morphology and palaeoenvironmental interpretation of deformed soft-sediment clasts: examples from...

ELSEVIER Sedimentary Geology 128 (1999) 293–306

Morphology and palaeoenvironmentalinterpretation of deformed soft-sediment clasts:

examples from within Late Pleistocene glacial outwash,Tempo Valley, Northern Ireland

Jasper Knight *

School of Environmental Studies, University of Ulster, Coleraine BT52 1SA, Co. Londonderry, Northern Ireland, UK

Received 10 November 1998; accepted 2 July 1999

Abstract

Glacial outwash, deposited during deglaciation of the late Devensian ice sheet, is present as a flat-topped valley fill inthe Tempo Valley on the southern flanks of the Fintona Hills, Northern Ireland. Sedimentologically, the outwash compriseswell-sorted and interbedded rippled to massive sands which record distal deposition within a proglacial water body. Bedsof ripple-drift cross-laminated sands contain deformed (folded and contorted) soft-sediment clasts which are composedmainly of silt and clay. The soft-sediment clasts were deformed prior to final deposition because clast a–b planes lieconformable to sand laminae which are undeformed. Morphological characteristics of the soft-sediment clasts, and theirfacies context, provide evidence for transport mechanisms, depositional environment, and processes of clast deformation.The soft-sediment clasts were transported into a proglacial water body by unidirectional water currents (¾1.5–2.5 ms�1). Sediment transport processes include sediment bypassing within the water column, a low bedload component, andgrain flow activity during waning flow stages. The overall morphology of soft-sediment clasts records between 1 and 3distinct phases of hydroplastic deformation prior to emplacement. The deformation phases are recognised on the basisof morphologically ‘unrolling’ the superimposed folds of the soft-sediment clasts. Deformation structures (i.e. fold style)and direction of the principal stress axis relative to clast axes suggest that clasts were reoriented with respect to waterflow direction following each deformation phase. Processes of deformation include folding-over of the clast along its baxis into two or more components, crumpling and abrasion of the outer margins of the b plane, and squashing of theclast c axis (some of which may be post-depositional deformation). The presence of silt- and clay-rich soft-sediment clastswithin the outwash succession suggests that they were ripped-up from shallow and irregular pools on the glacier forefield,into which fine sediments accumulated after flood or meltwater events, and transported distally into a proglacial waterbody. These inferences based on facies evidence and styles of hydroplastic deformation impact on reconstructions of localpalaeogeography, and the wider interpretation of similar soft-sediment clasts in the geological record. 1999 ElsevierScience B.V. All rights reserved.

Keywords: meltwater; deformation phases; palaeoenvironmental reconstruction; proglacial sediments; hydroplastic defor-mation

Ł Tel.: C44-1265-323153; Fax: C44-1265-324911; E-mail: [email protected][email protected]

0037-0738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 8 0 - 9

294 J. Knight / Sedimentary Geology 128 (1999) 293–306

1. Introduction

Discrete blocks or clasts of fine-grained intactsoft-sediments, which are not in situ, are often ob-served in many subaerial and shallow-water (fewdm depth) modern environments (i.e. Dickas andLunking, 1968; Karcz, 1969; Hall and Fritz, 1984;Allen, 1987a; Kale and Awasthi, 1993). Because oftheir fragile nature (Allen, 1982b, p. 547) they areless often found preserved within lacustrine, fluvialand marine sediment sequences (e.g. Little, 1982;Krainer and Poscher, 1990; Wang, 1992; McCabeand O’Cofaigh, 1995; Gibbard et al., 1996; Maizels,1997). However, these intraformational clasts canprovide information on clast transportation pro-cesses, sediment type on the glacier forefield, andmeltwater flow regime (i.e. Little, 1982; Diffendal,1984). The variable morphology and isolated oc-currence of these features have led to a range ofmorphogenetic terms being used (Picard and High,1973), depending on clast shape (i.e. degree of disag-gregation and rounding) and subordinately on grainsize. They have been variously termed rip-up clasts(Allen, 1982b; Wang, 1992; Gibbard et al., 1996;Maizels, 1997), mud clasts (Reineck and Singh,1980; Allen, 1987a; Selby and Evans, 1997), sed-iment clasts (Wilson, 1991), mud pebbles (Karcz,1969; Reineck and Singh, 1980), clay balls (Haas,1927), and armoured or unarmoured mud balls (Bell,1940; Reineck and Singh, 1980; Diffendal, 1984).Here the descriptive and non-genetic term ‘soft-sed-iment clast’ is used to describe these features. Vari-able terminology and sporadic identification from anumber of ancient and modern depositional settingssuggest that their significance as palaeoenvironmen-tal indicators has not yet been fully evaluated.

