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首页> 外文期刊>Journal of South American earth sciences >Lower Permian stems as fluvial paleocurrent indicators of the Parnaiba Basin, northern Brazil
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Lower Permian stems as fluvial paleocurrent indicators of the Parnaiba Basin, northern Brazil

机译：下二叠系茎作为巴西北部帕尔奈巴盆地的河流古流指示物

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摘要

A comprehensive biostratinomic study was carried out with abundant stems from the Lower Permian Motuca Formation of the intracratonic Parnaiba Basin, central-north Brazil. The fossils represent a rare tropical to subtropical paleofloristic record in north Gondwana. Tree ferns dominate the assemblages (mainly Tietea, secondarily Psaronius), followed by gymnosperms, sphenophytes, other ferns and rare lycophytes. They are silica-permineralized, commonly reach 4 m length (exceptionally more than 10 m), lie loosely on the ground or are embedded in the original sandstone or siltstone matrix, and attract particular attention because of their frequent parallel attitudes. Many tree fern stems present the original straight cylindrical to slightly conical forms, other are somewhat flattened, and the gymnosperm stems are usually more irregular. Measurements of stem orientations and dimensions were made in three sites approximately aligned in a W-E direction in a distance of 27.3 km at the conservation unit "Tocantins Fossil Trees Natural Monument". In the eastern site, rose diagrams for 54 stems indicate a relatively narrow azimuthal range to SE. These stems commonly present attached basal bulbous root mantles and thin cylindrical sandstone envelopes, which sometimes hold, almost adjacent to the lateral stem surface, permineralized fern pinnae and other small plant fragments. In the more central site, 82 measured stems are preferentially oriented in the SW-NE direction, the proportion of gymnosperms is higher and cross-stratification sets of sandstones indicate paleocurrents mainly to NE and secondarily to SE. In the western site, most of the 42 measured stems lie in E-W positions. The predominantly sandy succession, where the fossil stems are best represented, evidences a braided fluvial system under semiarid conditions. The low plant diversity, some xeromorphic features and the supposedly almost syndepositional silica impregnation of the plants are coherent with marked dry seasons. Thick mudstones and some coquinites below and above the sandy interval may represent lacustrine facies formed in probably more humid conditions. The taphonomic history of the preserved plants began with exceptional storms that caused fast-flowing high water in channels and far into the floodplains. In the eastern site region, many tree ferns only fell, thus sometimes covering and protecting plant litter and leaves from further fragmenta-tion. Assemblages of the central and western sites suggest that the trees were uprooted and transported in suspension (floating) parallel to the flow. Heavier ends of stems (according to their form or because of attached basal bulbous root mantle or large apical fronds) were oriented to upstream because of inertial forces. During falling water stage, the stems were stranded on riverbanks, usually maintaining the previous transport orientation, and were slightly buried. The perpendicular or oblique positions of some stems may have been caused by interference with other stems or shallow bars. Rare observed stems were apparently waterlogged before the final depositional process and transported as bedload. The differences of interpreted channel orientations between the three sites are expected in a braided fluvial system, considering the very low gradients of the basin and the work scale in the order of tens of kilometers. The mean direction of the drainage probably was to east and the flows apparently became weaker down-stream. This study seems to provide reliable data for paleocurrent interpretations, especially considering areas with scarce preserved sedimentary structures.
机译：对巴西中北部克拉通内帕尔奈巴盆地的下二叠统Motuca组丰富的茎进行了全面的生物地层学研究。这些化石代表了冈瓦纳北部罕见的热带至亚热带古植物记录。树蕨占主导地位（主要是铁提亚（Tietea），其次是Psaronius），其次是裸子植物，蝶藻，其他蕨类和稀有的苔藓植物。它们是经过硅化处理的，通常达到4 m的长度（通常超过10 m），松散地躺在地面上或嵌入原始的砂岩或粉砂岩基质中，由于它们频繁的平行姿态而引起了特别的注意。许多树蕨茎呈原始的直圆柱状至略呈圆锥形的形状，其他则有些扁平，裸子植物茎通常更不规则。在“ Tocantins化石树自然纪念物”保护单位的27.3 km的距离内，沿W-E方向大致对齐的三个位置进行了茎方向和尺寸的测量。在东部站点，有54个茎的玫瑰图表明相对SE的方位角范围较窄。这些茎通常存在附生的基部球根状根地幔和薄的圆柱形砂岩包膜，有时将其固定在几乎与茎干的侧表面相邻的位置上，渗入了蕨类松果和其他小植物碎片。在更中心的位置，有82个测得的茎优先向西南-东北方向定向，裸子植物的比例更高，而砂岩的交叉分层集表明古流主要流向NE，其次是SE。在西部站点，测得的42个茎中大多数位于E-W位置。主要是沙质演替，最能代表化石茎，这证明了半干旱条件下的辫状河流系统。低的植物多样性，一些干态特征以及据称几乎同位沉积的植物硅与干旱季节连在一起。含沙层以下和以上的厚厚泥岩和一些共喹啉岩可能代表在可能更潮湿的条件下形成的湖相。保留植物的植物学历史始于异常的暴风雨，暴风雨使河道中的高水快速流入，并泛滥成灾。在东部站点区域，许多树蕨仅倒下，因此有时覆盖并保护植物凋落物和叶子免受进一步破碎。中西部站点的集合表明树木被连根拔起并以平行于水流的悬浮（漂浮）方式运输。由于惯性力，茎较重的末端（根据其形状或由于附着的基部球根状根冠或大的顶端叶状体）朝向上游。在下降的水阶段，茎被困在河岸上，通常保持以前的运输方向，并被略微掩埋。一些茎的垂直或倾斜位置可能是由于与其他茎或浅条的干涉引起的。观察到的稀有茎在最后的沉积过程之前显然已被水淹，并作为床荷运输。考虑到盆地的坡度非常低，并且工作规模约为几十公里，在辫状河床系统中，预计这三个位置之间的解释河道方向会有所不同。排水的平均方向可能是向东，下游的水流显然变弱了。这项研究似乎为古流解释提供了可靠的数据，尤其是考虑到保留了稀少沉积结构的地区。

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