Mémoires pour servir à l'Explication Toelichtende Verhandelingen des Cartes Géologiques et Minières voorde Geologische en Mijnkaarten de la Belgique van België Mémoire No 21 Verhandeling N° 21
Subsurface structural analysis of the late-Dinantian carbonate shelf at the northern flank of the Brabant Massif (Campine Basin, N-Belgium).
by R. DREESEN, J. BOUCKAERT, M. DUSAR, J. SOl LLE & N. VANDENBERGHE
MINISTERE DES AFFAIRES ECONOMIQUES MINISTERIE VAN ECONOMISCHE ZAKEN ADMINISTRATION DES MINES BESTUUR VAN HET MIJNWEZEN
Service Géologique de Belgique Belgische Geologische Dienst 13, rue Jenner 13,Jennerstr.aat 1040 BRUXELLES 1040 BRUSSEL
Mé~. Expl. Cartes Géologiques et Minières de la Belgique 1987 N° 21 37p. 18 fig. Toelicht. Verhand. Geologische en Mijnkaarten van België blz. tabl.
SUBSURFACE STRUCTURAL ANAL VSIS OF THE LATE-DINANTIAN CARBONATE SHELF AT THE NORTHERN FLANK OF THE BRABANT MASSIF (CAMPINE BASIN, N-BELGIUM).
1. INTRODUCTION
1.1. Seismic surveys in the Antwerp Campine area 1.2. Geological - stratigraph ical setting 1.3. Main results
2. THE DINANTIAN CARBONATE SUBCROP
2. 1. Sei smic character 2.2. General trends and anomalies 2.2.1. Western zone 2.2.2. Eastern zone 2.2.3. Transition zone
2.3. Deformations 2.3.1. Growth fault 2.3.2. Collapse structures 2.3.3. T ectonics 2.3.3.1. Graben faults 2.3.3.2. Central belt 2.4. 1nternal and underlying structures 2.4.1. 1nternal structures 2.4.2. Underlying deep reflectors
3. RELATIONSHIP WITH OVERLYING STRATA 3.1. Upper Carboniferous 3.1.1. Namurian 3.1.2. Westphalian 3.2. Mesozoicum 3.3. Cenozoicum
4. GEOLOGICWORKING MODEL 4.1. Paleogeography 4.2. Structural evolution 4.3. Collapse structures and sinkholes
5. REFERENCES
ENCLOSURES 1. Location map seismic profiles ( 1974-1982) and cross-sections A to G .Il. lsochron map of the top of the Dinantian Ill. 1sochron map of the base of the Cretaceous IV. Cross-sections A to G
TABLES
-3-
SUBSURFACE STRUCTURAL ANAL YSIS OF THE LATE-DINANTIAN CARBONATE SHELF AT THE NORTHERN FLANK OF THE BRABAN"'F MASSIF (CAMPINE BASIN, N-BELGIUM).
by
1 2 2 3 4 R. DREESEN , J. BOUCKAERT , M. DUSAR , J. SOILLE & N. VANDENBERGHE
INTRODUCTION haut boreholes are the best documented (BLESS et al., 1976; 1981) (fig. 2). 1.1.- SEl SMIC SURVEYS IN THE ANTWERP The Heibaart- Loenhout borehole (He 1 and CAMPINE~AREA 1 bis) has been drilled in 1962 by PETROFINA. Coring was discontinuous. Lower Namurian strata underlying 1n table A (p. 36) a list is given of ali the seismic the basal Cretaceous unconformity were reached at surveys carried out in the a rea investigated. 1n particular 1052 m and the Dinantian at 1138 m. The Upper the recent campaigns 4, 5, 6 and more especially 7 were Devonian sandstones and marine shales sequence was usèd in our study. reached at 1461 m. The borehole penetrated the Si The different acquisition and processing para lurian subcrop at 1627 m. Between 1977 and 1980 meters of the reë\lnt dmp"àigns are listed in table B severa! boreholes (OZ H 1 - OZ H6) were drilled on (p.37). The weil seismic velocity surveys available are top of the Heibaart structure on behalf of DISTRIGAZ, indicated in table C (p. 37). in arder to test the possible reservoir qualities of the uppermost karstic zone in the Dinantian carbonates for subsurface gas storage. For a detailed description 1.2.- GEOLOGICAL - STRATIGRAPHICAL SETTING of the cored intervals of these boreholes (microfacies - microfauna - depositional environments) the reader is The studied area represents the southernmost part · referred to BLESS et al. (1981). The cored intervals of the Campine- Brabant Basin at the northeastern located at the top of the karstified llinantian limestone edge of the London- Brabant Màssif (fig. 1). The geolo have been attributed to the basal Warnantlan (V3b). gical framework of the area is made up of a sequence of Microfacies analysis of the cores of qoreholes OZ H2 slightly north-dipping Cenozoic and Mesozoic strata, and OZ H4 (S-flank of the Heibaart dame) pointed which unconformably overly steeper but also north out a deposition in an open marine shelf lagoon under dipping Devono- Carboniferous strata, which in their shallow water conditions, whereas the cores of the other <:urn cover a structurally more complex Cambro -Silu boreholes (ali located on the N-flank) indicated deposi rian basement (LEGRAND, 1968).· tion on tidal flats in very shallow lagoons with restricted water circulation and hypersaline waters. The Turnhout The Cenozoic deposits reach a thickness of about borehole (drilled between 1953 and 1955 for the Ins 600 m and comprise the following lithostratigraphical titut National de 1'1 ndustrie Charbonnière and the Belgian suite (from top to bottom) : Pleistocene Campine clays Geological Survey) reached the Upper Carboniferous and sands, the Neogene glauconitic Sands, the Boom at 1002 m and the top of the Dinantian at 2162 m. Clay, the Kallo Clays and fine Sands complex, the Lithological descriptions, especially of the Silesian, Lede-Brussel - Panisel Sands, the 1eper Clay and the have been published by DELMER ( 1962, 1963). The Landen- Heers Clays and Maris. reader is referred to BLESS et al. ( 1976) for a review The Mesozoic is essentially composed of Upper Cretaceous (Campanian- Maestrichtian) chalky lime stone and mari deposits (about 200 m thick). Present,ly at N{EB, Liège. The Paleozoic (Pre-Permian) subcrop h~s· been . 2 Billgian Geological Survey, Brussels. reached by severa! drillholes in the Campine area, 3 Distrigaz, N.V.- Brussels. East of Antwerpen, of which the Heibaart and Turn- 4 Presently at K.U. Leuven. -4-
0 50 Km
r.:-:-:1 l:..:...:..:..: Q - Dinantian r::::::::l c § ll1lTllflllT11l U p p e r . l.;_;_;_;_;_;_; ll.ll.I1Wlli.ll M i d d l e 0 evo n 1an ~ Aegir marker band--- ~ --'-'-'-'--"'-'"--'--'"------~ 8 ...... Lower Devonian Catharina marker band-A- ~ ~ Cambro- Silurien ~Sarnsbank marker band 3: ~Narhurian- -Pre- Cambrian ++#.f-.1. Belgian Dutch Boundary u Location of mapped subsurface
from Van Staalduinen et al. (1979) The Geology of the Netherlands
Figure 1. - General geological situation map. - 5-
~NHOUT TURNHOUT KESSEL BOOIS::HOT RILLAAR LOKSBERGEN HALEN
Na -581
V3c 1 1 V3a-b 1 1
------753
Lower ? Warnantian \ 1 -.810
Upper V3b W:lrnantian Cf Go!'
Lower Warnant ion V2b Livi an .... V3a ______---'"-=~-iz=CIOI- 937 ---- V2a? 563 -1429 ------957 -299 Motiniacian -592 -311
Dev? -612 V2a . D-376 .. · .. -695 -555 Via
-587 -1604 - 2407 Sil -619 VJ?
V1a
-684 ? -2508
NEDERLAND -2589 ?
