Origin of a washboard moraine of the Des Moines Lobe inferred from sediment properties

Geomorphology 248 (2015) 452–463

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Origin of a washboard moraine of the Des Moines Lobe inferred from sediment properties Suzanne Ankerstjerne a, Neal R. Iverson a,⁎, France Lagroix b a b

Department of Geological and Atmospheric Sciences Iowa State University, Ames, IA 50011, USA Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75005 Paris, France

a r t i c l e

i n f o

Article history: Received 19 January 2015 Received in revised form 7 July 2015 Accepted 8 July 2015 Available online 26 July 2015 Keywords: Glacier Moraine Till Fabric Preconsolidation

a b s t r a c t Geometric characteristics of the washboard moraines of the Des Moines Lobe (DML) of the Laurentide ice sheet agree with their proposed origin as crevasse-squeeze ridges, but study of their sediments is required to help further test this hypothesis. A 70-m-long, 3–5 m high section through a moraine ridge in central Iowa revealed till with irregular, isolated lenses of silt, sand, and gravel that dip to varying extents upglacier. The texture and density of the till are like those of the basal till of the DML studied elsewhere in Iowa, and preconsolidation pressures determined from tests on till and silt of the ridge indicate that it developed subglacially rather than at the glacier margin. Preconsolidation pressures additionally demonstrate that pore-water pressures in the bed supported most of the glacier's weight, which would have contributed to till mobility. Fabrics based on the anisotropy of magnetic susceptibility of 3125 intact till specimens collected at 125 locations in the section indicate two end-member states of strain that varied with location in the ridge and caused sediment mounding: simple shear that was directed downglacier along shear planes inclined upglacier, together with pure shear where an overlying crevasse allowed the sediment bed to extend upward and laterally. Meltwater that likely flowed along the crevasse deposited sorted sediments that were incorporated in till, deformed, and rotated. This positive test of the crevasse-squeeze hypothesis indicates that the DML was in longitudinal extension near its margin, reinforcing previous arguments that the lobe surged. The predominance of fabrics caused by simple shear demonstrates that crevasse filling was underway before the surge had fully halted. This study should prompt caution in using similar transverse ridges, such as those geophysically imaged in some submarine glacier forefields, as indicators of retreat rates. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Low-relief, transverse moraine ridges were deposited by the lateWisconsinan Des Moines Lobe (DML) of the Laurentide Ice Sheet (Fig. 1). We call them washboard moraines (e.g., Elson, 1957), with no genetic connotation intended, although they have also been called swell and swale pattern (Gwynne, 1942), minor moraines (e.g., Gwynne, 1951), and corrugated moraines (Stewart et al., 1988). In central Iowa, where they are especially prominent (Fig. 1B), they have heights of 1–5 m and wavelengths of 30–200 m (Gwynne, 1942; Foster and Palmquist, 1969; Stewart et al., 1988; Colgan, 1996). They are thus topographically more subdued than most ribbed moraines (e.g., Lundqvist, 1989; Lindén et al., 2008) but similar in scale to some De Geer moraines (e.g., Lindén and Möller, 2005). Stewart et al. (1988), who conducted the most thorough sedimentological study of the washboard moraines of the DML, concluded that ⁎ Corresponding author at: Department of Geological and Atmospheric Sciences, 253 Science I Hall, Iowa State University, Ames, Iowa 50011, USA. E-mail addresses: [email protected] (S. Ankerstjerne), [email protected] (N.R. Iverson), [email protected] (F. Lagroix).

http://dx.doi.org/10.1016/j.geomorph.2015.07.019 0169-555X/© 2015 Elsevier B.V. All rights reserved.

basal crevasses localized deposition by lodgment from ice prior to stagnation of the lobe. Others have interpreted the moraines as having developed after stagnation by filling of crevasses with basal and supraglacial sediments (Patterson, 1997; Jennings, 2006). Quite different alternative hypotheses are that these moraines formed in a subglacial environment characterized by longitudinal compression of ice and associated debris transport in transverse shear zones (Clayton and Moran, 1974; Colgan, 1996; Colgan et al., 2003) or at or near the glacier margin by seasonal pushing or thrusting (Gwynne, 1942, 1951; Lawrence and Elson, 1953). Whether these moraines formed seasonally at the ice margin is relevant to the origin of similar transverse moraines in submarine environments that have been interpreted in this way and used to calculate grounding line retreat rates of modern ice masses (Shipp et al., 2002; Dowdeswell et al., 2008). Recent spectral analyses of flow-parallel profiles across the washboard moraines of central Iowa using LiDAR data (Cline et al., 2015) indicated that moraine ridges are generally spaced with statistically significant periodicity. Dominant wavelengths range from 70 to 150 m, and the ridges display no systematic asymmetry, unlike most ice marginal moraines. Moreover, these LiDAR data, in conjunction with well logs, show that moraine trends deflect upglacier and converge as upglacier-

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Fig. 1. Study area. (A) Maximum extent of the DML at ~13,800 radiocarbon years before present (Clayton and Moran, 1982). (B) LiDAR image of washboard moraines, with the ridge segment of the study shown. The inset shows the Iowa footprint of the DML, with major end moraines black and moraine complexes gray, and the area spanned by the LiDAR image.

pointing cusps that are coincident with outwash trains—including trains deposited proglacially and buried in basal till by the overriding lobe—indicating that outwash was subglacial when it influenced moraine deposition. On the basis of these observations, Cline et al. (2015) suggested that the ridges formed by squeezing of till into transverse crevasses that extended to the bed. They suggested that overridden outwash trains reduced basal water pressure and thereby locally deflected crevasses by slowing glacier slip. This interpretation of the moraines as crevasse-squeeze ridges is significant because they are a landform indicative of glaciers that have surged and stagnated (Benn and Evans, 2010; Rea and Evans, 2011; Schomacker et al., 2014).

