Water escape fissures resembling ice-wedge casts in Late Quaternary subaqueous outwash near St. Lazare, Qukbec, Canada

GEOF H. BURBIDGE, HUGH M. FRENCH AND BRIAN R. RUST

Burbidge, Geof H., French, Hugh M. & Rust, Brian R. 1988 03 01: Water escape fissures resembling ice- wedge casts in Late Quaternary subaqueous outwash near St. h a r e , Quebec, Canada. Boreas, VOI. 17, pp. 33-40, Oslo. ISSN 0300-9483.

Post-depositional structures in late Quaternary subaqueous outwash near St. Lazare, QuCbec resemble ice- wedge casts but are interpreted as water escape fissures. Cryogenic origin is discounted because, in contrast with ice-wedge casts, the fissures have a higher depth to width ratio, do not form an intersecting network,

and do not exhibit adjacent upward turning of strata. In addition, their truncation by the sub-littoral unconformitydemonstrates formation before regression of the post-glacial Champlain Sea, under conditions in which ground ice development was highly unlikely. The fissures probably formed in response to elevated pore pressures caused by melting of remnant glacier ice or by liquefaction of deeper units. Excess pore water pressure initiated upward flow of dilute sediment-water mixtures that became concentrated in planar zones (fissures) along which they entrained and removed sediment. Slumping of the fissure walls followed, either during or after water escape. Minor faulting over cavities created by melting ice or water escape along fractures in underlying strata may have controlled the morphology of the fissures.

GeofH. Burbidgeand Brian R. Rust, Departmentof Geology, andHugh M. French, Departmentr of Geology and Geography, Uniuersiry of Ottawa, and Ottawa-Carleton Centre for Geosciencestudies, Ottawa, Ontario, Canada, KIN 6N5; 22nd May, 1987

BOREAS

Planar fissures cut stratified late Quaternary sand at high angles in a sand pit near St. Lazare, QuCbec (Fig. 1). They are similar to some of the ice-wedge casts reported from Pleistocene periglacial deposits by Dionne (1971, 1974), Gangloff et al. (1971), Johnson (1959), Katasonov (1973), Kol- strup (1980, 1986), Romanovskij (1973), and others. This paper interprets the structures in a non-cryogenic context, and emphasizes that cau- tion must be applied when similar structures are used in paleoenvironmental reconstruction.

Description of the fissures Few of the lower terminations and none of the upper terminations of fissures were observed, so our data on vertical extent are considerable under- estimates. Nevertheless, observed vertical extents range up to 4.5m. The fissures are variously oriented subvertical to steeply dipping planes, numbered according to their localities (Fig. 2). Fis-

Fig. 1. Location of sand pit (stippled) in which water escape fis- sures occur, near St. Lazare, Qukbec, Canada (UTM 664297 on

Fig. 2. Locations of water escape fissures, numbered from one to nine, indicated along the edge of the pit scarps. Fissure 9 was traced for 30 m along the floor of the pit, as indicated by the solid line on the diagram. Dip and strike symbols refer to the orientation of the most closely adjacent fissure. See Fig. 1 for

Canadian NTS mao 31Gh.1976). UTM reference.

34 Geof H . Burbidge et al. BOREAS 17 (1988)

sures 1,3 and 5 strike northwest, and 2 and 9 strike approximately northeast. The strikes of fissures 4, 6 ,7 , and 8 are unknown. The only observed junc- tion of fissures is that between 3 and 5, which coalesce at an acute angle (c30"). These fissures change strike by about 20" within three metres. Fissure 9 can be followed along the floor of the sand pit for a distance of 30 m; it has an arcuate trend, and varies in strike by 30" from one end to the other. In cross section, most fissures are close to the vertical but a few locally dip as low as 35". Fer- ruginous diagenetic bands cross the fissures, indi- cating that fissuring predates them.

