UNCONFORMITY SURFACES IN PELAGIC CARBONATE
ENVIRONMENTS: A CASE FROM THE MIDDLE BATHONIAN
OF THE BETIC CORDILLERA, SE SPAIN
Luis M. NIETO1, Francisco J. RODRÍGUEZ-TOVAR2, José M. MOLINA1, Matías REOLID1 & Pedro A. RUIZ-ORTIZ1
1
Departamento de Geología and CEACTierra, Universidad de Jaén, 23071 Jaén, Spain; e-mails: lmnieto@ujaen.es, jmmolina@ujaen.es, mreolid@ujaen.es, paruiz@ujaen.es
2
Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18002 Granada, Spain; e-mail: fjrtovar@ugr.es
Nieto, L.M., Rodríguez-Tovar, F.J., Molina, J.M., Reolid, M. & Ruiz-Ortiz, P.A., 2014. Unconformity surfaces in pelagic carbonate environments: A case from the Middle Bathonian of the Betic Cordillera, SE Spain. Annales Societatis Geologorum Poloniae, 84: 281–295.
Abstract: Integrative studies of sedimentary and palaeontological topics for unconformity surfaces are useful in basin analysis. A middle Bathonian unconformity surface in the Ammonitico Rosso facies cropping out in the La Mola Unit (Subbetic Domain, Betic External Zone) was studied by integrating ichnological, palaeotectonic, and sedimentological analyses to decipher sea-level, tectonic, and palaeogeographic conditions during its develop-ment. The trace-fossil assemblage mainly consists of the Glossifungites (Thalassinoides, Arenicolites, and Gastro- chaenolites) and Trypanites ichnofacies elements. Probable Ophiomorpha represents previous softground stages, Thalassinoides and Arenicolites were formed in firmground, Gastrochaenolites reflects an evolved firmground or early hardground, and Trypanites can be attributed to an incipient hardground. The degree of firmness, relative sea-level position, and continuity of deposition were related. The softground stage corresponds to a fall in relative sea level and continued deposition. The firmground (semi-consolidated substrate) probably reflects an extremely low sea level characterized by non-deposition, whereas the incipient hardground stage indicates an initial phase of relative sea-level rise, with an increase in marine current energy. The presence of two neptunian dyke systems reflects significant tectonic activity related to the transtensional deformation that affected the South Iberian Palaeomargin. Lateral variations in sedimentological and ichnological features recorded at similar discontinuity surfaces in nearby areas were considered and related to differences in bottom topography, with associated changes in sedimentation, and to the variable duration of the hiatus.
Key words: Unconformity, pelagic swell, softground, firmground, hardground, Middle–Late Jurassic, Betic External Zone.
Manuscript received 4 July 2014, accepted 5 November 2014
IN TRO DUC TION
A com par i son be tween Ju ras sic pe lagic swells, mainly char ac ter ized by the Ammonitico Rosso fa cies (red, nod u lar lime stones or nod u lar, marly lime stones, both with and with out ammonites), and cur rent ex am ples is com pli cated, mainly ow ing to chang ing palaeoceanographic con di tions be tween the Ju ras sic Tethys and Re cent ma rine pe lagic en -vi ron ments (e.g., Jenkyns, 1974; Mul lins et al., 1980; Bernoulli and Jenkyns, 2009; among oth ers). A re cent com par i son be tween the pe lagic swells in the Ex ter nal and In ter -nal Subbetic zones (Betic Cor dil lera, South east Spain) and the Han cock Seamount (Ha wai ian Ridge) and the Fie-berling Guyot (at 32° N, off the U.S. Pa cific coast) re veals that a del i cate bal ance be tween sed i ment de po si tion and win now ing de ter mined the sed i ment ac cu mu la tion rate in
these set tings (Coimbra et al., 2009). This agrees with the gen eral ge netic model pro posed for the Ammonitico Rosso fa cies (Jenkyns, 1974; Seyfried, 1978; Martire, 1992, 1996; Bertok and Martire, 2009, among oth ers), which con sid ers four main fac tors: (a) bot tom cur rents, (b) early ce men ta -tion, (c) bioturba-tion, and (d) grav i ta tional pro cesses.
Ammonitico Rosso fa cies are com mon in Med i ter ra nean Al pine chains and typ i fied the Mid dle and Up per Ju -ras sic of the Ex ter nal Subbetic (Seyfried, 1978; Molina, 1987; Rey, 1993; Nieto, 1997; Coimbra et al., 2009), as in other pe lagic Tethyan do mains (e.g., Elmi, 1981; Martire, 1992; Santantonio, 1993; Martire and Pavia, 2004; Basilone, 2009; Bertok and Martire, 2009). In the Betic Cor dil lera, these fa cies form the Up per Ammonitico Rosso For
-ma tion (Molina, 1987), dated as Bajocian–Lower Berria-sian (Vera, 2001). A typ i cal fea ture of these fa cies is their low sed i men ta tion rate (ref er ences above). In the Aus trian Alps, Jenkyns (1986) es ti mated sed i men ta tion rates be tween 0.5 and 1.5 mm/kyr and in the Apennines, sed i men ta tion rates be tween <1 and 6.5 mm/kyr. In the Subbetic Do -main, Vera (1989) cal cu lated sed i men ta tion rates be tween 1 and 5 mm/kyr, whereas Nieto (1997) de duced val ues rang -ing from 1 to 2 mm/kyr for the east ern Ex ter nal Subbetic Do main. In the case of the Oxfordian con densed de pos its in the neigh bour ing Prebetic Do main, Rodríguez-Tovar et al. (2010) in ter preted a sed i men ta tion rate of 1.5–1.6 mm/kyr. These low sed i men ta tion rates fa voured synsedimentary ce -men ta tion and the de vel op -ment of firmgrounds and hard-grounds (Tucker and Wright, 1990; Coimbra et al., 2009) as so ci ated with very long hi a tuses, in the or der of mil lions of years, but with out marked fa cies con trasts and/or an gu lar un con formi ties that might in di cate sig nif i cant tec toni cally driven en vi ron men tal changes. Fre quently, these hi a tuses are as so ci ated with a sin gle dis con ti nu ity sur face that splits lat er ally into sev eral mi nor dis con ti nu ities, sep a rated by con densed lev els.