-1339
Figure 2.- Stratigraphie correlation of the Dinantian weil sections in the Campine area. - 6-
of the microfaunal content and biostratigraphic con The Dinantian- Namurian unconformity might have clusions on the D inantian. M icrofaunal assemblages acted as a flat-lying sliding plane for the displacement (Foraminifera) indicated an V1a through V3c (Moli and the stretching of the Namurian strata. This growth niacian - Warnacian) age for the penetrated carbonates. fault may represent the northernmost limit of the None of the boreholes in the studied area has yielded shallow Dinantian carbonate shelf : farther to the a stratigraphically continuous sequence .of Dinantian North, Kulm-type Dinantian deposits might explain rocks thus far. the disappearance of the outspoken Dinantian lime From the frequent occurrence of stratigraphie gaps, stone reflector. unconformities, hardgrounds, breccias and reworked The preserved thickness of the Westphalian does microfossil assemblages in the studied boreholes N of not exceed 575 rn in the Turnhout borehole (DELMER, the Brabant Massif, it is clear that severa! epeirogenic 1962) whereas it is completely lacking on top of the movements have influenced the sedimentation pattern Heibaart structure. Obviously these thickness figures during late Dinantian times (BLESS et al. 1976, 1981). are determined by the structural dip of the Upper Continental facies have even been recorded from the Carboniferous and the post-Westphalian to pre-Upper Booischot borehole : conglomerates, sandstones, corn Cretaceous erosion. stones, paleosoils, ali dated V2a (LEGRAND, 1964; An overlap-sequence on top of one of the more BLESSetal., 1976). continuous sandstone reflectors might indicate a Na The Namurian is incomplete in the Campine Basin murian-Westphalian unconformable contact (VAN since the Pendleian and basal Arnsbergian strata are DENBERGHE, 1982). missing (BOUCKAERT, 1967). This stratigraphie hiatus The major and also the youngest tectonic fea is the result of a widespread regression. coinciding with orogenie movements of the sudetic phase (GRAULICH, tures in the studied area finally are the NNW-SSE 1962). This regression affected essentially the near directed tensional faults which affect the Carboniferous strata as weil as the overlying Meso-Cenozoic deposits. shore and inner shelf areas of the southern borders of These important faults are related to the Roer Valley ·the Brabant- Campine Basin. Graben boundary faults of the Netherlands Central The subsequent transgression gradually spread out Basin. They have been active at !east since the late across the shoals but was only completed in Kinderscou Paleozoic. During most of the Meso-Ceriozoic they tian times (BOUCKAERT, 1967; BLESSetal., l980). have been reactivated, sometimes in opposite senses, The age of the oldest Namurian strata in the Turnhout resulting in thickness variations of severa! Cenozoic borehole is Upper Arnsbergian ( E2c) whereas the strati formations in the NE-part of B.elgi!Jm (VAN STAAL graphical gap between Dinantian and Namurian extends DUINEN et al., 1979). into the Kinderscoutian most probably, in the Heibaart boreholes(BLESSetal., 1980, 1981). Thickness variation along the northern border of the 1.3.- MAIN RESULTS Brabant Massif is relatively small (500 -680 rn over a distance of more th an 100 km). The compilation of subsurface structural data However the preserved thickness of the Namurian has from severa! reflection seismic surveys, carried out been reduced to Jess than 70 rn over the particular between 1974 and 1982, resulted in a series of struc Heibaart antiform structure due to post-Westphalian tural isochron maps covering a surface of about 400 km2 pre-Cretaceous erosion. The Namurian succession in the North Campine a rea, south of the Du teh -Belgian becomes significantly thicker towards the central parts border. The isochron map of the top of the Dinantian of the basin as can be deduced from the more than reflects the structural complexity of the Late Visean 1800 rn thick sequence in the Rijsbergen-1 NAM regressive carbonate shelf. borehole (BLESS et al., 1976). The more precise nature .T~e surprising topography of the base of the Cretaceous of this deep Namurian basin, north of the Heibaart otherwise illustrates the erosion·al unconformity at the area, has been described by VANDENBERGHE, 1982 beginning of the Upper-Cretaceous transgression in (N flank Brabant Massif - 1980 seismic survey). This fluenced by the underlying Paleozoic strata. deep basin which mainly consists of silty mudstohes The most striking structural element observed on the top with sorne important sandstone levels, at !east towards D inantian structurai map, is the alternat ion of flattened, the top (VANDENBERGHE, 1982). is bordered to the wide, positive areas and narrow, relatively deep, synform south by an important north -dipping and roughly east belts with sorne more localized collapse structures (wes west striking listric-shaped fault, hereafter called the tern part of the stud ied a rea). F arther to east, the struc Hoogstraten fault, which runs south of Hoogstraten, tural picture becomes very complex, due to the interac Wortel and Weelde (VANDENBERGHE, 1984). This tion of severa! fault systems. The central, transitional !istrie fault, accompanied by adjacent" roll-over" struc a~e.a seems to be affected by extensive, but narrow, NS tures is interpreted as a growth fault, which developed d irected graben -1 ike deformations. during Westphalian times and. probably already from The structural .influenc_e of:the Paleozoic subcrop upon the late Namurian onwards. the covering Mesozoic deposits is weil illustrated in the - 7-
1 N 0 ~
E 0 ~~ 0 ~"' :§"' __ "0 "' .2! ~"' 0 .IJJ> a_"' ·"0 ~ ~ a.L,. 0 a_"' 0 L,. u ,C "0 ï~ 0 Qi 0 ë"' Ill ,0 ~ 0 ·a; 0 s 3: L,. u"' Ill ,!!l .E
.~ ~ i l ~J i
DZZ!
OOZ!
Figure 3.- lsobaths of the top Oinantian at Poederlee (from Finite Difference Migration, FDM, seismic profiles). -8-
enclosure IV, by the presence of sinkholes as weil as by 2.2.1.- Western zone · the occurrence of NS trends deviating from the general EW strike of the basal Cretaceous topography. The most striking feature of the western zone is the alternation of broad antiform and narrow depres From the structural relationships between Dinan sion belts, bordered by parallel longitudinal faults. tian-Silesian and Paleozoic-Meso-Cenozoic, a working The most obvious longitudi'nal depression belt is the madel can be derived for the paleogeographical and one running NW to SE, between the St-Antonius - structural evolution of the investigated a rea. Zoersel-Grobbendonk antiform and the Westmalle Vlimmeren-Lille antiform belt. The strike changes abruptly from WNW-SSE to NNW-SSE near Zoersel 2. THE DINANTIAN CARBONATE SUBCROP where the D inantian subcrop d isplays a pronounce kink or "nose". 2.1.- SEISMIC CHARACTER This depression belt has a lenght of more than 18 km and a width ranging from 250 to 1000 m. Within South of the 1istrie fault, the top of the D inantian the depression belt severa! minima occur, with maximum limestones is expressed as a pronounced, slightly dipping depths between 250 to 300 m (the depth time conver seismic reflector. By its high amplitude and lower fre sion is based on weil seismic data in the Heibaart and quency this reflector canin general easily be distinguish Turnhout boreholes (table C). ed from Upper Carboniferous reflectors. The Westmalle-Vlimmeren-Lille • antiform belt North of the Hoogstraten fault, the pronounced (width 1 to 3 km, maximum heigth of about 200 m) char acter of the top D inantian is !ost, except for sorne is subd ivided by transversal depressions into sever al short dipping intervals near the fault. elongated antiform units; the southernmost limb of this Although the top of the Dinantian limestones is ex antiform belt, the Wechelderzande-Lille-Poederlee anti pressed as a reflector of good quality, sorne zones are form complex, is separated from the adjacent Vorselaar not and the top D inantian reflector th en becomes dis antiforhl unit, by a (sub)transversal depression. Both continuous and in sorne cases even hard to recognize. units most probably have been former parts of a same These zones are in general restricted to narrow depres NW-SE elongated antiform flat. sion belts. Their origin is discussed in the next para The Poederlee antiform, in the southernmost graphs. part of the latter antiform complex, has recently been An intriguing feature, observed in ali the seismic selected as a potential subsurface gas storage target. surveys, is the almost complete absence of good reflec An exploration borehole on top of this Poederlee struc tors, if any, below the strong Dinantian reflector, with ture will be drilled for DISTRIGAZ and the BELGIAN the exception of one particular deep reflector, west of GEOLOGICAL SURVEY (fig. 3). Westmalle (see further). The Westmalle and Vlimmeren antiform struc This could result from the important increase tures are separated by a SW-directed extension of a in acoustic impedance at the contact of Namurian shales birdfoot-like lobated depression, right under the centre and Dinantian carbonates, reflecting almost ali the of Oostmalle. This depression belt extends from Vlim sei smic wave energy. meren to the W over more than 7 km, and reaches a maximum depth of about 200-250 m near Oostmalle. Furthermore a strong karstification and breccia Since the transversal fault bordered depressions do not tion occuring within the Dinantian, as seen in the dif substantially differ seismically from the longitudinal ferent boreholes, m ight have contributed to scatter ones, these depressions in fact eut a rectangular pattern the energy. into the top of the D inantian limestone. An EW directed antiform belt separates the Oost 2.2.