Lateral esker displacement and its regional implications
   
P�lamalm, SE Sweden This article will be published in September 1999 at Boreas
 




25 Feb. 99
Lateral esker displacement in southern Stockholm, east central Sweden, and its regional implications



Sedimentological investigations in P�lamalm, one of the few elongated, flat-topped, raised glaciofluvial deposits of the Stockholm area (malmar in Swedish) show that the deposit was formed in a full-tunnel subglacial environment during the early Preboreal. The study provides evidence for dynamic links between the morphology of a subglacial conduit, the regional subglacial discharge, and the regional ice sheet dynamics. The development and interrelations of eight distinguished lithofacies at P�lamalm provides evidence for the triggering mechanism responsible for the deposition of this 3-km-long glaciofluvial deposit. Highly deformed units of gravel are documented in close proximity to large deadice structures. The general morphology of the deposit and the lateral esker displacement are parts of a regional pattern. Similar, previously documented, observations in Jordbromalm, another elongated malm further to the east, suggest the occurrence of a sudden regional subglacial outburst, or j�kulhlaup, in the area. The sudden, intensive enhancement of water discharge is assumed to have caused the ice roof of the conduit to collapse. The high meltwater-pressure gradient caused a rapid lateral enlargement of the conduit. The subglacial tunnel took a new route because the original course was blocked by the large ice blocks which fell from the roof.
The steep flanks of the deposit, the presence of large dead-ice depressions along the central part of the deposit and the appearance of two different tunnel core facies in the main cross-section of the P�lamalm deposit support the suggested depositional model. The meltwater pulse was probably caused by a subglacial lake which suddenly discharged into the fractured frontal zone of the ice sheet. A probable late-glacial crustal rebound in response to the rapid deglaciation in the area may have been the triggering mechanism for the lake discharge.Subglacial deposits formed near the frontal zone of ice sheets which extend into the sea may reflect both terrestrial and glaciomarine processes. Studies of these deposits aimed at unravelling these glaciomarine-terrestrial relationships must consider various glaciodynamic factors, such as the impact of glacio-isostatic rebound during rapid deglaciation, subglacial hydrology, and bedrock topography. The effects of these parameters on the mass balance and supraglacial discharge systems are further complicated by the local geography. These factors are responsible for, among other things, the difference in the development of the supraglacial drainage systems between a marine and a continental ice sheet.
Detailed field investigations carried out during the last decade have improved our understanding of glacier bed systems and their sedimentary complexity. Theoretical considerations predict an increase in subglacial discharge with a deepening glaciomarine environment (cf. R�thlisberger & Lang 1987, Paterson 1994).
Despite their small areal coverage, eskers may be the only deposit which records important information regarding the spatial distribution of sediments and the hydrologic conditions in the subglacial environment of temperate ice sheets. The various glaciofluvial deposits in the Stockholm region provide a fairly clear pattern of the regional paleoenvironment for the last Scandinavian Ice Sheet deglaciation in SE central Sweden. Evidence from this region indicates that rapid deglaciation occurred in the interval from the late Younger Dryas to the early Preboreal (Fig. 1) (Lundqvist 1995). The processes controlling the sedimentary environment during rapid deglaciation are still not understood in detail because of the difficulty to accurately date coarse grain deposits. The glacial depositional environment south of Stockholm has been recently discussed (cf. Lundqvist 1995, Brunnberg 1995a), and these studies show that subglacial currents contributed much to the formation of the deposits. However, a regional perspective about the subglacial environment of the central Scandinavian Ice Sheet is still needed, as the extent and characteristics of events and their impacts on the subglacial depositional environment during the early Preboreal are not well known. The study of various mechanisms which could potentially trigger a sudden subglacial meltwater discharge and its consequent deposits would contribute to a better understanding of the mechanisms that govern the development of the ice-marginal zone.
The present study focuses on the sedimentary patterns and depositional processes of the early Preboreal succession, based on sedimentological studies of P�lamalm, which is one of the few large, flat-topped, gravel deposits of the area (malmar in Swedish). The different appearance of these flat-topped deposits in comparison to ordinary ridge-shape esker deposits, especially in P�lamalm, had been studied earlier this century (S. De Geer 1905). The occurrence of the P�lamalm deposit in the Uppsala esker system and its resemblance to deltas has resulted in it being classified as a subaquatic delta (Lundqvist 1990, Brunnberg 1995).
The glaciofluvial deposit in the P�lamalm gravel pit is an esker plain (cf. Lundqvist 1997), and consists predominantly of massive units of gravel and sand with minor interbedded silt in the lower part of the western section. The deposit is located in the south-eastern part of central Sweden and constitutes a roughly N-S trending sedimentary body.
