Depositional Transformation From Late Triassic To Early-mid Jurassic In North Qiangtang Basin And Its Signifiance

An important sedimentary transformation event occurred at late Triassic period(216±4.5Ma) in north Qiangtang Basin. There is a remarkably coincidence between sedimentary transformation event and large-scale volcanic eruption-sedimentary events in time. It is also consistent with the magmatism, large-scale regressions and biotic extinction events in the eastern Tethyan region. It is the reflections of the global tectonic events in the Qiangtang Basin. In this study, we choose the Tumengela formation, Nadigangri formation, and Quemocuo formation of Woruo Mountain section in the southern margin of the north Qiangtang Basin. It records the whole process of the sedimentary transformation event completely. By measured and observed the section in the field and combined the method of sedimentology, petrology, geochemistry and isotope chronology to study the sedimentary facies, sedimentary environment, provenance and tectonic setting of the Tumengela formation and Quemocuo formation. Following views have been summarized:First: The Woruo Mountain section has been identified the delta front subfacies, delta plain subfacies, away from the pyroclastic sedimentary subfacies of volcanic cone, alluvial fan, restricted platform and delta front subfacies form Tumengela formation to Quemocuo formation in turn.Second: Based on the division of sedimentary facies, the sedimentary period of Tumengela formation experienced an evolution from delta front subfacies to delta plain subfacies, which show a regressive sequence succession of upward shallowing as a whole. The sedimentary period of Quemocuo formation experienced an evolution from alluvial fan, restricted platform to delta front subfacies, which show a transgressive sequence succession of upward deeping as a whole. The sedimentary environment changed significantly form Tumengela formation to Quemocuo formation.Third: According to the detrital composition, major and trace elements, detrital zircon U-Pb age of sandstone to restrict the provenance of study area. The provenance changed significantly form Tumengela formation to Quemocuo formation, which shows a change form quartzose-rich rock to quartzose-rich mixed with intermediate-acid volcanic rocks. The main provenance areas may change from the central uplift belt of middle East to the middle west.Four: According to the major, trace and rare earth elements of sandstone to restrict the tectonic setting of study area. The tectonic setting changed significantly form Tumengela formation to Quemocuo formation, which shows a change form passive continental margin to continental island arc and active continental margin.