Late Wisconsinan Till Research Articles

Estimation of the representative elementary volume of a fractured till: a field and groundwater modeling approach

Fractured till is often represented as an equivalent porous medium (EPM) in groundwater models. Knowledge of the representative elementary volume (REV) is necessary for proper application of an EPM model. While REV estimation and hydraulic conductivity tensor determinations are common in fractured rock studies, they are rarely applied to materials with a permeable matrix such as fractured till. This study uses field fracture measurements, model simulations, and the FracKFinder toolbox to estimate the REV and determine hydraulic conductivity tensors for the fractured, late Wisconsinan till of the Dows Formation in central Iowa (USA), at depths of 1.0–1.5, 2.0–2.5, and 3.3–3.7 m. Results indicate that the REV of the till is directly related to both fracture density and transmissivity. REV size ranges from 4 to 5 m3 at depths of 1–2 m, where fractures are densest (P32 27.5 m2/m3) and most transmissive, to 2–3 m3 at depths of 3.3–3.7 m where fractures are least dense (P32 24.4 m2/m3) and least transmissive. Fracture hydraulic conductivity is five, four, and three orders of magnitude greater than matrix hydraulic conductivity at the shallowest to deepest depths of investigation, respectively. The results also suggest that hydraulic conductivity values estimated from permeameters and large-core laboratory experiments may not be of sufficient volume to represent the REV of fractured till.

Drumlinized tunnel valleys in south-central Ontario

A network of Late Wisconsin valleys deeply incise thick successions (up to 200 m) of Quaternary sediment and Paleozoic bedrock across south-central Ontario. The valleys have previously been interpreted as an integrated network of tunnel channels recording catastrophic releases of subglacial meltwater across the bed of the Laurentide Ice Sheet (LIS). Recent geologic investigations in the western part of the valley network integrate information gained from surficial sediment and landform mapping, continuously-cored boreholes that penetrate the entire Quaternary sediment succession in valleys as well as intervening uplands, and both water-borne and land-based geophysical surveys. Together, these data provide new insights and a refined interpretation of the genesis, timing and paleoglaciological significance of these valleys.Valleys within the study area are some of the largest observed within the regional network – up to 30 km long, 7 km wide and >175 m deep. They are oriented both north-south and in a radial pattern, varying from NE-SW in the southern part of the study area, to ESE-WNW in the north. An undulating Late Wisconsin till sheet (Newmarket Till) caps the regional uplands and is commonly observed along the flanks and floors of the valleys – all of which show evidence of a drumlinization phase that postdates excavation of most of the valleys. At least two generations of valleys are interpreted from cross-cutting relationships, with abrupt heads and locally perched downflow ends of some valley forms. Radiocarbon age determinations of organic material from both below and above the Newmarket Till constrain the timing of valley excavation to between 28.05 and 12.81 14C kyr BP. Borehole and outcrop data within the valleys reveal four stratigraphic units (SU1-SU4) that form the floor and sediment infill of the valleys. The Newmarket Till (SU1) is commonly underlain by deformed substrates. The till is absent in parts of some valleys and the lowermost parts of the valley fill commonly consist of coarse-grained gravels (SU2) that pass upwards into sands (SU3). The bulk of the sediment infill consists of glaciolacustrine silt-clay rhythmites (SU4) that pass upwards into organic- and mollusk-bearing nearshore and fluviodeltaic deposits that record drainage of proglacial lakes from the study area during deglaciation.It is proposed that these features are a previously unrecognized type of tunnel valley that developed subglacially during Late Wisconsin ice cover. Meltwater surpluses would be generated at the ice-bed interface due to the low-transmissivity substrate in the study area with flow preferentially routed into lows on the bed. Downward incision into confined and pressurized aquifers likely facilitated rapid headward erosion of the valleys enhanced by piping. Waning meltwater flow velocities/discharges allowed ice creep to re-occupy newly-formed valleys permitting renewed till deposition and subsequent drumlinization of the valleys. The valleys also provided efficient routing for meltwater draining the retreating ice marginal zone during deglaciation and allowed localised sand and gravel deposition as eskers and ice-proximal subaquatic fans in the Oak Ridges Moraine and within proglacial lakes that developed during ice retreat. However, there is no evidence for regional-scale catastrophic release of subglacial meltwater in the valley infills. This integrated dataset represents one of the most complete characterizations of subglacial valleys in a terrestrial setting in North America.

