growth and subsidence of carbonate platforms: numerical ... · 1582 claudia marella, riccardo...

1581

ANNALS OF GEOPHYSICS, VOL. 47, N. 5, October 2004

Key words lithospheric flexure – differential com-paction – Triassic

1. Introduction

The peculiarity of carbonate platforms is thatsediments can be produced only in a few metersof water depth. This notwithstanding, in many

regions of the world both past and present plat-forms commonly consist of several hundred me-ters of continuous successions of shallow-watersediments and these thick accumulations can beexplained only if accommodation space is con-tinuously created, that is if subsidence of the un-derlying ‘substratum’ and/or sea level rise con-tinuously occurs for relatively long time spans.

As concerns sea level variations, nowadayswe have a widely accepted knowledge of thefirst-order global fluctuations scale (Haq et al.,1987) and therefore, for almost any geologicaltime interval we can grossly quantify the con-tribution of this phenomenon in producing ac-commodation space. Whatever the case, be-

Growth and subsidence of carbonateplatforms: numerical modelling

and application to the Dolomites, Italy

Claudia Marella (1), Riccardo Caputo (2) and Alfonso Bosellini (3)(1) Dipartimento di Scienze della Terra, Università degli Studi di Camerino, Italy

(2) DiSGG, Università degli Studi della Basilicata, Potenza, Italy(3) Dipartimento di Scienze della Terra, Università degli Studi Ferrara, Italy

AbstractThe phenomenon of subsidence induced by the growth of carbonate platforms has been investigated with the aidof numerical modelling. The research aimed to quantify the relative contribution of this process in the creationof the accommodation space required to pile up thick neritic bodies. We analysed two end-member deformationstyles, namely the elastic behaviour of the lithosphere when locally loaded and the plastic-like reaction of a sed-imentary succession underlying a growing carbonate buildup. The former process, analysed using a modifiedflexural model, generates a regional subsidence. In contrast, the latter process, simulated by considering thecompaction occurring in soft sediments, generates a local subsidence. We attempted to quantify the amount anddistribution of subsidence occurring below and surrounding an isolated platform and in the adjacent basin. Themajor parameters playing a role in the process are discussed in detail. The model is then applied to the LateAnisian-Early Ladinian generation of carbonate platforms of the Dolomites, Northern Italy, where they are spec-tacularly exposed. Taking also into account the Tertiary shortening that occurred in the area, both local and re-gional subsidence contributions of major platform bodies have been calculated aimed at a reconstruction of themap of the induced subsidence. A major outcome of this study is that the accommodation space, that allowedthe accumulation of very thick shallow-water carbonate successions in the Dolomites, was only partially due tolithospheric stretching while the contribution given by the ‘local’ overload is as high as 20-40% of the total sub-sidence. Our results also shed some light on the water-depth problem of the Triassic basins as well as on thebasin-depth to platform-thickness relationships.

Mailing address: Prof. Riccardo Caputo, DiSGG, Uni-versità degli Studi della Basilicata, C.da Macchia Romana,85100 Potenza, Italy; e-mail: [email protected]

1582

Claudia Marella, Riccardo Caputo and Alfonso Bosellini

cause eustatic curves commonly show sometens of meters of continuous sea-level rise and,just in a few cases, up to 100-150 m, at a firstglance this natural phenomenon cannot explain,at least alone, the growth of thick platforms thatoccurred during many geological periods.

As a consequence, subsidence of the under-lying substratum must be claimed as the princi-pal mechanism for the generation of accommo-dation space during the build up of these sedi-mentary bodies.

Subsidence of the Earth surface can basical-ly occur as a consequence of three majorprocesses. Firstly, if the lithosphere is horizon-tally stretched, it becomes thinner (e.g., McKen-zie, 1978), the top and bottom interfaces be-come closer due to the occurrence of a large-scale pure shear and thus the Earth surface sub-sides. This process is mainly referred to as tec-tonic subsidence and typically occurs during arifting stage.

Secondly, if the crust and/or the lithospherebecome denser, a new lithostatic equilibriummust be reached. A typical example of thisprocess is the thermal subsidence associatedwith the cooling of the lithosphere in passivemargins (Sclater et al., 1971).

Thirdly, if the crust is locally overloaded,again new lithostatic conditions are required tokeep the equilibrium. Depending on the rheo-logic and mechanical behaviour of rocks, thisprocess can induce ‘local’ subsidence at thescale of the applied load as well as regional ef-fects at distances several times the size of theloaded area. This last case commonly occurs inregions where important topographic structureslike volcanoes and oceanic islands can be gen-erated on top of the Earth surface in relativelyshort time intervals. Typical examples are thefore-deep basins associated with the mountainchains and the post-glacial rebound of Scandi-navia.

In this paper the subsidence induced in thelithosphere and within the uppermost crust bythe growth of isolated carbonate platforms is in-vestigated. One of the principal aims of this re-search is estimating the amount and the distri-bution of subsidence during the growth of acarbonate platform. To achieve this goal we setup a numerical model compiled with the Matlab

software. In order to evaluate the role played bythe parameters considered, we performed sever-al tests.

A second important aim of this study is theapplication of the model to the carbonate plat-forms that grew in the Dolomites, NorthernItaly, during Middle Triassic that represent aspectacular and world famous example of car-bonate build-ups. As a major outcome, we cantentatively quantify the amount of subsidenceinduced by the platforms’ load and its distribu-tion in the palaeo-Dolomites region. Based onthe thickness of the neritic successions and con-sidering the calculated pattern of the ‘near-field’ contribution, the amount of subsidenceinduced by a ‘far-field’ process can be easilyobtained. According to the Middle Triassic geo-dynamics of the area, the tectonically inducedsubsidence can be roughly estimated and con-sequently we can have some idea on the amountof stretching which occurred during that timeinterval.

