Sedimentary Geology 220 (2009) 126–133

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Sedimentary Geology

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Significance of soft-sediment clasts in glacial outwash, Puget Sound, USA

Jasper KnightDepartment of Geography, University of Exeter, Cornwall Campus, Penryn, TR10 9EZ, UK

E-mail address: j.knight@exeter.ac.uk.

0037-0738/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2009.07.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 February 2009Received in revised form 11 May 2009Accepted 22 July 2009

Keywords:Soft-sediment deformationWisconsinan glaciationCordilleran ice sheetWashington State

Soft-sediment clasts composed of silt and clay are contained within glacial outwash sands in the PugetSound, Washington State, USA. The outwash was deposited during ice retreat of the Cordilleran ice sheetaround 17 cal kyr BP. The soft-sediment clasts have a distinctive and consistent morphology and dispositionwithin the sand beds. The sedimentology, sedimentary structures and presence of soft-sediment clastssuggest sand was deposited as proglacial outwash with silts and clays deposited in meltwater pools.Following drying-out of the pools and subaerial cracking, lumps of silt and clay were excavated by meltwaterand transported distally as soft-sediment clasts within high-density flows. The most likely final depositionalsetting is as a Salisbury-type ‘delta’ in which subaqueous outwash grades distally into deeper water. Thisinterpretation shows the power of soft-sediment clasts to inform on past processes and palaeogeography forwhich there is often little evidence in the geologic record.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Soft-sediment clasts (SSCs) are agglomerations (variously termedclasts, balls, pebbles or lumps, or rip-up clasts or blocks) of fine-grained sediments that are preserved as discrete, intact intraclastswithin unconsolidated sediments. They are also commonly found asintraclasts within lithified sedimentary rocks (Allen, 1982). Thepresence of SSCs reflects an evolutionary history comprising, intime-order, (1) initial deposition of fine-grained sediments; (2) break-up of the fine sediments into discrete clasts; (3) transport of clasts in adownstream direction; and (4) deposition of the clasts on top of, or atthe same time as, other coarser unconsolidated sediments.

As a diagnostic indicator of downstream reworking, SSCs canreveal information on clast transport, deposition and deformationprocesses, and thus regional-scale palaeoenvironmental setting. Noneof this information can be derived from the properties of the hostsediments, which only reflect site-scale environments and processes.Although SSCs sometimes have low preservation potential, theyprovide unambiguous evidence for sediment reworking (cannibalisa-tion) from older, previously-deposited sediments located upstreamfrom the site of deposition. As such, they are valuable in revealing thebigger environmental picture for which there is often little indepen-dent evidence.

SSCs have been commonly reported from many contemporarysedimentary environments, including proglacial forefields, alluvialfans, estuaries, river margins and coasts (e.g., Dickas and Lunking,

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1968; Stanley, 1969; Little, 1982; Diffendal, 1984; Allen, 1987a,b; Kaleand Awasthi, 1993; Knight, 2005). They have been reported lesscommonly as part of an unconsolidated sedimentary succession(e.g., Krainer and Poscher, 1990; Knight, 1999). In all theseenvironments, clast detachment takes place from a cohesive (silt/clay) source by processes that may include surface desiccation;subaqueous synaeresis cracking; dewatering and fluid migration;tectonic brecciation; hydrofracturing; or mechanical undercutting.These processes may occur singly or in combination in thesedifferent environments. Transport of the detached clasts awayfrom their site of origin takes place in a downstream direction andby a range of processes, determined largely by water availability andcurrent velocity (Allen, 1982). In all of these depositional settings,water-transported sands and gravels are dominant as the hostingsediments. The SSCs are located as intraformational elements withinthese sediments.

1.1. Sedimentary processes and soft-sediment clast morphology

The external morphology of the SSCs can reveal information onprocesses of clast detachment, transport and deposition. Duringtransport in particular, the clasts undergo plastic deformation bydynamic processes including fracturing, squeezing, squashing, foldingand pressing. The clasts also undergo rounding and abrading bytraction and grain–grain interactions with other transported sedi-ments, leading to their edges being extended and turned inwards,rounded, or armoured with coarser grains (Durian et al., 2007). Theseprocesses of plastic deformation result in changes to clast morphology(Knight, 1999), which can be quantified using standard measures ofaxial length, shape, roundness and angularity (e.g., Zingg, 1935;

Fig. 2. Generalised stratigraphic log of Esperance Sand sediment units, MarrowstoneIsland, showing the stratigraphic position of soft-sediment clasts described in the text(indicated by the star) and direction of cross-beds and ripple climb. Sediment faciescodes are included, after Eyles et al. (1983).