Interglacial Beds at Toronto, Ontario

Interglacial sediments have been known to occur at Toronto for about a century. There have been two main periods of attention: first by A. P. Coleman in the early twentieth century; and second mostly by the author and co-workers in the past quarter century. Attention was focussed early on the Don Formation because of its rich fossil assemblages. The Don Formation, consisting of gravel, sand, and clay, is commonly 6 to 9 m thick and has been encountered in outcrop only along the DonValley. However, excavations and borings indicate its presence under much of southern Metropolitan Toronto; it may continue northward along the Laurentian River Valley. Its only continuing, accessible exposure has been the Don Valley Brickyard. Early paleontological study emphasized molluscs, wood, leaves, and a few bones, which suggested a climate warmer than present. More recent studies have stressed microfossils. including pollen, diatoms, ostracodes. molluscs, Cladocera, insects, plant macrofossils, and microvertebrates. Altogether about 500 species have been identified, and the list is growing. Climatic indicators show that the Don Formation represents the declining temperatures of the waning half of an interglaciation. Although the Don Formation is beyond the range of radiocarbon dates and is undated, amino acid analysis on wood and shells support assignment to Sangamonian time. The overlying Scarborough Formation clay and sand, and the Pottery Road Formation sand contain mainly cold-climate fossils. These are in turn overlain by Early, Middle, and Late Wisconsinan tills and interbedded lacustrine sediments with corresponding radiocarbon and thermoluminescence (TL) dates.

Foraminiferal Distribution in the Central and Western Parts of the Late Pleistocene Champlain Sea Basin, Eastern Canada

Marine sediments from the late-glacial Champlain Sea have been sampled at 20 localities representing the deeper part of the basin, between Ottawa and the Rivière St-François, Québec. Foraminiferal assemblages have been extracted and a sequence of three deep water and two shallow water ecozones recognized. The lowermost zone (A) is characterized by Cassidulina reniforme, Islandiella helenae and I. norcrossi and represents a paleosalinity of 25 to 30%o. The overlying zone B is dominated by Elphidium excavatum. It represents salinities decreasing from 25 to as low as 10%o. The uppermost zone (C) contains only a sparse assemblage of a morphotype of E. excavatum. If suggests a paleosalinity of no more than 10%o. A mostly unfossiliferous silt and clay layer of variable thickness (post-C) occurs above zone C. It is probably lacustrine. Below zone A and above the Late Wisconsinan till there is a pre-A interval whose variable assemblages represent hyposaline environments east of Montréal, predominantly lacustrine conditions west of Montréal and alternating hyposaline/ lacustrine environments near and south of Montréal. Bottom water temperatures were probably "Arctic" (within a few degrees of 0°) from the pre-A interval up to zone B inclusively. The data from zone C are too poor to estimate temperatures. The shallow water zones indicate environments with high (zone EH) or low (zone EA) salinities but of shallower depths than the deep water zones. The existence of two sequences is interpreted as the result of (probably seasonal) water stratification. The data does not allow to determine the depth of the limit between the shallow and deep waters.

Glacial Stratigraphy of the Bulkley River Region: A Depositional Framework for the Late Pleistocene in Central British Columbia*

A depositional framework for late Pleistocene sediments in central British Columbia was developed from the composite stratigraphy of glacial sediments found in the Bulkley River region. Nonglacial deposits correlated to the Olympia Nonglacial Interval, are overlain in succession by sub-till, ice-advance sediments, Late Wisconsinan (Fraser Glaciation) till, and late-glacial sediments. Due to local erosion and depositional variability, some of the units are not continuous throughout the region and differ locally in their thickness and complexity. At the onset of the Fraser Glaciation, ice advance was marked by rising base levels in rivers, lake ponding, and ice marginal subaqueous deposition. Physiography and glacier dynamics influenced the position of drainage outlets, direction of water flow, and ponding. The region was completely ice covered during this glaciation and ice-flow directions were variable, being dominantly influenced by the migrating position of ice divides. Deglaciation was marked by the widespread deposition of fine-grained sediments in proglacial lakes and glaciofluvial sands and gravels at locations with unrestricted drainage.

