A paucity of research exists regarding the millennial-scale response of inland alluvial streams to abrupt base-level fall. Studies of modern systems indicate that, over short time scales, the ...response is a diffusion-like process of upstream-propagating incision. In contrast, evidence from the lower Chippewa River (LCR), located in the upper Midwest of the USA, suggests that autogenic controls operating over time scales of several millennia can overwhelm diffusion, resulting in incision that is prolonged and episodic. During the Last Glacial Maximum, the LCR drained the Chippewa Lobe of the Laurentide Ice Sheet to the glacial upper Mississippi River (UMR). As a meltwater stream, it aggraded and filled its valley with glacial outwash, as did its largest tributaries, which were also meltwater streams. Its nonglacial tributaries aggraded, too, filling their valleys with locally derived sediment. During deglaciation, the UMR incised at least twice, abruptly lowering the LCR's base level — ~15m at 16ka or earlier and an additional 40m at ca. 13.4ka. Each of these base-level falls initiated incision of the LCR, led by upstream migrating knickpoints. The propagation of incision has, however, been a lengthy process. The optically stimulated luminescence (OSL) ages of terrace alluvium indicate that, by 13.5ka, incision had advanced up the LCR only 15km, and by 9ka, only 55km. The process has also been episodic, resulting in the formation of fill-cut terraces (inferred from GPR surveys and exposures of terrace alluvium) that are younger and more numerous in the upstream direction. Autogenic increases in sediment load and autogenic bed armoring, the result of periodic tributary-stream rejuvenation and preferential winnowing of fines by the incising river, may have periodically caused knickpoint migration and incision to slow and possibly stop, allowing lateral erosion and floodplain formation to dominate. A decline in sediment flux from stabilizing incised tributary stream systems would have led to renewed knickpoint migration and incision when floods of sufficient magnitude to breach the channel armor occurred. Minimal floodplain development along the upper section of the present-day LCR, along with the channel morphology of an unstable wandering gravel-bed river immediately downstream from it, suggest that the river is still responding to the base-level falls that happened many millennia ago. The autogenic controls on the LCR's response to UMR incision are a direct consequence of the thick fills of noncohesive sediment that accumulated in its valley and the valleys of its tributary streams during the Late Wisconsinan, making the LCR a prime example of a former proglacial river that remains a paraglacial fluvial system.
•The response of former proglacial alluvial streams to base-level fall can be complex.•Autogenic variations in sediment supply and bed armoring are likely key controls.•The result is upstream propagating incision that is episodic and prolonged.•Abundant glacial-age alluvial fill likely affects the autogenic incision process.•Former proglacial streams can be characterized as paraglacial fluvial systems.
With concerns over rising global temperatures, ice sheet stability and implications for sea level rise, the responses of former ice sheets to past climate change provide useful insights into linkages ...between the atmosphere, cryosphere and oceans. One example is the behavior of the Laurentide Ice Sheet (LIS) during Heinrich Stadials, times of cooler sea surface temperature (SST) in the North Atlantic. Using existing cosmogenic surface exposure ages, varve chronologies, radiocarbon ages, and optically stimulated luminescence ages from the southern LIS margin, this report tests the hypothesis that the southern side of the LIS advanced during times of surface cooling of the North Atlantic. This test reconstructs the timing of local glacial maxima and the times immediately pro- and preceding them, from multiple areas and compares them to the timing of Heinrich Stadial 2 (HS2; 26.6–23.6 ka) and to the timing of Heinrich Stadial 1 (HS1; 19.3–15.3 ka).
During HS2 southern sector lobes (New England region, Huron-Erie Lobe, Lake Michigan Lobe, Green Bay Lobe and the Chippewa Lobe) advanced towards their maximum position, except for the Des Moines Lobe for which there is no applicable data. During HS1 these same lobes experienced substantial retreat, but the Des Moines Lobe advanced. Thus, the southern Laurentide margin behaved differently during the two Heinrich Stadials. This pattern may be attributed to differing east-west temperature gradients across North America and associated changes in atmospheric circulation.
•Southern Laurentide lobes advanced during Heinrich Stadial 2.•All but westernmost Des Moines Lobe retreated during Heinrich Stadial 1.•Des Moines Lobe advanced to maximum during Heinrich Stadial 1.•Pattern of opposite behavior during two Heinrich Stadials.•Possible east-west temperature or precipitation gradient across margin during HS1.