Although the spatial attributes of the DML washboard moraines are consistent with them being crevasse-squeeze ridges, no single attribute provides definitive support for this conclusion, highlighting the need to also study the sediments of the ridges. Previous work has indicated that they consist largely of till with isolated, channel-like sand and gravel deposits (Stewart et al., 1988). No single washboard ridge of the DML has been studied comprehensively, but till sampled from a handful of these ridges is interpreted to be mostly basal till, based on its textural homogeneity (Kemmis et al., 1981), high density (Kemmis et al., 1981), and commonly strong clast fabrics and bullet stones oriented in the glacier flow direction (Stewart et al., 1988). Basal till is present everywhere beneath the Iowa footprint of the DML, and this till is overlain

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by supraglacial till typically only in hummocky end moraines (Kemmis et al., 1981) (Fig. 1B, inset). No major differences between this regionally deposited basal till and that of the washboard moraines have been described. However, in the till of the moraines there seems to be a preferred tendency for sand and gravel layers within and above the till, both in ridge crests and adjacent swales (Stewart et al., 1988). An innovative aspect of the study by Stewart et al. (1988) was their use of magnetic till fabrics. They measured the anisotropy of magnetic susceptibility (AMS) of intact till samples and used their directions of maximum susceptibility to compute fabrics reflecting the alignment of fine magnetic grains. These fabrics generally mimicked those of clast long axes, including their tendency to parallel the glacier flow direction and plunge gently (10–30°) upglacier. However, only 3–12 samples were collected to construct each fabric, and a total of only seven such fabrics were measured. Moreover, Stewart et al. (1988) did not report the orientations of all magnetic susceptibilities, which if considered collectively yield considerably more information about deformation kinematics than clast fabrics (e.g., Shumway and Iverson, 2009). Most importantly, their interpretations could not benefit from later experimental studies of magnetic fabric development in sheared tills that connect the character of magnetic fabrics to the style, direction, and magnitude of deformation (Hooyer et al., 2008; Iverson et al., 2008). The goal of this study was to fully characterize a cross section through a particularly well-developed washboard moraine ridge of the DML (Fig. 1B), with the use of multiple tools to infer stress history and deformation kinematics. One of these tools is preconsolidation testing (e.g., Sauer et al., 1993), which has not been applied previously to washboard moraines and yields the maximum effective stress (total stress minus pore-water pressure) on sediments since their deposition. The other technique involved measuring the AMS of over 3000 intact till specimens to compute fabrics at 125 locations. By considering all three principal magnetic susceptibilities and basing interpretations on experimental calibrations of AMS fabric to till strain characteristics, we infer deformation patterns in the moraine. This information, together with ancillary measurements of sediment texture, till density, clast fabrics, and stratigraphy, allow an independent evaluation of the crevassesqueeze hypothesis for the origin of these moraines. An obvious drawback of our approach—studying a moraine ridge in great detail—is that the effort required for this comprehensive study, as well as the lack of natural outcrops, precluded studying more than one moraine. However, past studies have tended to present sedimentological data from several moraines at the expense of evaluations that were incomplete, spatially and methodologically. This comprehensive study of one moraine ridge, therefore, complements past work. This study also benefits from knowledge of soft-bed processes that has expanded significantly since the sedimentology of these moraines was last studied (i.e., Stewart et al., 1988; Kemmis, 1991).

2. Research questions Measurements were designed to answer two questions that bear on the crevasse-squeeze hypothesis: • Were the sediments of the moraine deposited subglacially and can ice-marginal deposition be ruled out? Ruling out ice-marginal formation would preclude for these ridges a variety of related hypotheses that have been proposed for the origin of washboard ridges, of the DML (Gwynne, 1942, 1951; Lawrence and Elson, 1953) and elsewhere (e.g., Ham and Attig, 2001). • Do these sediments show evidence of their upward deformation into a crevasse (Fig. 2)? Sediment could squeeze upward because of gradients in effective stress in the bed adjacent to an open crevasse; or alternatively, if crevasse-squeeze ridges form during forward motion of the glacier, deformation could involve shearing with an upward component as a transverse crevasse plows forward, scraping sediment from the soft bed. These two styles of deformation need not be mutually exclusive (Fig. 2).

3. Methods An especially prominent washboard moraine, about 16 km southeast of Ames, Iowa, was chosen for this study (Fig. 1B). The moraine is within 20 km of the three washboard moraines studied by Stewart et al. (1988) and within the area studied by Cline et al. (2015, see their Fig. 3). This moraine is unique in that a road was cut through it; otherwise roads in the area have been constructed over rather through moraines. The road trends north–south and obliquely cuts the moraine (Fig. 3A). Access to its interior is provided by an embankment on the east side of the road. About 0.5–1.0 m of soil and grass were cleared from it with a trackhoe to expose a cross section about 70 m across and 5 m high at the crest of the moraine (Fig. 3B). The borrow pit, road construction site, and quarry previously exploited by Stewart et al. (1988) are gone. The moraine consists of two segments that trend differently on each side of the cross section (Fig. 3B): 40° and 75° on the west and east sides of the section, respectively. The moraine is near a cusp, so its average trend differs slightly from being perpendicular to the regional flow direction, ~155°, as indicated by the trends of moraine crests upglacier and away from the cusp. As reference directions in stereonets showing till fabrics, we indicate the trends of the two moraine segments and the regional flow direction. Vertical profiles, 12 in total, were established at 5-m intervals across the exposure and numbered from south to north (Fig. 3C). Surveying the road through the moraine provided a reference for profiles. The

Fig. 2. Possible kinematics of bed extrusion into the base of a crevasse. The upstream side of the crevasse wall plows across the bed, resulting in piling of sediment near the upstream wall and thrusting, while the gradient in effective stress on the bed on each side of the crevasse causes upward extrusion of till.