- GENERAL STRUCTURAL TRENDS AND malle depression from the next one, the St-Lenaarts ANOMALIES depression belt, the minimum of which is situated SE of St-Lenaarts (maximum depth of about 200-250 m). The Dinantian subcrop (enclosure Ill: lsochron The main axis of this belt runs NW-SE whereas map of the top of the Dinantian) can be structurally its numerous lobes or extensions have a dominant EW subdivided into two main zones : a western zone in orientation.iust as their bordering longitudinal faults. which the strike grades from WNW-ESE in the north The NW-extensions of the St-Lenaarts trough are the to NNW-SSE in the south, and an eastern zone with a western limit of the next and most important antiform dominant NNW-SSE strike. An important NNW-SSE complex of the western zone, the Loenhout- Heibaart oriented fault system characterizes the transition be Rijkevorsel dome structure. tween bath structural areas. -9-
540 550 560 570 58(; 590 600 610 620
1 1 1 l 1 0 2 J
0,3
0,4
0,5
0,6
.. 07 '' J
0,8
0,9
1,0
1,1
1,2
1)
1,4 yellow : base of Cretaceous blue top of Dinantian 0 100 200 300 m orange: base of Tert iar y red taults
Figure 4.- Collapsed nature of the Dinantian top with accompanying subsidence till the Tertiary (8107 FDM). 800 790 780 770 760 750 740 730 720 710 700 690 680 670 660 1 1 1 1 1 1 1 1 1 1 1 l 1 1 1 o.2 ~ ~JJl1 ®llt~~ mw:~~~".~;~~.. ~;, (~r ~ .~m~: ~\~tl~~~~{~~~~"~;~~~~~wJ.~~~~~~~tmft~!fJH~~"b ij~·~n~ wm.~ . •ft ï!~~~ttm~tm ~~w .t~ .1{~ ;m(WWt!~~W'm tlWI 1 7 " • '" .. ·" " • " '"· .... ··nf"".·: ,.:::.;: ~m ~i.....imi•1 .. ~·· r"'=· · ·· ,.,...... · · · "· ...... · ''"' '"' ·' ·· "'' "'"' '· " · ' ~~ · ~ · '· ~~~'· • · :" • rl"-rml 1 l lW l(((IILIPII ..omi>IIIUI ~ l ;~m ••1 · · . t · · • .~ • mr 1
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,"" < "'" ...... :':" <' ' ~)~.'''Hi ·~l" m ;~ "' "'. ' : <' ··~·:•!'til>tif~~l 1 ,,.,,ti•'~'""''"''""'rdt'i~~~ ' ·":" ' l~\ . \~" ·•·•· ·: ·::: .. ·: :" '" ." :o,9 • 11'.'.\: . ù\1• ·... ·w , ln~ ...: :. : .. :~~ . . , , .. l'" ,. . ~n;r~~~ ... 1 ' , •• ·:r~~~ 'i'fu . : " , , ...... K< ,l\ll~~i't\. , ~J, .• . .' .. :.:,: ... (/{ .. .. tlî:î .~ . '~·"-'·' . ;re , :.tl~ .. '"':" ~~·! . . ~ .... , r trr'' ~m~•~ . . ., ~i ~='~ 'tel ~~~ ''"wL , o .... 0 ~~~-~ · ',\W• · ~1 · î!o l~: • · ".•( ·· · ·· ' ~~T ~·~~'j.'l'(!.>tm .u.:• · · • ~~ ,._.•.w~~~~~. ' 1 · . . . . ŒI~J & ~ · .··:· ~~ ~~ ·' .~,, ;,;,;.;.:. > Ht>.' ."': iii! !ltlll!.• :II'Jii! ·:, , 1,1 ' ' ~~i' [!l.~~ ..~~~.,,~~ 1:~ -'[!) ~ ~1\''11. ' rlk~ ~ '.!,1 ~ ' ;,.( t~ fè',,~ ~io.~. ~i) [L' · 7.1.~\l.~ , 7L ~ i'l ~ . t~ ~(~ ,•d~~l~~~~ ' tl ~:• l~ l>tll1 ~ ~ '" ' ''1 ra! ~~ ~ 1 2 ·'· ~c . ~r:..· . d~~ r;;.a: ~·. ~~[.~ ~ ~ t)t:~; . ··{( ~~ ~~:;r,•~ .. ~ ~~-! u.~: ~~ ~~~~~'""' , .. , , ··, " ··: ~ ~ ~ . ~! , , ·~QI.(· · • :;L~. ~~;~~ ~.:. iilio ~ ~m~lil . ;Ji; ... • . ~ • tl~~~ ~~ Ct~~. · ...... 7'1 ~jli! ., ~ 1: c>. • • .. . r;,) • a ·13 · · :.!1.' ·, · ~~~~~~~~~ ~~ ~~~ ~ ;~!r~ ~~~·~~~~l r~ ~ tl!~~~~ ~ &'i ' ' . . . tt~ {1 ,. ' iMi1 ~. , ~ ,;~ . ,;, , ' .t~.W~.:o;· ':rr. "" · ,~..,'M~ ~r... ~ ~t~~ ~ ~~~i~"Zf~~ , · :>l'~~~·!)'· ,~~.i~ ' ~- _...; ,. rn. ~ l~ ~~· J ·~ 1.4
yetlow base Cretaceous blue top Dinantian red faults
0 100 200 300 m
c; ...... r: ,.., ...... J: ...... ~ .. t--:..J ...... ----- : ...... __ - 11 -
JO 710 720 730 740 750 760 770 780 790
1 1 1 1 1 1 1 1 1 1 0,4
0,5
0,6
0,7
0,8
yellow base of Cretaceous blue top of Dinantian green Sarnsbank marine bed ? orange intra- Namurian reflector onlap 0 100 200 300 m brown intra -Dinantian reflector red faults
Figure 6.- Influence of the top Dinantian subsidence on the sedimentation during theWestphanian and the occurrence of an internai structure in the Dinantian underlying the Poederlee antitorm {8106.FDM). - 12-
940 930 920
1 1 L~ mm ~J" ~~ r.!~mt& rn;~ Q.! &.'.. \l Ç{•
Q7 t! ~~ tm 0,8
yellow base Cretaceous 0 100 200 300 m blue top Dinantian red fau lts
Figure 7.- Faults in the post-Dinantian sediment sequence induced by the subsiding Dinantian (8111 FDM). - 13-
0 820 810 800 790 780 770 760 750 740 1 1 1 1 1 1 1 1 1
0 100 200 300 rn yellow base Cretaceous blue top Dinantian green Sarnsbank marine ued ? orange: intra-Westphalian reflector red : faults
Figure 8. - lnfill of the subsiding zone as shawn by the thickness increases in different Upper Carboniferous intervals (8109 FDM - 14-
This complex comprises a main EW oriented dome wards the northwest, where it reaches a maximum width with an extension to the SE. lt is separated from its of about 3 km near Hoogstraten. southeastmost limb, the Rijkevorsel antiform flat, by The longitudinal bordering faults are not conti a narrow NE-SW directed depression, bordered also by nuous but they are made up of severa! successive fault longitudinal parallel faults. steps. ·W.ithin this faulted depression belt, sorne intern The latter flat also forms a connection to the ai structures are stiJl visible, such as a 4 km long, NS south with the Westmalle-Vlimmeren antiform corn directed antiform structure (between Vlimmeren and pl ex. Further to the north and the northeast otherwise Beerse). This depression belt widens northeast of Rijke the carbonate shelf is steeply dipping in the same di vorsel but the seismic interpretation of this area is very rection and hits the EW directed !istrie faults near preliminary because of the insufficient seismic coverage. Hoogstraten, where the Dinantian reflector disappears beneath or coïncides with the flat-lying plane. The Heibaart- Loenhout do me is the most pronounced an 2.3.- DEFORMATI,ONS tiform structure in the studied area, and has been tho roughly investigated by severa! seismic surveys and The faults affecting the top of the Dinantian car exploration drilling campaigns (see introduction). lts bonates can roughly be grouped into three main defor culmination point reaches about 300 m near Heibaart. mation mechanisms : 1. growth fault 2.2.2.- Eastern zone 2. collapse structures The dominant structural feature.s of the eastern 3. tectonics. zone are the NNW-SSE to NS strike of the. subcrop, and the eastward dip. ln the northeastern area the 2.3.1.- Growth fault tectonic framework becomes more complex. This major EW directed and N to NW - dipping The area west and northwest of Turnhout is !istrie fault runs at a distance of about 2 km to the north characterized by the presence of NNW-SSE elongated of the Loenhout-Heibaart dome, in an eastward direc flat antiforms (e.g. Gierle high, Merksplas high, Beerse tion, south of Hoogstraten, Wortel and Weelde. high) bordered or subdivided by very narrow, faulted and parallel synform structures (e.g. west of Gierle, For a detailed description of this growth fault west of Beerse, Vosselaar). These flat antiforms have a and of its bearing on the structural evolution of the length of about 10 km, a width of about 2 km and a Campine Basin during Silesian times, the reader is . maximum heigth of about 150 m. referred to VANDENBERGHE (1984). The depressfÔ~ belts are 150 to 500 m wide and The growth fault affects essentially the Upper they reach a maximum depth of about 200 m (e.g. west Carboniferous strata (a steep fault plane in the upper of Beerse). part, gradually flattening towards the base). The flat lying, slightly NE-dipping Dinantian-Namurian uncon To the nor.:Uleast of Turnhout the structural pic formity·Fias been used most probably as a sliding plane ture becomes very complex, due to the. presence of to displace the Silesian strata towards their depocenter severa! mutually intersecting fault systems (see further). in the north. As a result of these tectonic deformations the Dinantian subcrop has been subdivided into severa! structural blocks A series of E-W or WNW-ESE directed faults .with different dip and dip direction. between Turnhout and the Belgian-Dutch border, are interpreted as satellite faults of the main growth fault The last obvious antiform structure and adjacent and developed probably simultaneously with the major elongated narrow depression, are located north-north growth fault from Late Namurian on and mainly in the east of Turnhout. East of these structures, a real struc Westphalian. These faults have been transversally eut tural trend of the Dinantian subcrop is no longer dis by the younger graben-related faults. cernible.
