A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark

Sedimentology (1988) 35,9 15-937

A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark

L A R S H E N R I K N I E L S E N * , P E T E R N . J O H A N N E S S E N * a n d F I N N S U R L Y K T

*Geological Survey of Denmark, Thoravej 8, DK-2400 K0benhavn NV, Denmark; fGeoIogica1 Survey of Greenland, 0sster Voldgade 10, DK-13.50 K~benhavn K, Denmark

ABSTRACT

Several well-preserved Late Pleistocene spit systems occur uplifted in northern Jylland, Denmark. Their present-day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits. Two characteristic vertical sequences are recognized in the systems. The first (Sequence I) consists of a giant-scale cross-bedded foreset unit, overlain by topset and beach units, while the second (Sequence 11) consists of the foreset unit overlain by bar-trough and beach units. The two sequence types pass laterally into each other with a short overlap zone. They can be interpreted in terms of Meistrell’s (1966, 1972) model for spit-platform growth based on scaled wave tank experiments. The giant-scale cross-bedded unit corresponds to prograding of a coarse-grained subaqueous spit-platform while the topset, bar-trough and beach units reflect the growth of the subaerial spit. The alternation between sequence I and I1 reflects the inversely related growth of the spit and platform structures: when the rate of subaqueous platform progradation declines, the subaerial spit grows uniformly, and when the platform progrades uniformly spit growth declines. The model is probably only valid for relatively coarse-grained systems because only these deposits would have a relatively steep front. The water depth in which the spit system progrades and thus bottom topography, determines the thickness of the giant-scale cross-bedded foreset unit because the water depth over the top of the platform is relatively constant. If the water is less than a few metres deep the spit- platform is not developed as seen where the Late Pleistocene spit systems prograded over elevations of the sea bottom. Conversely, the correct recognition of spit-platform sequences allows precise determination of sea-level and water depth at the time of formation. Finally, the model adds one further mode of formation of giant-scale cross-bedding to those already known from fluvial transverse, lateral and point bars, subtidal sand waves and Gilbert deltas.

INTRODUCTION

In the present paper we report on Late Pleistocene spit systems, which have been isostatically uplifted to 20-30 m above sea-level following the melting of the Weichselian ice sheet. The identity as spits are clearly revealed from their topography and position (Figs 1 & 2). They form elongate ridges (the largest is 10 km long and on average 800 m wide) which are attached to a hilly glacial mainland at one end (Fig. 2). The ridges are superimposed on a plateau of much finer grained contemporaneous and slightly older marine deposits. The lithology and sedimentary structures of the spit deposits are well exposed in a large number of gravel pits.

The spit systems described here thus represent a rare example of an ‘ancient’ sedimentary system where the setting and geomorphology allows an immediate recognition of the basic environmznt and where both the detailed topographic expression and the internal sedimentological organization of the system can be studied (Johannessen & Nielsen, 1986).

The aim of the paper is twofold. First, the spit sequences are described and a general model is presented for coarse-grained spit systems prograding into relatively deep water.

Second, the exposures allow a test of Meistrell’s (1966, 1972) distinction into a subaerial spit and a

915

916 L. H . Nielsen, P . N . Johannessen and F . Surlyk

I-

@

-[--!---- ,""

57030

km 2

Glaclal landscape

0 Mlddle-Late Weichsel lan marlne p l a t e a u

Flandr lan marine p l a t e a u

Fig. 1. Geological sketch map of northern Jylland, Denmark, showing position of the study area (modified from Jessen, 1899, 1936).

subaqueous platform. Based on scaled wave-tank experiments, he postulated that the water depth above the platform remains constant irrespective of irregularities in shelf topography, that the structure is basically composed of foreset and topset beds, and that a subaerial spit ridge forms on top of the platform. The growth of the spit and platform structures are, in general, inversely related.

SETTING

The Middle to Late Weichselian spit systems described here are located in the eastern part of Vendsyssel, northern Denmark (Fig. 1). The morphology of Vendsyssel is composed of three major elements: (1) Weichselian glacial landscape, (2) late- glacial (Middle-Late Weichselian) marine plateau, and (3) post-glacial (Flandrian) low-lying marine plain (Jessen, 1899,1918). The latter element post-dates spit development and is not further considered.

The hilly glacial landscape is composed of ice- pushed glacio-marine, glacio-lacustrine and glacio- fluviatile deposits partly covered by till material.

The formation of the late glacial marine deposits is

due to the isostatic and eustatic changes caused by the gradual melting of the Weichselian ice cover. The weight of the ice depressed the former landmass. During melting of the ice the newly formed landscape was partly inundated by the sea, and only the highest parts were islands. Because of the unconsolidated nature of the material and the nearly barren nature of the land surface, the erosion by waves and rivers was very extensive, and many of the former islands form present-day hills limited by uplifted coastal cliffs (Fig. 2).

The melting of the glacial ice resulted in isostatic uplift of the area, and a general regression followed. The eustatic rise of sea-level balanced the isostatic uplift for several hundreds of years, probably due to intensive melting during the warm Balling period (13,000-12,000 yr BP). During this period of stable sea-level conditions, several spit systems were formed as an important element of coastal adjustment (Johannessen & Nielsen, 1984b). Eventually regression was resumed due to the continued uplift. Today the spit systems and the marine plateau are elevated 20-30 m above sea-level.

The age of the late glacial marine deposits in Vendsyssel falls within the range of 14,650- 11,360 yr BP as shown by 14C dating of shells (Tauber, 1966; Krog & Tauber, 1974; Knudsen, 1978). The age of the spit systems can be determined more precisely within this time-span. Three datings from the Voer- gird and Voldstrup plateau (Fig. 3), where the investigated spit systems are located, have given ages of 13,180k200 yr BP (K 887), 13,010+ 190 yr BP (K 903) (Tauber, 1966) and 13,130+ 140 yr BP (K 41 23) (Johannessen & Nielsen, I984a).

In the present paper we describe two of the spit systems which we have named after nearby settle- ments. They occur as long, relatively narrow topographic ridges which are limited by fine-grained bay deposits to the landward side and by contemporaneous and younger marine deposits to the seaward side. The Pudborg spit system, which is located south of the hill delimiting the Voergird plateau and the Voldstrup plateau (Fig. 3), is approximately 1 km long and 0.6 km wide and is well exposed in two gravel pits (locs 10 & 11). The Lyngsi spit system is located on the eastern part of the Voldstrup plateau, and it is about 10 km long and 0.5-2.5 km wide (Fig. 3). It is well exposed in six pits at the southern end (locs 1 to 6) and three pits at the northern end (locs 8, 9 & 13). A flood tidal delta related to the final stage of the Lyngsi spit system is identified at loc. 12. The roughly N-S oriented ridge, which formed the topographic

_y

I:

14 000- 13 300

i_ Ice margin

- Sea

Coasta l cl i f f

'x Stream

Tidal delta

1-d Glacial deposits

L_i deposits Late glacial marine

Late glacial sp i t -p la t form deposits

, 5 km

Fig. 2. A-G show the palaeogeographical evolution of the study area from about 15,000 yr BP to 12,500 yr BP. Light dotted ornamentation indicates glacial landscape and the gradually emerging late glacial marine plateau. Heavy dotted ornamentation indicates spit-platform systems. A roughly N-S orientated ridge which formed the topographic basis for growth of the main spit system was formed as a marginal ice push ridge in stage A. H is a simplified geological map showing the situation at the time when the spit systems and the bay were isolated from the sea as a result of rapid isostatic uplift. The subaerial part of ihe system during formation is indicated with black. The only difference from today is that the present coastline is situated slightly further eastwards.

918 L. H . Nielsen, P . N . Johannessen and F. Surlyk

Glacial deposits

Late glacial marine deposits

Late glacial spit-platform deposits

Raised coastal cliff

Local i ty number

+ Current rose Simplified 31 = no. of measurements sedimentological log

Dip direction of measured on topset beds

-'- Trough cross-bedding

DIP direction of giant-scale foreset Z E Planar cross-bedding

1 .2A.3 Facies/unit iL Cross lamination numbers

Fig. 3. Geological sketch map showing position of localities and simplified sedimentological logs with palaeocurrent data indicated. Three other spit systems are identified (but not named) in addition to the Lyngsi and Pudborg spit systems.

basis for growth of the main spit system, was formed as a marginal ice-push ridge in stage A (Fig. 2). Three additional spit systems are recognized from small exposures, morphological studies and water supply wells (Figs 2 & 3).

