Collaboratite Research: Rates of "Terminal" Early Cambrian Sea-Level Change, Apparent Polar Wander, and Faunal Turnover in Morocco and Avalon

[804988]从上寒武纪最低的局部三叶虫到中寒武纪三叶虫的出现，是生命史上的一个关键时期。E. Landing和sa . Bowring对这一事件间隔的研究表明，它持续了大约10 Ma，包括多达10个下寒武统三叶虫带，几个大旋回规模的沉积交替，下寒武统-中寒武统边界“Hawke Bay事件”，以及西冈瓦纳可能从热带到高纬度的漂移。我们建议量化冈瓦纳西部摩洛哥系硅质层序早-早中寒武世晚期海平面、动物和古纬度的变化速率。摩洛哥海平面—早中寒武世U-Pb时间标准将用于对比阿瓦隆大陆的层序地层学。摩洛哥下寒武统末端剖面：1)在页岩-碳酸盐岩交替中表现出2、3级海平面偏移；2)含有火山灰，其中一些含有锆石，其年代测定误差为？1 Ma及以下；3)包括适合古地磁工作和古地理重建的相。本项目将对上伊古因—伊萨芬组下寒武统三叶虫和小壳虫化石生物地层学、沉积史和微地层学进行精化，该层序在伊古因碳酸盐之上显示了页岩-碳酸盐大尺度旋回向内碎屑带的退步。S.A. Bowring将收集火山灰并确定年代，通过摩洛哥三叶虫带建立U-Pb地质时间标准，确定海平面漂移日期；估算沉积岩的堆积速率。地质年代学工作将使我们能够深入了解早寒武纪的动物更替率。三叶虫分布的数据将用于测试生物相中的协调停滞，并有助于理解进化古生物学的一个潜在重要过程。古地磁分析样本将补充J.L. Kirschvink在摩洛哥下寒武纪的研究。精确的下寒武纪U-Pb日期加上详细的古地磁和地层学工作，将可以确定古纬度变化的速率，并可以直接检验Kirschvinck et alli关于早-中寒武纪真正极移时期的假设。由于主要动物群的地方性和一些地理上广泛分布的三叶虫的最低局部发生率的历时性，下寒武纪的相关性是有争议的。U-Pb年代学提供的时间分辨率正在超越生物地层学，并提供了确定海平面漂移日期和在寒武纪古大陆之间进行关联的最佳潜力。阿瓦隆下寒武统（支系）含三叶虫的沉积被划分为两个沉积层，它们的成因被认为是表生的，但也可能反映了隆起的成分。上层序和上覆的中寒武统火山灰众多。我们将测定这些灰烬的年代，并寻找下支支灰烬，以提高与摩洛哥早寒武世末期动物序列和沉积历史的相关性。

9804988BowringA crucial interval in life history extends from the lowest local trilobites in the upper Lower Cambrian through to the appearance of Middle Cambrian-aspect trilobites. Work by E. Landing and S.A. Bowring on this eventful interval, with up to ten Lower Cambrian trilobite zones, several Grand cycle-scale sedimentary alternations, the Lower--Middle Cambrian boundary "Hawke Bay event(s)", and a possible excursion of west Gondwana from tropical to high south latitudes, indicates that it lasted about 10 Ma.We propose to quantify rates of late Early--earliest Middle Cambrian sea-level, faunal, and paleolatitudinal change in the of siliferous Moroccan sequences of west Gondwana. A Moroccan sea level-- and Early-Middle Cambrian U-Pb time standard will be used for correlation into the sequence stratigraphy of the Avalon continent. Terminal Lower Cambrian sections in Morocco: 1) show two, third-order sea-level excursions in shale-carbonate alternations; 2) contain volcanic ashes, some of which have zircons datable with an error of ? 1 Ma or less: and 3) include facies suitable for paleomagnetic work and paleogeographic reconstruction. This project will refine the Lower Cambrian trilobite and small shelly fossil biostratigraphy, depositional history, and microstratigraphy of the upper Igoudine--Issafen Formations, a sequence, which above the Igoudine carbonates shows a back-stepping of shale-carbonate macroscale cycles into the inner detrital belt. S.A. Bowring will collect and date volcanic ashes to establish a U-Pb geologic time standard through the Moroccan trilobite zonation, date sea-level excursions; and estimate accumulation rates of sedimentary rocks. Geochronologic work will allow insights into Early Cambrian faunal turnover rates. Data on trilobite distributions will be used to test for coordinated stasis in biofacies and contribute to the understanding of a potentially important process of evolutionary paleobiology. Samples for paleomagnetic analysis will complement J.L. Kirschvink's NSF-funded work much lower in the Moroccan Lower Cambrian. Precise Lower Cambrian U-Pb dates coupled with detailed paleomagnetic and stratigraphic work will allow determination of rates of paleolatitudinal change and permit a direct test of Kirschvinck et alli's hypothesis of a period of Early--Middle Cambrian true polar wander.Lower Cambrian correlations are controversial due to provincialism of key faunal groups and diachronous lowest local occurrences of some geographically widespread trilobites. The temporal resolution provided by U-Pb geochronology is overtaking that of biostratigraphy, and offers the best potential to date sea-level excursions and correlate them between Cambrian paleocontinents. The trilobite-bearing Lower Cambrian (Branchian Series) of Avalon has been divided into two depositional sequences, which are regarded as epeirogenic in origin but may reflect a eustatic component. Volcanic ashes are numerous in the upper sequence and overlying Middle Cambrian. We will date these ashes and search for lower Branchian ashes to improve correlation into the Moroccan terminal Early Cambrian faunal sequence and depositional history.