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Powers, 1953). The SSCs are particularly sensitive to transport-induced deformation because the clasts comprise deformable butcohesive sediments (mainly of silt or clay) that remain semi-consolidated during transport (Allen, 1982). The style of deformationcan inform on transport processes; the nature of grain–graininteractions during transport; and regional depositional setting(Knight, 1999). Despite the interpretive power of SSC characteristics,there has been little work on using SSCs to identify processes ofentrainment, transport and deposition.

This report examines the morphology and significance of SSCsfrom glacial outwash in the Puget lowlands, Washington State, USA.This report has three main aims: (1) to describe the glacial setting andsedimentology of the outwash; (2) to describe the morphology andcomposition of the SSCs contained within the outwash; and (3) toevaluate the significance of the SSCs for reconstructing regional-scaledepositional environments during formation of the outwash.

2. Glacial geology and geomorphology of the Puget lowlands

Glacial landforms and sediments in the Puget lowlands weredeposited mainly during the last advance–retreat cycle of the Pugetlobe (the Vashon stade of the Fraser glaciation) which was sourcedfrom ice dispersal centres located in the western British Columbiauplands (Easterbrook, 1969; Thorson, 1980; Easterbrook, 1992; Clarket al., 1993). During the Vashon stade, a piedmont lobe advancedsouthwards across Puget Sound and into the Puget lowlands(Anderson, 1968; Booth, 1986, 1994; Brown et al., 1987). Maximalice extent (at around 16,950 cal. yr BP) during the Vashon stade is wellknown in the Puget lowlands (Porter and Swanson, 1998), and datingcontrol is provided by radiocarbon ages on wood fragmentsincorporated into deposits of the advancing ice sheet (Porter andCarson, 1971; Porter and Swanson, 1998). During this ice advance,subglacial drainage channels transported sediments to ice margins(Booth and Hallet, 1993; Booth, 1994). These sediments weredeposited as a transgressive proglacial outwash unit (the EsperanceSand Member) that is laterally extensive across the Puget lowlands.Stratigraphically overlying the outwash is subglacial till (Vashon Till)deposited from the advancing ice and associated with formation ofdrumlins and flutes (Easterbrook, 1992). Overlying the Vashon Till is arange of sediments deposited during northward ice retreat during thesucceeding Everson Interstade. In the southernmost part of the Pugetlowland, these sediments were deposited in proglacial lakes as ice-

Fig.1. (a) Location of the Puget Sound area in northwestWashington State, USA, and southern(1993). (b) Location of key islands within Puget Sound. (c) Location of the exposure (starre

marginal deltas and morainal banks which, due to glacioisostaticdepression, are located at elevations above present sea-level (Thorson,1989; Porter and Swanson, 1998). Farther north, continued ice marginretreat opened out a glacial lake drainage channel on the northeasternedge of the Olympic peninsula (Thorson, 1989). Here, marine watersat a calving margin were able to circulate across the glacier front,leading to rapid northward ice retreat which was pinned on bedrockhighs and emergent islands within Puget Sound (Kovanen andSlaymaker, 2004). Deposition of Everson Interstade glacimarine tillwas associated with this ice retreat stage (Easterbrook, 1969). Overall,dynamic changes in relative sea-level driven by glacioisostaticrebound were important controls on stages of northward ice retreat,

most limit of the lateWisconsinan Cordilleran ice sheet, marked according to Clark et al.d) within Kilisut Harbor on Marrowstone Island.

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leading to formation of different shoreline and delta and outwashplain heights at different ice-front positions (Thorson, 1989; Dethieret al., 1995; Kovanen and Slaymaker, 2004; Mosher and Hewitt, 2004).In many places, Everson Interstade-age deposits are absent, suggest-ing stagnation zone retreat of the ice margin (Thorson, 1980;Easterbrook, 1992). A variable preserved thickness of EversonInterstade sediments likely reflects a combination of rapidly-changingice front position; changes in bathymetric relief and water currentsassociated with rapid proximal erosion and deposition; and dynamictectonic effects (e.g., Dethier et al., 1995).

