Coastal paleogeography of the Pacific Northwest, USA, for the last 12,000 years accounting for three-dimensional earth structure (original) (raw)
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Estimating Pleistocene Shorelines and Land Elevations for North America
Estimating paleo-shoreline extents for the late Pleistocene and early Holocene has traditionally been a difficult task to accomplish even at local scales. This fact is highlighted by the common use of the approximate Last Glacial Maximum (LGM) shoreline contour in the literature when regional or continental scale distributions are illustrated. While useful as a heuristic, the LGM shoreline is not accurate for most of the Paleoindian Period and can present problems for geographic information system (GIS) and other spatial analyses of site distribution and land use.
Earth-Science Reviews, 2015
For nearly a century, the Atlantic Coastal Plain (ACP) of the United States has been the focus of studies investigating Pliocene and Pleistocene shorelines, however, the mapping of paleoshorelines was primarily done by using elevation contours on topographic maps. Here we review published geologic maps and compare them to paleoshoreline locations obtained through geomorphometric classification and satellite data. We furthermore present the results of an extensive field campaign that measured the mid-Pliocene (~3.3-2.9 Ma) shorelines of the Atlantic Coastal Plain using high-accuracy GPS and digital elevation models. We compare our new dataset to positions and elevations extracted from published maps and find that the extracted site information from earlier studies is prone to significant error, both in the location and, more severely, in the elevation of the paleoshoreline. We also investigate, using geophysical modeling, the origin of post-depositional displacement of the shoreline from Georgia to Virginia. In particular, we correct the elevation of our shoreline for glacial isostatic adjustment (GIA) and then compare the corrected elevation to predictions of mantle flow-induced dynamic topography (DT). While a subset of these models does reconcile the general trends in the observed elevation of the mid-Pliocene shoreline, local discrepancies persist. These discrepancies suggests that either (i) the DT and GIA models presented here do not capture the full range of uncertainty in the input parameters; and/or (ii) other influences, such as sediment loading and unloading or local fault-driven tectonics, may have contributed to post-depositional deformation of the mid-Pliocene shoreline that are not captured in the above models. In this context, our field measurements represent an important observational dataset with which to compare future generations of geodynamic models. Improvements in models for DT, GIA and other relevant processes, together with an expanded, geographically distributed set of shoreline records, will ultimately be the key to obtaining more accurate estimates of eustatic sea level not only in the mid-Pliocene but also earlier in the Cenozoic.
Current Research in the Pleistocene, 2006
Estimating paleo-shoreline extents for the late Pleistocene and early Holocene has traditionally been a difficult task to accomplish even at local scales. This fact is highlighted by the common use of the approximate Last Glacial Maximum (LGM) shoreline contour in the literature when regional or continental scale distributions are illustrated. While useful as a heuristic, the LGM shoreline is not accurate for most of the Paleoindian Period and can present problems for geographic information system (GIS) and other spatial analyses of site distribution and land use. Fortunately, GIS data and sea-level depth estimations are now available that make modeling ancient shorelines and land elevations more practical at the continental scale. The ETOPO2 dataset of the National Geophysical Data Center (NGDC) is a 2-minute latitude/longitude grid (approximately 3.7-km spatial resolution at the equator) representing land elevations and seafloor bathymetry derived from the Global Land One-km Base Elevation (GLOBE) digital elevation model (Hastings and Dunbar 1998) and satellite altimetry and ship depth soundings for bathymetry (Jakobsson et al. 2000; Smith and Sandwell 1997). These data, which are easily integrated into a raster GIS environment for analysis, are distributed free online as raw data or customizable grids for specific areas (http://www.ngdc.noaa.gov/mgg/image/2minrelief.html). Using published sea-level curve estimates (Lambeck et al. 2002), it is relatively simple to use map algebra techniques in a raster GIS to reclassify the ETOPO2 grids to represent paleo-shorelines and land elevations. For example, the digital elevation model (DEM) for 13,000 CALYBP uses the sea level depth estimate of-75 m (Figure 1). To adjust the ETOPO2 DEM data, you
Late Pleistocene Archaeological Discovery Models on the Pacific Coast of North America
PaleoAmerica, 2019
The Pacific coast of North America is a hypothesized route by which the earliest inhabitants of the Americas moved southwards around the western margin of the Cordilleran Ice Sheet just after the last glacial maximum. To test this hypothesis, we have been using a stepwise process to aid in late Pleistocene archaeological site discovery along the coast. The steps involved include: (1) creating localized sea level curves; (2) generating detailed bare earth digital elevation models; (3) creating archaeological predictive models; (4) ground truthing these models using archaeological prospection; and (5) demonstrating that archaeological materials found date to the late Pleistocene. Here, we consider the use of these steps and how they have been employed to find late Pleistocene archaeological sites along the Pacific Coast of North America.