Ammonitico Rosso fa cies are usu ally as so ci ated with a re cord of di verse omis sion sur faces (Nieto et al., 2012 and ref er ences herein). In the Ex ter nal Subbetic Do main, dif fer ent au thors have re ported un con formi ties in the Ju ras sic re -cord and in the Up per Ammonitico Rosso For ma tion (Mo-lina, 1987; García-Hernández et al., 1989; Ruiz-Ortiz et al., 1997; O’Dogherty et al., 2000). Re cently, Reolid et al. (2010) and Nieto et al. (2012) have char ac ter ized five uncon for mity sur faces within this for ma tion in sev eral tecto-nic units (the Quípar, Lúgar-Corque, Cantón, Crevillente, and Reclot units) lo cated in the Lower–Mid dle Bathonian (Hg1), the Mid dle–Up per Bathonian (Hg2), the Lower– Mid -dle Callovian (Hg3), the Mid -dle–Up per Callovian (Hg4), and the Callovian–Oxfordian (Hg5). In some out crops, these au thors de fine a strati graphic break as Mid dle Bathonian–Mid -dle Oxfordian, in ter preted as the su per po si tion of the Hg1 to Hg5 un con formi ties.
The aim of this pa per is to im prove the in ter pre ta tion of un con formity sur faces de vel oped on car bon ate pe lagic swells on the ba sis of the in te gra tion of ichnological, pala-eotectonic (nep tu nian dykes), and sedimentological (fa cies and microfacies) anal y sis. An un con formity sur face in La Mola Unit out crops (east ern Subbetic Do main) was stud ied for this pur pose. The best out crop of this un con formity sur -face is lo cated in the north west faulted limb of the anticline, made up of Ju ras sic rocks (Fig. 1). The char ac ter iza tion and evo lu tion of sub strate firm ness, from softground to firm-ground and then hardfirm-ground con di tions are of ma jor in ter est for the in ter pre ta tion of sed i men tary evo lu tion, on the ba sis of their re la tion ship with sea-level con di tions (Christ et al., 2012; Nieto et al., 2012). The con clu sions could be ap plied to other West ern Tethys do mains where the sed i men ta tion of Ammonitico Rosso fa cies was im por tant dur ing the Ju -ras sic and un con formity sur faces were abun dant (Martire, 1992, 1996; Bertok and Martire, 2009; Basilone, 2009; among oth ers).
SED I MEN TARY CON TEXT
La Mola Unit lies in the east ern most Subbetic Do main (Nieto, 1997) (Prov ince of Alicante, south ern Spain; Fig. 1A, B). From a tec tonic point of view, it can be de fined as a Ju ras sic tec tonic win dow sur rounded by Tri as sic rocks (Fig. 1B). The Ju ras sic suc ces sion (Fig. 1C) shows strati -graphic fea tures typ i cal of the Ex ter nal Subbetic and of the In ter me di ate Do main, which in di cates that this unit had a palaeogeographic lo ca tion in the tran si tion be tween the Ex -ter nal Subbetic Do main and the In -ter me di ate Do main (Fig. 2). The Ex ter nal Subbetic Do main has been de fined as a row of shal low, pe lagic swells char ac ter ized by Mid -dle–Up per Ju ras sic Ammonitico Rosso fa cies (e.g., Vera et
al., 1988; Molina and Ruiz-Ortiz, 1993; Vera, 2001). The
In ter me di ate Do main, north of the Ex ter nal Subbetic Do -main, was a pe lagic trough where cal car e ous turbidites were de pos ited dur ing the Up per Ju ras sic (RuizOrtiz, 1983). Ac -cord ing to the pro posed model for the South Ibe rian Palaeomargin (Vera, 2001), the tran si tion be tween the two do mains is made up of sev eral halfgraben struc tures ar -ranged in a se ries of steps. These struc tures were con trolled by listric faults. Lat eral fa cies changes be tween Ammonitico Rosso fa cies and cal car e ous turbidites are ob -served there.
In the La Mola Unit, the low er most Ju ras sic stratigra-phic for ma tion is the Gavilán (Fig. 1C), dated as Hettan-gian–Lower Pliensbachian. This for ma tion cor re sponds to shal low car bon ateplat form fa cies, which crop out in the In -ter me di ate and Ex -ter nal Subbetic Do mains (Molina, 1987; Nieto, 1997; Ruiz-Ortiz et al., 2004). The over ly ing BaÔos For ma tion (Mid dle Toarcian–Aalenian) char ac ter izes the In ter me di ate Do main (Fig. 1C) and en com passes a lower mem ber of marlstones and marly lime stones and an up per mem ber of thin-bed ded dolostones. These fa cies rep re sent hemipelagic sed i ments (Ruiz-Ortiz, 1980). The dolostones de vel oped through sec ond ary dolomitization pro cesses (Nieto, 1997).
The Mid dle–Up per Ju ras sic Up per Ammonitico Rosso For ma tion (UAR For ma tion) is the most typ i cal for ma tion of the Ex ter nal Subbetic Do main (Figs 1C, 3). This unit com prises two mem bers. The lower mem ber is 10 m thick and de void of ammonites and macrofossils (Fig. 3). It has been dated as Bajocian–Lower Bathonian on the ba sis of cor re la tions with the near est tec tonic units (Nieto, 1997). Ac cord ing to the ter mi nol ogy of Martire (1996), this mem -ber is made up of red, bioturbated, pseudonodular marly-lime stones with in ter ca lated nod u lar or pseudonodular lime stones with lo cal par al lel lam i na tion (Fig. 3). All these lithologies are wackestone with “fil a ments” (F1 fa cies in Ta ble 1). This mem ber ends with a 40-cm-thick lime stone bed with F2 fa cies (in Ta ble 1).
