Sequence stratigraphy of Jurassic-Cretaceous boundary strata in Luanping, Northern Hebei, China

摘要

A sediment-continuous section of Jurassic-Cretaceous (J-K) boundary strata was found in Zhangjiagou of Luanping, northern Hebei Province, China. The boundary strata were systematically divided into 5 sequences, 13 subsequences and 139 parasequences by sedimentary cyclicities and facies analysis. According to stable time-ranges of subsequences and parasequences, some boundary ages can be marked as 142.90 Ma of the SQ Ⅰ (Dabeigou Fm) basal bound , 141.16 Ma of the SQ Ⅱ basal bound, 140.00 Ma of the SQ Ⅲ (Dadianzi Fm) basal bound, 138.24 Ma of the SQ Ⅵ (Zhangjiagou Fm) basal bound and 135.34 Ma of the SQ Ⅴ (Zhangjiagou Fm) top-bound. A sequence-stratigraphic framework of the Dabeigou- Zhangjiagou Fms has been constructed by tracing sequence-bounds, marker-beds and fossil zones. The basin-development and tectonic activities are studied based on sequence-stratigraphic evidence. 5 sequences represent 5 tectonic cycles and all the 6 sequence-bounds reflect tectonic activities. Tectonic activities clearly influence the basin-development that can be divided into 3 stages in the J-K transitional period. Each stage consists of a down-faulting-rising process, i.e. from strongly rising of basin-bottom, then violently faulting down (with volcanic eruptions) and to gently rising. The development shows a pattern of bottom down-faulting with basin narrowing and rising with spreading.