2. The numerical model

In order for subsidence to occur, the litho-sphere and/or the crust must deform either lo-cally or regionally. The deformation processcan occur according to three end-member be-haviours of lithospheric rocks: elastic, plasticand viscous.

To estimate the subsidence induced by thegrowth of carbonate platforms two effects havebeen considered. Firstly, the flexure of a thinelastic plate and, secondly, the plastic deforma-tion due to compaction of the underlying strati-graphic successions progressively buried.

As a first approach, we did not consider thepossible contribution associated with a viscousbehaviour because important effects in terms ofamount of deformation are negligible in rela-tively short time spans, say of the order of a fewmillion years (e.g., Ranalli, 1987).

The elastic behaviour of the lithosphere hasalready been recognised in several geodynamicconditions. Classical examples are the flexureinduced by the Hawaiian Islands chain (Tur-cotte and Schubert, 1982), the bending occur-ring in front of oceanic trenches along subduc-

1583

Growth and subsidence of carbonate platforms: numerical modelling and application to the Dolomites, Italy

tion zones (Karner and Watts, 1983; Wang,2001) or the deflection associated with conti-nental sedimentary basins (e.g. Walcott, 1970).

The elastic deformation is analysed here us-ing a flexural model of bending lithosphere sup-ported by a denser substratum. The permanentvolume reduction occurring in the sedimentarysuccession, which is progressively buried underthe increasing load of a growing carbonate plat-form, has been considered and consequentlymodelled as a plastic deformation. The two in-dependent deformational processes are thensummed up to calculate the induced subsidence.

2.1. Flexural model

The flexure of the lithosphere can beanalysed considering the bending of an isotrop-ic elastic plate of thickness h, much smaller thanthe two horizontal dimensions, subjected to asurface load located at the top and supported bya denser substratum (fig. 1). In such conditions,three forces are involved: firstly, the downwardfacing force exerted by the surface load; sec-ondly, the elastic force generated by the plate re-sisting bending; thirdly, the buoyant force due tothe replacement of dense asthenosphere rockswith less dense lithosphere rocks. Consideringthe three forces acting on the plate, the generalequation describing the deflection in the xy-plane is (e.g., Timoshenko and Goodier, 1951)

Dxw

x yw

yw gw L2 m i4

4

2 2

4

4

4

22

2 22

22

+ + + - =t t_ i( 2

(2.1)

where g is the gravity acceleration, ρm the den-sity of the asthenosphere, ρi the density of thereplacing materials (sediments and water), L isthe linear surface load and w is the deflection ofthe plate relative to the centre of load applica-tion. D represents the flexural rigidity that isequal to Te

3E/12(1−ν 2), where Te is the elasticthickness, E is the Young modulus and ν is thePoisson coefficient.

This equation can be applied to linear topo-graphic features like mountain ranges oraligned oceanic islands. Indeed, in this case,where the bending geometry is the same in anycross-section normal to the long dimension ofthe loading body the problem is reduced to aone-dimensional elastic bending

Ddxd w gw Lm i4

4

+ - =t t_ i . (2.2)

For a linear load along the y axis at x = 0 the gen-eral solution is (Turcotte and Schubert, 1982)

cos sinw w e x x/

x

x

0= +a a- a

a k (2.3)

where wx is the deflection at a distance x fromthe load, w0 is the maximum deflection at x = 0given by

Fig. 1. Simplified sketch of the numerical model used to estimate the subsidence induced by the growth of acarbonate platform: deflection and compaction processes occurring within the elastic and plastic layers, respec-tively, are indicated along with the main parameters. E: Young modulus; ν: Poisson coefficient; Te: equivalentelastic thickness; ρw: water density; ρp: platform rock density; A: platform dimensions; ρm: mantle density.

1584


w0= Lα3/8D (2.4)

and α is the flexural parameter α = [4D/(ρm+− ρi)g]1/4.

Keeping in mind that the aim of this study isto estimate the flexure of the lithosphere causedby a point load as a carbonate platform (bidi-mensional case), we also need to consider theshear stress induced in the yz-plane that obvi-ously plays an important role in reducing themaximum subsidence. According to Lowrie(1997), because a load with equal horizontal di-mensions causes a central depression that is lessthan a quarter the effect of the linear load a cor-rective coefficient is introduced in eq. (2.3),which thus becomes

. cos sinw w e r r0 25 /r

0= +a a- a

a k; E (2.5)

where r is the radial distance from the point load.We are well aware that this coefficient is an over-simplification of the mathematical aspects and itdoes not correspond to a rigorous solution of thedifferential equation. Nevertheless, we prefer tokeep the mathematics to a minimum for severalreasons. Firstly, because the principal aim of thepaper is to assess the order of magnitude of thisnatural phenomenon and not to quantify it exact-ly. Secondly, the values that some of the param-eters included in the above equations can assumewithin their possible ranges induce an even larg-er variability in the results. Accordingly, eq. (2.5)has been used to estimate as a first attempt theamount of lithospheric flexure due to the growthof a carbonate platform.

2.2. Compaction model

The plastic deformation is estimated by ap-plying a compaction model. The thickness reduc-tion of the sedimentary successions, buried underan increasing load has been calculated. The com-paction in sedimentary units commonly occursdue to the leakage of pore fluids and to a generalgeometric reorganisation of the particles consist-ing in the partial collapse of the grain framework,plastic deformation of soft fragments, and frac-turing and pressure solution phenomena (Rieke

and Chilingarian, 1974). At greater depths evenmineralogical transformations can contribute to aprogressive volume (i.e. thickness) reduction. Allthese factors lead to a reduction in porosity. An-other important factor that influences the porosi-ty reduction is the precipitation of cement mate-rial, though this process is more an obstacle tothe compaction process.

If analysed in detail, diagenetic compactionof sediments is not a simple plastic deformationmechanism, but for the purposes of this re-search and particularly at the scale we consider,as a first approximation this natural process canbe modelled as a plastic deformation (fig. 1).