TERMINOLOGY

Before the sedimentological description and interpretation is presented a few terms will be defined.

'A spit is a ridge or embankment of sediment

A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark 919

attached to the land mass at one end and terminating in open water at the other. It is younger than the land mass to which it is attached. The crest of the spit from the land outward for some distance rises above the water' (Evans, 1942).

Meistrell (1972) enlarged this definition as he differentiated between the spit platform and the spit : 'The spit platform is a large-scale primary sedimentary structure formed by sediment transport along the coast and is a rise over the shelf but below mean low tide. A spit is a sediment ridge on the spit platform, partly elevated over mean low tide, and the platform is established in advance of the spit formation'. This differentiation has proved to be significant and has been used in this study. Kumar & Sanders (1974) also used this differentiation in their classical description of tidal inlet sequences through spit systems.

FACIES AND UNITS

The spit systems described here contain three main facies which occur in a consistent stratigraphic order. As they form coherent sediment bodies analogous to lithostratigraphic units they are termed units 1, 2 and 3. Unit 2 is subdivided into two laterally interfingering units or facies (2A and 2B). The basic interpretation of the three units within the confines of a spit system is relatively straightforward and the units are thus for the convenience of the reader described under the headings: Unit 1, giant-scale platform foresets; Unit 2A, topset; Unit 2B, oblique bar-trough system; Unit 3, beach. They are described below in ascending stratigraphic order.

Unit 1: giant-scale platform foresets

Unit 1 consists of a giant-scale cross-bed, which has a maximum thickness estimated at about 10 m. It occurs in both the Lyngsi and the Pudborg spit systems. It has been observed in all localities except locs 7 and 8, where the underlying glacial deposits reach a high level, loc. 12 which is situated outside the spit systems, and locs 14 and 15, where the pits were not deep enough to reach down to the giant-scale foresets. In the proximal part of the Lyngsi spit system (locs 1-6) the unit is almost continuously exposed over an area of 500 x 300 m northward from the highlying glacial deposits, which formed an island during spit formation. The total length of unit l in the Lyngsi system

seems to be about 10 km in N-S direction and the width is up to 1.6 km corresponding to the dimensions of the present-day topographic ridge (Figs 2 & 3). This estimate is supported by information from water supply wells (Fredericia, 1984) and abandoned pits. In the Pudborg system (locs 10 & 11) unit 1 can be followed over an area of 400 x 300 m. Water supply wells, as well as field observations in the area make it probable that the unit has a total extension of approx. 1 x 0.6 km (Figs 2 & 3).

Only the upper set boundary of the giant-scale cross- bedded unit is exposed; the lower boundary occurs below the bottom of the pits and can thus not be observed directly. Up to 6 m of the upper part of the giant-scale cross-bedded set is exposed in the deepest pits. In the Lyngsa system the dip of the foresets decreases towards the bottom of the pits. This suggests that the foresets approach the lower set boundary tangentially not far below the bottom of the pits. This is confirmed from wells, which indicate that unit 1 is approx. 8 m thick in the area between pits 1 and 6. The exposure in the Pudborg system as well as the geological map (Jessen, 1899) makes it probable that the giant-scale set here is up to 10 m thick.

In the Pudborg spit system unit 1 is overlain gradually or erosively by a 1-2 m thick topset (unit 2A, Fig. 4). In the proximal part of theLyngsi spit system the foresets are truncated by a horizontal stone- strewn erosion surface, which forms the base for the overlying bar-trough deposits (unit 2B, Figs 5, 9A & 10).

The giant foresets of unit 1 are 5-100 mm thick. The thinner foresets consist of medium-grained sand while the thick foresets consist of coarse-grained sand and pebbles (Figs 4, 9 & lo). The foresets show an average dip of 20-25" with 15-30' as the total range. The dip direction is uniform within the same local area. In both the Lyngsi and the Pudborg spit system it is mainly towards the NW (Fig. 3). Pebbles and small cobbles constitute up to 30% of the coarse- grained foresets, with an average maximum size of approx. 40 mm (average of 10 largest pebbles) and a maximum clast size of 140 mm. Isolated fine-grained foresets occur consistently for each few meters of foresets. They are up to 0.2 m thick, consist of clayey silt and are strongly bioturbated.

Delicate vertical burrows of J or U-shape extend downwards from the fine-grained foresets into medium-grained foresets. They are 20-50 mm long and the shafts have a diameter of 1-2 mm.

Cross-bedded and cross-laminated intrasets commonly occur in the giant-scale foresets (Figs 6A, C &

920 L. H . Nielsen, P. N . Johannessen and F. Surlyk

Fig. 4. A. Complex giant-scale platform foreset formed by avalanching interrupted by phases of erosion. The foreset is overlain by coarse, gently dipping topset beds via a very complex transitional zone. Pudborg spit system (loc. 10, Fig. 3). Spade handle 0.7 m long (encircled). B. Detail of A photographed at a slightly later date. Note the extremely complex nature of the foreset- topset transition and slump structures on the cross-strata of the foreset (arrow). Divisions on scale 0.2 m long.

7). The intrasets may be so developed as to obscure the giant-scale foresets that they lie within. This is the case at the end of the Lyngsi spit system (loc 9, Figs 3 & 7). The foresets are often associated with slump structures where they reach the maximum angle of repose (Fig. 4B), and may also show water escape structures (Fig. 4A).

Coarse-grained lenticular sand bodies about 10- 50 m apart occur within the giant-scale foresets of unit 1. The bodies have erosive boundaries and are 1.5- 2.5m thick. They are 15-20m wide measured in

sections parallel to foreset dip direction and extend from the base of the exposure diagonally tip through the giant foreset to the top of the unit (Figs 8A & 9A). In sections parallel to foreset strike, the bodies are seen to be lens-shaped with a width of 8-25 m. The lower bounding surface is steeper and more irregular than the upper bounding surface. The lenticular sand bodies can be followed into the topsets of the Pudborg spit system, but are cut out erosively by unit 2B in the Lyngsl spit system (Fig. 10). They consist mainly of long, parallel, 10-40 mm thick, very regular, planar to

A Late Pleistocene coarse-grained spit-plaqorm sequence in northern Jylland, Denmark 92 1

W E

& Fig. 5. Field sketch of a section through deposits of the Lyngsi spit system (loc. 2, Fig. 3 ) . At the base, an obliquely-cut giant- scale spit-platform foreset (unit 1), with a true dip of 25-30" towards NW. Foresets are truncated by a pebble strewn erosion surface, which is overlain by low and high-angle cross-bedded bar deposits (unit 2B). Westwards towards the spit beach the bar deposits pass into cross-bedded trough deposits with wave and current ripple cross-lamination indicating gradual filling of the trough. The bar and trough deposits are overlain by coarse-grained beach deposits (unit 3) . Towards the left (W) they show possible overwash lamination, in the middle trough cross-bedding formed at the beach toe or inner rough zone (cf. Clifton er al., 1971), and to the right (E) swash-backwash lamination.

convex upwards laminae of coarse to very coarse- grained sand often separated by laminae of fine- grained sand less than 1 mm thick. The laminae which generally only dip a few degrees show a recurring, systematic variation in the magnitude and direction of dip (Figs 8 & 9). The coarse-grained sand laminae are thickest along the lower erosion surface and become gradually finer grained and thinner towards the upper erosion surface. They also show a marked lateral fining and thinning development (Figs 8 & 9). The interbedded thin fine-grained laminae become gradually thicker towards the upper erosion surface where they make up an increasing part of the lenticular sand body. The bodies thus show an overall upwards decreasing grain-size. The boundary between the fine- grained laminae and the coarse-grained laminae is sharp but mainly of non-erosive nature. The coarse- grained laminae are mainly non-graded. In a few cases, inverse-to-normal grading and floating pebbles are observed immediately above the lower erosion surface (Fig. l l ) , while some laminae show normal grading in the upper level of the sand bodies.