2.3.2.- Collapse structures 2.2.3.- Transitional zone The most conspicious structural element in the An extensive - at least 17 km long - faulted de western zone of the investigated area is the presence pression belt, runs from the North, near Hoogstraten, of longitudinal, transversal and lobate depression belts, to the southeast of Poederlee, and marks the transi partially with closed contours on the isochronmap tion between the two main structural areas mentioned (enclosure Ill). above. 1n ali these cases these depressions are bordered This graben-like depression is the narrowest in the by relatively steep and parallel normal faults, along south (about 500 m, NE of Poederlee) but widens to- which the Dinantian limestone reflector has been gra- - 15- 140 150 160 170 180 190 200 210 220 1 1 1 1 1 1 1 1 1
yellow base Cretaceous 0 200 400 rn blue : top Dinantian · red : fau lts
Figure 9.- Influence of the subsiding Dinantian on the overlying Upper Carboniferous and Cretaceous (8012). - 16-
400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 !' 0,0 0,1 0,2 . C,3 0,4 0,5 0,6
0,8 i Q9 1,0 1,1 1,2 1,3 : 1,4 1,5 1,6 1,7 1,S 1,9 2,0 2,1 2,2 . 2,3
2,4 2,5 2,6 2,7 2,8 2,9 .· 3,0 yellow base Cretaceous 0 200 400 m blue top Dinantian red fau l ts
Figure 10.- Nature of the Graben boundary fault (8004). 490 480 470 460 450 440 430 420 410 400 390. 380 1 1 1 1 1 1 1 1 1 1 1 1 1 ~-:: ' ' ' ' ' ' "' ' ' ' '' ~J <ù '· < <' w mw! ' ' '" '[!h~~l+~~,,~~ 0 6 < < < < < ~ < <- < < < < < < <<"<.<:,<.<.<<<- < <<.:~ ~<<,< < <,<:::<. <: ""- <<.: Cc c;·«·ç 'C < ( 1,3 1,4 n .. Jc:uw:...... o.J.. 1,6 yellow base Cretaceous 0 100 200 300 m blue top Dinantian green sarnsbank red faults Figure 11.- Nature of the central belt subsidence (8111 FDM). 750 760 770 780 790 800 810 820 830 840 850 860 870 880 1 1 lmiTTTTTTimmmrr 0,0 0,1 Q2 Q3 "'T1 ~ 0,6 )'J u ïïitiiiiiia;at' 0,7 2 ~ ~ 0,8 !; r;;rr~ (!) -~ 0... r+ :r ~·;;;-.;;;.,; (!) ~;fUt:~!~~~~: 0 1,0 (!) ~~· ... ::> r+ CIO ~ 1,1 CT (!) ;::; 1,2 en c: g- a: (!) ::> 1,4 l6 Cil 1,5 -0 1,6 1,7 1,8 ' 1.~...... ,·...-·,.~-,,·''~ ,,, • ' ~ 1 7 ' ''1:7~!.1 'Il.!..,, ' Ul~··'~····l>jNic~··/···•·,"""''7/'~f'<'n'"' ' '" '•' .• ;,l;>l /, ' ' '•··~·····,••i•~t7~.'~' ·"' .,, /.!>tt~~-·· "~~;;19 . " \ ) ~• • . l 1 ,., '''\' '. .. ~... ' ' ) • h) ',· ~· \1 ~ 1 ' ' ·, 1 ~~ ·' ' 1< ~., ~-~ ~ ~... ,~ .._ .. ~ . " ...... ,, ~~~.. .:. •.~Jï.ïr .... l\',1 ;w• . ·.• . . .~1'111-· ...11•-:.\"" 1. 'v.fiil2,0 yellow base Cretaceous blue top Dinantian 0 200 400 m red fau lts - 19 - dually or stepwise displaced downwards. From the differences in deformation intensity These bordering faults not only affect the Dinan . (different numbers of stratigraphie levels affected) we tian limestone and the Si lesian but generally a Iso the may presume that the dissolution has been active du ring Cretaceous and the basal Cenozoic formations as weil different time intervals since the end of the Visean : (fig. 4). first under vadose water conditions (during the final The deformation intensity inside the fault-bor Visean-early Namurian emersion) and later by subsur dered depressions shows lateral as weil as vertical va face groundwater circulation. riations : in many cases the Dinantian limestone reflec No typical collapse structures have been observed tor only, has been broken and/or displaced, whereas within the eastern structural unit, with the exception of the overlying strata have been rnoderately to strongly one NNW-SSE running depression belt, right north of bent (fig. 5). But often also the younger Silesian strata Turnhout, wherein the Upper Carboni'ferous strata are have been dislocated : the overlying Cretaceous and strongly bent and the base of the Cretaceous shows a Cenozoic strata are strongly bent, with decreasing con gentle concavity above the underlying deformed Dinan cavity towards the younger formations (fig. 4). The tian limestones (e.g. at the crossing of tines 8004 and youngest formations affected, as can be detected from 8012). At most a gentle inclination is observed of seismic tines, are of Late Eocene age. sorne Silesian reflectors. The basal Cretaceous remains 1nside the fault-bordered depressions it is often almost unaffected except for sorne N-S directed kinking difficult to trace a particular Silesian reflector from in its general E-W strike (enclosure Ill). one end to another. ln sorne cases this is obviously due The Jack of collapse features affecting the post to a difference in seismic character related to facies Silesian in the eastern zone might also be explained by changes during the deposition of that particular reflec an increase in thickness of the Silesian overlying the tor (fig. 6). Another rea son for this indistinctness is collapsing Din.antian. the apparant chaotic character of the deformed Silesian strata above dislocated Dinantian limestone, suggesting 2.3.3.- Tectonics broken or brecciated formations (fig. 7). Broken and brecciated basal Namurian siliciclastics 2.3.3.1.- The Roer Valley Graben boundary faults have also been observed in a cored section through the The presence of important NNW-SSE directed Namurian-Visean contact in weil DZH-1 on the Hei normal faults, northeast of Turnhout, adds considerably baart- RiJkevorsel dome. to the complexity of the eastern structural zone (LE On the other hand one may observe the presence GRAND, 1968). These faults are the westernmost re of contemporaneous infill, evidenced by the increased presentatives of an extensive NW-SE tending network thickness of sorne upper Namurian or basal Westphalian of longitudinal tensional faults in NE-Belgium, which reflector intervals, just above the depression center constitute Roer Valley Graben boundary faults, pro (fig. 8). longating the Netherlands Central Graben towards the southeast. Two of these graben faults have been ob These observations favour the idea of a synsedi served in the investigated area, northeast of Turnhout; mentary deformation mechanism during the Silesian. the first west of Ravels, the second east of Weelde. The This deformation continued into more recent times, latter has been related to the so-called Reijen Storing as shown for example by the unexpected topography (BOUCKAERT & VANDENBERGHE, 1980). They of the base of the Cretaceous unconformity in the can be followed to the north furthermore, across the western part of the survey area (enclosure IV). lndeed E-W striking Hoogstraten fault. These graben faults the roughly E-W striking basal unconformity plane not only affect the Carboniferous, but also the Creta displays a particular morphology of elongated depres ceous and the younger Cenozoic formations. Conse sions (e.g. around the "nase" of Zoersel) and more quently they represent the youngest tectonic deforma isolated, elleptical to lobate subsidence areas resembling tion in the studied area. Their fault plane is often sig sinkholes (e.g. Westmalle, Oostmalle, St Lenaarts). the moidal (fig. 10). The Carboniferous strata have been axes of which are located exactly above the faulted more displaced (greater offset) than the overlying for Carboniferous depression minima (enclosure Ill). mations, suggesting a rejuvenation of aider fault pla From their general morphology and especially nes. The graben-related fault, west of Ravels, consists from their structural relationship with the overlying of a series of normal step faults (with increasing offset strata, we may conclude that these particular deforma to the east) rather than of an single fault place (enclo tions are not tectonic in origin but developed rather con sure Ill and IV). tinuously over a long period, albeit with different inten Thickness and facies variations of the Meso-Ceno sity, suggesting collapse mechanisms. zoic strata have been recorded from shallow boreholes These collapse structures are most probably the on bath sides of the different graben-faults (north result of the dissolution of underlying carbonates and/ Campine area, LAGA, 1973; PAULISSEN, 1973) as or evaporites (see further). weil as from recent seismic surveys in the Campine coa! 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l'V 0 yellow base of Cretaceous blue top of Dinantian green Sarnsbank marine bed ? orange intra- Namurien reflector- onlap brown intra -Dinantian reflector 0 200 400 m red faults Figure 13. -,Internai structure of the Dinantian and the onlap contact of the Namurian (8106 FDM). - 21 - .._,. co en 0 E 0 0~ 0~ 0~ 8 ~-· M ,., 8N 0 0 N 0 M 0 0,.., <""') 0.._,._,.., 00 0 0 L()_ ~ <""') u el "": .., "'0 0 t> 1.0- Ql <""') ;;:: ~ o. Ql Ql 0 ..,; 0 l'- Ql <""') :; .!:!' lL 0 (/) CO- :J <""') 0 (1) c '- u a ...... 0 a ...... u ...... (1) (1) c 0 '- a en- u c (1) ,.., -'- 0 (/) (1) a.=: (/) a_Cll :J a 0 (1) a ..0 ...... -a 0 - .._,.o_ ;-; c 0 (1) ~ :J 0 -a (1) '- (1) >...o ..0 '- ~-.._,. 750 740 730 720 710 700 690 68-0 670 660 650 640 630 1 1 1 1 1 1 1 1 1 1 1 1 1 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1 1'.) 1'.) 1 ~~1,6 ~(~~~~~~-~~)~~-1,8 yellow : base Cret aceous 0 100 ZOO 300m blue top Dinant ian brown underlying deep reflector red faults -23- field (BOUCKAERT, DUSAR & VAN DE VELDE, Namurian, internai .structures of the Dinantian carbo 1981), suggesting reactivation of these graben-faults nate mass are either not visible or hardly indicated on during different stratigraphical intervals occasionally the seismic profiles. One exception is the apparent in opposite senses. internai reflector right under the antiform of Poederlee, about 125 rn/sec (approx. 250 m), under the top of the 2.3.3.2.