This study focuses on the relationships between sedimentological characteristics and the morphology of the deposit. The close field relationships and morphological similarities of this deposit and the other large gravel deposits in the area are subsequently used for a reconstruction of the successive sedimentary events. The aim of this paper is to analyse in detail the facies and facies transitions in a number of sections in the P�lamalm gravel deposit in order to determine the environmental conditions during the genesis of the deposits. Such a reconstruction can only be completed on a regional basis, by including previously documented data.
Quaternary geology of the area
The study area in the S�dert�rn Peninsula (Fig. 1) consists of crystalline Precambrian bedrock, which forms a fissure-valley landscape, with a moderate relief of up to 50 m. The highest peak reaches 110 m a.s.l. and the Quaternary sediments are directly deposited on the bedrock (M�ller & St�lh�s 1969). The youngest striations on the bedrock indicate ice movement towards the SSE. Clay-varve investigations (Brunnberg 1995a) suggest P�lamalm and other large sand and gravel accumulations in the area as evidence of accelerated ice retreat during the early Preboreal (Fig. 2). Conversely, Persson (1983) considered the deposits traces of a glacier readvance.
Further to the north is the 300 km-long Uppsala esker. A detailed study of the Quaternary deposits of the Stockholm area (G. De Geer 1932) suggested that the esker deposits are a time-regressive, continuous record of the meltwater discharge in subglacial tunnels. Clay varves were used as a tool for dating the meltwater events. The 1932 work followed an earlier, detailed study of the Uppsala esker system, which was done in P�lamalm, by S. De Geer (1905), who also documented the special morphological characteristics of the deposit. Later, G. De Geer (1932) introduced the idea of the regional phenomena of lateral esker displacement. Eriksson (1960) briefly mentioned the general morphology of the Jordbromalm deposit of the Stockholm esker. M�ller (1962) suggested episodic water discharge as the main factor for the deposition of glaciofluvial sediments in a subglacial tunnel environment. The present study was carried out at the P�lamalm site, which is located 40 km south of Stockholm (Fig. 3a) and is the largest glaciofluvial deposit of the area and the second largest deposit in the southernmost part of the Uppsala esker system. Formation of the southernmost deposit, Sorundamalm, about 12 km to the south has also been recently discussed (Mokhtari Fard & Jansson 1998). P�lamalm comprises a 3 km long (Fig. 3b) and 1.2 km wide glaciofluvial deposit (S. De Geer 1905). To the west it is limited by two lakes, the Stora and Lilla Skogssj�n Lakes. A former lake to the east, Kvarnsj�n, was filled in during gravel exploitation (Fig. 4). P�lamalm is a flat-topped glaciofluvial deposit with steep marginal slopes to the surrounding area, which consists of lowland and lakes. The deposit is formed on top of bedrock hills. In smaller deposits in the area, however, it is bedrock depressions which are filled with glaciofluvial deposits (cf. Mokhtari Fard et al. 1997). Postglacial beach deposits cover the glacigenic succession with an erosive contact at the P�lamalm site. The highest coastline of the Yoldia Sea was about 125 m above the present sea level (Brunnberg 1995).
The crest of the deposit is 80 m above the present sea level. Exploitation of the gravel pit started in 1960 and the facies described here were mapped from 1993-1997, at two topographic levels.
Methods
The relationship between the bedrock topography and the distribution of the sedimentary succession were studied. Various primary structures were documented to complete the detailed sedimentological analyses of each exposed wall in gravel pits while the commercial excavations continued. Some new outcrops were created for studies in scree-covered sections. Sediments were characterised using a descriptive facies model. Particle size analyses were done using spot samples, up to 5 kg, taken from beds of homogeneous cobble gravel to very fine sand and fines. Granulometric analyses consisting of dry sieving of gravel and sand particles were carried out. In order to establish lateral and vertical variations, both within individual beds and throughout the deposit, fabric analyses were performed. The main cross-section is shown in Fig. 5.
Sediment description and interpretation
The maximum thickness of the deposits at the P�lamalm site is about 30 m (M�ller & St�lh�s 1969). The exposed outcrops exhibit a variety of gravelly facies (cf. Fig. 5a). Thirty-two vertical profiles at P�lamalm were studied, mainly along the eight exposed sections at the site (A-H and R). The representative sedimentological logs of the site are depicted in Figs. 7-11.
The succession in the lower level accumulated in a bedrock depression and is about 3 m thick. The outcrop at this level consists mainly of fine-grained material. This part of the succession is topped by an erosional surface. The upper level consists of an approx. 10 m thick, mainly gravelly, succession. Various syndepositional defor-mations are documented in this level.
Eight lithofacies (A-H) are distinguished; their most important characteristics are described below:
A: heterogeneous, matrix-supported gravel
B: faulted and folded, graded gravel and sand
C: silty, rippled sand
D: graded gravel
E: clayey silt and silty sand
F: heterogeneous chaotic gravel
G: diamicton
H: pebbly sand.
Facies A
Facies A crops out in the lowermost part of section C1 and C 5 (Fig. 5, 7-8). The thickness varies between about 0.5 m in section C1 to 4 m at C5 (see Figs. 4 and 5 for the location of the sections). Facies A occurs directly on bedrock at C5. The lower boundary at C1 is not exposed. The upper contact with facies B is sharp. Facies A consists of heterogeneous, poorly sorted to massive, matrix-supported gravel.
The facies architecture in both outcrops is pseudoanticlinal. The maximum particle size of the measured clasts is approximately 1 m. The largest clasts are randomly distributed. Facies A is polymodal and the clasts are surrounded by a matrix of finer gravel and sand. Material finer than 2 mm constitutes less than 5% by weight.