Deglaciation of the Appalachian Plateau, northeastern Pennsylvania—till shadows, till knobs forming “beaded valleys”: Revisiting systematic stagnation-zone retreat

Glacial retreat from valleys in the moderate relief (300–500 m) Small Lakes Section of the Appalachian Plateau in northeastern Pennsylvania was characterized by episodic deposition of till in a series of knobs that formed “beaded valleys”. Individual valleys have a north to south series of till knobs alternating with wetlands or lakes at a spacing of one to five kilometers. Outcrop and well data, while small and few in any individual knob, when put together from the more than 1000 knobs mapped in region, show that the till knobs are typically 30 to 50 m thick. The knobs are cored by subglacial till (lodgment-deformation till) with a wedge of supraglacial till (flow or re-sedimented till) on the south sides, push structures in the interiors and north sides, and an overall veneer of “colluviated till” that thickens downslope on all sides. Glaciofluvial deposits are scarce, usually appearing as thin lenses on the flank of the knobs. The knobs are interpreted to be the periglacially and post-glacially modified remnants of recessional moraines. Individual till knobs were rapidly deposited in a few decades, probably through layer by layer stacking of deformation till and till block melanges. Active ice shearing over inactive ice could form an adverse slope where rapid till deposition could take place. The ice retreated systematically in a stagnation-zone retreat mode, with active ice leaving till knobs and 1–5 km wide stagnant-zone ice leaving the lake basins between the knobs. The till knobs can be connected from valley to valley, in lines perpendicular to the southwesterly ice flow, to delineate ice margin positions across the region. Valleys transverse to ice flow have “till shadows”, 30 to 50 m thick till deposits on the north or lee side of the valley. “One-sided” post-glacial bedrock gorges with bedrock on the south side and till on the north side, are ubiquitous in “till shadow” valleys and form as the stream incises down the bedrock-till contact. Till outcrops in such gorges show no evidence of weathered older till under the late Wisconsinan till. The till is predominantly lodgment-deformation till in the lower two-thirds of the section and mainly ablation and meltout till above that. On the adverse slope in the transverse valley the glacier did not deposit till, possibly because it was on a “till starved” relatively steep bedrock slope rather than a “till rich” deformable bed of a till knob.

Experimental Determination of Effective Diffusion Parameters in the Matrix of Fractured Till

Diffusion is often the dominant mode of solute transport in soils when advection is minimal. This paper describes the application of a radial diffusion cell method to estimate the effective diffusion coefficient ( D e ) and effective diffusive porosity (θDe ) for use in solute transport models for fractured-porous media. Twenty-four experiments were conducted for 28 d using three conservative solutes (Br, PFBA, and PIPES) on eight late Wisconsinan and Pre-Illinoian till samples from Iowa. The mean value of the total porosity (θT) of the till samples was 30.0%. Concentrations of the three tracers in the reservoir decreased with time and eventually approached equilibrium concentrations. A model simulated the observed concentration data and the modified goodness-of-fit ( d 1) values ranged from 0.878 to 0.950. Mean values of θDe from the model were 28.3 (Br−), 26.5 (PFBA), and 21.6% (PIPES) and there were significant differences in θDe among the three tracers ( p = 0.05). Mean values of D e were 5.6 × 10−10 m2 s−1 (Br−), 2.9 × 10−10 m2 s−1 (PFBA), and 1.3 × 10−10 m2 s−1 (PIPES). Values of D e differed significantly by compound and were significantly different ( p = 0.05) from the aqueous diffusion coefficient ( D ). Calculated mean values of the first-order mass exchange coefficient (α) were 8.4 × 10−7 (Br−), 4.1 × 10−7 (PFBA), and 1.6 × 10−7 s−1 (PIPES); they differed by compound ( p = 0.05) and generally decreased with increasing molecular weight of the tracer. This study confirmed that the radial diffusion cell method is an efficient method to estimate effective diffusion parameters necessary to accurately model solute transport in fractured till and soil.

