LATE PRECAMBRIAN MOELV TILLITE DEPOSITED ON A DISCONTINUITY SURFACE ASSOCIATED WITH A FOSSIL ICE WEDGE, RENDALEN, SOUTHERN NORWAY

JOHAN PETTER NYSTUEN

Nystuen. J. P.: Late Precambrian Moelv Tillite deposited on a discontinuity surface associated with a fossil ice wedge, Rendalen, southern Norway. Norsk Geologisk Tidsskrift, Vol. 56, pp. 29-56. Oslo 1976.

East of SjØlisand in Rendalen the Moelv Tillite rests on the Ring Sandstone with an erosional contact. wedge structure in the sandstone just beneath A the Moelv Tillite is interpreted as a fossil ice wedge. A conglomerate, con­ sidered to have been deposited as a glaciofluvial grave! in a proglacial en­ vironment, occurs at the base of the Moelv Tillite. tillite sheet within the A conglomerate is thought to be a flow till deposit. The clast content of the Moelv Tillite indicates a glacial transport from the S or SW. The origin of the discontinuity surface is considered to be related to a eustatic lowering of sea leve! prior to glaciation of the region. glacial deposition from a A grounded ice sheet is discussed, likewise the palaeoclimatic implications of periglacial phenomena in the Late Precambrian tillites of northern and northwestern Europe.

J. P. Nystuen, Institutt for geologi, Norges Landbrukshøgskole, 1432 Ås-NLH, Norway.

Since Holtedahl (1922) proposed a glacial origin for the Moelv Tillite, it has been generally accepted that this sedimentary unit originated by the rafting of till debris with floating ice (Spjeldnæs 1964, Bjørlykke 1966, 1974, Løberg 1970, Englund 1972, 1973). This interpretation rests essentially upon the observations, made in several areas, of a gradual transition from the underlying Ring Sandstone or shale equivalent upwards into the Moelv Tillite (Holtedahl 1922, 1953, Skjeseth 1963, Bjørlykke 1965, 1966, 1974, Englund 1972, 1973). However, recent investigations by the author in the eastern sparagmitc basin have revealed that in addition to subaqueous mudflow beds and glaciofluvial deposits, the Moelv Tillite includes both a clast-rich facies deposited as till from a grounded ice sheet and laminated shale beds with ice-rafted outsize stones. The object of the present paper is to describe a facies of the Moelv Tillite resting with a discontinuity contact upon the Ring Sandstone, together with an associated clastic wedge structure, and to discuss the sedimentolog­ ical and palaeoclimatic significance of the tillite and its contact relations. The field work was carried out during the summer of 1974. A brief report of the results has been given earlier (Nystuen 1975a). 30 J. P. NYSTUEN

Geological setting and stratigraphy The locality in which the contact between the Ring Sandstone and the Moelv Tillite is observed, is located at Sjølisand in Rendalen, east of Stor­ sjøen (Fig. l). The geology of the area was described by Holmsen (1966), and the bedrock has later been remapped by Nystuen & Sæther (Fig. 1). The sedimentary rocks, belonging to the Late Precambrian Hedmark Group (Bjørlykke et al. 1967) and the Cambrian, have been deposited in the

Sjoli- + sand + + + + + + + + + + + + � + 9- + .$'. ;,'ei. + �1 + + + 1 km

. l·

Thrust plane below the Ekre Sh ale Kvitvola nappe

Moelv Tillite Minor thrust plane ••• • •• Thrust plane below alloch­ ••• Osdal cgl. } thonous Hedmark Group Ring . Format ion D. Sandstone Cambrian Shale

Pre<:ambrian crystall ine roe ks GJ Vangstls Formation Fig. l. Geological map of the area east of the southem end of Storsjøen, at Sjølisand in Rendalen (for location, see key maps). Northeastem corner of the map sheet Even­ stad 1917 I, mapped by J. P. Nystuen and T. Sæther in 1972. The locality in which the contact between the Moelv Tillite and the Ring Formation is exposed west of Fuglåsen is marked with a circle. PRECAMBRIAN MOELV TILLITE IN RENDALEN 31

