rønø and kent 1999

Late Triassic structural evolution of the southern margin of theRingkbing-Fyn High, Denmark

Ole Rn Clausen*, Per Kent Pedersen

Department of Earth Sciences, University of Aarhus, DK-8000, Aarhus, C, Denmark

Received 16 December 1998; received in revised form 12 June 1999; accepted 17 June 1999

Abstract

A Late Triassic (Carnian) unconformity is observed on seismic sections and in wells along the southern margin of the

Ringkbing-Fyn High. The unconformity is characterised by low angle erosion on the central parts of the eastern Ringkbing-Fyn High (Mn High), and by a signicant onlap onto the unconformity surface. Structural reconstruction shows that theunconformity is related to dierential subsidence between the Ringkbing-Fyn High and the North German Basin. The

dierential subsidence initiated movements in the mobile Zechstein Salt, causing deformation of the cover sediments. Thevariability in dierential subsidence observed during the Triassic along the southern margin of the Ringkbing-Fyn High isinterpreted to reect the subdivision of the Ringkbing-Fyn High into a number of structure blocks. # 1999 Elsevier ScienceLtd. All rights reserved.

Keywords: Triassic; Denmark; Tectonics

1. Introduction

The objective of this study is to analyse the LateTriassic structural evolution of the southern margin ofthe Ringkbing-Fyn High with special emphasis onthe geological signicance of a Late Triassic unconfor-mity observed in wells and on seismic sections. Thestudy integrates observations on sedimentology, faciesevolution, and hiatuses made in wells in the south-east-ern part of the Danish area with structural interpret-ations of seismic sections. The study uses releasedconvectional 2-D seismic surveys (GY84 T, PRKL80,WGC79, WGC81), high resolution seismic surveyDA98 (owned by the Department of Earth Sciences,University of Aarhus), and well logs, cuttings andcores from the wells: Ringe-1, Sllested-1, Rdby-1,Kegns-1, rslev-1, Tnder-1, Lgumkloster-1,Slagelse-1, R-1 and S-1 (Fig. 1).

1.1. Structural setting of the study area

The study area encompasses the northern margin ofthe North German Basin and the southern part of theNorwegianDanish Basin (Fig. 1). In Triassic timesnorthern and central Europe formed a large land-locked epicontinental basin, the Northwest EuropeanBasin (Ziegler, 1990), which was composed of numer-ous sub-basins, such as the NorwegianDanish Basinand the North German Basin separated by structuralhighs such as the Ringkbing-Fyn High (Fig. 1). Westof the Central Trough area, the Silverpit Basin islocated south of the Mid North Sea High in a similarsetting to the North German Basin (Coward &Stewart, 1995).

The Ringkbing-Fyn High forms part of the gener-ally eastwest striking system of highs running acrossthe North Sea area (Ziegler, 1990). The individualblocks of the Ringkbing-Fyn High are referred toaccording to the nomenclature given by Vejbk (1997)(Fig. 1). The Ringkbing-Fyn High is characterised asan area where the crystalline basement is shallow com-pared to the adjacent basins. The Ringkbing-Fyn

Marine and Petroleum Geology 16 (1999) 653665

0264-8172/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.PII: S0264-8172(99 )00026 -4

* Corresponding author. Tel.: +45-89-42-25-18; fax: +45-89-42-

25-25.

