Hydration diapirism: a climate-related initiation of evaporite mounds in two continental Neogene baslns of central Spain

MANUEL HOYOSI, MIGUEL DOBLAS l , SERGIO SÁNCHEZ-MORALl, JUAN CARLOS CAÑAVERAS 1, SALVADOR ORDOÑEZ'-, CARMEN SESÉI, ENRlQUE SANZl &

VICE"[';TE MAHECHAI. IMuseo Nacional de Ciencias Naturales, Consejo Superior de Investigaciones Científicas

(CS.l.c.;, Josi Gutiare:Abas("(d 2, 2f1006 Madrid. Spain !Departamento de Ciencias Ambientales y Recursos Naturales, Dívision de Geología.

Universidad de Alicante, 03690 Alicante, Spain

Abstrlld, Wllile rrany halokinetic mode", conslder rhat a thlCk overburdea is necessary bcfore Jiapni:'i\l develops, other ,lUdie"how ttnu :.all migllt hegin ti' llltWe soca ~flef <.lepo~it¡(ln in T~spon.,c lo diffcrelll fac\CIrs .e.g. regional extension). We describe IWO continental Neogene ba-ms incemrat Spcin. cuaracterized by thtec Mioccne sedimenrarv unit-. whcre unuwally early illitlation of evaporite mounds or 'incipjcnt pillow>' .x:curs. with no (lverburden on ¡OP of lhem. The ;¡r¡tum..rus suggcsung thc eaely hulging of lhe evaporitcs "f the l.ow..r Unil. forming 3

p"t"currlief oerorc and during the deposirion of thc lntermediate Unit, rely on certam cuaructer­isl\CS ollhe palaeokarcts developcd on the roo! uf thc wwer Unir. as weHas in the nanre, dispo­,ilion, thiekne,s, and ,yn,edll1elJl;¡ry prccesses of Ihe Intermediale 1Jnil. :','Id" f~Il11" tell,ion gashcs. and collapse structures develop withln lhe ev~r"riTc mound,. and these dcformatiol) teanuv mighl be related 1" lhélT upliIt histor;. ThI: agent of iniriarton suggested in this papa for the evapmite mounds 'hydraticn diapirism': or a volume increase related to the hydrauon of anhy­Mile to gYP,ullI ha, not been l'reviou~ly recognize<J in lhe literature. A series 01 sedimentologi­cal and palaeontclogical argumcnts flll1h .., ~Il¡;g ..st !hat thi" ..hemi..:a! rc"CÜ"" was tr¡¡;~clell by a chrnatic changc at the boundary betwcen the Luwer and the Inrc-rnediate Units.

Tirrung and agents 0\ initiauon 01 üiapirism are two loading. large-scale thcrmal convecrion of salr, of the most inreresring topics in salt tectonics. The local hear sources. graviry gliding and spreading halokmetic rnoce: of Trusheim í 1960\ ror graviry­ (Jackscn & Talbot 1986). sub-sal: deformauon. and driven salt tectonics considered rhat a thick ovcr­ thin-skinned extensión and contracuon (woodbury burilen was e~sen¡iaJ ter t~ gencration of pillowv. el <JI. 1980: Jackson & Talbot 1994). and their subsequenr evolution into diupirs. Even if In tlus paper, we study twu continental Neogcne ¡his eoly notiün nas been maintained by sorne basins (Madrid and Cnlatayud}, characrerized by authcrs (Jenyon 1(86), a growing body uf cvidence three sedimemary units: a 'Lower Unn", compnsing Ircm seismic stucies (Hughes 1968) and quantita­ up to 500 m of Agenian and Aragonian scdimcnts tive rcccnstrucuons oí cross-sections (Wu et al. with detrital deposns in the borúers. pcscing 10 1990) teoos ro indicare that salr might begin tu evaporitcs in the centre: and up to ISOm of detriral move soon after dcposition, cven below thin over­ and carbonare sediments. consututing the "ínter­hurdens O;¡cksof\ & Talbot 1<)')4: Jacksoll & medrare and Upper unírs' of the Mioccne in central Vendeville ]994). According lo lackson& Vende­ Spain (Hoyos & López-MarlineJ, \985; Calvo er al. ville (1994). 'early' initiation 'refers lo the time 19QQ). soon after salt deposition when overburden was Th..,t" evaporite moullds ~how ~miquc feMures in thinner than 1 km'. In Ihi~ sen se. lhe con,,·ept is their evolutiotl in relation to the :iming and agents Iinked to the existence Df al leasl ~I)me uycrburden. of initiation 01' diapirisffi. In lhe first place, tltese normally between 350 and 1000 m (1ackson & mounds began to mo\e unusually early in their Ven<ie-viJ[" 199·1 ¡. hJSlory. I,C. when no overburden whalsoever existed

Tite possible agenl of initiation of salt pillows is on top of them. Several authon have previol1sly lhe other debalable subJecl. and il is OD<: of lne le:Jsl recogni,_cd the exi¡.;tence of palaeokarsts. upwelling understood a~pects of salt tectonics (Jackson & phe1.omena. ¡md the occurrenec of internal highs in Vendcvllle 1994). Several proce'<sn ha\'e Ix:en the Madrid Basin (Alberdi el al, 1983; Martín suggested to initiate salt movemenl: erosional Escorza 1983: Hoyos el ul. 1985; Calvo el al. 1984; unloading (Evans 1993). sedimentar:< differential Ordoñez & Glrcía Jel Cura 19921.