Interpretation
The poorly sorted facies A indicates contemporaneous deposition of matrix and clasts. Poor sorting is attributed to the variable flow character in the centre of subglacial tunnels (cf. Saunderson 1977). The bent (convex upward) configuration of the bedding is a common phenomenon in esker cross-sections and is usually interpreted as a partially syndepositional, partially postdepositional feature, resulting from slumping and faulting of material as the ice walls supporting the esker melt away (G. De Geer 1932; McDonald & Shilts 1975). Some anticlinal bedding structures in eskers has, however, been interpreted as primary (Flint 1971). Lack of faults in the P�lamalm sections also suggest a primary origin of the pseudoanticlinal configuration.
Facies B
Facies B occurs at sections A, B, C1, C2 and C5 (see Figs. 4 and 5 for location of the sections) and has a stratigraphic position above facies A, bounded by a sharp contact. The upper boundary with the postglacial beach deposits is erosive, so that many structures are cut by the overlying postglacial gravel sequence. In section A1 a thickness of about 4 m of facies F occurs on top of facies B (Fig. 7). The contact surface is erosive and sharp.
With a maximum thickness of 10 m facies B is thicker than other facies in the studied sections. It comprises massive or bedded sands and matrix-supported, massive to graded gravels with some local silt beds of minor importance. The maximum observed thickness occurs at sections C1 and C2. Some massive cobble units have a thickness between 0.1 and 5.5 m. Intercalated pebble units and parallel-bedded coarse-grained sand are frequent in some sections. Most of facies B outcrops exhibit fining-upward sequences ranging from very coarse into fine sand. The other sand beds are well sorted. Most gravel beds exhibit clear bedding; they are commonly graded. Sand beds are about 1 cm in thickness.
The thickness of the pebble beds is most commonly approximately 5 cm. In one section (C2), a detached body of deformed, fine-grained, well bedded sand was observed (Fig. 9). This isolated unit is an extreme result of the intense deformation that is commonly present, mostly in the form of faulting (normal faults, thrusts and boudinage structures) and folding of the gravel units (which consequently lost their primary sedimentary structures at some places). Collapse structures are also present. Deformed beds of cobbly gravel are particularly frequent in section C1-2 (Fig. 10). The slump structure in section A2 (Fig. 11), contributes to the locally chaotic appearance, although the sedimentary architecture as a whole is fairly uniform, with widespread sheet-like and slightly undulating beds.

Interpretation
Sedimentary structures in the sand are almost entirely restricted to parallel lamination and grading. The long axes of gravel particles show a preferred orientation indicating transport towards 220�-240. Sets of both fine sand and cobbles are found sporadically; such sets are normally graded. The poorly sorted, sandy gravel and pebbly sand can only be explained by deposition from high-concentration debris flow and grain-flows (Postma 1984). The abrupt change in the angle of the dip between successive beds of sand and gravel is explained by shifts in the direction of the flows. This is particularly well documented in section C2 where the contact between facies A and B is visible; a change in the current direction of the meltwater stream apparently led locally to a decrease in the energy.
Coarsening upward successions in some parts of facies B reflect delta progradation (cf. Cohen 1979). The appearance of normally graded gravel is due to rapid deposition from a turbulent suspension at the base of a turbidly current transporting pebble and cobble grains. The succession suggests that deposition was mainly in the upper flow regime and that the currents were almost unidirectional with some minor directional changes. The relatively strong current inferred from the sedimentary structures and the paleocurrent data suggest that facies B is part of a tunnel deposit covering the sediments of facies A that were deposited in the centre of a subglacial tunnel (cf. Fig. 8). As there is minimal postdepositional disturbance of the sediments, it is unlikely that they were deposited on top of ice. This rules out an englacial depositional environment (Banerjee & McDonald 1975). The sand and gravel alternations are interpreted as a product of deposition of bedload or deposition by debris flows. The presence of listric faults and boudinage structures indicates deformation under tension, which can be ascribed to the rapid loading of the deposits. The high-angle normal and thrust faults result from collapse near free faces or from melting of buried ice (Rust & Romanelli 1975). The slump structure was probably formed due to slope instability after the melting of a buried ice block.
Facies C
Facies C is limited to section D (cf. Fig. 4) and is characterised by the finest particles of the site and has common small-scale structures. An erosional contact separates this facies over a length of about 30 m (sections D2-D3) from the underlying facies D; the base is not exposed in other parts of section D. The facies no longer shows its complete development, as the upper boundary is erosive. The overlying material forms part of facies B, but the transition becomes obscured more towards the north, which hampers the analysis of the lateral development of facies C and its boundaries. No deformation has been observed in facies C. Many beds are graded, with silty sand becoming dominant in the top parts. Parallel lamination and/or climbing ripples (ripple-drift cross-lamination) are present in most beds. The ripples indicate paleocurrent flow towards the south. The average thickness of the rippled beds decreases towards the top of the succession. In section D1, a cross-section of a minor channel was observed (Fig. 12). The width of the channel was about 20 m, and its height about 180 cm. The infill consists of sets of fining upward sand beds with occasional drapes of silt and very fine sand over the ripple structures.