Internal erosion surfaces are common close to the lower bounding surface. Occasionally they extend laterally to the margin of the sand body and merge with the main lower bounding surface (Fig. 9A). Near the lower erosion surface the coarse-grained laminae contain up to 30% pebbles as well as transported shells of Hiatella arctica and Macoma calcarea. The pebbles are of the same grade as those occurring in the giant foresets, and in some cases clasts of clay and silt occur immediately adjacent to giant-scale foresets of the same fine-grained composition.

Unit 2A: topset layers

Unit 2A always overlies unit 1. It is exposed in three localities (locs 10 & 11 in the Pudborg spit system, and at loc. 9 in the distal part of the Lyngsg spit). The unit is 1-2 m thick and can be followed laterally over the full extent of each exposure (50-100 m). The unit is developed rather differently at the three localities with respect to lithology and structures.

The marginal part of the Pudborg spit system (pit 11, Fig. 3) is characterized by beds of silt and fine- grained sand showing parallel lamination alternating with cross-lamination, and silt draped symmetrical sand ripples frequently exhibiting chevron cross- lamination. Additionally, these fine-grained strata are characterized by small delicate vertical J- or U-shaped burrows as described from unit 1.

In the proximal and central part of the Pudborg spit system (loc. 10, Fig. 3) unit 2A consists of 0.05-0.40 m thick, interbedded, laterally continuous sandy and pebbly layers. The layers dip 2-7" towards the NW in the same direction as the foresets of the underlying unit 1. The boundaries between the sand layers and the pebbly layers are well-defined and parallel (Fig. 6B). The sand layers are parallel stratified with subordinate 0.2-0.3 m thick trough cross-bedded sets with foreset dip towards the NW, roughly in the same direction as in the giant-scale foresets of unit 1 (Fig. 5). The pebbly layers show a diffuse stratification.

At relatively regular intervals (every 5-20 m) distinct 0-05-0-20 m thick silt and fine-grained sand layers can be followed from the topset layers downwards into the platform foresets. They often contain

Fig. 6. Details from platform foresets and topset. (A) Intrasets in platform foresets formed by both oscillating and unidirectional currents. True foreset dip 25" towards N W , Lyngsi spit system (loc. 1, Fig. 3). Handle of trowel 0.12 m long. (B) The lower part shows a strike parallel section through platform foresets (l), with true dip of 20-25" towards N W . The upper part shows a strike parallel section through gently (2-7") N W dipping coarse-grained topset beds (2A). Central part of the Pudborg spit system (loc. 10, Fig. 3). Divisions on scale 0.2 m. (C) Section through the distal end of the Lyngsi spit system (loc. 9, Fig. 3). The transition from the platform foresets (unit 1) (which occur immediately below the section) to the topset is dominated by trough cross-bedded pebbly sand. The main palaeocurrent direction is seawards, towards E and SE and reflects strong tidal (ebb) current activity at the later stage of spit growth where only a narrow inlet connects the open sea and the bay (Figs 2, 3). Handle of trowel (encircled) 0.12 m long.

A Late Pleistocene coarse-grained spit-pla form sequence in northern Jylland, Denmark 923

n =72 n=50

Fig. 7. Sketch from the distal end of the Lyngsi spit system (loc. 9, Fig. 3) showing section roughly parallel to the dip direction of the giant-scale platform foreset (i.e. approximately parallel to the length of the spit), showing palaeocurrent directions from trough cross-bedding in the topset (right) and in the platform foqset (left). The former shows transport seawards towards E and SE, while the latter are bimodal with a dominant direction towards the bay (W). Palaeocurrent direction of the platform foreset is towards N.

delicate vertical J- or U-shaped burrows as described from unit I . The boundary between unit 2A and unit 1 is often very complex (Fig. 12). Fig. 12 shows, in a general form, four main unit 1 to 2A transitions based on observations from locs 9 and 10. The boundary is erosive in most cases.

Unit 2A passes up-dip into the overlying deposits of unit 3.

In the distal part of the Lyngsi spit system (loc. 9) unit 2A deviates somewhat from the one described from the Pudborg spit system. In loc. 9 both unit 1 and unit 2A are trough cross-bedded intrasets composed of coarse-grained, occasionally pebbly sand (Figs 6C & 7). The foresets of the intrasets of unit 2A dip in easterly directions towards the ancient Kattegat Sea while the foresets of the intrasets of unit 1 dip in westerly directions towards the Voldstrup bay (Fig. 7).

Unit 2B: oblique bar-trough system

This unit is 1-2 m thick, overlies unit 1 and passes laterally into unit 2A. In the Lyngsi spit system it is

exposed in the southern older part of the spit system at localities 1-6, and in the northern younger part at locality 8. The unit can be followed through the different pits in the southern part over an area of 500 x 300 m. In the marginal part of the Pudborg spit system the unit is exposed at loc. 1 1.

The unit is dominated by upwards coarsening, fine- to coarse-grained sand. It consists of alternating planar and trough cross-bedded sets, cross-laminated sets, silt draped symmetrical ripples and parallel laminated beds. In a few silty and clayey layers small delicate vertical J- or U-shaped burrows occur.

The planar cross-bedded sets are 0.15-0.55 m thick and either have steep (20-30") or low-angled (5-15') foresets dipping obliquely towards the margin of the spit. The former tends to occur in the upper part of unit 2B while the latter characterizes the lower part of the unit. The steep foresets consist of medium- to very coarse-grained sand, occasionally with pebbles. Fore- set laminae are distinct and have angular or tangential lower contacts. Bottomsets are thin (up to 80 mm) or absent. Reactivation surfaces are often present.

The low-angle cross-strata are several metres long, concave upwards and pass asymptotically into bottom-

9 24 L. H . Nielsen, P. N . Johannessen and F. Surlyk

Fig. 8. (A) Steep NW-dipping platform foreset to the right truncated by irregular erosion surface which is overlain by hummocky cross-stratified chute deposits possibly representing storm wave reworked material eroded from the platform edge. Lyngsi spit system (loc. 2, Fig. 3). (B) Detail of hummocky cross-stratified sand deposited above and away from erosion surface in platform foresets (as in A). The slightly dipping laminae to the right can be interpreted as backset bedding on a hummocky bedform at the foreset slope deposited from high-density flows spilling over the platform edge or from material eroded from the upper part of the platform foreset. Note the subtle interplay between erosion and deposition on the left margin of the hummock (arrowed). Ruler 0.4 m long. Pudborg spit system (loc. 11).

A Late Pleistocene coarse-grained spit-platform sequence in northern Jyllund, Denmark 925

Fig. 9. A. Section through the proximal part of the Lyngsi spit system (loc. 1, Fig. 3) showing giant-scale platform foreset (unit 1) overlain by bar-trough deposits with a basal pebble strewn erosion surface (unit 2B). White rectangle shows position of Fig. 9B. One coarse-grained sand body (chute deposits) occurs interbedded with the platform foreset deposits. The laminae within the sand body fine and thin away from the platform foresets. An internal erosive surface is arrowed. The cross-strata of the platform foreset (unit 1) continue to the right of the sand body. The sand body is interpreted as formed by storm wave reworking of material eroded from the platform front and deposited in a chute channel which cuts down through the platform surface and platform front. Low-angle cross-strata in unit 2B are several metres long, concave upwards and pass asymptotically into thick bottomsets. One single low-angled foreset is outlined by three arrows. Divisions on scale at lower centre 0.2 m long. (B) Detail of A (indicated by white rectangle). Deposits of an oblique bar-trough system (unit 2B) from the Lyngsi spit (loc. I , Fig. 3). The units labelled b-t consist of bar foresets and trough deposits, while the unit labelled t consists of the deposits of one or more troughs. The spit beach was situated to the W. The lowermost b-t unit shows both low and high-angle cross-strata. Note two beds (arrowed) with climbing ripple cross-lamination in the middle of the t unit. The upper b-t unit consists mainly of high-angle cross-strata dipping towards W (into the photo).