w��;_����------�L�-�-/;·;��b�200-30 0m ' ...... · . \ ...... ·. ·. .:.·. . . · ..· .·...·· · Sa ...... -...... ·. : · 1 ...... \ '-..-:� ..... ',:..:,·�·�· 100 150 .... "-/ 4 - m ', ' ::: ' ,...... I"V ' :: ..... ;..f" ', • • • • • ' . . .•. 3 - 15 m A .•.•.•:•:•=-·:•. 2 b O-30m5 \ . \ . ' . \ ' • • min.300 ' • m ' • • \ • 2a \ • \ \ \ • \ B Fig. 2. Simplified stratigraphical columns of the Late Precambrian and Cambrian sedi­ mentary strata deposited on the crystalline basement (l) and (B) at the western (A) margin of the eastern sparagmite basin (units represented within map of Fig. 1). The Hedmark Group (2-5): (2) Ring Formation, (2 a) arkose and feldspathic sandstone, (2 Osdal Conglomerate Member; (3) Moelv Tillite; (4) Ekre Shale; (5) Vangsås b) Formation, (5 a) Vardal Sandstone Member, (5 b) Ringsaker Quartzite Member; (6) Cambrian shales.

western part of the eastern sparagmite basin and partly upon the adjacent Precambrian basement (Fig. 2). This basement forms a narrow ridge of crystalline rocks (granite, a pli te, gabbro) separating the eastern basin from a larger, western one (Fig. 1). The sandstones and the Osdal Conglomerate Member (Holtedahl 1921:31, Holmsen 1956:50) of the Ring Formation are considered by the author to be mainly of fluvial origin, deposited in extensive alluvial fans and plains. The Moelv Tillite is a clast-rich, grey or red-brown facies, interpreted as having been deposited from a grounded ice sheet. At Andrå, 20 km N of Sjølisand, this facies is overlain by a clast-poor tillite (Holmsen 1956:30), probably of subaqueous origin. Ekre Shale, a postglacial formation, consists of red and greyish-green silty shale with interbeds of fine-grained sandstone. In the present area, the contact to the Moelv Tillite is either soil covered or tectonically dis­ turbed. A radiometric age determination has given a minimum age of 630 m.y. for the Ekre Shale (Rankama 1973:91). A new sedimentary cycle started with the deposition of the coarse-grained, feldspathic Vardal Sandstone Member (Fig. 2). By the end of this cycle, the Precambrian rock terrain surrounding the basin was reduced to a peneplain, on which the uppermost part of the Ringsaker Quartzite Member was subsequently deposited during a transgression. This unit is succeeded by fossiliferous Cambrian shale (Fig. 2). 32 J. P.NYSTUEN

B

m 1 o

5 N .. :.··· ·· . : . : : ......

o o 5

Se a le of Figs. A & 8

A

Fig. 3. Profiles at the locality west of Fuglåsen showing the contact relations between the Ring Sandstone and the Moelv Tillite and the main lithological features of the formations. Fold axes of folds in profiles A and B c. N 80g lOg E and of folds in profile C c. N 375g 15g N. Profiles A and B are vertical projections of the lower (A) and upper (B) parts of the hillside; their locations are shown in the vertical section through the folds of profile C. Note the cross bedding at locality for Fig. 8. PRECAMBRIAN MOELV TILLITE IN RENDALEN 33

Top of Fig. B c

Bottom B of Fig.

Top of A . Fi . _, ·. .. . ·: .. g . . .

. · ..

w L------�------� O 2 5 50m

s

lnterpreted lithologic border (and eroded) Discontinu ous ly exposed lithologic border c Continuously exposed lithologi border Ti l lite } Moe lv Ti l lite Poor ly sorted conglomerate

F:"'"'l Ring Sandstone; bedding plan e & L::..:..W laminati on indicated 34 J.P.NYSTUEN

Fig. 4. Weakly stratified and poorly sorted conglomerate of glacio-fluvial origin at the lower part of the Moelv Tillite. Note the presence of pronounced spherical and well­ rounded quartz pebbles among the generally poorly rounded detritus. Section vertical to stratification. Diameter of coin: 2.9 cm.

During the Caledonian orogeny, the basin-deposited sequence was partly thrust beyond the original margins of the basin; in the present area the thick basin sequence (Fig. 2B) was thrust above the thin autochthonous sedimentary units (Fig. 2A). The allochthonous sedimentary rock sequence was subsequently folded with one fold axis running NE-SW and one NNW­ SSE. Severe tectonic deformation has occurred locally along thrust planes, but primary sedimentary features are otherwise generally well preserved. Erosional remnants of the Kvitvola Nappe, consisting of Late Precambrian metamorphic sandstones, are present as small outliers (Fig. l). The struc­ tural history of the eastern sparagmite basin has been outlined by Nystuen (1975b).