The physical processes of soft-sediment clast for-mation are not fully understood (Picard and High,1973; Allen, 1982b). Intact beds of fine-grainedsoft sediment may be broken up either by subaerialshrinkage (desiccation), or ripped up subaqueouslyby strong currents (Allen, 1987b). Brittle soft-sedi-ment clasts formed by desiccation may be reworkedand redeposited under a rising water plane, but thishas not been commonly described and the preserva-tion potential of this clast type is low (Moore, 1914).Clasts ripped up subaqueously from cohesive muddybeds in rivers and at estuary mouths (Picard and

High, 1973; Allen, 1987a), or by meltwater emanat-ing from ice margins (Maizels, 1993, 1997; McCabeand O’Cofaigh, 1995), have a higher preservationpotential because they deform hydroplastically or ina quasi-solid state (Elliott, 1965). Allen (1982a) (pp.69–71) recognised that fluid stressing on a sedimentbed (which is a likely condition for removal andtransport of a soft-sediment clast with hydroplasticbehaviour) is caused by a partial pressure differencebetween the flow velocity of the overlying fluid andthe frictional resistance of the bed. This physical re-lationship has implications for reconstructing waterflow regimes and other environmental variables.

This paper addresses two key issues which hinderthe interpretation of soft-sediment clasts in the ge-ological record: (1) the processes of formation anddeformation (changes in morphology) of soft-sedi-ment clasts; and (2) the relationship of soft-sedimentclasts to their surrounding (host) sediment, particu-larly in terms of particle support mechanisms duringtransport, and processes of final sediment deposi-tion. Lack of consensus on the interpretation of sub-aqueous sediment transport and deposition processesfrom the geological record has impeded palaeoen-vironmental reconstruction in some depositional set-tings (Shanmugam, 1996, 1997).

2. Aims and methods

This paper has two main aims: (1) to describethe morphology and sedimentary setting of soft-sed-iment clasts which are hosted within glacial outwashin the Tempo Valley, Northern Ireland; and (2) todetermine how soft-sediment clasts were formed,deformed, and emplaced within the host sand beds.

Glacigenic landforms in the Tempo Valley weremapped geomorphically using standard mappingsymbols (Savigear, 1965) and sediment exposureswere catalogued in the field using facies codes (Mi-all, 1977; Eyles et al., 1983). Sedimentary environ-ments were reconstructed on the basis of palaeodi-rectional indicators, sediment type and structures,and inferred processes of deposition. Sedimentsand soft-sediment clasts were examined in detailat Dooneen. The original (here termed ‘pre-defor-mational’) shape of individual soft-sediment clastsat Dooneen was determined by geometrically ‘un-

J. Knight / Sedimentary Geology 128 (1999) 293–306 295

rolling’ them about their axes of deformation in orderto identify the presumably flat pre-deformational bedsurface. In this context ‘deformation’ refers to theprocesses by which the originally flat bed was dis-rupted by erosion, and the fragments transported andshaped prior to emplacement in the host sediments.Sequential stages of clast deformation are identifiedon a morphological basis where later deformationstages have altered earlier deformation signatures,and given the notation D1, D2: : :Dx (i.e. oldest toyoungest). Some clasts were disaggregated in diluteCalgon solution and examined for grain size distri-bution using the pipette sampling method (BritishStandards Institution, 1975).