Post-glacial sea-level change along the Pacific coast of North America
Quaternary Science Reviews, 2014
Sea-level history since the Last Glacial Maximum on the Pacific margin of North America is complex and heterogeneous owing to regional differences in crustal deformation (neotectonics), changes in global ocean volumes (eustasy) and the depression and rebound of the Earth's crust in response to ice sheets on land (isostasy). At the Last Glacial Maximum, the Cordilleran Ice Sheet depressed the crust over which it formed and created a raised forebulge along peripheral areas offshore. This, combined with different tectonic settings along the coast, resulted in divergent relative sea-level responses during the Holocene. For example, sea level was up to 200 m higher than present in the lower Fraser Valley region of southwest British Columbia, due largely to isostatic depression. At the same time, sea level was 150 m lower than present in Haida Gwaii, on the northern coast of British Columbia, due to the combined effects of the forebulge raising the land and lower eustatic sea level. A forebulge also developed in parts of southeast Alaska resulting in post-glacial sea levels at least 122 m lower than present and possibly as low as 165 m. On the coasts of Washington and Oregon, as well as south-central Alaska, neotectonics and eustasy seem to have played larger roles than isostatic adjustments in controlling relative sea-level changes.
Journal of Geophysical Research, 1990
Marine terraces are prominent landforms along the southern Oregon coast, which forms part of the forearc region of the Cascadia subduction zone. Interest in the Cascadia subduction zone has increased because recent investigations have suggested that slip along plates at certain types of convergent margins is characteristically accompanied by large earthquakes. In addition, other investigations have suggested that convergent margins can be broadly classified by the magnitude of their uplift rates. With these hypotheses in mind, we generated new uranium series, amino acid, and stable isotope data for southern Oregon marine terrace fossils. These data, along with terrace elevations and two alternative estimates of sea level at the time of terrace formation, allow us to determine terrace ages and uplift rates. Uranium series analysis of fossil coral yields an age of 83 _ 5 ka for the Whisky Run terrace at Coquille Point in Bandon, Oregon. A combination of amino acid and oxygen isotope data suggest ages of about 80 and 105 ka for the lowest two terraces at Cape Blanco. These ages indicate uplift rates of 0.45-1.05 and 0.81-1.49 m/kyr for Coquille Point and Cape Blanco, respectively. Late Quaternary uplift rates of marine terraces yield information about deformation in the overriding plate, but it is unclear if such data vary systematically with convergent margin type. In order to assess the utility of the southern Oregon uplift rates for predicting the behavior of the Cascadia subduction zone, we compared late Quaternary uplift rates derived from terrace data from subduction zones around the world. On the basis of this comparison the southern Oregon rates of vertical deformation are not unusually high or low. Furthermore, late Quaternary uplift rates show little relationship to the type of convergent margin. These observations suggest that local structures may play a large role in uplift rate variability. In addition, while the type of convergent margin may place an upper limit on possible uplift rate, greater upper limits serve to increase the range of possible uplift rates. In the case of the southern Oregon coast, variability in uplift rate probably reflects local structures in the overriding plate, and the rate of uplift cannot be used as a simple index of the potential for great earthquakes along the southern Cascadia subduction zone.
The Holocene, 1998
Intertidal stratigraphy has been instrumental in demonstrating the hazard posed by great earthquakes at the Cascadia subduction zone, but inferring an earthquake history from interbedded sequences of peat and mud is complicated by many factors that influence sedimentation and relative sea-level change on both tectonic and nontectonic coasts. Rapid-to-sudden rises in relative sea level marked by sharp contacts between intertidal peat and overlying mud or sand may reflect coseismic coastal subsidence and tsunami deposition or, alternatively, nonseismic hydrodynamic changes in estuaries. Reconnaissance coring at 16 sites in the marshes fringing a narrow, protected tidal inlet of Coos Bay, supplemented by diatom and 14 C analyses at four sites, reveals a stratigraphic record too fragmentary and ambiguous to distinguish seismic from hydrodynamic causes for more than three of the 10 rises in relative sea-level identified. Only three sharp contacts have the wide extent and evidence of substantial (Ͼ0.5 m) submergence that distinguish them from similar contacts produced by nonseismic processes. Correlation with stratigraphic sequences at other estuaries shows that the fringing marshes suddenly subsided and were partially buried by tsunami sand during a great plate-boundary earthquake about 300 years ago. Similar contacts were produced by earthquakes about 1500-1800 years ago, and perhaps about 2400-2700 years ago. Other earthquakes with substantially less subsidence may also have occurred, but evidence is too ambiguous to reconstruct a more complete history.