Fig. 10. (A) Section through the proximal part of the Lyngsl spit system (loc. I , Fig. 3). Giant-scale NW-dipping platform foreset at the base (unit I ) truncated by extensive planar pebble strewn erosion surface, overlain by bar-trough and beach deposits (units 2B-3). Platform topset (unit 2A) is not preserved. (B) Detail of A. Note the regular development of the cross- strata of the platform foreset and the erosion surface.

sets which can be up to 0.3 m thick (Fig. 9). They contain abundant cross-laminated fine- to medium- grained intrasets. The low-angle cross-strata are commonly silt-draped, and symmetrical sand ripples with internal chevron-structures are preserved on the foresets. The relatively regular lateral distribution of drapes give the sets a ‘bundle wise upbuilt’ appearance (cf. Raaf et al., 1977). Intrasets up to 0.15 m thick characterized by steep cross-strata are in some cases seen within cross-sets of low-angle foresets (Fig. 9B). In these cases a low-angle foreset constitutes the lower boundary of the steep cross-strata, while the upper boundary is often marked by a reactivation surface above which low-angle foresets are developed again (Fig. 5, right part).

In the marginal part of the Pudborg spit system

planar cross-bedded sets with 0.15-0.30 m thick topsets are seen. The topset laminae are several metres long and pass gradually into the gently dipping foreset laminae.

Superjacent sets of planar cross-bedding are separated by up to 0.5 m thick strata characterized by trough cross-bedded, medium- to coarse-grained sand, cross-laminated fine-grained sand and silt draped fine- grained, symmetrical sand ripples. The foresets in the trough cross-bedded sets dip roughly in the same direction as the elongation of the spit. In some cases the planar cross-bedding gradually passes laterally into strata with symmetrical ripples, cross-laminated and trough cross-bedded sand (Fig. 5). In some exposures the planar cross-bedding is absent and trough cross-bedding is the dominant structure.


Simple

Compound

2A 1

- Fine- grained drape

5m

Fig. 12. Schematic diagrams showing four main types of transition from platform foresets (unit 1) to topset (unit 2A) based on field observations. The top sketch shows how a cessation in formation of avalanche foresets is followed by aggradation by suspension fall-out of fine-grained material. Resumed foreset progradation starts at a topographically slightly higher equilibrium level.

The second sketch illustrates the case where active platform aggradation is initiated by migrating megaripples forming cross-bedding after a period of still-stand. Platform progradation was first resumed when the megaripple reached the platform edge.

The third sketch shows a case where platform progradation is followed by erosion of the platform edge, and aggradation by suspension fall-out of fine-grained material. Resumed progradation as in the first sketch.

The bottom sketch shows a more complex sequence representing platform progradation, erosion, suspension aggradation, ‘active’ aggradation, erosion, suspension aggradation, and resumed platform progradation from the new, higher topographic equilibrium level.

Fig. 11. Three types of grading within storm sand laminae in the erosive sand bodies in the platform foreset (unit 1). (A) Inverse to normally graded laminae. (B) Non-graded laminae, which are the most common type. (C) Normally graded laminae near the upper boundary of a sand body.

928 L. H , Nielsen, P. N . Johannessen and F. Surlyk

Unit 3: beach deposits

Unit 3 is 0.5-1.5 m thick and is the topmost unit. It always overlies units 2A or 29 and it is present at all localities, but is often poorly exposed due to the applied quarrying technique. It consists of very coarse- grained pebbly sand and pebble beds. The transition from the lower units is ir, most cases gradual. When unit 3 overlies unit 2A, the lower boundary is difficult to define as grain size and structures do not change significantly. Where it overlies unit 2B the boundary is placed at the change from sand- to pebble-dominated lithologies. The upper boundary is the present-day land surface, which morphologically resembles beach ridges (Andersen, 1961).

The unit is characterized by 0.1-0.3 m thick alternating beds of very coarse-grained sand and pebbles, dipping slightly (4-8”) towards the sea (Fig. 13). Generally, the beds have sharp boundaries. The base of the pebble beds is most commonly erosive. Internally, a diffuse stratification is seen dipping parallel to bed boundaries. The sand beds also show parallel lamination, dipping even less than the set boundaries. Consequently, the internal lamination is truncated by the base of the pebble bed (Fig. 13). The stratification of the sandy beds consists of 10-20 mm thick layers of medium-grained sand alternating with 5-10 mm thick layers of coarse-grained sand and granules. Pebbles are often concentrated in distinct layers. Occasionally the internal stratification dips slightly in the opposite direction to the bed boundaries.

Downwards, the alternating sand and pebble beds pass into trough or, less commonly, planar cross- bedded sets. The former show some variation in palaeocurrent direction although the direction parallel to the spit-growth direction dominates. The latter have steep foresets which dip in the opposite direction to the slightly inclined bounding surface. In places the foresets are overlain by topset laminae dipping a few degrees in the opposite direction to the foresets.

The sandy and pebbly beds are often topped by a coarse, poorly stratified pebbly bed. It has an erosive base strewn by the largest pebbles. Seaward pebble imbrication has been noted in a few cases.

FACIES INTERPRETATION

Unit 1: giant-scale platform foresets

The giant-scale foresets which dip mainly towards the NW in the longitudinal direction of the spit systems

are interpreted as formed by avalanche processes down a steep subaqueous spit platform front.

The J-form burrows described from the fine-grained foreset drapes indicate rather long quiet periods where organisms have settled on the spit platform front without being disturbed by avalanching. When coarse sedimentation was resumed due to changes in wind conditions (increasing wind forces from E or S), it is thought that the infauna of the front was buried and killed, as escape traces have not been found.

Intrasets are interpreted as having been deposited by both oscillating and unidirectional currents trans- porting sediments upwards, downwards and along the steep subaqueous spit platform front.

The giant-scale foresets and topsets are nearly masked at the northern point of Lyngsi spit system (loc. 9, Fig. 3) due to the dominance of trough cross- bedding (Figs 6C & 7). The formation of these intrasets is discussed later in the section on topset beds (unit 2A).

The spit platform front was extremely unstable and was commonly modified by slump processes (Fig. 4). This is particularly the case where the foresets have reached the maximum angle of repose. Small ‘fans’ or ‘cones’ located at the base of the spit platform in a recent tidal inlet have been described by Kumar & Sanders (1974), who connected them with slump processes.

Progradation of the spit platform was occasionally punctuated by the formation of coarse, lenticular sand bodies with erosive boundaries. The bodies contain clasts eroded from fine-grained drapes of the foresets and are thus clearly related to platform margin erosion. Their flat lenticular shape in sections parallel to foreset strike suggests deposition in wide shallow chutes. The regular internal alternation between fine- and coarse-grained laminae suggest deposition from pulsatingcurrents. Lamination along the chute margin have a convex-upward, mounded configuration resem- bling hummocky cross-stratification (e.g. Dott & Bourgeois, 1982). This similarity is accentuated by the presence of a basal erosive boundary with a lag of pebbles and shells. Shells have otherwise not been observed in the spit system although they occur in abundance in the underlying fine-grained deposits. This indicates that sea-floor erosion also contributed to the formation of the sand bodies.

It is thus suggested that the erosive bodies were formed during intense storms and two related mechanisms are proposed to explain their genesis. These mechanisms can be considered as end members of a series of processes.

930 L. H . Nielsen. P . N . Johannessen and F . Surlyk

When storm waves approached perpendicularly to the platform, they rose and broke very quickly when the steep platform front was approached. In this way a very strong turbulence was created and a large water transport took place over the margin of the platform towards the end of the spit. The downflow currents became concentrated in chutes at the platform margin, and large amounts of sand could be transported in suspension through the chutes to the frontal slope and base of the platform. The current will expand over the sea bottom and the sediment will be deposited at the sea bottom in front of the platform. When the storm begins to abate, current expansion will take place over the platform front itself and the sediment will fall out of suspension and be deposited in the chutes. The lamination dipping towards the platform foresets are thus interpreted as backset bedding formed by overloaded, high-density flows on the upslope side of the initial base-of-slope mound.

It is also possible that the erosive bodies formed as a result of localized wave scour and concentration of large clasts occurred on the higher parts of the giant- scale foresets and the bodies thus represent sediment eroded from above. In this case the internal structure of bodies may be described as hummocky cross- stratification formed on a dipping surface (Fig. 9A). The presence of chutes suggests, however, that the first explanation is valid (in some cases at least) and that it is possible that the two interpretations represent end members of a series of storm processes of decreasing energy levels active in the platform front area.