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Fig. 3. Moraine ridge of the study. (A) LiDAR image of the moraine, with the cleared section through it (white bar) and the contrasting trends of the moraine segments (dashed lines) on either side of the section. Regional flow direction is based on orientations of washboard moraines immediately upglacier. (B) The section studied, viewed looking southeast, after removal of grass and soil. (C) The stratigraphy of the section, vertically exaggerated, and locations of sampling profiles.

stratigraphy across the section was described; and within each profile, horizontal, flat shelves (~0.1 m2) were cut every 0.2 m for sampling. 3.1. Till texture, density, and clast fabrics Till of the moraine was studied in three standard ways. The grain-size distributions of 31~500-g samples from three vertical profiles (2, 8, 12, Fig. 3C) were measured using sieve and hydrometer analysis following the ASTM protocol (ASTM, 2007). In 29 locations till density was measured over volumes of 300–800 cm 3 using the excavation method (Arshad et al., 1996). Clast fabrics (e.g., Benn, 1995) were measured within six profiles (2, 4, 6, 8, 10, 12, Fig. 3C). At each profile, a vertical surface (~1.0 m2) was exposed. For each fabric, 25 gravel- or cobblesized elongate clasts (aspect ratios N 1.5) were measured. 3.2. Preconsolidation pressure Preconsolidation pressure is the maximum effective stress that sediment has been subjected to since its deposition and reflects its degree of consolidation. To measure preconsolidation pressure in the usual way, a confined, intact, sediment specimen is saturated with deaired water and

subjected to incremental increases in axial stress. Resultant excess pore pressure is allowed to dissipate as the granular skeleton consolidates after each loading increment. Initial loading increments are small and result in primarily elastic deformation. At larger stresses, the specimen permanently consolidates owing to irrecoverable rearrangement of grains. The stress at which a specimen begins to deform permanently is the preconsolidation pressure (e.g., Bowles, 1986). Intact specimens of silt and till were sampled for preconsolidation testing. Silt specimens were collected from lenses near and within profile 5 (Fig. 3C). A flat, horizontal surface was excavated in a silt lens, and a metal sampling ring (635 mm diameter, 20 mm thick) was pressed into the surface. Till was generally sampled differently, owing to its tendency to fracture along fissile partings during ring insertion. Three intact blocks of till (~0.3 m per side) were collected and then subsampled and trimmed in the laboratory so that sampling rings could be pressed snugly around the subsamples. This till was collected from near the top of profile 1 (Fig. 3C). The silt and till specimens were saturated with deaired and deionized water and consolidated under a loading sequence of 12, 25, 50, 100, 200, 400, 800, 1600, and 3200 kPa using fixed-ring oedometers (Bowles, 1986). Results were analyzed using the empirical method of

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Casagrande (1936) to obtain the preconsolidation pressure of each specimen (see Hooyer and Iverson, 2002, for a description).

3.3. AMS fabrics AMS, which can provide a quantitative proxy for the orientation and alignment of nonequant magnetic particles, was used as the primary tool for analysis of till deformation. Between 24 and 26 intact till samples were collected from each shelf of the 12 vertical profiles for AMS analysis, with each set of samples spanning an area of ~ 0.01 m2 and a horizon ~20 mm thick. Uniformly oriented samples were collected by pressing 18-mm plastic cubic boxes with 1-mm wall thicknesses into the shelf (Fig. 4). The boxes were carefully excavated, capped, and labeled. A total of 3125 AMS samples were collected from 125 shelves. The magnetic susceptibility of the samples was measured using a Geofysika Kappabridge KLY-3S magnetic bridge. This device subjects each sample to a uniform magnetic field of known strength at 15 orientations. The strength of the induced field is characterized by the susceptibility, k: the ratio between the applied and induced fields in a particular orientation. The susceptibility varies with orientation because some particles are more easily magnetized in one orientation than another (Tarling and Hrouda, 1993). Hysteresis experiments and tests that measure the dependence of magnetic susceptibility on temperature, like those described by Hooyer et al. (2008), indicate that the AMS carrier mineral in the DML till is magnetite, with grains 20–100 μm in diameter being most important. Magnetite is a mineral dominated by so-called shape anisotropy (Tarling and Hrouda, 1993), such that grains have the strongest susceptibility parallel to their long axes. Thus, susceptibility orientations of intact samples of the DML till are indeed a proxy for grain alignment. The AMS of a single sample is commonly visualized as an ellipsoid, with the three axes corresponding to the maximum (k1), intermediate (k2), and minimum (k3) susceptibilities. Box insertion during sampling can, to some extent, disturb till. However, results of ring-shear experiments with various tills, in which till was sampled using the same box-insertion technique as in this study, yielded self-consistent, strong fabrics clearly related to shear deformation and unrelated to the orientation of box insertion (Hooyer et al., 2008). Moreover, field AMS fabric studies in other basal tills using the same sampling technique yielded k1 orientations concordant with traditional particle fabrics (Thomason and Iverson, 2009; Gentoso et al., 2012), so box insertion does not cause significant disturbance. We cannot rule out that freezing and thawing or biotic activity may have disturbed fabrics at the shallowest depths sampled. However, the effect of such activity would have been to weaken fabric strength without a

