Facies analysis, sequence stratigraphy and chemostratigraphy of the Sete Lagoas Formation (Bambui Group), northern Minas Gerais State, Brazil: evidence of a cap carbonate deposited on the Januária basement high

ABSTRACT: Sedimentary rocks of the Sete Lagoas Formation, exposed in the left margin of the São Francisco river (Minas Gerais State, Brazil), were deposited on the Januária-Itacarambi basement high. They show both lateral and vertical rock stacking along continuous outcrops, allowing us to carry out detailed facies analysis and sequence stratigraphy studies. Our studies also integrate data from geological mapping, macro and microscopic petrography and high-resolution C and O isotope analysis. Eight facies and four facies associations make up a sequence composed by a transgressive tract in the base, and a high stand tract in the upper portion, separated by a maximum flooding surface. The high stand tract shows a progradation stacking from the basement high apex towards progressively deeper basement areas. This stratigraphic framework, associated with others stratigraphic and isotopic features, indicates that the now exposed Januária basement high also represents a paleo-high during the sedimentation event. Aragonite pseudomorphs and dolomites coupled with δ13C values of -5‰ characterize the basal carbonate of the transgressive tract as a cap carbonate. Records of the Cloudina fossil, recognized in the high stand tract, indicate a Late Ediacaran age for the upper portion of the studied stratigraphic sequence.摘要：出露于巴西米纳斯吉拉斯州圣弗朗西斯科河（São Francisco River）左岸的塞特拉戈斯组（Sete Lagoas Formation）沉积岩，沉积于雅努阿里亚-伊塔卡兰比基底隆起（Januária-Itacarambi basement high）之上。该套沉积岩在连续露头中展现出侧向与垂向的岩石叠置特征，为开展精细岩相分析（facies analysis）与层序地层学（sequence stratigraphy）研究提供了良好基础。本次研究整合了地质填图、宏观与微观岩石学、高分辨率碳氧同位素分析的相关数据。研究识别出8种岩相（facies）与4种岩相组合（facies associations），构成一套以海进体系域（transgressive tract）为底部、高位体系域（high stand tract）为上部的层序，二者以最大海泛面（maximum flooding surface）相分隔。高位体系域呈现出从基底隆起高点向逐步加深的基底区域的进积堆叠特征。该地层格架结合其他地层学与同位素特征表明，现今出露的雅努阿里亚基底隆起在沉积时期同样属于古隆起（paleo-high）。海进体系域底部的碳酸盐岩以文石假晶（Aragonite pseudomorphs）与白云石为特征，其δ¹³C值为-5‰，属于帽碳酸盐岩（cap carbonate）。在高位体系域中发现的克劳迪奴虫化石（Cloudina fossil）记录，表明本次研究的地层序列上部形成于埃迪卡拉纪晚期（Late Ediacaran age）。