NSF Award
9805177
9805177<br/>Landing<br/><br/>A 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.<br/><br/>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.<br/><br/>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.
