two remarkable examples of portuguese neogene bioeroded … - two remarkable examples of... ·...

Artigo original

Original Article

Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/209 Comunicações Geológicas (2016) 103, Especial I, 121-130ISSN: 0873-948X; e-ISSN: 1647-581X

Abstract: The study of bioeroded rocky paleoshores applied to the practicalresolution of paleontological and geological issues, already showed itsusefulness. The study of the geological record of this kind ofpaleoenvironments in Portugal, with particular interest in Neogeneoccurrences, has resulted in a better understanding of the geological historyof the study areas in the context of the regional tectonics evolution and themain Neogene transgressive phases. Two Neogene bioeroded rocky shores(Foz da Fonte, with new data being presented herein, and Oura), remarkablefor both their extension and associated fossil record, have been analysed andthe bioerosion ichnoassemblages identified. The bioerosion trace fossilsinclude structures produced by clionaid sponges, polychaete and sipunculidannelids, acrothoracican cirripedia, endolithic bivalves, and echinoids. Allthe bioerosion structures correspond to the boring activity of endolithic andmobile epibenthic organisms. From an ethological point of view, dwellingstructures (domichnia) represented by the ichnotaxa Entobia, Caulostrepsis,Maeandropolydora, Trypanites, Rogerella and Gastrochaenolites aredominant, while those related with a combination of feeding habits such astrapping - food farming and food searching, agrichnion-pascichnia(compound trace fossil Ericichnus-Circolites) are the less common. Arelatively diverse assemblage of in situ body fossils has also been identifiedassociated with these bioeroded surfaces, mainly made up of oysters,barnacles and colonial corals.

Keywords: bioerosion; Paleoichnology; endolithic communities; rockypaleoshores; Neogene; Portugal.

Resumo: O estudo dos paleolitorais rochosos aplicado à resolução práticade questões paleontológicas e geológicas, já mostrou a sua utilidade. Oestudo do registo geológico deste tipo de paleoambientes em Portugal, comparticular interesse durante o Neogénico, resultou num melhor entendimentoda história geológica das áreas de estudo no contexto da evolução tectónicae das principais fases transgressivas neogénicas. Foram analisados dois litorais rochosos Neogénicos bioerosionados (na Fozda Fonte, em relação ao qual há novos dados, e em Oura), notáveis quer pelasua extensão, quer pelas associações fossilíferas a eles identificados, e assuas estruturas bioerosivas identificadas. De um modo geral, estas estruturasbioerosivas correspondem à actividade perfurante de organismos endolíticos(endobentónicos litófagos) e à actividade raspadora de organismosepibentónicos vágeis. Os organismos produtores destas estruturasbioerosivas pertencem a grupos muito diversificados sendo possívelencontrar entre eles esponjas clionídeas (produtoras de estruturas atribuídasao icnogén. Entobia), anelídeos poliquetas (Caulostrepsis eMaeandropolydora), bivalves endolíticos (Gastrochaenolites) eequinodermes (Circolites, Ericichnus).Do ponto de vista etológico dominam as estruturas de habitação, domichnia(representadas por estruturas atribuídas ao icnogén. Entobia, Caulostrepsis,Maeandropolydora e Gastrochaenolites), encontrando-se em menor númeroevidências de cultivo de alimento-pastoreio, agrichnion-pascichnia (estruturacomposta Ericichnus-Circolites). Associados a estas superfíciesbioerosianadas foi identificada uma associação relativamente diversificada

de somatofósseis in situ composta, sobretudo, por ostras, balaníneos e coraiscoloniais.

Palavras-chave: bioerosão; Paleoicnologia; comunidades endolíticas;litorais rochosos; Neogénico; Portugal.

1. Introduction

Until approximately two decades ago, the geological record of ancientrocky shores was still considered a geological rarity (Boucot, 1981).Nevertheless, studies on this topic have grown in number since,showing that the occurrences of fossil rocky shores - environmentsthat in the geological past provided optimal conditions for boring andencrusting organisms - are much more common than previouslythought, spanning a wide range of rock substrates and ages.

In Portugal, the study of bioeroded rockgrounds, being thoserelated with Neogene rocky shores the most common, starts to bearfruits. The oldest reference to Portuguese Neogene trace fossils inlithified substrates goes back to Choffat (1903-04) who recognizedthe presence of clavate borings made by Pliocene bivalves on LowerJurassic dolomitic blocks in Senhora da Vitória promontory, 11 kmnorth of Nazaré (Central-West Portugal). In the nineteen eighties,Pais (1982) and Antunes (1984) briefly commented on the presenceof bivalve borings on contact surfaces between Jurassic andNeogene formations in Southern Portugal. However, it was Silva et

al. (1995; 1999) who presented for the first time evidence ofNeogene bioerosion structures clearly indicating the occurrence ofancient rocky shores in several places in. Domènech et al. (1999)studied the bioeroded megasurface at Oura (Albufeira), one of themost extensive bioeroded surfaces known from the geologicalrecord so far, identifying it as representing a transgressive surfacecorresponding to an important intra-Miocene stratigraphicdiscontinuity with vast implications for the understanding of thepaleogeographic and tectonic (Betic) evolution of the southernPortuguese Algarve Basin.

