Sedimentary features of a Weichselian glaciolacustrine delta LARS B. CLEMMENSEN AND MICHAEL HOUMARK-NIELSEN
BOREAS
Clemmensen, Lars B. & Houmark-Nielsen, Michael 1981 WDI: Sedimentary features of a Weichselian glaciolacustrine . delta. Boreas. Vol. 10, pp. 229-245. Oslo. ISSN 0300-9483. Inclined beds of sand, granules, pebbles and cobbles resembling a glacio-tectonically tilted sequence were shown by sedimentological studies to constitute the 10-12 m thick foreset beds of a glaciolacustrine Gilbert-type delta in Kyndby, North SjaAland, Denmark. The foreset beds are slightly curved, dip 10-28" SE, and display a bundlewise structure with alternating coarse-grained cobble-rich and fine-grained sandy units. The occurrence of ascending megaripple cross-bedding and climbing ripple cross-lamination in the sandy foresets can be ascribed to strong backflow currents formed by the lee-side vortice. The foreset beds are underlain by flat-lying tine-grained sand, silt and clay (bottomset beds), and are overlain transitionally or erosively by 2-3 m of flat-lying sand, pebbles and cobbles (topset beds). In the transition zone between foreset beds and topset beds, various delta distributary channel units occur. The delta probably formed in a partly ice-dammed lake in connection with the general retreat of a Weichselian advance from the north ('Norwegian ice'). Lars B. Clemmensen & Michael Houmark-Nielsen. Institut for almen Geologi, &ter Voldgade 10,
DK-1350KQbenhavn K , Denmark: 23rd April, 1980 (revised 12th September, 1980).
The Weichselian stratified drift successions of Denmark have been much studied in recent years from glacial-dynamic and glacial-stratigraphic points of view (see e.g. Berthelsen 1973, 1978; Houmark-Nielsen & Berthelsen, in press). Detailed sedimentological studies aiming at a better understanding of the depositional environments have been few in number since the early work of Andersen (1931). It is hoped that the present paper will help to rectify this situation by describing the wellexposed Gilbert-type delta sequence in Kyndby, North Sjselland (Figs. 1 and 2). The lithology, sedimentary structures and palaeocurrents of the deltaic facies are described and related to depositional processes. Special emphasis is given to backflow cross-bedding of unusually large size in the foreset beds. A reconstruction of the environment during deposition of the Kyndby delta is proposed and a model for annual sedimentation at the delta front is given. Finally, the palaeocurrents and provenance-dependent features within the delta systems are discussed with respect to glacial stratigraphy. The authors' fieldwork, on which the paper is based, was carried out during a series of visits to Kyndby gravel pit in 1977, 1978 and 1979.
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The sequence at Kyndby The glacial sequence at Kyndby (Fig. 3, column A) is composed of a basal stratified delta sequence 10-12 m thick unconforrnably overlain by two tills of various thickness separated by 2-3 m of coarse-grained, fluvial deposits. The sequence beneath the upper till has suffered slightly from glacio-tectonic deformation (Fig. 3, column B). Compositionally the glacial sequence (Fig. 3, columns C-D) can be divided into an upper unit rich in palaeozoic rock fragments (upper till) and a lower unit poor in palaeozoic rock fragments comprising the delta beds, the lower till and the overlying glaciofluvial deposits. Directional evidence in the glaciodeltaic and glaciofluvial deposits, such as &-plane imbrication, foreset bed orientation and measured palaeocurrent evidence from sedimentary structures, divides these sediments (Fig. 3, column D) into a lower SSE-directed sequence (the delta beds) and an upper W-directed sequence comprising the coarse-grained fluvial deposits between the tills. Directional evidence from the tills, i.e. long-axis orientation of the clasts (Fig. 3, column C), suggests ice flow from the NE (lower till) and from the SE (upper till), both tills possessing longitudinal fabrics of P2-type in the sense of Lindsay (1970).