Interpretation.
The fine-grained facies C, with its horizontally laminated sand, rippled sand and draped lamination, represents typical deposition by turbidity currents under conditions of decreasing velocity in a high-frequency internal wave environment (Eyles et al. 1987). The migration of the ripples across the sections indicates a gradual reduction with subsequent increase in flow velocity towards the south.
Waning flow velocities led to the development of ripple-drift cross-lamination with a steeper climb angle when settling from suspension became more important. Finally, when the weaker tail of the turbidity currents moved over the site, suspension deposition started to dominate and drapes started to form. Such sequences are common on the lower parts of glaciolacustrine delta fronts, where turbidity underflows associated with meltwater streams occur (Gustavson et al. 1975).
Facies C may thus be considered as a low-energy deposit in a stagnant-water body. The gradual reduction in the thickness of the rippled beds reflects an ongoing decrease in the strength of the currents. Nemec (1995) considered the responsible mechanism for a similar sequence to be internal waves caused by wave loading on the bottom of the basin. Section D, to which facies C is restricted, is present only on the lower morphologic level of the P�lamalm gravel pit (cf. Fig. 5), underneath a 10 m thick gravelly succession of facies B. This succession can be explained in two ways: (1) it reflects the fine-grained, most distal part of a delta foreset in front of the delta, or (2) it is a remnant of a minor subglacial channel, most of which was eroded afterwards. The topographic position of section D - i.e., in the bedrock depression - in combination with the presence of the diamicton succession that starts at the top of the bedrock, makes the latter option the more likely one.

Facies D
Facies D occurs in the lowest part of section D (cf. Fig. 12). The lower limit is not exposed (covered by scree). Facies D is overlain by facies C; the contact between these two facies is sharp and erosive. Facies D consists of 1 m of graded, coarse to medium sand and pebbly gravel with a sandy matrix. The sand and gravel succession shows planar bedding with some minor, irregular cross-bedding, and gradually fines upward.

Interpretation
The restricted occurrence (in a bedrock depression) below facies C suggests a subglacial channel origin for this facies. Subglacial channels occur under temperate glaciers and ice sheets (cf. Eyles et al. 1983) and are known from many glaciated areas (Lundqvist, pers. comm.). Ehlers (1996) argued that the only position where sediments deposited in subglacial channels can be preserved is in bedrock depressions. These subglacial channels are generally small (Brodzikowski & Van Loon 1987).
Facies E
Facies E is exposed only in section R, in the remote southern part of the site. This section is situated about 4 km to the south of the main section at P�lamalm (cf. Fig. 4); peat is being excavated here instead of gravel. The entire facies is about 150 cm thick. Facies E consists of clayey silt to silt and fine sand. The beds show wavy to parallel lamination and low-amplitude ripples (Fig. 13). The laminae are sometimes thinner than 1 mm, and there seems to be no essential difference between these thin laminae and the thicker ones; beds up to 2 cm thick show identical characteristics as these laminae. Both the laminae and the thin layers typically display normal graded bedding, so that the succession consists of cycles with relatively light-coloured lower parts that gradually become darker and finer grained towards the top.
A specific feature is the sporadic occurrence of isolated stones and boulders, up to 85 cm in diameter, that are randomly scattered throughout these fine-grained sediments. The fine-grained sediments underneath these stones are bent downwards.

Interpretation
The distinct parallel lamination in the sandier parts of facies E has all the characteristics of parting lineation (cf. Allen 1970), typical for planar lamination formed in rapidly flowing water under conditions of the upper flow regime.
The planar lamination within the finer-grained silty sediment, however, cannot be explained this way. It must have resulted either from deposition under lower plane-bedded conditions (below the critical current velocity for ripple formation), or from settling from suspension clouds (Merritt et al. 1995). Both types of geneses may apply to these sediments but, in my opinion, settling from suspension in quiet water was the dominant process, representing the lowest possible energy conditions, presumably in the distal environment of a proglacial lake. The minor fluctuations in grain size within the laminated fines reflect pulses in the sediment supply over short but undefined time intervals.
The scattered cobbles and boulders are interpreted as dropstones, partially on the basis of their scarce, isolated occurrences, partly on the imprints that are present underneath them, which are interpreted as 'impact craters'.
Facies F
This chaotic gravel facies occurs mainly in the eastern part of the P�lamalm site, in sections C6 to C8 (cf. Fig. 5 for location). The sediments of this facies are deposited directly on top of the Precambrian bedrock. An isolated outcrop of facies F is exposed in section A1 where it covers the facies B sequence (Fig. 7). An erosive contact separates facies F from the overlying wave-washed gravel of post-glacial origin. The geometry of bodies of facies F is highly irregular (Fig. 14a). The entire facies attains a maximum thickness of approximately 6 m. Facies F consists of matrix-supported massive gravel.
The clasts are rounded to subrounded (Fig. 14b). The units in the lower part of the facies tend to be fairly massive, but in the upper parts of some sections the gravel becomes roughly bedded, sometimes showing crude cross-stratification. The succession shows a coarsening upward tendency in sections C6-8. The textural variations within sets of cobble-sized gravels are considerable. Inhomogeneous sets of cobbles up to 0.2-1 m thick occur almost exclusively within multi-storey massive gravel successions with a non-erosive, gradual lower boundary. The upper contact surfaces are transitional. In section A1, to the western flank of the deposit, the matrix of facies F is reduced and crude bedding is documented (Fig. 7).