3. Study area

The Tempo Valley (15 km long) is located onthe southern margin of the Fintona Hills in North-ern Ireland (Fig. 1). The western flank of the valleyis dissected by the Tempo–Sixmilecross fault whichseparates two geological provinces. Devonian OldRed Sandstone and associated conglomerate bedsoutcrop west of the fault and in the Fintona Hills,whereas Carboniferous limestones and shales arepresent in the Clogher Valley lowlands east of thefault. Glacigenic sediments within the Tempo Val-ley include sand-dominated spreads which have beendissected by deep (<30 m), valley axis-parallel melt-water channels found especially on the western sideof the valley (Fig. 1). In the Clogher Valley the out-wash overlies subglacial ridges (Rogen moraines),but in upland areas (>140 m OD) it onlaps ontobedrock. Overall, landform and sediment distribu-tions record ice retreat southwards and westwardstowards the Lough Erne Basin where the ice finallydisintegrated (J. Knight, 1997). Successive glaciallakes (at ¾190 m, 155 m, 120 m and 90 m OD)were impounded between the Fintona Hills and theretreating ice margin. The possible depths of glaciallakes may have varied from a few metres to >20 m,and their relationship to ice margins is unknown asno ice-marginal moraines or eskers are observed inthe Tempo Valley. Soft-sediment clasts are presentwithin outwash spreads at Dooneen, which are re-lated to a 155 m lake level (Figs. 1 and 2).

4. Facies description and interpretation

The Dooneen exposure (16 m long, 4 m high)is located 6 km northeast of Tempo and forms partof a flat sediment surface (0.05 km2) which lies at155 m OD (š1 m) (Fig. 2). The exposure comprisesstacked, well-sorted, massive to cross-bedded andrippled fine- and medium-grained sand (Sm, Sl, Sr,Ss facies) (Fig. 2). Sand beds (0.2–1.4 m thick) aretabular, laterally continuous across the exposure andseparated by both sharp and poorly defined planarcontacts which vary laterally. Cross-bed dip direc-tion records palaeoflow towards 040º (i.e. up-valley),which is consistent with flow indicators elsewhere inthe Tempo Valley (J. Knight, 1997, 1998). No chan-nel structures, scours, marked erosional horizons,coarse lags or mud beds are observed. Calcareousconcretions are present within a single bed of finesand (Sm facies; 0.6 m thick). The concretions (J.Knight, 1998) are generally bladed in morphology(<20 cm ð 11 cm ð 3 cm dimensions) with flat-lying a–b planes which are oriented parallel to thedirection of cross-bed deposition.

Soft-sediment clasts (Fig. 3) are present withincross-bedded and laminated sand beds (Ss, Sr facies)which overlie the bed containing concretions. Thesand beds (individually 10–30 cm thick) comprisea couplet of horizontal and sinusoidal laminationswhich change upwards into type-A ripple-drift cross-laminations (Jopling and Walker, 1968), recordingpalaeoflow consistently toward the northeast. Thisbed couplet is repeated in different parts of the ex-posure but generally occurs at the same stratigraphiclevel. Soft-sediment clasts lie within and are con-formable with laminations in the rippled sand beds.However, no associated clay drapes are found. Com-monly (75% of all occurrences), clasts are alignedtransverse to the strike of climbing ripples with theprincipal axis of deformation (D1, defined above)located on the up-flow side of the clast. No erosionalscours are observed developed in the surroundingsand adjacent to the soft-sediment clasts.

The overall sediment succession at Dooneenrecords the deposition of water-sorted sediments bygrain flow and mass flow processes into a shallowproglacial lake environment (Church and Gilbert,1975). This is evidenced by sediment stacking, lat-eral changes from one facies type to another, facies’


Fig. 1. Location and deglacial geology of the Tempo Valley study area, Northern Ireland. Ice retreat stages are reconstructed mainly fromsand and gravel extent and inferred glacial lake levels.


Fig. 2. Sedimentology of the Dooneen exposure, Tempo Valley, Northern Ireland.

tabular geometry and stratification, and degree ofsediment sorting (Ashley, 1995). Absence of sand–mud couplets, sand grain size variability and beddeformation as by glaciotectonism, and the presenceof climbing ripple sequences, suggest active circu-lation of dense undercurrents (Jopling and Walker,1968). The well-sorted nature of sediment in theclimbing-ripple beds also implies that depositionwas ice-distal with a constant sediment and meltwa-ter supply, and that sediment sorting resulted fromflow bypassing during passage through the watercolumn. The strong unidirectional component of theclimbing ripples also implies strong and unchanging(perhaps seasonally related) current circulation pat-terns (strong underflows and surface return currents)and an absence of controlling bottom topography.