To mitigate saltwater flooding, the waterfront and downtown areas of Port Angeles, Washington were built-up with up to 8 m of anthropogenic fill beginning in 1913. Shoreline modification continued into the present as this important natural deep-water harbour along the Strait of Juan de Fuca was developed for maritime industries. This and other historical activities obscured at least two historically occupied villages and burial sites of the indigenous Coast Salish Klallam people. Since these archaeological sites remain buried beneath the modern Port Angeles waterfront knowledge of the distribution of buried landforms, coastal zone processes, and estimates of site preservation and modern disturbance potential is needed for archaeological identification and preservation efforts. We created a model of the fill thickness by combining data from: (i) field observations, where the thickness of the fill could be observed directly in the landscape; (ii) topographic differences between pre-fill sounding maps and present-day LIDAR-determined elevations; and (iii) ground-penetrating radar (GPR) surveys. The GPR surveys also helped to reconstruct the now buried palaeoenvironment by identifying tidal lagoons, beach berms and stream channel features beneath the fill layer. The history of post-glacial sea-level change, here impacted by global eustasy, glacio-isostatic and tectonic factors is the first control on the development of quasi-stable coastal landforms suitable for long-term human occupation. Knowledge of past landscapes is a critical component in the development of future archaeological site catchment 'predictive' models based upon the spatial distribution and stability of landforms and resource accessibility prior to the Euro-American historic period of intensive shoreline modification. The geophysical and geomorphic identification and spatial reconstruction of buried landforms also provides needed insight into the geology of the subsurface and its control on the flow of groundwater and contaminants across the nearshore environment.
A sea-level database for the Pacific coast of central North America
Quaternary Science Reviews, 2015
A database of published and new relative sea-level (RSL) data for the past 16 ka constrains the sea-level histories of the Pacific coast of central North America (southern British Columbia to central California). Our reevaluation of the stratigraphic context and radiocarbon age of sea-level indicators from geological and archaeological investigations yields 600 sea-level index points and 241 sea-level limiting points. We subdivided the database into 12 regions based on the availability of data, tectonic setting, and distance from the former Cordilleran ice sheet. Most index (95%) and limiting points (54%) are <7 ka; older data come mainly from British Columbia and San Francisco Bay. The stratigraphic position of points was used as a first-order assessment of compaction. Formerly glaciated areas show variable RSL change; where data are present, highstands of RSL occur immediately post-deglaciation and in the mid to late Holocene. Sites at the periphery and distant to formerly glaciated areas demonstrate a continuous rise in RSL with a decreasing rate through time due to the collapse of the peripheral forebulge and the reduction in meltwater input during deglaciation. Late Holocene RSL change varies spatially from falling at 0.7 ± 0.8 mm a À1 in southern British Columbia to rising at 1.5 ± 0.3 mm a À1 in California. The different sea-level histories are an ongoing isostatic response to deglaciation of the Cordilleran and Laurentide Ice Sheets.
Present-day vertical deformation of the Cascadia Margin, Pacific Northwest, United States
Journal of Geophysical Research, 1994
We estimate present-day uplift rates along the Cascadia Subduction Zone in California, Oregon, and Washington in the Pacific Northwest, United States, by utilizing repeated leveling surveys and tide gauge records. These two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast. Uplift rates are extended inland through east-west leveling lines that connect the north-south line along the coast to the north-south line along the inland valleys just west of the Cascades. The results are summarized as a contour map of present-day uplift rates for the western Pacific Northwest. We find that rates of present-day uplift vary latitudinally along the coast and inland valleys, as well as longitudinally along transects connecting the coast to the inland valleys. Long-term tidal records of Neah Bay, Astoria, and Crescent City indicate uplift of land relative to sea level of 1.6 • 0.2,0.0 ß 0.2, and 0.9 • 0.2 mm/yr, respectively (• 1 standard error). Unlike previous estimates of relative sea level change at Astoria, we adjust for discharge effects of the Columbia River, including human management influences. After approximating an absolute framework by using 1.8 ß 0.1 mm/yr to compensate for global sea level rise, results indicate that much of the western Pacific Northwest is rising at rates between 0 and 5 mm/yr. The most rapid uplift rates are near the coast, particularly near the Olympic Peninsula, the mouth of the Columbia River, Cape Blanco, and Cape Mendocino. Two axes of uplift are identified: one trends northeast from the southwest Oregon coast, and the other trends south-southeasterly from the Olympic Peninsula to the Columbia River. The Puget Sound vicinity and a small east-west region from the north central Oregon coast to the inland Willamette Valley are subsiding at rates up to 1 mm/yr. We interpret the overall pattern of rapid present-day uplift to be generated by interseismic strain accumulation in the subduction zone. This interseismic elastic strain accumulation implies significant seismic hazard.