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230 Lars B . Clemmensen and Michael Houmark-Nielsen
Fig. I . Location and Weichselian stratigraphyof the gravel pit near Kyndby, North Sjrelland. The deltaic sequence is unconformably overlain by an NE-till with associated . glaciofluvial sediments. This latter sequence is unconformably overlain by an SE-till. The distribution of Norwegian tills of Weichselian age on Sjrelland and adjacent areas is based on Berthelsen (1978) and Houmark-Nielsen's observations. .
Glacio-tectonic deformation gave rise to small-scale overturned folds in the strata below the upper till, reaching as far downwards as the top of the delta beds. With a NEMW-orientated fold axis (42/0) and northwesterly vergence, the folding is most probably related to movement from SE of the ice sheet which deposited the upper till. Thus, kinetostratigraphic classification (cf. Berthelsen 1978) enables the establishment of two stratigraphic units which can be correlated to equivalent stratigraphic units of regional distribution (Houmark-Nielsen 1980). The underlying delta sequence is left unclassified; these deposits cannot be connected to any of the tills observed in the gravel pit. The NE-till (lower till) and the coarse-grained deposits closely associated with this till constitute the NE-unit, whereas
the SE-till (upper till) forms an SE-unit to which the deformation in the sequence below belongs.
Sedimentary facies The Weichselian sequence in Kyndby is dominated by inclined beds (dipping 10-28") of sand, granules and stones resembling a glacio-tectonically tilted sequence of outwash sediments (Fig. 2) as described elsewhere (e.g. Ruegg 1976; figs. 13 and 14). The inclined beds grade upwards, however, into flat-lying coarse-grained sediments and likewise grade downwards into flatlying fine-grained rhythmites, leaving little doubt that the sequence is an undeformed Gilbert-type delta. This interpretation is supported by a dis-
Glaciolacustrine sedimentary features 23 1
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,
\ photo
t
N
Fig. 2. General view of the Kyndby gravel pit showing the
10-12 m high foreset beds overlain by the 2-3 m thick topset beds. The person at the bottom of the foreset beds indicates the scale. Sketch below shows general outline of the pit (May 1978) and features the strike and dip of the foresets.
tinct coarsening-upwards in grain size from the bottomset beds via the foreset beds to the topset beds. The occurrence of gradational contacts between the three depositional units furthermore indicates simultaneous or penecontemporaneous deposition. Between the well-developed foreset beds, which show a remarkable variety of sedimentary features, and the topset beds there is commonly a zone with channel structures. In the following the foreset beds, bottomset beds, channel sediments and topset beds will be described and interpreted in turn. Foreset beds Description. - Generally speaking, the foresel beds (Fig. 2) are at least 200 m wide, about 10 IT high, slightly curved in vertical section, and divided into various subunits by slight angulai unconformities and abrupt grain size changes. Thus several fine-grained sandy and coarsegrained conglomeratic subunits can be disting uished often arranged in well-defined sedimentary sequences. The sandy units with cm- to dm-thick foresets (fine-, medium- or coarse-grained with scattered pebbles) show even lamination, ascending and
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Glaciolacustrine sedimentary features
233
Fig. 4. Sedimentary features of the foreset beds. A. Giant-scale foreset beds of conglomerate, pebbly sand and sand dipping towards the SE. Small-scale climbing ripple lamination of presumed backflow origin occurs in several horizons: see handle (90 cm) of spade for scale. B. Foresets of pebbly sand and sand dip towards the SE. Note the occurrence of several climbing megaripples of presumed backflow origin in the sandy and in the more pebbly units: see handle (12 cm) of trowel for scale.