The up per mem ber is 5 m thick and from base to top con sists of thin-bed ded lime stones, marly lime stones, red nod u lar marly lime stones, a thick lime stone ho ri zon and pseudonodular lime stones. All these lithologies cor re spond to wackestone to packstone of bioclasts (F3 fa cies in Fig u -re 3 and Ta ble 1). This mem ber has been dated as Up per
Fig. 1. Geo graphic and geo logic fea tures of study area. A. Lo ca tion of La Mola Hill. B. Geo log i cal sketch map of La Mola Unit. C. Com pound strati graphic sec tion of Ju ras sic suc ces sion of La Mola Unit.
Oxfordian on the ba sis of ammonites (Nieto, 1997). In the lower and mid dle parts of the mem ber (Figs 1C, 3), the pres ence of Sowerbyceras tortisulcatum and Ochetoceras sp. al -lows as sig na tion to the Bifurcatus Zone (first biozone of the Up per Oxfordian, af ter Ogg, 2004, Fig. 4). In the up per part of this mem ber, Idoceras planula, Taramelliceras sp., and
Mesosimoceras sp. iden tify the Planula Zone, which is the
last biozone of the Up per Oxfordian biostratigraphy, af ter Ogg (2004) (Fig. 4). The bound ary be tween the lower and up per mem bers of the UAR For ma tion is a sur face with trace fos sils and nep tu nian dykes that rep re sents the Mid dle Bathonian–Mid dle Oxfordian un con formity.
The up per most Ju ras sic for ma tion in the La Mola Unit is the Toril For ma tion (Kimmeridgian–Lower Berriasian, Nieto, 1997), which is made up of cal car e ous turbidites and peb bly mudstones. This is the most rep re sen ta tive for ma tion of the In -ter me di ate Do main (Ruiz-Ortiz, 1980); it cor re sponds to an apron de pos ited on the slope of an Ex ter nal Subbetic swell (Nieto, 1997). The Villares For ma tion (Fig. 1C) is the first strati graphic unit of the Cre ta ceous, made up of al ter nat ing marls and marly lime stones with sev eral sand stone in ter ca la -tions, es pe cially at the bot tom of the for ma tion.
METH ODS
Dif fer ent meth ods were ap plied to ana lyse the fa cies, nep tu nian dykes, and trace fos sils in or der to char ac ter ize the Mid dle Bathonian–Mid dle Oxfordian un con formity in the La Mola Unit.
The geo log i cal map (scale 1:50,000) 871 Elda (IGME, 1978) was re viewed and mod i fied (Fig. 1B). Field work was car ried out to log two crit i cal sec tions of the tec tonic unit and 80 rock sam ples were col lected for thin-sec tion study to com plete field fa cies anal y sis of the Ju ras sic for ma tions. Twenty sam ples were taken at the un con formity sur face to ana lyse the (mi cro)fa cies and the tex ture of the ma te ri als form ing the nep tu nian dykes. Macrofauna (ammonites) are gen er ally scarce in all the Ju ras sic for ma tions of the La
Fig. 2. Palaeogeographic re con struc tion of the South Ibe rian Palaeomargin. A. Palaeogeographic map of the South Ibe rian Palaeomargin for the Late Kimmeridgian (sim pli fied from Vera, 2001). B. Palaeogeographic sec tion of the South Ibe rian Palaeomargin with lo ca tion of the La Mola Unit (slightly mod i fied from Vera, 2001).
Fig. 3. De tailed strati graphic sec tion of the Up per Ammonitico Rosso For ma tion (UAR Fm) in the La Mola Unit.
Mola Unit. How ever, around 12 spec i mens were col lected to date the up per mem ber of the UAR For ma tion. Other strati graphic units were dated by fa cies cor re la tion with the near est tec tonic unit (Reclot Unit). The biozonation used is that es tab lished by dif fer ent au thors for the Subbetic, cor re lated with that pro posed by Ogg (2004) for the SubMed i -ter ra nean Do main (Fig. 4).
Only nep tu nian dykes at right an gles to bed ding (and there fore orig i nally subvertical) were con sid ered. The orig i nal strike of the dykes was re corded at the sur face af ter re -stor ing the un con formity sur face to the hor i zon tal (i.e., af ter cor rect ing for tilt ing). The ro ta tion of the tec tonic unit dur ing the Al pine Orog eny was not con sid ered ow ing to the ab -sence of palaeomagnetic data from the Ex ter nal Subbetic units in this area.
Ichnological anal y sis of the un con formity sur face in -volved de tailed out crop ob ser va tions, in clud ing quan ti ta tive and qual i ta tive fea tures, such as ori en ta tion, shape, length, and di am e ter of in di vid ual bur rows, branch ing an gles, bur -row walls, as well as den sity, dis tri bu tion, and cross-cut ting re la tion ships. Ichnofossil sam ples were taken for petrogra-phic stud ies, es pe cially in thin sec tion to ana lyse bur row texture (mar gins and infill). More data about ichnological anal y sis were dis cussed by Rodríguez-Tovar and Nieto (2013).
SEDIMENTOLOGICAL
AND ICHNOLOGICAL FEA TURES
OF THE MID DLE BATHONIAN–MID DLE
OXFORDIAN UN CON FORMITY
SUR FACE
The un con formity sur face oc curs at the top of a 40-cm-thick highly lithified lime stone bed, com pris ing packstone with dis or ga nized “fil a ments” (Bositra buchi) (F2 fa cies, Ta ble 1; Fig. 5A), lo cated in the up per most part of the lower mem ber of the UAR For ma tion. This lime stone bed lo cally dis plays a pseudonodular tex ture ac cord ing to the ter mi nol -ogy of Martire (1996). The main fea ture of this tex ture is the tran si tional bound aries be tween the nod ules and the sur -round ing ma trix. The up per mem ber of the UAR For ma tion over ly ing the un con formity sur face (Figs 1C, 3) has abun -dant ammonites (Figs 3, 5B). The tex tures are wackestone to packstone with dif fer ent bioclasts and some
Globulige-rina tests (F3 fa cies, Ta ble 1).