systematic effect on fabric orientation. We rely upon only fabric orientations to interpret deformation patterns. The AMS ellipsoids from each set of samples from a platform were used to determine a fabric. The orientation of maximum clustering of each of the three principal susceptibilities (k1, k2, and k3) is represented by a V1 eigenvector (Mark, 1973), with the degree of clustering about V1 given by the eigenvalue, S1: S1 = 1.0 reflects perfect alignment, and S1 = 0.33 indicates no alignment (isotropically distributed orientations). Eigenvectors and eigenvalues were calculated based on the orientations of each of the principal susceptibilities. Although analyzing orientations of principal susceptibilities separately, rather than using tensor statistics, can lead to V1 eigenvectors for k1, k2, and k3 orientations that are not orthogonal, this problem is significant only for poorly clustered data. For this reason, we elect not to break from the tradition in glacial geology of using eigenvectors and eigenvalues to characterize fabrics, but we compute eigenvectors only if principal susceptibility orientations cluster sufficiently (e.g., 1 − (S2/S1) N 0.5). An exception is in Fig. 1 of the online supplementary material, where to highlight this issue we plot eigenvectors for principal susceptibility orientations regardless of their degree of clustering. Results of previously conducted ring-shear experiments on tills with AMS controlled by magnetite grains (Hooyer et al., 2008; Iverson et al., 2008), as in the DML basal till, provide a foundation for relating magnetic fabrics to shear strain direction and magnitude. Progressively increasing shear strain increases k1 fabric strength at an exponentially decreasing rate up to a critical strain, 10–30, beyond which the fabric strength remains steady (Hooyer et al., 2008; Iverson et al., 2008). Results of these experiments provide guidelines for interpreting AMS fabrics resulting from simple shear (Fig. 5A): • All three principal susceptibilities are clustered. • Orientations of k1 and k3 cluster in the longitudinal flow plane, with k1 parallel to the direction of shear. • Orientations of k2 cluster normal to the longitudinal flow plane and in the shear plane. • Orientations of k1 plunge upglacier 18–30°, relative to the shear plane.

Deformation, however, can also include or be dominated by pure shear. Fabrics that result largely from pure shear can be inferred by considering the rotation of passive lines in a body, with shortening in one direction and extension in the two other directions. In this case only orientations of k3 are clustered, with clustering parallel to the axis of shortening. Orientations of k1 and k2 are not clustered and instead form a girdle normal to the axis of shortening (Fig. 5B). The distinctly different two patterns of clustering in Fig. 5 illustrate a major advantage of AMS fabrics over traditional particle fabrics: simple and pure shear can be more readily distinguished.

4. Results 4.1. Stratigraphy

Fig. 4. Collecting and marking sample boxes from an excavated platform for AMS analyses.

The moraine is composed of sandy till with isolated bodies of sand, silt, and gravel (Fig. 3C). The till consists of gravel-sized clasts in a dense, calcareous matrix that is highly fissile and densely fractured, with oxidized fracture surfaces. Soil, 0.60–1.0 m thick, at the top of the cross section is underlain by a thin layer of till (b0.4 m thick). This till overlies a continuous lens, up to 1 m thick, of fine-grained sand that extends horizontally ~ 30 m across the crest of the moraine. Several smaller irregularly shaped sand and silt lenses occur across the exposure. Most of these lenses have apparent dips to the north. A horizontal, trough-shaped lens consisting of gravel, sand, and silt layers (Fig. 3C) is present between profiles 1 and 2 near the south end of the section.

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Fig. 5. Simple and pure shear end members with orientations of principal susceptibilities that develop after sufficient total strain, if magnetite grains control AMS. (A) AMS ellipsoid for simple shear and a sample fabric diagram from a ring shear experiment on a till with magnetite as its principal magnetic mineral, showing orientations of principal susceptibilities (k1 open squares, k2 triangles, and k3 solid circles) plotted on a lower hemisphere stereonet. (B) AMS ellipsoid for the case of pure shear, with shortening along one axis and extension along the other two axes, and a sample fabric diagram from this study, showing orientations of principal susceptibilities.

4.2. Till texture, density, preconsolidation pressures, and clast fabrics The till in the exposure contains, on average, 16% clay, 29% silt, 49% sand, and 6% gravel, with standard deviations of approximately ±5%. The till's texture is relatively uniform spatially and does not vary systematically with depth. On a USDA soil texture classification ternary diagram, individual till samples plot in a field at the intersection of loam, sandy loam, and sandy clay loam (Fig. 6). The till's textural range can be compared with that of the two well-studied tills of the DML (tills of the

Fig. 6. Soil texture ternary diagram with till data from moraine profiles and fields for tills of the Morgan (supraglacial) Member and Alden (basal) Member of the DML's Dows Formation (Kemmis et al., 1981).

so-called Dows Formation): the Alden Member basal till, present nearly everywhere over the footprint of the lobe in Iowa, and the Morgan Member supraglacial till, present in end moraines of the lobe (Kemmis et al., 1981). Although slightly sandier, the texture of the till of the moraine mirrors the homogeneous texture of the basal till of the Alden Member and is far more homogeneous than the supraglacial till of the Morgan Member. The average dry bulk density of the till of the moraine is 1.82 ± 0.19 g cm−3, without systematic spatial variations. Comparing till densities for the moraine with those reported by Lutenegger et al. (1983) for the Alden Member and Morgan Member tills of the DML (Fig. 7) indicates that the density of till of the moraine generally matches the higher density of the basal till of the Alden Member.

