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

Abstract

To reveal the development characteristics and distribution of gravity flow sedimentary system under micro-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 deposits 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.

Keywords

• gravity flow deposit
• slope break belt
• paleogeomorphology
• channel-lobe complex
• Triassic Yanchang Formation
• Ordos Basin