Fmm AL>!)? G. 1..BU'DhU. D, J. & DA\1'[)N.I, ied,). 1996. Sdr T~{"//J/\I(S.

GeologicalSOClCly Spedal Pub!icatjr¡n No, lOO. pp. 49---Ó3. 49

50 M. HOYOS ET AL.

ID'<>_nWiC.

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----,...\~'f.---,.'

• o 3D'm

iI30"e'~ DOuoternary, O Pliocene; ~Mlocene uppEr unlt . ~Miocen"

intermedio'" uni!; SMiocene lower unil; OLawe, lo uppee Mlocene

clastics; IlIIIIIIlPaloeogene; m Mesozoic (carbonates), BPclQeozoic

(plutonios ond metomorphicsl; ~ Thrusl tccns

Fig. l. Schematic geologica! map of the Madrid Basin showing major stratigrapbic unns. l-F is lhe hne of the eross­secnon in Fig. 8.

It is suggested that the agent of initiation of the evaporite mounds is related lO an increase in volume associated with the chemical reaction anhy­drite + water -> gypsum. This chemically induced disturbance i5 tenned 'hydranon diapirisrn", and, to OUT knowledge, it has not been identified previously as a trigger in the initiation of diapirism. We further suggest that hydration diapirism is the result of a climanc change in the study area.

Tsui & Cruden (1984) described the effect of hydration of anhydrite to gypsum in the génesis of anticlinal structures. One mechanism suggested for the formation of the intestíne-like enterolithic folds is related to volume changes. sucn as the hydration of anhydrite to gypsum (Jackson & Talbot 1994). Oedometer tests carried out by Nuesch et al. (1984) have shown that the hydration of samples with shale and anhydríte might yield dramatic volume íncreases of up to 120%. Geological hazard assess­ment in the northeastem pan of Spain (Ebro Basin),

suggested that the hydration of anhydrite to gypsum was responsible for retes up to I cm a-' of ground elevation change (Ordoñez et al. 1990), Cümete­related salt tectonics has been describen in 1ranian salt glaciers, which have been shown ro ñow after rainfall at rates much raster Ihan predicted by experiments (Talbot & Jarvis 1984).

Tbe Madrid Basin The Neogene Madrid Basin occupies more than 10 000 km2 in the centre oí the Iberian Peninsula (Fig. 1). It is bounded by ditferent geological units: the Iberian Variscan Massif to the north, west, and south, with the NE-SW orientated Spanish Central Range and the E-W trending Toledo Mountains (Palaeozoic metamorphics and granites). To the east, the Madrid Bas¡n is bounded by the NW~SE

orientated Ibenan Range, and the N-S trending

51 J-IYDRATlON DJAPIRI5M

ThíckneseMADRID B A SI N® w E m

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. A A A A ' ••" g .• ,~ A A A A t, A -:. •

Q Evaporites G Detritol evoportres gCorbonales

r::.-::.-lMu<.lstones 1-;-~-;-IGro~els ond slInds O cccrse clcatíc so Karstic deposits

Fig.2. Simplified Iahostratigrapbic sketchevdepícting the sedimentary series of the Madrid and Catauvud Ba,in~.

'Duce majar units are recognized during tbe Miocene. separated by discontinuities: Lower Uní!. Intermediare Unir, and Upper Unit.

Altornira Range, both characrerized by Mesozoíc clasucs and evaporites. The Palaeogene ccntinenral carbonates and detrital sediments (Martín Escorza basin occupied a much wider area in central Spain 1983: Calvo el aJo 1990). as compared with the present sbape of the Madrid

The Madrid Bas¡n is ñlled with a sequence of Bas¡n. The sedimentar)' infilling 01" the Madrid Tertiary sediments, showing a marked thickness Basin. as a true and separate tectonic rrough, is asvmmetry: 3000 m in the west and 1500 ro in ihe Miocene in age, and it was related to the Alpine east. The basal part to the west and in the centre of uplift of its surrounding ranges. The Miocene seor­the basin displays Cretaceous carbonates, while in ments occupy most oí the areal exrem of the Madrid the east mese are in places underlain by Triassic Bas¡n. The thickness oí the complete Neogene evaporites (Qúerol 1989). The marine Mesozoic series ranges between 800 and 1200 m. basin occupied the eastern half of the Iberian Penin­ Three main Miocene sedimenrary units have sula, aad the Madrid urea corresponded to the west­ been distinguished in che Madrid Basin (Fig. 2A; ern margin of this basin. Mesozoic deposits are Alberdí el al. 1983; Cañaveras 1994): Lower Unir overlaín by a thick succescion of Palaeogene Intennediate Unit. and Uppcr Unir. These units are

52 ~ HOYOS sr AL.