E-mail address: [email protected] (O.R. Clausen)

High was formed during a pre-Zechstein stretchingphase in which the Ringkbing-Fyn High area sueredlesser stretching than the adjacent basins (Vejbk,1997). It cannot be excluded that the pre-ZechsteinRingkbing-Fyn High was fault bounded since theseismic resolution beneath the Top pre Zechstein sur-face generally is poor. However, the faults at the Toppre-Zechstein surface indicates little or no fault controlof the post-Zechstein dierential subsidence (Fig. 1)and the dierential subsidence between the basins andthe high thus has had the character of regional inex-ion. The Ringkbing-Fyn High formed a barrier andseparated the Southern and Northern Permian Basinsin Late Permian time (Clark & Tallbacka, 1980), andwas inundated in early Triassic times (Pedersen, 1998).Thinning of Triassic deposits across the Ringkbing-Fyn High has previously and mainly been interpretedas due to slower subsidence on the Ringkbing-FynHigh than in the North German Basin andNorwegianDanish Basin and partly due to erosiondue to Mid-Jurassic relative uplift of the Ringkbing-Fyn High (Sorgenfrei, 1969; Bertelsen, 1980;Cartwright, 1990; Kockel, 1995).

The subsidence of the NorwegianDanish Basin hasbeen attributed to thermal subsidence following pre-Triassic rifting (Srensen, 1986; Vejbk, 1990).However, the Horn Graben, which is located in the

eastern North Sea (Fig. 1), shows active rifting andconsequent deposition of a very thick Lower andMiddle Triassic succession (Bertelsen, 1980; Vejbk,1990; Clausen & Korstga rd, 1993). The SorgenfreiTornquist Zone, which is a major strike-slip zone deli-miting the sedimentary basin to the north-east (Fig. 1),experienced right-lateral transtensional movementsduring the Triassic (Mogensen, 1994; Vejbk, 1990;Michelsen, 1997).

Analysis of the character and cause of the thinningof the Triassic succession is the main objective of thepresent study.

2. Triassic stratigraphy

The Triassic succession in the Danish area has beensubdivided lithostratigraphically by Bertelsen (1980)and revised by Pedersen (1998) (Fig. 2). The subdivi-sion is based on signicant changes in facies, which arereected in changes in lithology (Pedersen, 1998). Thelithostratigraphy dened south of the study area forthe German area by Beutler and Schu ler (1987) is cor-related with the Danish lithostratigraphy by Pedersen(1998) as shown in Fig. 2. Correlation with the UKlithostratigraphy is hampered by poor biostratigraphiccontrol and lateral continuity across the central parts

Fig. 1. Map showing the study area, dominated by the Norwegian Danish Basin and the North German Basin separated by the Ringkbing-Fyn

High. The NorwegianDanish Basin is bounded to the north-east by the SorgenfreiTornquist Zone. Wells presented in this paper are shown as

lled circles. Seismic sections used in this study are indicated by full lines. The names of the blocks of the Ringkbing-Fyn High is from Vejbk

(1997) as: ENBl: East North Sea Block, GrBl: Grindsted Block, GlBl: Glamsbjerg Block, StBl: Stigsns Block, MH: Mn High.

O.R. Clausen, P.K. Pedersen /Marine and Petroleum Geology 16 (1999) 653665654

of the North Sea Basin. However, the Schilfsandsteinis interpreted to correlate with the Arden SandstoneMember (Mercia Mudstone Group) in the UKsouthern North Sea (Warrington & Ivemey-Cook,1992). The Triassic succession in southern Denmark isconstituted by Lower Triassic redbeds (Bunter Shale,Bunter Sandstone and rslev Formations), MiddleTriassic carbonates (Falster Formation), UpperTriassic redbeds, sandstone and evaporites (Tnderand Oddesund Formations) and Upper TriassicLowerJurassic marine shales and sandstone (Vinding andGassum Formations) (Fig. 2). The lateral facies vari-ations in the southern Danish area and in the northernpart of the North German Basin are so small and gra-dual that a correlation of the units is possible(Bertelsen, 1980). This mainly non-marine succession ischaracterised by several essentially synchronous mar-ker beds, such as the basal evaporite bed of the rslevFormation (Figs. 2 and 3).

