Andrew Leier

Detrital zircon geochronology and provenance of the Neoproterozoic to Late Devonian Franklinian Basin, Canadian Arctic Islands

More than 1800 detrital zircon uranium-lead (U-Pb) ages collected from Franklinian Basin sedimentary strata of the Canadian Arctic Islands provide important insights into the depositional and tectonic evolution of the northern margin of Laurentia from the late Neoproterozoic to the Late Devonian. The Franklinian Basin succession is composed of strata with three distinctly different U-Pb age provenance signatures, which have implications for the tectonic and paleogeographic evolution of the entire Arctic region. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations of 1750–1950 Ma and 2650–2800 Ma, which are consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland–northeast Canadian Shield. The Lower Silurian to Lower Devonian Danish River Formation contains a dominant population of 900–2150 Ma detrital zircons with scattered Archean ages. The 900–2150 Ma zircons were likely transported axially along the foreland basin of the East Greenland Caledonides (Caledonian orogen) and deposited in a deep-water basin between the Pearya terrane and northern Laurentian margin. Middle Devonian to Upper Devonian strata contain detrital zircon populations of 900–2150 Ma, similar to the Danish River Formation, but these units also contain 370–450 Ma and 500–700 Ma detrital zircons. The 900–2150 Ma zircons were likely derived from the East Greenland Caledonian Mountains, the uplifted foreland of the East Greenland Caledonides, and the Pearya terrane. The population of 370–450 Ma detrital zircons potentially comes from uplifting granites in the Caledonian Mountains and Pearya terrane. The 500–700 Ma detrital zircons were likely derived from the continental landmass responsible for the Ellesmerian orogen. The 500–700 Ma age of the zircons suggests that the northern landmass likely had a connection to rocks of the Timanide orogens, located in the Timan Range of northwestern Russia. A dominant population of 365–450 Ma and 500–700 Ma ages in Upper Devonian strata suggests that the Pearya terrane and the northern continental landmass became the dominant source by the end of Franklinian Basin sedimentation. Because detrital zircons are often recycled from older strata into younger deposits, these data provide the basis for understanding the sedimentary provenance of younger units of the Sverdrup Basin and sedimentary wedges along the present Arctic continental margin.

Sediment dispersal in an evolving foreland: Detrital zircon geochronology from Upper Jurassic and lowermost Cretaceous strata, Alberta Basin, Canada

The Alberta foreland basin is a classic example of a retro-arc foreland basin, yet the early stages of its development remain poorly understood. Several contrasting hypotheses have been proposed to explain the source areas and dispersal patterns of sediment in western Canada during the Late Jurassic initiation of the foreland basin. Here, we use detrital zircon uranium-lead (U-Pb) geochronology, sandstone petrography, paleocurrent measurements, and regional correlations to reconstruct the early basin evolution, including sediment provenance and depositional history. These data indicate sediment in the early foreland basin was delivered via two principal sedimentary systems: a south-to-north axial river system, and transverse fluvial systems that emanated from the adjacent Cordillera. Accordingly, sandstones of the Jurassic foreland, associated with the Minnes Group and equivalent Kootenay and Nikanassin formations, are divided into two informal groups, type 1 and type 2. Type 1 sandstones are mature quartz arenites, present along the entire north-south length of the Alberta Basin, and generally at the base of the succession. Type 1 sandstones have zircons with age populations between 980 and 2000 Ma, similar to sediments of Jurassic and Lower Cretaceous strata in the western United States. These deposits are interpreted to have been derived from southern sources and transported axially to the north along the earliest foredeep of the Cordilleran foreland basin. Type 2 sandstones by contrast, are less mature, containing higher quantities of chert and lithic fragments, and are dominated by 1765–2100 Ma zircons with a smaller population at 2500–2800 Ma. The zircon age populations of type 2 sandstones are similar to populations recorded in the Neoproterozoic to Triassic miogeocline strata of the adjacent fold-and-thrust belt. Type 2 sandstones are common in the western, orogenic side of the basin, but they extend eastward across the basin in fluvial sediments in the upper portion of the succession. Changes in provenance and sediment composition are associated with the evolution of paleodrainages and the increasing importance of Cordilleran erosion to the sediment budget. The progressively greater influx of orogen-derived material relative to subsidence displaced the axial fluvial system toward a more cratonward-position. The collected data support the hypothesis that much of the sediment was initially transported northward by an axial drainage network, followed by Cordilleran-sourced sediments fed by transverse river systems. The present study attempts to unravel predictable patterns of sediment dispersal in evolving foreland basins, while testing whether clear changes in sediment composition of the first clastic pulse of sediment are related to hinterland exhumation, changing drainage divides, weathering processes, or varied provenance.