Use of Soils to Identify Glacial Deposits of Various Ages East of Glacier National Park, Montana, U.S.A.

Degree of soil development increases with age of parent materials in the zone of confluence of continental and alpine piedmont glaciers east of Glacier National Park, Montana. Surficial deposits are assigned to three, possibly four, glacial events on the basis geomorphic and pedologic evidence. Based on over 100 profiles examined, soils in late Wisconsin till and outwash are typically Cryoborolls and Cryochrepts with 0- to 62-cm-thick, brown cambic (Bw) horizons and 50+- cm-thick calcic (Bk) horizons with Stage II to II+ carbonate morphology. Soils in late Illinoian or early Wisconsin sediments are Argiborolls or Argic Cryoborolls with 34- to 92-cm-thick, dark yellowish brown argillic (Bt) horizons and those in Illinoian sediments are strongly developed Paleudolls with 51- to 54-cm-thick, yellowish red argillic horizons. Soils on Number 1 and 2 Bench remnants (considered Pliocene to early Pleistocene in age) are very strongly developed Paleudolls with about 2.5-m-thick, leached, yellowish red argillic horizons. Properties which best reflect relative soil age include thickness of argillic horizon, depth of leaching, depth of oxidation, Harden profile index, Harden rubification index, and clay accumulation index.

DEGLACIATION OF A SOFT-BEDDED LAURENTIDE ICE SHEET

We present a series of numerical reconstructions of the Laurentide Ice Sheet during the last deglaciation (18–714Cka) that evaluates the sensitivity of ice-sheet geometry to subglacial sediment deformation. These reconstructions assume that the Laurentide Ice Sheet flowed over extensive areas of water-saturated, deforming sediment (soft beds) corresponding to the St. Lawrence lowland, the Great Lakes region, the western prairies of the U.S. and Canada, and the Hudson Bay and Hudson Strait regions. Sediment rheology is based on a constitutive law that incorporates experimental results from late Wisconsin till deposited by the Laurentide Ice Sheet which suggest only mildly nonlinear viscoplastic behavior. By varying the effective viscosity of till, we produced four reconstructions for the ice sheet during the last glacial maximum 1814Cka, and two reconstructions each of the ice sheet at 14, 13, 12, 11 and 1014Cka. We also produced one reconstruction for 9, 8.4, 8, and 714Cka. Reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl-shaped depression over Hudson Bay and thin ice (<1000m above modern sea-level) over the western and southern margins. Those reconstructions that assume a higher effective viscosity of till in the Hudson Bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over Hudson Bay and a more northerly position of the central ice divide. These two different geometries may represent ice-sheet orographic changes associated with a Heinrich event. Further increases in effective viscosity of till, approaching the effective viscosity of ice, would result in a high, monolithic ice dome centered over Hudson Bay, reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology.Modeled ice-surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds. Our reconstructions with higher effective till viscosities in Hudson Bay also agree with the ICE-4G reconstructions (Peltier, 1994), which are based on inversion of relative sea-level data, for the early part of the last deglaciation (18–1314Cka), but then depart significantly from ICE-4G beginning at about 1214Cka due to differing assumptions of the history of deglaciation. Modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50–55m by a soft-bedded Laurentide Ice Sheet; this would increase as the effective viscosity of till increases. Subsequent ice-volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at Barbados, New Guinea, and Tahiti, but suggest that the Laurentide Ice Sheet was not the source of meltwater pulse 1A.