D

HYDR,A.TlON D1APIRISM

separated by [WO sedimentary dísconünumes The Lower Unit (600-1000 m of Agenian tu the Middle Aragonian deposits) is characterized by the follow­ing etemems trom tne border s te lhe centre (Ordoñez el al. 1991; Ordoñez & Garcfa del Cura 1992; Sanchez-Moral ('I al. 19\).1): alluvial fans with a máximum width of 50 km in the northwestem bordcr (these fan~ are only 3--4 kíu wide in rhe southcrn and eastern bordersj: satine rnudflats and playa lakes with transiuonal facies; central evapor­itic facies. depositad in saline lakes with shallow warers. The evanoríuc .tornain has an appro)l;imale thickncss of 500 m and, from the base to the 10p, consists of the following sequence (García del Cura 1979; Ordcñez esal. t\j9I)~ (11 a lowermost secrion with altemating anhydrite, halne, gjauberire. poly­halite and magnesitc, enriched towards the borders of Ihe basin wilh e~dy diagenetic glauherite. massíve thenardite, and occasional reddish muds with halite andlor glaubenre (Ordoñez & Garcra del Cura 1994); anll (2) un upper pan where anhydrite and gYPSUr:l are thc most abundant phascs. with cccasional halite. Anhydme and halite rarely outcrcp. but thcy have been obserced in oorenoles. Anhydrite presente nodular and laminar Iabcics, with rnany vanettes of habit (Ordoúez el al. 1991). Secondary gypsurn derived Ircm the hydrauon of anhydrite i~ the dominant evaporite and is the on!y type e:<posed at th.. surfacl". w¡th a thickness of up to ]20 m (primary gypsum is nt'"gligible): lhis secondary gypsum (ommon!y displays p!>eudo­morphi( fabrks and relicts of anhydrite.

The lntertnediate L'nit spans the middJellate Aragonian to the early Vallcsian, with thickne~ses

between 30 aod 200 m. AlIuvial fans outcrop along the borders. whiJe c1a,tics and lacmtrine and palus­trine carbonates occur in the centre. Finally, the Uppel' Uníl ¡lale Vallesian to the L\te Turolian) displays fluvial and ,hallow-water carbonates. with a ma~imum lhicknes~ C'f 50 m. The Miocene sedi­mcnts are lClcally overl~in by P\iocene c1a"lics and -carbonates, ;lnd alluviaJ aud tluyil,J QUUltrnary sediments «().-30 m). A series of palaeokarsts and collap,e 'dolincs' jor slnkholcs) afe found in Ihi1>; basin on top of the Lower Unít. as welJ as higher in the series (Figs ::lA .loo :lC: Hoyos el ul. 1985;

HO:io~ & Lópcl-Mal1ínCL \985: López-Marunez <'1 al. 1987J.

The triangular shape oi the Madrid Rasin ¡, ccndmoned by a series oí boonding ',hruse rauks (NE-SW, Spanish Central Range: E-W, Toledo Mountams: and N-S. Altomira Range) of late variscan age (Doblas ¡1)91), whicu were reactrvated dunng the Alpme formation of the Madrid Basin (Fig. l: Martín Escorza 1983). Tbe lateral facies vanauons of the Miocene sedirnerus, anrí rhe present orientation of the fluvial ncrwork were c'ontrolled by a series uf NE-SW and E-W smkíng faults (Alía 1960; Martín Escorza 1976, 1983). In conuust to the bounding faults, the enes fcund within rhe Madrid Basín are mostly normal. Block ulting pbenomena and lcrge-scale genlle Iolds are typically associared with the NE-SW fault trends within the basin

folds, faults, tensión gashes. tiJted blocks and collapsc srructures are found within the eVllporiles 01' the Madrid Basin. and -hrec rnajor hyporheses have been suggested te expla¡n these deformation features (Martín Escorza 1983): (1) halokinesis acting UPC)I) Ihe gypsum layers (Alía 19601: (2) deformation events retatec to external tectonics (Capote & Peméndez Casals 1978); (3) a mixed scheme involvmg ba ...ernem faull'" octing. upon weaker gypsum layer~ (Martin Escor.za 1983 j. In this papcr, we prupuse lhal the defonnation can be explained by \he up\ift oí lb;: evapurite muund.

The possible Alpine tectonic phases which gave rise to this Neogene trough afe poorIy constr;:uned (Martín Escorza 1983) as: (1) lhe prc·!\1¡ocenc sed;­ffients are reslricted lo the borders. and they ~how

strong facies variatíons; (2) mos( of the cominental sediments display a poor faunal conlent; and (3) mos! of the discordances are local in charader, !lnd only tVio major sedimentar) discontinuities bound­ing the Miocetle unils are of regional extenl (Antunes el al, 1987). In Ihis senSe, recent attempts to unravel the tectonic history of \he Madrid Basin using fault population analysrs (Vicrnte el af. 1990, in press; Calvo el al. 1991) are difficult to accepl. Moreover, \he data used by these authors are critically unconstraint'"d in several ways: (1) mosl of the faults they meas",c are fUVlld in the sUITounding

