Lahar facies of the Latest Paleozoic Arbasay Formation: Geomorphological characters and paleoenvironment reconstruction of Northern Tian Shan, NW China

摘要: Abstract Texturally well-preserved lahars are exposed in the Latest Paleozoic Arbasay Formation, Northern Tian Shan. The consist primarily of two lithofacies: massive, poorly lithified diamictites and stratified, moderately gravelly sandstones. can be generally divided into subfacies, i.e., matrix-supported clast-supported diamictites. Most structureless nongraded, however some crudely stratified beds with a certain grading have also been found. They thick contain large clasts up to 3 m dimension. sandstones display much finer particle size wedge or lenticular geometries. Large absent within them sorting characters better than Despite different grading, matrix lithofacies appear homogeneous. is sandy mudstone. comprise rhyolites, dacites, andesites, andesitic basalts basalts, same co-existing volcanic rocks, suggesting they originate from cognate volcanics. disorganized supposed deposit turbulent flood pyroclastic surge. sandstone interpreted as sand-rich flows hyperconcentrated during waning stage mass-flow event. overall characteristics deposits suggest dominated alluvial fan environment. rapidly accumulated small, normal faults bounded depocenters. Formation lahars, together geochemistry clearly prove that were deposited post-collisional extensional regime. LA-ICP-MS zircon dating intercalated fallout tuff sample provided an age 314.4 ± 3.4 Ma (MSWD = 1.6), at Late Carboniferous. Therefore, North Shan Ocean must closed before 314 Ma, initial basin type should faulted basins rather foreland basins.