Interpretation
The variation in the texture and structure of facies F is striking. The irregular pattern strongly resembles the subaquatic gravel-flow cross-stratification that typically occurs in ice-contact sedimentary bodies, as a subglacial or ice-marginal product of limited reworking of coarse and heterogeneous glacial debris by high-energy meltwater flows. The large-scale cross-bedded sediments are interpreted as a result of sliding-bed formation (cf. Saunderson 1977). In a full-tunnel environment, a decrease in the subglacial discharge induces the bed form to change to large sand waves or dunes, producing large-scale cross-stratification. This mechanism of bed sliding is, however, not effective if the tunnel is not full of water. A comparison between esker and pipe experimental sedimentation results (Saunderson 1977) shows that relationships exist among particle diameter, mean velocity and transition velocities separating various modes of transport. In large pipes, the sliding-bed mechanism is only effective for coarser particles (Fig. 15).
The considerable variation in texture and grain size within facies F reflects a rapid variation in hydraulic conditions. The characteristics in the eastern part of the site (sections C6-8) suggest that the hydraulic conditions of this part of the depositional system in P�lamalm were unique and differed from those in the other parts. A different flow type would have resulted from confinement of water and sediment within a subglacial conduit wall. Hanvey (1989) interpreted a similar gravel succession with coarse clasts and rounded cobbles as a subglacial conduit sequence suggesting a conduit within which materials accumulated. The conduit at the P�lamalm site must have been large, and only very turbulent, high-energy conditions may explain transport of cobbles with a diameter such as present in facies F.
The overall inversely graded nature of these stratified deposits is considered to reflect a lateral increase in conduit size and its hydraulic activity through time. Hershey (1897) described the process by which such structures are formed in a subglacial environment: in an ice-walled conduit, scouring and melting of the wall by the passing meltwater increases the amount of both available water and sediments carried along by the current. This in turn increases the energy of the current. A change in the hydraulic pressure of the subglacial drainage system results.
A succession like that of facies F might, however, also be explained by deposition in a tunnel mouth. Although there are similarities between the characteristics of the gravel of facies F and those of tunnel-mouth fan deltas (cf. Cheel & Rust 1982), this interpretation is not applicable in P�lamalm, for the following reasons:
* there is no fine-grained succession present on top of facies F. Such a succession should be present in any tunnel-mouth sequence, because a fining upward succession would be deposited in an ice proximal zone during the retreat of the glacier;
* the absence of well sorted, stratified sand and gravel suggests high-concentration, plastic flow (Middleton & Wilcock 1994). Such flow would inhibit differential deposition of clasts in a closed conduit system. The above considerations suggest that facies F has a closed-conduit origin.
Facies G
Facies G forms the lowest exposed part in section G in the northern outcrop of the P�lamalm site. Only 2 m of this thick, massive, dark grey, matrix-supported diamicton of originally unknown thickness is exposed. The lower contact is not exposed. The upper boundary is also unknown, because the contact surfaces with the overlying sediments are covered by talus scree. The matrix consists of silty fine sand with some pockets of coarse sand, with rounded clasts of up to 50 cm in diameter. The succession is not graded but a rough stratification is visible in the lower part of the section.

Interpretation
The massive diamicton is inferred to have been rapidly deposited from a highly concentrated dispersion (cf. Brennand 1994). It was most likely deposited subglacially in the core of a tunnel, as indicated by the massive texture and the variety of the clasts. Saunderson (1977) reported a facies with a similar texture; he interpreted the deposit as a poorly sorted gravelly-sand sliding-bed facies in an esker, formed under full-pipe flow conditions. Facies G is therefore considered to represent a subglacial conduit-core deposit. The location of the deposit (section G in the northern part of the site; see Fig. 5) suggests a continuation of the core deposits exposed further to the south in the main section at C1.
Facies H
Facies H consists of a poorly sorted, massive, matrix-supported sandy gravel. The facies is present only in section C8, and is about 8 m high and 20 m wide. In the eastern part of section C8, where several sets of minor thrust faults and other types of deformation are present, the succession overlies facies B unconformably. The individual beds dip towards 80�-120�. Layers of bowl-shaped fine sand occur intercalated between the gravels. The bases of these sandy beds are parallel but gradually become less clear towards the top. Bedded units of pebbly sand are also present; they generally fine upward. Irregular lenses of unsorted sand and gravel are present, as well as pockets (up to half a metre in diameter) of well sorted pebbles in a poorly sorted sand matrix. The clasts are mainly rounded. Individual beds often show abrupt lateral and vertical variations in particle size. They range up to 20 cm in thickness, but lateral changes in thickness are common. Pebbly sand with crude horizontal bedding is present, more frequently in the lower than in the upper part of the section.

Interpretation
The pebbly sand units resemble mass-flow deposits described from channel-fill sequences in glaciolacustrine pro-delta and ice-proximal glaciomarine environments (cf. Dardis & Hanvey 1994). Similar massive, matrix-supported, poorly sorted facies have been reported in open-channel deposits (Ringrose 1982), in proximal outwash deposits (Boulton & Eyles 1979), and in hyperconcentrated flood flow deposits (Lord & Kehew 1987), as well as in full-pipe closed-conduits (Saunderson 1977). The heterogeneous, crudely stratified gravel facies suggests the latter as a probable depositional mechanism. The limited size of the clasts, the presence of fine sand, silty sand and pockets of pebbly diamicton pockets reflect the low-discharge conditions in the subglacial tunnel. The variations in the texture reflect discharge pulses taking different routes, probably more or less parallel to the previous courses where the sedimentary surface had become less suitable for ongoing sedimentation because deposition had made the surface locally higher than in the direct vicinity.




