The Poplar-River alluvial fan in the sunken of western Heshituoluogai Basin of northwestern in China has an area of 327.6 km2, a radius of 25km, a smooth 7‰-4‰ sloping surfacies, and it’s a modern gravelly alluvial fan... [ view full abstract ]
The Poplar-River alluvial fan in the sunken of western Heshituoluogai Basin of northwestern in China has an area of 327.6 km2, a radius of 25km, a smooth 7‰-4‰ sloping surfacies, and it’s a modern gravelly alluvial fan which sediments are mainly formatted by braid-rivers. In modern, the Poplar River, dominated by active structure, is interrupting the alluvial fan. Two huge cross sections (15-18km), along the axial of the fan, are formatted by the Poplar River. Facies analysis of 2-40m high exposure at 72 sites reveals that the fan is built by several flows and mostly by stream-flow processes.
According to different fluid property and lithofacies association, twelve lithofacies can be recognized and five typical facies can be confirmed. Facies A (in proximal), dominated by debris flow, consist several cycles. One cycle contain sand-matrix-supported coarse to very coarse pebbly gravel in lower part and conglomeratic sand trough with large-scale cross beds in the upper part. Facies B, between proximal and intermediate part, is dominated by sheetflood and mainly consist numerous troughs. In the outcrop, the troughs extend long and 0.5-2.5m thick, and they are lateral and vertical superposition. Facies D (in intermediate) is dominated by stream-flow. There are numerous large-scale troughs intersected each other. This facies, is characterized by lateral superposition of numerous bar and channel deposites, suggesting deposition in braided rivers. Facies C, the transitional zone between facies B and facies D, is dominated by sheetflood and streamflow. In the section cross, sheetflood depositions (wide-shallow groove) alternate with streamflow depositions (narrow-deep groove), and it exists distinct erosion surfaciess between the two kinds of depositions. Facies E represented poorly-drained floodplain with wet land in the distal part, which has rich vegetation. The primary depositions are fine-mudstone sediments in wet land, sheet anemoarenyte, and local gravel channel sediments.
From the research of outcrop sites，the characteristics of sediments can been recognized. Notable proximal-to-intermediate trends include a change in facies A, dominated by debris flow, and facies B, dominated by sheetflood, sediments in the fluvial succession from thickest(at least 80m) to thinnest (at least 20m); facies D, from the proximal to intermediate, include a change in an increasing proportion of successin from 0% to 90%, and an average grain size from coarse gravel to sand. Facies E tend to associate with adjacent facies, such as paludal facies, lake, floodplain etc. the distal facies E don’t have typical sedimentary characteristics.
Topics: Physical sedimentary processes , Topics: Fluvial depositional systems