234 Lars B . Clemmensen and Michael Houmark-Nielsen FORESET E
BEDS W MASSIVE OR IMBRICATED
1
PARALLEL LAMINATION CLIMBING RIPPLES
DESCENDING MEGARIPPLES
m SCOUR-AND-FILL STRUCTURES
ASCENDING MEGARIPPLES
II PARALLEL LAMINATION
I
Fig. 5. Fining-upwards sequences within the foreset beds. Each sequence is initiated by parallel bedded conglomerate or pebbly sand (I); this is succeeded by medium sand with large-scale ascending megaripple cross-bedding (11). and finally at the top medium-fine sand with large-scale descending megaripple cross-bedding (111). Note the occurrence of a few scour-and-fill structures between units I1 and 111 and the occurrence of parallel bedding and small-scale climbing ripple lamination at the very top of unit 111. The fining-upwards sequence could be seasonal. Sketch after photograph.
descending megaripple cross-bedding, climbing ripple lamination and isolated scour-and-fill structures. The ascending and descending megaripple cross-bedding is generally of roughly tabular shape, but formsets are also seen (Fig. 4). The cross-bedding may occur isolated, in small groups, or in well-defined cosets (Fig. 5). In one case, a fining-upwards sequence with ascending megaripple cross-bedding is overlain by descending megaripple cross-bedding (Fig. 5). These structures may consist of rather wellsorted mainly medium-grained sand (Fig. 5), or show marked grain size differentiation (Fig. 4). In the latter case, coarse- and medium-grained sand make up the bulk of the cross-bedded sets; scattered granules and small pebbles occur as a minor component. The granules and pebbles apparently represent avalanche sediment on the delta front, this material sometimes transgressing the front of the ascending megaripples. The climbing ripple cross-lamination (Figs. 4 and 5) forms cosets of fine sand up to 25 cm thick which commonly have a large lateral continuity (at least several metres). The structures can be
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classified as type A, with eroded stoss sides (cf. Gustavson et al. 1975), and they may ascend (a common situation) or descend the delta foreset beds. The trough-formed isolated scour-and-fill structures commonly are 20-50 cm wide and are filled with concordant or oblique layers of sand, granules and small pebbles. There is commonly a grain size fining-upwards within the scours. A grain size analysis of one sandy layer with a mean of 0.33 mm indicates that the sediment is composed of a fine-grained population interpreted as suspension fall-out and a coarsegrained population thought to be formed by avalanching processes in well-sorted sand (Fig. 6). Two types can be recognised within the coarsest-grained subfacies. One, the most common, is composed of matrix-supported conglomerates. The pebble segregation (cf. Clifton 1973) commonly lies between 10 and 30 or 30 and 50. The stones, which range between a few centimetres and 10-15 cm, are randomly orientated (larger clasts) or aligned with their long axis subparallel to the dip of the foreset beds. These matrix-supported conglomerates vary in thickness between a few centimetres and about 50 cm, and commonly possess a great downslope continuity (at least 5-10 m). The other, less common type is characteristically composed of clast-supported conglomerates with coarse sandy or gravelly matrix. These layers occur in 3-5 m long and 5-20 cm thick lenses or in smaller pockets. They may show longitudinal and inverse grading as well as imbrication (both a and b-axis imbrication). The two types are often intergradational, occurring within the same foreset bed (Fig. 4). Grain size analysis of the matrix-supported conglomerates indicates that they are composed of three grain size populations (Fig. 6). Material smaller than about 1 mm was transported in suspension over the delta front while the coarser material (two populations) at the same time was transported down the foreset slope by avalanching processes. Directional data from the foreset beds have been compiled in Figs. 3 and 7. It can be seen that the delta prograded towards the southeast. The ascending. megaripples were directed either obliquely to the delta front (i.e. towards the west) or in precisely the opposite direction (i.e. towards the northwest). The descending megaripples were directed towards the east.
Glaciolacustrine sedimentary features 235
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Fig.6. Grain size analysis of three samples from the foreset beds. Note the occurrence in samples A and B of an upper well-sorted suspension fall-out population (smaller than I mm), a middle poorly sorted avalanche population, and a basal better sorted avalanche population (larger than 3 4 mm). Note also the well-sorted suspension fall-out population (smaller than 0.18 mm) in sample C in association with a very well-sorted avalanche population.
A
i.
' number
n=25
Fig. 7. Palaeocurrent data from the foreset beds. A. Intraset ' cross-bedding. B. Foreset beds.