Structural features
Pseudo-anticlinal structures
Pseudo-anticlinal forms are found in sections C1 and C5; they are up to 1 m in diameter and consist of low-angle, arched or anticlinal bedding structures in the massive, clast-supported, heterogeneous gravel of facies A. In the more eastern section (C5), the structure is exposed best; it is covered unconformably by stratified gravel of facies B. High-capacity currents are required to remove the boulders and cobbles in the arch-shaped gravel (cf. Brennand 1994). Williams (1983) estimated a current velocity of at least 2 m/s for transport of boulders in the range of 1 m. Such large clasts are primarily transported by rolling (cf. Rust 1972), which allows an interpretation of the paleocurrent direction from the direction of the a-axes. The direction thus determined indicates transport roughly towards the south. The presence of sediments deposited by traction, saltation and suspension would suggest a turbulent fluid mechanism.
Compressional faults
The most striking deformation structures are observed in the graded, sandy gravels of facies B, and in the gravelly sands of facies H. The bedding of these units is sometimes rotated into a vertical direction and cut by high-angle compressional faults. The compression caused a shortening in a N-S direction in facies B and H, in sections C2 and C3 respectively. The maximum observed displacement along a single fault plane is 30 cm.
Kettle holes
Several kettle-hole depressions occur in the central part of the P�lamalm site, along its longitudinal axis (see Fig. 3b & 4). The largest one is located about 2 km north of main section C. The depression is 32 m deep and 350 m in diameter (S. De Geer 1905). In 1967 this deadice depression was recognised as exceptionally large, and it became a protected natural monument (cf. Miller & Robertsson 1966). More than ten smaller depressions, mainly concentrated in the northern part of the site, have been recognised as kettle holes. Some of these depressions form minor lakes and bogs; they surround the northern end of the site (M�ller & St�lh�s 1969). It is most likely that, at some stage, the basal zone of the glacier in the conduit system was heavily charged with blocks of ice falling from the roof of the tunnel to form these dead-ice structures. This is the sole acceptable explanation for the presence of the mentioned depressions. The fact that these structures were preserved proves that the dead-ice blocks forming these large holes must have remained unmelted during the entire time span of the deglaciation and sediment reworking processes (M�ller & St�lh�s, 1969).
Slump structures
The only documented slump structure is in the gravel succession of facies B in section A2 (Fig. 11). The original succession was a fining-upward set of cobbly to fine sand with an original thickness of about 4 m. The deformed succession is covered by an undisturbed unit of sandy gravel belonging to facies B. Several mechanisms could explain the formation of the slump structure: (1) a shock affecting the water-saturated, coarse-grained material may have triggered movement along the steeply sloping surface; (2) rapid deposition of gravel on the slope may have created instability, resulting in slumping; (3) failure of silt and fine sand deposited on a surface sloping at more than 15 may have resulted in slumping (cf. Lowe 1976); (4) melting of a buried ice body may have caused collapse of the overlying sediments by fluidisation of the sediments through increased pore-water pressure. Considering the local situation and the geological context, the explanation of melting a dead-ice block seems the most likely. Further to the north of the area more deadice structures are documented in the glaciofluvial deposits (Lundqvist pers. comm.). This may suggest consequent conduit roof collapses in the course of deglaciation of the Stockholm area.
Discussion
The distribution of lithofacies in a landform that is interpreted as an esker system, reflects the depositional environment of either a subglacial conduit or in a tunnel mouth in stagnant water (Banerjee & McDonald 1975, Saunderson 1975, Cheel & Rust 1982, Goldthwait 1989). Full-pipe flow conditions may be present in the subglacial tunnel (Church & Gilbert 1975, Bannerjee and McDonald, 1975, Saunderson, 1977, Ringrose 1982, Warren & Ashley 1994). Fan and delta structures are formed where glaciofluvial sediments are deposited in front of a tunnel mouth (cf. De Geer 1897) in a more or less stagnant water body. An extensive review of the older literature on the geomorphology and sedimentary texture of eskers was provided by Warren and Ashley (1994).
Evidence for a subglacial origin of the P�lamalm deposits
The most important factors favouring the accumulation of sediment in subglacial passages are a large discharge and low surface gradient during the final stages of deglaciation, when melting and down-wasting of the ice are greatest (Flint 1971). The shape of the subglacial passages carrying water and sediment are determined in particular by the supply of water to the passage, and the thermo-erosion of the walls. Together, accumulation rate, current characteristics and tunnel shape largely determine the internal and external structure of the sedimentary body that will later become an esker.
The external and internal structure of eskers are now well known. The deposits at the P�lamalm site can be interpreted as part of an esker; topographic maps do not allow any other interpretation. One might question, however, whether these sediments could also be interpreted as formed in a subglacial passage or a tunnel mouth without the morphological data. One must conclude that the characteristics of each individual sedimentologic lithofacies distinguished here allows multiple interpretations, although the combination of the characteristics of, and the spatial relationships between these facies supports a subglacial or tunnel mouth environment. Some deposits might be interpreted as small deltas or fans, with special reference to the occurrence of deadice structures in such accumulations, if they were considered out of their geological context. The absence of fining downstream in the main parts of the deposit, and the wide range of particle sizes (in combination with the sedimentary structures showing a variety in flow regimes) would, however, argue against a deltaic origin. The wide range of particle sizes, as well as irregular masses of coarse-grained material in the uppermost facies in the eastern and western parts of the main section, i.e. facies B and F at sections C1 and C7 respectively, are, in addition, strong indications for a subglacial origin. Furthermore, the 'floating' sand and cobble masses in the sands and silts of facies B and F (Fig. 11) are most easily explained as supplied en bloc. The combination of the various types of data thus indicates deposition by full-pipe flow in a subglacial conduit (cf. Saunderson 1977). The pseudo-anticlinal structure of the deposit and slumping of the side slopes, are clearly primary (cf. Sharp 1953), which is additional support for deposition in an ice-walled subglacial conduit.