In the Danish and North German basins, theTriassic lithostratigraphic units are generally chronos-tratigraphically signicant as shown in several studiesboth in the Danish and the German areas (e.g.Bertelsen, 1980; Aigner & Bachmann, 1992; Pedersen,1998). The lithostratigraphic subdivision in the dier-ent wells is based on petrophysical log characteristics

supplemented by cuttings and cores when available.Due to the continental character of the sediments, thebiostratigraphic information from the boreholes is lim-ited to absent.

3. `Late Triassic unconformity'

Beutler and Schu ler (1978, 1987) interpreted anunconformity, the `Late Triassic unconformity', withinthe upper Triassic in the Rdby-1 and rslev-1 wells,which they correlated to the `AltkimmerischeHauptdiskordanz', an angular unconformity observedat outcrops and in wells in the eastern part of theNorthwest European Basin (Beutler, 1995). In a revi-sion of the upper Triassic lithostratigraphy byPedersen (1998), the `Late Triassic unconformity' wasshown regionally to occur within the upper part of theTnder Formation, e.g. in the Sllested-1 at the baseof thick sandstone interval (Fig. 3). The interpretationof Pedersen (1998) involved all available wells andpositioned the unconformity within the CarnianTnder Formation (Figs. 2 and 3). This interpretationdiers from Beutler and Schu ler (1978, 1987), who inthe Rdby-1 and rslev-1 positioned the unconformityat the top of the Lower Norian O-3 member of the

Fig. 2. Triassic lithostratigraphic scheme, compiled from Beutler and Schu ler (1987), Aigner and Bachmann (1992) and Pedersen (1998).

O.R. Clausen, P.K. Pedersen /Marine and Petroleum Geology 16 (1999) 653665 655

Fig.3.Lithostratigraphic

logcorrelationofthelate

Lower

toUpper

Triassic.Note

the`Late

Triassic

unconform

ity'in

therslev-1,Rdby-1

andSlagelse-1

wells

andthinningoftheTnder

Form

ationin

theSllested-1

well.Thepositionofthelithostratigraphic

boundaries

isdened

byfacies

shifts

interpretedonthebasisofwelllogs,coresandcuttings.


Fig.4.Seism

icsectionsshowingthegeometry

oftheTriassic

successionfrom

theNorthGermanBasinto

theMnHigh.Thesection(a)is

acomposite

sectionfrom

twoseismic

surveys.

The

section(c),whichisanenlarged

part

of(a)ties

totherslev-1

wellwheretheunconform

ityisobserved

attheerosivetopoftheFalsterForm

ation.Note

theonlappingreectors

oftheinternal

reectors

inthesuccessionoverlyingtheunconform

ity.Themap(b)showsthesouthernmargin

oftheRingkbing-FynHigh,thepositionofthemajorsalt-structures(from

Vejbk&

Britze,

1994)andlocationofthewellsandseismic

sectionsusedhere.


Fig. 5. Structural reconstruction of the seismic section from Fig. 4. (a)(g) shows the geometric reconstruction of the section. The dashed line at

the basement in (f) and (g) shows the unfaulted Top pre-Zechstein if assumed that the faulting took place during the Triassic. (h) Shows the dip

of the basement from the dierent reconstructed sections superimposed unto each other. The lines `f' and `g' dips less that the other lines which

indicated that the major change in dip of the basement takes place between section (e) and (f). See text for further discussion.


Oddesund Formation (see Pedersen (1998) for adetailed discussion). Pedersen (1998) thus argues thatlocally erosion of the upper part of the FalsterFormation took place e.g. in the rslev-1 well (Fig. 3).The erosional character at the top of the FalsterFormation is also shown by erosional truncations onseismic sections (Fig. 4). In the Danish area, the `LateTriassic unconformity' is thus interpreted to be locatedat the top of the Falster Formation in the rslev-1and Rdby-1 wells (as indicated in Fig. 2).