However, it is not until the year 2008 that the study of thesetopics really takes off. In this year Santos and co-authors publisheda paper describing and discussing several middle to upper Miocenerocky shore bioeroded surfaces on the southern Portuguese Atlantic

Two remarkable examples of Portuguese Neogene bioeroded

rocky shores: new data and synthesis

Dois exemplos notáveis de paleolitorais rochosos bioerosionados

do Neogénico de Portugal: novos dados e síntese

A. Santos 1*, E. Mayoral 1, C. M. da Silva 2, M. Cachão 2

© LNEG – Laboratório Nacional de Geologia e Energia IP

1 Departamento de Geodinámica y Paleontología, Facultad de CienciasExperimentales, Campus de Excelencia Internacional del Medio Ambiente,Biodiversidad y Cambio Global, CEI-Cambio"2 Instituto Dom Luiz e Departamento de Geologia, Faculdade de Ciências,Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal*Corresponding author/Autor correspondente: [email protected]

122 A. Santos et al. / Comunicações Geológicas (2016) 103, Especial I, 121-130

coast (Algarve) and the Mediterranean coast of Spain. This workprovided a synthesis drawing significant connections betweenmarine erosional surfaces bearing trace-fossil assemblages andaspects of regional tectonic evolution in the context of Neogenetransgressive events in the two sectors (Santos et al., 2008).

In Cachão et al. (2009) the authors revisit the study of the Ourabioeroded surface, and based on a detailed stratigraphical andpaleoichnological analysis confirmed the existence of an importantintra-Miocene stratigraphic gap (ca. 3 Ma hiatus), represented by arazor-sharp erosional contact that separates the two main Neogenelithostratigraphic units in the coastal area of the Algarve andreconstructed the sequence of events related to this episode basedon the trace fossils.

Santos et al. (2010) studied a rocky-shore platform marking thebeginning of the Neogene stratigraphic record of the Lower TagusBasin at Foz da Fonte, some 30 km south of Lisbon, featuring asuccession of bioerosion events that occurred during the earlyMiocene (middle Burdigalian). Trypanites and Rogerella boringsare documented for the first time in the Atlantic region of Iberia andthe diversity of Gastrochaenolites ichnospecies is higher than foundpreviously elsewhere in Portuguese outcrops. In their paper, theauthors show that the studied unconformity surface with itspaleoichnological features marks the former Miocene mean sealevel in that region, in that particular time, and demonstrate that thestudy of former rocky shores may be successfully used to solvebroader geological issues. Based on data from Foz da Fonte, the generalapplicability of the photogrammetric method to paleoichnologicalanalysis of bioeroded surfaces was tested, and Space Point Patternanalysis was used to describe patterns of trace-fossil locations andto check whether there was a significant occurrence of clustering inthe study area (Cachão et al., 2011).

More recently, the study of bioeroded rocky surfaces wasextended to volcanic islands of the North Atlantic archipelagos.Santos et al. (2012) described for the first time extensive bivalveborings from basalt rockgrounds in the North Atlantic volcanicislands of Macaronesia, specifically Porto Santo (Madeira archipelago,Portugal) and also Santiago Island (Cape Verde). The discovery inthese bioeroded surfaces of fossils of specimens belonging to thefamily Pholadidae, as well as Mytilidae (Lithophaginae), demonstratesthat both were active boring bivalve on volcanic rocks.

Lately, Santos et al. (2015) provided the first case study of anew trace fossil produced by sea-urchins used as a paleoenvironmentaltool, indicating boring activity of termophile rock-boring echinoidpreserved in the fossil record. In this work two new bioerosionichnospecies related to the activity of echinoids were described (inhonor of Richard Bromley and Ulla Asgard: Ericichnus bromleyi

and E. asgaardi).The present paper is structured to be used as a field trip guide

of the post-congress field trip of the fourth International Congresson Ichnology to be held in Idanha-a-Nova (Portugal). The aim ofthis paper is to report new data related with the occurrence ofNeogene bioeroded surfaces in the Portuguese geological recordand provide a synthesis of the knowledge of two remarkableMiocene bioeroded surfaces: Foz da Fonte (Central West Portugal)and Oura site (southern Portugal).

2. Geographical and geological setting

The study area covers the Central West and southern Portuguesecoast (Foz da Fonte and Oura outcrops, respectively). The studiedsurfaces and the bioerosive structures affecting them are early tomiddle Miocene in age although the affected substrates are madeup of rocks of different ages from Mesozoic to Cenozoic.

The Foz da Fonte bioeroded surface is located on the Atlanticcoast of the Setúbal Peninsula, Central West Portugal, about 30 km

southwest of Lisbon (Fig. 1). This surface, as well as the bioerosionstructures affecting it, are Miocene in age and developedunconformably on lower Cretaceous limestones (Albian age) beingoverlain by lower Miocene conglomeratic and calcarenitic sediments(Silva et al., 1995; 1999; Manupella et al., 1999; Santos et al., 2010).