The North American Ice Sheet Complex (NAISC; consisting of the Laurentide, Cordilleran and Innuitian ice sheets) was the largest ice mass to repeatedly grow and decay in the Northern Hemisphere ...during the Quaternary. Understanding its pattern of retreat following the Last Glacial Maximum is critical for studying many facets of the Late Quaternary, including ice sheet behaviour, the evolution of Holocene landscapes, sea level, atmospheric circulation, and the peopling of the Americas. Currently, the most up-to-date and authoritative margin chronology for the entire ice sheet complex is featured in two publications (Geological Survey of Canada Open File 1574 Dyke et al., 2003; ‘Quaternary Glaciations – Extent and Chronology, Part II’ Dyke, 2004). These often-cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka (all ages in uncalibrated radiocarbon years) using a combination of geomorphology, stratigraphy and radiocarbon dating. However, by virtue of being over 15 years old, the ice margin chronology requires updating to reflect new work and important revisions. This paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies. We also update the original radiocarbon dataset from the 2003/2004 papers with 1541 new ages to reflect work up to and including 2018. A major revision is made to the 18 ka ice margin, where Banks and Eglinton islands (once considered to be glacial refugia) are now shown to be fully glaciated. Our updated 18 ka ice sheet increased in areal extent from 17.81 to 18.37 million km2, which is an increase of 3.1% in spatial coverage of the NAISC at that time. Elsewhere, we also summarize, region-by-region, significant changes to the deglaciation sequence. This paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of Dyke et al. (2003) and Dyke (2004) where new information is lacking; this is a pragmatic solution to satisfy the needs of a Quaternary research community that requires up-to-date knowledge of the pattern of ice margin recession of what was once the world’s largest ice mass. The 36 updated isochrones are available in PDF and shapefile format, together with a spreadsheet of the expanded radiocarbon dataset (n = 5195 ages) and estimates of uncertainty for each interval.
•Updated deglaciation sequence using regional studies/expertise and radiocarbon data.•Region-by-region overview of significant ice margin changes from 18 ka to 1 ka.•New 18 ka 14C (∼21.7 ka cal.) ice sheet is 3.1% larger than previous estimates.•36 updated isochrones (PDFs/shapefiles) and radiocarbon dataset (n = 5195 ages).•Ice margins of Dyke et al. (2003) retained where new information is lacking.
Newly available high resolution elevation data across North America and Europe have revealed previously unknown glacial landforms. This study focuses on the morphology and stratigraphy of a variation ...of tunnel channels with unique morphology that we have named parallel tunnel channels (PTCs). They are found in northeastern Indiana, northwestern Ohio, and southeastern Michigan, which were previously glaciated by the Huron-Erie Lobe (HEL) of the Laurentide Ice Sheet (LIS). PTCs are discontinuous, quasi-parallel pairs of depressions separated by a continuous central ridge that can extend upwards of 50 km, cross-cut multiple recessional moraines, and transition into standard tunnel channels. The PTCs range in width from 200 to 900 m, and the depressions range in width from 100 to 200 m and in depth from 3 to 10 m. Using water well logs and multiple electrical resistivity surveys, we determined that the stratigraphy beneath and surrounding the PTCs consists of a lower layer of sand and gravel (confined aquifer) and an upper layer of New Holland till, both of the Lagro Formation. A vibracore and multiple resistivity surveys reveal that depressions are filled with ~4 m of lacustrine sediment and occasionally are floored with gravel channel deposits. The depressions are erosional in origin, rather than a product of ice block melt-out and are cut into the upper surface of the New Holland till. We interpret the PTCs as subglacial, erosional landforms, similar to tunnel channels, which represent distinct pathways transporting subglacial meltwater from the interior to the ice margin. Identification of these landforms has several important implications for understanding the deglaciation of the HEL: 1) the major recessional moraines may not record the final deglaciation of the HEL (i.e., are palimpsest features); 2) a large portion of the HEL must have stagnated to preserve the subglacial landscape including the PTCs; and 3) the combined network of PTCs, tunnel channels, and proglacial spillways may represent a partially complete subglacial drainage network with distinct morphological changes as meltwater was transported from the interior to the margin of the HEL. An initial formational model is proposed for the formation of the PTCs by way of a supra- and/or subglacial lake draining through an ice-walled subglacial tunnel, but additional work is required to understand the distinct pattern of depressions observed.