- The central belt Dinantian (fig. 6). This internai reflection is not a A particular graben-like deformation affects the multiple. This particular reflector is essentially subhori Dinantian and Silesian formations in the central, tran zontal, with a very slight dip to the E and looks like the sitional zone of the survey area. This deformation apparent prolongation underneath the antiform of the belt can be traced over more than 17 km, shallowing Dinantian reflector. Of special interest is the presence gradually to the South, where it probable disappears of a distinct uplift of the reflector right under the cul to the east of Poederlee. lt apparently widens towards mination point of the Poederlee high limit. the northwest (northeast of Rijkevorsel) and seems ln the Heibaart dome sorne internai structures to terminate at the EW-running Hoogstraten fault al have also been observed, which directly underly the though the limited seismic coverage there makes any top of !he Dinantian. These reflections are not as clear interpretation inconclusive. as in the Poederlee area : they are rather discontinuous This extensive NNW-SSE running fault-bordered and show a wavy seismic character, with variable dip; depression belt is shawn in profile by the seismic lines laterally they stop at the flanks of the antiform Dinan 8111-8110-8109-8101. tian reflectors (fig. 13). The bordering faults consist of several longitu These reflectors might be interpreted as disconti dinal fault steps tapering off into one another (e.g. nuous internai stratification, although the indistinctness east of Vlimmeren, enclosure Il). of these reflectors makes an interpretation very difficult. The dip, measured in the cores on top of the Heibaart This graben-like deformation is characterized by antiform structure reached 10° at maximum. an. im·portant offset of the Dinantian limestone along relatively steep, normal faults (especially important in 2.4.2.- Underlying deep reflectors the north) which traverse the Silesian without affecting the basal Cretaceous unconformity. The latter displays Most of the apparent deeper reflectors under the mostly a gentle kinking just above the underlying faults top of the Dinantian reflector, are related to multiple (enc.losure Ill). reflections from overlying strong Meso-Cenozoic re flectors. The asymmetrical base of this "graben" is appa rently composed of severa! Dinantian limestone blacks, A particular genuine deep reflector nevertheless dipping in opposite directions. These internai structu has been observed at a distance of about 450 to 600 misee res (synantiformal) are visible for instance west of below the top of the Dinantian limestone (fig. 14, 15). Beerse-Merksplas (enclosure Il, IV). Although this reflector has not systematically been mapped, it is visible throughout most of the seismic The overlying Silesian is almost unaffected by this li nes in the extreme. west and south of the investigated internai blockfaulting but di'Splays sorne subsidence area, where it runs apparently subparallel to the over and/or infill phenomena (thickness variations of sorne lying top of the D inantian. reflector intervals) (fig. 11, 12), which would suggest at least a partially synsedimentary origin and an early The exact origin and the stratigraphical position (Upper Carboniferous) age for this deformation. of this deep reflector is unknown. A post-Paleozoic reactivation of the faults is pro bable, as deduced from the k inks in the basal Cretaceous unconformity strike. Anyhow, this central deformation 3. RELATIONSHIP WITH OVERL YING belt differs essentially from the western structural zone by the more important offset of the Dinantian reflector, STRATA by its asymmetric outline and by the less pronounced subsidence of the overlying strata. 3.1.- UPPER-CARBONIFEROUS 3.1.1.- Namurian 2.4.- INTERNAL AND UNDERLYING STRUCTURES The contact between the top of the Dinantian carbonates and the overlying Namurian mudstones is 2.4.1.- Internai structures of the Dinantian limestone a well-defined limit on the geophysical logs (BLESS et al., 1981). Because of the important acoustic impedance contrast and the loss of energy through scattering at the Cored samples of boreholes DZH-1 and DZH-3 irregular (karstified) contact between Dinantian and consist of dark~grey silty mudstones with silty laminae. -24- Dips of 4° to 8° have been measured. Similar litholo of strong continuous reflectors of rather high frequency, gies have been described from the Turnhout borehole. characteristic of alternating mudstones, sandstones and The Namurian generally displays a rather wavy thin coal-seams. seismic character with weak and discontinuous reflec The thickness of the Westphalian is varying accord tors, as compared to the more pronounced and more ing to the underlying Dinantian-Namurian topography; continuous reflectors of the overlying Westphalian. it also wedges out towards the west and the south. No The onlap of the basal Namurian strata upon the Westphalian strata have been encountered in the bore Dinantian relief illustrates the stratigraphical gap be hales on top of the Heibaart structure. tween both stages. This angular unconformity is parti Just as the underlying Namurian, the Westphalian cularly obvious on the flanks of both the Heibaart strata have been affected by collapse deformation in and Poederlee antiform structures (fig. 13, 14). side the fault-bordered Dinantian depression belts. Outside these particular areas the onlap is less The deformation intensity decreases from the base to clear : here the Narriurian apparently conformably the top, varying from broken reflectors (e.g. the Sarns overlies the Dinantian and only a very weak unconfor bank reflector) to only gently bent reflectors (fig. mity may locally be detected. - 8, 16). The Namurian sediments have also intensively 1n the latter case it is not uncommon to observe been deformed inside the faulted depression belts of a different seismic character as weil as thickness diffe the underlying Dinantian : strongly bent or even broken rences inside and outside the faulted depression belt, reflectors characterize the Namurian above the Dinan related there to facies changes and infill phenomena tian depression minima; very locally one may observe during the deposition of the different Westphalian also some infill phenomena. sediments (2.3.). These deformations are indicative of a synsedi mentary mechanism during or shortly after the depo 3.2.- MESOZOICUM sition of the strata corresponding to these particular An isochron map of the base of the Mesozoic · Namurian reflectors. illustrates its structural relationship with the Carbo The Namurian strata wedge out to the west and niferous. are completely absent in the extreme west-part of the The general strike of the basal unconformity is studied area (e.g. profiles 8108-8113) : here a direct E-W with a very slight dip to the NE. 1n the Loenhout contact is observable between top D inantian and basal Heibaart area the basal Cretaceous unconformity plane Cretaceous (enclosure 1V). gently dips at 1.20' to the NNE (BLESS et al., 1981). Above the Heibaart and Loenhout type highs, the 1n the western structural zone, longitudinal depression youngest Namurian (and especially the Westphalian) belts and more isolated depression minima occur just strata tend to be remarkably parallel to the top of the above the faulted depression belts affecting the top of Dinantian. Hence a gentle reactivation of these domal the Lower Carboniferous. Particularly obvious is the structures at the end of the Namurian base of the West presence of extensive NW-SE running depression mi phalian cannat be ruled out. nima, with closed contours, such as near Westmalle, Oostmalle and St Lenaarts. 1n the northern part of the survey area, an impor tant sedimentary basin developed- from Namurian times Normal faults, discontinuous and with small onwards. The southern border of this deep Namurian offset, delimit these basal Cretaceous depression belts basin coïncides with the E-W tending listric fault (2.3.). and minima. They are clearly related to, if not coincid The axis of its depocenter runs approximatively perpen ing with, the bordering faults of the underlying faulted dicular to the general strike of the D inantian shelf, ex Carboniferous synforms (enclosure IV). cept for the northwestern most part of the studied area ft is noteworthy furthermore that neither the (Heibaart-Loenhout region) where both run parallel. Heibaart dame nor the Poederlee high have influenced, in any way, the general morphology of the overlying 3.1.2.- Westphalian basal Cretaceous unconformity plane. Ali of the other antiform structures shawn on the basal Cretaceous The contact between the Namurian and the West isochron map otherwise (such as the Vorselaar-Wechel phalian formations is characterized on the seismic pro derzande high, the Westmalle-Vlimmeren high, the files by a strong reflector which can easily be traced Rijkevorsel high, etc.) are clearly influenced by the throughout most of the investigated area. This reflector presence of the underlying antiform structures in the has been correlated with the Sarnsbank marine band and Dinantian (compare bcith the enclosures Il and Ill). an underlying sandstone unit, as compared to borehole These observations then would imply two different data from Turnhout (DELMER, 1962) and Meer (VAN mechanisms for the origin of the antiform structures DENBERGHE, 1984). in the studied area. A non-tectoniè, post-sedimentary The Westphalian is charàcterized by the presence 640 650 660 670 680 690 700 710 730 740 750 760 770 0,8 0,9 1 Il.) 01 üa yellow base Cretaceous 0 100 200 300 m btue top Dinantian red faults Figure 16.