Lithology of the Moelv Tillite and its contact to the Ring Formation The contact between the Moelv Tillite and the Ring Formation is exposed on the hillside west of Fuglåsen (Fig. l). The beds are folded along the area's two main fold axes, NE-SW and NNW-SSE (Fig. 3). The Ring Formation consists of a medium- to coarse-grained well-sorted sandstone in 30-200 cm thick beds. (The formal name Ring Sandstone is used below.) A faint, parallel lamination is present in most beds, and tabular cross bedding is developed in others. Well-rounded and highly PRECAMBRIAN MOELV TILLITE IN RENDALEN 35

Fig. 5. Lithology of tillite bed occurring within the conglomerate at the lower part of the Moelv Tillite (see Fig. 3). Note the presence of well-rounded quartzite pebbles and cobbles with spheroidal shape together with mostly angular to subangular clasts of granite and aplite. Section subparallel to stratification. Diameter of coin: 2.9 cm. spherical 'floating' quartzite stones up to 7-8 cm in diameter occur widely dispersed throughout the beds. The Ring Sandstone is disconformably overlain by a conglomerate. The boundary cuts the bedding planes, the parallel lamination and the cross bedding, and thus it is evidently of erosional origin. Detailed mapping along about 200 m of the contact surface indicated that the erosion was essentially confined to the uppermost preserved bed of the Ring Sandstone. Locally the erosion appears to have penetrated 3-4 m below this stratigraphical level (Fig. 3). The conglomerate is massive to weakly stratified, and poorly sorted (Fig. 4). The lowermost 0.5-1.0 m of the conglomerate is dominated by granules and pebbles smaller than 2 cm. Perthitic microcline and granite fragments comprise up to 40 % of this fraction. In the westemmost part of the locality (Fig. 3A) a grey, practically un­ sorted, tillite-like rock (Fig. 5) occurs within the conglomerate in a bed whose thickness increases eastwards. At about 400 m from its westemmost outcrop, this bed seems to grade into the clast-rich, main facies of the Moelv Tillite (Fig. 6), though the junction itself is not exposed. Nevertheless, the field relations strongly suggest that this interbed within the conglomerate is indeed a tillite sheet, wedging out laterally away from the 10-15 m thick unit of the clast-rich tillite facies. That part of the conglomerate lying above the tillite bed in the west (Fig. 3A) probably thins out eastwards. 36 J. P. NYSTUEN

Fig. 6. Lithology of the clast-rich, ordinary facies of the Moelv Tillite. Angular to subangular clasts of granite, aplite, and felsite occur along with dominantly rounded and well-rounded clasts of vein quartz and grey quartzite. Elongate fragments tend to be preferentially orientated with their apparent long axes in a NE-SW direction. Section nearly vertical to the stratigraphical boundary. Diameter of coin: 2.9 cm.

According to the stratigraphy and structural geology of the area, either tillite or Ekre Shale overlies the conglomerate in the westernmost part of the area. The tillite interbed exhibits a slight deficiency of the finer fractions as compared with the main unit of the tillite rock (Figs. 5, 6), but its lithology appears relatively homogeneous. The contact between the tillite bed and the adjacent conglomerate is usually transitional within a zone of 5-15 cm. In the upper contact zone a faint, discontinuous stratification is locally developed within the tillite. The tillite bed clasts, up to 40 cm in diameter, are preferentially orientated with their a-b planes parallel to the bedding surface. Elongate clasts in the main tillite unit tend to be orientated with their longest visible axes in the NE-SW direction (Fig. 6), but in this locality the a-fabric is probably of tectonic origin. The observations presented above prove that the conglomerate which lies above the discontinuity surface must be part of the Moelv Tillite. The coarse detritus in the conglomerate has been transported and deposited by running water whose hydraulic properties corresponded to the upper flow regime. Its intimate association with the tillite facies proves that glacial conditions prevailed in the depositional area, and hence the conglomerate is thought to be a glaciofluvial deposit. Furthermore, from the differences in sedimentary features between the Ring Sandstone and the fluvial con- PRECAMBRIAN MOELV TILLITE IN RENDALEN 37

glomerate of the Moelv Tillite, it must be concluded that the discontinuity surface marks an abrupt change in the physical parameters of sediment transport and deposition.

Source rocks of the Moelv Tillite and direction of glacial transport The conglomerate and the tillite contain clasts from the same lithologies, though in unequal amounts (Fig. 7A).