Unit 2A: topset beds

Unit 2A was deposited under the influence of waves as shown by the high degree of lateral continuity of bedding, good segregation of sand and gravel and silt- draped symmetrical ripples which is characteristic of the unit in the Pudborg spit system (cf. Clifton, 1973).

The trough cross-bedded medium- to coarse-grained sand with foresets showing the same transport direction as the underlying giant foresets (unit 1) are interpreted as deposited by 3-D megaripples which migrated on the platform towards the front.

The intricate relationship between unit 2A and the giant-scale foresets of unit 1 illustrates the complex depositional history of the subaquatic spit platform. The presence of erosion surfaces extending downwards from unit 2A through the foresets indicates that the platform front often was subject to erosion. The

edge of the platform is especially prone to erosion as incoming waves suddenly reach the bottom here. The draping of the erosion surfaces reflects a subsequent quiet period during which the suspended material was deposited on the platform. The presence of J- or U- shaped burrows indicates a certain duration of these quiet periods before progradation was resumed.

The boundary between topset and foreset display a series of vertical jumps of up to one metre. This is clearly seen in sections parallel to foreset dip direction (unit 1; Fig. 12). These jumps have been formed in several different ways and are interpreted to represent changes between progradational and aggradational phases of platform construction. When the formation of foresets was resumed after an aggradational phase it took place from a higher level corresponding to the level of aggradation (Fig. 12).

Meistrell (1972) observed in his tank experiments that the platform establishes an equilibrium and thus sustains a constant water depth, but the conditions determining this equilibrium were not indicated. It is obvious that conditions such as wave parameters, sediment supply and coast parallel currents must have a decisive influence on the equilibrium. The changes between progradation and aggradation which we have observed (Fig. 12) reflect an adaptation to changed equilibria caused by variations in the above-men- tioned conditions.

The clear bimodal distribution of intraset dip directions in unit 2A and in the foresets (unit 1) in the distal part of the Lyngsii spit system (loc. 9) suggest that the platform front was influenced by tidal currents or by differences in set-up of the water caused by wind or both. A mainly tidal control is preferred as a flood- tidal delta has been recognized north of the point of the Lyngsii spit system (Nielsen & Johannessen, 1984). At the time when the prograding Lyngsii spit system reached loc. 9 the connection between the open sea and the bay was reduced so much that the tidal currents through the small strait became so concentrated that they could redeposit the major part of the material which was carried to the northern end of the spit by the coast parallel currents along the side of the spit. The trough cross-bedded, coarse-grained sets are thus interpreted as deposited by 3-D megaripples which migrated upon the spit platform top and front in directions toward the open sea and toward the bay, respectively, indicating that the ebb and flood currents were separated. Where the small strait was widened towards the Voldstrup Bay and Kattegat, respectively, flood tidal and ebb tidal deltas were formed (loc. 12, Fig. 3) (Johannessen & Nielsen, 1984b).

A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark 93 1

Unit 2B: oblique bar-trough system

Foreset orientations in the planar cross-bedding of unit 2B show that straight-crested bars migrated towards the exposed eastern and western beaches of the Lyngsi and Pudborg systems, respectively. The 3- D megaripples which generated the trough cross- bedding in contrast migrated more or less parallel to the spit beaches.

The steep foresets in the planar cross-bedded sets are interpreted as formed by avalanching. A distinct asymmetry and a well-established slipface are very often characteristic for the innermost bar in many recent coast parallel bar systems (Dabrio 1982; Dabrio & Polo, 1981; Davidson-Arnott & Greenwood, 1974, 1976; Davis et al., 1972; Hine, 1979; Hine & Boothroyd, 1978). The low water depth results in an increasing frequency and intensity of breaking waves. Accordingly, the steep planar avalanche type foresets are interpreted as being generated by waves plunging over strongly asymmetrical near shore bars developing strong horizontal translation currents which carried a high amount of sediment across the bar crests resulting in bar migration (cf. Davidson-Arnott & Greenwood, 1976).

The presence of cross-lamination, symmetrical ripples and fine-grained drapes show that the long, low-angle dipping planar foresets have not been formed by flow separation and avalanche, and that waves played an important role during sedimentation. From recent barred coasts it has been described how spilling waves on crests of outer bars produce asymmetrical currents resulting in a pulsating sediment transport over the symmetrical or slightly asymmetrical bars (e.g. Davidson-Arnott & Green- wood 1976). The gradually increased water depth on the lee side of the bar and the loss of energy due to spilling of the waves result in progressively weaker bottom currents towards land. The sediment which by-passes the bar crest is deposited as gently land- wards-dipping foresets, which pass (coastwards) into thick fine-grained bottom sets (cf. Davidson-Arnott & Greenwood, 1976). A preserved example of this is seen on Fig. 5. The frequent fine-grained foreset drapes reflect calm weather periods.

Steep bar foresets are mainly present in the upper part of unit 2B, while the low-angle foresets are more frequent in the lower part. This vertical trend is interpreted as a result of coastal progradation, whereby outer bar deposits are overlain by inner bar deposits.

The interpretation of the bar foresets suggests that

the thick and widespread cross-laminated bottom sets represent trough fill in a bar-trough system.

Silt-draped symmetrical ripples are interpreted as wave-generated ripples, while most cross-lamination seems to be generated by unidirectional currents. The currents which passed through the troughs originated from the net water movement across the bars generated by obliquely incoming waves. The very frequent trough cross-bedded sets are interpreted as the result of migration in the trough of 3-D megaripples during periods with relatively strong currents. The strong currents were probably generated when plunging waves on the bars caused a very large water transport across the bars.

Detailed examination of palaeocurrents from locs 1 to 6 and 8 suggests that the bars were obliquely orientated to the spit coast. The trough cross-bedded sets in the trough fill show a dominance of northerly current directions (Fig. 3). Only three measurements show a southerly direction. This suggests that the bars were attached to the spit coast during constructive periods and thus prevented southwards current flow. The strong currents through the troughs were generated by oblique incoming waves during southerly and easterly winds. A similar regulating influence on the trough currents has been described from recent bar- trough systems by Van den Berg (1 977) and Hunter et al. (1 979). The current-generated cross-lamination also shows a distinct northerly mode, but southerly directions have also been measured. The latter were probably caused by weaker and relatively rare currents formed during periods with dominant northerly winds or by eddies formed by the dominant north-flowing currents.

Bar asymmetry in general appears to be most pronounced near the land-attached end, while the coast distant end only is slightly asymmetric with a gently dipping slipface (e.g. Davidson-Arnott & Greenwood, 1974, 1976; own observations on recent active oblique bars).

The presence of steep foresets within sets dominated by low-angle foresets suggests that plunging waves during strong onshore winds caused development of a steep slipface in the shore distant end, too. In these cases water transport across the bar crests probably was so extensive that sinuous or lunate megaripples were the only bedform in the trough. When wave energy ceased, migration of the bars stopped. The steep foresets were truncated by an erosion surface which re-established the gentle landward sloping slipface.

932 L. H . Nielsen, P. N . Johannessen and F. Surlyk

Unit 3: beach deposits

This unit forms the top stratum in the topographic ridges of each spit system. The coarse grain-size of the unit suggests deposition under high energy conditions. The dominantly parallel stratification in the alternating pebbly and sandy beds is interpreted as representing the upper plane bed phase of the upper flow regime. The small bed lenticularity and relatively good segregation of the pebbly and sandy beds suggests that the deposits were generated by waves (Clifton, 1973).

The stratification and the bed boundaries at locs 1- 6 strike parallel to the crest of the 10 km long ridge. This suggests wave transport and reworking along the full length of the ridge, and similar structures have been recognized from recent coasts in the swash- backwash zone (Thompson, 1937; Clifton, 1969; Cliftonetal., 1971 ; Reineck &Singh, 1973; Semeniuk & Johnson, 1982). The scattered occurrences of laminae sets dipping oppositely to the general direction, forming type A sequence of Thompson (1937), are interpreted as representing berm lamination.