40

Fig. 3. Sorne~xample, 0:- palaólkarst~ unJ eollapse doline, in ¡h~ lwo spanish continental b~sins. (A) Palaeokarst wi¡h an exolu,tic depre~s,on IíJI~d willt in "¡tu .~dimellt". dcvelop<:d <>n Ihe H",f uf lhe gyp~um uepo'li.' of lhe Lower Uni! (~adrid Ba..in: Arganda; Long: 3"26'W: La!: 4O"¡S'N). The wiJth of th~ scclion is 13 m. (B) Collapse lloline affecting¡he gypsull1 of tite Lowcr Unil (húri7,ontai ,edimcnl~ to the kft). filled with fine/y banded carlxmates of lhe lntennedíate Uníl (foldeJ sellime~ts lOlhe rigt.t), in lhe Cahlayud Basin (MUllcbrega. Lúllg~ 1"4 I 'W; Lal' 41'IS'N). Note the synsedimenlUTy ehatacter of ¡hese late! deposil:;. with respecl lo the cClllapse event (lhe thielness of the balld~<l <'arbonot".' inerea<~' toward, lhe nghn, tlle width of the scchon is 5 m. (e) Collap~e doline affectillg a series of tUllIr, ~nd palumine ~arbonales of Ihe lnternediale Unil in (fleM;¡dri¿Ba~ln (Va\le<:as; Long: )<33'W; Lat·

02J'N). (D) Large-,~ale collapse lloline wlthin lite Calalayud Bósin (Mullcbrega; Long: 1"41'W; La! 41"15'NJ. TIte subltúrimntalligltl-grey sediments surn,unding lhe doline are gyp~um llepo~i¡s of the Lower Uni\. The doline i, lilled Wilh dClrilal and ¡;arbOn~le sediment~ 01 Ihe lipfl':-l' Ul\il

M. HOYOS ET AL.

\ o

\

\ /

/' NQrmQI fQulls

Fíg. 4. Schernatíc geological map of theCalawyud Ba!;in showing majO{ stratigraphic units. 1-1' is the line of the cross-secüon shcwn in Fig. 9.

ranges (and not within the uasln). and they are often oí late vanscan age, with "sunposedly" Alpine reacuvanons (only relative ages are available): (2) the number and senses of movement since the late varíscan are unknown. as no detailed kinematic analyses oí slickenside planes have been under­takcn; (3) these aumors use stress inversión rnethods which have been found to be based on 'mvalid assurnptions' (Pollard t'f al L993~ Cashman & Ellis 1994); and (4) these fault analyses only account for the crassíc horizontal srresses, ignoring the relevance of vertical stresses associated with halokinesis and the upwelling of the salt mound (particularly in the case of me faults found withm che basin). Using these unconstrained data, Vicente el al. (1990, in press) and Calvo er al. (1991) propose three deformarion episcdes for me Madrid Basin. including a final extensional phase following the deposition of the lntermediate Unir.

However, severa! arguments suggest that compressional tectonícs acted during the whole Miocene: (1) me bounding reverso faults of the Spanish Central Range and the Toledo mountains were active until al least the upper MíocenesPliocene (Martín Escorza 1990); (2) the Iberian Range lo the easr displays a series of

Miocene to Pliocene folds. overríding the Tertiary sediments of the Madrid Basin; (3) a series of compressional srructures are found in the roof of the Miocene Upper Unit, Le. contraetional karstic tubes, small-scale folds. etc.; (4) the normal faults, which are restricted to me evaporites of the Madrid 8asin and do not affect the substratum. only reñecc localized extensíonat stresses related to the uplift of the evaporite moond; (5) the presenr-day geotec­roníc setting of central Spain is compressional-, the Madrid 8a,ín constirutes an imraplate trough subject te the eonvergence of África and Burasta during the Míocene, with NW directed indentation in the southeastern comer of the Iberian Península (Doblas et al. 1991). Compression was transmítted towards central Spain thrcugb thin-skinned thrust­ing, involving a fundamental detachmenr at ITÚd­crustal level, with the Spanish Central Range and the Toledo Mountains acnng as 'pop-ups' (Banks & Warburton 1991; Doblas etal. 1991).

The Calatayud Basin The Calatayud 8asin consurutes an interrnontane basin elongated parallel te the NW-SE onenrared Iberian Range (Fig. 4). The Iberian Range is an

55 HYDRATlON DIAPIRISM

CARBON ATES (V PPER U N Ir)......_...... ""'7

NW S E/'

Cl ASTI CS \ (l N YE RM EO IAT E:\

..~...Y. ~_~.~.~ ~ .~~~ ITS l \

F íg. S. Complex collapse/chaotic structure in the Calatayud Basin (Maluenda; Long: 1°37' W; Lat: 41°17'N ). The detrital and carbonate dcposits of the Intermediare and Upper Units collapsed on rop 0 1' thc gypsum 01'the Lower Unit.

intraplate fold -and -thrust bclt composed of Palaeo­zoic and Mesozoic rocks, whic h underwent a three­stag c evolution: (1 ) rifting and sedimentation during the Mesozoi c: (2) crustal thickening and th in-skinned thru sting duríng the Palaeogene co rnpress ion; and (3) pos t-orogenic tran stension and crus tal thinning, from the Neo gene to the prcsent (Capote 1983; Sim ón 1990) .