The un con formity sur face has a flat mor phol ogy and does not show ev i dence of min er al iza tion (crusts or im preg -na tions). The sur face is char ac ter ized by the ab sence of macrofossils and by rel a tively abun dant trace fos sils. Ac cord ing to the geo chron ol ogy pro posed by Ogg (2004), the hi a -tus re lated to the strati graphic break had a du ra tion of 9.2 Ma (Fig. 6).
Nep tu nian dykes
Two types of nep tu nian dykes with straight, sharp rims were rec og nized. The first type in cludes thin dykes, with a mean dis tance of 5 mm be tween the mar gins and an infill of red micrite (mudstone). Some dykes have dog tooth white sparite that de vel oped on the mar gins and grew into the
Ta ble 1
Main fea tures of fa cies re corded in the lower and up per mem ber of the Up per Ammonitico Rosso For ma tion in the La Mola Unit
No Facies Allochems Sedimentary structures Member
F1 Wackestones of “filaments”
“Filaments” (Bositra buchi), peloids, spicules, foraminifera (Globochaete sp., Valvulina sp.,
Lenticulina sp.)
Burrows infilled by peloids, parallel
lamination Lower
F2 Packstones of “filaments”
“Filaments” (Bositra buchi), aptychi, sponge spicules, gastropods, ammonite embryos, radiolaria,
foraminifera (Dimorphina sp., Lenticulina sp.)
Disorganized “filaments” Lower (40 cm thick limestone bed) F3 Wackestones to packstones
of bioclast
Globuligerina sp., gastropods, ammonite embryos,
sponge spicules, aptychi Burrows infilled by peloids Upper
Fig. 4. Cor re la tion be tween the biozonation pro posed for the Ex ter nal Subbetic Zone by dif fer ent Span ish au thors and that pro posed by Ogg (2004) for the SubMed i ter ra nean do main. Geo -chron ol ogy ac cord ing to the same au thor. L – lower, M – mid dle, U – up per.
dyke cen tre; red micrite fills the rest of the dyke (Fig. 7A, B). The sec ond type of dyke is wider nep tu nian dykes, with around 10–40 mm be tween the mar gins. They are filled by a packstone com posed of “fil a ments” (Bositra buchi) with peloids, radiolaria, cri noids, foraminifera (Globuligerina), and other bioclasts (Fig. 7C, D), sim i lar to the F2 fa cies.
Af ter re stor ing the un con formity sur face to the hor i zon -tal, two main nep tu nian dyke strikes were re corded: N–S and N140–150 E (Fig. 7E). The lat ter cut the N–S dykes and some branched trace fos sils. The an gle be tween the two nep tu nian dyke sys tems is about 35°.
Trace-fos sil com po si tion
Ichnological anal y sis of the sur face by RodríguezTovar and Nieto (2013) was mainly based on the mor pho log i cal fea tures of the trace fos sils. De spite the poor pres er va -tion of trace fos sils on the sur face due to diagenetic ef fects (which makes ichnotaxonomic de ter mi na tion dif fi cult), sev -eral ichnogenera were iden ti fied (see Rodríguez-Tovar and Nieto, 2013, for a de tailed char ac ter iza tion of the ichno-taxa). These ichnogenera be long to two well-dif fer en ti ated
mor pho log i cal groups: branched and cir cu lar struc tures. Ta -ble 2 shows the size of each of these mor pho log i cal groups, ac cord ing to Rodríguez-Tovar and Nieto (2013).
The branched struc tures are char ac ter ized by poorly pre served T- and Y-shaped, tu bu lar hor i zon tal forms, with only oc ca sional net works. Iso lated, sim ple struc tures are prob a bly the seg ments of branched forms. Two ichnogenera have been dif fer en ti ated in this group (Rodríguez-Tovar and Nieto, 2013): (a) Ophiomorpha, with ir reg u lar mar gins, a ferruginous halo be tween the bur row and the sur round ing sed i ment (Fig. 8A, B), and a sed i ment infill of sim i lar com -po si tion to the host ma te rial (Fig. 8C, D); this ichnofossil has pel let-lined walls to re in force bur rows. (b)
Thalassinoides, with Y to Tshaped branched struc tures, sharp mar
-gins, and nei ther a ferruginous halo nor in fill ing.
The cir cu lar struc tures have been ob served as paired or as in di vid ual holes (Fig. 8E, F), and as signed to
Arenicoli-tes, GastrochaenoliArenicoli-tes, and Trypanites. Paired struc tures char ac ter ized by sharp, cir cu lar open ings and the ab sence of spreite be tween the pairs of holes are prob a bly Arenicolites (Fig. 8E, F). In in di vid ual holes, the ab sence of ver ti cal cross-sec tions pre cludes the ob ser va tion of clavate or tears droplike forms, but the lo cal pres ence of a cal car e ous lin ing of the holes, ar ranged as paired or sim ple ap er tures, al -ways with in di vid ual holes touch ing one an other, al lows their as sig na tion to Gastrochaenolites (Fig. 8A). Sim ple, nar row cir cu lar struc tures with sharp bor ing mar gins and no co her ent, cal car e ous lin ing were as signed to the bor ing
Try-panites (Fig. 8E).
Lo cally, Thalassinoides is cross-cut by both nep tu nian dykes and cir cu lar struc tures (Fig. 7A, B), prob a bly Gas tro-
chaenolites. Arenicolites is also cross-cut by nep tu nian
dykes.
IN TER PRE TA TION
Evo lu tion of the sub strate con sis tencyof the un con formity sur face
The in te gra tion of sedimentological and ichnological fea tures per mits the in ter pre ta tion of the sur face evo lu tion. One of the most im por tant con trol ling fac tors of the trace fos sils at this sur face is sub strate con sis tency, with clear dif -fer ences as so ci ated with each ichnotaxa.
Ophiomorpha is a prod uct of bur row ing in sands; the
pel let lin ing re in forces walls and aids in sta bi li za tion (Shinn, 1968; Frey et al., 1978). The ichnofossils Ophiomorpha
nodosa are re corded in softgrounds prior to the de vel op ment
of firmgrounds (e.g., MacEachern et al., 1992). Un like sand -stone en vi ron ments, in car bon ates the lithification pro cess is quick, so the soft-sub strate stage is usu ally brief and firm-ground con di tions are achieved rap idly. There fore,
Ophio-morpha prob a bly de vel oped mainly in the fi nal stages of
softground con di tions (Fig. 9A).