Vigorous underflow circulation may also be drivenby, and reflect, thermal stratification of the glaciallake (Ashley, 1995).

5. Characteristics of soft-sediment clasts

5.1. Morphology and grain size

Six soft-sediment clasts (coded A, B, D–F, H;see Fig. 3 for codes) were examined in detail forsize, external shape, deformation style and grain sizecomposition (Tables 1 and 2; Fig. 4). Pre-defor-mational clast shapes are mainly blades or discs(Zingg, 1935). Post-deformational clast shapes in-clude equant and roller forms which suggest bed-


Fig. 3. A representative selection of soft-sediment clasts found at Dooneen. Scale bar is in 2 cm increments. From left to right, clasts arenumbered A–C (top row) and D–I (bottom row).

load rounding and abrasion, or oscillatory (back andforth) motion (i.e. Dickas and Lunking, 1968; Stan-ley, 1969; Kale and Awasthi, 1993). In all cases atDooneen clast thickness (dimension of the pre-defor-

Table 1Pre- and post-deformational axial dimensions (mm) of soft-sediment clasts at Dooneen, Northern Ireland

Attribute Clast A Clast B Clast D Clast E Clast F Clast H

Pre-deformationala axis 130 100 70 75 50 45b axis 65 67 50 55 45 25c axis 24 8 14 15 15 7Ratio b=a 0.50 0.67 0.71 0.73 0.90 0.55Ratio c=b 0.37 0.12 0.28 0.27 0.33 0.28Clast shape Blade Disc Disc Disc Disc Blade

Post-deformationala axis 85 67 50 55 50 45b axis 65 50 29 29 30 25c axis 35 35 20 20 15 10Ratio b=a 0.76 0.74 0.58 0.52 0.60 0.55Ratio c=b 0.53 0.70 0.68 0.68 0.50 0.40Clast shape Disc Equant Roller Roller Blade Blade

Clast shape after Zingg, 1935.

mational c axis) increases with overall clast size(area of the pre-deformational a–b plane). Clastsare composed mainly of silt-sized particles (¾75%)(Table 2; Fig. 4). Consistency of grain size between


Table 2Grain size data for soft-sediment clasts, Tempo Valley, NorthernIreland

Grain size class % Total mass

clast A clast B clast E clast F

Medium sand 0.24 0.29 0.09 0.35Fine sand 0.76 0.77 0.87 1.29Very fine sand 13.80 13.55 13.08 11.59Coarse silt 28.61 25.19 25.49 23.03Medium=fine silt 48.48 50.26 52.10 53.33Clay 8.04 9.87 8.31 10.34

Total 99.93 99.93 99.94 99.93

Fig. 4. Grain size compositional spectra for selected soft-sedi-ment clasts (phi scale).

the different clasts analysed suggests that they werederived from the same population and source area.

5.2. Deformation style

On a geometric basis, D1 deformation has hadthe most profound influence on the final shape ofall soft-sediment clasts identified (Table 1; Fig. 5).Later deformation stages (D2 and D3) have succes-sively minor impacts on clast shape, and may reflectthe development of more stable and rounded ge-ometries over time (rollers, discs, equant clasts) (i.e.Bell, 1940; Kale and Awasthi, 1993). The axis aboutwhich deformation occurred during stage D1 is gen-erally located (5 of 6 cases) near the middle of clasta–b planes. D1 deformation therefore overturned partof the flat clast upon itself (Fig. 5). Two clasts of

bladed pre-deformational shape (clasts A, B) haveD1 fold axes aligned perpendicular to clast a axes,showing that clast a–b planes during D1 deformationwere aligned parallel to water flow. In contrast, threeclasts (D–F) show D1 deformation perpendicular tothe pre-deformational b axis, but these clasts wereall disc-shaped with low elongation (b=a >0.70).The remaining clast (H) shows D1 deformation atan angle to both a and b axial planes. In all caseslater deformation (D2, D3) occurs obliquely to D1