The depth of the basin cxceeds 1500 m and the lower part is unknown, except for thc Pala eogene scdimcnts. The Palaeogene on ly ou tcrops in the northwest of the basin, and consis ts of alluvial fan deposits with conglomerates, lutit es and marls. As in thc Madrid 8asin, thc Miocenc serie s consists of three units (Fig . 2B). The Lower Unit is co rnposed of more than 500 m of Agcnían age sedíments with clastics in the borders, progressívcly passin g lo evaporites in the ce ntre with palaeokarst s in their

roof (Hoyos & López Martínez 1985). The evapor­itic facies consi st es sentially of gypsu rn and associated soluble saline phases (an hydrite and ha litc), with occasional intercalations of lutit es and do lomitic marl s whieh become more abundant towards the margins of the basin. Secondary gyp sum is thc dominant exposed evaporite, with a thickness of IIp to 200 m. Secondary gypsum is dcrived from the hydration of anhydrite, as revealed by petrography (Ort í el al . 1994). Bo reholes in thé central part of the basin reveal thick deposi ts of anhy drit e (more than 400 m) and sorne halite , undcrneath the secondary gypsum. Bo th the Inter­mediare Un it (Agcnian to Turolian agc, with maxi­mum thickness around 160 m) , and the Upper Unit (35 -75 m of Turolian lo Ruscinian age deposits ), consist of clastic sediments and lacu strine and palustrine carbonate rocks. The Mioccne is covered

56 M. HOYOS ET AL.

~AIIUVial taos; Ofluvial and distal alluvial detrit¡cs; o Lake mm1ln

wllh trons;~iQnal tccíes , ~lake oreo wilh evcporttes , ~loke meo wil\l

carbonates; Qmergent ikor~tified rene wilh usscciüted lalero', cfustlcs

___ Thrust foull'

o 60,,,,

Fig.6. The Madrid Basin during tne lower and middle Miocene. lA) Palaeogeographic map, saowing lile facies distnbution al the end (Ir die Lower Unn. ¡81 Palaeogeograpbic map. showing lhe facies distribution during the middlc part "r the Intemlcdime Vnit; an uplifted evaponte maulld is shown in the ct'nlra/ sector of the basm. (e) Isopach map of the lruermedíare Uní! (modified from Vicente et al. in press), highligbting the position (lf {he ~vap"rite mound

by Pho/Quaternary clasücs and carbonates. As in the Madrid Basin. dofines.. palaeokarsts and ccllapse strucurres are found during the whole Mioceno evolution of Calatayu.; (Figs 3D. 3D, and 5).

During the Miocene, the Ealatayud Besin under­went extensional rcctonics, together with a dextral stnke-slip componenr along NW-SE bounding faults. The geotectonic seníng of the Catarayud Basin is rarber complex, being the result of mrec factors: (1) soumward-directed cumpression from the Pyrenees (Simón 1990): (2) north- lOnorthwest­directed indentation from the southeastem comer of the Betic cordilleras (Doblas el al. 1991: López­Ruiz et al. 19IB ) ; and (3) generalized low-angle extensión disrupting (he western Mediterranean (Valencia and Alborán basins: (Doblas & Oyarzun, 1989a. b, 19(0)

Gravirational folds and normal faults are cornmonly found in the central evaporitic secrcrs of thc basin, associated with the uptift of the rnound. Abundant sets of fractures and rension gashes filled with ñbrous gypsum only exíst in the dry mudflat arcas surruunding the evaporire mound.

Early initiation 01 evaporite mounds in central Spain in the Madrid and Calatayud Basins, internal highs devejoped at the sne of the prevíous evaporitic depocenues oí the Lower Unit, cocvally with the initial pulses uf the Intermediare Unir (Figs 6 and 7). Pour dífferem arguments clearly indicare rhe early initiation of these evaporite mounds when no overburden covered the deposits.

HYORATION OJAPIR1SM

AlluVlcl fans

® Fluviol ond d'.. ter oPuviol closti~s

o t.oke mon;llr' wnn I.onsilionol facies

Q Loke oreo wilh evopnrltes

§ Locuslrine corno­notes

o Pctusf rine cor uo , notes

Emergen!! ~ofslifi€d ~OM with osso­c.cteo 10l'"al closlics

® / Nem-e! fuults

Fig.7. The C~la¡ayud Ih"in during ¡he h:>wcr and mjddl~ Mioccne. PaJacog~ographie rnaps ,howing Ihe fades distribunon "1 the end of the Lcwer Umt (A) and the beginrnng ofthc Intermediare Unit (R) (8) shows an uphned eV~[lOrile mound in lil~ central PW1l'f the basin.

Firvtly, a series of palaeokersts with exokarstic depressions is developed on top of (he Lower Urar gypsum (Figs 3A and 8), These dolines are filled wuh autcchthoncus karsuc deposits (clay, marl. dclomite. and secondary gypsum). corning from the aheration uf gypsum.

