Ward, Brent

This thesis advances understanding of late Cenozoic landscape evolution and glaciation in southernmost South America using continental sedimentary deposits and landforms in the Lago Cardiel region in the foothills of the southern Patagonian Andes and along the Atlantic north and south of the Strait of Magellan. The evolution of the landscape in these two areas was determined through landform mapping and relative chronologic landform correlations. Paleomagnetic characteristics of late Cenozoic sediments and basalt flows and the stratigraphy and sedimentology of Pleistocene glacial sediments in sea cliffs and anthropogenic exposures provide a chronology and evidence of depositional environments during Pleistocene glaciations. The landscape in the Lago Cardiel area changed significantly following the last major period of tectonic uplift at the end of the Miocene. Large west-trending valleys that incise Miocene-aged basalt were abandoned by their formative rivers about 4.4 Ma. The closed basin that contains Lago Cardiel began to form on the relict plain surface before 4.0 Ma and grew in size throughout the Pliocene and Pleistocene by a combination of erosion by small streams, deflation, colluviation, and possibly tectonic collapse. Drainage reorganizations occurred at about 4.0 Ma and 3.6 Ma, most likely initiated by increased aggradation or isostasy during Pliocene glaciations. Eolian, fluvial, and mass-movement processes continued to alter the landscape throughout the Pleistocene with higher rates during glacial periods. Evidence of at least three glaciations is recorded in the stratigraphic exposures at the Atlantic Coast and the shores of the Strait of Magellan. At Cabo Vírgenes and Bahía Posesión, two glacial drift units were deposited in a grounding-line environment. These sediments are normally magnetized and date to the Brunhes Chron (<0.78 Ma). The Tres de Enero highway cut exposes three subglacial tills deposited during the Great Patagonian Glaciation (GPG) – two normally magnetized tills that I assign to the early Brunhes Chron and a lower reversely magnetized till deposited during the Matuyama Chron (2.581-0.78 Ma). The reversely magnetized till and other reversely magnetized GPG sediments indicate that the earliest Pleistocene glaciations occurred before 0.78 Ma. In the Río Gallegos Valley, a 0.86 Ma basalt flow caps a thick unit of normally magnetized glaciofluvial gravel, which was probably deposited during the Jaramillo Subchron (1.075-0.991 Ma). This thesis provides a timeline for the evolution of the landscape of the Lago Cardiel region from the Miocene to the present. It also contributes to our understanding of the age and depositional environments of GPG and post-GPG 1 glacial events in the Strait of Magellan region by documenting the magnetic polarity of glacial sediments throughout the region.

Quaternary glacial and non-glacial sediment exposed at White River and Silver Creek provide a record of environmental change in southwest Yukon for much of the late- Middle to Late Pleistocene. Eighteen sites at White River, located beyond the marine oxygen isotope stage (MIS) 2 glacial limit, contain thick accumulations of till, loess, peat, gravel and glaciolacustrine silt and clay, with tephras, paleosols, plant and insect macrofossils and large mammal fossils. Radiocarbon ages and eleven tephra beds constrain two tills to MIS 4 and 6. These tills correlate to the Gladstone and Reid glaciations and represent the penultimate and maximum all-time limits of the St. Elias lobe of the northern Cordilleran Ice Sheet. Two peat beds located between these tills indicate that interglacial conditions existed in the area during MIS 5e and 5a. Pond sediment deposited during mid-MIS 5 suggests that the sites were covered by an open birch tundra at this time. The MIS 3/2 transition was marked by a treeless, dry steppe- tundra populated by mammoth, horse and bison. The eleven Silver Creek sites, located ~200 km up-ice, contain a similar record of glacial and non-glacial sediment. Infrared-stimulated luminescence (IRSL) and radiocarbon dating constrain the glacial deposits at these sites to MIS 2, 4, either MIS 7 or 6, and to two Early to Middle Pleistocene, Pre-Reid glaciations. Tilting of glaciolacustrine beds of up to 1.9 mm/yr may be from uplift along the Denali fault since MIS 7. Pollen and macrofossils analyses from overlying MIS 3-aged sediment suggest that the environment was dominated by herbs and forbs, with few shrubs and almost no tree pollen at this time. Combined, the White River and Silver Creek sites contain a record of glacial and non-glacial conditions in southwest Yukon since the Middle Pleistocene. The glacial limits in southwest Yukon are markedly different from those in central Yukon. In southwest Yukon, the glacial limits are closely-spaced and were more extensive in the Middle to Late Pleistocene than in the Late Pliocene and Early Pleistocene. In central Yukon, glacial limits are separated by up to 300 km and were most extensive in the latest Pliocene and Early Pleistocene. This suggests that different forcing mechanisms controlled the extents of the St. Elias and Selwyn lobes during successive glaciations. Boundary conditions such as varying substrates, topography, moisture pathways and atmospheric circulation likely had a greater affect than tectonics and sea level on these glacial limits throughout the Plio-Pleistocene.