S foreset beds
Lambert net
236 Lars B . Clemmensen and Michael Hournark-Nielsen
Interpretation. - The foreset beds formed by deposition on a slipface and probably represent the slope of a large Gilbert-type delta formed during the infilling of a lake (cf. Edwards 1978). Progradation of the delta was towards the southeast and probably took place in several stages; @is is suggested by the occurrence of internal angular unconformities and grain size finingupwards sequences in the foresets. Several authors have described the specific processes of sedimentation at delta fronts (see Elliott 1978 for review). The precise manner in which the river outflow and the basin water mix is considered of prime importance for the depositional processes, and using the analogy of jet, Bates (1953) described three types of river inflow: (a) immediate three-dimensional mixing at the delta front (homopycnal flow), (b) surface flow (hypopycnal flow), and (c) underflow (hyperpycnal flow). Gilbert-type deltas should only form in situations with immediate mixing of the water bodies and corresponding rapid deposition of the fluvial sediment load. In a recent paper by Pharo & Carmack (1979) the sedimentation processes of a recent lacustrine delta are divided into (1) delta progradation, (2) episodic sediment density surges, and (3) river plume dispersion, which may be of homopycnal, hypopycnal or hyperpycnal type, but also influenced by the Coriolis effect. In their model, steep Gilbert-type foresets form in direct response to delta progradation processes and are not influenced by the river plume dispersion pattern. In agreement with these ideas the steep and coarse-grained foresets in Kyndby are explained in the light of delta progradation processes only. It would appear in this connection to be worthwhile using the results from laboratory experiments on flow and sediment transport patterns over the lee faces of megaripples and microdeltas (Jopling 1%1, 1963, 1%5, 1967; Allen 1965). The present Gilbert-delta could in fact be viewed as a giant-scale bed-form prograding into standing water. By direct analogy with the above mentioned experiments, the flow pattern over the Kyndby delta front is divided into (a) zone of no diffusion, (b) zone of mixing, and (c) zone of backflow. The zone of no diffusion, which is the uppermost one, can be regarded as the continuation of the main stream. In the zone of mixing, which has an intermediate position, macroturbulence and some reverse circulation occur. Final-
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DRAPED LAMINATION
CLIMBING RIPPLES, TYPE A
Fig.9
RIPPLE FORESETS
LENTICULAR BEDDING
CLIMBING RIPPLES, TYPE B
0.2m] MUD
SAND
Fig. 8. Graphic log of parts of the bottomset beds.
ly, in the zone of backflow, which occupies the lower part of the water body beyond the delta front, circulation is reverse and directed up the delta front. This zone of backflow continues only to a certain distance beyond the delta front to the locus line of zero velocity. Beyond this point circulation is directed away from the delta front. The ascending megaripples reported from the delta foreset beds could accordingly have formed by these backflow currents at the leeside of the delta front, and should, if this interpretation holds true, be regarded as a giantscale example of an ‘interwoven’ set in the terminology of Boersma et al. (1968). The descending megaripples could similarly be viewed as c d o w cross-bedding (cf. Boersma etal. 1968). The fact that most descending megaripples occur within the inclined foreset beds and not within horizontally bedded toesets, seem to indicate another origin, however. It is therefore suggested that most of the descending cross-bedding formed by megaripples climbing down the delta front during a period with lack of
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Glaciolacustrine sedimentary features
237
Fig. 9. Sedimentary features of the bottomset beds. A. Alternating horizons with climbing ripple lamination and draped lamination. At the bottom a horizon with ripple lamination of possible wave origin. B. Detail of A. Note the arrangement of the foresets in the ? wave riooles and the continuous nature of the clav in the draned laminatinn
238 Lars B . Clemmensen and Michael Houmark-Nielsen
flow separation and the existence of a simple lakeward directed under flow. Before accepting this interpretation of the ascending cross-bedding as backflow structures, however, it is necessary to consider the hydrodynamics. The discharge during peak flood must have been considerable, as indicated by the numerous 10-15 cm large stones in the topset beds and also in the foreset beds. These stones were transported to the delta front by strong currents (up to 5-6 m s-I, cf. Sundborg 1956) and were then transported down the delta slope by avalanching processes. At the same time, sand and granules were carried in suspension by the main stream (cf. Harms et al. 1975; fig. 7-3) and transported past the delta front by the stream entering the lake body. Beyond the delta front this material was in many cases caught in by the lee-side vortice, and assuming that the back-flow currents of this system were sufficiently high, back-flow megaripples formed and climbed the delta front. According to Jopling (1961), backflow current velocity constitutes between 114 and 1/5 of the main flow current velocity. During peak discharge the back-flow would therefore have reached up to 1.5 m s-I. This value is much higher than the 0.4 m s-' needed to form megaripples at 3-4 m water depth (Harms et al. 1975, Fig. 2-1 1). It thus appears likely that the main flow that built up the delta was able, at least during the higher stages of discharge, (1) to carry sand and granules in suspension beyond the delta front, and (2) to create a back-flow at the lee-side of the delta front strong enough to form sandy megaripples climbing the delta front.