The site described in this paper is located near to the northeasternedge of the Olympic peninsula where the change in depositionalenvironment during the Everson Interstade (from proglacial/lacus-trine to marine) first took place. Sediments described in this paper arelocated at Kilisut Harbor (at 48°05.23′N, 122°42.91′W) on thenorthwestern side of Marrowstone Island, Puget Sound (Fig. 1).Sediments are present discontinuously across the northern part of the

Fig. 3. Photos of sediments at Kilisut Harbor. (a) Trough cross-bedded sands at the bottom29 cm long. (b) Massive sand beds overlain by swaley cross-stratified beds, at the top of th

island and thin to the south where they onlap rising bedrock. Here,interbedded sands and gravels (b10 m thick) were deposited duringthe Everson Interstade. These sediments show preserved primarybedding structures, including planar and trough cross-bedding; theyhave not been deformed by ice readvance; and flat upper erosionalsurfaces at Marrowstone Island correspond in elevation to those onthe adjacentWhidbey Island, and are due to contemporaneousmarineplanation during ice retreat (Dethier et al., 1995; Kovanen andSlaymaker, 2004; Mosher and Hewitt, 2004).

3. Site sedimentology

On the northwest side of Marrowstone Island, sediments (b18 mthickness in total) comprise laterally continuous and planar beds ofsorted fine sand to granules (Fig. 2). The beds (b1.2 m thick) aremassive to well stratified with generally flat-lying erosional contacts,and show well developed ripples, drapes and trough cross-bedding

of the succession, with dispersed British Columbia erratics. Trowel for scale (centre) ise succession. The sediments in this photo are 3.5 m high.

Fig. 5. Plot of a–b plane dip of isolated soft-sediment clasts at Kilisut Harbor (n=25).The large white arrow shows the resultant vector.

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(Fig. 3a). Associated with some trough cross-bedding reactivationsurfaces (McCabe and Jones, 1977) are occasionally subroundedgranitic and metasedimentary pebbles (b6 cm diameter) of northerly(British Columbia) provenance. Ripple-massive sand couplets (15–25 cm thick) are also present. Symmetric and asymmetric climbingripples (sensu Jopling and Walker, 1968) are directed consistently to130°. The uppermost part of the profile comprises laterally-contin-uous massive and swaley cross-stratified sand beds which havesharply erosional contacts (Fig. 3b).

Soft-sediment clasts are located within sand beds in the lower partof the sediment succession (Fig. 2). Here, beds (20–60 cm thick)comprise moderately-sorted medium sand (median grain size360 µm) which is massive to planar stratified and cross-bedded(Fig. 4). These beds do not show any internal grading or ripples. Thecross-beds dip to 285°. Isolated, non-touching SSCs are mainly flat-lying parallel to primary bedding, and are located on top of andconformable to bed surfaces (Fig. 4a). There is little evidence forsignificant erosional scouring around or beneath the SSCs. Theseisolated SSCs form horizontal lines in section, and are laterallycontinuous (over some tens of metres). Measured a–b plane dips ofthese isolated clasts are shown in Fig. 5. Where SSCs touch oneanother, an imbricate fabric is sometimes developed (Fig. 4b). Theseimbricated clusters comprise up to 25 individual clasts which arelocked together but are not cemented. SSCs are only present low in thesediment succession, and are not observed elsewhere.

Fig. 4. Photos of soft-sediment clasts (SSCs) at Kilisut Harbor (see Fig. 2 for stratigraphicposition). (a) Isolated SSCs located parallel to bed contacts. Trowel for scale is 29 cm long.(b) Imbricated SSCs with sand armour (below the pencil). Pencil for scale is 15 cm long.

