A quantitative model for predicting sedimentary-system apexes based on modern fluvial grain-size data and its application to paleogeographic reconstruction

Abstract

Determining the location of the apex of a sedimentary system can provide important spatial constraints on paleogeographic reconstructions. To solve the difficult problem of predicting the apex of subsurface sedimentary systems, this study establishes a quantitative mathematical model of grain size with distance from the apex of the modern sedimentary system by statistically analyzing the grain size of gravel in the riverbed of the Shule River. To evaluate the accuracy of the mathematical model, the research takes the Golmud River depositional system in the Qaidam Basin and the Dinar River depositional system in the Tarim Basin as examples, where the position of each actual apex is known for the two systems. The estimated apex position in the Golmud River depositional system is 4.79 km from the actual apex position, with an accuracy rate of 95.3%. In the Dinar River depositional system, the estimated apex position is 3.88 km from the actual apex position, with an accuracy rate of 94.8%. Based on the testing of these two examples, the results of the evaluation largely verify the accuracy of the mathematical model. The mathematical model was applied to the geological record. The research takes the Jurassic Karazar Formation in the southern margin of the Junggar Basin in western China as the target. Based on the mathematical model and combines the conglomerate grain size from the Nananjihai River outcrop of the Karazar Formation and conglomerate grain size from the core of Tianan 1 well, and completes the prediction of apex position of the sedimentary system of the Karazar Formation. Results have helped to give better geographical constraints on the system and apex location of the southern margin in the Junggar basin.