Fig. 7. Distribution of dry bulk density of till samples from the moraine and for tills of the Morgan (supraglacial) Member and Alden (basal) Member of the DML's Dows Formation (Lutenegger et al., 1983).

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Consolidation testing of till and silt specimens yielded preconsolidation pressures of 67–390 kPa (Fig. 8). In contrast, modern overburden pressures over the range of sampling depths are significantly smaller, 20–57 kPa, so effective stresses on the till and silt specimens were higher in the past than they are presently. Thus, the till is overconsolidated. Preconsolidation pressures are generally higher for silt specimens than for till specimens (Fig. 8). Average values for silt and till are 286 kPa and 149 kPa, respectively, although a silt specimen yielded the lowest preconsolidation pressure. Till clast fabrics for each of six profiles across the moraine are fairly consistent (Fig. 9). Fabrics are clustered and unimodel, with S1 eigenvalues ranging from 0.61 to 0.82. Azimuths of maximum clustering, as indicated by V1 eigenvectors (Mark, 1973), are roughly perpendicular to one of the two moraine crest segments with strongest fabrics perpendicular to the segment west of the moraine (Fig. 3A). Four of the six fabrics have V1 eigenvectors that plunge gently upglacier.

profiles (Fig. 1, online supplementary material) clustering and girdling of principal susceptibilities are evident. Although both of the fabric patterns illustrated in Fig. 5 are present in the section and apparent in the depth-aggregated data of Fig. 10 and in individual fabrics (Fig. 1 of online supplementary material), most data indicate a tendency toward clustering of all three principal susceptibilities, with k1 plunging upglacier. The upglacier plunge is clearly seen if we consider only fabrics with k1 orientations that are sufficiently strongly clustered (S1 N 0.75) and plot orientations of k1based V1 eigenvectors from those fabrics on a single stereonet (Fig. 11). The orientation of maximum clustering of V1 orientations is 314° (within 4° of being perpendicular to the moraine-crest segment on the west side of the section) with an upglacier plunge of 55°.

5. Discussion 5.1. Subglacial origin

4.3. AMS fabrics A great diversity of fabric strengths and patterns are illustrated if orientations of principal susceptibilities (k1, k2, k3) and their corresponding V1 eigenvectors are plotted for all of the 125 locations (Fig. 1 of online supplementary material). Owing to a lack of systematic depth variability within the profiles, we focus on horizontal variability along the 70-m-long section. To do so, in Fig. 10 we plot all data from a given profile on a single stereonet and thereby compare fabrics among profiles. Each stereonet displays orientations of principal susceptibilities of 200 to 350 till samples, depending upon the height of the profile (1.5–3.0 m). Clear variations in depth-aggregated fabric character are displayed along the section (Fig. 10). At its south end in profiles 1 and 2, orientations of the three principal susceptibilities are clustered (see ternary Benn diagrams, Fig. 10), with k1 clusters plunging 45–55° to the northwest or upglacier. Immediately to the north—particularly in profile 3 but also more weakly in profile 4—the pattern is very different: only k3 orientations are clustered, with orientations of k1 and k2 forming a girdle parallel to the crest of the moraine on the east side of the section (Fig. 3A). The girdle in profile 3 delineates a plane that dips steeply to the north-northwest. Still farther north, in profile 6 and in profiles 8–11 on the north side of the moraine crest, all three principal susceptibilities are again clustered, with orientations of k1 plunging 5–60° upglacier. Aggregating the data of profiles 5, 8, and 12 results in principal susceptibility orientations too isotropic to be useful, although locally in those

Fig. 8. Preconsolidation pressures determined from intact silt and till samples from the moraine, compared with the modern overburden pressure of overlying sediment.

Data from this study demonstrate that washboard moraines of the DML formed subglacially—an obvious requirement of the crevassesqueeze hypothesis. Grain-size data provide the simplest evidence: the grain-size distribution of the till is similar to that of the basal member of the DML (Alden Member) studied elsewhere in Iowa (Fig. 6). Basal tills of the U.S. Midwest are characterized by homogenous grain-size distributions (Johnson et al., 1971; Kemmis et al., 1981; Lutenegger et al., 1983), owing to the tendency for basal sediment transport processes to eliminate primary heterogeneities (Benn and Evans, 1996). The till of the moraine is slightly sandier than the Alden Member till, which may reflect till mixing with sand deposits in the moraine (Fig. 3C). The till density and the preconsolidation pressures reinforce that the ridge formed beneath the ice rather than adjacent to it. The range of till density is similar to that of the Alden Member and convincingly larger than that of the supraglacial Morgan Member till (Fig. 7). Preconsolidation pressures exceed modern overburden pressures by ~25–335 kPa (Fig. 8). Post-glacial erosion has reduced the height of washboard moraines by as much as 0.6–1.2 m (Burras, 1984), so that modern overburden pressures may be underestimated by 13–21 kPa (assuming a wet density of ~ 2000 kg m − 3 for overburden sediment). Another uncertainty is that although the till samples were moist when collected, they were above the water table, so they had lower moisture contents than during loading by the glacier. Such drying can increase preconsonsolidation pressures (e.g., Mickelson et al., 1979; Tulaczyk et al., 2001) by increasing matric suction (making pore pressure more negative)—an effect that increases with decreasing grain size. Vanapalli and Oh (2011) measured matric suction, as a function of water content and compaction stress, in till with a clay content of 28% and hence finer grained than that of the till of this study (16% clay). Extrapolating their data to compaction stresses equal to the modern overburden pressure on the till of this study indicates matric suction b 80 kPa was likely during drying of the DML till and thus insufficient to account for most of the measured preconsolidation pressures. Moreover, generally higher preconsolidation pressures of the silt (Fig. 8), which is coarser than the fine fraction of the till, argue against drying being an overriding factor affecting the preconsolidation data. Therefore, effects of post-glacial erosion and drying on effective stress fall short of accounting for the measured degree of overconsolidation, indicating that the silt and till of the moraine were loaded by the glacier. These data also preclude the possibility that the moraine sediments were deposited subglacially but subsequently pushed into a moraine at the glacier margin. Deformation of basal, overconsolidated sediments associated with proglacial push moraine formation would have caused sediments to dilate, yielding till densities systematically lower than those of the Alden Member. In addition, deformation of overconsolidated