Bottomset b e d s Description. - The foreset beds are underlain by and laterally replaced down dip by flat-lying fine-grained bottomset beds up to 6 m thick (Fig. 8). The most common structure here is climbing ripple lamination type B of Gustavson et al. (1979, but the draped lamination (Fig. 9) of the same author is also common. Additional structures include: thin parallel laminated clay layers, climbing ripple lamination type A (with truncated foresets), sinusoidal lamination, horizontal lamination, wave-ripple lamination and disturbed lamination (Fig. 9). A typical small-scale sequence is composed of climbing ripple-lamination type B overlain by draped lamination or a clay laver. A more complete sequence seen in
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N
1
number
Fig. 10. Palaeocurrent data from the climbing ripple lamination in the bottomset beds.
one case constituted: horizontal lamination (base), climbing ripple-lamination type A, climbing ripple-lamination type B, sinusoidal lamination, and draped lamination (top). The bottomset beds are intercalated at the top with various fine-coarse sand beds with largescale trough or planar (microdelta) cross-bedding. Directional measurement of the climbing ripple lamination indicates palaeocurrents towards the ESE or the WSW (Fig. 10). Interpretation. - The fine-grained nature of the sediments indicates deposition from suspension in a low-energy lake environment. Only once does the lake bottom appear to have been affected by wave action. The climbing ripples formed from decellerating currents flowing southwards away from the delta front (turbidity currents). The material of these turbidite currents could be riverborne sediments transported into the lake as an underflow, or redeposited material (relocated by sediment density surge processes, cf. Pharo & Carmack 1979) initially dumped on the delta front. The small-scale sequences composed of sand with climbing ripples overlain by draped lamination of silt or clay could be annual (cf. Gustavson et al. 1975), but more likely represent shorter events in the infilling of the lake.
Channel sediments Description. - A large number of channel structures interfere with low-angle dipping foreset beds near their upper contact with the topset beds (Fig. 11). The channel structures are espe-
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Glaciolacustrine sedimentary features 239
Fig. 11. Sedimentary features of one of the channels. The inclined bedding of coarse-grained material ( I ) is interpreted as the foreset beds of a microdelta, and the small-scale ripple-lamination in the sand body at the bottom of the channel (2) is interpreted as bottomset sediments deposited by backflow and coflow currents associated with the progradation of the microdelta into the depression. Large arrow indicates direction of delta progradation; small arrow indicates direction of backflow. Handle (90cm) of spade indicates scale.
cially common in the most distal (i.e. southeastern) part of the gravel pit. The channel widths vary between 0.5 m and 2-3 m. The infill of the channels varies from rather large pebbles and granules to fine sand. Most channels are characterised by channel-fill cross-bedding (i.e. short large-scale cross-bedded sets with curved laminae), but other channels possess more complex structures, e.g. various types of microdelta cross-bedding many with well-developed differentiation into foresets and bottomsets (Fig. 11). Interpretation. - The channels represent fluvial erosion and deposition on the delta front. Possible agents are secondary fluvial cross-currents or more likely anastomosing small distributary channels from the main delta-building system (cf. Theakstone 1976: fig. 2). Many of the channels were apparently filled with sediment in several stages, and lateral accretion resulting in deposition of micro-delta cross-bedding occurred locally.
