The Neoproterozoic and Early Paleozoic Tectonic and Environmental Evolution of Alaska and Northwest Canada.

摘要

Neoproterozoic and early Paleozoic sedimentary deposits of the North American Cordillera record large fluctuations in global biogeochemical cycles, the establishment and diversification of multiple eukaryotic clades, the fragmentation of the supercontinent Rodinia, and the protracted development and subsequent demise of the western and northern Laurentian passive margins. Here, I put forth new tectono--, bio--, and chemo-stratigraphic models for the ∼780-540 Ma Windermere Supergroup of western North America and "pre-Mississippian" stratigraphy of northern Alaska that refine previous models for the Neoproterozoic and early Paleozoic tectonic and environmental evolution of Alaska and northwest Canada.;First, I present an updated model for early Windermere (780-720 Ma) sedimentation in NW Canada through a detailed study of the Callison Lake Formation of the Mount Harper Group, spectacularly exposed in the Coal Creek and Hart River inliers of the Ogilvie Mountains of Yukon, Canada. Twenty-one detailed measured stratigraphic sections are integrated with geological mapping, facies analysis, and new Rhenium-Osmium (Re-Os) geochronology to provide a depositional model for the Callison Lake Formation. Mixed siliciclastic, carbonate, and evaporite sediments record a complex subsidence history in which episodic basinal restriction and abrupt facies change can be tied accumulation in marginal marine embayments formed in discrete hangingwall depocenters of a major Windermere extensional fault zone. New organic-rich rock Re-Os ages of 752.7 +/- 5.5 and 739.9 +/- 6.1 Ma bracket Callison Lake sedimentation and constrain early Windermere sedimentation in NW Canada to post-date the eruption of the Gunbarrel Large Igneous Province by ∼30 million years and predate the successful rift-drift transition by ∼200 million years. In order to accommodate coeval extensional and compressional tectonism, abrupt facies change, and Neoproterozoic fault geometries, I propose that NW Canada experienced strike-slip deformation during the ∼740-660 Ma early fragmentation of Rodinia.;Second, I integrate carbon and oxygen isotope chemostratigraphy, sequence stratigraphy, geochronological data, and microfossil biostratigraphy from the Callison Lake Formation to highlight the potential for margin-wide correlation of Neoproterozoic successions in North America. Here, I also report the discovery of abundant and well-preserved vase-shaped microfossils in the Callison Lake Formation, dated with Re-Os geochronology at 739.9 +/- 6.1 Ma, that share multiple species-level taxa with a well-characterized and coeval assemblage from the Chuar Group, Grand Canyon, Arizona dated with U-Pb on zircon from an interbedded tuff at 742 +/- 6 Ma. The overlapping age and species assemblages from these two deposits suggests biostratigraphic utility, at least within Neoproterozoic basins of Laurentia, and perhaps globally. Sequence stratigraphic data from the Callison Lake Formation and other basal Windermere successions in northwest Canada delineate four major depositional sequences that are broadly coeval with similar stratigraphic packages in the ∼780--720 Ma Chuar-Uinta Mountain-Pahrump basins of the western United States. The new Re-Os age also confirms the timing of the Islay carbon isotope excursion (ICIE) in northwest Canada, which predates the onset of the Sturtian glaciation by >15 million years. Here, I hypothesize that this carbon isotope excursion represents a primary perturbation to the global carbon cycle and explore a number of models for its origin related to the duration of the excursion. Together, these data provide global calibration of sedimentary, paleontological, and geochemical records on the eve of profound environmental and evolutionary change.;Finally, I present an updated model for the origin of the Arctic Alaska--Chukotka microplate, a composite Cordilleran "suspect" terrane that comprises the greater portion of the modern continental margin of the Amerasian Basin of the Arctic Ocean, through a detailed study of pre-Mississippian stratigraphy in the Shublik, Sadlerochit, and British Mountains of the northeastern Brooks Range, Alaska. An exotic, non-Laurentian origin of Arctic Alaska--Chukotka has been proposed based on paleobiogeographic faunal affinities and various geochronological constraints from the southwestern portions of the microplate. Here, I report new early Paleozoic trilobite and conodont taxa that support a Laurentian origin for the North Slope of Arctic Alaska, as well as new Neoproterozoic--Cambrian stratigraphic correlations and igneous and detrital zircon geochronological data, that are both consistent with a Laurentian origin and profoundly different from those derived from similar-aged strata in the southwestern portions of Arctic Alaska--Chukotka. The North Slope terrane is accordingly interpreted as allochthonous with respect to its current position in northwestern Laurentia, but most likely originated further east along the Canadian Arctic or North Atlantic margins. These data demonstrate that Paleozoic construction of the composite Arctic Alaska--Chukotka microplate resulted from juxtaposition of the exotic southwestern parts of the microplate against the northern margin of Laurentia during protracted Ordovician(?)--Carboniferous Caledonian and Ellesmerian tectonism.