Thalassinoides is found in a wide va ri ety of sub strates
from soft to hardgrounds (Fig. 9A, B, C and D), with vari a tions in bur row ge om e try ac cord ing to firm ness, and is com mon dur ing the early and mid dle stages of hardground de -vel op ment (Myrow, 1995). Thalassinoides is one of the most com mon ichnotaxa in the Glossifungites ichnofacies,
Fig. 5. Some ex am ples of microfacies. A. Packstone of “fil a -ments” (F2 fa cies, Ta ble 1) of the lime stone level (up per most bed of the lower mem ber of the UAR For ma tion), at the top of which the strati graphic break stud ied is lo cated. B. Wackestone to packstone of bioclasts (F3 fa cies, Ta ble 1) in the lower part of the up per mem ber of the UAR For ma tion.
which de vel oped in sta ble, co he sive sub strates (firmgrounds), unlithified, es pe cially semicon sol i dated car bon -ate firmgrounds (Fig. 9B, C, D) or sta ble, co he sive, par tially dewatered muddy sub strates (Pem ber ton and Frey, 1985; Lewis and Ekdale, 1992; MacEachern et al., 1992, 2007; Pem ber ton et al., 1992, 2001, 2004; Pem ber ton and Mac Eachern, 2005; Rodríguez-Tovar et al., 2007). In the case stud ied, the time re la tions be tween Ophiomorpha and
Tha-lassinoides are un cer tain. How ever, these two ichnofossils
were prob a bly not con tem po rary be cause Ophiomorpha in -di cates unlithified sed i ment, whereas Thalassinoides nee-ded a sta ble, co he sive sub strate (semi-con sol i dated, Fig. 9), ac cord ing to the above au thors.
Arenicolites was re corded from softgrounds to firmgrounds (Fig. 9A, B), but not in hardfirmgrounds, be ing a char -ac ter is tic tr-ace of the Glossifungites ichnof-acies (Pem ber ton
et al., 2001). Con sid er ing that the sed i men ta tion rate is very
low in pe lagic car bon ate en vi ron ments, par tial lithification of the sub strate is quick. There fore, the softground stage is short, which makes it likely that Arenicolites were pro duced in firmgrounds (Fig. 9A, B).
Gastrochaenolites has usu ally been in ter preted as a re
-sult of bor ing in lithic sub strates (Kelly and Bromley, 1984), and is there fore re lated to rockgrounds and hardgrounds. How ever, this ichnogenus also has been rec og -nized in firm sub strates, in some cases to gether with typ i cal com po nents of the firmground as so ci a tions, such as
Thalassinoides isp. (Carmona et al., 2006, 2007). More
over, firmground Gastrochaenolites is con sid ered a com mon struc ture in the Glossifungites ichnofacies (e.g., Pem -ber ton et al., 2001). Ekdale and Bromley (2001) showed that the tracemaker of Gastrochaenolites could work in sub -strates with dif fer ent de grees of con sis tency. In the study case, the pres ence of car bon ate lin ing in some spec i mens could cor rob o rate par tial sub strate lithification. In ad di tion, this ichnofossil cuts Thalassinoides (Fig. 9A), prov ing that
Gastrochaenolites de vel oped later and prob a bly in an evolved firmground sub strate, nearly at hardground con di -tions (Fig. 9A, C).
The pres ence of Trypanites could ev i dence a sig nif i cant change in sub strate firm ness re lated to an in crease in sub -strate con sis tency (Fig. 9A, B, D). Trypanites is an ichno-taxon ex clu sive to lithified sub strates such as hardgrounds and rockgrounds, be ing the ep o nym trace fos sil of the
Try-panites ichnofacies (Seilacher, 1967) and the Entobia
ich-nofacies (ac cord ing to Bromley and Asgaard, 1993). The pres ence of Trypanites is as so ci ated with tracefos sil as sem -blages dom i nated by deep-tier bor ing (e.g., Lewis and Ekdale, 1992), re veal ing long-term bioerosion on a sur face ex posed for an ex tended pe riod (in this ex am ple over 9.2 Ma; Fig. 6). This is the max i mum pos si ble time span, but the sur face could have formed in a much shorter time af ter sev eral events not reg is tered in the rock re cord. Very small bor ings are ev i dence of shal low bioerosion, and phys i cal abra sion is dis counted, given the ab sence of ex am ples of deeper-tier ac tiv ity (Fig. 9A, B).
Fig. 6. Chronostratigraphic chart where the Mid dle–Up per Ju ras sic suc ces sion at the La Mola Unit is cor re lated with other strati graphic sec tions (see num bers) within dif fer ent tec tonic units in the Subbetic Do main (par tially mod i fied from Nieto et al., 2012). Geo -chron ol ogy af ter Ogg (2004). BEZ – Betic Ex ter nal Zones, L – lower, M – mid dle, U – up per.
Con se quently, the tracefos sil as sem blage re veals sub -strate evo lu tion at the top of a pe lagic swell (Fig. 9A):
Ophiomorpha char ac ter izes softgrounds; Arenicolites, Tha- lassinoides, and Gastrochaenolites are com mon in the Glo-ssifungites ichnofacies firmgrounds (Pem ber ton and MacEachern, 1995; Pem ber ton et al., 2004); and only
Trypanites in di cates in cip i ent hardground de vel op ment.
The ab sence of char ac ter is tic fea tures of ma rine hard-grounds, such as macroencrusters or min er al iza tion (Fürsich, 1979), prob a bly in di cates an ex tremely low sed i men ta -tion rate or even a mi nor ero sional phase (sug gested by the ab sence of ver ti cal shafts in Thalassinoides).