Downturning of strata Most fissures show a downward displacement of strata on either side of a central zone (Figs. 3A and 3B). This displacement, or downturning, extends laterally outward from the central zone to an aver-

age distance of 10 to 15 cm. The downturning is either smooth and regular, increasing evenly from no displacement at the edge of the deformed zone to maximum displacement along the plane of the fissure, or it comprises a series of small, stepped normal faults which generally approximate a smooth curve. In a few cases there is no down- turning associated with a fissure, or the down- turning is asymmetric, occurring on one side only (Fig. 4). In the latter case, the undisturbed strata are on the underside of a fissure where it dips as low as 45".

The microfaults and downturning of the strata near the fissure are one of the most common characteristics of ice-wedge casts reported in the literature on Pleistocene periglacial deposits (Black 1976; French 1976:236-245; Pissart 1970; Washburn 1980). In the case of ice-wedge casts (or casts of secondary infill, in the terminology of Gozdzik 1973) faulting results from a loss of sup-

Fig.3A. PartoffissureZ, about3.5 mfrornthetopofthesection. Downturning is symmetric, with some microfaulting. Sediment is well sorted very fine to medium cross-laminated sand. Scale in

Fig.3B. UpperpartoffissureZ. Notedownturningofbedsadjac- ent to the fissure and small, stepped, normal faults. Scale in dm.

A m

BOREAS 17 (1988) Water escape fissures 35

Fig. 4. Fissure 3, in vertical section. Note absence of downturning of beds on right (lower) side fissure. Fissure dips about 45" at this point. Scale in dm.

port as the ice wedge melts. In addition, Johnsson (1959) reported that asymmetric downturning of adjacent strata is common in ice-wedge casts.

The central zone The width of the central zone of fissures varies from 1 mm to (very rarely) more than 2 to 3 cm, and exceptionally to 6cm, with an average width of about 2 mm. The central zone has some small scale relief in the form of irregular undulations. whose Fig. 5. Central part Of fissure 6. Note 8 CIII thick bed (A) of well

amplitude is less than 3 ci in most cases. fie sedi- sorted, structureless find sand, above which there is slight upturning of the sediments adjacent to a P 1 cm wide centralzone. Below themassive sandbed, strataaredownturnedoneitherside within the 'One is to that Of the

rounding or nearby strata. Where it is thick, the central zone is well laminated to vaguely laminated

of the fissure. See text for explanation of differences above and MOW unit A. Scale in dm.

parallel to the fissure walls. The lower termination of a fissure was uncovered in only one vertical section. There, the fissure gradually tapered to nothing within otherwise undisturbed cross-lami- nated very fine sand. The lateral termination of a fissure was observed in plan view at the eastern end of fissure 3. This was also a gradual tapering of the central zone and adjacent downturned strata.

Clastic dikes associated with fissures Fissures 6, 7, and 8 have a spacing of about 3 m between them and all change in aspect at a specific horizon within the sediments. This horizon is marked by a bed, of varying thickness, of well sorted structureless fine sand (Fig. 5, A). The form of the fissures above this sand bed is that of multiple irregular, subvertical, sand-filled planar sheets. These have either a thin (2-5 mm) central zone with adjacent slight upturning of strata or, more rarelv. are thicker and cut the sediment sharulv

and cleanly, like clastic dikes. For example, fissure 6 continues upward in this manner for 70cm, changing in dip upwards from vertical to 60°, and cuts a bed of convoluted rippled sand (Fig. 5). From that point upwards the fissure is discon- tinuous but thicker (5-10 mm) and dike-like, and consists of structureless fine sand.

Fissure 7 (not illustrated) resembles fissure 6 below the massive sand bed, and above it has avery short (10 cm) section with upturned strata, above which the fissure is thick (2 cm), massive and dike- like, ending abruptly 10 cm above the top of the section with upturned strata. The fissure re- appears 40 cm above this in the form of a straight, sub-vertical, 2 cm-thick clastic dike, which con- tinues upward for more than 50cm. These examples are interpreted to indicate that the water escape fissures changed from a dilute erosional

36 Geof H. Burbidge et al.

form below horizon A (in Fig. 5) to a more dense sediment-water mixture above this level, which deflected the strata in the direction of fluid move- ment by shear drag. This phenomenon is attributed to the permeability of the massive sand unit (Fig. 5, A) which bled off water from the main fissure below. This resulted in decreased pressure and increased density of the fluid within the fissure, and hence caused a difference in fissure properties above the permeable zone.