Secondly, a series of detrital gypsum sedirnents, which cortstitute the first elements of the Iraer­mediare Unir, were deposued away trcm the central evaporitic palaeoreliefs. in the surrounding troughs (Fig. ti). This dirección of sedimentation is oppcsed

to the ene which prevails in the borcers of thc basin i.e. direcred towards the centre of thc basín. These 1WO argurnents clearly indicatc ernergence and weathering of tite gypsum, accnmpanied by a reía­tive drop or the phrenric lcvel. thus implying the presence of a palaeorelief in 1he centre of rhe basin.

Thirdly, synsedimentary structuees are obscrved

(slumping srructures. localized mercases in uuck­ness. and carbonatic .nnaclasts). affecting the basal carbonates of the Intermediare Unir. which accorn­modere themselvcs within the karstic gypsum dcpressions (Fig. 3BJ This clearly rcveals thar subsidence and collapse. associatcd with the karsrification of lhe gypsum, ccntinucd withm the dolines. until the complete fossllizuuon of the evap­orite upwelungs.

The final argumcnt is related ro the variation in thidJ1CSS of the Irnermediate Unit (Figs 6C, 9 aud 10), which is a mínimum en top of rhe evaporitic mounds (0-15 rm and mcreases away from them (60--1 SO m in its decpest parts).

The aboye cbservatrons reveal that the evaporite masses in central Spain hed becn uplifted very early in their histcry. nnd it is argued that mese upwellings or mounds might constitute 'mcipient

- -

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58 M HOYOS ETAL

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WG'Mites, ~M.sczoic corb010t •• , 5'3JPaloeo~e.'e clo.I,cs and e"oporite.; WAlluvial loo c~n~lom~rotes,

OFlo",O! oo~ ~1.Tol nl'""o> ""<tic., 1"::~ :"\C.h~ •• rn \lolo","i~ <>,,~ ",U~Of¡"C ma<., 8\0..1,11<11 ~v~o~,IIU,

E:;:::']E,OPOfjTH¡ ~Korstic Mposito, =C> Upllft of the ,¡IIQ\'I5

Fig.8. Schemauc cross-secnons showing me evolurion of th.. Marlr.d Basln fmm lhe cnd of the Lowel Unít ¡A) lO lile ¡ir,1 stages ot the lntermediate Unit (8).

pillows', with máximum diamerers of 30 and 10 km. and mínimum amplitudes of 1JI) and óO m for Madrid and Calatayud. respectively.

Hydration diapirism as the egent of Initiation of the evaporite mounds We suggest that 'Hydration Diapirism", or the tran­sitian of anhydrite te gypsum hy the chemical reac­tion anhydrite + water -> gypsum. may be the agenr uf iniriat.on of the evaporire mounds in central Spain.

We further propose rhat the unusual inñux of ....úter responvible for íhe 'hycmtion". wnich occurred in borh Spanish basins during the rransi­tinn frMI the Lcwer Unit to thc Intermediare Uuir

)0).. /';L', '1> , ,

'. ~" ". " o , , , "" ~-

c:.5J~o.I""O''''''' "~'omo'o~io., O Tmo .. hoo<l fooi";

~ Ca'boo"'", @ Lo~., UM, ® In'.,,,,.,,O!. Unif,

(Ramblian/Aragunian), may be related lo a climanc ¡;hange fmm arid ¡;ondi.tilJn;;, towards more hurmd­ity. Three indcpendent sedimentolcgical and palae­onrclogical arguments support this interprctation. (1) There are changes in the lacustrine sedimen­tacion. with evapnnres of rhe Lower Unit replaced by more extensive carbonate deposits of the ínter­mediare Unit (borh units are separated by a sedi­mentary discuntinuity). This can only be explained by an incrcascd influx of mereortc waters. (2) Changes in the fossil record are present, wrth 'mofe humid' associarions, including a series of Upper Ramblian smaú mammAls in the MN3 zone of Calatayud (mainly Gliridae and Eomyidae; {De Bruijn 1967j); aad, the fir'<.l arrival of Iarge mammals (Mustodontidae, Proboscidea (De Bruijn 1967~ Bcné ~I al 14i10; Junco ti al, L985). together

CJ C'Q'hc,; tEl:''''~o'it.' "ith ~olo.o~o"ts;

@upper Un'!

Flg. \l. Schernatic transverse cross-secnon of the Calatayud Basin. showing the preseru arrangemem of the Moceoe d.-.posí¡ional systems. and highhghting (he varianon in rhickness of che Inrermed.atc Unít. i.e. a mininmm on tcp <Jf the evaporne upwelljng, and .ncreasing away from it.

I

59 Ii YDR ArI ON DlA PIRIS;-,l

CA" eOU~ [$

IUPPER UN'"

, ~lo>$ U "

~,!"_E_" _~ .~ '.~ I

t'l¡:. Jll. Palacorclie f dc"Cloped onthc roo f or lile gY P~ lIm of thc Lo.....cr Unit in thc Calalayud Basin (Torre , ; Long : I035'W: Lal: 4 1"2ü 'N). 1ñe pholograph shows thc vananon in Ihi<:kncloS of rhc lntennediatc Unlt. which ís a minimum abo..... lhe gYP' 1I1\l uplifl (lo lile right) " nd inCkascs ¡'W;lYfj-nm it \lo lllt:: leh ).

wit h the abundaucc in this lasl zo ne of thc srnall nmmrual Laxol,.,iI (a ripanun Ochotonidae (Albcrdi e t <1/. 19X5). in the Rarnblian/Aragonian tra nsition bctwcen thc MN3 and M N4 zon es . (3) Van dcr Meulen & Daam s ( 1992) havo shown that the rod ent ussc m blagc s nf the Calata yu d Bas¡n defi ne a m áximum in the hu rnidity dur ing the Rarnblian/ Aragonian time jopan.