The inclination of the stratification in the sand beds shows that the swash zone dipped only a few degrees seaward during deposition, whereas the dip of the stratification of the pebbly layers as well as the bed boundaries show that the swash zone had a steeper gradient during deposition of the pebbly beds. This change in foreshore gradient is probablyjust a function of grain-size (Komar, 1976) and thus may only reflect indirectly a change from a gentle fair-weather profile to a steeper rough weather profile. The regular change which is present in some places (Fig. 13) may represent seasonal variation between a summer and winter profile.

The gently dipping swash zone passes downwards and laterally in a seawards direction into relatively small coarse-grained trough cross-beds (Fig. 13). The interfingering of the swash zone lamination and the trough cross-bedding indicates that the latter was formed in the ‘swash-trough transition zone’ described from recent barred coasts (Davidson-Arnott & Green- wood, 1976; Hunter et al., 1979) or the ‘inner rough zone’ of non-barred coasts (Clifton et al., 1971; Semeniuk & Johnson, 1982).

The planar cross-bedding with foresets dipping oppositely to the swash lamination is interpreted as representinglongshore bars migrating up the foreshore and finally welding to the beach, causing a progradation of the shore and preservation of planar cross- bedded sets (Dabrio, 1982; Davis & Fox, 1972; Davis et a/., 1972; Hine, 1979; Hine & Boothroyd, 1978).

The slightly seaward-dipping lamination above the foresets probably represents wave reworking by swash-backwash of the partly exposed stoss-side of the swash bars or possibly the steeper toe of the beach in the inner rough zone of Clifton et a/. (1971).

The up to 1 m thick pebbly layer topping unit 3 is characterized by a lower erosive boundary with a concentration of the largest clasts, and by diffuse internal stratification, and poorly developed imbrication. The stratification and the imbrication planes dip in the same direction as the underlying swash lamination. Komar (1976) has described similar features from recent beach ridges formed in the backshore during strong onshore winds. Jessen (1899, 1936) and Andersen (1961) have mapped beach ridges on the present spit systems on the basis of surface morphology. Accordingly the uppermost part of unit 3 is interpreted as storm-generated beach ridge deposits. Fig. 14 gives a slightly generalized interpreted section through these deposits in the upper part of the Lyngsi spit system.

VERTICAL SEQUENCES

Two characteristic vertical sequences occur in the spit-platform system. Sequence 1 consists of platform foreset, topset and beach (units 1-2A-3). Sequence 11- consists of platform foreset, bar-trough and beach (units 1-2B-3). The two sequences pass laterally into each other, but the zone of overlapping is relatively short and has only been seen exposed in one locality. A transitional sequence between I and I1 can thus also be recognized. It consists of units 1-2A-2B-3, where the combined thickness of 2A and 2B corresponds to the thickness of unit 2A in sequence I or unit 2B in

--=l BEACH RIDGE

SWASH BAR

TRANSITION TO

INTERMEDIATE PART

SHORE DISTANT PART

STORM DEPOSITS

Fig. 14. Generalized, synthetic sequence of the upper units of the Lyngsk spit system combined from several exposures.


sequence 11. The two sequences are discussed and contrasted below.

Sequence I (units 1-2A-3)

The vertical sequence of giant platform foresets (unit 1)- topset (unit 2A) - beach (unit 3) has been observed in the Pudborg spit system (locs 10 & 1 1 ) and in the

I IilI

Sand $ n :

Sand ij n a

distal end of the Lyngsa spit system (loc. 9). It represents a clear shallowing-upward trend, formed by the progradation of a spit system.

Sediment transport from the proximal source area to the distal end of the spit system took place along the seawards side of the spit. The topset (unit 2A) was deposited on the platform in front of the end of the spit, where the long-beach current expanded, and the

II

Sand ji n 2

I I/II II Beach ridge

Swash bar

Platform --I topset

Bar-trough

Platform foreset

PLATFORM PROGRADATION

SPIT PROGRADATION I IiII II - 7 7 7

3 2 A / 2 0 / Z

1

Fig. 15. Dynamic model for coarse-grained spit-platform system prograding into relatively deep water. Sequence I represents a situation where the spit has prograded almost to the end of the platform. The platform top is thus exclusively formed by the topset beds (unit 2A). Sequence I1 represents a stage of platform progradation with maximum distance from the end of the spit to the end of the platform. The platform top is thus in this case represented exclusively by the oblique bar-trough deposits (unit 2B). Sequence 1/11 marks a transitional stage where an oblique bar-trough system is being welded to the spit-beach and overlying the topset beds (unit 2A). Below is shown three consecutive states in platform and spit progradation showing the inversely related phases of platform and spit progradation.

934 L. H . Nielsen, P. N . Johannessen and F . Surfyk

wave energy per unit length along the spit decreased because of refraction (Fig. 15). Spit progradation occurred when swash bars on the platform turned around and towards the end of the spit where they became welded to the coast as described for recent systems by Hine (1979).

In the LyngsA spit system, however, tidal currents had a strongly modifying effect on the northern end of the system when a narrow tidal inlet was formed before the bay was completely closed off (Figs 2 & 3).

The LyngsA platform is up to 1.6 km wide. Evans (1942) noted that the platform is a relatively stable structure in recent systems, although the subaerial spit constantly changes in form and position on the platform following the weather conditions. It is thus likely that the spit in the LyngsA system frequently changed position on the platform. An alternation between large platform segments composed of fine- and coarse-grained foresets, respectively, has been observed both along and across strike of the platform foresets. This can best be explained as representing shifting positions of the spit and its beach on the platform. Coarse-grained, pebbly foresets are only formed where the end of the spit is close to the platform front (Fig. 15). However, rapid alternations between fine and coarse-grained foresets are probably due to short-term changes in wave dynamics.

Sequence I1 (units 1-2B-3)

The vertical sequence of platform foreset, bar-trough and beach (units 1, 2B and 3) have been observed in the proximal part of the LyngsA spit system (locs 1-6, Fig. 3). It represents progradation of the spit beach and its bar-trough system over the platform (Fig. 1 5 ) . This seawards progradation was towards the east at a right angle to the northward progradation direction of the still active end of the spit (Fig. 3).

The upper part of the sequence corresponds to the ideal sequence of a prograding barred nearshore system of Hunter et al. (19793. The progradational sequence above the platform deposits is initiated with a horizontal pebble-strewn erosion surface which truncates the top of the giant platform foresets. The erosion surface has been traced for at least 350 m. A comparable erosion surface was shown by Hansen (1951, p. 185) from a locality in the proximal south- eastern part of the Pudborg spit system (Figs 2 & 3). The erosion surface is overlain by trough cross-bedded deposits, and the lower set boundaries merge with the erosion surface. Palaeocurrent measurements show a strong mode towards NNW parallel to the length of

the spit-platform system. This suggests that the erosion surface was formed by 3-D megaripples migrating in the troughs parallel to the shore.

The ideal vertical sequence for an oblique bar- trough system is likewise initiated by a subhorizontal erosion surface which is formed by rip channel erosion (Hunter et al., 1979). However, this possibility can be ruled out in the present case as offshore-directed palaeocurrent directions are not represented (Fig. 3).

DYNAMIC MODEL FOR

SEQUENCES PROGRADATIONAL SPIT-PLATFORM

The two types of vertical sequences described above (Sequence I and 11) make it possible to develop a general, dynamic model for progradational coarse- grained spit-platform systems (Fig. 16). The basic, non-dynamic interpretation of the system is based solely on morphology and sedimentary features. The terminology is based on Meistrell’s (1966, 1972) concept of a subaquatic spit-platform and a subaerial spit. Theseelements were also recognized by Kumar & Sanders (1 974) in their study of the Fire Island Inlet sequence.

Our data furthermore confirm Meistrell’s (1966, 1972) observations on the growth dynamics of the spit and spit-platform. These concepts are consequently incorporated in our model.

Meistrell (1966, 1972) demonstrated in his wave tank experiments that sediment upon reaching deeper water at the end of a ‘headland’ beach was deposited in a successive series of foreset beds. This physical extension of the beach is the beginning of the platform structure. As the platform continued to increase in length Meistrell could recognize three major charac- teristics: (1) the depth of the water above the platform remained constant irrespective of irregularities in shelf topography, (2) the structure was basically composed of foreset and topset beds, and (3) a spit ridge formed on the top of the platform. For each of his experiments, at a point in time of the development of the platform, a spit started to form on the top of the platform.

