Subglacially formed clastic dikes

Subglaciallyformed clasticdikes by Eiliv Larsen and Jan Mangerud

Larsen, E. and Mangerud, J., 1992: Subglacially formed clastic dikes. Sueriges Geologisha Undersdhning, Ser. Ca 81, pp. 163-170. ISBN 91-7158-518-4. Clastic dikes formed by downward injection or infilling of sediments from the sole of glaciers are reported from many sites around the world. They are of two main types: till dikes consisting of material from the overlying till; and sorted dikes consisting of clay to gravel that can be massive or laminated parallel to the walls. Dikes have been reported to be as much as 2.5 m wide and up to 20 m long. Subglacial dikes can form by crack and fill, by squeeze-in of till (or other plastic material), or by injection of a water/sediment mixture. All these processes require that the base of the glacier is at the pressuremelting point. However, the substrate could be either frozen or unfrozen, the latter situation probably being most common. Crack and fill is obvious in cases where dikes were formed in hard bedrock, but this mechanism can also operate in unlithified and unfrozen substrate. Squeeze-in of till or other plastic material is driven by the weight and movement of the overlying glacier. Injection of a water/sediment mixture into the underlying host sediment takes place as several successive pulses, with widening ofthe fissure and simultaneous deposition of each lamina. The latter mechanism requires a steep hydraulic gradient into the subglacial sediments. This may be obtained under different glaciohydrological situations. However, near-margin position with pore pressure in the subglacier sediments controlled by low water level outside the terminus may perhaps be the most common situation where such gradients are established. Eiliu Larsen, Geological Suruey of Norway, P.O. Box 3006 - Lade, N-7002 Trondheim, Norway. Jan Mangerud, Uniuersity of Bergen, Department of Geology, Section B, All6gt. 41, N-5007 Bergen, Noru.tay.

The term clastic dike is used for any feature filled with clastic material, and cutting through a host rock. Clastic sill has been used to describe intrusive sheets more or less parallel to the host rock bedding. Any ofthe types ofclastic dikes described below may have a distal end that can be regarded a clastic sill. We thus consider clastic sill to be a sub-type of clastic dikes. Clastic dikes intruded from below or above occur in genetically very different sedimentary rocks (Blatt et aI. 1980, Reineck & Singh 1980). Dike formation has also been demonstrated to have taken place in both consolidated and unconsolidated host rock. In consolidated rocks a fracture must form before the infrlling (e.g. Peterson 1968). If the host sediment is unconsolidated at the time of dike formation, formation may either take place as crack and fill, as squeeze-in of plastic sediment or as injection of a liquid water/sediment mixture, with the latter two mechanisms displacing the host sediment during deposition. In the present paper we describe dikes occuring in glaciogenic environments. Dikes are frequently formed by upward intrusion. However, here we only describe types where intrusion occurred from above. We also restrict the discussion to dikes that originate from basal tills, or otherwise can be proven to have a subglacial origin.

Classification and occurrence Clastic dikes formed beneath glaciers have been reported from many sites (Tab. 1) including Permo-Carboniferous glacial sediments in South Africa (von Brunn & Talbot 1986), Pliocene sediments in Alaska (Armentrout 1983), Pleistocene sediments of the Laurentide, Scandinavian,

Scottishand European4ps ice sheets(e.g.Anderson 1940, Dionne & Shilts 1974,Amark 1986,van der Meer 1980) and Holocenesediments in Norway (Worsley & Alexander 1975). Based on published descriptions (Tab. 1), including sites where we have personal experience (Mangerud & Skreden 1972, Mangerud el al. 1981, Larsen & Ward 1992),we classify subglacial dikes accordingto texture and structures of dike-fill material in two main groups: Till dikes consisting of massive diamicton/till and sorted dikes consistingof clay, silt, sand or gravel. The sorted dikes may either be laminated or massive.Between these end members there are transitional types. Most reports are on till dikes (Tab. 1), implying that this is the most common type. However, we suspect that the other types are overlookedor misinterpretedas e.g. ice wedgecasts, as was the case for some dikes in western Norway (Mangerud & Skreden 1972) reinterpreted as injected dikes by Mangerud et al. (1981).

Till dikes Till dikes are here referred to as downward penetrations of till from a till bed into underlying sedimentsor bedrock. This definition includes till wedgesas describedby Dreimanis (1969,this volume),but it is wider as it also encompassesall clastic dikes of till regardlessof shape. Since Dreimanis (1969)related the orientationof till wedgesto ice movement, there have been many reports on such dikes (Tab. 1). Till dikes had. however.been observedand describedprior to this date (Goldtwait & Kruger 1938,Anderson 1940,Lundqvist 1967).Further evidencefor the re-

163

E. LARSENAND J. MANGERUD

3 m

25

movement, but may be vertical, at least in their upper portions (Dionne & Shilts 1974). The host for a till dike may be any consolidated or unconsolidated material already in place when the site was overridden by a glacier. In most reported cases(Tab. 1), however, the host is a sandy sediment.

