Sedimentary characteristics and model of gravity flow channel-lobe complex in a depression lake basin: A case study of Chang 7 Member of Triassic Yanchang Formation in southwestern Ordos Basin, NW China

To reveal the development characteristics and distribution of gravity flow sedimentary system under mi-cro-paleogeomorphologic units of the Chang 7 Member of Triassic Yanchang Formation in the southwestern Ordos Basin, on the basis of the restoration of the paleogeomorphological form of the Chang 7 depositional period by the impression method, each micro-paleogeomorphologic unit was depicted in-depth, and the characteristics and development models of gravity flow de-posits in the study area were studied in combination with outcrop, core, mud logging and log data. The results show that: (1) The paleogeomorphology in the Chang 7 depositional period was an asymmetrical depression, wide and gentle in the northeast and steep and narrow in the southwest. Three sub-paleogeomorphologic units were developed in the basin, including gentle paleo-slope, paleo-slope and paleo-depression, and they can be further subdivided into eight micro-paleogeomorphologic units: bulge, groove, slope break belt, plain of lake bottom, deep depression of lake bottom, paleo-channel, paleo-ridge of lake bottom, and paleo-uplift of lake bottom. (2) There are 9 types of lithofacies and 4 types of lithofacies assemblages of Chang 7 Member. According to lithofacies composition and lithofacies vertical combination, the gravity flow deposit is further divided into 5 types of microfacies: restricted channel, unrestricted channel, natural levee, inter-channel, lobe. (3) Paleogeomorphology plays an important role in controlling sediment source direction, type and spatial distribution of sedimentary microfacies, genetic types and distribution of sand bodies in Chang 7 Member.

Automated summary

This study reveals the development characteristics and distribution of gravity flow sedimentary system in the southwestern Ordos Basin during the Triassic Yanchang Formation. The results show that the paleogeomorphology of the Chang 7 depositional period was an asymmetrical depression, and various lithofacies and microfacies were identified.