It is worth stressing that these channel structures are associated almost exclusively with the transition zone between low-angle dipping foreset beds and topset beds. These delta sediments with distributary channels probably characterised a period of lake infilling somewhat late1 than the steeper and more proximal Gilbert delta foreset beds. Topset beds Description. -These beds (Fig. 12) are characterised by two main lithologies. The most common is clast-supported conglomerate with frequent imbrication. The stones vary in size between 5 and 20 cm and the matrix is composed of coarse sand and granules. This lithology occurs as irregular sheets 5-50 cm thick and several metres long, and alternates with somewhat thinner sheets of medium and coarse sand. These sandy layers are characterised by horizontal lamination or rare large-scale semiplanar or troughformed cross-bedding.
240 Lars B . Clemrnensen and Michael Hournark-Nielsen
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Fig. 12. Sedimentary features of the topset beds. A. Gravel-sand couplets of the topset bed overlying the steeply dipping foreset beds. Note bundlewise structure of the foreset beds and the rhythmic appearance of the topset beds. Thickness of exposed sequence approx. 5 m. B. Topset beds with imbrication in the conglomerates and large-scale cross-bedding in the sandy units. Thickness of exposed sequence approx. 5 m.
Glaciolacustrine sedimentary features
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241
N
N
A
Fig. 13. Palaeocurrent data from
I S
the topset beds. A. Cross-bedding. B. Imbrication.
imbrication ( a/b-plane)
Directional measurements in the topset beds have been compiled in Fig. 13, where it can be seen that current flow was towards the south. Interpretation. - The coarse-grained nature of the topset beds and the lack of well-developed fining-upwards cyclothems point to a braided river origin. If the facies and sequence are compared with those of Miall (1978), is the best fit with the Scott type braided river of this author. The topset beds therefore seem to have been deposited in a braided river with coarsegrained longitudinal bars, lag deposits and subordinate cross-bedded sand wedges.
Depositional model Based on the above description and discussion a depositional model for the glaciolacustrine delta in Kyndby is proposed (Fig. 14). It is possible to distinguish three closely related 'depositional events in the infilling of the lake, all part of the same major progradating process. During the first stage, relatively steep (dip 2&28") foreset beds prograded towards the southeast into a 10-15 m deep lake (Fig. 14A). Fine-grained rhythmites (bottomset beds) were at the same time deposited in the distal part of the lake. At a somewhat later stage, apparently when the prox-
net 0
imal part of the lake was already infilled by delta sediments and the lake was more shallow, the depositional slope of the foreset beds was less steep and distributary channels migrated over the delta top and the delta front (Fig. 14B). The delta foresets were linked upcurrent with a braided river plain. As the last surviving part of the lake became sediment-filled, these outwash plain sediments (topset beds) prograded towards the south over the lake foreset beds (Fig. 14C). It is suggested that the anastomosing distributary channels should be viewed as the most distal part of the prograding outwash plain. The model also shows the existence of a lee-side vortex system during the early stage of delta progradation (Fig. 14A). This vortex pattern created backflow currents apparently strong enough to transport the available material (mainly medium-coarse sand) towards and up the delta front in the form of megaripples. At the same time, the more coarse-grained material (granules and stones) was transported down the delta front by avalanching processes. Delta progradation probably took place in several episodes, as suggested above by the occurrence of angular unconformities within the foreset beds, and especially by the occurrence of at least 3 m-thick fining-upwards sedimentary sequences in the foreset beds. These finingupwards sequences in the foreset beds (Fig. 5)
242 Lars B . Clemmensen and Michael Houmark-Nielsen
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Fig. 14. Tentative reconstruction of depositional environments during deposition of the delta sequence at Kyndby. A. Early stage of delta progradation. B. Late stage of delta progradation. C. Final stage of lake infilling and progradation of braided river plain.