The mechanical compaction of sedimentsdepends on the buried depth (i.e. lithostaticload) and on the differential stresses. If thecompaction is the only working process, thethickness of the solid component remains con-stant and therefore the ratio between solidity(1−φ ) and the thickness reduction is linear(Baldwin and Butler, 1985). In this study weuse a simple porosity-depth function as pro-posed by Athy (1930). Ruby and Hubert (1960)showed that the porosity should be an exponen-tial function of depth in the form

e cz

0=z z - (2.6)

where φ 0 is the initial porosity and c is the com-paction factor.

In this study we follow the approach proposedby Sclater and Christie (1980), which allows esti-mation of the depositional (i.e. original) thicknessof a sedimentary succession from the present-daythickness as directly measured in the field. Forexample, if we assume a total overburden thick-ness that caused a certain compaction, inversionof eq. (2.6) allows to ‘de-compact’ the deposits.Accordingly, we can calculate the thickness forany depth of burial and from the difference be-tween this calculated value and the present-daythickness we obtain the amount of compactionthat occurred during any time interval.

3. Parameter definition

A key step in any numerical modelling isthe choice of the most suitable and correct val-

1585


ues for each parameter and variable introducedin the calculations. Many of these values can beeasily inferred from the abundant literature onthe topic (e.g., Jaeger and Cook, 1979; Turcotteand Schubert, 1982; and references therein),while in some cases the choice of the numericalvalue is more difficult because the lack of geo-logical, mechanical and geodynamic informa-tion does not allow a tight constraint of the pa-rameter. In these cases, a more or less widerange of values has been tested.

3.1. Elastic deformation

Table I lists all the numerical values andranges attributed to the parameters used in theprogram. However, in the following, only themost important parameters will be discussed.

As concerns elastic deformation, one of themost important parameters is the elastic thick-ness, Te, which generally does not represent theentire lithosphere but only that part behavingelastically. Indeed, one of the major problemsin defining this parameter is that the lithosphereis not perfectly elastic as assumed in the theoryfrom which eqs. (2.1) to (2.5) are derived. Infact, the possible occurrence of hotter and/orless competent volumes within the lithosphere,

diffuse creeping phenomena and the possiblecrust-mantle decoupling de facto strongly re-duce the real elastic behaviour of the litho-sphere. In order to account for these rheologicalheterogeneities within the lithosphere, a per-fectly elastic but thinner lithosphere is com-monly considered, that is an ‘equivalent elasticthickness’ is introduced in the calculations.

For the oceanic crust the elastic thickness isa function of age and roughly coincides withthe isotherm of 600 C° (McNutt, 1988). In con-trast, the continental lithosphere does not en-tirely depend on temperature and pressure con-ditions (Burov and Diament, 1995).

The thermal state of the continental litho-sphere is one important factor among othersthat determines the value of the equivalent elas-tic thickness. Other factors are the state of thecrust-mantle interface, the thickness and pro-portions of the mechanically competent crustand mantle and the local curvature of the platethat is directly dependent on the bending stress-es (Burov and Diament, 1995).

Burov and Diament (1995) document thatthe continental lithosphere elastic thickness hasa wide range of values between 5 and 110 km.These authors also emphasise the occurrence ofa bimodal distribution characterised by twopeaks at 10-30 km and 70-90 km, respectively.

Table I. List of parameters, their symbols and values used during numerical modelling. The values in the lastcolumn are those selected for the model application to the Late Anisian-Early Ladinian carbonate platforms ofthe Dolomites, Northern Italy. The selected values for the initial porosity and the compaction factor refer to theBellerophon Formation (BEL) and the Werfen Formation (WER), respectively.

Parameter Symbol Unit Value or range Selected value

Young modulus E Pa 1010-1011 1010

Poisson coefficient ν - 0.22-0.25 0.25Equivalent elastic thickness Te km 10-50 21Mantle density ρm kg/m3 3000-3500 3500Replacement density average of ρw-ρp kg/m3 1100 1100Platform density ρp kg/m3 1800-2200 2200

Initial porosity φ0 - 0.3-0.7 BEL 0.6WER 0.5

Compaction factor c m−1 2 ⋅10−4-7 ⋅10−4 BEL 5 ⋅10−4

WER 2.7 ⋅10−4

1586


Similarly, McNutt (1988) have compiled a data-base of elastic thickness values reported in theliterature for the continental lithosphere undermajor mountain chains. The minimum valuethey collected is 15 km, the maximum 30 km.

Based on the gravity anomalies of the Kr-ishna-Godavari Basin in India and the inferredload induced by the sedimentary infill, Krishnaet al. (1999) suggest that the best fitting valueof the equivalent elastic thickness to explain theobserved gravity anomalies is about 30 km.Similar values were assumed to calculate thetectonic subsidence and the crustal flexure inthe Chaco Basin, Bolivia (Coudert et al., 1994).

Accordingly, though the elastic thicknessstrongly depends on the broader and specific ge-odynamic conditions to be numerically simulat-ed, as a first attempt for continental lithospherewe chose a range of values between 10 and 50km. The variability of Te has a twofold effect.Firstly, by decreasing the equivalent elasticthickness the maximum deflection increases al-most proportionally and, secondly, the peripher-al bulge moves closer to the central depressionand becomes more pronounced (fig. 2a).

Also the Young modulus is inversely pro-portional to the maximum deflection, though itseffects are less important because by varying Eof one order of magnitude (from 1010 to 1011

Pa), w0 varies of about ±25% (fig. 2b).

Another influencing parameter is the mag-nitude of the point load that is directly propor-tional to the maximum deflection (eq. (2.4)).On the other hand, this parameter is a functionof i) the initial distribution and extension of theneritic conditions (i.e. the growing carbonateplatform), ii) the amount of aggradation, iii) theamount of progradation and iv) the selected val-ue for the density of platform rocks. All thesefactors will be further analysed and discussed ina later section where the numerical model is ap-plied to real examples of carbonate platforms.