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Fig. 9. Fabrics based on clast long axis orientations (solid squares) from six of the 12 profiles of the section. Crosses show orientations of maximum clustering (V1), different styles of dashed lines show the trends of the two moraine segments (Fig. 3A), and arrows point in the regional glacier flow direction determined from washboard moraines upglacier (Fig. 1B). Mean sampling height above the datum and fabric strength (S1) are also shown for each profile.

till, unloaded by ice, would have reset preconsolidation pressures to normally consolidated values (e.g., Holtz and Kovacs, 1981). High pore-water pressures within the bed must have partly supported the weight of the glacier, consistent with the conclusions of previous studies of the DML's basal till (Hooyer and Iverson, 2002). Reconstructions of the lobe indicate that the ice thickness over central Iowa was 100–200 m (Clark, 1992; Hooyer and Iverson, 2002), resulting in total normal stresses of 900–1800 kPa on the bed. Average preconsolidation pressures for silt and till were much smaller, 286 kPa and 149 kPa, respectively. Pore pressure at the bed of the DML must, therefore, have supported most of the glacier's weight, consistent with the fast

flow suggested for the lobe (e.g., Clayton et al., 1985). Higher preconsolidation pressures in the silt could reflect lower pore pressures associated with higher permeability of the silt or a tendency to disturb the structure of the till while sampling it, causing artificially low preconsolidation pressures. The conclusion that sediments in the moraine are derived from under the glacier indicates that the sand, silt, and gravel bodies in the moraine are derived from water flow that was subglacial. Sorted sediments have been commonly attributed to meltwater deposition in channels at the ice-bed interface (Clark and Walder, 1994; Benn and Evans, 1996; Evans et al., 2006). Such deposits, if subsequently deformed and rotated

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Fig. 10. Aggregated AMS fabric data for each profile in the moraine. Orientations of the three principal susceptibilities (k1 red, k2 green, and k3 blue) are shown on lower hemisphere stereonets. Different styles of dashed lines show the trends of the two moraine segments (Fig. 3A), and arrows point in the regional glacier flow direction determined from washboard moraines upglacier (Fig. 1B). Fabric-shape ternary diagrams (Benn, 1994) illustrate the degree of clustering or girdling of the orientations of each of the principal susceptibilities, as based on S1, S2, and S3 eigenvalues.

subglacially, could explain the upglacier dipping sand, silt, and gravel lenses of the ridge (Figs. 3C, 10). Stewart et al. (1988) believed sand at the surfaces of the washboard moraines they studied was aeolian. Till above the extensive horizontal sand layer at the top of the moraine of this study (Figs. 3C, 10) precludes post-glacial aeolian deposition as its origin, although this till does not preclude overriding of proglacial aeolian sediments during an undocumented advance of the lobe that could have

occurred after the lobe reached its maximum extent at the Bemis moraine (Fig. 1B, inset). 5.2. Deformation Three sources of evidence indicate that the sediments of the moraine have been deformed: the orientations of lenses of sorted sediment, clast

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Fig. 11. V1 eigenvectors of 57 k1-based fabrics that satisfy S1 N 0.75. The solid circle shows the orientation of maximum clustering. Different styles of dashed lines show the trends of the two moraine segments (Fig. 3A), and the arrow points in the regional glacier flow direction determined from washboard moraines upglacier (Fig. 1B).

fabrics, and AMS fabrics. If the crevasse-squeeze hypothesis is correct, evidence should indicate a state of strain reflecting upward motion of till into a crevasse. Upward motion in the absence of basal sliding would need to occur exclusively by extrusion into the crevasse owing to effective stress gradients in its vicinity, causing compression and shortening of till perpendicular to the moraine crest and upward and possibly lateral till extension into the crevasse. On the other hand, in the presence of basal sliding, extrusion could also result from mounding of sediment associated with downglacier-directed simple shear with an upward component (Fig. 2). Many lenses of sorted sediments dip steeply in the upglacier direction (Figs. 3C, 10). This orientation could have developed from overturning during simple shear, such that originally horizontal sand bodies were rotated more than 90°; from pure shear with shortening parallel to the flow direction, causing lenses to dip upglacier; or from components of both of these strain end-members (Fig. 2). Some sedimentary features, such as the channel deposit near the south edge of the moraine, appear to have been deformed little (Figs. 3C, 10). The assemblage of highly deformed features in spatial proximity to undeformed deposits highlights heterogeneity of deformation, in possibly both space and time. Clast orientations have azimuths clustered roughly perpendicular to either the eastern or western moraine-crest segment (Fig. 9), with V1 eigenvectors that cluster about the regional glacier-flow direction. Such clustering is consistent with flow-parallel simple shear, as demonstrated in field (Benn, 1995; Benn and Evans, 1996) and laboratory (Hooyer and Iverson, 2000) investigations. This simple shear interpretation of clast fabrics, however, is not definitive. For example, NW–SE extension of the bed, with bed contraction in the other two dimensions could have also yielded unimodal fabrics like those of Fig. 9. AMS fabrics provide the more detailed and definitive picture of strain patterns in the moraine. Fabrics with clustered k1, k2, and k3 orientations dominate the moraine obliquely upglacier (north) from its crest and in the two southernmost profiles sampled (Fig. 10). The simplest interpretation of these fabrics is that they developed by simple shear. For that state of strain, clusters of k1 and k3 orientations lie in the longitudinal flow plane (Fig. 5A), indicating that shear was directed perpendicular to either the eastern or western moraine crest segments