Glossifungites is the most com mon ichnofacies, used to
de limit semi-lithified sub strates that orig i nated ei ther by subae rial ex po sure or by burial and sub se quent ex hu ma tion. Col o ni za tion of the dis con ti nu ity sur face by the
Glossifun-gites suite oc curs un der ma rine con di tions dur ing a
deposi-tional hi a tus. This re la tion ship be tween the Glossifungites ichnofacies and strati graphic dis con ti nu ities has been ana -lysed in nu mer ous pa pers (e.g., MacEachern et al., 1992, 1999; Pem ber ton et al., 1992, 2001, 2004; Pem ber ton and MacEachern, 1995; MacEachern and Bur ton, 2000; Zonneveld et al., 2001; Gingras et al., 2002; Holz, 2003; Di
Celma and Cantalamessa, 2005; Gibert and Robles, 2005; Buatois and Encinas, 2006; Cantalamessa et al., 2006; Rodríguez-Tovar et al., 2007).
Eustatic con text
Pe lagic swells have been con sid ered sed i ment-starved en vi ron ments, with the Ammonitico Rosso re garded as the most char ac ter is tic fa cies (Bosellini, 1989; Martire, 1992). Along these lines, Martire (1992) pro posed a se quen tial strati graphic model to ex plain the gen e sis of this fa cies that can be ap plied to the in ter pre ta tion of the UAR For ma tion (Fig. 10). This au thor pro poses that Ammonitico Rosso se d -i men ta t-ion -in pe lag-ic swells oc curred -in transgress-ive and, es pe cially, highstand sea-level con di tions. In this con text, cur rent ac tiv ity was lower and the prod ucts of pe lagic sed i -men ta tion may be pre served. The lower mem ber of the UAR For ma tion, char ac ter ized by wackestone with “fil a -ments”, could be re lated to sim i lar sea-level con di tions.
The highly lithified bed at the top of the lower mem ber of the UAR For ma tion, with a pseudonodular tex ture, could have formed in shal low seas. In this en vi ron ment, bot tom wa ters were prob a bly well ox y gen ated, so infauna should be abun dant and bioturbation of the sed i ment sig nif i cant, as in di cated by the tex ture of the bed (“fil a ments” and peloids). The par tial lithification of the de pos ited sed i ment was the re sult of the low sed i men ta tion rate (0.20 mm/kyr for the lower mem ber of the UAR For ma tion; Nieto, 1997; Nieto et al., 2012); there fore, softground con di tions and the first stage of firmground con di tions de vel oped at the top of the highly lithified bed.
The softground con di tions could in di cate a rel a tively low sea level, with a slight in crease in ma rine cur rent en ergy prob a bly con trol ling win now ing of fine-grained sed i ment or con cen tra tion dur ing a low sed i men ta tion stage (Fig. 10). These con di tions fa voured the pres ence of Ophiomorpha, in which wall re in force ment aided in bur row sta bi li za tion (Fig. 10). The sed i ment sur face evolved into a semicon sol i
-Fig. 7. Ex am ples of trace fos sils cut by nep tu nian dykes (ND). A. Nep tu nian dyke infilled by white sparite. B. Nep tu nian dyke made up of red micrite. C, D. Packstone com posed of “fil a ments” (Bositra buchi) of nep tu nian dykes. E. Di a gram show ing the main strikes of nep tu nian dykes af ter re stor ing strat i fi ca tion to the hor i -zon tal (n – num ber of mea sure ments).
Ta ble 2
Sizes of dif fer ent ichnofossils re corded at the un con formity sur face stud ied (data from Rodríguez-Tovar and Nieto, 2013)
Diameter (mm) Length (mm) Width (mm) Thickness of the lining (mm)
Branched structures 25–15, usually 20 180–130, maximum 250 – –
Circular structures Paired holes 10–20 – 85–35 1–2
Fig. 8. Some ex am ples of ichnofossils in the stud ied un con formity sur face. A. Hor i zon tal trace fos sil show ing Y-shaped branches (Ophiomorpha) with a limonitic halo and bor ing with a cir cu lar sec tion (prob a bly Trypanites). B. T-shaped trace fos sil and limonitic halo. C. Microfacies (“fil a ment” wackestone) with bioturbation in the infills. D. Thinsec tion show ing the re la tion be tween the infill with bur -row and the hardground sur face. E. Poorly pre served branched struc ture and cir cu lar struc tures as signed to Gastrochaenolites. F. Cir cu lar sec tions grouped by pairs (Arenicolites) and Y-shaped trace fos sil (Thalassinoides).
dated firmground and early hardground, with
Thalassinoi-des, Arenicolites, and Gastrochaenolites ichnofossils
(Fig. 10). Cur rent en ergy prob a bly in creased, caus ing sed i -ment win now ing and lead ing to firmer sub strates, a halt to sed i men ta tion, and ero sion ex hum ing the con cealed firm-ground. These con di tions are typ i cal of very low sea lev els, but with out karstification struc tures. Fi nally, the de vel op ment of a hardground, with Trypanites, was likely as so ci ated with en hance ment of these prior con di tions: a nearab sence of sed i men ta tion and a com par a tive in crease in ero -sion of the seafloor, cut ting the ver ti cal gal ler ies of some ichnofossils and fa vour ing the ex hu ma tion of firmer sub strates. The lat ter phase re veals a sig nif i cant in crease in ma -rine cur rent en ergy, pos si bly as so ci ated with the ini tial stages of a rel a tive sea-level rise (Fig. 10).
The com plex un con formi ties re corded at the Mid dle– Up per Ju ras sic tran si tion of the Tethyan do mains were re -lated to dif fer ent transgressive-re gres sive cy cle bound aries. Hardenbol et al. (1998) rec og nized a ma jor se quence bound ary (Bat5), dated as Late Bathonian, that bounds two sec ond-or der se quences, cor re spond ing to Early–Late Aalenian to Mid dle–Late Bathonian and Mid dle–Late Bathonian to Mid dle Oxfordian. These au thors rec og nize a eustatic sea-level fall prior to the Bat5 se quence bound ary
and a sub se quent sealevel rise. Also in the Subbetic Do -main, Nieto et al. (2012) cor re late the Hg1 un con formity with a rel a tive sea-level fall, fol lowed by a sea-level rise. The sur face stud ied in the La Mola Unit could be in ter preted as the re sult of low sea-level con di tions to gether with the pro cesses re lated to the sub se quent sea-level rise (in creased en ergy, sed i ment bypass ing and po ten tial ero sion). Dif fer -ent au thors (Cecca et al., 2005; Ramajo and Aurell, 2008) have shown that a low sed i men ta tion rate is typ i cal of the Mid dle Ju ras sic pe riod in the Tethyan do mains, be cause there is a tran si tion from rift ing to postrift ing; as a con se quence, a global re duc tion in car bon ate pro duc tiv ity is de -tected dur ing the Late Bathonian–Early Oxfordian.