The depositional history
Any suggested depositional model for the P�lamalm gravel pit must account for the small spread in paleocurrent directions, the presence of several dead-ice depressions along the axis of a sedimentary body with a flat-top ridge morphology and steep flanks, and lateral faulting parallel to the axis of the body. The unusually flat upper surfaces is one of the most characteristic features of the eskers in the Stockholm area. In combination with other topographic data, it seems thus beyond any reasonable doubt that the study area forms the southernmost part of an esker. This is, however, in contrast with the general ridge form of eskers (Lundqvist pers. comm.). The accumulation of the deposit must have taken some time, although clay-varve studies suggest a rapid deglaciation of the area during the early Preboreal (Brunnberg 1995a). The study area endured an isostatic uplift of at least 8 m during the early Preboreal (cf. Hedenstr�m 1996). M�ller (cf. M�ller & St�lh�s 1969) has discussed several alternatives for concentration of large boulders on top of the deposit at P�lamalm (Fig. 3b). It is here suggested that the intensive post glacial erosion as well as outwashing of finer-grained material resulted in the over representation of these large boulders on the surface of the deposit.
Considering these factors, one would expect that the 3 km long deposit should have developed an inclined surface or, alternatively, a step-like upper surface reflecting raised beaches. This is, however, not the case. One of the main characteristics of the P�lamalm deposit in specific, as already mentioned by S. De Geer (1905), is its unusually flat surface (Fig. 4b). Hence, the flat surface is likely a primary structure. This argues for glaciological control of the morphology of the deposit.
Studies in recent glaciers show the most important characteristic of a conduit system to be its instantaneous response to variations in the drainage-water pressure (Shreve 1972). Plastic deformation of subglacial conduits in response to variations in the hydraulic drainage system was recently studied in detail (cf. Menzies 1995). Studies indicate that the potential energy released by water passing through a subglacial conduit is capable of enlarging the cross-section of the conduit faster than the subsidence of the roof by plastic flow (Hooke 1984). Consequently, the roof slowly deforms creating an ecliptic conduit. The ongoing widening of the conduit makes it likely that the roof will eventually collapse (cf. Lundqvist 1997). Although there is no direct evidence for the presence of an ice roof at the time of the formation of the deposit at P�lamalm, it must be kept in mind that the water level of the Yoldia Sea that covered the adjacent proglacial area at the time was about 45 m above the present topography (cf. Svensson 1991), so that the sediments either were deposited in a roofed conduit or must have accumulated in water of several dozens of metres deep. The abundance of deadice structures along the axis of the deposit can only be explained by collapse of the tunnel roof. The first option seems therefore by far the most likely; there was a wide ice tunnel full of water (cf. Brennand & Sharpe 1993).
The deposition of facies F in the eastern part of the P�lamalm site (sections C5-7) and the erosion of the main channel in which the body accumulated, indicate the action of a powerful, short-lived, sudden subglacial discharge event, which interrupted the deposition of the sandy gravel of facies B. On the basis of the sedimentary structures and the facies described above, it seems probable that this event was a sudden change in the course taken by the subglacial tunnel. The new course, possibly caused by lateral thermo-erosion, was to the east of the previous location (Fig. 16). It cannot be established with certainty that the new route was created first, followed by abandonment of the first route; it is also possible that the original route could no longer be followed, which forced the water to 'excavate' a new tunnel.
Whatever the exact cause/effect relationship, the conduit system at the base of the ice sheet is envisaged to have become heavily charged with a huge flux of water. The unstable ice roof of the tunnel then must have released large ice blocks, which fell on top of the subglacial deposits and blocked the previous shallow conduit in the western part of the deposit. Further to the east of the settled ice blocks, where the bedrock is locally deeper, the new path for the increased water mass was either in addition to or replaced the blocked path completely. The new path followed by the subglacial meltwater caused more turbulent flow to the eastern part of the deposit, where the main sedimentological feature is the `plastic` accumulation and deformation of coarse-grained gravel beds which reflect the high velocity and turbulence of the flow in the new conduit. Shortly after the waning of this intensive but short-lived pulse, the roof of the tunnel collapsed and covered the deposits. Near the end of the deposition of facies F, the ice melted and the glacial environment yielded to a post-glacial erosional system in which post-glacial sediments were deposited on the flat-topped tunnel deposit. An intensive erosional phase following the deglaciation must have removed all of the material on top of the succession and probably the topmost part of the tunnel succession as well.
The presence of large boulders on top of the deposit (cf. Fig. 4b) has been discussed earlier (M�ller & St�lh�s 1969). The present investigation favours postglacial erosion for over representation of the large particles, while finer deposits are transported further to the south. Some of the blocks may also have been deposited as dropstones (M�ller & St�lh�s 1969, p. 76).
The depositional environment at the P�lamalm site cannot be viewed in isolation, as there is another esker deposit (Jordbromalm) in the area, (Fig. 2, at about 10 km east of P�lamalm), which belongs to the Stockholm esker. These parallel esker systems form an integral part of the glacial depositional environment. Only a fairly general, schematic morphological description of the Jordbromalm is, however, available (Eriksson 1960, p. 46) because the deposit is totally excavated. Frequent deadice structures and the documented primary structures reported from the Jordbromalm deposit suggest a subglacial origin for this deposit (cf. Eriksson 1960). Contemporaneous formation of these deposits is inferred, as both deposits occur at the same latitude (Fig. 2). The similarity between the morphology of P�lamalm and the adjacent Jordbromalm, as well as the documented lateral widening of the Jordbromalm deposit around a dead-ice depression (Eriksson 1960), suggest a single sudden increase in the subglacial discharge water on a regional scale. Such a discharge would have affected the conduits forming the Uppsala and the Stockholm eskers. Thus, a regional event must have had a dramatic impact on the subglacial hydraulic system of the ice sheet at the time.