(30º–70º). Where D1 deformation acted to decreaseb=a by ‘squashing’ (i.e. become more blade-like;clasts D–F), D2 and D3 deformation occurred whilethe a–b planes of the deformed clasts were alignedparallel to water flow, similar to the D1 deformationof clasts A and B (Fig. 5). D2 deformation of clastsA and B occurred when the D1 axis was alignedboth transverse to water flow direction and locatedon the down-flow side of the clast. In clast B, D2 de-formation occurred when a small, rounded lump ofsediment (4 cm ð 4 cm ð 3 cm dimensions) becamelodged in the open up-flow ‘mouth’ of the clast. Therounded nature of this lump suggests it rolled alongthe bed as traction load, but it shows no sand armour.The impaction of the lump, and possibly associatedchanges in fluid stress around the clast, help form theuppermost ‘rim’ of the clast, which is overturned inthe direction of water flow (Fig. 5).

6. Discussion

The preservation of deformed soft-sediment clastswithin a glacial outwash succession has implicationsfor the interpretation and palaeoenvironmental sig-nificance of such features. These implications areaddressed with respect to the derivation of the siltand clay comprising the soft-sediment clasts, andthe formation, deformation and emplacement of theclasts.

6.1. Derivation of sediments comprising thesoft-sediment clasts

No in situ silt or clay beds are observed in theTempo Valley area. Silt=clay facies are generallyindicative of deposition in quiet-water (ice-distal)environments unaffected by strong or rapidly fluc-


Fig. 5. Morphology of soft-sediment clasts through deformation stages D1–D3, showing direction of deformation and associated waterflow. For clarity and comparison purposes all soft-sediment clasts are shown with D1 directed from the top of the page. Note that D3

deformation is present only in clasts B, D and F, and that clast sizes are proportional.

tuating currents. However, the soft-sediment clastsare present within sand beds that reflect more activemeltwater environments. Therefore the silt=clay fa-cies are likely to have formed in shallow, isolatedpools on the former glacial forefield located be-

tween the ice margin and the water body into whichthe soft-sediment clasts were later emplaced. Fine-grained sediments from receding floodwaters mayhave accumulated in these ephemeral depressions(Maizels, 1993).


An alternative source of fine-grained sedimentsmay include material frozen on at the base of theice sheet and deposited at the ice margin during re-treat. For example, laminated muddy sediments areknown to comprise part of the debris-rich basal icelayer extruded at some Alaskan and Greenland icemargins during ablation and ice retreat (P.G. Knight,1997). In Austria, water-transported and roundedsoft-sediment clasts, composed of subglacial di-amict, were redeposited within proglacial outwashas ice retreated (Krainer and Poscher, 1990). Thesoft-sediment clasts described from Dooneen arefree of clasts and ice-melt structures, and are morefinely laminated than modern debris-rich basal icefacies. Therefore, although reworking and round-ing by proglacial meltwater was important (Churchand Gilbert, 1975; Krainer and Poscher, 1990), thesoft-sediment clasts were not derived from the re-treating ice sheet directly. A glacial forefield settingfor sediment deposition is consistent with evidencefor the deposition of fine-grained slack-water sedi-ments on the forefields of Icelandic glaciers (Lawler,1991).

6.2. Formation and entrainment of individualsoft-sediment clasts

The commonest mechanism by which mud bedsare broken up is by subaerial desiccation, whichproduces a laterally extensive pavement of inter-locking angular blocks (Picard and High, 1973;Allen, 1987a,b). The pre-deformational bladed ordisc shapes of individual clasts at Dooneen (Table 1)are consistent with those produced by subaerial des-iccation (i.e. Allen, 1987b), and furthermore mayreflect the uniform grain size of the clasts and theirfracture along laminae-parallel desiccation planes(Allen, 1986). These soft-sediment clast shapes arealso produced where the ground surface is irregular,and in water pools of irregular shape and=or size(Allen, 1987a,b). Both these aspects characterisestony and unvegetated glacial forefields (Lawler,1991). Clasts may have a–b planes aligned in thedirection of slope dip (Allen, 1986, 1987b). Thismay suggest that elongate clasts in which D1 is per-pendicular to a–b axes (i.e. clasts A and B) wereripped-up from descending slopes. Clasts producedby desiccation are brittle and unlikely to be pre-

served during or following transport (Allen, 1986,1987b). This, and the hydroplastic deformation styleof the Dooneen soft-sediment clasts (discussed be-low), suggests strongly that desiccation either did nottake place, or that the mud beds were exposed onlylong enough for the clast shapes to become defined(Moore, 1914).

