滇东南法郎组含锰地层沉积相分析

Sedimentary facies of the manganese-bearing strata in the Falang Formation,southeastern Yunnan

关键词:  法郎组  浊积扇  碎屑流  锰矿

Key words:  Falang Formation  turbidite fan  debris flow  manganese ores

摘要:

滇东南中三叠统法郎组是重要的含锰地层,前人对其沉积环境的认识却存在着深水浊流沉积和浅水潮坪沉积的争议。本文通过对建水-开远地区典型剖面的研究,认为法郎组为浊积扇沉积。法郎组底部为碳酸盐滑塌沉积、碎屑流沉积与泥岩粉砂岩互层,中上部逐渐由外扇渐变为中扇辫状水道和辫状水道间沉积,顶部为内扇补给水道和溢岸沉积。相对海水深度总体表现为在拉丁早期逐渐加深,到拉丁中晚期有逐渐变浅。锰矿层主要发育在底部的碎屑流沉积中,其形成可能与拉丁早期相对水深的增高有关。

Abstract:

There is still debate concerning the sedimentary environments of the Middle Triassic Falang Formation as important manganese-bearing strata in southeastern Yunnan: a deep-marine turbidite fan or a shallow-water tidal flat.The authors in this study contend,on the basis of representative sections in the Jianshui-Kaiyuan region,that the Falang Formation represents the turbidite fan deposits.The lowermost part of the Falang Formation is made up of the carbonate slump deposits,debris flow deposits and the interbeds of mudstone and siltstone,followed by the outer-fan to mid-fan braided channel and braided interchannel deposits in the middle-upper part,and inner-fan recharge channel and overbank deposits in the uppermost part of the Falang Formation.On the whole,the sea water became progressively deeper during the early Ladinian,and shallower during the middle and late Ladinian.The manganese ores are mostly hosted in the debris flow deposits in the lowermost part of the Falang Formation,implying that the origin of the manganese ores may be associated with relative sea-level rise during the early Ladinian.