3.1. Morphology and composition of soft-sediment clasts

The SSCs have a wide measured a-axis length range of 6–110 mm(n=130) (Fig. 6a). Clasts are dominantly (48%) of tabular or discshape with fewer equant (28%) and roller/prolate clasts (15%)(Fig. 6b). The distribution of SSC morphological types is consistentwith previous studies (e.g., Knight, 2005). There is no relationshipbetween SSC size and shape. In detail, the characteristics of SSCs showsome consistency over this wide size range (Fig. 7). Here, clasts of allsizes are armoured by coarse sand grains, occasionally by pebbles upto 6 mm diameter, that have been pressed onto clast surfaces. Somewell-rounded smaller clasts (b1.5 cm diameter) have also beenpressed against larger clasts. All clasts show protruding elementsthat have been edge-rounded during transport (cf. Durian et al.,2007). In particular, disc-shaped and equant clasts have prominentoverturned edges. Fig. 8 shows top, bottom and side-view sketches ofselected clasts that illustrate their morphological characteristics.Fig. 8a shows a roller-shaped clast that has a shortened b-axis, givingcontorted and compressed ends to the clast, caused by compressionduring rolling. Fig. 8b and c shows equant and tabular clastsrespectively that have extended and overturned sides. These clastsare almost perfectly circular in plan view but do not show evidencefor having been squashed into this shape.

Some clasts were dissected vertically through the b–c plane. Insection, the clasts show thin (sub-mm-scale) laminations that may bediscontinuous or slightly deformed and lie parallel to the clast's a–bplane. No distinctive coarse-fine couplets are seen. When disaggre-gated, the clasts (n=3) are dominated by a mean grain size in therange 6–20 µm, and median diameter values in the range 8–13 µm.Grain size distributions are unimodal and normally distributed. Theclasts have a low CaCO3 content by dissolution with dilute HCl (1.4–3.5%, n=3). Some clasts (n=3) were disaggregated using 6% H2O2,washed and sieved, and examined under the microscope for thepresence of microfossils. The samples were not fossiliferous.

4. Discussion

4.1. Development of soft-sediment clast morphology

The sedimentary and morphological characteristics of the SSCsat Marrowstone Island are very distinctive, and similar regardless ofclast size (Fig. 7). The internal sedimentology of the SSCs suggestsdeposition in a quiet-water environment unaffected by coarsesediment input or turbulent underflows, such as meltwater pools onan outwash surface that are relatively stable over several yearsduration. The absence of clear varves within the SSCs suggest thatthese pools were not affected by a seasonal ice cover.

Fig. 6. (a) Column graph of a–b–c axial dimensions of soft-sediment clasts (n=130). (b) Zingg plot of the axial ratios of soft-sediment clasts (n=130).

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The mechanisms associated with initial formation of the SSCs areuncertain. A likely origin is by seasonal drying-out of meltwater poolsand subsequent formation of subaerial contraction (desiccation)cracks (e.g., Allen, 1982, 1987b; Weinberger, 2001). Following crackformation, SSCs were excavated and undercut by renewed (seasonal)meltwater activity. The size of SSCs closely corresponds to the spacingof surface desiccation cracks (Karcz, 1969). Variation in the size of theSSCs recovered, but little variation in clast roundness, reflectsvariation in the spacing of desiccation cracks in a single source area,rather than differences in clast transport distance (cf. Bell, 1940;Diffendal, 1984). Crack spacing is known to vary from the outside toinside of a water pool as the process of drying out proceeds (Allen,1987b). The presence of horizontal desiccation cracks directed alongsediment layers can account for the tabular (bladed) shape of manySSCs (Allen, 1987a; Weinberger, 2001).

The deformed edges of the SSCs show clearly that the clasts werenot frozen during dissociation and transport. The deformation andsand armouring can develop over very short distances from source

(Little, 1982; Faimon and Nehyba, 2004). Examination of the style ofdeformation (Figs. 7 and 8) shows that the clasts had their edgesupturned, and in some cases overturned, during transport. This isseen by the presence of a raised rim around many clasts, in particu-lar discs. This argues in favour of clast rolling over a relatively hardand smooth substrate (leading to deformation along a single axialplane), rather than tumbling over and over (leading to deformationalong all axial planes). The association of SSCs with reactivationsurfaces suggests active shearing at the base of a viscous debris orgrain flow.