Initiation of soft-sediment clast movement is ahydrodynamic problem. Under unidirectional flowsflat, bladed or discoid clasts may be hydrodynami-cally stable where their long axes are aligned parallelto flow direction (Allen, 1982a). Clasts may be in-corporated into the flow when ‘flipped’ over by anincreased pressure difference between the clast up-per surface and the bed. This may be caused bychanges in flow velocity or direction, or by changesin basal turbulence related to flow velocity or thick-ness of the water layer (dm-scale). However, thestyle of D1 deformation, and the requirement for theclasts to have been in a hydroplastic state to permitsurvival of these deformation stages, suggest thatfolding about the D1 axis occurred while part of theunderside of the clast (geometrically about half clastarea) was still attached to the bed. The clasts weretherefore removed from the bed by turbulent waterflow (probably by ripping-up) during or immediatelyafter D1 deformation. Turbulent flows may have in-creased fluid pressure beneath the upstream edge ofthe clast (Allen, 1982a). This scenario, requiring tur-bulent, erosive floods across the glacier forefield, isconsistent with the episodic outburst floods (jokulh-laups) observed at some modern ice margins. InIceland, jokulhlaups incorporate, transport and roundsoft-sediment clasts into more distal outwash sandsand gravels (Maizels, 1993, 1997). Detailed pro-cesses of clast incorporation and transport within theflow are unknown.

6.3. Deformation of soft-sediment clasts

D1 deformation reduced a axis lengths of allclasts by ¾25–35%. D2 and D3 deformation affectsonly the leading (upstream) edge of the soft-sed-iment clasts, suggesting that clasts were activelyreoriented within the flow between the D1 and D2

episodes. The rounded shape and unarmoured stateof the lump of sediment which impacted into clastB suggest that traction transport occurred across a


relatively hard bed free of loose sediment grains. Itmay also suggest that the lump was highly cohe-sive and not prone to disaggregation, either in theambient water or by comminution against other bed-load particles. Flow velocities required to keep thesediment lump in bedload transport are of the range1.5–2.5 m s�1 (Reineck and Singh, 1980) whichis comparable to seasonal runoff events or episodicfloods into the glacial forefield (Jopling and Walker,1968; Lawler, 1991; Ashley, 1995; Maizels, 1997).The curled ‘rim’ (D3) seen on the uppermost sur-face of clast B is also observed on clast F (Fig. 3)and reflects deformation of the clast’s upstream edgeby water while aligned transverse to flow. All de-formation represented by stages D1–D3 occurred byplastic modification of clast shape by folding, notbrittle fracture during bedload transport, despite thehighly variable pre-deformational thickness of clasts’c axes (7–24 mm). Individual laminations within thesoft-sediment clasts are consistent throughout, defor-mation fold axes generally show no tension cracksindicative of desiccation, and evidence for minorsquashing of the clasts (i.e. clasts A, D) can be at-tributed to loading during emplacement. In only onecase (clast H) were tension cracks oblique to theD1 axis observed (Fig. 5). Overall, these character-istics suggest that the clasts remained plastic (andnot frozen) both internally and externally through-out all deformation stages. Supporting the style ofD1 deformation, they also suggest that clasts werenot desiccated prior to initial ripping-up from thebed, or later disaggregated by ambient water. Ab-sence of armour and small-scale structures producedby interactions of the bedload component suggest(1) a hard bed, (2) very rapid bypassing of smaller(sand) particles around the bedload, and (3) absenceof other particles in bedload transport. The defor-mation stages identified from clast morphology weresequential in time but may have occurred during asingle flow event (few hours duration).