Dynamically, in general, the growth of the spit and platform structures are inversely related and occur in alternating cycles. Thus, when the rate of growth of the platform declines, the spit grows uniformly, whereas when the platform grows uniformly, the rate of growth of the spit declines (Meistrell, 1966, 1972).

The two characteristic sequences (I and 11) and the


Fig. 16. Dynamic three-dimensional morphological-sedimentological model for coarse-grained spit systems prograding into deep water. 1, Spit platform; 2, oblique bar-trough system; 3 , spit beach.

volumetrically less important transitional sequence (I/ 11) recognized in this study are interpreted as reflecting alternating cycles of growth of the spit and spit- platform system (Fig. 15).

The sedimentological mechanism for the growth pattern will now be described in more detail.

In the constructive phase of the development of a spit system, obliquely incoming waves will create an oblique bar-trough system on the platform in the surf zone along the seaward side of the spit. Sand is transported forward to the edge of the platform by trough currents and migration of bars, while pebbles are transported in the swash-backwash zone along the spit coast. New giant-scale foresets are formed by avalanche down the steep platform front causing a progradation of the platform.

As a result of erosion of the mainland, the spit system will be eroded in the proximal end, as fast as the coast on the mainland retreats. Further ahead along the spit there is a balance between erosion and deposition. Net deposition will occur where the coast begins to turn away from the sea and towards the bay.

As a result of the bending of the coast, the waves will be refracted causing a reduction of the wave energy per unit length of the coast. At the same time there is an expansion of the coast-parallel currents over the platform, because the controlling effect of the subaerial spit gradually ceases. This combination causes high sedimentation on the platform along the end of the spit, implying that the 'refracted' bars will

be wider and longer and the troughs shallower (Hine, 1979). This modification of bars and troughs will be more and more extensive, the more the spit coast bends away from the sea.

In the case where the distance from the tip of the spit to the edge of the platform is very short, the surface of the platform will be relatively steep (up to 7") and consist of coarse-grained sediment (Fig. 15). Because bar-trough systems only generate on flat coasts with much lower slope (Komar, 1976), a bar- trough system will not exist in front of the spit. Consequently, a prograding coarse-grained spit system with a short platform will generate a characteristic sequence consisting of a gravelly giant-scale cross- bedded foreset, overlain by dipping stratified, gravelly topset beds, which again are overlain by coarse- grained beach deposits (Fig. 15, sequence I).

From the point on the spit coast where the bar- trough system begins to bend around the tip of the spit, and to the point where the bar-trough system is destroyed, the bars will migrate towards and weld onto the spit coast. In this manner successive bars will contribute to a seaward progradation of the spit tip. An idealized sequence for this part of the spit system will consequently consist of sandy giant foreset and topset overlain by sediments deposited in the bar- trough system (Unit 2B), (Fig. 15, sequence 1/11). Gradually, as the bars migrate up the shoreface, they will emerge and become swash bars, with swash- backwash lamination generated on their seaward side.

936 L. H . Nielsen. P. N . Johannessen and F. Surlyk

In cases where the distance from the tip of a spit to the edge of the platform is large and the surface of the platform consequently has a small slope, the bars would be able to ‘refract’ the whole way around the tip of the spit resulting in a forward spit progradation as the bars merge with the beach. Consequently, sequence I1 (Fig. 15) will be formed all along the tip of the spit and sequence I will not be generated.

Since the tip of a subaerial spit often changes its position on the platform the distance from the tip to the edge of the platform will vary. Consequently, longitudinal sections through a spit system may show an alternation between sequence I and 11. Transport directions and the degree of modification of the preserved bars thus indicate in which position on the curved tip of the spit the sequence was formed. A sequence formed on the seaward side of the spit will tend to show a much more condensed depositional history than a sequence formed in front of the spit, because of the much larger progradation rate at the end of the spit compared to the progradation rate on the seaward side of the spit.

The 10 km long Lyngsi spit system prograded across a large bay which it almost closed off (Figs 2 & 3). In this final stage the topset beds and the giant foresets are nearly masked by medium- to large-scale trough cross-beds consisting of pebbly sand (Fig. 7). Characteristically, cross-bedding in the giant foreset and the topset indicate sediment transport into the bay and out of the bay, respectively, although bimodal current directions are seen in both units. The change in configuration of the giant foreset and topset in the distal end of the spit system is related to an increased influence of tidal currents caused by narrowing of the bay mouth and the formation of a tidal inlet. The tidal currents became concentrated and redeposited a significant portion of the material carried to the end of the spit by longshore currents along the seaward side of the spit. Kumar and Sanders (1974) have described a similar internal structural configuration of the platform from a spit situated in a tidal setting with tidal range of c. 2 m.

APPLICATIONS AND LIMITATIONS OF THE SPIT MODEL

The model has allowed the prediction of three additional Pleistocene spit systems in northern Den- mark (Johannessen & Nielsen, 1984a).

If the sequences are fully preserved the precise sea-

level and water depth can be determined for the time when the spit system prograded (Johannessen & Nielsen, 198413).

The model is presumably only valid for spit systems composed of relatively coarse-grained sediment, because only such deposits have a steep platform front (Zenkowitch, 1967). Furthermore, the water depth in which the spit system progrades determines the thickness of the giant-scale cross-bedded set. If the water depth is small-less than a couple of metres- no spit platform will be developed. This was the case where the Pleistocene spit system prograded over elevations on the sea bottom.

The preservation potential of spit systems is considered to be relatively high in micro-tidal regimes but decreases with increasing tidal range and the systems are rarely or never developed under macro- tidal conditions.

Giant-scale cross-bedding has been described from aeolian dunes (e.g. Glennie, 1970), fluvial mid-channel bars (Coleman, 1969), alternate side bars (McCabe, 1977) and point bars (Ori & Ricci Lucchi, 1981), subtidal sand waves (e.g. Allen, 1980, 1982), fan deltas and Gilbert deltas (e.g. Clemmensen & Houmark- Nielsen, 1981; Gilbert, 1885; Surlyk, 1975). The model for spit-platform systems presented here adds further mode of formation of this impressive sedimentary structure. Special care should be taken to distinguish between giant-scale cross-bedding of marine Gilbert delta origin and spit-platform origin as these structures taken out of context show a high degree of similarity.

ACKNOWLEDGMENTS

This study is based on thesis work carried out by Nielsen and Johannessen at the University of Copen- hagen under the supervision of Surlyk. Radiocarbon dates were determined by H. Tauber at the 14C dating laboratory of the Geological Survey of Denmark and the Danish National Museum. We wish to express our gratitude to H.E. Clifton, L.B. Clemmensen and C.J. Dabrio for constructive criticism of the manuscript.

We thank B. Larsen and M. Larsen for typing the manuscript, J. Lautrup for photographic illustrations, and B. Sikker Hansen for artwork. The paper was written while Surlyk was recipient of a research professorship awarded by the Danish Natural Science Research Council.


REFERENCES

ALLEN, J.R.L. (1980) Sand waves: a model of origin and internal structure. Mar. Geol., 26, 281-328.

ALLEN, J.R.L. (1982) Sedimentary Structures: Their Character and PhysicalBasis. Vol. 11. Elsevier, Amsterdam, 663 pp.

ANDERSEN, S.A. (1961). Geologisk f0rer over Vendsyssel. Historisk Samfund for Hjsrring Amt. Ksbenhavn. 207 pp.

BERG, J.H. VAN DEN (1977) Morphodynamic development and preservation of physical sedimentary structures in two prograding recent ridge and runnel beaches along the Dutch coast. Geol. Mijnb. 56, 185-202.

CLEMMENSEN, L.B. & HOUMARK-NIELSEN, M. (1981) Sedi- mentary features of a Weichselian glaciolacustrine delta. Boreas, 10,229-245.

CLIFTON, H.E. (1969) Beach lamination-nature and origin. Mar. Geol., 7 , 553-559.