1 o o% �::""7'T.-:-:-"T":""':"7":'1 . Grey quartzite ·.; :. � :. : . :: .. . : . 90 Vein quartz Apt i te & felsite 80

Granite 70 Gabbro 60 Alkali fetdspar . . 50 : : ·.:.: -�. � 8 60 °/o 40

50 congtomerate 30 40 30 20 20 1 o 10 o ASASRRWR A SASRRWR O n 220 143 189 n= 110 n= 85 A 1 2 3

60 .,. tit lite 50 40 30 20 10 o AASASRRWR A S ASR R WR A SASRR WR n= 184 n:58 n=110 Fig. 7. Clast and clast roundness in the Moelv Tillite. A. Frequencies of clast in the conglomerate (1), in the thin tillite bed within the conglo­ merate (2), and in the tillite of the main tillite unit (3). B. Roundness of the various clast lithologies in conglomerate and tillite (total from

2 & 3). A = angular, SA = subangular, SR = subrounded, R = rounded, WR = well­ rounded (Pettijohn 1957:59). 38 J. P. NYSTUEN

Clasts of grey quartzite and vein quartz are characterized by a high degree of roundness in both the tillite and conglomerate (Fig. 7B). The quartzite clasts are ellipsoidal in shape while the well-rounded quartz pebbles possess a high sphericity (Figs. 4, 5). Some quartzite types have preserved remnants of their clastic texture, but the majority are metaquartzites which have suffered complete recrystallization with the growth of muscovite flakes and epidote crystals. None of the types are known from the local Precam­ brian basement along Storsjøen. The less metamorphosed quartzites may have been derived from the Trysil-Dala Sandstone which at present out­ crops to the east of the eastern sparagmite basin. The nearest known meta­ quartzite sources are located in the Solør - Kongsvinger - Romedal districts, 80-140 km south of the basin (Hjelle 1960, Gvein 1967, Nystuen 1969a, Gvein & Sverdrup 1973, Gvein et al. 1973) and in Varmland and Dalarne in Sweden (Magnusson et al. 1958). Granite, aplite, and gabbro form another cleast group which is character­ ized by pronounced angularity (Fig. 7B). All of these rock types are common within the crystalline basement along the eastern side of Storsjøen. F elsite may be petrogenetically related to the aplite or to quartz porphyries, a few pebbles of which have been recognized. Volcanic rocks of these lithologies occur in the Trysil area east of the basin (Holtedahl 1921) and in the El­ verum district 70 km south of it (Nystuen 1969b). Within the basement rocks of the Atnasjø window north of the western sparagmite basin, quartz porphyries form the granite border facies (Ofte­ dahll952:11), and the same has been observed by the author in the Trysil area. Consequently, one cannot exclude the possibility that the quartz porphyry and felsite clasts may be mutually related to a common source rock within the local granitic basement. The present conclusion is that the debris material of the Moelv Tillite consists of two subpopulations: The clasts of grey quartzite and in part those of vein quartz representing distant source rocks and probably of multicyclic origin. The clasts of igneous origin derived from the local crystalline basement south and west of the basin and which underwent only one sedimentary cycle, mainly by glacial transport. The clast assemblage thus indicates that the glacier ice, from which the Moelv Tillite of the present locality was deposited, covered the basement ridge between the eastern and western sparagmite basin and moved in a northerly or northeasterly direction. The higher proportion of quartzite and quartz clasts in the conglomerate than in the tillite rock (Fig. 7A) may reflect different source areas of the two facies, or alternatively an irregular distribution of various types of terrigeneous detritus within a common source area. A third possibility is that the conglomerate material represents reworked till where the most easily comminutable clasts have been broken down by powerful glaciofluvial streams. The high content of microcline grains and granite fragments in PRECAMBRIAN MOELV TILLITE IN RENDALEN 39

the granule and coarse sand fractions of the conglomerate may favour this suggestion.

Fossil ice wedge In the southwesternmost part of the locality (Fig. 3A) a wedge-shaped, clastic dyke of coarse-grained detritus occurs in the uppermost bed of the Ring Sandstone, just beneath the erosional surface at the base of the conglomerate belonging to the Moelv Tillite (Fig. 8A). The wedge structure is tectonically cut by a minor shear-plane, and the two segments are dis­ placed c. l cm relative to each other. The wedge, being exposed in a ver­ tical section and on a smaller subhorizontal surface, is orientated almost perpendicularly to the bedding plane and the erosional contact, and runs in an E-W direction. There are no structures indicating soft sediment deforma­ tion of the contact surface between the formations. The wedge is 5 cm wide at the top of its exposure and thins downwards to c. l cm at a depth of 60 cm. At this level the wedge structure is diffuse and difficult to outline, in part due to the disjointing of the bedrock here. However, there is no sign of any rewidening and downward continuation below c. l m from the top. The wedge structure is therefore presumed to

lO cm

..'· .