Elastic deformation is also influenced bythe density contrast that determines the buoyantforce induced by the replacement of the as-thenosphere mantle rocks with less dense mate-rials (fig. 1). Indeed, while the density of the re-placement materials is close to unity, consistingfor more than 95% of water, the replaced man-tle rocks can range between 3000 and 3500kg/m3. Whatever the case, the final result is notstrongly affected.

3.2. Plastic deformation

As concerns deformation associated withcompaction, the most important parameters arethe initial porosity and the compaction factor.Initial porosity, φ0, can strongly vary from more

Fig. 2a,b. a) Influence of the equivalent elastic thickness (Te ) on the deflection. The larger the value, thestronger the layer, the less important the maximum deflection. b) Influence of the Young Modulus on the de-flection. Note the inverse proportionality between the Young modulus and the maximum deflection, the shift andthe magnitude of the peripheral bulge.

a b

1587


than 60% to less than 15% (Pryor, 1973; Camp-bell et al., 1974). Indeed, these extremely vari-able values of porosity are associated withstrong variations in the possible amount ofporosity reduction and therefore in compactionand the consequent subsidence. This parameteris particularly influential during the initial bur-ial history of sediments (fig. 3a), though atdepths greater than 5 km, all curves tend to con-verge to a similar value.

The second important parameter playing arole during the compaction process is the so-

called compaction factor, c, in eq. (2.6) (fig.3b). In fact, although all c-curves have the com-mon tendency to a progressive reduction inporosity, at great depths, say more than 5 km,small and large values of c differ up to almost20%. But even more important is the influenceof this parameter during the burial evolution.For example, during the first 1000 m of burial,a compaction factor of 8 ⋅10−4 induces about30% of porosity reduction, while a value of2⋅10−4 is associated with only a 10% porosityreduction, that is of sedimentary compactionand thus of induced subsidence. This parameterstrongly depends on the lithology of the sedi-ments and especially on their texture character-istics.

A further influential factor as concerns theamount of induced subsidence is obviously theoriginal thickness of the sedimentary unit un-dergoing compaction. Indeed, it is clear thatwhatever the compaction factor and the initialporosity are, the larger the original thicknessthe larger the final amount of induced subsi-dence (fig. 3c).

Again with reference to the plastic deforma-tion and considering a simplified 3D truncated-conical shape for carbonate platforms, it is ob-vious that the amount of burial of the underly-ing sedimentary units varies with the positionrelative to the platform limits. Indeed, com-paction is nil outside the platform toe of theslope and it is maximum inside the ring defin-ing the shallow-water depositional area. There-fore, the slope angle directly influences the di-mension of the annular gradient for the amountof compaction occurring in the sediments un-derlying the carbonate build up. Carbonate plat-forms in the Dolomites commonly range be-tween 25° and 45°.

4. Applications to Triassic platforms of the Dolomites

As mentioned in the introductory notes, thenumerical model has been applied to some ofthe most spectacular and well-known examplesof carbonate platforms in the world: theDolomites of Northern Italy. Indeed, during theMiddle and Upper Triassic, the whole region

Fig. 3a-c. Reduction of porosity (i.e. compaction)associated to progressive burial: a) with variable ini-tial porosity and constant compaction factor(c = 4⋅10−4); b) with variable compaction factors (c)and constant initial porosity (φ 0=0.5). c) Subsidenceinduced by compaction of a ‘plastic’ layer, for differ-ent initial thicknesses and material properties, as afunction of the overburden.

a b

c

1588


was characterised by several episodes of fastgrowing isolated carbonate buildups. The di-verse generations of platforms generally differin thickness, amount of aggradation and progra-dation and their relative rates. An extensive lit-erature exists on the topic (see, for example,Bosellini et al., 1996 and references therein).

In order to test the numerical model, amongthe several generations of platforms that can berecognised in the Triassic succession of theDolomites, we selected the so-called pre-vol-canic Late Anisian to Early Ladinian platformswhich are 400-800 m thick and show highaggradation rates and relatively simple pre-growth stratigraphic conditions.

The relatively short time interval (i.e. LateAnisian-Early Ladinian) of platform growth,lasting probably less than 2 Myr (Gianolla et al.,1998), assures that the assumption of a geologi-cally restricted temporal time window is satis-fied and therefore the viscous contribution to de-formation can be neglected (e.g. Walcott, 1970;Beaumont, 1978).

Due to the strong influence of the equivalentelastic thickness in the numerical modelling,we paid special attention to the selection of thisparameter. As previously discussed, the elasticthickness of the continental lithosphere de-pends on numerous factors. Burov and Diament(1995) showed that the thermal state of the lith-osphere, the crust-mantle interface, the thick-ness and proportions of the mechanically com-petent crust and mantle and the local curvatureof the plate, which is directly related to thebending stress, are all crucial properties indefining the equivalent elastic thickness of a re-gion. Nevertheless, following Burov and Dia-ment (1995), an estimate of the equivalent elas-tic thickness of the Dolomite region duringMiddle Triassic can be attempted. Based on theavailable data for the Hercynian orogeny thataffected the area during the Devonian-Car-boniferous period, the thermal age of the litho-sphere at the time of platform growth was about100 Myr (Del Moro et al., 1980; Zanferrari andPoli, 1992). Based on this age and if we assumea realistic value for the Triassic crustal thick-ness of about 20-25 km as an average, follow-ing Burov and Diament (1995) it is possible toestimate a Te between 20 and 22 km. As a mean

value, we selected 21 km for our calculations,bearing in mind a ± 5% of variability inducedby this parameter.