BOREAS 17 (1988)

Origin of the fissures Similarity to faults The common downturning of strata adjacent to the central fissure resembles drag along a fault (fault drag folds or distributive faulting). Fault-related drag, however, has the opposite sense on either side of the fault. In all cases the deformation associ- ated with the fissures described here has the same sense (downwards) on both sides of the fissure. This, together with the fact that there is no net displacement of strata across the fissure, eliminates the possibility that they originated through faulting alone. The possibility that the fissures are very recent phenomena, resulting from relaxation or stress release during excavation, can also be dis- missed, because the fissures neither offset the dia- genetic ferruginous bands, nor run parallel to the face of the excavation. In addition, one fissure is truncated by the sub-littoralunconformity (Fig. 6).

Evidence for and against thermal contraction cracking Our current understanding of ice wedges indicates that only a few years of permafrost conditions are necessary for polygon networks and incipient ice wedges and veins to develop (Mackay 1986). Ther- mal contraction cracking usually occurs in areas of continuous permafrost (Black 1976; Mackay 1974, 1975). Seasonal frost cracking may also occur in non-permafrost areas due to severe winter freezing (Washburn et al. 1963; Svensson 1977). The fis- sures at St. Lazare, if formed by thermal con- traction cracking, would represent only the early stages of wedge development because of their nar- rowness and the fact that they do not form a well- developed polygonal network.

Many workers have postulated that following deglaciation a period of severe climatic cooling

Fig. 6. Top of fissure 9, truncated by the erosional unconformity which represents the lower limit of sediment reworking by littoral processes in the Champlain Sea. As discussed in text, this clearly indicates that fissure formation predated emergence. Angular dark clasts are rip-ups of clayey silt. Shovel handle 12 an across.

occurred in the Ottawa-St. Lawrence Lowlands, and that periglacial conditions either persisted well into the Holocene or returned after a period of climatic amelioration (Davis 1969; Dionne 1971; Gangloff 1970; Lagarec 1973; Lasalle 1976; Rich- ard 1971; Poltzer 1953; Terasmae 1959). Pleis- tocene periglacial features were reported from southern QuCbec by Dionne (1974), Gangloff (1970), and Lagarec (1973). Although these reports have been disputed (Andrews 1973; Dionne 1974; Hughes 1965; Prest 1970), it is poss- ible that the St. Lazare area may have been exposed to a few hundred years of periglacial con- ditions at some point after regression of the Cham- plain Sea.

Several considerations suggest that the fissures are not cryogenic. First, with a depth range of up to more than 4.5 m, they are deeper than most ice wedges (Black 1976,1983; French 1976; Harry et al. 1985). Ice wedges are rarely more than 4 m deep; the depths of seasonal frost cracks, as re- corded by Washburn et al. (1963) are less than 1 m. In the Mackenzie Delta, for example, they are rarely more than 2.5 m deep (Mackay 1986). In addition, ice wedges usually occur only in ice- rich fine grained sediments (silts and clays). The St. Lazare fissures are in sand, although finer sediments are present in the local succession. In

BOREAS 17 (1988) Water escape fissures 37

modern permafrost regions, ice wedges form in silt and clay much more readily than in clean sand. In the Western Canadian Arctic and Siberia, for example, wherever frost wedges occur in coarse material they are invariably better developed in nearby fine grained deposits (Leffingwell 1919; Romanovskij 1977; French et al. 1982; Harry et al. 1985).

According to Black (1975:ll) single wedges are suspect anywhere. No other frost cracks or wedges have been reported in the Vaudreuil area (S. H. Richard, pers. communication 1983), and Dionne (1974) notes that ice-wedge casts have not been observed in areas previously submerged by the Champlain Sea. A further consideration is that frost wedge networks tend to develop, especially in the initial stages, with near-orthogonal inter- sections between wedges (Mackay 1974, 1975, 1985). Only two out of the nine fissures at St. Lazare were observed to meet, and they coalesce at an acute angle. Even if they represented the very earliest stage in the development of a frost wedge network, a non-orthogonal intersection of fissures does not fit the generally accepted morphology of frost wedge networks (Lachenbruch 1962).