- Slightly bent Upper Carboniferous reflectors overlying a subsidence zone in the Dinantian (8101 ). -=-< -26- deformation mechanism · su ch as the subsiding and/or observations· support the idea of a non-tectonic, conti collapsing of neighbouring carbonates (= the faulted nuous deformation rilechanism, which has been active depression belts) would be typical of the relatively si nee the end of the Visean. broad and flat antiform structures bordered by small and relatively deep depression belts as discussed in part 2. The particular Poederlee and Heibaart dornes otherwise must have been positive areas (paleorelief), 4. GEOLOGICAL WORKING MODEL already before the deposition of the Upper Carboni ferous. This latter idea is also supported by the visible The combination of borehole data and subsurface onlap of the older Namurian strata upon both the Di structural elements (seismic interpretation) allows the nantian highs. proposition of a paleogeographical and structural work 1n the central and eastern structural zones of the ing model for the Dinantian shelf in the Antwerp Cam investigated area, the basal Cretaceous unconformity pine area. This hypothetical model needs to be con displays pronounced N-S to NNW-SSE kinking of its firmed nevertheless by new borehole data and further general E-W strike. These kinks are especially well _seismic exploration. developed east of Merksplas. They surmount or coïn• cide with underlying NNW-SSE trending fault planes in 4.1.- PALEOGEOGRAPHY the Dinantian and Silesian (bordering faults of depres sion belts), which would suggest a gentle post-Carboni The boreholes which penetrated the Dinantian ferous reactivation of these structural tines. The major carbonates in the Campine revealed the presence of the deformations affecting the basal Cretaceous, are the following particularities : graben-related tensional faults which run northeast of - paleosoils and associated detrital sediments (Kessel, Turnhout. The first one, west of Wortel is well.-develop Booischot); ed, whereas the second one, east of Weelde is only hard - silicifications (Halen, Turnhout, Kessel, Heibaart); ly indicated and cannot be traced further down to the - breccias (Turnhout, Heibaart, Kessel, Booischot); south. A direct contact finally between the Dinantian - internai angular unconformities (Loksbergen); and the basal Cretaceous is visible in the extreme west of the studied area, west of Westmalle (seismic profiles - paleokarst (Booischot, Heibaart); 8108 and 8113) : here the Silesian is completely remov - reworked microfossils (Turnhout, Halen); ed by erosion along a roughly NNW-SSE-trending tine. - irregular contacts, erosional unconformities, hard- 1n the uppermost part of the Booischot borehole, grounds ( Heibaart). a paleokarst has been observed within V2a limestones, Stratigraphie correlation moreover between these filled up with a breccious mixture of chalky lime,stone boreholes showed important variations in thickness and black clay, the latter of possible Wealdiari~ age. of the different Visean stratigraphical units suggesting This might represent the only evidence thus far unstable sedimentary regimes. for the presence of Lower Cretaéeous in the Antwerp Microfacies analysis furthermore of cored inter Campine (LEGRAND, 1964). vals on top of the Heibaart dome, demonstrated that restricted marine to sublagoonal depositional envi ronments prevailed during tate Visean times. 3.3.- CENOZOICUM The abundant authigenic quartz crystals observed The relationships between the Dinantian struc within Livian to Warnantian algal-stromatolithic lime tures and the Cenozoic sediments are restricted to rela stones (e.g. in Heibaart and Turnhout) are related tively small local subsiding areas within ·the westl:!rn either to hypersaline conditions (B LESS et al., 1981) structural zone. or to vadose water circulation, acid fresh waters derived from adjacent emerged areas (Labofina report, 1976). A remarkable subsiding (bending) of sorne Ceno zoic reflectors occurs just above the depression centers Ali of the former observations indicate thus a (with closed contour) of the basal Cretaceous uncop shallow to very shallow epicontinental sea, bordered to formity, which in their turn overly the faulted Dinan the S by relatively flat, deeply weathered non-carbonate tian collapse centers. The deformation intensity is va hinterland on the Brabant Massif, which has been affect ed intermittently by epeirogenic movements. riable from one subsiding centre to another, as weil ~s vertically : the concavity of the affected reflectol-s Subtropical to semi-arid climatological conditions decreases towards the younger formations. moreover allowed the development of soils on the hin The youngest . Cenozoic formations affected by terland,whereas vadose waters provoked synsedimentary underlying collapse phenomena, .as 'shown by s.eismiç silicification .in the nearby unconsolidated carbonates, sections, are dated as uppermost E"oc~ne (fig. 4L Thesê as weil as karstification in....the emerged lithified carbo nates. -27- 1 HINTERLAND 1 INNER SHELF jsHOALI OUTER SHELF sw intertidal flots oolitic NE lagoonal ponds sands si!iciclastics evaporites ? reels? V1• VZ paleosoils algol mats DEVON lAN SILURIANDINANTIA~f~~~~~~~~~~~~~~~~~~~~~~~ii~~~~~~;;~;;;;~ BLOCK- FAULTED BASEMENT 2 RUNOFF DISSOLUTION FRONT ::·.:-.·J·-. ~~.... -~-,-; ...... _,._ VADOSE CIRCULATION - 1) 1) 1) WITHDRAWAL OF THE SEA ...... EPEIROGENIC UPLIFT ? 3 SUBMERGED KARST RELIEF GRADUAL DROWNING INTERSTRATI FIED LIMESTONE SCARPS EVAPORITIC ROCKS ?? COLLAPSED LIMESTONE • DROWN!NG OF KARST TOPOGRAPHY 4 SUBS!DENCE OF OVERBURDEN R.D. 1983 COLLAPSE AFfER BURIAL Figure 17.- The different stages in the paleogeographical evolution of the Dinantian shelf (working model). 1. Late Vi sean regressive shelf; 2. Earl y Namurian emersion- Subaerial exposure; 3. Namurian transgression invading a karst topography; 4. Subsidence and collapse phenomena by ongoing groundwater circulation (SW-NE crossections trough paleogeographical sketch of fig. 18). ~ 1 dolincY ""CXl PRE- KARST KARST Figure 18.- Hypothetical paleogeographical sketch for the Dinantian subcrop. -29- lt is suggested that the pré-Namurian topography around the inner shelf margin. These hills are capped consisted of a solution-etched limestone relief. This so or partly capped by biostromal deposits, submerged lution-topography developed shortly after a significant · reet-banks and emergent reefs (pinnacle reefs and drop in sealevel, encompassing at !east the Pendleian knolls). (corresponding to the stratigraphical gap observed in Playa lagoons and swampy mudflats are present the boreholes Turnhout and Heibaart). on the seaward margin of the karstic hinterland. At that moment acid run-off from the non-car The post-Oinantian - pre-Silesian karst topogra bonate hinterland in the southwest flooded into the phy originated much in the same way as the karst emerged shelf lagoon carbonates in the east and north determined shelf relief in Southern British Honduras, east (fig. 18). Maximum solution therefore occured as described and illustrated by PURDY (1974) (fig. along the lithological contact of siliciclastics-carbonates. 18), although a real guarding rim of reet barriers is not Subsequent collapse resulted further int the appearance present in our recontructed model. Similar processes of limestone scarps, as illustrated by the scarp along nevertheless must have been active an the emerged the "no se·· of Zoersel. Dinantian carbonates during early Namurian times Karst development closely followed the with (fig. 17). drawal of the sea to the east-northeast and preferentially After the Pendleian-time interval, the karstified affected highly soluble (inter-supratidal) carbonates or Dinantian shelf has gradually been drowned by the even evaporites. This could explain the remarkably pa Namurian transgression, which invaded the emerged ralle! orientation of ridges and troughs indicating belts shelf from the east. Because of this graduai drowning, of similar 1ithology para li el to the paleo-coastl ine the westernmost areas of the emerged shelf have been (=the Zoersel scarp). longer exposed to solution activity than the eastern The adjacent shelf lagoon was bordered to the east regions. This could perhaps also account for the ap and northeast by a discontinuous belt of more or less parent structural tl ifferences between both structural isojated shoals (the Heibaart and Poederlee dornes) zones. which represent structurally-controlled positive areas : the Heibaart 1 borehole penetrated condensed Devo nian and Carboniferous deposits, and a relative! y shallow 4.2.