•Description of a variation of tunnel channel morphology from southern Great Lakes region•Landforms composed of discontinuous paired depressions offset by a central ridge•Cross cut multiple recessional moraines of the Huron-Erie Lobe•Connect to traditional tunnel channels and outwash channels at the ice margin•Evidence of a subglacial drainage network extending from interior to marginal outwash channels
Optical dating and mapping of eolian sand in the Upper Mississippi River basin provide important constraints on the timing and extent of major paleoenvironmental shifts south of the Laurentide Ice ...Sheet during the last deglaciation. Twenty nine optical ages from 10 sites document a major period of eolian sand activity ca. 18–14 ka on the Iowan Erosion Surface in eastern Iowa and the northern Driftless Area in west-central Wisconsin. This age range coincides with the end of accelerated mass wasting within and adjacent to the Driftless Area, sand wedge formation in the northern part of the Driftless Area, a major transition in speleothem growth in a cave in the southern part of the Driftless Area, major retreat of the Des Moines and Lake Michigan lobes, and the youngest ages of tundra-like floral remains in the study area. A regional increase in sediment availability due to a change in surface and subsurface hydrology associated with the degradation of permafrost may explain the alignment of ages from multiple sites >200 km apart. Factors that allow for the movement of sand, including low ground cover percentage and well-drained surface conditions, were maximized immediately after degradation of permafrost. Eolian sand was sourced from several distinct geomorphic settings, including fluvial (fill terrace) sediments from both glaciated and unglaciated fluvial catchments and colluvium derived from Cambrian sandstone. Overlap of optical ages within error between 18 and 14 ka suggests a regional eolian response rather than localized responses due to changes in sediment supply. Eolian sand activity occurred within a Picea-Larix woodland / parkland with few modern analogs.Optical ages on eolian sand from the western glacial Lake Wisconsin basin indicate dune activity in the early Holocene ca. 12–10 ka, more than 5 ka after the drainage of glacial Lake Wisconsin and ca. 5 ka younger than dune activity in the adjacent northern Driftless Area. Vegetation density in the early Holocene must have been reduced because sample sites are west of sediment sources from the Wisconsin River and ages lag the degradation of permafrost by several thousand years. It is not immediately clear the trigger for vegetation density change, but it may relate to a combination of factors including high seasonality, xeric soil moisture regime and invasion of Pinus and Quercus into the region in the early Holocene.
The Franklin 7.5-minute quadrangle is situated near the maximum limit of Wisconsin Episode glacial deposits insoutheastern Johnson County. Till of Wisconsin Episode age (Trafalgar Formation) ...dominates the surficial geology of thequadrangle. Areas of Wisconsin Episode outwash (Atherton Formation) are found in the Sugar Creek, Youngs Creek, andHurricane Creek valleys and within former meltwater pathways not occupied by modern streams. Kame deposits (sand andgravel) are found in the northern half of the quadrangle and are part of a larger kame complex present in central JohnsonCounty. Holocene (post-glacial) alluvium is found in the valleys of Sugar, Youngs, Nineveh, Buckhart, and Ray Creeks andtributaries. Unconsolidated sediment thickness generally increases from southwest to northeast across the quadrangle, fromless than 15 ft along the southwestern margin of the quadrangle to over 200 ft thick in the far northeastern corner of thequadrangle. This transition of bedrock topography represents the buried northern end of the Knobstone Escarpment. Twosignificant bedrock paleovalleys exist in the quadrangle, one in the south-central part of the quadrangle which parallels theburied Knobstone Escarpment and one in the northeast corner of the quadrangle which is part of a large bedrock valleyextending north into Marion County. The Quaternary sediments in the western half of the quadrangle are underlain byMississippian Borden Group siliciclastic rocks. In the eastern half of the quadrangle, the Devonian New Albany Shale andMuscatatuck Group carbonate rocks subcrop under thick Quaternary sediment cover. The Wisconsin Episode maximumlimit is located within one mile south of the southern boundary of the quadrangle. Radiocarbon ages from the adjacentTrafalgar 7.5-minute quadrangle indicate the Laurentide Ice Sheet reached its maximum extent just after 23,700 years ago. Asecond glacial limit, marked by the Crawfordsville Moraine in the adjacent Trafalgar 7.5-minute quadrangle,crosses the southern end of the Franklin 7.5-minute quadrangle, albeit diffuse. The Crawfordsville Moraine represents thelimit of a readvance which occurred 21,700 years ago. This preliminary geologic map is an interim geologic map product thatdocuments progress in mapping the Quaternary geology of Johnson County, Indiana. The lithologic classification of theQuaternary units of Indiana was used in assigning map units.
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