Fig. 8. A. Wedge structure (outline) in the Ring Sandstone (RS), just below the contact of the conglomerate at the base of the Moelv Tillite (MT). Visible part of stick 68 cm. B. Interpretation of the contact relation between the wedge structure and the overlying conglomerate. The deformed pattern of cross bedding is schematized from structure visible on polished slabs and acetate peels (see Fig. 9). 40 J. P. NYSTUEN

Fig. 9. Acetate replica of the contact zone between the Ring Sandstone and the coarser­ grained wedge structure. A faint cross bedding, defined by alternating laminae and thin layers of slightly different content of feldspar (light) and quartz (black), is bent down­ wards against the wedge structure. Note the transitional boundary between sandstone and wedge structure. close at a depth of about 70 cm. Detachment and removal of large blocks from the bedrock at the top of the sandstone prevent inspection of the contact between wedge and conglom:erate (Fig. 8A). However, the wedge structure has not been observed to penetrate the conglomerate, and the interpretation of the junction is as shown in Fig. 8B. The boundary between the wedge and the adjacent sandstone is irregular and transitional, seen on a small scale. The weakly developed cross bedding of the sandstone is cut by the wedge, and the laminae are bent downwards along the contact (Fig. 9). Sand grains from the sandstone have inter­ mingled with the coarser wedge material along the wedge walls (Fig. 9). This coarser fraction consists of coarse sand grains, granules, and pebbles smaller than c. 10 mm. It is composed of quartz, microcline perthite, and some granite and quartzite fragments. Elongate particles display a pro­ nounced vertical fabric of their longest visible axes (Fig. 10). The coarse grains have evidently fallen into a wedge-shaped fissure in the cohesive but unlithified sand deposit from the overlying gravel bed. In sedimentary sequences, dyke- and wedge-shaped clastic bodies pene­ trating bedding planes can originate in many ways. Several such conceivable modes of origin (desiccation crack, intrusive dyke, tectonic fissure, fissure formed by settling or landslide) have been appraised but rejected as very unlikely on account of the form and textural composition of the wedge structure and its palaeoenvironment on a nearly horizontal sand plain. PRECAMBRIAN MOELV TILLITE IN RENDALEN 41

N S o- -o

/ ' 50 50

l 100

Fig. 10. Fabric diagram showing orientation of the apparent long axes of sand and

granule grains in the clastic wedge structure, measured on a vertical section. n = 100.

The wedge is very similar to structures presently being formed as thermal contraction-cracks in regions with very cold climates. Reviews on classifica­ tion and discussions on the origin of such structures have been given by Mangerud & Skreden (1972), Katasonov (1973), Mackay & Black (1973) and Romanovskij (1973). During the cold winters in periglacial areas with perennially frozen ground, thin wedge-shaped and vertical fissures may be opened in the ice­ cemented ground in response to tension induced by thermal contraction. Crack widening in any one winter is in the range about 0.5 to 5 mm (Lachenbruch 1960, 1962, Pewe et al. 1969, Black 1973). In early spring the cracks are filled from above with melt water which then freezes, sealing the cracks below the thawed layer. During the next winter the ground tends to fracture a1ong the previously formed ice-filled veins, and another supp1y of ice is added in the spring when surface water flows into these cracks and freezes. By several such cycles, repeated during a number of years, ice wedges are formed. By intersecting, ice wedges produce a po1ygonally pat­ terned ground. When the permafrost thaws and the ice wedges melt, the fis­ sures are filled in by clastic material from above and from the sides, thus resulting in fossil ice wedges. According to Johnsson (1959: 15) a true fossil ice wedge may be identified by the following features, seen in a vertical section: the filling material ap­ pears to have come from above; the sides usually dip downwards; the stones in the wedge are vertically orientated; the wedge structure must be widest at the top; the wedge should be more or less vertical. To this list other evidence should be added: a breadth:depth ratio of 1:10 or less (John 1973:19 1), and the bedding in the adjacent sediment is deformed and usually downwarped along the wedge walls (Pewe et al. 1969:63). 42 J. P. NYSTUEN

Glaciofl uviat currents

2 P�r m af rost

Activ� tayer

1 P�rmafrost

Fig. 11. An interpretation of the sequence of the fossil ice wedge development in the Ring Sandstone beneath the Moelv Tillite's basal conglomerate.