- STRUCTURAL EVOLUTION Silurian basement. A similar structurally-controlled Two important structural directions can be ob high is supposed to occur in Poederlee. served on the lsochron map of the Dinantian reflector This shoal-1 ine represents most probably the (enclosure Il) : first an E-W to NW-SE direction which inner shelf margin : beyond this line the dip of the abruptly takes a N-S orientation near Zoersel; secondly, shelf increases considerably (e.g. northeast of Loen directions perpendicular to the former, and which appa hout). Farther to the east and northeast, the topogra rently radiate around the Zoersel high (western structu phy of the NE-dipping Dinantian outer shelf has strong ral zone). ly been affected by synsedimentary faults, which deve These structural !ines subdivide the subcrop into loped from the early Silesian on, as weil as by post a chess-board-like configuration of small highs and Silesian graben-related faults. A solution morphology troughs. Sorne. of these blocks were already structural nevertheless developed most probably also upon this highs before the Visean (e.g. Heibaart dome). supposed outer shelf, as demonstrated by the presence 1n our opinion th ose structural li nes represent of karst phenomena in the Turnhout borehole in the a fault-pattern related to a block-faulted basement. Upper Vi sean (Warnantian) carbonates. The karstification of the emerged carbonate shelf From a morphological point of view, this hypo has been guided most probably by joints along these thetical paleogeographical reconstruction of the Antwerp fault paths, along which the vadose waters could easily Campine a rea, shows sorne affinities with the Late Oua penetrate the carbonate mass, and could have possibly ternary Yucatan shelf in the southern Gulf of Mexico reached underlying or interstratified evaporites. This (LOGAN et al., 1969). The Yucatan Peninsula consists would explain the presence of narrow collapsed de of a low undulating karst plain that has no surface pressions perpendicular to the normal parallel highs drainage. Soil-filled depressions support dense rain and depression belts (see the Poederlee high for ins forest. During Wisconsin glaciation times, the karst tance). lnterstratified collapse breccias have been margin plain extended over the area which is now the observed in the Turnhout and Hal~m boreholes, espe inner shelf, with a solution rim and conical karst deve cially at the Livian-Warnantian contact. loping close to the present shelf margin. Much of the Yucatan plateau has been submerged by the post After drowning of the karst topography by the Wisconsin marine transgressions to form a shallo\N invading Namurian sea, the solution of carbonates was open clastic-free continental shelf. A series of e1on still proceeding due to continuous groundwater circu gated rocky hills, forms a discontinuous raised rim lation. This rriechanism provoked collapse and subse- -30- quent subsidence of thé overlying strata. Dissolution as paleosurfaces, suggesting temporary subaerial expo and collapse breccias are therefore to be expected vvithin sures (SWENNEN et al., 1981, 1982; JACOBS et al., these deformed depression belts. 1982). A reactivation of these faults could also explain More than 100 subsidence pits (so-called "puits the uplift of some small blacks such as the Heibaart and naturels" or subjacent karst collapse dolines) have been Poederlee highs, during early Westphalian times most recorded in the Mons Basin (extreme west of the Namur probably. Synclinorium) where they affect the Silesian and over The presence of an important NNW-SSE-directed lying formations (Cretaceous depression of the Haine graben-like deformation (transitional zone) could also Valley) (DELMER & VAN WICHELEN, 1980). Their be interpreted as the result of tensional movements extension is limited. They were known to the coal along some of these deep-seated lineaments. These mining industry as "cylindrical accidents" or "cir tectonic movements, accompagnied by synsedimentary cular faults"). deformation, could characterize an early stage of the These subsidence pits were only recently related Asturian phase (early Westphalian ?). to the dissolution of evaporitic rocks which were caver- The post-Westphalian continental phase eroded . ed by Warnantian carbonates (DELMER, 1977); it was the Dinantian-Silesian subcrop dawn to the actual the d iscovery of an unexpected thick sequence of anhy level before being covered again by Upper Cretaceous drite (about 600 m) in the St Ghislain borehole ( 1976) marine sediments. which favoured this idea for the origin of these bizarre deformations in the Haine Basin. This nodular anhy The eastern structural zone became affected fi drite is underlying V3b-V3c limestones in which slump nally by graben-boundary faults, especially during ing and brecciated levels frequently occur (DEJONGHE, Cênozoic times. DELMER & GROESSENS, 1976). ln the nearby Douvrain borehole ( 1979) a dissolution breccia has 4.3.- COLLAPSE STRUCTURES AND SINKHOLES been encountered at a stratigraphie level corresponding to the interval with anhydrite of the St Ghislain bore Collapse structures have been seismically identified hale (LECLERCO, 1980). for the first time in the subsurface of the Antwerp The Ghlin borehole ( 1980) otherwise penetrated Campine area. an abnormal thick sequence of Lower Turonian maris The interpretation of these structures is compati (over 350 m) and brecciated Silesian rocks, which re ble with the recent observation of paleokarst phenomena present the infill of a subsidence pit. This pit origi and associated solution collapse breccias in the Upper nated most probably after the dissolution of carbonates Devonian and Carboniferous around the Brabant Massif. and evaporites on top of, or interstratified within, the Warnantian carbonates (DELMER et al., 1982). The They have been described from different stratigra authors suggested that the subsiding rate inside the phie levels suggesting different periods of updoming or pit was irregular, and that the pit must have acted as eustatic sealevel fluctuation, subaerial exposure and a sediment trap during Cretaceous times. subsequent karstitfcation. DElMER ( 1977) noted the analogy finally between Boreholes at Turnhout and the Heibaart dame the Wealdian topography of the Haine Valley and the already indicated that the Dinantian s~crop is karsti evaporite-buried karst of the Athabasca Carbonate fied in the Antwerp Campine area (BOt:JCKAERT & in N-Canada or the "cenote" landscape of Yucatan VANDENBERGHE, 1980). "-- (cenote: water-filled collapse doli~e). Important paleokarst and solution collapse brec Very dramatic furthermore has been the record cias affect Frasnian limestones in the Visé area, along of recent collapse phenomena in the Tournaisis region the eastern border of the Brabant Massif. They deve (1970-1977) where combined geological-hydrological loped during Famennian times and have gradually been particularities of the subsurface provoked numerous drowned by the Dinantian transgressions. The breccias collapses of the overburden, resulting into the forma originated most probably after the dissolution of Lower tion of "puits naturels" reaching the surface (De Frasnian evaporites (on top of the Givet Limestones) ROUBAIX & LEGRAND, 1977). The top of the (POTY, 1981, 1982). Dinantian limestone lies here at a depth of less than Dolostone breccias near the Tournaisian-Visean 100 m. The numerous karst pits, which occur ali over (lvorian-Moliniacian) boundary in the eastern parts the solution-etched paleosurface have been filled with of the Namur and Dinant Synclinoria as weil as in the Wealdian, Cretaceous and Cenozoic deposits. After a Vesdre area, have been interpreted as evaporitic collapse drastic lowering of the groundwater level inside the breccias. They are often capped by columnar or palissa karstified Dinantian limestone (more than 1 m per de calcite which are interpreted as either a pseudomor year) the aquifer collapsed and provoked a graduai phy after gypsum or speleothems. Dedolomitization subsiding and subsequent downfall of the overlying zones on top of these collapse breccias are interpreted formations. - 31 - Sorne of the paleokarst phenomena at the Di GRAULICH, J.M., 1962. La phase sudète de l'orogenèse varis nantian-Silesian contact are still visible in outcrop : que dans le synclinorium de Namur à l'est du Samson. in abandonned limestone quarries along the Meuse Bull. Soc. Belg. Géol. 71 (2) : 181-198. Valley, at Tramaka for instance. Here paleokarsts cavi JACOBS, L., SWENNEN, R., VAN ORSMAEL, J., NOTE BAERT, L. & VIAENE, W., 1982. Occurrence of pseu ties on top of Warnantian c.arbonates are filled with domorphs after- evaporitic minerais in the Dinantian Namurian shales, at the very base of which irregular carbonate rocks of the Eastern part of Belgium. Bull. beds of lead mineralizations have been observed (JA Soc. Belg. Géol. 91 (2) : 105-123. COBS et al., 1982). LABOFINA S.A. - U.C.L. Lab. Paléontologie : Rapport sur : La biostratigraphie du Dinantien, Bassin de Campine. I.R.S.I.A. project E 751548, septembre 1976. LAGA, P., 1973. The Neogene deposits of Belgium. Guide ACKNOWLEDGEMENTS book for the field meeting of the Geologists associa tion Londen (31 March-3 April 1973). Belg. Geol. Dienst: 1-31. The authors are indebted to Mr. BODEMANN, LECLERCO, V., 1980. Le sondage de Douvrain. Serv. Géol. OTTO, ROSSA, SCHWARZ, WERNER of PRAKLA - Belg. Prof. Paper, 170 : 1-51. SEISMOS, Hannover, for their stimulating interest LEGRAND, R., 1964. Coupe résumée du forage de Booischot. and advices during the seismic campaign and its inter Bull. Soc. Bel_g. Géol. 72: 407-409. pretation. LEGRAND, R., 1964. Sondage de Booischot. Ann. Mines Belg. : 462-463. LEGRAND, R., 1968. Le Massif du Brabant. Mém. Expl. Cartes Géol. Min. Belg., 9: 1-148. REFERENCES LOGAN, B.W., HARDING, J.L., AHR, W.M., WILLIAMS, J.O. & SNEAD, R.G., 1969. Carbonate sediments and reefs. Yucatan shelf, Mexico. Am. Ass. Petrol. Geol., Mem. BLESS, M.J.M., BOUCKAERT, J. et al., 1976. Dinantian rocks 11 : 1-355, Tulsa, Oklahoma. · in the subsurface north of the Brabant and Ardenno PAULISSEN, E., 1973. De morfologie en de Kwartairstratigra Rhenish Massifs in Belgium, the Netherlands and the fie van de Maasvallei in Belgisch Limburg. Kon. Acad. Federal Republic of Germany. Meded. Rijks Geol. Dienst, Wetenschappen, Letteren, Schone Kunsten Belgie, N.S., 27 (3) : 81-195. Klasse der Wetenschappen, XXXV (127) : 1-257, Brussel. BLESS, M.J.M., BOUCKAERT, J. et al., 1980. Pre-Permian POTY, E., 1982. Paléokarst et brèches d'effondrements dans le sedimentation in NW-Europe : Pre-Permian deposi Frasnien Moyen des environs de Visé. Leur influence tional environments around the Brabant Massif in Bel dans la paléogéographie dinantienne. Ann. Soc. géol. gium, the Netherlands and Germany. Sedim. Geology, Belg., 195:315-337. 