were apparently deposited during periods with gradually decreasing flow strength. It is possible that this flow pattern could be seasonal, as recent runoff near glaciers is characterised by
anomalously high nival (snowmelt) flood in July followed by a much reduced flow in late summer (cf. Church & Gilbert 1975). Thus, during summer, peak discharge and high flow velocities
Glaciolacustrine sedimentary features
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I
DEPOSlTlONAL
MODEL 1
I
LOW ENERGY LOW STAGE MEDIUM-FINE SAND TRANSPORT SUPERIMPOSED MEGARIPPLES
MEDIUM ENERGY MEDIUM STAGE COARSE-FINE SAND TRANSPORT BACK- FLOW MEGARIPPLES
243
I
HIGH ENERGY HIGH STAGE COBBLE-PEBBLE TRANSPORT AVALANCHING
~~
Fig. I S . Tentative depositional model of ? seasonal sedimentation pattern at the delta front.
pebble- and cobble-rich foresets were possibly deposited (Fig. 151). A lee-side vortex was developed but megaripples possibly constructed by this system were apparently choked by the continuous avalanche of coarse material down the delta front. Later during the summer, when discharge and flow velocities were somewhat reduced, it is likely that mainly sand and fine gravel were transported to the delta front. A lee-side vortex still existed and was apparently strong enough to create back-flow megaripples climbing the delta front (Fig. 1511). Finally, during late summer, discharge and flow velocities were so low that the lee-side vortex had apparently become insignificant. Medium and fine sand were now transported over the delta top and down the delta front as a simple underflow creating descending megaripples (Fig. ' 15111).
Palaeocurrents and glacial stratigraphy The relationship between meltwater flow directions and the flow directions of the ice-sheet from which the meltwater originated is not always straightforward, though data from the classic papers by Ussing (1903) and Harder (1908), and recent publications by Larsen et al. (1977) point out that the palaeocurrent pattern is directed away from the glacier front but follows the pre-existing proglacial topography. The glacial stratigraphic reliability of palaeocurrent measurements in proglacial sedi-
ments is related to many features, e.g. grain size and depositional topography. Coarse-grained sediments deposited on a plane topography should give the most reliable measurements in this context. Thus the braided river topsets, which were deposited on a plane landscape, give the best estimate of the meltwater flow direction and the glacier front position, and within these deposits measurements on imbrication should give the lowest variability (cf. Bluck 1974). The lakebound delta foresets, in spite of their coarse grain size, are considered less reliable than the topsets with regard to overall meltwater flow direction, because the delta foresets could prograde into the lake both longitudinally and laterally, and, furthermore, the lake could have any orientation with regard to the glacier front. In the present case, however, there is a fairly good agreement between palaeocurrent measurements in the topsets and the foresets. The delta sequence in Kyndby is difficult to relate to a specific Weichselian glacial advance or readvance. Measurements of palaeocurrents in topsets, foresets and bottomsets have revealed that the dominant sediment transport was towards the southeast or south. In agreement with the above considerations it is therefore suggested that the delta sequence was deposited by southward flowing meltwater that originated from an ice-sheet lying north of the area. Since the delta beds are overlain by deposits from the NE-ice (Main Weichselian Advance), it is logical to relate the @cussed foreset beds to the melting of an oldemeichselian, Norwegian ice-sheet (Fig. 3). The large number of Norwegian indicator
244 Lars B . Clernrnensen and Michael Hournark-Nielsen
boulders (Oslo region) led