Evidence for recent groundwater flow through Late Wisconsinan till near Toronto, Ontario

The Northern till is a thick (~50 m) Late Winsconsinan diamict unit that occurs throughout south-central Ontario. The till has generally been regarded as massive and uniform, with a very low vertical hydraulic conductivity. It is similar to many other till units of mid-continental North American glaciated terrain in that it is believed to inhibit recharge to underlying aquifers and afford a high degree of protection to these aquifers from surface and near-surface sources of contamination. Standard methods of estimating hydraulic conductivity (K) for the Northern till, such as laboratory testing of core samples (other studies) and rising–falling head field piezometer tests (this study and other studies), characteristically yield values on the order of 10–11 to 10–9 m/s. Typically, these values indicate advective traveltimes through the till on the order of hundreds to thousands of years. In contrast, isotopic evidence (2H, 18O, and 3H) from till pore waters indicates the presence of modern (post-1952) waters at depths of up to 50 m, suggesting either that certain facies of the till are considerably more permeable or that minor sand lenses or hydrogeologically active secondary permeability structures are locally important. In some areas, vertical flow velocities may approach 1 m/year. By comparing pore-water isotopic data from cores acquired using mud (sodium bentonite) and dry rotary methods, this study further demonstrates that representative pore-water samples can be obtained using a drilling fluid providing care is taken in preparing core samples for analysis. Key words: till, aquitard, permeability, recharge, contaminant transport, isotopes.

Absence of Glaciation in Illinois during Marine Isotope Stages 3 through 5

A10Be inventory and14C ages of material from a core from northernmost Illinois support previous interpretations that this area was ice free from ca. 155,000 to 25,000 yr ago. During much of this period, from about 155,000 to 55,000 yr ago,10Be accumulated in the argillic horizon of the Sangamon Geosol. Wisconsinan loess, containing inherited10Be, was deposited above the Sangamon Geosol from ca. 55,000 to 25,000 yr ago and was subsequently buried by late Wisconsinan till deposited by the Lake Michigan Lobe of the Laurentide Ice Sheet. The Sangamonian interglacial stage has been correlated narrowly to marine oxygen isotope substage 5e; our data indicate instead that the Sangamon Geosol developed during late stage 6, all of stages 5 and 4, and early stage 3.

Evidence for recent groundwater flow through Late Wisconsinan till near Toronto, Ontario

The Northern till is a thick (~50 m) Late Winsconsinan diamict unit that occurs throughout south-central Ontario. The till has generally been regarded as massive and uniform, with a very low vertical hydraulic conductivity. It is similar to many other till units of mid-continental North American glaciated terrain in that it is believed to inhibit recharge to underlying aquifers and afford a high degree of protection to these aquifers from surface and near-surface sources of contamination. Standard methods of estimating hydraulic conductivity (K) for the Northern till, such as laboratory testing of core samples (other studies) and rising–falling head field piezometer tests (this study and other studies), characteristically yield values on the order of 10–11 to 10–9 m/s. Typically, these values indicate advective traveltimes through the till on the order of hundreds to thousands of years. In contrast, isotopic evidence (2H, 18O, and 3H) from till pore waters indicates the presence of modern (post-1952) waters at depths of up to 50 m, suggesting either that certain facies of the till are considerably more permeable or that minor sand lenses or hydrogeologically active secondary permeability structures are locally important. In some areas, vertical flow velocities may approach 1 m/year. By comparing pore-water isotopic data from cores acquired using mud (sodium bentonite) and dry rotary methods, this study further demonstrates that representative pore-water samples can be obtained using a drilling fluid providing care is taken in preparing core samples for analysis. Key words: till, aquitard, permeability, recharge, contaminant transport, isotopes.

The southern limit of Cordilleran ice in the Colville and Pend Oreille valleys of northeastern Washington during the Late Wisconsin glaciation