The `Late Triassic unconformity' has not beenrecognised on seismic sections in southern Jutland, inspite of the fact that a hiatus interpreted in wellsencompasses a considerable period with non-depo-sition and local erosion in the eastern part of theRingkbing-Fyn High (Fig. 2). The regional extentof the `Late Triassic unconformity' and the dierencein the Upper Triassic succession encountered in thetwo closely spaced Rdby-1 and Sllested-1 wells willbe discussed below on the basis of seismic interpret-ation.

3.1. Age of the `Late Triassic unconformity'

The hiatus in the Rdby-1 and rslev-1 wells at thetop of the Middle Triassic Falster Formation,described above, encompasses a period from theLadinian (upper Falster Formation) to the LowerNorian (O-3 member of the Oddesund Formation;Figs. 2 and 3). In the Slagelse-1 well, the calcareousdeposits of the Falster Formation are abruptly over-lain by arenaceous deposits of the upper part of theTnder Formation (Fig. 3). The Tnder Formation inthe Sllested-1 well is relatively thin compared to else-where (Fig. 3). In the Sllested-1 well, the calcareousdeposits of the Falster Formation are gradually over-lain by arenaceous redbeds of the Tnder Formation,similar to the facies transition encountered further tothe west (Fig. 3; Pedersen, 1998). Likewise, the upperboundary of the Tnder Formation is a gradual faciestransition from arenaceous deposits to the anhydriticredbeds of the overlying Oddesund Formation (Fig. 3).The base of this upper arenaceous succession is a pro-nounced log-break with an abrupt increase in gammaray readings, which is interpreted as a `Late Triassicunconformity' within the middle part of the TnderFormation (Fig. 3).

The hiatus observed in the Rdby-1 and rslev-1wells encompasses in the nearby Sllested-1 well onlythe middle part of the Tnder Formation (Fig. 3).The abrupt facies changes observed regionally bymeans of well logs and cuttings positioned within theTnder Formation is interpreted to correlate with the`Late Triassic unconformity'. The `Late Triassicunconformity' is thus interpreted to be of Carnianage (Fig. 3).

4. Structural evolution

Based on interpretation of seismic sections (Fig. 1),calibrated by the wells investigated, the Mesozoicstructural evolution (with special emphasis on theTriassic structural evolution) of the southern ank ofthe Ringkbing-Fyn High can be unravelled. We haveselected three composite sections which run from theNorth German Basin onto the Ringkbing-Fyn Highas indicated in Fig. 1 for presentation. The sectionscover an area from the east coast of Jutland to theeast coast of the island of Falster. The sections aretied to the wells located in the study area.

4.1. The rslev-1 area

Fig. 4(a) gives an interpreted seismic cross-sectionextending from the rslev-1 well in SSWNNE direc-tion and crosses the southern margin of the MnHigh, a part of the Ringkbing-Fyn High (Figs. 1 and4). This composite seismic section shows that the seis-mic basement (the Top pre-Zechstein) dips to thesouthsouthwest and that the post-Zechstein sedimentsare cut by a major fault (Fault A, Fig. 4(a)) and aminor fault-zone (Fault-zone B, Fig. 4(a)). Fault Adoes not cut the Zechstein evaporites and is located atthe southern ank of a salt roller composed ofZechstein salt. A fault at the Top pre-Zechstein islocated beneath the salt roller but there are no indi-cations of a possible genetic control of the Top pre-Zechstein fault on the initiation of the salt roller.Fault-zone B, is composed of opposed dipping faultswhich detach at the Top Zechstein. A fault (or exure)cutting the Top pre-Zechstein surface is interpretedbelow the fault-zone B (Fig. 4(a)).

4.2. Structural reconstruction of the rslev area

The interpreted seismic section shown in Fig. 4(a) isrestored in time to evaluate subsidence patterns (Fig.5) using the reconstruction software RESTORE,designed for structural reconstruction in areas withmobile salt (Schultz-Ela & Duncan, 1991). The recon-struction software assumes a xed geometry for thebasement through time. The dip of the basement canbe changed in order to adjust for salt movements anddeformation of the cover sediments. This gives controlon the amount of salt present in the section (i.e.enables us to account for salt moving in and out ofthe section). If faulting of the basement takes placecontemporaneously with the faulting of the cover sedi-ments, it can be accounted for by manually recon-structing the basement geometry and continuing thereconstruction back in time using the reconstructedbasement geometry.