We will now describe lhe thrce-sta ge evolu non modcl involv cd in thc ' Hydrarion Dia pir ism ' hypothcsis.

T he first stage corresponde re the 'Dcposition Stage (Fig. 11M. involvlng thc dcposiuon uf the evaporitcs within the satine lakes in the centre of thc husin s. cha rac tcr izcd by un in ternnl drain age and dry condiuons (Figs 6A and 7A).

T he be ginning o f IhE: scco nd stagc . or ' Up wc\ l ing Stagc' (Fig. 1 IR ). is markcd by a major change in rhe sedimc ntaucn. wit h an augmentcd inñux o f meteonc waters . during the first pulse, u f dcposi­tion uf the lntcrmedia tc Unir. This chang e prod uced

a dro p in thc salinity, which would llave trig gercd an uugmcnted water actrvity in thc lake, thus favourin g the stability of thc hydrated sa line phascs ovcr the anhyd rous one s. According to Plumme r et al. (1988) and Sanchez- Moral (19(4 ). a water activity uf 0.77 7 marks the boundary betwccn the stabililY uf anhydrire tbc jow it) an d gy psum (above il). In this scns e. the trcsh warers. sub satura rcd in satr. that reachcd rhe íacust nec area induc ed a rehydnuion o f the surlidal anhydrite depos hs, which would be tm ns for rncd 10 gy psum ( gY P ~>Ífi c ati(Jn). Thís proc ess usually induces a 6 1% volu me incrca sc (Sonncnfeld 1984) , Thc erystalfizatlo n pressurc triggercd by the h yd ration uf a nhydritc to gy psum depends on the sa rururion o f thc sys tem in gypsum (Win klc r 1975 ). and it mig ht rangc bctween 282 and 334 atrnoxphcres, within the expccrcd tempe ra­tures uf conrincnral sedimcntary envinmmems (O and 50°C ). T hc volumc íncreasc is furt her acce ler ­arcd by th e prcsence uf alkaline s ulphate s in drs so ­lution (Conlcy & Bundy. 1958), which exist in both

60 M. HOYOS ET AL

r=:2l 80s,o mor'.LO

~ (;'0'.1' ond '000<

f=-_~::1 MuOSNne,

O O U O

Aohyd,,'.

1'01". ~"d gl"ub";'.

Gyp,"",

","ritol .'o~~rl1es

~ 1.10'\' ano limeslon ••

-iE- Ao"yd,M-----p.Gypsum

.-/' Su,he',", "o'" __ Fo"'!.

/ E'~"o" 01 the e.Q~o"".s

~ Up"ellioq o' 'h~ e"opa"!e mound

~ -.otOt;c dep".its

Sub.'dooo.

fil:. 11. Highly scbemetic diagrams showiug the uiree-srage evolution of lhe Madrid an,j Calatayud Basins. ",¡lh their associated evaporite mounds. A: LO\!"H Unit. B: "early/middle" Intermediare Unlt. C: "late" Iraermediate Unit.

Spanish basins (Ordoñez el al, \991; Sénchez­Moral el al. 1993; Ordoñez & Oarcra del Cura 1994). II can be shown that the hydration or anhy­drite to gypsum occurred jusi after the deposition or the Lower Unit: Ihis ISrevealed by üetrital second­ary gypsum pebbles wi1h anhydritc nuclei. which are found in [he base of the Inrermediate Unít deposited in the troughs surroonding the evaporite upwellings, as well as by the climatic argumente which irnply an inflox uf mereoríc water. This increase in volume of the rnost sarñcial anhydrite layers changin¡;: lOsecortdary gypsum would trigger a first upwelling of the previous evaporitic depo­centre, as well as a depositional rrough surrounding it (Figs 68, 7B and 8). The emerged secondary gypsum would be weathered and the eroded sedi­ments would be washeé away from the central palaeorelief. wbile kar srificaticn, subsidence, couapse and disscluuon would have taken place. In this scnse. the chemicel transformation of anhydrite 10 gypsum would continué, reaching progressively deeper evaporite laycrs and bence increasing the upwelling. Two mejor arguments explain why the layers of anhydrite did not expand by the same amoun! in aH the basin i.e. why [he uplifl was greater in the middle part of [h!:evaporite basin (see also Fig. llB). (1) The anhydrile was not uniformly

disrribured areally in the salme lake. as abundan! clay intercalatíons (distal facies of alluvial fans) existed rowards the boréers of the satine Iake. In this sense, a úifferential volume increase was estab­lished between the central pan (higher) and the borders (lower] of the evaporire basin. (2) The uplift was enhanced in the centre 01' the basin by a series of everna taking place in the troughs surrounding Ihe evaporite mounds - the partial dissolution of the anhydrite, and the uepcsition of the initial sedi­mcnts of the Interrnedrate Unit - which exerted a differential loading in these ureas, and buried the external parts of the salíne Iake, mus increasing [he effect of the meteoric waters in the central and uncovered secticn of the evaporite dornain.