Probably the most significant site-specific evi- dence against frost cracking at St. Lazare is that the top of one fissure is truncated by overlying wave- reworked sediments (Fig. 6). This indicates that the fissures predate the regression of the Cham- plain Sea, and restricts frost crack development to a hypothetical brief period during regression of the Champlain Sea when water was shallow enough to permit development of winter sea ice from the water surface down to the top of the sand, but before the sand surface was at wave base. In our opinion this is very improbable, because sea ice is rarely thick enough to extend below wave base, and because ice would have insulated the sand from the large and sudden temperature drops necessary to initiate thermal contraction cracking.

Origin of the fissures by water escape Water escape structures form in unconsolidated sediments as a result of expulsion, generally upward, of excess pore water accompanied by readjustment of all or part of the sediment into a more stable and tightly packed fabric. The escap- ing pore fluid tends to be localized within the sedi- ment, creating zones in which it moves at relatively high velocity, which can entrain sediment and gen- erate water escape fissures, or pillar structures

(Lowe & LoPiccolo 1974). This process is termed fluidization, defined by Lowe and LoPiccolo as a process in which ‘the individual sediment grains become partially or entirely supported by the drag of upward moving water’. This is distinct from liquefaction, where ‘loosely packed, metastable grains are shaken loose from each other and briefly settle unsupported through the pore fluid’. Kunii & Levenspeil (1969) defined a point at which the upward velocity of the pore fluid becomes high enough to entrain the sediment grains, thus ini- tiating fluidization. Fluidized intrusions are char- acterized by turbulent flow (Lowe 1975), and a low sediment-water ratio (Gill & Kuenen 1958). At lower velocities, fluid escape takes place by seep- age through the sediment, which remains undis- turbed. The grain size of the sediment is the most important factor controlling its susceptibility to fluidization, fine grained sand being most easily fluidized (Lowe 1975). Fluidization is most likely to occur in sands deposited at high rates of sedi- mentation because of high initial pore water content. Structures resulting from fluidized inclusions are clastic dikes and sills (Lowe 1975).

Lowe (1975) defined five different types of water escape pillars. All include both cylindrical and planar forms. Cylindrical pillars (or water escape pipes, Plint 1983a), differ from planar, or sheetlike, pillars in morphology only, not in gen- esis or internal structure. Lowe’s second, or ‘B’ type pillars include large (up to ‘many’ metres), planar vertical and subvertical forms. Internally, the pillars may be massive or faintly laminated. Peterson (1968) noted that lamination parallel to the walls is common within clastic dikes.

Lowe (1975:173) stated that ‘the upward flow of sediment and water within fluidization channels is commonly accompanied by a more or less continuous subsidence of surrounding grain- supported sands . . . and occasionally by slumping of the pillar walls . . . similar to spouting beds of industrial fluidization systems’. Two of Lowe’s figures (1975: Figs. 6a and 6b) illustrating type B pillars show downturning ofithe sedimentsimmedi- ately adjacent to the pillar walls, in a manner simi- lar to the St. Lazare fissures. Plint (1983a, b) described downturning in fluidization pipes in Eocene sands. He noted that downturning occurs as a series of microfaults (Plint 1983a: Fig. 7) and described collapse of adjacent bedding into the fluidized core of a dewatering pipe (Plint 1983b: Fig. 6). Gill & Kuenen (1958) illustrated down- turning of beds at the marein of DiDes of sand vol-

38 Geof H. Burbidge et al. BOREAS 17 (1988)

canoes (which are type B pillars of fluidization channels by Lowe’s definition).

