A giant sandy sediment drift in early Silurian (Telychian) and its multiple sedimentological prosess

摘要

This study describes a regional-scale sedimentary feature in the Upper Yangtze region of China that is identified as a Telychian age (438 Ma) sediment sandy drift that is 500-800 km long, 100-150 km wide, and 10-20 m thick. Graptolite zones is applied to recognize Telychian sandstone beds and reconstruct an isochronal stratigraphic framework for the study area. It is possible to observe numerous traction structures in Telychian sandstone beds, including ripples, cross-bedding, cross-laminations, wavy-laminations and horizontal-laminations and Bouma sequence. The palaeocurrent direction is towards SSW (200-240), which is measured by sedimentary structures (cross-bedding, cross-laminations and ripples) in outcrops. Besides, the alignment of elongate particles in thin section, and the distribution characteristics of heavy minerals also indicate a SSW palaeocurrent direction. This direction is parallel to palaeo-coastline, which is a characteristic feature of bottom current. A multiple sedimentary processs is identified based on analysing fades of each sections. Contourite-dominated sequences are observed in channel-related seaway while turbidite sequences are observed in shelf near palaeo-coastline. Based on provenance analysis, the Telychian drift (channel reworked sandy drift) is the production of a mixed depositional system (first turbidite deposition and then bottom current deposition). The turbidite sand on the shelf provides material source for the sandy drift, and the deposit is transported from the shelf to the channel (parallel to ancient coastline). The provenance direction is identified by analyzing the distribution of heavy minerals. The unstable heavy minerals content is higher in the turbidites samples (near provenance region) than the channel reworked sandstone samples (far away from provenance region). This Telychian deposit is a rare example of an ancient channel reworked sandy drift.