During the reconstruction, the thickness of the


Zechstein salt deposits was assumed to be constant in

the rslev-1 well since only 20% of the Zechstein ismobile halites in the rslev-1 well. The halite is con-

centrated in two layers of 35 m and 30 m thickness

out of a total 369 m of Zechstein. The salt thickness tothe south of Fault A (Fig. 4(a)) i.e. beneath the hang-

ingwall of Fault A, was assumed too thin or to be con-

stant through time, as salt generally ows away from

the area beneath the hangingwall of a fault in thecover sediments.

Fault A (Fig. 4(a)) dips to the south and detaches

at the top of the Zechstein. The reconstruction inFig. 5 shows that activity along Fault A was mainly

the result of salt accumulating beneath the footwall

and that only a very small volume of salt was with-

drawn from the hangingwall area. A gentle saltroller thus developed during the Triassic and the salt

structure became accentuated during the post-Triassic

and was very active during the Cretaceous. Thereconstructed geometry shows a thinning of the

Oddesund Formation (light shaded in Fig. 5(e)) at

the footwall of fault A which indicates that the salt

roller below the fault accentuated during depositionof the Oddesund Formation. It is possible that the

salt from the area indicated with an asterisk in Fig.

5(e) moved downdip into the salt roller according tothe model of Roberts, Price and Olsen (1990),

thereby creating a depression in which bi-directional

onlap is observed on the seismic sections (Fig. 4(a)).

However, it cannot be excluded that salt moved intothe salt roller perpendicular to the section. The de-

pression may thus have been generated due to inter-

action between salt movements and regional tilt ofthe basement.

The dip of the basement through time is summar-

ised in Fig. 5(h), where the position of the rslev-1well is used as a reference point. Fig. 5(h) shows

that the dip of the basement changes signicantly

between f and e, which corresponds to the period

when the `Late Triassic unconformity' is interpretedin this area. The basement geometry was xed

during this reconstruction. However, it is possible

that the faults were active during the Triassic, since

early Triassic stretching of the basins is observed(Vejbk, 1997; Srensen, 1986). Assuming that the

salt volume was relatively constant, a reconstruction

of the eect of the basement faults on the dip ofthe basement was undertaken. The dashed line at

the basement in Fig. 5(g) and (f) thus indicates the

geometry of the Top pre-Zechstein with a constant

salt volume. It clearly shows that although the Toppre-Zechstein may have undergone faulting during

the Triassic, it has not aected the timing of the

southward tilting of the basement as indicated inFig. 5(h).

4.3. Triassic erosion and non-deposition in the rslevarea

Truncationed reector terminations evident on theseismic sections indicate that erosion took place at the

top of the Falster Formation (Figs. 4(c) and 5(a)).

During the structural reconstruction, the top Falster

Formation reector was extrapolated to the north as

indicated by the dashed line in Fig. 5(e). The extrapol-

ation of the Top Falster Formation suggests that in

the rslev-1 well approximately 50 to 75 m sediment

was eroded from the top of the Falster Formation

(Fig. 5(e)). This is in accordance with the observed

thickness dierence of the Falster Formation between

the Sllested-1 well (see Fig. 3), where seismic sections

show no evidence for erosion at the top of the Falster

Formation, and the rslev-1 well. Furthermore, reec-

tors onlapping the `Late Triassic unconformity'

observed south of the rslev-1 well, emphasise that

the missing upper Tnder Formation, and lower andmiddleOddesund Formation in the rslev-1 well as

compared to the Sllested-1 and Slagelse-1 wells

(Fig. 3), indicate that the rslev-1 area was an area

of non-deposition prior to the deposition of the O-3

member of the Oddesund Formation. Thus, the hia-

tus observed in the rslev area contains an erosional

as well as non-depositional component with the

major part interpreted as being due to non-depo-

sition. This suggests that the area around the rslev-

1 well was located above the erosional baselevel

whereas sedimentation was continuous in the area

south of the rslev-1 well.