The southern limit of the lobes of the Cordilleran ice sheet in the Colville and Pend Oreille valleys of northeastern Washington during the Late Wisconsin (Fraser) glaciation has been placed at widely different positions by various investigators. The confusion that resulted in these different positions is due to the fact that both lobes, at times, terminated in glacial lakes and much of the terminal area of the Pend Oreille River lobe was swept by floods from glacial Lake Missoula. Evidence, presented in this paper, of the southern limit of these two lobes during the Late Wisconsin glaciation consists of (i) the southern limit of Late Wisconsinan till, (ii) the southern limit of lateral meltwater channels, (iii) meltwater channels whose headward areas coincide with the southern limit of Late Wisconsinan till, and (iv) reconstructed ice-surface profiles. This evidence suggests that the southern limit of the Colville lobe is represented by a moraine near the town of Springdale, whereas that of the Pend Oreille River lobe is near the town of Newport. The timing and extent of Late Wisconsinan deglaciation in the Colville Valley are provided by radiocarbon ages and Late Pleistocene tephras at sites upglacier from thfe terminal position. These data demonstrate that the Colville lobe had retreated at least 50 km by about 12 450 BP. In the Pend Oreille Valley, the presence of a Glacier Peak tephra layer near the town of lone indicates that the Pend Oreille River lobe had retreated at least 70 km prior to deposition of the tephra.

Seismic reflection, borehole and outcrop geometry of Late Wisconsin tills at a proposed landfill near Toronto, Ontario

The search for new landfill sites in the Greater Toronto area of southern Ontario, Canada, is producing a wealth of data regarding the subsurface stratigraphy and geometry of Late Wisconsin (&lt;25 ka) till deposits. Till strata are favoured as landfill substrates because of their wide surface extent, thickness (maximum ~60 m), high degree of overconsolidation, apparently massive character, and low permeability. However, problems are emerging where surface contaminants have migrated through till deposits into underlying aquifers along poorly understood transport paths. This paper reports the results of a detailed shallow seismic reflection investigation of a proposed 275 ha landfill site 40 km northeast of Toronto near Whitevale, where previous hydrochemical analysis and hydrogeological monitoring identified rapid vertical recharge of contaminated surface waters through Late Wisconsin tills up to 60 m thick. Seismic reflection data are ground truthed by drilling (36 holes; total drilled 3157 m), coring (1600 m), downhole geophysical logging, and outcrop data. The site stratigraphy at Whitevale consists of an uppermost Late Wisconsin till (Halton Till) separated from a lower till (informally named Northern till) by a silt, sand, and gravel complex. Seismic reflection profiles identify the presence of well-defined reflectors within the Northern till, which are correlated in outcrop with laterally extensive erosion surfaces overlain by sheet-like sands and gravels, up to 1 m thick, and boulder concentrations. Erosion surfaces and associated sediments record episodic scouring by subglacial meltwaters and provide potential "hydraulic windows" for the movement of surface contaminants through the till into underlying aquifers.

Contemporary Groundwater Methane Production from Pleistocene Carbon

AbstractRelative to surface systems, microbial activities in groundwater have been poorly studied, especially with regard to methanogenesis. Past studies have reported high concentrations of CH4 in groundwater in the late Wisconsinan till and Wisconsinan loess of central Iowa. The objectives of this work were to assess whether CH4 is presently produced in the till and loess, and, if so, to characterize the spatial variability of CH4 production rates. Groundwater and sediment sampies were collected at several locations within the Des Moines Lobe in central Iowa. Methane concentrations in groundwater ranged from 0.04 µmol L−1 at 1 m to 2600 µmol L−1 at 7 m. Incubations of deep sediment obtained from core samples revealed active CH4 production was occurring, although high variability was observed. Highest specific CH4 production rates were associated with buried particulate C fragments of Wisconsinan age. Results of our study indicate methanogenesis is an active process in the sediments beneath the Des Moines Lobe, that the substrate for methanogenesis is of late Wisconsinan age, and that the high spatial variability of CH4 production is controlled by the heterogeneous distribution of particulate organic C fragments in the glacial sediments.

