Sedimentology and stratigraphy of Neoproterozoic-lower Paleozoic carbonate-siliciclastic succession of the southwesternmost Amazon Craton, state of Rondônia, Brazil

ABSTRACT: Facies and stratigraphic analysis were carried out in Neoproterozoic-Lower Paleozoic carbonate-siliciclastic deposits of Cacoal and Pimenta Bueno formations exposed on basement rocks and into the Pimenta Bueno Graben, northwestern portion of Parecis Basin, southwesternmost Amazon Craton. The redescription and redefinion of this succession confirmed the previous interpretation for the Cacoal Formation as a Marinoan (~ 635 Ma) cap carbonate. The Cacoal Formation is subdivided here in two units separate by sharp contact found exclusively overlying Mesoproterozoic crystalline basement rocks: 1) a homonymous formation characterized by diamictites, sandstones and siltstones with dropstones interpreted as glacio-marine deposits; and 2) the Espigão d’Oeste Formation that consists of dolostone, dolomitic stromatolites, dolostone-siltstone rhythmite and siltstone interpreted as shallow to moderately deep platform deposits. The Ordovician to Silurian Pimenta Bueno Formation is a filling of Pimenta Bueno graben and overlies locally the Meso and Neoproterozoic rocks. This unit consists in diamictites, sandstones, siltstones and pelites interpreted as glacial-marine and tide- to storm-influenced platform deposits, recording a glacio-eustatic regressive-transgressive event. This new stratigraphic proposal modify the current stratigraphy for the Parecis Basin and suggest, at least, two levels of glaciation exposed in the sothwesternmost Amazon Craton related to the Marinoan and Late Ordovician-Early Silurian events.摘要：对出露于基底岩石之上、位于亚马逊克拉通（Amazon Craton）西南端帕雷西斯盆地（Parecis Basin）西北部皮门塔布埃诺地堑（Pimenta Bueno Graben）内的卡科阿尔组与皮门塔布埃诺组的新元古代-下古生代碳酸盐岩-硅质碎屑岩沉积开展了相和地层分析。本次对该沉积序列的重新描述与厘定，证实了此前将卡科阿尔组归为马里诺冰期（Marinoan，约6.35 Ma）帽碳酸盐岩（cap carbonate）的解释。研究将卡科阿尔组划分为两个以截然接触为界的单元，二者仅覆于中元古代结晶基底岩石之上：1）同名组，以含坠石（dropstones）的杂砾岩（diamictites）、砂岩与粉砂岩（siltstones）为特征，被解释为冰海沉积（glacio-marine deposits）；2）埃斯皮冈多奥斯特组（Espigão d’Oeste Formation），由白云岩（dolostone）、白云质叠层石（dolomitic stromatolites）、白云岩-粉砂岩韵律层以及粉砂岩组成，被解释为浅至中等深度的台地沉积。奥陶纪至志留纪的皮门塔布埃诺组为皮门塔布埃诺地堑的充填沉积，局部覆于中元古代与新元古代岩石之上。该岩系由杂砾岩、砂岩、粉砂岩与泥质岩（pelites）组成，被解释为受冰川-海洋及潮汐-风暴影响的台地沉积，记录了一次冰盖型海平面变化（glacio-eustatic）驱动的海退-海进事件。本次提出的新地层方案修正了帕雷西斯盆地当前的地层格架，并表明在亚马逊克拉通西南端至少存在两期冰川作用，分别对应马里诺冰期以及奥陶纪晚期-志留纪早期冰川事件。