Conclusions
Esker deposits in the Stockholm area are late deposits formed in regions of low relief, when the ice was relatively thin and the flow was sluggish (cf. Flint 1971; Shreve 1985, Lundqvist 1987, Holmlund & Fastook 1993). The lateral esker displacement in southern Stockholm is interpreted as the response of the subglacial depositional environment to a sudden discharge of subglacial meltwater flow. The presence of a chain of deadice structures along the axis of the esker can be explained by tunnel roof collapse. The event that caused a catastrophic discharge of subglacial meltwater would have ultimately led to disintegration and collapse of the ice sheet (cf. Eyles et al. 1997, Shoemaker 1991, 1992). The glaciofluvial deposits at P�lamalm and the excavated large gravel deposit in Jordbromalm, represent subglacial full-tunnel deposits which record a sudden, powerful subglacial discharge or j�kulhlaup in the marginal zone of the Scandinavian Ice Sheet. The complex lithostratigraphy and surficial morphology of the P�lamalm site can be reconstructed to represent a primary subglacial-tunnel phase, the collapse of the tunnel roof as a response to a sudden increase in the meltwater discharge, and the opening of a new outlet for the drainage of the excess water. The bedrock morphology forced the abandonment of the previous tunnel and the opening of the new one in the frontal zone of the ice sheet. A rapid postglacial shore displacement has already been suggested for the study area (Risberg et al. 1991, Svensson 1991).
Recent investigations (Mokhtari Fard in prep.) suggest a short-lived late-glacial isostatic rebound while the study area was still covered by the ice in the marginal zone of the ice sheet at a short distance from the adjacent Yoldia Sea. The delayed response of the crust to both the rapid ice retreat in the southern parts of the area (at the end of the Younger Dryas) and the sudden unloading of the crust as a consequence of the dramatic drainage of the Baltic Ice Lake are the probable causes of this event in early Preboreal.
A subglacial lake is assumed (Mokhtari Fard in manus) to have been established prior to the deglaciation in the M�laren depression, an east-west elongated bedrock depression with an area of about 11,00 km2.(cf. Fig. 1) which occurs to the north of the study area.
As a consequence of the inferred rapid uplift, the fractured ice conveyed the sudden meltwater discharge from the subglacial lake to the frontal zone of the ice sheet. The existence of the extensively elongated flat - topped P�lamalm and Jordbromalm is the evidence for this short-lived event.

ACKNOWLEDGEMENTS

This article forms part of the author�s doctoral thesis at Stockholm University. The research was part of a regional project � The Middle Swedish Ice Marginal Zone and the global climate changes at the transition Pleistocene- Holocene� financed by Swedish Natural Science Research Council (NFR) and led by professor Bertil Ringberg.
I am grateful to Jan Lundqvist and Eve M. Arnold for a careful review of the manuscript and the valuable suggestions which improved the text. I am indebted to Peter Jansson for field discussions and the review of an early draft of this paper. Bertil Ringberg and Ann- Marie Robertsson reviewed the manuscript and gave valuable suggestions. Thanks are due to Jehanders Grus Co. and Lars Svensson for access to the outcrops in P�lamalm during 1993 to 1997 and for information on the bedrock of the site. I am grateful to Chris Griffiths (Swansea, U.K.) for the valuable field discussions and assistance. A field grant from Earthwatch Institute (Boston) is acknowledged. Log R (Fig. 6) is modified after the results of the field course in sedimentology (1997). Stimulating discussions with students of this course is greatly appreciated. The author benefited from several discussions with Lars Brunnberg. Laszlo Madarasz created the illustrations.

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