Analysis For Depositional Facies, Palecurrent And Provenance Of The Chang 8 And Chang 6 Member In Xifeng-Heshui Region, Ordos Basin

Depositional systems and facies, paleocurrent directions and provenanceof the Chang 6 and Chang 8 Members of Yanchang Formation, Upper Triassic in Xifeng-Heshui region from the Ordos basin are discussed, and depositional model, temporal and spatial distribution of the sedimentary facies are revealed in this paper, based on field work of geological survey and measurements of outcrops in the peripheral area of the Ordos basin, observation and description of 72 cores, analyses of drilling and logging data, observation and identification of thin-sections by point counting, measurement of light and heavy minerals in sandstones, analysis of trace element and rare-earth elements of mudstones and clay minerals of sandstones. Many new theories and technological methods related to sedimentology, petroleum geology, lithofacies and paleogeography, geophysical and geochemistry are used in the paper.Sedimentary model was built by the analysis of depositional systems and facies. Fan-delta front sub-facies is exit in the western of the study area and braid river deltas front sub-facies is exit in the south in chang8 periods of late Triassic. There are braid river deltas front sub-facies in the chang6 periods, in the west and southwest of the study area.Paleocurrent and provenance research result suggests that major paleocurrent direction was from the southwest to northeast and from south to north during the Chang 8 and Chang 6 Member of Yanchang Formation, Upper Triassic deposited. The provenance from the southwest for the Chang 8 and Chang 6 Members was originated from the Longshan Group of the Middle Proterozoic, the Liziyuan Group and Huluhe Group of the Sinian-Qrdovician System in southwest of the Ordos basin. The provenance in south and southeast came from the Permian-Triassic clastic rocks and Sinian-Qrdovician limestones and dolomites. In addition, the Taihua Group in basement of the North China Plate may contribute to some part of the provenance.There were some differences existed in the Chang 6 period and in the Chang 8 period. As Qinling orogenic belt has been uplifting during Chang 6 period sincePermian, south and southeast provenance came from the Permian-Triassic clastic rocks and Sinian-Qrdovician carbonate rocks injected to the study area and their influences to the provenance became more and more obvious.