Continental-scale detrital zircon provenance signatures in Lower Cretaceous strata, western North America

Lower Cretaceous strata and the underlying sub-Cretaceous unconformity in western North America record a profound, but poorly understood change in sedimentation patterns and basin dynamics in the Cordilleran foreland basin. To better understand the regional sedimentary systems and provenance during Early Cretaceous time, we sampled 10 Lower Cretaceous sandstone and conglomerate units that overlie the sub-Cretaceous unconformity in Canada and the United States for detrital zircon uranium-lead (U-Pb) geochronology. These Lower Cretaceous strata contain two distinct detrital zircon U-Pb age signatures. A “northern” signature, present in strata in Alberta and British Columbia, contains zircons with ages of ca. 120 Ma and 1850 Ma, and is composed of zircons from the Cordilleran arc and grains recycled from strata of the Canadian miogeocline. A “southern” signature, present in strata from southwestern United States to central Montana, contains zircons with ages of ca. 160 Ma, ca. 250–650 Ma, and ca. 1040 Ma, and consists of zircons from the Cordilleran arc and grains recycled from late Paleozoic strata and Mesozoic eolianite units in the western United States. We propose that the differences in detrital zircon U-Pb age populations between northern and southern areas of western North America are due to differences in zircon populations in the sediment source strata exposed in the contemporaneous thrust belt, and possibly a subtle paleohydraulic divide in Montana. These distinct provenance signatures along the Cordillera suggest that the mechanisms responsible for Early Cretaceous changes in foreland basin dynamics occurred along the length of the Cordillera in both the U.S. and Canada.

Quantifying Dextral Shear on the Bristol-Granite Mountains Fault Zone: Successful Geologic Prediction from Kinematic Compatibility of the Eastern California Shear Zone

For regional kinematic compatibility to be a valid boundary condition for continental tectonic reconstructions, there must be tests that validate or invalidate kinematic model predictions. In several reconstructions of western North America, the displacement history of the Mojave block continues to be unresolved. The magnitude of displacement along the Bristol-Granite mountains fault zone (BGMFZ), which is the eastern margin of the Eastern California Shear Zone (ECSZ) in the Mojave block, is a key example of a long-standing kinematic prediction that has defied a positive field test until now. The ECSZ is a network of late Neogene and Quaternary right-lateral strike-slip faults that extend from the Gulf of California north through the Mojave Desert, linking Pacific–North America plate motion with Basin and Range extension. This network of faults accounts for ∼15% of post-16-Ma plate transform motion. Geologic estimates of net dextral offset along the Mojave portion of the ECSZ (53 ± 6 km) are approximately half that measured to the north in the Owens Valley–Death Valley region (∼100 ± 10 km). Previous geological estimates of BGMFZ slip range from 0 to 15 km. Models of right-lateral displacement that are based on kinematic compatibility suggest 21–27 km of BGMFZ displacement. We map and describe a tuff- and gravel-filled paleovalley offset by the BGMFZ. The orientation of the Lost Marble paleovalley is constrained by the position of gravel outcrops, provenance, and tuff anisotropy of magnetic susceptibility. Reconstruction of the paleovalley indicates at least 24 km of post-18.5-Ma dextral offset, confirming a significant, previously undocumented component of dextral slip in the Mojave portion of the ECSZ.

Sandstone provenance and insights into the paleogeography of the McMurray Formation from detrital zircon geochronology, Athabasca Oil Sands, Canada

The Lower Cretaceous McMurray Formation of northeastern Alberta hosts most of the bitumen resources of the Athabasca Oil Sands. Despite its importance, the sedimentary provenance and corresponding Early Cretaceous paleodrainage system associated with these fluvial deposits remain poorly understood. We collected 18 sandstone samples from five cored wells drilled in the McMurray Formation and analyzed these for detrital zircon uranium–lead (U–Pb) geochronology. Together, these samples yield detrital zircon U–Pb age populations of less than 250, 300–600, 1000–1200, 1800–1900, and 2600–2800 Ma. Almost all of the samples contain detrital zircons with ages of 300–600 and 1000–1200 Ma, which were originally derived from the Appalachian and Grenville provinces, respectively, of eastern North America. Lowermost strata of the McMurray Formation are characterized by relatively small fluvial channel deposits and detrital zircon ages of 1800–1900 and 2600–2800 Ma, which suggest a limited paleodrainage area that includes the adjacent Canadian shield. In contrast, channel deposits in the middle–upper part of the formation are relatively large and contain abundant Appalachian- and Grenville-derived detrital zircons. These data suggest that the paleodrainage system of the McMurray Formation evolved over time, increasing in size between deposition of the lowermost units and the middle–upper deposits. Detrital zircons from the Appalachian and Grenville regions may have been transported directly to western Canada during the Cretaceous or recycled multiple times prior to their deposition. Detrital zircons from the Cordillera (<250 Ma) are restricted to the northern part of the study area, which suggests that a tributary may have joined the main trunk fluvial system in this area.

Wind-driven reorganization of coarse clasts on the surface of Mars

Coarse (pebble to cobble sized) clasts on the intercrater plains of the Mars Exploration Rover Spirit landing site exhibit a nonrandom (i.e., uniformly spaced) distribution. This pattern has been attributed to the entrainment and redistribution of coarse clasts during extreme wind events. Here we propose an alternative mechanism readily observable in wind tunnels and numerical models at modest wind speeds. In this process, coarse clasts modify the air fl ow around them, causing erosion of the underlying substrate on the windward side and deposition on the leeward side until a threshold bed-slope condition is reached, after which the clast rolls into the windward trough. Clasts can migrate across an erodible substrate in repeated cycles of trough formation and clast rolling, “attracting” or “repelling” one another through feedbacks between the local clast density, substrate erosion and/or deposition rate, and substrate elevation. The substrate beneath areas of locally high clast densities aggrades, building up a topographic high that can cause clasts to repel one another to form a more uniform distribution of clasts through time. This self-organized process likely plays a signifi cant role in the evolution of mixed grain size eolian surfaces on Earth and Mars.