[email protected](1893) to the assumption that the lower till of N E Sjaelland was deposited by a Norwegian ice-stream. From the northern part of Sjaelland and adjacent islands, Berthelsen (19781, when summing up the latest results of glacial stratigraphy, reported the occurrence of a so-called Norwegian till underlying the NE-ice deposits. This till is characterised by small amounts of Palaeozoic material, Norwegian indicator boulders, and the admixture of a marine foraminifera1 assemblage of early Weichselian interstadial origin. Glacialtectonic features and till fabric indicate icemovements from the north. The Kyndby delta beds are characterised by small amounts of Palaeozoic material, and indicator counts in the foresets show strong domination of Norwegian indicators (Oslo: 68 %, Dalar: 32%, Baltic: 0% of a total of 38). Since Norwegian till is present farther south than the Kyndby area, it is unlikely that the delta beds originated from the first Norwegian advance. However, the Norwegian ice seems to have readvanced southwards beyond the coastal parts of NE Sjaelland (Sj@rring 1973; HoumarkNielsen 1980; Jensen 1977) and it is likely that the delta beds of Kyndby mark the melting, and the largest extension, of this readvance. Since older (Saalean?) tills of Baltic composition (high Palaeozoic contents) crop out north of the Kyndby area, the composition of the delta beds suggests that erosion of older strata by the meltwater was rather restricted and that the material could therefore have been deposited from the Norwegian readvance into a glacial lake partly bounded by dead-ice masses from the first (main) Norwegian advance. Thus, the sequence at Kyndby uncovers only parts of the complete glacial stratigraphy found in the northern Sjzlland and Storebaelt region (Houmark-Nielsen 1980), where the Weichselian sequence, resting upon till-material of Saalean derivation, comprises till and glaciofluvial deposits from the Norwegian (N-unit), the Main (NE-unit), the Eastjutlandic (E-unit), and the Beltsea (SE-unit) advances. Since the delta sequence is covered by two tills and glaciofluvial deposits, it is most likely that the features of the glacial landscape surrounding the Kyndby area (cf. Milthers 1948) are ascribed to later or the latest glacial event. The presenhalacial landscape around Kyndby is apparently not related to this (presumably) the first Weichselian glaciation in northern Denmark from which no
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geomorphological signs of ice-marginal zones have been reported from north Sjaelland.
Conclusions (I) The Weichselian sequence at Kyndby is composed of a basal delta sequence unconformably overlain by a NE-till and a glaciofluvial sequence related to the Main Weichselian Advance, which in turn are overlain unconformably by an SE-till related to the youngest ice-stream covering the area. (2) Palaeocurrent studies, general stratigraphic position, and counts on erratics indicate that the delta was deposited in a lake formed by meltwater from a Weichselian Norwegian ice. (3) Given the wide scatter of the palaeocurrents in the delta system, it is obvious that glacial stratigraphic interpretations on palaeocurrent data should only be made after detailed analysis of the depositional environments involved. (4) The Weichselian lake was filled from NW and N. The first stage in the infilling constituted a Gilbert-delta with steep foreset beds. In the second stage of infilling less steep delta foresets formed and numerous small distributary channels cut the delta front. The final stage in the lake filling was the covering of the delta sequence by coarse-grained braided river sediments. ( 5 ) Internal build-up of the foreset beds as m-thick fining-upwards sequences suggests seasonal control of sedimentation. (6) The occurrence of ascending megaripple cross-bedding and climbing ripple cross-lamination in the foresets can be ascribed to strong back-flow currents formed by the lee-side vortex. (7) Giant-scale undeformed delta foreset beds are not easily distinguished from glacio-tectonically tilted sequences in poor exposures or in borings. However, if the following features can be observed, a Gilbert-delta origin is likely: - the occurrence of flat-lying sequences above and below the dipping beds - grain size coarsening-upwards in the sequence as a whole - a consistent palaeocurrent pattern in the inclined and flat-lying sequences. Acknowledgements. -The authors are indebted to John Collinson (Keele) and to A. Berthelsen and C. Pulvertaft (Copenhagen), who read the paper critically and made valuable suggestions. Thanks are also directed to Annemarie Brantsen and Pernille Andersen, who typed the manuscript, and to C. Rasmussen, who made the drawings.
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