The next step was quantifying the load pro-duced by each carbonate platform and its area ofinfluence. It is therefore necessary to identifyand locate the several buildups belonging to thisMiddle Triassic generation of platforms and todefine the dimensions of each carbonate body.Based on literature data (Gianolla et al., 1998,and references therein), published geologicalmaps (Leonardi et al., 1968; Caputo et al., 1999)and available unpublished data (M. Stefani andP. Gianolla, pers. comm.), we inferred the possi-ble initial dimensions of the different platforms,the amount of aggradation and that of prograda-tion which characterised the growth of the sever-al Late Anisian-Early Ladinian carbonate bodies.Due to the locally deep erosion and the tectonicdeformation that occurred in the Dolomites, thereconstruction of the geometric parameters(thickness and lateral extension) has sometimesbeen inferred. However, due to the relatively

Fig. 4. The present-day distribution of the LateAnisian-Early Ladinian carbonate platforms consid-ered for modelling in the present note. The filled cir-cles tentatively represent the size of the initial nuclei,while the external circles (radial pattern) are propor-tional to the final size (maximum progradation). Thegeneral geometry of the bodies is obviously highlysimplified. Platform labels indicated in figure corre-spond to table II. Major thrusts affecting the regionare also represented.

1589


strong influence of these parameters on the cal-culated subsidence, we attempted a conservativeinterpretation keeping the inferred dimensions tothe minimum values. Accordingly, in fig. 4, thenuclei of the considered platforms are highlysimplified and represented as filled circles whosedimensions are roughly indicative of the possibleinitial size.

We emphasise that in some cases the consid-ered platforms, whose corresponding values arereported in table II, merged during their growthdue to convergent progradation thus forming aunique larger carbonate body. This is certainlythe case, for example, of the Civetta-San Lucano-San Martino platforms (P14-P15-P16 in fig. 5;Bosellini et al., 1996) and the Gran Vernel-Mar-molada platforms (P8-P9 in fig. 5; Stefani andCaputo, 1998). However, in terms of producedload and thus of induced subsidence, our separateand simplified carbonate bodies are roughlyequivalent to more realistic platform shapes.

With respect to the plastic contribution to de-formation, an important aspect of numericalmodelling is the selection of the parameterscharacterising the underlying sedimentary unitsthat are supposed to have undergone compaction

Table II. List of parameters used for modelling the 16 carbonate platforms. Label: platform label used in figs.6 and 7; a – initial platform radius (m); b – final platform radius (m); c – final platform thickness (m); d – pres-ent-day thickness of the Bellerophon Formation underlying the platform (m); e – present-day thickness of theWerfen Formation underlying the platform (m); f – present-day thickness of the post-Werfen to pre-Late Anisiansuccession; g – estimated maximum overburden on top of the Werfen Formation.

Label Name a b c d e f g

P1 Croda dei Baranci 1800 3250 400 260 420 400 4220P2 Tre Scarperi 1500 3100 400 310 420 400 4310P3 Cadini di Misurina 1250 2500 400 310 410 400 4240P4 Putia-Sass Rigai 2600 4000 650 140 200 150 2300P5 Sassolungo 1200 1950 450 215 140 100 1750P6 Sella 900 1600 450 220 80 100 1780P7 Sciliar-Catinaccio 3000 5200 500 150 290 130 1780P8 Gran Vernel 1250 2500 750 230 170 150 1700P9 Marmolada 1300 2150 750 235 160 150 1880P10 Ombretta 3000 4800 800 220 200 150 1750P11 Latemar 2400 3000 750 180 320 150 1650P12 Viezzena 2000 2950 750 210 310 150 1650P13 Agnello 2100 3400 750 145 340 150 1600P14 Civetta 3200 4500 500 205 220 500 2840P15 San Lucano 2900 4300 500 190 250 500 2540P16 San Martino 3200 5200 600 160 300 450 2330

Fig. 5. Tentative palinspastic reconstruction of theDolomites and assumed location and distribution ofthe carbonate platforms during Late Anisian-EarlyLadinian. Platform labels correspond to table II. Thedotted pattern shows the outer limit of the coalescedplatforms.

1590


during the growth of the carbonate buildups.First of all, we have to define which formationscould have undergone some compaction. In-deed, the pre-Late Anisian stratigraphy of theDolomites shows that on top of the Palaeozoicmetamorphic basement the sedimentary units ofsome interest for our model due to their thick-ness, lithology and area distribution are the ValGardena Sandstone (Middle Permian), theBellerophon Formation (Late Permian) and theWerfen Formation (Schythian). Other sedimen-tary units also exist but they mainly consist ofcarbonate layers, like for example the ContrinFormation, or they are coarse-grained like theSesto and Ponte Gardena Conglomerates. In theformer case, diagenesis usually occurs soon af-ter deposition thus impeding any important com-paction during burial; while in the latter case, thecompaction factor is generally extremely lowand so not influential in our calculations. More-over, as concerns the Val Gardena Sandstone, thetime elapsed between sedimentation and thegrowth of the platforms investigated in this pa-per was probably sufficiently long to allow an al-most complete diagenesis. Therefore their possi-ble contribution to subsidence as induced bycompaction is likely to be negligible.

According to geological and stratigraphicdata (e.g., Bosellini, 1968), the present-daythickness of the Bellerophon and Werfen for-mations have been inferred in correspondencewith the several carbonate platforms (table II).

However, in order to estimate the amount ofcompaction suffered by these two sedimentaryformations during the growth of the Late Anisian-Early Ladinian platform generation and the con-sequent amount of induced subsidence, it is alsonecessary to know their pre-platform burial histo-ry and particularly the thickness of the strati-graphic succession already overlying the Bel-lerophon and Werfen units just before the LateAnisian. Therefore, the amount of the post-Scythian to pre-Late Anisian overburden existingin correspondence with each considered platformhas been also inferred (Assereto et al., 1977; Ma-setti and Trombetta, 1998; Gianolla et al., 1998)and used during numerical modelling (table II).

There is no way to directly measure the ini-tial porosity (φ0) and the compaction factor (c) ofthe two above-mentioned sedimentary units.