Characteristics and origin of coarse gold in Late Pleistocene sediments of the Cariboo placer mining district, British Columbia, Canada

The Cariboo placer mining district (1000 km 2) sited in the Interior Plateau of central British Columbia, Canada, is the premier placer gold mining district of the Province. Gold is recovered from three Late Pleistocene sedimentary facies: postglacial fluvial gravels (< 10 Ka), Late Wisconsin till (ca. 25–10 Ka), and “older” fluvial gravels (>25 Ka). This study reports the morphology (size, roundness, sphericity) of 1636 gold grains, ranging in size from 0.25 to 17 mm, recovered from 19 placer mines. Older gravels contain the smallest gold grains (mean grani size 1.53 mm), grains of intermediate size occur in till (2.23 mm) and the coarsest gold occurs in postglacial gravels (2.34 mm) with a mean of 1.93 mm for the mining district as a whole. The most common grain shapes are sub-rounded, discoidal (14.73% of the grain population), sub-angular, discoidal (10.88%), and sub-rounded, sub-discoidal (9.59%); the most angular grains occur in postglacial gravels. In-situ growth of coarse, angular grains is indicated by a “composite” grain structure, consisting of aggregates of gold particles welded together by high-grade (Ag = < 2%) filamentous gold; in-situ coarsening may be reliant on organic complexing agents produced below a dense forest cover. An evolutionary sequence of grain form, from angular aggregates to rounded “pumpkin seed” grains, is suggested. Rounded grains commonly show a crystalline structure which may result from the cold hammering of gold during transport; fracturing along crystal boundaries is common. Gold grains may undergo cycles of coarsening, rounding, diagenesis and breakup in response to repeated recycling through Pleistocene sedimentary environments.

Hydrogeology and redox geochemistry of CH4 in a Late Wisconsinan Till and Loess Sequence in central Iowa

The hydrogeology and redox geochemistry of CH4 in two hydrogeological settings within a late Wisconsinan till and loess sequence in central Iowa were investigated using CH4, Eh, dissolved O2, NO3‐N, NH4‐N, dissolved Fe, SO4, H2S, dissolved organic carbon (DOC), particulate organic carbon (POC), δ13C‐CH4, δ2H‐CH4, δ13C of dissolved inorganic carbon (DIC), and 14C−CH4. Depleted δ13C‐CH4 and δ2H‐CH4 values, enriched δ13C‐DIC values, and the lack of higher hydrocarbons suggest that the CH4 is microbial and is formed by CO2 reduction. Carbon 14 dates of CH4 indicate that 14,000‐ to 17,000‐year‐old POC and DOC in the 1oess and till compose the substrate for methanogenesis. Production of CH4 probably began after deglaciation at about 12,000 years B.P. and has continued to the present. Groundwater recharge, particularly in the upland topographic position at the site, has decreased CH4 concentrations in the upper part of the till.

Effects of Acidification on the Chemical Composition and Layer Charge of Smectite from Calcareous Till

AbstractThe objectives of the study were to determine the chemical composition and layer charge of smectite found in calcareous till of the Interior Plains region of western Canada and to examine the effects of acidification on alteration of the smectite. Samples of acidified and non-acidified (calcareous) late-Wisconsin till were obtained from four soil pits located immediately adjacent to an elemental sulfur block located in southern Alberta. Samples of the surface material (0–10 cm depth) had been subjected to extreme acidity for 25 years due to the oxidation of elemental sulfur and displayed pH values of about 2.0. Samples of the till obtained at depth (65–75 cm) remained calcareous with pH values between 7.3 and 7.6. A combination of analytical methods was used to determine the chemical composition of the smectite found in the samples. The layer charge of the smectite was determined independently using X-ray diffraction data for n-alkylammoniurn saturated specimens. Smectite found in the non-acidified calcareous material was characteristic of montmorillonite with a low content of Fe and very little substitution of Al for Si in the tetrahedral sheet. The smectite had a structural formula of M+0.40(Si3.96Al0.04)(Al1.56Fe3+0.10Mg0.33)O10(OH)2, which compared well with a mean value for layer charge of 0.399 mol(−)/O10(OH)2 determined using X-ray diffraction data for n-alkylammonium treated specimens. Smectite remaining in the till material subjected to extreme acidity underwent incongruent dissolution with a net loss of layer charge and preferential loss of octahedral Mg.