Detailed deformation style and processes duringeach deformation phase is shown using the exampleof clast B (Fig. 6). Overall pre-deformational clastshape is consistent with a desiccation origin (e.g.Allen, 1987b) but deformation style (showing hy-droplastic behaviour) and the ragged margins of theclast suggest that subaqueous ripping-up occurred.This is in keeping with the other clasts examined

(Fig. 3). The clast was folded approximately in halfalong an open fold by D1 deformation (Fig. 6).Incomplete closure of this fold may be due to a de-crease in flow velocity or thickening of the fold axiswhich acted to reduce stress to the fold limbs. The D1

fold axis was located on the up-flow side and the fold‘mouth’ on the down-flow side, which is hydrody-namically more stable. Water stress from the up-flowside during this phase may have opened out the foldmouth still further. This is evidenced by the squashedmiddle portion of the D1 fold, and the splayed sidesof the clast (Fig. 6). D2 deformation involved theimpaction of the mud lump into the open clast mouthduring active traction transport. Water velocity dur-ing this deformation phase is likely to have beenabove that required for mud lump traction, but belowthat required for transport of the soft-sediment clast.The trajectory of the mud lump into the clast, and itsrounded morphology, may indicate a different sourcedirection and erosional history. The uppermost foldlimb of the soft-sediment clast is more deformedby mud lump impaction than the lowermost limb(Fig. 6). This may reflect both the nature of theimpact (rolling) and fluid stressing of the uppermostclast surface (D3 deformation). Overall, the defor-mation history of clast B records changes in flowvelocity and clast orientation relative to flow.

6.4. Emplacement of deformed soft-sediment clasts

The soft-sediment clasts were emplaced synde-positionally with and conformable to the surround-ing rippled sand beds. The preserved nature of thesoft-sediment clasts suggests that they rolled intoplace with little erosive transport. No equivalentstudies have addressed this, but based on this degreeof preservation, transport from initial ripping-up todeposition site may have been over a few tens ofmetres. Synchronous deposition of the contrastingsoft-sediment clast and rippled sand bed componentsimplies a number of suspension and traction pro-cesses (Jopling and Walker, 1968). Fahnstock andHaushild (1962) state that rippled sand beds, asso-ciated with lower flow-regime conditions, are un-likely to contain a coarse bedload component whichis transported mainly by upper flow-regime flows.Therefore the presence of soft-sediment clasts andupsequence changes in ripple type may be associ-


Fig. 6. Annotated sketches of clast B (Fig. 3) showing clast morphology and deformation characteristics.

ated with changes in flow regime as by changesin flow velocity or sediment concentration (Joplingand Walker, 1968). Outsized and freighted clasts, ofwhich soft-sediment clasts are one example, havebeen associated with a number of flow types includ-ing grain flows, mass flows, debris flows and densitycurrents (e.g. Stauffer, 1967; Postma et al., 1988;Shanmugam, 1996, 1997; Selby and Evans, 1997).These flow types vary in rheology along a continuumaccording to flow density. Depending on flow rhe-ology, clasts in these flows are supported variouslyby a combination of inter-grain dispersive pressure,matrix strength, buoyant lift and flow turbulence(Shanmugam, 1996). The synformational context ofsoft-sediment clasts within rippled beds suggests thatthese processes did not occur singly. At Dooneen it islikely that grain flows=debris flows and density cur-

rents occurred in combination with traction and sus-pension fallout as sediment input and water velocitychanged over time. These processes may also showsimilar sedimentary signatures, such as massive sandbeds (i.e. Stauffer, 1967; Shanmugam, 1996) whichseparate rippled beds at Dooneen (Fig. 2). Changesin flow density through the water column may berelated to the mechanism of sediment input intothe system (through suspension fallout or turbulententrainment by bottom currents).

7. Depositional model and palaeoenvironmentalimplications

The morphology of soft-sediment clasts, their de-formation style and position within the sediment


succession, can be used to help reconstruct the over-all depositional setting and regional palaeoenviron-ments, including palaeogeography and hydrologicalregime. On the basis of these data, four stages ofenvironmental change in the Tempo Valley can beestablished, supported by local and regional fieldevidence and data from modern analogues. Fieldevidence and other data are discussed in detail else-where (J. Knight, 1997, 1998).