However, by considering comparable present-day sediments (e.g., Campbell et al., 1974; Scla-ter and Christie, 1980) these parameters havebeen tentatively assumed for the Bellerophonand Werfen units. The selected values of initialporosity are 0.6 and 0.5, respectively, while thoseof the compaction factor are 5⋅10−4 and 2.7⋅10−4,respectively (table I).

Furthermore, in order to correctly calculatethe amount of subsidence induced by com-paction, the total burial overlying the Permo-Scythian formations just before the occurrenceof the Oligocene-Quaternary widespread ero-sion must be estimated (table II). Accordingly,following the procedure proposed by Sclaterand Christie (1980), we calculated the thicknessof the two modelled sedimentary units just be-fore the growth of the Late Anisian-Early La-dinian carbonate platforms. These values havebeen used as reference values for the calculationof the induced compaction (viz. subsidence).

5. Subsidence map

It is clear that the present-day distribution ofthe investigated platforms does not correspond tothe original pattern. It is well known that mainlyduring Cainozoic times the Dolomites sufferedconsiderable deformation (e.g., Doglio- ni andBosellini, 1987). However, in order to accountfor the large-scale effects of the elastic deforma-tion induced by each single carbonate body, wehad to perform a palinspastic restoration to ob-tain the original distribution. We are well awareof the complex tectonic evolution that affectedthe region both in terms of intensity, distributionand kinematics of the different deformationalevents recognised in the area (e.g., Caputo,1996). Nevertheless, as a first approximation wecan assume that the bulk of shortening which oc-curred in the Dolomites has a mean N-S direc-tion (Doglioni, 1987; Doglioni and Bosellini,1987; Castellarin et al., 1992; Caputo, 1997; Ca-puto et al., 1999). Accordingly, we firstly identi-fied the major thrusts affecting the region and theamount of shortening they caused and then wereconstructed a simplified palaeogeographicmap of Middle Triassic times (fig. 5) slightlymodifying that proposed by Bosellini (1996).

1591


Based on this palinspastic restoration andthe consequent re-location of the carbonateplatforms we thus performed the calculations ofthe induced elastic deformation for the broaderDolomite region.

As a first step, we calculated the subsidenceinduced by a single platform at different stagesof the whole process. Figure 6a-d shows an ex-ample of subsidence evolution in four timesteps, relative to the Sciliar-Catinaccio platform(P7). Both local and regional contributions areevident, while the gradient in between the twopatterns is an effect of the prograding slope.

The same calculations have been performedfor all investigated platforms and all contribu-tions have been summed up to generate the totalsubsidence map represented in fig. 7. This maprepresents the regional distribution of theamount of subsidence induced by the growth ofthe Late Anisian to Early Ladinian carbonatebuildups. Also in this case, both local and re-gional patterns are evident.

Fig. 6a-d. Four time-steps map (a to d) represent-ing the progressive subsidence induced by thegrowth of a single pre-volcanic platform: the exam-ple corresponds to the Sciliar-Catinaccio (P7 in figs.4 and 5 and in table II). The contour lines of the in-duced subsidence are in meters (see grey scale).

Fig. 7. Map of the calculated subsidence induced bythe growth of all Late Anisian-Early Ladinian carbon-ate platforms. Both elastic (regional) and plastic (lo-cal) contributions are evident and can be easily quan-tified. The round geometry of the local contributionsis due to the simplified shape of the carbonate bodiesassumed in the model. The contour lines of inducedsubsidence are in meters (see grey scale). The maxi-mum regionally induced subsidence is concentrated inthe Central Dolomites and amounts to about 220 m,while local contributions range between 40 and 90 m.

6. Concluding remarks

Due to the uncertainties in the selection ofsome of the parameters used in the numericalmodel and according to their more or less im-portant influence in the calculations as dis-cussed in a former section of the paper, it isclear that our final results should be consideredas a first order approximation of the investigat-ed natural phenomenon. Nevertheless, these re-sults clearly show that the subsidence inducedby the growth of the Late Anisian-Early Ladin-ian carbonate platforms in the Dolomites gavean important contribution for generating the ac-commodation space required for these thicksuccessions of shallow-water sediments.

a b

c d

1592


Based on the distribution and the diversethickness values of the platforms (table II), wecan tentatively reconstruct the map of the totalsubsidence occurred in the Dolomites during theMiddle Triassic that caused the necessary ac-commodation space (fig. 8a). According to our

calculations, the subsidence induced by thegrowth of carbonate platforms gives an impor-tant contribution as high as 20% to 40% of thetotal amount of subsidence. Consequently, thiscontribution should not be disregarded especial-ly when dealing with stratigraphic, palaeogeo-

Fig. 8a-c. a) The estimated pattern of the total sub-sidence in the Dolomites during Middle Triassic asinferred from the thickness of the carbonate bodies(table II). b) Map of the subsidence induced by thegrowth of the platforms (simplified from fig. 7). c)Pattern of the tectonic subsidence obtained from theformer two maps and likely induced by a far fieldextensional stress field. The map emphasises the ex-istence of a NE-SW oriented graben affecting theCentral Dolomites. Dashed lines represent the twomajor border faults (or fault systems). Contourslines are in metres in all three maps (see correspon-ding grey scales).

a b

c

Fig. 9a,b. Regional (a) and local (b) subsidence ofa carbonate platform (Bosellini, 1984). In the formercase, the water depth of the basin roughly corre-sponds to the thickness of the carbonate platform. Inthe latter case, with a similar amount of aggradation,the adjacent basin is much shallower. 1: deep-waterdeposits; 2: platform slope deposits; 3: internal plat-form deposits.