CLIFTON, H.E. (1973) Pebble segregation and bed lenticularity in wave-worked versus alluvial gravel. Sedimentology,

CLIFTON, H.E., HUNTER, R.E. & PHILLIPS, R.L. (1971) Depositional structures and processes in the non-barred high-energy nearshore. J. sedim. Petrol., 41, 651-670.

COLEMAN, J.M. (1969) Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3, 129-239.

DABRIO, C.J. (1982) Sedimentary structures generated on the foreshore by migrating ridge and runnel systems on microtidal and mesotidal coasts of S . Spain. Sediment. Geol.,32, 141-151.

DABRIO, C.J. & POLO, M.D. (1981) Flow regime and bedforms in a ridge and runnel system, SE. Spain. Sediment. Geol., 28,977109.

DAVIDSON-ARNOTT, 'R.G.D. & GREENWOOD, B. (1974) Bedforms and structures associated with bar topography in the shallow water wave environment, Kouchibouguac Bay, New Brunswick, Canada. J. sedim. Petrol, 44, 698- 704.

DAVIDSON-ARNOTT, R.G.D. & GREENWOOD, B. (1976) Facies relationships on a barred coast, Kouchibouguac Bay, New Brunswick, Canada. In : Beach and Nearshore Sedimenta- tion (Ed. by R.A. Davis Jr. & R.L. Ethington). Spec. Pubis Soc. econ. Paleont. Miner., Tulsa, 24, 149-168.

DAVIS, R.A. JR & FOX, W.T. (1972) Coastal processes and nearshore sand bars. 3. sedim. Petrol., 42,401-412.

DAVIS, R.A. JR., Fox, W .T., HAYES, M.O. & BOOTHROYD, J.C. (1972) Comparison of ridge and runnel systems in tidal and non-tidal environments. 3. sedim. Petrol., 42,

DOTT, R.H. JR. & BOURGEOIS, J . (1982) Hummocky stratification: significance of its variable bedding sequences. Bull. geol. Soc. Am., 93,663-680.

EVANS, O.F. (1942) The origin of spits, bars, and related features. In: Spits and Burs (Ed. by M. L. Schwartz), pp. 53-72. Dowden, Hutchinson and Ross, Stroudsburg, P.A.

FREDERICIA, J. (1984) Geologisk basisdatakort 1 : S O 000 nr. 1317 I, 1317 11. Danmarksgeologiske Unders0gelse.

GILBERT, G.K. (1885) The topographic features of lake shores. Ann. Rept. U S . Geol. Surv., 5,75-123.

GLENNIE, K.W. (1970) Desert sedimentary environment. Developments in Sedimentology, 14, 222 pp. Elsevier, Amsterdam.

20,173-187.

41 3-421.

HANSEN, S. (1951) Ekskursion B. Nordlige Jylland 23-29 maj 1951. Medd. DanskGeol. Foren., 12, 181-186.

HINE, A.C. (1979) Mechanisms of berm development and resulting beach growth along a barrier spit complex. Sedimentology, 26,333-351.

HINE, A.C. & BOOTHROYD, J.C. (1978) Morphology, processes, and recent sedimentary history of a glacial outwash plain shoreline, southern Iceland. J . sedim. Petrol., 48,

HUNTER, R.E. & HILL, G.W. (1977) Nearshore current pattern of South Texas: an interpretation from aerial photographs (abstract). Bull. Am. Ass. Petrol. Geol., 59, 798.

HUNTER, R.E., CLIFTON, H.E. & PHILLIPS, R.L. (1979) Depositional processes, sedimentary structures, and pre- dicted vertical sequences in barred nearshore systems, southern Oregon coast. J. sedim. Petrol., 49,711-726.

JESSEN, A. (1899) Beskrivelse ti1 Geologisk kort over Danmark. Kortbladene Skagen, Hirtshals, Frederik- shavn, Hjsrring og Lskken. Danm. geol. Unders., 1 rckke 3,368 pp.

JESSEN, A. (1918) Vendsyssels Geologi. Danm. geol. Unders., 5 rzkke, 2,260 pp.

JESSEN, A. (1936) Vendsyssels Geologi. Revised edition. Danm. geol. Unders., 5 rzkke, 2,195 pp.

JOHANNESSEN, P.N. & NIELSEN, L.H. (1984a) Senglaciale oddesystemer og deres implika f ioner for de Senglaciale havniveau andringer i 0st Vendsyssel, Danmark. Unpub- lished Thesis, part 2, Ksbenhavns Universitet, Danmark. 32 PP.

JOHANNESSEN, P.N. & NIELSEN, L.H. (1984b) Den geologiske udvikling af et Senglacialt marint plateau i 0st Vendsyssel, Danmark. Unpublished thesis, part 3, Ksbenhavns Univ- ersitet, Danmark. 41 pp.

JOHANNESSEN, P.N. & NIELSEN, L.H. (1986) A sedimentological model for spit systems prograding into deep water. (Abstr.) 7th Reg. Mtg Krakow, 1986,4 pp.

KNUDSEN, K.L. (1978) Middle and Late Weichselian marine deposits at Nsrre Lyngby, northern Jutland, Denmark, and their foraminifera1 faunas. Danm. geol. Unders., 2 rzkke, 112,44 pp.

KOMAR, P.D. (1976) Beach Processes and Sedimkniation. Prentice-Hall, Englewood Cliffs, 429 pp.

KROG, H. & TAUBER, H. (1974) C-14 chronology of Late- and Postglacial marine deposits in North Jutland. Danrn. geol. Unders., Arbog 1973,93-105.

KUMAR, N. &SANDERS, J.F. (1974) Inlet sequence: a vertical succession of sedimentary structures and textures created by the lateral migration of tidal inlets. Sedimentology, 21,

MCCABE, P.J. (1977) Deep distributary channels and giant bedforms of the Central Pennines, North England. Sedimentology, 24,27 1-290.

MCKENZIE, P. (1958) Rip current systems. J . Geol., 66, 103- 113.

MEISTRELL, F. J. (1966) The Spit-Platform Concept: Labora- tory Observation of Spit Development. Unpublished Mas- ter's Thesis, University of Alberta, Edmonton, Alberta,

MEISTRELL, F.J. (1972) The spit-platform concept: laboratory observation of spit development. In: Spits and Bars (Ed. by M.L. Schwartz), pp 225-283, Dowden, Hutchinson and Ross, Stroudsberg, PA.

901-920.

49 1-532.

46 PP.


NIELSEN, L.H. & JOHANNESSEN, P.N. (1984) En sedimentolo- gisk model for Pleistocene oddesystemer, Elst Vendsyssel, Danmark. Unpublished thesis, part 1, Knbenhavns Univ- ersitet, Danmark, 68 pp.

ORI, G.G. & Ricci LUCCHI, F. (1981) Giant epsilon bedding in coarse-grained point bars of late Pliocene fan delta systems, Int. Ass. Sedim., 2nd Reg. Mtg., Bologna

RAAF, J.F.M. DE, BOERSMA, J.R. & GELDER, A. VAN (1977) Wave generated structures and sequences from a shallow marine succession. Lower Carboniferous, County Cork, Ireland. Sedimentology, 4, 1-52.

REINECK, H.-E. & SINGH, I.B. (1973) Depositional Sedimen- tary Environments- With Reference to Terrigeneous Clastics. New York, Springer Verlag, 439 pp.

SEMENIUK, V. &JOHNSON, D.P. (1982) Recent and Pleisto- cene beach/dune sequences, Western Australia. Sediment. Geol., 32,301-328.

SURLYK, F. (1975) Block faulting and associated marine sedimentation at the Jurassic-Cretaceous boundary, East Greenland. NPF-Northern North Sea Jurassic Symp., 7 , 1- 31.

TAUBER, H. (1966) Danske kulstof-14 dateringsresultater 11. Medd. Dansk Geol. Foren., 16, 153-176.

THOMPSON, W.O. (1937) Original structures of beaches, bars and dunes. BUN. geol. SOC. Am., 48, 723-752.

ZENKOVITCH, V.P. (1967) Processes of Coastal Development. Oliver and Boyd, London. 738 pp.

(Manuscript received 9 November 1987: revision received 21 March 1988)

A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark

Documents

Transcript of A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark