Triassic strata with more than 10000 m in thickness are dominated by marine sediments in northern Three-River area of eastern Tibet. Composed of one or two dominant components of clastic, carbonatic and volcanic rocks, the sediments are characterized by abyssal sediments formed by debris flows and turbidities of terrigenous, basinal and volcanic sources, and accumulated in Jomda arc basin and Sinda back_arc basin. The deep-water sediments seem to be the response to active tectonics, and hence are of indicating significance to trace tectonic activity in Three-River area.