新疆昌吉喀拉扎山U形谷的发现及其古气候意义

U-shaped valleys found in Mount Kalazha,Changji,Xinjiang,and their paleoclimate significance

笔者等在新疆乌鲁木齐西南40 km的喀拉扎山脊北侧发现十几条并排分布的U形谷,而U形谷之间以刃脊相隔。这些U形谷发育在上侏罗统巨厚层长石砂岩露头表面,据其形态和分布特征属于冰川悬谷。这个发现揭示了喀拉扎山脊以南曾经发育过海拔2000 m的冰帽,而冰帽的发育时间为更新世,更可能是晚更新世。中国西北尚无第四纪冰帽的报道,本文的发现为中国西北地区第四纪冰川和古气候研究提供了新的材料。由于新疆比青藏高原海拔低、更加干旱,新疆U形谷的发现也为青藏高原更新世大冰盖的存在提供了新的佐证材料。

Here is a huge anticline of Jurassic—Cretaceous terrestrial clastic rocks outcropped to the southwest of Urumqi about 60 km away.The fold is more than 60 km long in the E—W direction and more than 20 km in width.The fold axis passes through the peak of Mount Kalazha(about 2000 m ASL),40 km away from Urumqi,exposing the Upper Jurassic sandstones,arkose of Kalazha Formation.We investigated this anticline and found a series of U-shaped valleys on the north side of the ridge.There are about 15 U-shaped valleys adjacent to each other and separated with sharp aretes in between.Every U-shaped valley is about 200~400 m in width at the ridge top and gradually narrows down the slope,extending northward for more than 450 m to the end where the vertical Jurassic thick sandstone strata are changed to red conglomerates of Lower Cretaceous.The U-shaped valleys are then abruptly turned into V-shaped valleys downward and continue their courses across a wide terrace.The U-shaped valleys are typical glacial hanging valleys,suggesting that a snow field and ice cap once existed to the south of Mount Kalazha with the altitude of more than 2000 m during the Pleistocene.Because the U-shaped valleys and the aretes are still clearly intact at the outcrop of Jurassic arkose that are not resistant to weathering and erosion,we prefer that the fossil snow field and ice cap existed in the Late Pleistocene although we still lack material to give the exact time.There has not been any evidence reported in scientific articles or any other media that an ice cap once existed in the northwest region of China,and even a debate about a Pleistocene Icesheet in the Tibetan Plateau has lasted for a century without end.Our fieldwork reveals the erosion evidences for a Pleistocene ice cap in Xinjiang,and this means that there were some continental glaciers once developed in northwest of China during the Pleistocene time.Furthermore,because the temperature is obviously lower and the precipitation is much higher in Tibetan Plateau than those in Xinjiang area,our finding can also provide collateral evidence for the research on the glaciation in the Plateau.