Discovery of Tillite in the Late Proterozoic at Jixian County of Tianjin,North China

Key words: 
• Jixian county /
• Late Proterozoic /
• Xijingyu Formation /
• tillite /
• glacier

Abstract: In the Middle and Neoproterozoic in Jixian county,Tianjin in the north-central part of the North China Plate,a set of breccia was developed above the Jingeryu Formation of the QingBaikou system and the Fujunshan Formation of the Early Cambrian. There is no definitive evidence of whether it belongs to the Late Proterozoic or Early Cambrian. The author speculates that it is the Late Proterozoic and named the formation the "Xijingyu Formation." The sedimentary characteristics of the breccia in the Xijingyu Formation are described in detail through field measurements and thin slice observation,and its genesis and provenance were analyzed. The results show that the Xijingyu Formation is 155 m thick and can be divided into 9 sections according to the change of breccia composition and size. The composition of the breccia is mainly dolomite,with a small amount of chert and limestone; the formation is blockshaped,without bedding or layering;the breccia content is 80%-90%,disorderly arranged,and mixed in size,The sorting and roundness are poor,and it is generally angular and sub-angular. The interstitial is mainly dolomite mud and silt,and the structural maturity is extremely low. The parent rock of the breccia is mainly the Wumishan Formation from the Middle Proterozoic,and a small amount is from the Tieling Formation. This breccia is glacial in nature and is a typical tillite. The discovery of the tillite is of great significance for restoring the characteristics of the Late Proterozoic Earth's climate, and its emergence may be related to the "Snowball Earth".