Four stages of ice-flow occurred in Howard’s Pass during the late Wisconsinan McConnell glaciation. The first stage is marked by ice growth from local cirques. During the second stage, an ice divide developed east of the Nahanni River, with ice flowing southwest across Howard’s Pass. Ice sheet growth continued during stage 3 and the ice divide migrated southwest into the Logan Mountains. At this time ice flowed northward across the study area. Stage 4 is marked by deglaciation and more topographically influenced ice-flow. This last phase of ice-flow is the most important for drift prospecting in the valley bottoms. Conversely, drift transport directions at higher elevation are likely remnant from earlier stages of ice-flow. A mobile-metal-ion survey over a known deposit returned promising results, supporting the potential of this geochemical technique in other drift-covered areas of Howard’s Pass.

Chapter: The natural landscape of the North Shore of Vancouver is a mountainous one extending from sea level to ~1400 m. Land below ~400 m has been undergoing increasing urbanization since the 1950s. Development has encroached on areas subject to natural hazards such as floods, debris flows, slope failures, and coastal inundation. We will visit examples of these urban hazards, discuss problems of hazard identification in a forested landscape, and review urban planning and engineering responses to hazard management. Book: This volume, prepared for the 126th GSA Annual Meeting in Vancouver, Canada, offers guides to trips in the Cascadia subduction zone. The active tectonism of the region has had a profound effect on the bedrock and surficial geology of the area, and on human interactions with the geologic environment. These themes are reflected in the trips associated with the meeting. Trip topics relate to bedrock geology, volcanism and Cordilleran glaciation and deglaciation, as well as human interaction with the natural environment. The trips that discuss human interaction cover archaeology, natural hazards and the urban environment, as well as the role that local geology and tectonism have played in shaping colonization of the region since the last glaciation. The field guide volume has something for everyone!

Chapter: This field guide describes a three-day trip from Vancouver, British Columbia, to the Wells Gray–Clearwater volcanic field (WGCVF) in east-central British Columbia. The WGCVF is the site of transitional to alkali olivine basaltic volcanism erupted over the last three million years. The small volume magmas (<1 km3) erupted along preexisting normal faults related to the late stages of Cordilleran terrane amalgamation, along the boundary between the miogeoclinal and pericratonic rocks of the Kootenay terrane and the allochthonous Slide Mountain and Quesnellia terranes west of ancestral North America. The magmas are highly enriched in incompatible elements, especially large-ion lithophile elements, and are interpreted as the result of low degrees of partial melting of a heterogeneous, metasomatized mantle. Upon ascent through the crust, they carried up both crustal and mantle xenoliths. During the eruptive period of the WGCVF, at least four glacial periods have occurred. The interplay between volcanism and glaciation is captured in the wide range of volcanic features found in the region. Field trip participants will view numerous diverse volcanic landforms and deposits: from tuyas to ice-marginal valley-edge deposits, volcanoclastic-lacustrine deposits, and associated pillow lavas and hyaloclastites. <p>Book: This volume, prepared for the 126th GSA Annual Meeting in Vancouver, Canada, offers guides to trips in the Cascadia subduction zone. The active tectonism of the region has had a profound effect on the bedrock and surficial geology of the area, and on human interactions with the geologic environment. These themes are reflected in the trips associated with the meeting. Trip topics relate to bedrock geology, volcanism and Cordilleran glaciation and deglaciation, as well as human interaction with the natural environment. The trips that discuss human interaction cover archaeology, natural hazards and the urban environment, as well as the role that local geology and tectonism have played in shaping colonization of the region since the last glaciation. The field guide volume has something for everyone! --From publisher description.