The depression, in which relatively thick upper

Tnder Formation and lower and middle Oddesund

Formation were deposited, was probably caused by

minor salt withdrawal contemporaneously with (andprobably triggered by) a change in dip of the base-

ment. The analysis of Vejbk (1997) and Pedersen

(1998) shows that the area north of the Ringkbing-

Fyn High also suered increased subsidence during the

deposition of the upper Tnder Formation and

Oddesund Formation compared to the Ringkbing-

Fyn High area. The Ringkbing-Fyn High was thus

hinged along both the northern and the southern mar-

gins. The internal reectors of the Oddesund

Formation shows bi-directional onlap onto the mar-

gins of the depression (Figs. 4(a) and 5(e)). The

Oddesund Formation contains two distinct evaporite

beds (Bertelsen, 1980; Pedersen, 1998) which indicates

that the bedding was originally horizontal (or near

horizontal). The bi-directional onlaps thus indicate

that the depression was generated prior to the depo-sition of that part of the Oddesund Formation, which

conrms a pre-Oddesund Formation age of the struc-

ture.


4.4. The Sllested-1 and Rdby-1 area

The seismic section WGC80-8021 was interpretedusing the Sllested-1 and Rdby-2 wells (Fig. 6) ascalibration points which tie to the section via WGC80-8022 and WGC79A-7925. The Rdby-2 and Rdby-1wells are both located on the Rdby salt structure andshow very similar evolution whereas the Sllested-1well is located adjacent to a salt wall (Fig. 6). The seis-mic interpretation constrains the log correlation of theearlier described `Late Triassic unconformity' (Fig. 6).The Rdby-1 well shows deeper erosion at the `LateTriassic unconformity' than is observed in theSllested-1 well (Fig. 3) in spite of the more basinwardposition of the Rdby-1 well compared to theSllested-1 well. This is interpreted to be due to the lo-cation of the Rdby-1 with respect to the underlyingsaltstructures (Fig. 6). The Rdby salt structure mayeither have uplifted the area around the Rdby-1 wellor have generated a relative depression similar to thedepression south of the rslev-1 well. That a complexinteraction between salt-movements and basin tec-tonics inuenced the unconformity is also suggested bythe position of the Sllested-1 well relative to a salt-wall (Fig. 6).

The area shows a geological evolution very similarto the rslev section (Fig. 4(a)) where salt-movementand basement subsidence generates the topography inwhich the sediments were deposited just above theunconformity. There are no erosional truncationsobserved in the north-western part of the section(Fig. 6), but low-angle erosion truncations cannot beexcluded and the seismic interpretation indicates a sig-nicant thinning of the Triassic strata located belowthe unconformity in the north-eastern part. It is alsopossible that periods of non-deposition took place asinferred in the rslev-1 area.

4.5. The south-western margin of Funen

The area around and north of the Kegns-1 well isillustrated by four seismic sections (Fig. 7). The sectionGY84 T-8 (Fig. 7(a)) shows that salt-movementscaused relative uplift of a salt structure south-west ofthe Kegns-1 well. There are a large number of ero-sional truncations at the Top Trias. The Kegns saltstructure was uplifted in Post-Triassic time as indicatedby the constant thickness of the Tnder and older for-mations. Furthermore, no indications of onlap intern-ally in the Triassic succession onto the Kegns saltstructure are observed. The section WGC79-7917 (Fig.7(b)) strikes parallel with the south-western border ofthe Glamsbjerg Block (Fig. 1) and shows no signicantchanges in thickness of the rslev, Falster and TnderFormations outside the faulted area. The complexfault pattern in the centre of the section is due to sec-