These uplifting processes would go on until the progressive fossilizauon of both the surrounding depositional troughs and the evaporrtic palaeorelief during thc test seage ('Burial Stage': Pig- IIC). This occurred in the opper Aragonian in Madrid and in rhe Turolian in Calarayud. with the final deposi­tional pulses of the Intermediate Unit.

Although mosr of the upwelling activity Wa.3 concentrated in botn basins during the lnrerrnediate Uni!, !he evaporitic mounds were pal1ially reacti­valed later (Hoyos el 11'- 1979; Martín Es<:orza 1983; Cañaveras el al. 1991), pal1icularly in Ihe

61

Madrid Bas¡n. where palaeokarsts developed on top of the Interrnediate Unit. and a generalized dome­shaped topography was established in the centre of the basin during the Pliorene/Quarernary periodo These reacnvatíons might be relared to differential erosión accompanying the fluvial downcuning whích occurred during the Pliocene/Quatemary period. and to the partial dissolution of the evapcr­ites in the bcrders of the mounds.

Sorne additional argurnents that further suppon the hydratíon cíiapinsm model are the fullowing. (1) 11Ie climatic chango responsible for the tnñux of fresh water in the sedimentary environment, and hence the hydration of anbydntc. ts the only ccmmon factor which exists between Calatayud and Madrid during the Mioeene. (2) According to Tsu¡ & Cruden (1984), water bodies such as lakes and streams near {he anhydritie strara are prerequisues for hydration to occur. and ttus is prccisely the snu­ation in the two Spanish basins.

We should atso meraron the importance of 'per descensum' mereoric waters in relation te the halite horizons, which are found in the lower parts of the evaporitic series at Calatayud and Madrid. ln mis <ense, it is well known thar the introducuon of brines may lead te an immediate and large acceler­ation of the creep rate of halire (Jackso» & Talbot 1994), henre facifitating the possible flow and upwclling of the ';'1.1t.

Condusions Early initiation of evaporjle mounds occurred in 11'10 continental basins in central Spain, immedi­atcly following Ihe deposition of the evaporitcs, alld when no overburden had been deposited on top of thero. The argumenls lhat allow us to identify lhi~

incipient diapinsm rely on the presencc of palaeokarsts drveloped in the TOof of Ihe Lowcr Unit, as well as in the synsedimemary strllctures, disposition, Ihickness, and scdimcntological char­acteristin 01 the Intennediale Unil. It is interesting 10 nole th,u Jecreasing amounts of nvcrburdcn have been advocated to initiate diapirism. While Tru,heim (1960) considered thal a thick overbunkn was nccessary, lachon & Vendeville (1994) recog­nize that salt might begin w well up with a thin overburden (350-1000 m). In this paper, we suggest an extreme situatiun, whcrc no overburden whatso­~ver is required for halokinesis lO begin.

To our knowledge, Ihe agcnl of initiation sllggested fur Ihe }.Iadrid and Calatayud Basitls has no! bren cited previously. Hydration has resulted from lhe cherni.:aJ transition of anhydrilC to gypsum. Se\'cral .~edimentological and palaeonto­logical argumems fllrther suggest that this chemical reaction was triggered by a climatic change al the

boundary between the Lower and rhe Intermediare Urms.

Thcse mounds or 'incipient pillows' developed within continental-derived evaporitic series (mostly anhydrite and gypsumr with no hydrocarbon poten­tial, in contrast to the typical thick, marine-denved series (mostly halite. with minar anhydrite and argillaceous and potash-rich beds) classically associated with diapirisrn. ln this sense. the Madrid and Calatayud Bnsins might provide unique ctues to the incipienc stages of diapirism, offering alterna­tíve mechentsms for these uplifts. As pointed out by Bishop (197R): "There is no rnetbod currently known 10describe the formation of incipienr diapirs because they are without houndary conditions".

The different deformation features found within the tWO Spanish basins (collcpse structures, faults and folds), which werc prevrously ascribed to a complcx variety or changing tectonic stresses (Calvo et al. 1991), can now be explained as related 10 a simple cause: rne upwelling of evaporice mounds. In this sense. this papel' highlighrs rhe importance 01' venical-related stresses in the evolu­tion of the two Spanish Neogene continental bastns. originally proposed for tue Madrid Basm by Alía (1960, 1978).

Financia! SlIpport was provided by thc D"c..·ción Genera! d~ Investigación Científica y Técnica through ProJecl PB­IN-(lf)~7 awarded lo M. Hoyes. W~ tbank 1. Daviscn. 1. AI~('Ip, O. Graversen, R. Evans. C. TalbOl and M. Jackson for meir rcv.cw, 01'an earlier versión of thís manuscrípl. Sre~;al rbanks are dm: lO J",é Arroyo for Ibe drafting \,r lhe figure>.

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