Sorted dikes

Fig. 1. Till dike with off-shootesof laminateddikes at Voss.W. Norway after Mangerud& Skreden(1972).Thesewere interpreted as fossil ice wedgesby Mangerud& Skreden(1972),but werereinterpretedas dikesby Mangerudet al. (L981). lation to ice movement is presented by Dreimanis (this volume). Till dikes normally consist of the overlying till, showing moderate to no evidence-of sorting. However, parts of the dike may be sorted (e.g. Amark 1986). This may appear as a dike consisting of a till core with sorted material on both sides, which in most cases was entrained from the host sediment when the dike formed (Fig. 1). Till dikes may also have off-shootesof laminated dikes (Fig. 1). In other casesthe dikes consist of diamicton near the overlying till and gradually become more sorted and frner grained with depth (Humlum 1978). Till dikes vary greatly in length from some 20 cm (Dionne & Shilts 1974) to more than b-6 m (Amark 1986), and in width from a few cm up to, rarely, 2.5 m (Amark 1986).When formed in unconsolidatedsediments they typically taper downwards; the tapering and connection to the overlying till are the two most important criteria to conclude downward penetration of dike fill material. Till wedges(sensuDreimanis 1969,this volume) appear to have a three-dimensionalcrescenticform (Mdrner lg7L, Humlum 1978). They typically dip downglacier (Fig. 2, Dreimanis 1969, this volume, Mtirner 1972, Iiumlum 1978) and strike perpendicular to the direction of ice

t&

Sorted dikes normally range in grain size from clay to sand, but are occassionally gravelly (Mangerud et al. 1981,Amark 1986).They are commonly frnely larninated with either irregular lamina or lenses, or long continuous lamina. When narrow, they may be massive. They taper downwards and their subglacial origin is usually concluded becausethey originate from a basal till. Sorted dikes range from vertical through horizontal (sills), and are often curying with the deep end being horizontal or near horizontal. They are reported to be more than 20 m long (Fig. 3, Mangerud et al. L98l) and up to 1.2 m wide (Amark 1986).The dike boundaries may be very sharp (Fig. 4, Larsen & Ward 1992).A narrow transitional zoneis also reported (Mangerud & Skreden 1972),formed as a result of impregnation of silt or clay in the host sand, thus indicating unfrozen host sediment. Dikes normally taper downwards with several branches and off-shootes (Figs.1,5). Laminations are parallel to the dike (Mangerud & Skreden l97Z,Mangerud et al. 1981,Amark 1986,Larsen & Ward 1992)both in steeply inclined and horizontal dikes (Figs. 4, 5, 6). The individual lamina commonly are thin and discontinuous (Mangerud & Skreden 1972) showing evidenceof erosion during intrusion of individual lamina (Fig. 6). However, 4-5 m long, very regular lamina with thickness from 1 to 10 mm are observed(Fig. 5). Clearly, the laminations proves a repetitive processoperating during formation of these types of dikes.

Formation of subglacial dikes In the following we will frrst discuss the conditions in the substrate: was the host rock frozen or unfrozen when the dikes were formed? Secondly,we consider the conditions in the sourcearea: was the base ofthe glacier at the pressure-melting point or colder? Finally, we discuss three proposed mechanisms for formation of subglacial dikes: Cracking of the substrate and penecontemporaneousor subsequentfilling with till (Dreimanis 1969, this volume)

Fig. 2. Vertical crosg sections of till dikes according to Miirner (1972). Arrow indicate direction of ice movement.

SUBGLACIALLY FORMED CLASTIC DIKES

Table 1. Recorded dikes formed subglacially by downward

penetration into a host'

Features

Site

Author(s)

Till dikes.

Roslin. Scotland.

Anderson, 1940.

Till(ite) dikes.

Icy Bay, Alaska.

Armentrout,1983.

Till dikes.

M illstone Point, Connecticut.

Black. 1983.

Till dikes.

Chaudi€reValley, Qudbec.

Dionne & Shilts, 1974. Elson, 1975.

Till dikes.

Nova Scotia.

Dredse& Grant,1987'

Till dikes.

S. Ontano.

Dreimanis. 1969. This volume.

Till dike.

Sjodalen, Norway.

