RUI: Terrane Analysis of Silurian Reefal Carbonates, Alaska

9417907 SOJA许多地质学家认为，“外来”地壳碎片或地体在大陆边缘的堆积是过去6,000万-1亿年来北美西部大陆增长的主要构造过程。由于增生构造产生了复杂的地质关系，北美西部这一构造拼贴的完整地质史尚不清楚。古生物地理研究在阿拉斯加至关重要，因为保存在那里的化石出现在这一复杂的吸积地体带中，并占据着目前位于北美动物群省份与环太平洋和环北极地区之间的中间地理位置。为了更好地了解阿拉斯加地体的起源和地质历史，古生物学家一直在寻求识别独特的化石套件，这些化石套件基于占据相似栖息地的动物群的共同亲缘关系，将某些地体连接在一起，或与大陆物源区建立生物地理联系。以这种方式使用古生物地理数据在解决相互竞争的模型中反映的争议方面具有巨大的潜力，这些模型描述了北美西部，特别是阿拉斯加的构造演化。拟议的项目旨在通过调查阿拉斯加东南部的志留纪生物礁和与珊瑚礁相关的碳酸盐矿床，以确定它们与阿拉斯加西南部志留纪石灰岩的地质关系，从而建立在PI之前的研究基础上。阿拉斯加东南部出露的岩石属于亚历山大地体，它在早-中古生代作为火山活动岛屿存在于古太平洋的一个未知地点，而阿拉斯加西南部的沉积物(尼克松福克地体)代表北美大陆的原始但已被肢解的一部分。这两个地区的地层共有志留纪海绵动物群，这些动物群保存在不寻常的叠层石REF中。在阿拉斯加东南部，志留纪碳酸盐岩已在分散的地点进行了详细的研究，但主要只在亚历山大地体南部的一个相对较小的区域进行了研究。拟议中的研究将使人们能够对亚历山大地体其他部分暴露的志留纪岩石进行详细调查，特别是在勘察研究发现保存完好但尚未研究的生物群的地方。涉及本科生的野外作业将集中于描述岩石，并从测量和拍摄的地层切片中系统地收集化石。样品将从现场运来提取化石，并在实验室准备成用于岩石学目的的薄片和抛光板材。岩石学、岩石学和古生物化学数据的汇编将成为确定保存下来的生物群的古生态、环境背景和动物群亲缘关系的基础，并确定与阿拉斯加西南部志留纪岩石的岩石和化石成分的相似程度。证明阿拉斯加东南部不止一个相对较小的地区的化石多样性与阿拉斯加西南部是共同的，这将代表着我们对亚历山大地体早期地质历史的理解的重要进步。这些数据将有助于记录支持亚历山大地体靠近北美西北部这一假设的证据，从而允许两个地区之间的动物群交流。这些数据对于提供亚历山大地体和北美之间明确的古地理联系将是重要的。由于类似的志留纪海绵沉积也出现在俄罗斯的乌拉尔山脉，拟议中的研究将为探索海道使生物在志留纪晚期沿北美(劳伦蒂亚)和欧洲(波罗的海)北部边缘迁徙的想法奠定关键基础。

9417907 Soja The accretion of "exotic" crustal fragments, or terranes, to continental margins is the tectonic process believed by many geologists to have been responsible for continental growth along western North America in the last 60-100 million years. Because of the complex geologic relationships produced by accretionary tectonics, the complete geologic history of this tectonic collage in western North America is still unknown. Paleobiogeographic research is of critical importance in Alaska because fossils preserved there occur within this complex zone of accreted terranes and occupy a present geographic position intermediate between the faunal provinces of North America and the circum-Pacific and circum-Arctic regions. To gain a better understanding of the origins and geologic histories of terranes in Alaska, paleontologists have been seeking to identify distinctive suites of fossils that, on the basis of shared affinities in faunas which occupied similar habitats, link certain terranes together or establish biogeographic connections with continental source areas. Using paleobiogeographic data in this way has enormous potential for resolving the controversies reflected in competing models that describe the tectonic evolution of western North America, in general, and Alaska, in particular. The proposed project aims to build upon the PI's previous research by investigating Silurian reef and reef-related carbonate deposits in southeastern Alaska to determine their geologic relationship to Silurian limestones in southwestern Alaska. Rocks exposed in southeastern Alaska belong to the Alexander terrane, which existed during the Early-Middle Paleozoic as volcanically active islands at an unknown site in the ancient Pacific Ocean, whereas deposits in southwestern Alaska (Nixon Fork terrane) represent an original, although dismembered part of the North American continent. Strata in these two areas share in common distinctive Silurian sponge faunas preserved in unusual stromatolite ree fs. In southeastern Alaska, Silurian carbonates have been examined in detail at scattered localities but principally only in one relatively small area in the southern part of the Alexander terrane. The proposed research will enable detailed investigations of Silurian rocks that are exposed in other parts of the Alexander terrane, especially where reconnaissance studies have revealed well-preserved, but as yet unstudied biotas. Field work involving undergraduate research students will focus on describing rocks and collecting fossils systematically from measured and photographed stratigraphic sections. Samples will be shipped from the field for the extraction of fossils and to be prepared in the lab as thin-sections and polished slabs for petrographic purposes. Compilation of petrologic, petrographic, and paleontolgic data will form the basis for determining the paleoecology, environmental setting, and faunal affinities of the preserved biotas and for identifying the degree of similarity in rock and fossil composition with Silurian rocks in southwestern Alaska. Demonstrating that a diversity of fossils from more than one relatively small part of southeastern Alaska is shared in common with southwestern Alaska will represent an important advancement in our understanding of the early geologic history of the Alexander terrane. These data will help to document supporting evidence for the hypothesis that the Alexander terrane was in close proximity to northwestern North America allowing faunal communication between the two localities. Such data would be important in providing a definitive paleogeographic link between the Alexander terrane and North America proper. Because similar Silurian sponge deposits also occur in the Ural Mountains, Russia, the proposed study will lay critical groundwork for exploring the idea that a seaway enabled migration of organisms along the northern rims of North America (Laurentia) and Europe (Baltica) in the Late Silurian.