How ever, some lo cal con di tions re lated to tilted tec -tonic blocks can not be ex cluded. Ar eas where the con -densed lev els were pres ent can be in ter preted as be ing more pro tected from ma rine cur rents, sed i ment win now ing, and ero sion, thereby pre serv ing the scarce sed i ment. Where the con densed sec tions are ab sent, as in the La Mola Unit, the hi a tus as so ci ated with a strati graphic break was long. It could be ex plained as hav ing arisen in shal lower ar eas where the ac tiv ity of ma rine cur rents as well as low pro duc tion of the car bon ate fac tory did not fa vour sed i ment ac cu -mu la tion.
Fig. 9. Model for change of the un con formity sur face in the La Mola Unit. Com par i son with other mod els. A. Change model of the un -con formity sur face in the La Mola Unit mainly ac cord ing to dif fer ent de grees of firm ness. B. Change model of the un -con formity sur face in the Reclot Unit (East ern Ex ter nal Subbetic; data from Reolid et al., 2010). C. Change model of a dis con ti nu ity sur face in the High At las Do main (data from Christ et al., 2012). D. Change model of the dis con ti nu ity sur face at the top of the Amuri Lime stone (Mid dle Oligocene, New Zea land. Mod i fied af ter Lewis & Ekdale, 1992). Oph – Ophiomorpha, Are – Arenicolites, Tha – Thalassinoides, Gas – Gastrochaenolites, Try – Trypanites.
Tec tonic con text
At the dis con ti nu ity stud ied, nep tu nian dykes are in dic a tive of synsedimentary tec ton ics, prob a bly re lated to ten -sional de for ma tion sys tems (Basilone, 2009; Bertok and Martire, 2009; Nieto et al., 2012). Data from the same in ter -val stud ied in this re search in other Subbetic tec tonic units also in di cate that synsedimentary tec ton ics was an im por tant con trol in sed i men ta tion in the east ern Subbetic (Fig. 6). The suc ces sions stud ied in the La Mola Unit, Reclot Unit (sec -tion 1; GPS co or di nates: 38°20'52.83" N; 0°55'26.68" W), Cantón Unit (sec tion 5; GPS co or di nates: 38°17'49.75" N; 1°00'52.27" W), and Lúgar-Corque Unit (sec tion 8; GPS co or di nates: 38°12'15.17" N; 1°11'25.60" W) only re cord a long hi a tus. How ever, in sec tion 9 of the Quípar Unit (GPS co or di nates: 38°04'27.86" N; 1°48'32.79" W) and sec tion 6 of the Lúgar-Corque Unit (GPS co or di nates: 38°14'12.77" N; 1°07'49.04"W; Fig. 6), five un con formi ties (Hg1 to Hg5) bound ing con densed se quences have been de scribed (Nieto
et al., 2012). These sig nif i cant strati graphic dif fer ences
plain could be the re sult of half-graben de vel op ment as a con se quence of block ro ta tion by listric faults (Vera, 2001; Nieto et al., 2012).
This in ter pre ta tion is in ac cor dance with Ziegler (1989), who showed that the Mid dle–Late Ju ras sic tran si tion was a tec toni cally ac tive pe riod with a change from extensional to transtensional con di tions in the West ern Tethys. In the East -ern Ex ter nal Subbetic (Quípar Unit; Fig. 6), Nieto et al. (2012) char ac ter ized nep tu nian dykes re lated to Hg1 (top of the Lower Bathonian), Hg2 (top of the Mid dle Bathonian),
and Hg5 (top of the Callovian). In the un con formity sur face stud ied here, cor re lated to Hg1, the pres ence of nep tu nian dykes re vealed that the pe lagic swell prob a bly was af fected by a mixed ten sional and shear de for ma tion phase. Ac cord -ing to Han cock (1985, 1994), this type of frac ture sys tem, form ing a di hed ral an gle rang ing be tween 10° and 50°, are hy brid frac tures with mixed ten sional and shear de for ma -tion, which is in agree ment with the hy poth e sis of Nieto et
al. (2012). How ever, it should be noted that the La Mola
Unit, like other Subbetic units, was af fected by sev eral tec -tonic stages, only re corded at a sin gle un con formity. On the ba sis of the re la tion ship be tween the nep tu nian dykes and the ichnofossils (es pe cially the branched ichnofossils), the most im por tant de for ma tion stage should be sub se quent to the de vel op ment of these trace fos sils. Vera (2001) and Nieto et al. (2012) show that around Mid dle Bathonian to Oxfordian, the South Ibe rian Palaeomargin was af fected by transtensional de for ma tion re lated to crustal sep a ra tion be -tween the Af ri can, Ibe rian, and Mesomediterranean plates. In the Subbetic, this sit u a tion in volved an in crease in pe lagic swell de vel op ment and there fore a sig nif i cant re duc -tion in sed i men ta -tion rates and a rel a tive sea-level fall.
DIS CUS SION: COM PAR I SON WITH
OTHER GEO LOG I CAL DO MAINS
De tailed anal y ses of Mid dle–Up per Ju ras sic hard-ground sur faces re corded in the East ern Ex ter nal Subbetic
(de vel op ing in pe lagic swells in the South Ibe rian Palaeomar gin; West ern Tethys; Fig. 6) re veal sig nif i cant lat eral fa cies changes, mainly re lated to spe cific lo ca tions and dif fer -ences in palaeobathymetry (Reolid et al., 2010; Nieto et al., 2012). Lat eral vari a tions can also be ob served in the trace-fos sil as sem blages (Fig. 9).