This feature is well developed in many Pleistocene fossil ice wedges (e.g. Svensson 1964, Hillefors 1966, Donner et al. 1968, Bergersen & Pollestad 1971, Mangerud & Skreden 1972). All the criteria on fossil ice wedges referred to above are in accordance with the observed features of the present wedge structure. In areas with perennially frozen ground, clastic wedge structures can also originate as ground wedges, which are thermal tension cracks having been filled primarily with sediment (Dylik 1966:282), normally by eolian sand in arid Arctic climates in front of glaciers (Pewe 1959, Black 1973, Romanovskij 1973:258). The textural composition and structures of the present wedge exclude this mode of origin. In regions with seasonally frozen ground, frost cracks may be opened in the ground and filled partly or completely with ice (Washburn et al. 1963, Dylik & Maarleveld 1967). During spring, meltwater may carry fine sand and silt into the cracks before the ground thaws (Dylik 1966:248, Pewe et al. 1969:49). By repeated recurrence of the cycle within one and the same initial fissure, the clastic wedge grows in width and depth and may PRECAMBRIAN MOELV TILLITE IN RENDALEN 43 give rise to an internal vertical lamination (Dylik 1966). Though the dimen­ sions of the present wedge correspond to those of some Pleistocene clastic wedges considered to be seasonal frost cracks (Dionne 1975), neither the structural nor the textural features of such sediment-filled cracks compare very well with the observations of the wedge structure in the Ring Sand­ stone. The conclusion to be drawn from the above discussion is that the most probable interpretation of the wedge structure is that it represents a true fossil ice wedge. The major objection to this interpretation is that only one single wedge structure has been observed. This may be due to the scarceness of exposures or to primary causes such as erosion of the wedge­ bearing sand horizon or internal structural and textural rearrangement during compaction. Fig. 11 illustrates a possible sequence in the development of the fossil ice wedge. It developed in the topmost bed(s) of the Ring Sandstone with subsequent erosion of the active layer by glaciofluvial currents, depositing a gravel bed. When permafrost thawed, the wedge was filled by sand caving in from the side and coarser particles falling into it from above.

Significance of the discontinuity surface, and deposition of the Moelv Tillite The erosional contact between the Ring Sandstone and the Moelv Tillite shows that the western marginal parts of the eastern sparagmite basin had been subaerially exposed, at least locally. This is also consistent with a fluvial origin for the Ring Formation. The shallowing of the eastern sparag­ mite basin in the period preceding the glaciation was probably caused by the withdrawal of sea water into ice sheets, and by the rapid influx of terri­ geneous detritus during the deposition of the Ring Formation. The Ekre Shale succeeds the Moelv Tillite in both the eastern and western sparagmite basins (Skjeseth 1963), indicating an increased water depth. The postglacial character of the Ekre Shale suggests that this deepening was a eustatic re­ sponse to glacial ice melting. Though the crust of the glaciated areas must have experienced isostatic recovery, its effect in the stratigraphic record of the sparagmite region in Southern Norway has not been clarified. Similar eustatically controlled sedimentation sequences have been postu­ lated for the Late Precambrian glaciation in Finnmark (Reading & Walker 1966, Banks et al. 1971) and Scotland (Spencer 1971). The sub-tillite uncon­ formity described in the present paper differs from that below the lower tillite in Finnmark which has been ascribed to tectonic movements (Holte­ dahl 1918, Føyn 1937) and to isostatic adjustments and glacial erosion (Reading & Walker 1966). The Långmarkberg Tillite in Sweden (re-named the Dabbsjon Formation by Gee et al. (1974:392)) contains clasts derived from the underlying sandstone units, and Kulling (1942, 1955) thought this was due to an episode of regional uplift with subsequent glacial erosion. 44 J. P. NYSTUEN

