Filamentos microbianos em estromatólitos e laminitos da Sequência Balbuena III (Maastrichtiano/Daniano) da Formação Yacoraite na Sub-bacia Metán-Alemania, na região de Salta, Argentina, e seus significados paleoambientais

ABSTRACT Salta Basin is located in the northwest of Argentina and its origin is related to an intracontinental rift which was formed during the Cretaceous and early Paleogene. It is subdivided into four sub-basins (Lomas de Olmedo - east; Sey - west; Tres Cruces - north; and Metán-Alemania - south) that were filled by sediments from the Pirgua (synrift phase), Balbuena and Santa Bárbara (both in sag phase) Subgroups. The Subgroup or Supersequence Balbuena is divided into four sequences: Balbuena I, II, III and IV, from bottom to top. This research identified microbial filaments in scanning electron microscope images from stromatolites and laminites samples of Balbuena III Sequence (Maastrichtian/Danian), Yacoraite Formation, in the Metán-Alemania Sub-basin, situated in Cabra Corral dam region, district of Coronel Moldes, Argentina. The occurrence of these filaments, identified in limestones formed in dry weather periods and deposited in the closed lake interval of the studied sequence, indicates the influence of biological activity in the formation of these rocks. The identification of microbial filaments, preserved from cyanobacteria structures, supports the palaeoenvironmental interpretation, since the occurrence of these microorganisms is limited to the photic zone and is common to stressful environments. The laminites of the studied area have been interpreted as formed in supralittoral mudflats in the vadose zone, while the stromatolites were formed in sublittoral environment, being constantly submerged. In addition to the microbialites, the Balbuena III Sequence consists of carbonates, siliciclastic and mixed facies which have been deposited in a lacustrine environment.

摘要 萨尔塔盆地（Salta Basin）位于阿根廷西北部，其成因与白垩纪至古近纪早期形成的陆内裂谷密切相关。该盆地被划分为四个次级盆地：东部的洛马斯德奥梅多（Lomas de Olmedo）、西部的塞伊（Sey）、北部的特雷斯科鲁塞斯（Tres Cruces）以及南部的梅坦-阿莱马尼亚（Metán-Alemania），盆地内充填了裂谷期皮尔瓜（Pirgua）亚群、坳陷期巴尔布埃纳（Balbuena）亚群与圣巴巴拉（Santa Bárbara）亚群的沉积物。巴尔布埃纳亚群（或超层序）自下而上可划分为巴尔布埃纳I、II、III、IV四个层序。本研究在阿根廷科罗内尔莫尔德斯县卡布拉科拉尔坝库区的梅坦-阿莱马尼亚次级盆地内，于亚科拉伊特组（Yacoraite Formation）巴尔布埃纳III层序（马斯特里赫特阶/达宁阶）的叠层石与纹层岩样品的扫描电镜图像中，识别出了微生物丝状体。上述丝状体发现于干旱期形成的石灰岩中，赋存于研究层序的封闭湖相沉积层位内，表明生物活动对这类岩石的形成具有显著影响。经鉴定，这些保存完好的微生物丝状体属于蓝细菌（cyanobacteria）结构，这类微生物的产出局限于透光带且常见于胁迫环境中，该特征可为相关古环境解释提供支撑。研究区的纹层岩被解释为形成于渗流带的潮上泥坪环境，而叠层石则形成于持续淹没的潮下环境。除微生物岩外，巴尔布埃纳III层序还包含了沉积于湖相环境的碳酸盐岩、硅质碎屑岩及混合岩相。