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

9805177登陆生活史上的一个关键时期是从当地最低的三叶虫开始的。E.Landing和S.A.Bowring对这一重大事件的研究表明，这一事件持续了大约10 Ma。我们建议量化冈瓦那西部早中寒武世晚期-早期-早期的海平面、动物群和古纬度的变化速率。摩洛哥海平面和早中寒武世U-Pb时间标准将用于对比阿瓦隆大陆的层序地层学。摩洛哥陆相下寒武统剖面：1)显示了页岩-碳酸盐蚀变中的两次三级海平面漂移；2)含有火山灰，其中一些含有可测年的锆石，误差为？Ma或以下；3)包括适合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的生物地层学、沉积史和伊古丁-伊斯萨芬组上部的微观地层学，伊古丁碳酸盐之上的序列显示出页岩-碳酸盐大尺度旋回的倒退，随后是下-中寒武统边界砂岩的进积，并将这些旋回追踪到内部碎屑带。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地质时间标准，确定海平面漂移的日期，并估计沉积岩的堆积率。地质年代学工作将使人们能够深入了解早寒武世的动物群周转率。关于三叶虫分布的数据将被用来测试生物相的协调停滞，并有助于理解一个潜在的重要的古生物学进化过程。用于古地磁分析的样本将补充J.L.Kirschvink在摩洛哥下寒武统低得多的NSF资助的工作。精确的下寒武统U-Pb测年结合详细的古地磁和地层学研究将有助于确定古纬度变化的速率，并允许直接检验Kirschvinck等人关于早中寒武世真正极地游荡时期的假说。由于关键动物群组的地域性和一些地理上分布的三叶虫的历时最低出现率，下寒武统的对比存在争议。U-Pb年代学提供的时间分辨率正在超过生物地层学，并为确定海平面漂移的日期和寒武纪古陆之间的对比提供了最好的可能性。阿瓦隆下寒武统(布兰奇统)含三叶虫的沉积层序被划分为两个沉积层序，它们的成因被认为是陆生的，但可能反映了海平面成分。上位层序和中寒武统上覆有大量火山灰。我们将测定这些骨灰的年代，并寻找下布兰奇骨灰，以提高与摩洛哥末期早寒武世动物群序列和沉积史的对比。

9805177LandingA crucial interval in life history extends from the lowest local 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 fossiliferous 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. Termainal 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 ? 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 followed by progradation of Lower--Middle Cambrian boundary sandstones, and trace these 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 an 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.