滇中地区晚三叠世的重力流沉积及其地质意义

LATE TRIASSIC SEDIMENTARY. GRAVITY FLOW DEPOSITS IN CENTRAL YUNNAN AND THEIR GEOLOGICAL SIGNIFICANCES

上三叠统云南驿组一段下部是钙质浊积岩,上部为陆源碎屑浊积岩及少量火山碎屑浊积岩;二段的角砾状灰岩有坍塌和重力滑动两种成因,夹钙质浊积岩。罗家大山组一段由火山碎屑质的碎屑流、浊流和颗粒流沉积组成,夹基性熔岩。它们是滇中裂谷的记录,物源区在西边,构成中国西南部三叠纪古特提斯的一部分。

The Upper Triassic along Xiangyun—Midu, central Yunnan is named, in ascending order, the Yunnanyi, Luojiadashan and Baitutian formstions. The lower part of Member One, Yunnanyi Fm., may be calcareous turbidite, overlying conformably the Mido Triassic Tuanshanchun Fm. The majority of upper part of the member seems to be composed of terrigenous turbidite, and the subordinate volcaniclastic turbidite can be found near Xiangyun Country. The lower part of Member Two, Yunnanyi Fm., is characterized by calcarous conglomerates, which could be separated into two types. One is slump breccia, with the limestone gravels being from the Beiya and Zhongwo Fms. in the west (Heqing—Dali). Another maybe result from penecontemporancous gravity sliding, with the wrinkle marks indicating the paleocurrent eastwards. Sandwiched in the conglomerates is the calcareous turbidite, which becomes, ascendingly, shallow marine carbonate rocks. The member one of Luojiadashan Fm. is mainly composed of volcaniclastic sediments, with the interlayer of basaltic lava firstly discovered. The conglomerate seems to be debris flow deposits, with gravels (limestone, silicalite, granite, etc.) implying the source region in the west. In addition, the sediments of turbidity cur- rent and grain flow are well developed in the member.The sedimentary gravity flow deposits demonstrate that there might exist a rift (the central Yunnau rift) during Late Triassic, which might link the Songpan—Ganzi Trough in western Sichuann with the Youjiang rift in western Guangxi. The Triassic period might be a stage of Paleo-Tethyan relic oceanic basin, namely, the paleo-Tethys during Triassic might be composed of several relatively small oceanic basins sand some rifting troughs in southwestern China, including the central Yunnan rift. All of them might be closed and continent-continent collision occurred during the Indosinian Orogenesis, and the molasse of the Baitutian Fm. might be deposited in a hinderiand basin.