Depositional System Analysis Of Early Cretaceous Chengzihe-Muling Formation In Jixi Basin,Northeastern China

Started with the previous research achievements, guided by sedimentological and seismic stratigraphy theory, supported by abundant outcrop data and detailed core observation, combined with electrofacies and seismic facies analysis as well as paleocurrent directions and heavy mineral analysis, this paper systematically studied the sedimentary facies types, the provenance directions and the allocation relationship of depositional system within Jixi basin. The sedimentary and tectonic features of Jixi basin and the characteristics of the Great Sanjiang unified lake basin were compared, the relationship between these two basins was also discussed, which provided sedimentological basic foundation for the determination of the nature of Great Sanjiang basin during Chengzihe and Muling formation. The main achievements of this paper are as follows:1. The types of depositional systems of Jixi basin during the period of Chengzihe-Muling Formation in Early Cretaceous were identified and classified.In this study, a mass of outcrop observations were carried out, three outcrop sections were depicted in detail and the core of Well Jixi6was described. From these samples, we concluded that the basin mainly developed delta facies-based coal measure strata during Chengzihe and Muling Formation. Combined with electrofacies and seismic facies analysis, we considered the basin had mainly developed delta facies which could be further identified as delta plain subfacies and delta of front phase subfacies, and lacustrine facies which was mainly the shore-shallow lacustrine subfacies, Consequently we concluded that during the period of Chengzihe-Muling Formation, Jixi basin prevalently developed delta and lacustrine subfacies, which could be further divided into delta plain subfacies, delta of front phase subfacies and shore-shallow lacustrine subfacies.2. The main provenance system of the basin was determined.By analyzing paleocurrent and heavy minerals, the location of provenance was determined. The research showed that:during the period of Chengzihe Formation, paleocurrent characteristics revealed that provenance mainly came from northwest, west and southwest of the basin, and gradually heading east while the west one divided into two branches at Hengshan uplift, entered into south and north part of the basin respectively. After analyzing depositional system of heavy minerals and comparing the types of circumbasin mother rock, west provenance was thought to be primarily from the orogenic belt of Xiaoxingan-Zhangguangcai Range. Progradation configuration in the seismic profiles pointed out that upon the southeast of the basin there existed another subordinate provenance with less influence. During the period of Muling Formation, the paleocurrent characteristics revealed that the main traits formed in Chengzihe Formation were mostly inherited, i.e. the basin still had two separate provenances. The features of heavy mineral assemblage reflected that the types of mother rock from the west part of the basin were consistent with orogenic belt of Xiaoxingan-Zhangguangcai Range, which indicated that the west provenance mainly came from the aforementioned orogenic belt, while the mother rock types from southeast site were consistent with Yanji fold belt, illustrating that southeast provenance mainly came from it, gradually augmented its influence on development of Jixi basin from Chengzihe Formation to Muling Formation.3. The allocation relationship of depositional systems of Jixi basin was analyzed.The allocation relationship of depositional systems of Jixi basin in Chengzihe Formation and Muling Formation had been identified through sedimentary facies analysis and provenance analysis. It was difficult to control the whole basin accurately due to the limited cores and low-resolution seismic data. Therefore, the analysis of the sedimentary facies and paleogeography mainly relied on the existed cores, outcrops, electrofacies and seismic profiles to fill the blank. We thought that the basin revealed its sedimentary facies as follows:it gradually turned into shore-shallow lacustrine subfacies from delta plain subfacies from east to west during Chengzihe Formation. The dominant provenance was provided by Zhangguangcai Range along with another provenance originating from south of the basin carrying its sediments around Well Jishenl. During the period-of Muling Formation, the allocation relationship and depositional systems were fundamentally consistent with that of Chengzihe Formation:the main facies were still shore-shallow lacustrine subfacies but with larger influence area. Meanwhile upon the west and southeast edge there developed delta plain subfacies and delta of front phase subfacies. The provenance succession remained the same. Due to the extension of water area, the provenance located near Zhangguangcai Range shrank while the south one gradually gained dominance, resulting in a system with shore-shallow lacrustrine subfacies in dominance accompanied by delta of front phase subfacies extending to lake at the west and southeast of the basin.4. It is concluded that Jixi basin was part of the Great Sanjiang unified lake basin during the period of Chengzihe-Muleng Formation.Based on data collected from outcrops, the Jixi basin primarily developed a sort of delta plain subfacies-based coal measure stratas and no typical marginal facies were discovered in the outcrops. In terms of provenance, Jixi basin mainly benefited from two areas:the Xiaoxingan-Zhangguangcai Range and Yanji fold belt; in respect of tectonics, the brink faults of Jixi basin mostly had thrust style which destroyed the basin. During Chengzihe-Muling Formation period, these faults hardly revealed their control on the sedimentary systems of the basin. After Muling Formation or Dongshan Formation, they started to destroy and reconstruct the structure of the basin, eventually matched the characteristics the Great Sanjiang unified lake basin. According to all of the aforementioned information, we reasoned that the Great Sanjiang basin should be a unified large-scale depression in Early Cretaceous. Jixi basin was not independently developed, instead, it was part of the unified large-scale depression called Great Sanjiang basin.