Glacial dispersal of heavy minerals in late Wisconsinan till, central New Brunswick

Drift prospecting surveys in New Brunswick have previously had limited success in recognizing dispersal plumes through geochemical or mineralogical analyses of tills. This study was undertaken in the Todd Mountain - Trout Lake area of central New Brunswick to define possible constraints on glacial dispersal and assist in related prospecting activities. Dispersal patterns for specific heavy minerals in the silt to fine sand (0.125 mm - 0.250 mm) fraction of 63 till samples were investigated over a 12 km x 25 km area. Twelve minerals, confirmed by Scanning Electron Microscopy, were selected for study on the basis of local geology and ease of recognition.&#x0D; &#x0D; In this area of New Brunswick, eastward-trending dispersal plumes can be distinguished over short distances (&lt;5 km) only for rare indicator minerals that characterize specific sources (e.g., coticules or zircon grains). When sampled at intervals of 100 m the mineral dispersal patterns more clearly reflect the nature and trend of units in the bedrock. Correlations between mineral content and geochemistry are poor because of glacial mixing of variable source materials. Some poor chemical correlations may also be due to weathering and hydromorphic dispersion. In this area of central New Brunswick, prospecting programs utilizing the fine-grained till fraction require close sample spacing and accurate identification of till facies.&#x0D; &#x0D; RÉSUMÉ&#x0D; Les levés de prospection par les dépôts glaciaires au Nouveau-Brunswick ont auparavant rencontré des succès mitigés dans la reconnaissance des patrons de dispersion glaciaire par les analyses géochimiques ou minéralogiques des tills. La présente étude a été entreprise dans la région mont Todd - lac Trout, dans le centre du Nouveau-Brunswick, afin de foumir des données sur la dispersion glaciaire et d'assister les activités de prospection associées. Les patrons de dispersion de certains minéraux lourds dans la fraction silt à sable fin (0.125 mm - 0.250 mm) de 63 échantillons de till onlélé étudiés pour une région de 12 km par 25 km. Douze minéraux, dont l'identification a été confirmée par microscopie éléctronique, ont été choisis pour l'étude en raison du contexte géologique et de leur facilité d 'identification.&#x0D; &#x0D; Pour cette région du Nouveau-Brunswick, il est possible de distinguer des patrons de dispersion de direction est, sur de courtes distances (&lt;5 km), seulement pour de rares minéraux indicateurs caractéristiques de certaines sources (comme les grains de coticule ou de zircon). À des intervalles d'échantillonage de 100 m, les patrons de dispersion minéraux reflètent plus nettement la nature et l'orientation des unités du socle. La corrélation entre le contenu en minéraux et la composition géochimique est faible en raison du mélange glaciaire de matériaux de diverses sources. Il y a de faibles corrélations géochimiques qui peuvent aussi être dues à l'altération supérficielle et à la dispersion par l'eau souterraine. Dans cette région du centre du Nouveau-Brunswick, les programmes d'exploration qui utilisent la fraction fine du till néccssitent une grille d'échantillonage serrée et une identification précise des faciès du till.&#x0D; &#x0D; [Traduit par le journal]

A gravel/diamicton lag on the south Albertan prairies, Canada: Evidence of bed armoring in early deglacial sheet-flood/spillway courses

A clast-supported, massive diamicton to poorly sorted, massive gravel (Gm/Dcm) bed in the area around Dinosaur Provincial Park, southern Alberta, Canada, provides a means of identifying early deglacial flood courses. This is especially useful where flood and spillway courses have very subtle geomorphic definition and are difficult to map from aerial photographs. The Gm/Dcm bed conformably overlies a brown, late Wisconsinan till and rarely overlies bedrock and lacustrine sediments. The geographic distribution of the bed is restricted to flat to gently undulating prairie on either side of the Red Deer River where shallow floods and spillway melt waters were concentrated upon deglaciation. The bed has also been observed along the banks of the South Saskatchewan River at Medicine Hat, 90 km to the south, and is therefore regionally extensive. Because the Gm/Dcm bed has none of the attributes of supraglacial sediment and possesses a clast fabric and sphericity very similar to those of underlying till, it is interpreted as a winnowed gravel/diamicton lag, which was produced by low-competence melt-water floods along shallow depressions in the prairie surface after the drainage of local proglacial lakes. After enough of the fine-grained matrix of the late Wisconsinan till was removed to create a lag