Garnes & Bergersen,1977. Bersersen& Garnes, 1983.

Till dike.

Svstofte. Denmark.

Humlum.1978.

Till dike.

Godsya, Norway.

l-andvik & Mangerud, 1985.

Till dikes.

SkAne. Sweden.

M6rner.1972.

Till dikes.

Nova Scotia.

Mdrner,1973.

Till dike.

Karakoram, Pakistan.

Owen & Derbyshire, 1988.

Till dikes.

W. Connecticut.

Schafer. 1969.

Till dikes.

Murten, Switzerland.

van der Meer. 1980.

Till dikes, transitionalto sorted dikes. Sorted. laminateddikes. Clastic sills.

SkAne,Sweden.

Amark, 1986.

Till dikes. Sorted, homogeneousdikes' Sorted, laminated dikes. Clastic sills.

North Scituate, Rhode Island.

Birma-q,1952.

Till dikes. Sorted. laminated dikes.

Cranbrook. British Columbia.

Brosteret al., 1979.

Till dikes, transitional to sorted laminated dikes.

Eigeroya, Norway.

Garnes, 1976.

Till dikes. Sorted. laminated dikes.

New Hampshire.

Goldtwait & Kruger, 1938. Kruser. 1938.

Till dikes. Sorted dikes.

Jlmtland. Sweden.

Lundqvist, 1967.

Till dikes. Sorted, laminateddikes.

Voss, Norway.

Manserud & Skreden, 1972.

Till(ite) dikes. Clastic sills.

Natal, S. Africa.

von Brunn & Talbot 1 9 8 6 Talbot & von Brunn 1987 Talbot & von Brunn 1 9 8 9

Sorted, laminateddikes. Clastic sills.

Ovre Setesdal,Norway.

Blystad,1978.

Sorted. laminated dikes.

G6tallvdalen, Sweden.

Hillefors, 1969.

Sorted. laminated dikes

Godcrya, Norway

[:ndvik.

Sorted, laminated dikes.

Skorgenes, Norway.

larsen & Ward, 1992. This paper.

Sorted. larrinated dikes. Clastic sills.

Barstadvik, Norway.

This paper.

Sorted, laminateddikes.

Bersen, Norway.

Manserudet al., 1981.

Sorted. laminateddikes.

Holandsfiord, Norway.

Worslev & Alexander, 1975.

1982.

165

E. LARSEN AND J. MANGERUD

7 1 0 Distance from lhe road

1

3

1

6

or other sediments; squeeze-in of till (Miirner 1972) or other plastic material; and, injection of a liquid water/ sediment mixture (Mangerud et al. I98l). The two latter mechanisms have several similarities with the main difference being that injection or other transport in a liquid phase causes sorting and bedding, whereas squeezing or other deformation of plastic material doesnot.

Frozen vs. unfrozen substrate Birman (1952) reported dikes cutting through both strongly weathered and underlying unweathered bedrock and that the extent ofweathering had no effect on the distribution ofdikes. This suggests that the substrate was frozen during cracking so that both the loose weathered part, which has rheologic properties similar to unconsolidated sediment, and the bedrock had the same properties. In other cases determination of thermal conditions in the substrate is more difficult. Very sharp dike boundaries on dikes deposited from a water/sediment mixture has been taken to indicate frozen substrate (e.g. Amark 1986). The argument is that the frozen substrate prevents water with clay and silt from penetrating the host sediment. We will rather turn the argument around, and state that impregnation along the dike (Fig. 5) indicates that the host sediment was unfrozen, allowing clay-silt laden water to escape(Mangerud et aI. l98L). The sharp angles at some of the bends of the dike at Skorgenes,W. Norway (Figs. 4, 5) could also indicate frozen ground. We assume, however, that both sharp boundaries and sharp-angle bends

166

19m

Fig. 3. Distribution of laminated dikes in one ofthe excavations at Fjosanger, Bergen (Mangerud ef al. t98l; Fig. 7). The dikes at 16 m from the road follow tension cracks in a syncline.

may also be formed in unfrozen sediment if the permeability of the host is low, or if the water content of the injected water/sediment mixture is low. The latter assumption is strongly supported by impregnation along dikes contemporaneous with the sharp angle dikes at Skorgenes. The strongest arguments for unfrozen conditions are small scale folding along the dike (Humlum 1978) and irregular dike course with branching (Mdrner 1972), and the above mentioned impregnation. Field indications of the environment during ice advance have also been used to determine whether the ground was frozen or unfrozen. For example, in most cases when ice advanced into deep water, the substrate was unfrozen. Still, it is very diffrcult to prove that the reconstruction is valid for the time when the dikes were actually formed (e.g. Dionne & Shilts 1974). We conclude that except for the type of evidence presented by Birman (L952), there are no unambiguous crit€ria for frozen ground during dike formation. From a glaciological point ofview it is easiest to postulate an unfrozen subglacial bed. With the load of a thick glacier on the sediment, and if pore water is drained, the internal friction/ cohesion of the sediment will increase. Thus, the shear strength will increase, and therefore also the possibility of forming tension cracks in unfrozen sediments.