Fig. 6. Seismic section showing the geometry of the successions from

the North German Basin onto the Stigsns Block. The unconformity

has here the character of an onlap surface. The map shows the pos-

ition of the section and wells in relation to underlying salt structures

(from Vejbk & Britze, 1994).


tion parallel faults, which strike NWSW and detachalong the Top Zechstein surface. The seismic sectionWGC79-7918 (Fig. 7(c)) strikes eastwest and the sec-tion DA98-46 (Fig. 7(d)) continues across the LilleBlt to the shore of Funen. All sections emphasisethat no signicant dierential basement-controlled sub-sidence took place during deposition of the TnderFormation in the area around the island of Als. In the

Ringe-1 well, which is located at the centre of theGlamsbjerg block (Fig. 1), the Tnder Formation isabsent and the Falster Formation is overlain by theSlagelse Member. Previous seismic interpretations car-ried out by Sorgenfrei (1966) in the area around theRinge-1 well shows that the Triassic succession is con-formable and shows no evidence of lateral thicknesschanges. Newer seismic sections conrm this. The

Fig. 7. Seismic sections from the area around the island Als showing the geometry of the Triassic succession. The position the Kegns-1 well is

also shown (a). The consistent thickness of the early and middle Triassic succession around the Kegns pillow and the erosional truncation near

the base Cretaceous (BC), at a horizon which corresponds to the Top Triassic, indicates that the salt movements generating the Kegns pillow

took place after deposition of the Tnder, Falster and rslev Formations. No signicant change in thickness of these Triassic deposits is

observed in the other section running along the island of Als (b and c) and across the Lille Blt (d). See text for further discussion. For location

of the sections see the inserted map. See Fig. 4 for legend.


absence of the Tnder Formation in the Ringe-1 wellthus suggests that at least the central parts of theGlamsbjerg Block were topographically high duringdeposition of the Tnder Formation or that erosion ofthe Tnder Formation (if deposited) took place priorto deposition of the Slagelse Member, as suggested forthe area around the rslev-1 well.

5. Discussion

5.1. Variations in structural evolution along the southernmargin of the Ringkbing-Fyn High

The `Late Triassic unconformity' observed in thestudy area is interpreted as related to basementinvolved relative uplift of the eastern part of theRingkbing-Fyn High, causing the initiation of move-ments of Zechstein salt in basinward areas. The rela-tive uplift of the Ringkbing-Fyn High is interpretedas due to heterogeneous Triassic stretching of the area(less stretching at the Ringkbing-Fyn High comparedto the basins) and later rapid thermal subsidence of

the basins both north and south of the Ringkbing-

Fyn High (Srensen, 1986, Vejbk, 1997). Late

Triassic tectonics are also observed in the German

Basin located south of the study area (Wolburg, 1969;

Dittrich, 1989, 1991; Beutler, 1995). Clausen and

Korstga rd (1993, 1994) show in the Horn Graben area

the presence of a major unconformity at the base of

the Jurassic succession in accordance with Cartwright

(1990). Furthermore, a major intra-Triassic surface

was interpreted as marking the change in dominant de-

position within the Horn Graben to the area south of

the Ringkbing-Fyn High (Clausen & Korstga rd,

1993). The lithostratigraphic subdivision of the wells

R-1 and S-1 in the Horn Graben (Fig. 1), according to

the principles dened by Pedersen (1998), shows that

the intra-Triassic surface is located within the Tnder

Formation, and thus most probably corresponds to

the stratigraphic level of the above described Late

Triassic unconformity (Fig. 8). The analysis of EWstriking detaching faults within Horn Graben shows

that a change in dip of the Top pre-Zechstein surface

took place during the late Triassic and is thus compar-

able with the southward tilt of the North German

Fig. 8. Well section showing the reinterpreted correlation of the lithostratigraphy related to the seismic subdivision of Clausen and Korstga rd