In the locality at Fuglåsen in Rendalen, the Moelv Tillite has been laid down in a terrestrial position, and the basal glaciofluvial conglomerate has probably been deposited in a proglacial environment when the glacier ice advanced from areas lying to the S and SW. The interbedded tillite layer might have been formed either by an intermittent advance of the ice front above the proglacial gravel beds, or as a flow till. In the first case, it would be expected that the till sheet was formed by the ice reworking the gravel, a phenomenon described from recent glaciers (e.g. Price 1969:21) and Pleistocene deposits (e.g. Bergersen & Garnes 1971:102), or that the contact between the tillite interbed and the conglomerate was more sharply marked or deformed by glacial tectonics (e.g. Fromm 1965:32, Lundqvist 1967, Hillefors 1969). From recent glaciers in Spitsbergen, Boulton (1968, 1971) has described flow tills forming as mud flows of englacially transported till debris that melts out on the snouts of wasting glaciers. The sheets of flow till may interdigitate with and grade into proglacial sediments. The following features of the present tillite sheet accord with those reported from flow tills in Spitsbergen and from Pleistocene deposits considered to be flow tills (Harts­ horn 1958, Hester & Du Montelle 1971, Marcussen 1973): wedge-shaped form, thickness and magnitude of lateral extent (c. 400 m); transitional contact with glaciofluvial sediments; lithological composition similar to the main tillite unit; textural composition bearing evidence that part of the debris has suffered washing; and preferred orientation of a-b planes of clasts parallel with the bedding surface. Hence, it is concluded that the tillite interbed is a flow till deposit which probably has been formed in a period when the ice front was stationary or in a stage of temporary retreat. The local occurrence of this facies of the Moelv Tillite suggests that the prog1acial sedimentation was related to a tongue-shaped protrusion of the advancing ice sheet and to topographic features in the ice-covered areas.

Palaeoclimatic and palaeolatitudinal problems of the Late Precambrian glaciation Fossil ice wedges have been reported from several other tillite units occupy­ ing similar stratigraphical positions, formed during glaciations at the end of the Precambrian some 650 m.y. ago. In the Port Askaig Tillite of Scot­ land, polygonal sandstone wedges, considered to be of permafrost origin, are very abundant at several stratigraphical Ievels (Spencer 1971). Fossil ice wedges are described from the Wilson Breen Tillite of Spitsbergen (Chu­ makov 1968:116), from the Smalfjord Formation in Finnmark (Edwards 1975), and patterned ground and fossil ice or sand wedge structures occur in the glaciogenic sediments of the Bthaat Ergil Group of West Africa (Trompette 1973). In northern Europe, a periglacial environment has furthermore been suggested for the Late Precambrian glaciation of northern Sweden (Kulling 1972:266). In all these localities, the permafrost PRECAMBRIAN MOELV TILLITE IN RENDALEN 45

must have developed in lowland areas dose to sea level, comparable with the permafrost that originated in the low plains of Europe and North America in front of the advancing Pleistocene ice sheets (Embleton & King 1969:230).