4.2. Depositional processes and environment of the soft-sediment clasts

The SSCs observed at Marrowstone Island contrast in their size andmorphologywith thewell-sorted sandswithinwhich they are located.The presence of the SSCs conformably within these sands and theirsand armour (Fig. 7) suggests that the SSCs and sands weretransported together (e.g., Bennett and Bridge, 1995). Furthermore,

Fig. 7. Detailed photo of a representative selection of soft-sediment clasts from Kilisut Harbor. Clasts are placed showing their a-axes uppermost. The scale bar is in 2 cm increments.The photo was taken by Nigel McDowell (University of Ulster).

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the absence of localised erosional scouring beneath the clasts, orsediment drapes on top of the clasts, suggests these sediments werealso deposited at the same time. The processes that are likelyassociated with the coeval transport and deposition of these differingsediment types vary up-section. The presence of isolated and non-touching SSCs within massive sand beds likely record high-densityfluidal flows or turbidites in which quartz sand grains are transportedin a dense, turbulent carpet at the bottom of the water column(Shanmugam, 1996; Lønne, 1997). Here, SSCs are supported by highdispersive pressure during flow activity. Imbricated and clusteredSSCs within planar stratified sand beds (Fig. 4) suggest transport anddeposition as traction bedload within sheetflows (e.g., Brayshaw et al.,1983; Iseya and Ikeda,1987; Bennett and Bridge,1995). This took placein a shallow water environment and related to episodic sedimentpulses from the ice margin (e.g., Gustavson and Boothroyd, 1987;Maizels, 1993; Krzyszkowski, 1994). It is likely that the shallow dipangle of the sand sheets records the angle of repose of these water-saturated sediments. Upward changes in sedimentary structures,including the development of sand ripples, reflect variations in flowregime (Jopling and Walker, 1968; Iseya and Ikeda, 1987; Maizels,1993; Bennett and Bridge, 1995).

Coeval transport and deposition of SSCs and sands are controlledby variations in water depth and sediment supply, which togetherdetermine flow viscosity (Postma, 1986). The most likely depositionalsetting is a proglacial forefield in which fines deposited in proglacialpools (forming the SSCs) are reworked to a more distal and deeperwater environment by episodic meltwater events that are able toliberate these sediments and mix them with better-sorted sandsduring transport (Landvik and Mangerud, 1985; Postma, 1986;Maizels, 1989, 1993). This environment, of a proximal proglacialforefield grading into a deeper-water setting distally, is best describedas a Salisbury-type ‘delta’, which comprises a subaqueous outwashplain (Salisbury, 1892) dominated by channelised and cut-and-fill

sands. This interpretation can also account for the presence of ice-proximal kettleholes on Puget Sound islands (Thorson, 1980; Kovanenand Slaymaker, 2004; Mosher and Hewitt, 2004). The interplay withinPuget Sound between sediment supply, sea-level change andsedimentary processes likely reflects these controls on Salisbury-type ‘delta’ development, and was likely transitional to a full-marinesetting distal from the ice margin (Dethier et al., 1995).

5. Conclusions and wider implications

Soft-sediment clasts provide information on events, processes andenvironments prior to, and outside of, their final site of deposition. TheSSCs reported here provide clear evidence for sediment reworkingalong an energy gradient in a waterlain depositional setting outside ofthe retreating late Wisconsinan Cordilleran ice margin. The SSCsprovide unambiguous evidence for cannibalisation from previously-deposited sediments of markedly different sedimentary character tothe host sediments. As such, the SSCs are valuable in revealing thebigger environmental picture for which there is often little preservedevidence. For example here the SSCs show that, during the EversonInterstade, there was a depositional gradient between proglacialoutwash and full glacimarine conditions. This has implications forreconstruction of palaeogeography including the position of icemargins and open-water, and their changes over time. Furtherexamination of SSCs from sensitive ice margins will help identifyregional gradients in forefield depositional environments and pro-cesses. More widely, this study shows the power of allochthonousSSCs within unconsolidated or lithified sediment sequences forpalaeoenvironmental and palaeogeographic interpretation.

Acknowledgements

I thank reviewers Nick Eyles and Dori Kovanen for their comments.

Fig. 8. Annotated sketches of the top, bottomand sides of three representative soft-sediment clasts, illustrating theirmorphological characteristics and dimensions (sketches by the author).

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