1593


400-800 m thick overlying platforms has a mag-nitude of some tens of meters, while its gradientis distributed within the progradation zone, thatis between the nucleus of the platform, wherecompaction is maximum, and the toe of theslope, where the compaction is nil. It is thusclear that the Middle Triassic subsidence in-duced by compaction generated a ‘very local’effect. Therefore, the latter hypothesis for the

Fig. 10. Map of the calculated subsidence occurredduring Middle Triassic and reduced to its present-daydistribution after considering the Tertiary contrac-tion.

graphic and geodynamic reconstructions of theDolomites. Moreover, this local overload by car-bonate platforms induces a positive feedback be-cause any increased subsidence induces furthergrowth and consequently a larger load onto thelithosphere. The quantification for the first timeof this contribution is a major result of this paper.

Obviously a remote stress field, whose ori-gin is probably associated with a process oflithospheric stretching, induced the remaining60-80% of subsidence. By combining the mapof the total subsidence (fig. 8a) with that of theinduced subsidence (fig. 8b) it is possible toobtain a map of the tectonic subsidence (fig.8c). According to fig. 8c, the Central Dolo-mites are characterised by a 20 km wide NE-SW trending graben where the two borderfaults (or fault systems) show a vertical dis-placement of about 200 m. It is well knownthat immediately after the growth of the LateAnisian-Middle Ladinian carbonate platforms,the Central Dolomites were affected by a shortbut intense volcanic event. It is noteworthy thatthe two major magmatic bodies (Predazzo andMonzoni) documented in the area, partially in-truding the platforms and associated with vol-canic edifices, are located exactly along thesouthern border of the graben. It is likely thatthe rising of the magma and the emplacementof the plutons were facilitated by a crustal scaleextensional structure.

Further geologic and palaeogeographic im-plications of this study also regard the depth ofthe surrounding and/or adjacent basins. In fact, ifa platform subsides solidly with the adjacentbasin, the water-depth of this basin grossly corre-sponds to the thickness difference between plat-form carbonates and basin sediments. In contrast,if a platform subsides independently the formerequation is not valid (fig. 9a,b). The results ob-tained from the application of the proposed nu-merical model to the Middle Triassic evolution ofthe Dolomites certainly gave a contribution tounravel this problem and to choose between thetwo hypotheses. Indeed, the approach followedallows the differential subsidence between the ar-eas with a direct overload induced by the plat-form and the adjacent basin to be quantified.

According to the results presented in this pa-per, the differential subsidence induced by the

a

b

1594


occurrence of an ‘independent’ subsidence (fig.9b) is supported by our numerical modelling.However, because the differential subsidencewe calculated is distributed along a distance ofseveral hundreds of metres to some kilometres,corresponding to the dimension of a progradingslope, its influence in terms of thickness varia-tions and original geometry of the sedimentarybodies is probably not detectable with geologi-cal means.

These results can be easily extended to othergenerations of carbonate platforms of the Dolo-mites as well as elsewhere in the stratigraphicrecord.

A second major outcome of this research isthe reconstruction of the present-day distributionof the subsidence induced by the local overload,after re-considering the Tertiary shortening thataffected the region (fig. 10). With the exceptionof the local ‘troughs’ directly caused by, and un-derlying the platforms, lateral variations in theamount of induced subsidence is generally lim-ited, being about 50 m across the whole Dolo-mite region. Consequently, also in this case,even specific and dedicated geological investiga-tions will probably not disclose any significantinformation in the stratigraphic record docu-menting these small variations relative to thedepth of the basin surrounding the platforms orconcerning the thickness of the basinal deposits.

Acknowledgements

We thank to M.R. Gallipoli, M. Mucciarel-li, D. Albarello for discussions on the numericalmodelling and to M. Stefani and P. Gianolla forsupporting unpublished data about the Triassicplatforms of the Dolomites.

REFERENCES

ASSERETO, R., C. BRUSCA, M. GAETANI and F. JADOUL

(1977): The Pb-Zn mineralization in the Triassic of theDolomites. Geological history and genetic interpreta-tions, L’Industria Mineraria, XXVIII, 1-15.

ATHY, L.F. (1930): Density, porosity and compaction of sed-imentary rocks, Bull. Am. Ass. Petrol. Geol., 14, 1-24.

BALDWIN, B. and C.O. BUTLER (1985): Compaction curves,Bull. Am. Ass. Petrol. Geol., 69, 622-626.

BEAUMONT, C. (1978): The evolution of sedimentary basins

on a viscoelastic lithosphere: theory and examples,Geophys. J. R. Astron. Soc., 55, 471-497.

BOSELLINI, A. (1968): Paleogeologia pre-anisica delleDolomiti Centro-Settentrionali, Atti Acc. Naz. Lincei,CCCLXV, 1-33.

BOSELLINI, A. (1984): Progradation geometries of carbonateplatforms: example from the Triassic of the Dolomites,Northern Italy, Sedimentology, 31, 1-24.

BOSELLINI, A. (1996): Geologia delle Dolomiti (Ed. Athe-sia, Bolzano), pp. 192.

BOSELLINI, A., C. NERI and M. STEFANI (1996): Geologiadelle Dolomiti. Introduzione geologica. Guida all’es-cursione generale, in 78a Riunione Estiva Soc. Geol.Italiana, 16-18 Settembre, San Cassiano, pp. 120.

BUROV, B. and M. DIAMENT (1995): The effective elasticthickness (Te) of the continental lithosphere: what doesit really mean?, J. Geophys. Res., 100, 3,905-3,927.

CAMPBELL, R., F. CORTS et al. (1974): Well evaluation Con-ference – North Sea, Schlumberger, France, pp. 171.

CAPUTO, R. (1996): The polyphase tectonics of EasternDolomites, Italy, Mem. Sci. Geol., 48, 93-106.

CAPUTO, R. (1997): The puzzling regmatic system of East-ern Dolomites, Mem. Sci. Geol., 49, 1-10.

CAPUTO, R., M. STEFANI and G. DAL PIAZ (1999): Contrac-tional and transcurrent tectonics in the MarmoladaGroup (Dolomites, Italy), Mem. Sci. Geol., 51 (1), 63-77.