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(Fig. 10, profiles 1, 2, 8–11). Ring-shear experiments indicate that k1based eigenvectors plunge upglacier 18–30° from the shear plane (Fig. 5A) and that the average upglacier plunge of k1-based eigenvectors in the moraine was 55° (Fig. 11). These observations indicate that shear planes were not horizontal but on average dipped ~25–37° upglacier, consistent with downglacier-directed shear with an upward component or thrust-like deformation. Finally, noting that k2 orientations cluster in the shear plane and perpendicular to the shearing direction (Fig. 5A), k2 orientations indicate that shear planes also laterally dipped gently to the northeast (Fig. 10, profiles 1, 8–11). In contrast, in profiles 3 and 4 downglacier from the moraine crest, AMS fabrics indicate that deformation was closer to the pure shear end-member of deformation (Fig. 10). For that state of strain, orientations of k3 are clustered parallel to the most compressive principal stress and the resultant principal orientation of shortening in the bed (Fig. 5B). Orientations of k1 and k2 are parallel to principal orientations of extension and form a girdle that is particularly well-developed in profile 3 (Fig. 10). The girdle is parallel with the trend of the moraine segment on the eastern side of the crevasse (Fig. 3A). The girdle of k1 and k2 orientations, therefore, likely reflects upward and lateral extension of sediment that occurred during its extrusion into the plane of a crevasse that extended to the bed. Interpreting the k1–k2 girdles of profiles 3 and 4 (Fig. 10) as being the result of upward and lateral till extrusion into a crevasse is also consistent with the dip of the plane of the girdles: steeply upglacier, like transverse crevasses in glaciers that have recently surged (e.g., Lawson et al., 1994). These AMS fabrics indicative of longitudinal compression with upward and lateral till extension do not support deposition of till by lodgment, as advocated by Stewart et al. (1988) for these moraines. 5.3. Origin The results of this study support the crevasse-squeeze hypothesis for the formation of the washboard moraines of the DML—a well-established process in some modern glacial environments (e.g., Mickelson and Berkson, 1974; Sharp, 1985; Rea and Evans, 2011). Weak basal till under low effective stresses was mobilized and mounded by a combination of downglacier shear along planes inclined upglacier and upward extrusion. Fig. 12 generalizes the state of strain in the moraine, bearing in mind that the relief of Iowa washboard moraines has been reduced since deglaciation (Burras and Scholtes, 1987). The pure-shear fabrics of profiles 3 and 4 on the downglacier side of the ridge likely reflect upward extrusion where the crevasse opened onto the bed. Such crevasses would have conducted meltwater and deposited sorted sediments along their trends. These sediments were deposited with the till and during building of the ridge were deformed, rotated, and displaced. Four nearby washboard moraine ridges of the DML also consist of basal till with interbedded sorted sediments, which prompted Stewart et al. (1988) to infer that basal crevasses localized till and fluvial deposition. Their till fabric data were limited—2–5 clast fabrics per ridge and a total of seven magnetic fabrics at two sites—but like our data indicated that much of the till of the ridges was sheared in approximately the glacier flow direction. Thus, the crevasse filling that led to basal moraine formation was assisted by basal ice motion and so occurred prior to complete ice stagnation. We infer that crevasse filling began during the surge but in its late stages, to allow preservation of the ridges commonly attributed to post-surge stagnation (e.g., Sharp, 1985; Benn and Evans, 2010; Rea and Evans, 2011). 6. Conclusions The washboard moraine ridge of this study formed by shearing and extrusion of bed sediment upward into a crevasse—a process consistent with geometric characteristics of washboard moraines in the area (Cline

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Fig. 12. State of deformation in the moraine, inferred from AMS and clast fabrics and reinforced by orientations of sorted sediment bodies, with simple and pure shear contributing to extrusion of till into the basal mouth of a crevasse.

