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

9804988鲍林生命史上的一个关键时期是从下寒武统上部最低的当地三叶虫到中寒武世外观三叶虫的出现。 工作E。兰丁和SA Bowring在这段重要的时期内发现了多达10个下寒武统三叶虫带、几个大旋回规模的沉积交替、下-中寒武统边界的“霍克湾事件"以及西冈瓦纳可能从热带向南高纬度的漂移，表明它持续了大约10 Ma。和古纬度的变化。 摩洛哥海平面和早中寒武世U-Pb时间标准将用于阿瓦隆大陆层序地层学的对比。 终端下寒武统部分在摩洛哥：1）显示两个，第三阶海平面漂移页岩碳酸盐交替; 2）包含火山灰，其中一些有锆石测年误差？1 Ma或更少：3）包括适合于古地磁工作和古地理重建的相。 该项目将完善下寒武统三叶虫和小壳化石生物地层学、沉积历史和上Igoudine-Issafen组的微地层学，该序列在Igoudine碳酸盐之上显示出页岩-碳酸盐宏观循环向内部碎屑带的后退。S.A.鲍林将收集火山灰并测定其年代，以通过摩洛哥三叶虫分带建立U-Pb地质年代标准，测定海平面漂移的日期，并估计沉积岩的积累速率。 地质年代学的工作将使人们能够深入了解早寒武世动物群的周转率。 关于三叶虫分布的数据将用于测试生物相的协调停滞，并有助于理解进化古生物学的一个潜在的重要过程。 用于古地磁分析的样品将补充J. L. Kirschvink的NSF资助的工作在摩洛哥下寒武统的更低。 精确的下寒武统U-Pb测年加上详细的古地磁和地层工作，将允许确定古纬度变化的速率，并允许直接测试Kirschvinck等alli的假设的一个时期的早-中寒武世真正的polar wander.Lower寒武统的相关性是有争议的，由于provincialism的关键动物群和历时最低的本地发生的一些地理上广泛分布的三叶虫。 U-Pb地质年代学提供的时间分辨率正在超越生物地层学，并提供了最好的潜力，以确定海平面漂移和寒武纪古大陆之间的相关性。 阿瓦隆含三叶虫的下寒武统（Branchian统）被划分为两个沉积序列，它们被认为是造陆成因的，但可能反映了海平面变化的成分。 火山灰在上层序和上覆中寒武统中大量存在。 我们将确定这些灰烬的年代，并寻找下布兰奇期的灰烬，以提高摩洛哥终端早寒武世动物群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.