Paleoenvironments of a regressive Devonian section from Paraná Basin (Mato Grosso do Sul state) by integration of ichnologic, taphonomic and sedimentologic analyses

ABSTRACT: Studies that integrate ichnologic, taphonomic and sedimentologic data result in more accurate paleoenvironmental inferences than isolated approaches. Most of paleontological studies regarding Devonian from Paraná Basin were conducted in the southern part of the basin (Paraná state), precluding taphonomic or ichnologic studies to the northern part, and even its macrofossils content is understudied. This study analyzes paleoecologic and depositional conditions represented by trace fossils, macrofossils and sedimentary facies in a regressive Devonian section from Paraná Basin, Mato Grosso do Sul state, Brazil. Seven ichnofabrics (Macaronichnus, Psammichnites, Arenicolites-Skolithos, Cylindrichnus-Skolithos, Zoophycos, Rhizocorallium-Palaeophycus, and Chondrites ichnofabrics) and three taphofacies (T1: parautochthonous to allochthonous preservation; T2: Autochthonous preservation; and T3: time-averaged autochthonous to allochthonous association) were diagnosed. The studied sections are positioned in a highstand systems tract (HST) exhibiting dominance of sandy facies, and four sub-environments were defined: foreshore; shoreface; storm-dominated shoreface to transitional offshore; and offshore. The dominance of foreshore to shoreface settings in a HST corroborates a shallower context in relation to the southern part. However, similarities in the facies and ichnofacies stacking, as well in the macrofossil content suggest that the hypothetical division between two sub-basins (Apucarana and Alto Garças Sub-basins) was not complete until early Emsian.

摘要：相较于单一研究方法，整合遗迹学（ichnologic）、埋藏学（taphonomic）与沉积学（sedimentologic）数据的研究可获得更为精准的古环境推论。此前针对巴拉那盆地（Paraná Basin）泥盆系的古生物学研究多集中于盆地南部的巴拉那州，导致盆地北部的埋藏学与遗迹学研究存在空白，其大型化石（macrofossils）的相关研究亦较为匮乏。本研究针对巴西南马托格罗索州（Mato Grosso do Sul state）巴拉那盆地一处退积型泥盆纪剖面展开分析，通过遗迹化石、大型化石与沉积相（sedimentary facies）揭示其古生态与沉积环境特征。本次研究共识别出7种遗迹组构（ichnofabrics）：麦卡洛迹（Macaronichnus）、砂迹（Psammichnites）、砂囊迹-石针迹（Arenicolites-Skolithos）、圆柱迹-石针迹（Cylindrichnus-Skolithos）、动藻迹（Zoophycos）、孔雀尾迹-古藻迹（Rhizocorallium-Palaeophycus）及小球迹（Chondrites）；同时划分出3种埋藏相（taphofacies）：T1为准原地至异地埋藏组合，T2为原地埋藏组合，T3为时间平均准原地-异地埋藏组合。研究剖面赋存于高位体系域（highstand systems tract, HST）内，以砂质相（sandy facies）为主，据此可划分出4种亚沉积环境：前滨（foreshore）、临滨（shoreface）、风暴控制型临滨至过渡远海（storm-dominated shoreface to transitional offshore）以及远海（offshore）。高位体系域内前滨至临滨环境占主导，表明该区域相较于盆地南部水体更浅。然而，沉积相与遗迹组构的垂向叠置模式以及大型化石组合特征均显示，直至埃姆斯早期（early Emsian），阿普卡兰纳次盆地（Apucarana Sub-basin）与阿尔托加尔萨斯次盆地（Alto Garças Sub-basins）之间的次级盆地分隔并未完全形成。