It seems most likely that the fissures at St. Lazare are a form of water escape feature. Their intimate association with definite water escape structures (exhibiting upturned strata and clastic dikes) is strong circumstantial evidence. We believe, there- fore, that downturning of the beds resulted from failure of the fissure walls due to removal of under- lying water and sediment as a result of upward movement of escapingpore water. This flowwould have been relatively dilute, and failed to support the adjacent fissure walls. At the end of the water escape event, further slumping of the fissure walls occurred, completely filling the space formed by the removal of the slice of sediment within the cen- tral core. Where the fissures dip at angles sig- nificantly less than 90°, the beds on the lower side of the fissure were supported from below and slumping occurred only on the upper side. For- mation of the narrow vertically laminated central core now visible in places would have occurred at this point during residual, more viscous, fluidized flow.

The high pore water pressures necessary to drive the water escape process may have been provided by remnants of glacier ice buried at some depth within the sediments (beneath the fissures). This would have charged the sediments with high pore water pressures. The existence of buried ice is not thought to be a requisite for the formation of these fissures, however, because liquefaction of deeper sand units could also have raised the pore water pressure in the overlying sediments. Liquefaction could have been spontaneous, or could have been triggered by seismic activity within the Ottawa- St. Lawrence graben system (Kumarapeli & Saul1 1966). Upward relief of this pressure might have taken place first by seepage, then by concentration of seepage flow into planar channels, possibly along minor faults. The velocity of seepage flow eventually surpassed that necessary for fluid- ization, and formation of the water escape fissures was initiated.

In view of the relative rarity of large planar de- watering structures reported in the literature, the question arises as to why the release of excess porewater pressure took place at St. Lazare by way of planar rather than the more common cylindrical forms. Although there is little or no total displacement of the strata across the St. Lazare fissures, stresses favoring the development of minor faults within the sediment may have

controlled their formation. Such stresses could have existed over cavities within the sediment formed from melting of buried ice blocks or, simply, by volume loss during water escape itself.

Thorson et al. (1986) attributed fissures in gla- cigenic sands in eastern Connecticut, U.S.A., to liquefaction and water escape, although they were formerly identified as ice-wedge casts. Features similar to those described in this paper occur in well sorted fine glacial outwash at West Runton, Norfolk, England. Previously regarded as ice- wedge casts, they have been re-interpreted recently as dewatering structures (Charles Cruickshank, pers. comm. 1986). According to Cruickshank, dewatering was initiated by ground- water movement ahead of a subsequent (Anglian) ice advance.

Conclusions The recognition of ice-wedge casts in Pleistocene sediments is an important aid in paleoenviron- mental reconstruction, because they imply the for- mer existence of perennially frozen ground, or permafrost. The fissures at St. Lazare show some similar phenomena, but other, more diagnostic features resulted from fluid escape mechanisms in a subaqueous environment. Black (1983) has described such features as pseudo ice-wedge casts.

Understanding the genesis of ice wedges and water escape fissures provides criteria for their dif- ferentiation in cast form. Ice wedges grow laterally by increments, physically displacing and com- pressing adjacent sediments. This is accommo- dated by an upward displacement of strata, sometimes accompanied by folding of beds. In con- trast, water escape fissures form by the removal of a slice of sediment (and probably the ejection of material at the surface). No upward folding of strata adjacent to the fissures can form. If observed, such a feature would eliminate water escape as the formative process. However, this flexure of the strata may not always survive the subsidence and downfaulting accompanying melting of an ice wedge.

The similarities in micro-morphology between water escape structures and ice-wedge casts may be such that regional and sedimentological studies are necessary to distinguish between them.

Acknowledgements. - The authors thank Gkrard Chevrier for unlimited access to the Chewier sand pit. Very able field assist- ance wasprovided byL. P. Bennett. Webenefitedfromdiscussion

BOREAS 17 (1988) Water escape fissures 39

with D. R. Sharpe, J. Shaw, and L. P. Bennett, and review of an earlier version of the manuscript by D. R. Lowe. We thank S. H. Richard(Geologica1SurveyofCanada) fordrawingthe fissures to our attention, and the Natural Science and Engineering Research Council of Canada for financial support.

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