27 (1): 1-81. PURDY, E.G., 1974. Karst-determined facies patterns in BLESS, M.J.M., BOONEN, P., DUSAR, M. & SCILLE, P., British Honduras : Holocene carbonate sedimentation 1981. Microfossils and depositional environment of model. Am. Ass. Petrol. Geol. Bull. 58 : 825-855. Late Dinantian carbonates at Heibaart (Northern Bel gium). Ann. Soc. géol. Belg.; J04: 135-165. SWENNEN, R., VAN ORSMAEL, J., JACOBS, L., OP DE BEECK, K., BOtlCKAERT, J. & VIAENE, W., 1982. BOUCKAERT, J., DUSAR, M. & VAN DE VELDE, E., 1981. Dinantian sedimentation around the Brabant Massif, Exploration for coal in the Neeroeteren-Rotem area sedimentology and geochemistry. ln : The Pre-Permian (Campine Coalfield of the Campine-Brabant Basin, around the Brabant Massif. Publ. Natuurk. Gen. Limburg, NE-Belgium) : preliminary results of a seismic survey XXXII (1-4): 16-23,64-69. carried out in December 1980 - January 1981. Ann. Soc. géol. Belg., 104: 281-289. SWENNEN, R., VIAENE,W.,JACOBS, L.& VAN ORSMAEL, J., 1982. Occurrence of calcite pseudomorphs after gypsum DEJONGHE, L., DELMER, A. & GROESSENS, E., 1976. in the Lower Carboniferous of the Vesdre region (Bel Découverte d'anhydrite dans les formations anténamu gium). Bull. Soc. Belg. Géol. 90 : 231-247. riennes du sondage de Saint-Ghislain. Bull. Acad. Roy. Belg. (Cl. Sei.) : 80-83. VANDENBERGHE, N., 1982. Carboniferous North of the Brabant Massif in the area of Heibaart, Turnhout and DELMER, A., 1962. Coupes de sondages du Bassin Houiller du Meer; seismics and boreholes. Publ. Nat. Hist. Gen. Nord de la Belgique. Sondage de Turnhout (120). Ann. Limburg, 32 (1-4) : 24-26. , Mines Belg,: 101-138. VANDENBERGHE, N. & BOUCKAERT, J., 1980. Geologische DELMER, A., 1963. Cartes des Mines du Bassin Houiller de la beschouwingen van geothermische mogelijkheden in Campine. Ann. Mines Belg. : 739-754. Noord-Belgie. Belg. Geol. Dienst Prof. Paper 1980, DELMER, A., 1977. Le Bassin du Hainaut et le sondage de 168:1-21. Saint-Ghislain. Serv. Géol. Belg. Prof. Paper, 1977 : VANDENBERGHE, N., 1984. The subsurface geology of the 6: 1-12. Meer area in North Belgium and its significance for the DELMER, A. & VAN WICHELEN, P., 1980. Répertoire des occurrence of hydrocarbons. Journ. Petroleum Geolog. puits naturels connus en terrain Houiller du Hainaut. 7 ( 1 ) : 55-66. Serv. Géol. Belg. Prof. Paper, 1980, 172 : 1-79. VAN STAALDUINEN et al., 1979. The geology of the Ne De ROUBAIX, E. & LEGRAND, R., 1977. Effondrements du therlands. Meded. Rihks. Geol. Dienst, 31 (2) : 949. sol dans le Tournaisis. Serv. Géol. Belg. ~ / ~ ./ ( \ No40Jiand ' ..... '"'··· /.// e '!.•"'"'• ..... ~ ..._ •• ''\'~. ~.-~ / -~i...... t··.,... ~ .~ ..... •'t ... .:: ...... :· ., ~ n ~ "'-:- ."-Jo' .. luri'\Mut• ç B ~ %. 0 :l 3 ~ '"':"é' '?.. N ~· 3 Fi (j) -g ~ ëô "' --" ~ l """-" 10 ~ Q) SBSMIC CAMPAIGN '\981-1982 5. sELGIAN GEOLOGICAL SIJRVEY -015TRIGAZ SA Q '-"""""""" of_, -~~71.·""'""- ,.-A-G "helen ~ • $CALE ) !11\..lf' -· . • 1 ~"" t. ~ 0 rr, \..~\'l,t.aa.'t• )> g \~ Gl •?ulderbO$ • # ·"".·'"./ 1 m :::1 ë"' ;;; 0 ::r(") 0 :::1 3 Ql w "C w 0...., ...::r ...CD 0 "C S, ~ ...::r CD 0 :ï Ql ...:::1 ii:ï :::1 SE/SMIC CAMPAIGN 1981-1982 BELG/AN GEOLOGICAL SURVEY- 0/STR/GAZ S. A /sochron map of the top of the Dinantian ~ SE!SMIC PROFILES AND TRACE NUMBERS !Ines 1-19 CAMPAIGN 1974-1975 DISTRIGAZ 5 A -GEOSTOCK llne N7801 CAMPAIGN 1978 BELGIAN GEOLOGICAL SURVE:Y l1nes 8003-8014 CAMPAIGN 1980 BELGIAN GEOLOGICAL SURVEY lmes 8100-8113 CAMPAIGN 1981-1982 BELGIAN GEOLOGICAL SURVEY -DISTRIGAZ SA •Geo ~·w~- ISOCHRON(20msec EQUIDISTANCE) ..&...~~ FAULTS SCALE m :l. 2. 0 "'c: Cil ii> 0g. a :l ~_./ 3 w "0'" ~ s.,.. ::r "'C" Ill 0"' ~ ...... ::r "' Q ..,...... _ ~ Q) ~ 0 c: !" SEISMIC CAMPAIGN 1981-1982 ·~---- BELGIAN GEOLOGICAL SLRJEY -DISTRIGAZ S. A T1elen . lsochron mop of the bose of the Cretoceous ------SEISMIC PROFILES AND TRACE NUMBERS \ines 1-19 CAMPAIGN 1974~ 1975 DISTRIGAZ SA - GEOSTOCK llne N7801 CAMPAIGN 1978 BELGIAN GEOLOGICAL SURVEY \~nes 0003-8014 CAMPAIGN 1980 BElGIAN GEOLOOCAL SJRVEY bnes 8100-8113 CAMPAIGN 1981-1982 BELGIAN GEOLOGICAL SURVEV- DISTRIGAZ S A •Geo ISOOiRON{20msec ECll.lllSTANCE) -··-...L...L..L.J...... L..L. FAULTS CONTACT BETWEEN HF CF ONANTIAN AND BASE OF CRETACEOUS 3 SCALE km R ŒEESEN 1983 .~ ...... "s Lv; ''l: ' ~ E~: CRETACEOUS [: OINANTIAN t" Er:~ CRETACEOlJS m (")"' 0 DINANT! AN "'c: Cil :< SE[: 1 CRETACEOUS Q 0 SILESIAN ~ "' ~ [)NANTI AN aô' )>... 0 Cl cRETACEOUS SILESIAN ONANTIAN Sect1on F-F' SE[:- SEISMIC CAMPAIGN 1981-1982 BELGIAN GEQOGICAL SURVEY- DISTRIGAZ S, A CRETACEOUS r Structural cross- sections (A tc G) Based on isochron maps (Top of Dinantian- Base of Cretaceous) SILESIAN 0 2 3 HORIZONTAL SCALE km OINANTIAN TIME-DEPTH CONVERSION TABLE (2-way t1mel RIRNJ:OOT 512Q borehole 1 HEIBAABT Hl boteho!e SSE[:~ 200m511C = 160m 200msec: = IS1m 400m5ec • 365m :n>msec:= 273m CRETACEOUS 600msec:= SSOm 'OOm!«• 37Srr, EO:lm5« • 820m aoomsec = eoom 1000m5eC= 1190m !l(l)msec = 968m SILESIAN 1200mset: 1590m IOX!msec = 1\24m 1400msec: 1995m 1100msec: 145Bm 1600mse<:= 23Sl'n 1300m,..,=l507m OREESEN R. 1983 OINANTIAN -36- Table A.- SEISMIC SURVEYS IN THE CAMPINE AREA E AND NE OF ANTWERP CAMPAIGN SURVEYORS +no OF SURVEY COMMISSIONERS PURPOSES / TARGEi 1. Campine Belge SEISMOS n° 881 Belg. Geol. Survey - unconformity at the base of the 1953-1956 Cretaceous - presence of Permo-Triassic - fault systems within the Cenozoic - delimit the S-border of coal bearing strata. 2. Campine Belge SEISMOS nO 1367 Société Campi noise de subsurface mapping of the Garbo ni- 1961-1962 Recherches et d'explo- ferous limestone further elaborated rations Minérales (SCREM) in 3. 3. Bree-Kessenich-Heibaart Compagnie Générale de Société Cam pino ise de Re -the antiform trend of the Heibaart 1963 Géophysique cherches et d'Explorations VIimmeren area, as shown by a Minérales (SCREM) gravimetrie survey (1962, SCREM) (hydrocarbon exploration). - presence of deeper reflectors 4. Heibaart 1974-1975 PRAKLA-SEISMOS nO 741044 DISTRIGAZ S.A. - delimit the Heibaart antiform (gas storage project) - thickness of the covering Upper Carboniferous strata. 5. Heibaart 1978 PRAKLA-SEISMOS no 781310 Belg. Geol. Survey -investigation of karstified water- infilled limestones at the top of the Dinantian -investigation of the NE-slope of the Heibaart Loenhout antiform struc trure (geothermal energy project) 6. N-flank Brabant Massif PRAKLA-SEISMOS n° 801311 Belg. Geol. Survey, Un iv. - methodology study for distin 1980 of Utrecht, E.E.C. guishing between karst-affected and compact limestone (Dinantian top) (geothermal energy project) 7. Oostmalle 1981-1982 PRAKLA-SEISMOS n° 811321 Belg; Geol. Survey, -the detection of appropriate struc DISTRIGAZ, S.A. tures S a nd E of the Heibaart struc ture in the karstified Visean lime stone (gas-storage project). -37- Table B.- ACQUISITION PARAMETERS OF RECENT SEISMIC CAMPAIGNS IN THE STUDIED AREA (VIBROSEIS-reflection surveys by PRAKLA-SEISMOS, Hannoverl CAMPAIGN Heibaart 1974-1975 Heibaart 1978 N-flank Brabant 1980 Oostmalle 1981-1982 COMMISSION ER DISTRIGAZ, SA, Brussels Geol. Survey Belgium Geol. Survey Belgium Geol. Survey Belgium GEOSTOCK, Paris University Utrecht, EEC DISTRIGAZ SOURCE Parameters Sweep 18-65 HZ, 10 s 20-100 Hz, 7 s 18-98 Hz, 11 s 15-90 HZ, 20 s NO of vibra tors 3 3 3 3 Vibrator spacing 20 rn 7,5m 10m 10,44 m Vibrator step width 7m Om Om 2,61 rn Length vibrator pattern 89 rn 15m 20 rn 28,71 rn vertical stacking 8-fold 8-fold 8-fold 4-fold vibrator centres 50 m 30 rn 40 m 40m RECEIVER Parameters N° of groups 48 24 120 120 Geophones per group 24 or 36 12 12 18 group length 55m 11 rn 20 rn 22m geophone array in 2 or 3 rows,equally in 1 row, equally in 1 row, equally in tapered array (34m) spaced (50 m) spaced (40 ml spacel (20 ml (in 1 row, equally spaced (34) geophone type (!ines 8100,8103, 8105 only). geophone type SM4- (10Hz) SM4- 510Hz). SM4- (10Hz) SM4- (10Hz) RECORDING Parameters record length 13 s 9s 14 s 24 s sampling rate 4 ms 2 ms 2 ms 2 ms 50 Hz notch filter in not not not Low-cut filter 12 Hz,18 dB/oct 12 Hz, 18 dB/oct 12,5 Hz, 12 dB/oct 12,5 Hz, 12 dB/oct High-cut filter 62 Hz, 72 dB/oct 124Hz, 72 dB/oct 125Hz, 72 dB/oct 125 Hz, 72 dB/oct Gain constant 42 dB 42dB 42 dB 42 dB PROCESSING Parameters 1. Routine processing ali lines (19) ali !ines (3} ali !ines (15) ali !ines (15) (PRAKLA-SEISMOS) display 2. Coherency fil ter dis play not not alllines (15) ali fines (15) 3. Wave equation line 2 li ne 7801 8011 propate !ines 8102-8106-8107 migration 8111-8112 Table C.- SEISMIC VELOCITY SURVEYS 1. Turnhout (17 E/225, S120) (Rapport sur les mesures de vitesses sismiques dans les sondages 121-Meeuwen et 120-Turnhout 1953-54 SEISMOS no 734). 2. Heibaart 1 (7E/178, S129) (Rapport de supervision et d'interprétation concernant l'étude sismique de Heibaart. Rapport nO 4: Stockage souterrain de gaz en Belgique, DISTRIGAZ, GEOSTOCK, 1974/75). 3. Meer (7E/205, 81491.