Composition and sediment dispersal pattern of the Upper Triassic flysch in the eastern Himalayas, China: significance to provenance and basin analysis

Abstract

The paleogeography and basin type of Upper Triassic flysch (Langjiexue Group) in the eastern Himalayan Orogen are disputed. In order to shed new light on the flysch’s origin, we applied different sedimentological methods. Assemblages of heavy minerals and clastic components of sandstones were utilized to determine the primary depositional composition. Heavy mineral indices, S/M ratios (thickness of sandstone + siltstone “S” versus slate/mudrock “M”), and paleocurrent data were combined to reveal the sediment dispersal pattern and the location of the source areas. In the analyzed sandstones, heavy minerals such as zircon, rutile, tourmaline, apatite, and anatase are most common, and zircon is predominant (most over 60 %). ZTR values range from 60 to 98 % and systematically increase southward. As a provenance-sensitive parameter, RuZi values vary in large magnitude and are significantly higher in both the east and west (>20 %) than in the center. The majority of S/M ratios decrease from north to south, suggesting an overall decrease in grain size to the south. Paleocurrent directions vary between 120° and 270° (main vector 205°, and 185° after 20° counterclockwise correction), displaying a radial-curved pattern. Variable heavy mineral assemblages indicate different sources, and the sandstones fall in the “recycled” and “mixed-arc orogeny” fields of Dickinson triplots, together supporting the view of multiple sources. Results of the ZTR values, S/M ratios, and paleocurrent directions illustrate a dispersal pattern, corresponding to a submarine fan system. The provenance and submarine fan dispersal pattern along with the basin configuration (deep basin with oceanic affinities) suggest that the Langjiexue Group accumulated in a remnant basin between Lhasa, Greater India, and Australia, where the sediments dispersed into the basin toward the developing orogen/suture zone and not away from the orogen, challenging the provenance direction for the traditional remnant basin model.