CASTELLARIN, A., L. CANTELLI, A.M. FESCE, J.L. MERCIER,V. PICOTTI, G.A. PINI, G. PROSSER and L. SELLI (1992):Alpine compressional tectonics in the Southern Alps.Relationships with the N-Apennines, Ann. Tectonicae,6 (1), 62-94.

COUDERT, L., M. FRAPPA, C. VIGUIER and R. ARIAS (1994):Tectonic subsidence and crustal flexure in the NeogeneChaco basin of Bolivia, Tectonophysics, 243, 277-292.

DEL MORO, A., F.P. SASSI and G. ZIRPOLI (1980): Prelimi-nary results on the radiometric age of the Hercynianmetamorphism in the South-Alpine basement of theEastern Alps, N. Jb. Geol. Paläont. Mh., 707-718.

DOGLIONI, C. (1987): Tectonics of the Dolomites (SouthernAlps-Northern Italy). J. Struct. Geol., 9 (2) 181-193.

DOGLIONI, C. and A. BOSELLINI (1987): Eoalpine andmesoalpine tectonics in the Southern Alps, Geol. Rund.,76, 735-754.

GIANOLLA, P., V. DE ZANCHE and P. MIETTO (1998): Triassicsequence stratigraphy in the Southern Alps (NorthernItaly): definition of sequences and basin evolution, in-Mesozoic and Cenozoic Sequence Stratigraphy of Eu-ropean Basins, SEPM Spec. Publ. 60, 719-747.

HAQ, B.U., J. HARDENPOL, P.R. VAIL, R.C. WRIGHT, L.E.STOVER, G. BAUM, T. LOUTIT, A. GOMBOS, T. DAVIES,C. PFLUM, K. ROMINE, H. POSAMENTIER and R. GIAN

DU CHENER (1987): Mesozoic-Cenozoic Cycle Chart,Version 3.1A.

JAEGER, J.C. and N.G.W. COOK (1979): Fundamentals ofRock Mechanics (Chapman and Hall, London), 3rd edi-tion, pp. 593.

KARNER, G.D. and A.B. WATTS (1983): Gravity anomalyand flexure of the lithosphere at mountain ranges, J.Geophys. Res., 88, 10,449-10,477.

KRISHNA, M.R., S. CHAND and C. SUBRAHMANYAM (1999):Gravity anomalies, sediment loading and lithosphericflexure associated with the Krishna-Godavarin Basin,eastern continental margin of India, Earth Planet. Sci.

1595


Lett., 175, 223-232.LEONARDI, P. (1968): Le Dolomiti. Geologia dei Monti tra

Isarco e Piave (Ed. Manfrini, Rovereto), 2 vols.LOWRIE, W. (1997): Introductory geophysics (Cambridge

University Press, London), pp. 354.MASETTI, D. and G.L. TROMBETTA (1998): L’eredità anisica

nella nascita ed evoluzione delle piattaforme medio-tri-assiche delle Dolomiti occidentali, Mem. Sci. Geol.,50, 213-237.

MCKENZIE, D.P. (1978): Some remarks on the developmentof sedimentary basins, Earth Planet. Sci. Lett., 40, 25-32.

MCNUTT, M.K. (1988): Variations of elastic Plate Thicknessat Continental Thrust Belts, J. Geophys. Res., 93,8,825-8,838.

PRYOR, W.A. (1973): Permeability-porosity patterns andvariations in some Holocene sand bodies, Am. Ass.Petrol. Geol. Bull., 57, 162-189.

RANALLI, G. (1987): Rheology of the Earth. Deformationand Flow Processes in Geophysics and Geodynamics(Allen & Unwin), pp. 366.

RIEKE, H.R. and G.V. CHILINGARIAN (Editors) (1974): Com-paction of Argillaceous Sediments, Dev. Sedimentol.,16, pp. 424.

RUBY, W.W. and M.K. HUBERT (1960): Role of fluid pressurein mechanics overthrust faulting, II. Overthrust belt ingeosynclinal area of Western Wyoming in light of fluid-pressure hypothesis, Bull. Geol. Soc. Am., 70, 167-206.

SCLATER, J.G. and P.A.F. CHRISTIE (1980): Continentalstretching: an explanation of post mid-Cretaceous sub-

sidence of the central North Sea Basin, J. Geophys.Res., 85, 3711-3739.

SCLATER, J.C., R.N. ANDERSON and M.L. BELL (1971): Ele-vation of ridges and evolution of the Central EasternPacific, J. Geophy. Res., 76, 7,888-7,915.

STEFANI, M. and R. CAPUTO (1998): Stratigrafia triassica etettonica Alpina nel Gruppo Marmolada-Costabella(Dolomiti Centrali), Mem. Soc. Geol. Italiana, 53,263-293.

TIMOSHENKO, S. and J.N. GOODIER (1951): Theory of Elas-ticity (McGraw-Hill, New York), pp. 506.

TURCOTTE, D.L. and G. SCHUBERT (1982): GeodynamicsApplications of Continuum Physics to GeologicalProblems (J. Wiley and Sons, New York), pp. 450.

WALCOTT, R.I. (1970): Flexural rigidity, thickness and vis-cosity of the lithosphere, J. Geophys. Res., 75, 3,941-3,954.

WANG, W.H. (2001): Lithospheric flexure under a criticallytapered mountain belt: a new technique to study theevolution of the Tertiary Taiwan orogeny, Earth Plan-et. Sci. Lett., 192, 571-581.

ZANFERRARI, A. and M.E. POLI (1992): Il basamento su-dalpino orientale: stratigrafia, tettonica varisica ealpina, rapporti copertura-basamento, Studi GeologiciCamerti, 1992/2 (spec. vol.), 299-302.

(received October 6, 2003;accepted March 4, 2004)

growth and subsidence of carbonate platforms: numerical ... · 1582 claudia marella, riccardo...

Documents