In pre vi ously stud ied sur faces, es pe cially those in the Reclot Unit sec tions (Fig. 9B), the most abun dant trace fos sil is Thalassinoides, coex ist ing with other trace fos sils re -corded as paired ver ti cal tubes, as signed to Arenicolites or
Diplocraterion, and iso lated tubes, in ter preted as bor ings of
the Trypanites ichnofacies (Reolid et al., 2010; Nieto et al., 2012). On the ba sis of the dif fer ent kinds of ichnofossils re -corded (Trypanites with ei ther Arenicolites or
Thalassinoi-des), the pro gres sive sub strate lithification went from a soft-
ground to a firmground or hardground (Reolid et al., 2010; Nieto et al., 2012). Sev eral sig nif i cant dif fer ences can be rec og nized with re spect to the sur face stud ied in the La Mola Unit, par tic u larly the re cord of Ophiomorpha and
Gastrochaenolites and the less abun dant Trypanites (Fig. 9A, B). The sig nif i cance of sub strate con sis tency re -lated to trace-fos sil as sem blage and its evo lu tion prob a bly re flect a change from com par a tively softer, more un sta ble sub strates as so ci ated with the sur face in the La Mola Unit to in cip i ent hardground con di tions. These ichnological fea tures, and their cor re spond ing in ter pre ta tions, are in agree -ment with other sedi-mentological data re corded in the Reclot Unit (Reolid et al., 2010; Nieto et al., 2012), but ab -sent in the sec tion at the La Mola Unit. For in stance, in the dis con ti nu ity sur face at the La Mola Unit, there is an ab -sence of fea tures, such as Fe-Mn oncoids and crusts re lated to well-de vel oped strati graphic breaks, and of taphonomic fea tures of the macroinvertebrate as sem blages, ev i denc ing long ex po sure of the re mains on the seafloor (such as corrasion, fac et ing, and encrustation).
In com par i son with other car bon ate en vi ron ments, the di ver sity of trace fos sils as so ci ated to dif fer ent steps in seafloor evo lu tion is higher. For in stance, in the Ju ras sic of the Cen tral High At las (West ern Tethys, Mo rocco), Christ
et al. (2012) show that Thalassinoides is the trace fos sil that
char ac ter izes soft and firmground stages in seafloor evo lu tion, and Gastrochaenolites is re lated to hardground con di -tions (Fig. 9C). Lewis and Ekdale (1992) ana lysed the Amuri Lime stone (Mid dle Oligocene, New Zea land; Fig. 9D), de pos ited in a pe lagic en vi ron ment. These au thors show that Thalassinoides is the trace fos sil as so ci ated with softgrounds to hardgrounds (see Lewis and Ekdale, 1992, fig. 14); this last ground also is ac com pa nied by Trypanites.
CON CLU SIONS
An in te gra tive anal y sis of sedimentologic, stratigraphic, palaeotectonic, and es pe cially ichnological data ob -tained from un con formity sur faces in the Ex ter nal Subbetic Zone (La Mola Unit, Betic Cor dil lera) de vel oped in pe lagic car bon ate-swell en vi ron ments re veals the im por tance of sea-level con di tions, tec tonic con text, and palaeogeogra-phic lo ca tion in their gen e sis. The use of trace fos sils for char ac ter iz ing the evo lu tion of seafloor fea tures is clearly
ex em pli fied in the mid dle Bathonian–mid dle Oxfordian un -con formity sur face stud ied, and has wide ap pli ca bil ity to the Ju ras sic Ammonitico Rosso fa cies, where un con formi ties are fre quently re corded.
The pres ence of Ophiomorpha char ac ter izes softground con di tions and un sta ble sub strates. The as so ci a tion of
Tha-lassinoides, Arenicolites, and Gastrochaenolites re veals an
in crease in firm ness, with firmground de vel op ment. Fi nally,
Trypanites de notes an in cip i ent hardground stage. The evo
-lu tion in sub strate con sis tency is re lated to dif fer ent sea-level phases and, there fore, to char ac ter is tic ma rine-cur rent en er gies and vari able con ti nu ity of de po si tion. Softground to early firmground stages could in di cate rel a tively low sea lev els, with lowen ergy cur rents and com par a tively con tin -u o-us de po si tion. S-ub se q-uent firmgro-unds or early hard-grounds de vel oped at ex tremely low sea lev els (rel a tively) with higher-en ergy cur rents, which could be re spon si ble for sed i ment win now ing and there fore for non-de po si tion. The ini tial phases of sea-level rise were marked by hardground con di tions, with higher-en ergy cur rents that re sulted in sea-floor ero sion. Dif fer ences in sedimentological and ichnological fea tures with re gard to pre vi ously stud ied dis con ti -nu ity sur faces from nearby ar eas (e.g., the Reclot Unit), are re lated to vari a tions in the palaeogeographic lo ca tion and bot tom to pog ra phy, de ter min ing changes in de po si tion and in the prob a ble du ra tion of the as so ci ated hi a tus. Ichnofossil abun dance is very high in the Subbetic com pared with other car bon ate en vi ron ments, which aids the im proved es tab lish -ment of se quences in seafloor evo lu tion.
Ac knowl edg ments
This re search was car ried out with the fi nan cial sup port of re -search pro jects CGL2012-33281, CGL2009-10329 (both of the Ministerio de Economía y Competitividad), P08-RNM-03715 (Junta de Andalucía), UJA2011/12/17 (Universidad de Jaén) and the Re search Groups RNM-178 and RNM-200 (Junta de Andalucía). We would like to thank A. Uchman, L. Martire, and the ref er ees (B. Bádenas and R. Aubrecht) for their com ments, which im -proved the orig i nal manu script. We are also grate ful to An to nio Piedra, a tech ni cian at the Laboratorio de Geología of the Universidad de Jaén for prep a ra tion of the thinsec tions. We are also in -debted to Chris tine Laurin for the Eng lish ver sion of the text.
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