et al., 2015). The grain-size distribution of the moraine's till, its density, and preconsolidation pressures point uniformly to the ridge having formed subglacially rather than at the ice margin. Preconsolidation pressures additionally indicate that pore-water pressure supported most of the glacier's weight, weakening till and increasing its mobility. AMS fabrics reveal that both simple shear of bed sediments along planes inclined upglacier and local pure shear of sediments associated with their upward extrusion into a crevasse contributed to ridge formation. AMS and clast fabrics, which demonstrate shear deformation directed downglacier, indicate that the glacier was not fully stagnant when the ridge formed, in agreement with less extensive fabric data from other washboard ridges in the area (Stewart et al., 1988) and suggesting that the ridges had already began to form during the late stages of the surge. The crevasse-squeeze origin of washboard ridges in central Iowa indicates that the DML was in longitudinal extension even near its margin at its maximum extent. Propagation of a surge wave to the margin provided the necessary transient extension to develop transverse crevasses. The thin ice of the lobe, as indicated by reconstructions (Clark, 1992; Hooyer and Iverson, 2002), and the high basal water pressure that contributed to mobile till and basal slip would have helped keep crevasses open at the bed by inhibiting creep of ice into them. This evidence that the DML surged toward its maximum extent is consistent with the radiocarbon chronology of the lobe that indicates rapid advance rates (Clayton et al., 1985; Clark, 1994), other landforms of the lobe interpreted to be the result of widespread glacier stagnation after surging (Kemmis, 1991; Patterson, 1997), thin ice and associated low basal shear stresses when the ice was fully extended to the Bemis Moraine (e.g., Clark, 1992), and fossil evidence that the local climate was relatively mild when the lobe advanced (Baker, 1996; Bettis et al., 1996; Schwert and Torpen, 1996). The finding that the moraines formed subglacially with evidence of shear directed roughly perpendicular to their trends supports their use as flow direction indicators in geomorphic reconstructions of the lobe (Clark, 1992; Hooyer and Iverson, 2002). However, in areas north of the Altamont and Algona moraines (Fig. 1B, inset), washboard moraines could reflect flow during later advances of the lobe to those former margins. Thus, use of washboard moraines in these areas as flow direction indicators in reconstructions of the lobe at its maximum (e.g., Clark, 1992; Hooyer and Iverson, 2002) remains inadequately justified. Although these washboard moraines provide good evidence of surge-like motion, they provide no means of calculating retreat rates, as ridge spacing reflects crevasse spacing rather than margin positions. In contrast, transverse submarine ridges that are geometrically quite similar to those of this study have been interpreted—based largely on their regular spacing—as having formed seasonally at retreating grounding lines and have thus been used to calculate retreat rates of

grounding lines (Shipp et al., 2002; Dowdeswell et al., 2008). Early studies of the DML washboard moraines resulted in similar inferences and calculations of retreat rates (Gwynne, 1942; Lawrence and Elson, 1953). The results of this study, therefore, are cautionary for those tempted to interpret, without supporting sedimentological or geotechnical data, transverse moraine ridges as sequentially formed at glacier margins. Acknowledgments We thank L. Blocker, J. Byers, and M. Mathison for their help in the field and B. Tikoff for the use of his Kappabridge at the University of Wisconsin-Madison. J. Gage there helped facilitate the AMS measurements. We also thank V. Schaefer for access to the Iowa State University Soil Mechanics Laboratory and C. Lloyd and J. Frevert for graciously allowing access to their property. The comments of D. Mickelson, A. Schomacker, and two anonymous reviewers improved the paper. References Arshad, M.A., Lowery, B., Grossman, B., 1996. Physical tests for monitoring soil quality. In: Doran, W., Jones, J. (Eds.), Methods for assessing soil quality. Madison WI, 1996. Soil Sci. Soc. Am. 49, pp. 123–141 (special publication). ASTM, 2007. Standard Test Method for Particle-size Analysis of Soils, D422-63. American Society for Testing and Materials International, West Conshohocken, PA http://dx.doi. org/10.1520/D0422-63R07E0. Baker, R.G., 1996. Pollen and plant macrofossils. In: Bettis, E.A., Quade, D.J., Kemmis, T.J. (Eds.), Hogs, Bogs, and Logs: Quaternary Deposits and Environmental Geology of the Des Moines Lobe. Iowa Department of Natural Resources, pp. 105–109 (Guidebook Series 18). Benn, D.I., 1994. Fabric shape and the interpretation of sedimentary fabric data. J. Sediment. Res. A64, 910–915. Benn, D.I., 1995. Fabric signature of subglacial till deformation, Breidamerkurjokull, Iceland. Sedimentology 42, 735–747. Benn, D.I., Evans, D.J.A., 1996. The interpretation and classification of subglacially deformed materials. Quat. Sci. Rev. 15, 23–52. Benn, D.I., Evans, D.J.A., 2010. Glaciers and Glaciation. 2nd edition. Hodder Education, London. Bettis, E.A., Quade, D.J., Kemmis, T.J., 1996. Overview. In: Bettis, E.A., Quade, D.J., Kemmis, T.J. (Eds.), Hogs, Bogs, and Logs: Quaternary Deposits and Environmental Geology of the Des Moines Lobe. Iowa Department of Natural Resources, pp. 1–79 (Guidebook Series 18). Bowles, J.E., 1986. Engineering Properties of Soils and Their Measurement. McGraw-Hill, New York. Burras, C.L., 1984. Characterization of Soils Associated With Minor Moraines, Story County, Iowa M.S. thesis Iowa State University. Burras, C.L., Scholtes, W.H., 1987. Basin properties and postglacial erosion rates of minor moraines in Iowa. Soil Sci. Soc. Am. J. 51, 1541–1547. Casagrande, A., 1936. The determination of the preconsolidation load and its practical significance. Proceedings of the First International Conference on Soil Mechanics and Foundation Engineering 3, pp. 60–64. Clark, P.U., 1992. Surface form of the southern Laurentide Ice Sheet and its implications to ice-sheet dynamics. Geol. Soc. Am. Bull. 104, 595–605. Clark, P.U., 1994. Unstable behavior of the Laurentide Ice Sheet over deforming sediment and its implications for climate change. Quat. Res. 41, 19–25. Clark, P.U., Walder, J.S., 1994. Subglacial drainage, eskers, and deforming beds beneath the Laurentide and Eurasian ice sheets. Geol. Soc. Am. Bull. 106, 304–313.

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