In regions with existing permafrost, ice wedges are found to be actively growing in areas with a mean annua} air temperature of about -5 to -6°C or colder (Pewe et al. 1969, Brown & Pewe 1973) or a mean annua! ground temperature of corresponding values (Katasonov 1973, Romanovskij 1973). These data, together with the recognition in northern and northwestern Europe of tillites having been deposited by grounded ice sheets (Reading & Walker 1966, Bjørlykke 1967, Banks et al. 1971, Spencer 1971, Kulling 1972) seem to conflict strongly with palaeomagnetic studies which indicate that this part of the world, induding Greenland, was situated in low latitudes during the Late Precambrian (Girdler 1964, Tarling 1974). An extensive glaciation with permafrost conditions at low altitudes and dose to the equator ought to imply that ice sheets covered all the continents of the Earth (Spencer 1971 :69); such a world-wide glaciation in the Late Pre­ cambrian has been suggested and discussed by many authors (see Schermer­ horn 1974:674). However, as pointed out by many of those discussing this problem (e.g. Harland 1964: 124), the frequent association of tillite beds with strata indicating a warm dirnate in the Late Precambrian rock sequence must reflect rapid dimatic variations. Harland (1964: 124) thought this might be due to rapid changes in the level of solar radiation, whilst Roberts (1971) suggested that a Late Precambrian glaciation resulted from an anti-green­ house effect due to the locking up of co2 in carbonate beds in the period prior to the glaciation. Carbonate beds, however, also occur within and dose above tillites and other possibly glaciogenic units, as well as beneath them (Schermerhorn & Stanton 1963, Wilson & Harland 1964, Dunn et al. 1971, Spencer 1971, Binda & Van Eden 1972, Stewart 1972, Kroner & Rankama 1973, Birkelund et al. 1973). Crowell & Frakes (1970), reviewing and discussing factors supposed to initiate ice ages, conduded that glaciations primarily result from an in­ creased albedo in response to the interaction between air-ocean systems, continental arrangement, and land elevation. These factors were thought to have been fundamental during the Late Palaeozoic ice age when polar and high-latitude regions of Gondwanaland were glaciated during that super­ continent's migration across the southern rotational pole. Crowell & Frakes (1970:203-4) suggested a similar nonsynchroneity of the Late Precambrian glaciations. The locations of the glacierized areas might have moved with respect to the continental drift of Pangea in a manner similar to the migra­ tion of glaciation centres in the Late Palaeozoic. Using a similar line of argument, Crawford & Daily (1971) also maintained that a worldwide syn­ chroneity of Late Precambrian glaciations was very unlikely. While Piper (1973) conduded that the Late Precambrian tillites of Africa were deposited in low latitudes, a revised polar wander path constructed 46 J. P. NYSTUEN on the basis of data from all Gondwanic continents by McElhinny et al. (1974) demonstrated that the Late Precambrian to Ordovician glaciations in South America and Africa affected high-latitude regions in different areas at various times, as the south rotational pole migrated relatively across the supercontinent from south to north. McElhinny et al. (1974:560) stressed the importance of large and rapid shifts in the polar wander paths in the time range 750--600 m.y. and suggested that such changes in the pole posi­ tions might account for the widely distributed tillite horizons of this period. Williams (1974, 1975) emphasized two enigmatic features of the Late Precambrian glacial climate: its remarkable inequability, particularly with respect to temperature, and a preponderance of grounded ice sheets ap­ parently in low palaeolatitudes. He postulated that if the Earth possessed a much greater obliquity of the ecliptic in the Late Precambrian, this would, during an ice age, cause the equatorial regions to be preferentially glaciated, and tropical regoliths and warm water carbonate facies to be located at all latitudes in association with glaciogenic beds. Schermerhorn (1974), reinterpreting most Late Precambrian tillites as mass flow deposits, concluded that no Late Precambrian ice age (or ages) had existed at all, and that only local highland areas had been glaciated. Thus, at the present stage of our knowledge concerning the problems of the Late Precambrian glaciations, there is considerable uncertainty about fundamental data such as genetical interpretations of till-like beds, absolute ages of the strata, stratigraphical correlations, and palaeolatudinal positions. Both actualistic and non-actualistic principles have been employed in the discussions; the hypothesis of 'an expanding Earth' (Carey 1975) may also have relevance to the debate. On1y additional, critically examined basic data from various parts of the world can provide a basis for so1ving one of the most intriguing problems in the Earth's history.

Conclusion and summary

The discontinuity surface between the Ring Sandstone and the Moelv Tillite is considered to have been formed by erosion consequent to eustatic lowering of sea level at the beginning of the Late Precambrian glaciation. Permafrost conditions prevailed in the subaerially exposed areas and were responsible for fossil ice-wedge formation. The glacial deposition started with proglacial sedimentation of glaciofluvial grave! beds during a sta­ tionary stage of a generally advancing ice sheet. A tillite layer, interbedded in the glaciofluvial conglomerate, probably originated as a flow till in the proglacial environment. After intermittent proglacial sedimentation, the ice sheet readvanced into the more central parts of the eastern sparagmite basin. The lithology and roundness of clasts in the conglomerate and tillite suggest that the local glacial transport was from the south or southwest, thus mixing long-transported and multicyclic rock debris with more locally PRECAMBRIAN MOELV TILLITE IN RENDALEN 47

derived material. The deglaciation was succeeded by a rise in sea level and deposition of the Ekre Shale. During the Late Precambrian, the northern and northwestern regions of Europe were covered by ice sheets from which lodgement till was deposited in lowland areas dose to sea level. An arctic climate with permafrost period­ ically gave rise to ice-wedge growth. A number of hypotheses have been put forward to explain the origin and extent of Late Precambrian glaciation. Our present knowledge is insufficient to permit any definite conclusions to be drawn about these problems, and additional information is needed.

Acknowledgements - Acknowledgement is made to the Norwegian Research Council for Science and the Humanities (NAVF) which financed the field investigations as part of the project 'S edimentological facies analysis of the sparagmite basin in southern Norway'. The author wishes to thank cand. real. O. F. Bergersen, Dr. K. BiØrlykke, and Dr. J. Mangerud for discussions, and the two latter for critically reading an earlier draft of the manuscript. A. Read, B. A., kindly corrected the English text. March 1975

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