Basal thermal regime fill is only deposited beneath ice at the pressure melting point. Thus, both the till dikes and the sorted dikes, which

SUBGLACIALLY FORMED CLASTIC DIKES

Fig. 4. Courge of the main dikes at Skorgenes, W. Norway after Larsen & Ward (1992). Note the sharp corners. Individual lamina could be traced for most of the length of this dike system.

requires water, show that the sole ofthe glacier was at the pressure melting point. We will therefore here consider the conditions for formation of dikes in frozen substrate as there is a conflict between the concluded temperature conditions at the glacier sole and a frozen substrate, and thus this hypothesis has been met with some scepticism. Dreimanis (1969, this volume) interpreted till dikes to have formed by cracking of frozen glound due to glacial drag and penecontemporaneousinjection oftill into these cracks. This theory was adopted by Miirner (1972, L973) and many subsequent workers. Based on basal thermal considerations, Worsley (1973) and Humlum (1978) strongly objected to this idea. They both argued that a glacier overriding permafrost would be frozen to its bed. Consequently, glacier movement would be by internal deformation rather than basal sliding and no friction cracks would develop. Moreover, water and unfrozen debris along the glacier base would be absent. The freld evidence

of Birman (1952) presented above do, however, suggest that dikes can form in frozen ground, and Dreimanis (this volume) may be correct when stating that this is most common for till wedges (sensu Dreimanis 1969, this volume).

Crack and fill Crack and fill is the only possible mechanism in the cases where dikes formed through hard bedrock. Glaciogenic clastic dikes formed through bedrock are reported from at least three sites (Birman L952, Broster et al. L979, Black 1983).Black (1983) presented arguments that fractures in bedrock opened due to pressure release as the overlying ice wasted, and that the fractures were filled with diamicton under stagxant ice. Broster et al. (L979) argued that dikes in bedrock were formed under sliding ice; of the sev-

Fig. 5. Closer view of the dike complex in Figure 4. Taken after the section wall had been cleaned further back. Note the delicate laminations and sharp dike boundaries. Where the lower right dike bends around a stone and the upper branching dike communicate with the main dike, clay and silt have dissipated into the host sand demonstrating that the substrate was unfrozen during dike formation. Note also small scale faulting above the boulder dying out with depth.

l6'7

E. LARSENAND J. MANGERUD

eral sets of fracture orientations, only the one set which was dipping downglacier and was proven to be caused by glacially induced displacement of bedrock blocks, contained glacial debris. Dreimanis (1969, this volume) postulated that glacial drag caused cracking in frozen ground and squeeze-in of till to form till wedges. As discussed above, several authors have opposedto the postulation of frozen ground. We also assume that unfrozen sediments may have high enough

shear strength for cracking to occur. The till dike described by Dionne & Shilts (L974) suggeststhat brittle deformation caused by an overriding glacier occurred in unfrozen sand. The strike ofthe dike was oriented perpendicular to ice movement, and host sediments adjacent to the dike were displaced in a downglacier direction; both demonstrating fracturing and infilling due to the overriding glacier (Dionne & Shilts 1974). The unfrozen state of the host sediment is concluded from the fact that the glacier ended in 100 to 130 m of water (Dionne & Shilts 1974).Also dikes describedby Amark (1986) and Larsen & Ward (1992) are oriented perpendicular to ice movement, and the fractures were probably formed by tension due to basal friction. At the site described by Larsen & Ward (1992), we have demonstrated small scale reverse faulting (Fig. 5) associated with dikes and caused by glacial drag. Also, one of the dikes partly opened as one fissure to its full width and was infilled with inclined sand lamina (Fig. 7). Mangerud ef al. (1981) described laminated dikes that fingered out along tension cracks in a small syncline in sand. In most caseswhere a crack and frll mechanism is proposed in non-consolidated sediments, we assume that this was not a simple process where the crack was opened to its full extent and subsequently frlled. Rather, (some of) the dikes followed pre-existing cracks that became widened by the hydrostatic pressure ofthe injected fluid, or by mechanical erosion of the walls (Mangerud et al. t98L). Probably the widening by the hydrostatic pressure ofthe fluid, or of the plastic material in the case of till dikes, is the most important mechanism, simply becauseerosion of the walls would not give a net increase of the dike volume, only a re-distribution of eroded sediment within the dike.