(1) The Tempo Valley was occupied by aproglacial lake partly separated from the ice mar-gin by a narrow (< few hundred metres wide)deglaciated forefield. The lake is likely to have beenpartly ice-contact because of the presence of deep(¾30 m) and high-energy delta deposits near theFintona Hills (J. Knight, 1997). The absence of lakestrandlines and ice-rafted sediment in the region sug-gests that the lake either did not exist long or thatwater levels fluctuated (i.e. Ryder, 1991). The fore-field comprised of shallow pools of standing waterand braided, migratory streams flowing over sandysediments, consistent with modern analogues (i.e.Maizels, 1993). The Dooneen depositional modelsuggests the importance of high-magnitude meltwa-ter events, again consistent with these analogues.

(2) The soft-sediment clasts originated as mudbeds (¾3–5 cm thick) within isolated pools on theglacial forefield. From clast size and shape thesepools are likely to have been small (area of 2–5m2) and formed as a result of irregularities of theunderlying sediment surface, and possibly filled withwater during waning-flow (slack water) flood stagesfrom the ice margin (i.e. Maizels, 1997). Likely,subaerial exposure of these sediments occurred priorto D1 deformation which is attributed to high-energyfloods from the ice margin.

(3) It is hypothesised that flooding at Dooneencaused removal of soft-sediment clasts and D1 de-formation. Clast reorientation relative to water flow,and later D2–D3 deformation, may be characteristicof more mature (i.e. distal) flows with a better-devel-oped bedload component. Development of more sta-ble clast shapes (i.e. rollers; Table 1) is consistentwith clast reorientation and rounding during bedloadtransport.

(4) The development of both rippled and massivesand beds, which are conformable and occasion-ally interbedded, suggests a combination of traction

and grain flow processes (Jopling and Walker, 1968;Shanmugam, 1996). Soft-sediment clasts may havebeen swept ahead at the nose of the incoming flow(i.e. Hampton, 1979; Postma et al., 1988) and buriedsyndepositionally by waning-flow ripple beds. Theconsistent direction of climbing ripples away fromthe inferred ice margin suggests unidirectional flowvectors behind the flow nose. It is therefore possi-ble that the entire sediment succession at Dooneencould have been deposited by a few high-magnitudedepositional events.

8. Conclusions and wider implications

(1) The occurrence, morphology and deformationstyle of soft-sediment clasts within proglacial out-wash in the Tempo Valley, Northern Ireland, recordan event sequence comprising (a) the ripping-upof mud beds by strong, shallow currents across aglacier forefield, (b) clast transport with between 1and 3 phases of syndepositional deformation, and (c)emplacement within rippled sand beds in a proglacialwater body.

(2) The hydroplastic style of soft-sediment clastdeformation, and directional component of clastfolding, show that the clasts moved intermittentlyas dispersed bedload across a relatively hard, sandysubstrate. Clasts were reoriented relative to flow fol-lowing each deformation phase, possibly by a ‘pulse’of increased fluid velocity.

(3) The massive and rippled sands, in which thesoft-sediment clasts are hosted, were deposited fromdense undercurrents. Rippled sand beds reflect unidi-rectional bottom currents during waning flow stages.Soft-sediment clasts were rolled syndepositionallyinto these beds in response to meltwater ‘pulses’.

(4) The presence of soft-sediment clasts atDooneen, and their inferred processes of deforma-tion, have wider implications for the interpretation ofsuch features in the geological record. For example,clast deformation and rounding processes are oftendescribed (e.g. Diffendal, 1984; Kale and Awasthi,1993), but little attention has been paid to sedimentsource area and palaeogeography. This issue is par-ticularly important, as at Dooneen, where soft-sed-iment clasts are hosted within contrasting materi-als which have a different sedimentary history (e.g.


Krainer and Poscher, 1990) and which may suggestdifferent palaeoenvironmental interpretations. Thereis therefore a need to consider soft-sediment clastsource area, and deformation and transport history, inoverall facies interpretation. Other unknown factorsinclude the physical processes of soft-sediment clastformation (rip-up versus desiccation), and mecha-nism of clast support during transport (dispersivepressure, turbulent lift).

Acknowledgements

Fieldwork was funded by a DENI CAST awardwith the Department of the Environment (North-ern Ireland). Nigel McDowell processed the photos.Peter Wilson assisted with grain size analysis andhelped locate references. Marshall McCabe com-mented helpfully on several drafts of the script. Iam very grateful to the editor Keith Crook and twoanonymous referees for their comments.

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