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The Piedras de Afilar Formation (Neoproterozoic, Uruguay): Sedimentology and provenance of a key unit for SW-Gondwana paleogeography
Journal of South American Earth Sciences ( IF 1.8 ) Pub Date : 2021-01-22 , DOI: 10.1016/j.jsames.2021.103176
Karina Pamoukaghlián , Claudio Gaucher , Daniel G. Poiré , Robert Frei

The Piedras de Afilar Formation (PAFm) is a late Neoproterozoic sedimentary succession autochthonous to the Tandilia Terrane, part of the Río de la Plata Craton. It overlies with erosional unconformity a Paleoproterozoic igneous and metamorphic basement. According to facies analysis and lithostratigraphy, the PAFm is divided into three members for which detailed petrography, X-ray diffractometry and geochemistry studies were carried out: (1) the Cuchilla Alta Member, made up of yellowish-white quartz arenites and subarkoses with through, planar and hummocky cross-stratification, ripples, interference ripples and flute casts; (2) the Arroyo del Junquito Member, comprising dark grey, red and green shales and siltstones, with wavy lamination and wrinkle structures; and (3) the Arroyo de la Tuna Member, made up of grey limestones, grey to green marls, rhythmites and dark shales. Dolerite sills, up to 50 m thick, intrude the succession, and affect clay mineralogy and rare earth composition near the contact. A deepening, fining- and thining-upward sequence in a shallow marine paleoenvironment is evidenced by facies analysis. Ce anomalies are negative at the base of the Arroyo del Junquito Member and become positive up section, indicating increasingly anoxic conditions.

Paleocurrents show a bimodal oblique pattern, with predominant paleocurrents to the north-east and subordinately to the east, suggesting a provenance from the west, which agree with detrital zircon interpretation. Sm–Nd analyses of shales yielded TDM model ages between 2.53 and 1.55 Ga, which indicate a mixed provenance from Paleoproterozoic units of the Río de la Plata Craton and from a proto-Andean, Mesoproterozoic belt at its western border, in accordance with available detrital zircon ages for sandstones of the PAFm. The influence of a recycled orogen as source area is also shown by sandstone petrography and trace element composition of shales. The correlation with other Ediacaran successions of the Río de la Plata Craton, such as the lower Arroyo del Soldado Group in Uruguay and lower Sierras Bayas Group in Argentina, is strengthened by the new data. All units were deposited on a stable shelf developed at the eastern margin of the Río de la Plata Craton.



中文翻译:

Piedras de Afilar组(乌拉圭新中生代):沉积学和西南部冈瓦纳古地理学关键单元的物证

Piedras de Afilar组(PAFm)是新元古代晚期的沉积演替,与Ríode la Plata Craton的一部分Tandilia Terrane有关。它以侵蚀不整合覆盖了古元古代火成岩和变质基底。根据相分析和岩相地层学,PAFm分为三个部分,对其进行了详细的岩石学,X射线衍射和地球化学研究:(1)Cuchilla Alta成员,由黄白色石英编织物和亚黑石构成,通过,平面和垂直交叉分层,波纹,干涉波纹和长笛铸型;(2)Arroyo del Junquito成员,由深灰色,红色和绿色的页岩和粉砂岩组成,具有波浪状的层压和皱纹结构(3)Arroyo de la Tuna成员,由灰色石灰石,灰色至绿色的泥灰岩,节律植物和深色页岩组成。厚达50 m的钙铝石基岩侵入岩层,并影响该接触层附近的粘土矿物学和稀土成分。通过相分析,证明了浅海古环境中的加深,细化和稀疏向上序列。Ce异常在Arroyo del Junquito成员的底部为负,并变为向上的正剖面,表明越来越多的缺氧条件。

古洋流呈双峰倾斜模式,东北部以东为主,古风以东为主,暗示着西部的物源,与碎屑锆石解释相符。页岩的Sm–Nd分析得出T DM模型年龄介于2.53和1.55 Ga之间,表明根据PAFm砂岩可用碎屑锆石年龄,来自里约拉普拉塔克拉通古元古生界单元和其西部边界的原始安第斯中元古生界地带的混合来源。砂岩岩相学和页岩微量元素组成也表明了造山带作为源区的影响。新数据加强了与拉普拉塔克拉通河Riao la la Plata Craton的其他埃迪亚卡拉纪系的相关性,如乌拉圭的较低的Arroyo del Soldado组和阿根廷的较低的Sierras Bayas组。所有单位都存放在Ríode la Plata Craton东边的稳定架子上。

更新日期:2021-01-28
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