Squeeze-in of till or (other) plastic sediment

Fig.6A

Cracks with a low angle will only be slowly filled with till and other low viscosity plastic materials if gravity is the only driving force. Thus, till is probably squeezedin by the weight and movement of the glacier (Dreimanis 1969).

Fig. 6. Laminated dikes at Voss, W. Norway (A) (from Mangerud & Skreden 1972), and Barstadvik, W. Norway (B). Note that the lamina in the Voss dike are relatively continuous, but wavy and with indications of erosion between individual lamina. The lamina in the dike from Barstadvik are less continuous and varies more in grain size, but erosion is evident. FiC.6B

168

SUBGLACIALLY

FORMED CLASTIC DIKES

Fig. 7. Close-upofpart ofthe dike at Skorgenesshowinginclined sand lamina that was frlled into a crevassthat openedin the host sand (Neptunian dike at this particular place).Note that the lower sand frll is more frne grainedthan surrounding host whereasupper frll material is coarserthan the sandto the sides.The thin silt betweenthe dike sandand the host sandwas injectedafter the crevassehad beenfilled,

Miirner (L972) proposed that till was squeezedinto unfrozen sediment, without a preceeding cracking, and this interpretation has been adopted by subsequentworkers (e.g. Garnes 1976, Humlum 1978). Evidence cited to be lndicative of the squeeze-in mechanism are irregular dikes with several offshootes and branches, and small-scale deformational structures in the host along the dike boundary (Mijrner 1972,Garnes 1976,Humlum 1978).

kfection

of a liquid water/sediment

of the fissure and simultaneous deposition of each lamina. At the site Skorgenes,we could trace individual lamina for at least 4-5 m (Figs. 4, 5), showing that the main deposi"slow" prograding, tional processis not a but rather simultaneous deposition in much of its length. A key observation is that some of the dikes have a low angle dip or are even horizontal (Figs. 3, 4). These fissures would have been closed by the weight of the overlying sediments under normal hydrostatic pressure. The only "roof' and promechanism known to us that can lift the duce the lamination is injection of liquified sediments with considerably higher hydrostatic pressure than the host sediment, and with a large hydraulic gradient along the dike (Mangerud et al. 1,981).The opening of the frssure probably was a propagation by hydraulic splitting. As described above, the dikes partly follow pre-existing cracks, but this is possibly not always the case. We concludethat the laminated dikes are formed by an injection of sediment laden water downwards into the sediments. However, steep hydraulic gradient downwards is an abnormal situation. All laminated dikes start from or penetrate basal tills. Thus, we conclude that they were formed subglacially (Mangerud et aI. L98L). The hydraulic gradient beneath a glacier is normally downglacier, resulting in a downglacier water flow. The amount of flow through underlying beds is determined by the permeability and thickness of these beds, but this "normal" situation will not provide the necessary conditions for the formation of dikes. However, at the base of a glacier the hydrostatic pressure may vary considerably through time due to changing glaciohydrological conditions, and probably a steep downward hydraulic gradient may be established in several different situations. Here we will only mention the interpretation given by Mangerud et aI. (1981) which in addition to near margin conditions also accounted for stratigraphy and properties of the subglacial sediments. They described a sandy sediment below a nearly impermeable till. In a near marginal situation the pore pressure in the subtill sediment may be controlled by relatively low water level outside the glacier front. Due to low permeability ofthe till, the hydrostatic pressure could be much higher above the till, especially if it rapidly increased.Formation of a crack in the till. will in this situation cause water to spill down into the sand and form a dike. The model of Mangerud et al. (L98L), is probably not the only one accounting for steep hydraulic gradients, but it may indeed be applicable to all sites discussedin this paper.

mixture

The following discussion will be limited to the laminated dikes, but the general conclusionsmay have a wider application. The lamination and the sorting in each lamina show that they were deposited by water. The lamination parallel with the walls demonstrates that each individual lamina had well-defrned boundaries during deposition, since a wider frssure would have resulted in horizontal laminations (Mangerud & Skreden 1972). Thus, the dikes were formed by several successivepulses, with widening

Acknowledgements. The work was frnancially supported by the Norwegian Research Council for Science and the Humanities (NAVF). Figures were drawn by J. Ellingsen and G. Gr9nli. B. Ward read the manuscript critically and corrected the English language. To these persons and NAVF we extend the most sincere thanks.

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E. LARSEN AND J. MANGERUD

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