning Province and Hebei Province are interpreted as aeolian deposits resulting from basin's basement uplift of the Tuchengzi Formation. Metamorphic core complex in YinshanYanshan or nearby areas in the Late JurassicEarly Cretaceous previously reported suggests an extensional tectonic background at that time, and it is probable that the coupling of lithospheric thinning was impacted mainly by Pacific plate subduction northwestward in the Late JurassicEarly Cretaceous. This research suggests that the Tuchengzi Formation is the sediments of rift basin and deposited in an extensional tectonic background of the Late JurassicEarly Cretaceous. A new dynamic model of the evolution of basins of Tuchengzi Formation was proposed finally.

Key words: YinshanYanshan, the Late JurassicEarly Cretaceous;Tuchengzi Formation, facies, provenance, paleocurrent, uplift, intracontinental rift basin