(1993). The horizon TR1-4 are of Triassic age, and TPZ equals the Top pre-Zechstein. The seismic surfaces are correlated to the same lithostrati-

graphic unit in both wells, which conrms the assumption that the lithostratigraphic subdivision used in this study is chronostratigraphic signi-

cant. The TR4 is a marked unconformity and correlates to a surface internally in the Tnder Formation, and is important since it marks the

change in subsidence pattern in the Horn Graben area (Clausen & Korstga rd, 1993). See text for further discussion.


Basin observed in south-eastern Denmark. Assumingthat the constant thickness of the Late Triassic succes-sion in the areas Als-Funen and Southern Jutland indi-cates no dierential subsidence, the areas in southernJutland and the isles Als and Funen dier from theareas in south eastern Denmark and the Horn Graben.Therefore, the stratigraphic record of the Ringkbing-Fyn High suggests that the individual blocks, consti-tuting the Ringkbing-Fyn High, underwent dierentstructural evolution, as already suggested bySorgenfrei and Buch (1964). The subsidence patternalong the Ringkbing-Fyn High is thus suggested tobe a consequence of intra-plate deformation due to theoverall EW extension of the European continentduring the Triassic.

5.2. Eect of the salt structures

The reconstruction of the section in the south-east-ern Danish area shows that mobile salt had an import-ant role during deposition of the Late Triassicsuccession by modifying the accommodation space.Best, Kockel and Scho neich (1983) show that the saltstructures located in the southern part of the HornGraben were similarly active in the Late Triassic tothose in the south-eastern Danish area. However, thisis in contrast to the observations in southern Jutlandand the Als-Funen area where the salt structures areinterpreted to be generally younger. The correspon-dence between change in dip of basement and saltmovements as observed in this study corresponds tothe observations made along the southern rim of theRingkbing-Fyn High in the Danish North Sea(Clausen & Korstga rd, 1996) and along the southernmargin of the Mid North Sea High in the UK sector(Coward & Stewart, 1995). A similar trend is observedalong the northern margin of the Danish Ringkbing-Fyn High (Britze, Japsen & Langtofte, 1992; Vejbk,1997).

6. Conclusions

The integration of well-logs and seismic sectionsenables the geological analysis of a Late Triassicunconformity and the structural evolution. The analy-sis has shown that:

1. The dierential subsidence between the Ringkbing-Fyn High and the North German Basin during theLate Triassic varies along the strike of theRingkbing-Fyn High. A signicant change in dipof the basement is observed in the south easternpart of the Danish area and in the Horn Grabenarea. This contrast with southern Jutland where noindications of dierential subsidence or salt move-

ments during the Late Triassic are observed.2. Late Triassic erosion is shown by an unconformity

characterised by local low-angle erosion at the crestof the Ringkbing-Fyn High and onlap onto theunconformity at the Ringkbing-Fyn High.

3. Salt-structures in the North German Basin areactive during the Late Triassic, but restricted toareas where a signicant change in basement dip isobserved. It is therefore inferred that the change inbasement dip controlled the movements ofZechstein salt. The halokinetic events inuenced thedeposition of Late Triassic sediments by generatinglocal depressions and highs in the area south of theRingkbing-Fyn High.

4. The subsidence pattern of the Top pre-Zechsteinsurface varies along the southern margin of theRingkbing-Fyn High. This indicates that the subdi-vision of the Ringkbing-Fyn High into structuralblocks played a major role into the evolution of theTriassic succession in the Danish area.

Acknowledgements

The referees are thanked for valuable comments,which improved the manuscript. Egon Nrmark isthanked for the seismic processing of the seismic lineDANA98-46, which is part of the Danish NaturalScience Research Council 9701761. Per K. Pedersenwas nanced by the Faculty of Science, University ofAarhus.

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