On the other hand, the megasurface of Oura is located on thesea cliff to the west of the sandy beach of the locality with the samename and about 3 km east of Albufeira, on central Algarve (Fig. 2).This surface may be followed, intermittently, for about 1.5 km alongthe Oura sea cliffs, and is one of the largest bioeroded surfacesknown in the fossil record (Domènech et al., 1999; Santos et al., 2008;Cachão et al., 2009). This surface, as well as the bioerosion structuresaffecting it, are Miocene in age and developed unconformably onthe contact between the middle Miocene biocalcarenite of theLagos-Portimão Formation (middle to late Serravalian age) below,and the upper Miocene siliciclastic sediments of the CacelaFormation above (Cachão and Silva, 1992; 2000).

3. Description of the Miocene bioeroded rocky shores

3.1 Foz da Fonte (Sesimbra, Central West Portugal)

The Foz da Fonte site shows a highly bioeroded rock surface, whichcorresponds to a subhorizontal transgressive surface cut through apreviously emerged topographic feature showing karstifiedcarbonate relief (see Silva et al., 1999; Santos et al., 2010). Thebioeroded surface is located about 10 m above present day meansea level, and was produced over Lower Cretaceous limestonesduring a transgressive trend that occurred in this region during the

Fig. 1. Geographical and geological setting of the Foz da Fonte site (Central WestPortugal). Age symbols and stage abbreviations after Harland et al. (1989): K1Apt-Alb – Lower Cretaceous, Aptian-Albian; N1Bur – Lower Miocene, Burdigalian.

Fig. 1. Localização geográfica e enquadramento geológico dos afloramentos de Foz daFonte (Centro Oeste de Portugal). Símbolos de idades e abreviaturas dos andaressegundo Harland et al. (1989): K1Apt-Alb – Cretácico Inferior, Aptiano-Albiano;N1Bur – Miocénico, Burdigaliano.

Portuguese Neogene bioeroded rocky shores 123

early Miocene (Santos et al., 2010) giving rise to a low angleunconformity (Fig. 3).

The rocky surface is extensively bored, showing numerous tracefossils in 80-90% of its area (Santos et al., 2010; Cachão et al.,2011). The most prominent trace fossil found is the characteristic,club-shaped, bivalve boring Gastrochaenolites preserved as concaveepireliefs on the surface (Figs. 4a, b). Furthermore, the morphologicalanalysis of the bioerosion structures revealed a total of elevenichnospecies belonging to seven ichnogenera (Table 1). Theseinclude structures produced by polychaete annelids (Caulostrepsis

isp.), clionaid sponges (Entobia isp.), sipunculid annelids(Trypanites weise Mägdefrau, 1932), acrothoracican cirripedia(Rogerella isp.), endolithic bivalves (Gastrochaenolites torpedo

Kelly and Bromley, 1984, Gastrochaenolites lapidicus Kelly andBromley, 1984, Gastrochaenolites ornatus Kelly and Bromley,1984) and echinoids (Ericichnus asgaardi Santos and Mayoral,2015, Circolites kotoucensis Mikuláš, 1992) (Figs. 4c, e, f).

The bioeroded surface also presents evidences of encrustingorganisms, such as the pycnodontic bivalve Pycnodonte squarrosa,preserved with its original shell, basal plates of balanomorphs, andcolonial corals in situ. Several bivalves sometimes grew over theborings, indicating a later larval settlement, and others are alsoperforated by boring bivalves, representing a second colonizationand bioerosion phase (Fig. 4d).

The trace fossil assemblage present on the Foz da Fonte rockyground is assigned to the Entobia subichnofacies (MacEachern et

al., 2007) of Bromley and Asgaard (1993), of the Trypanites

ichnofacies of Frey and Seilacher (1980), corresponding to a veryshallow marine rocky substrate environment with a minorsedimentation rate (Santos et al., 2010). The quantitative analysisof the total surface bioerosion allowed further conclusions namelyon possible alignment of the early Miocene paleoshore and possiblesuccessional patterns (Cachão et al., 2011).

Recently, Santos et al. (2015) described a new ichnospeciesrelated with rock-boring echinoids (Ericichnus asgaardi) for thissame surface (Fig. 4e). In what concerns the Cretaceous limestoneaffected by the bioeroded surface, it is characterized exclusively byOphiomorpha nodosa Lundgren, 1891. The information availableon these occurrences is summarized in Table 1.

Recent field work on the northern end of the Foz da Fonte coastalinlet, some 450 northeast of the previously described occurrence (Fig.5a), revealed a new stretch of this very same bioeroded surface.Apparently, this new exposure is the result of a very energetic coastalerosion event that took place in February-March of 2014. The erosionsignificantly reduced the pocket sandy beach of Foz da Fonte,removing the sediments of the talus normally covering the base thesea cliff at this location, and exposing the continuation of theCretaceous/Miocene unconformity and its bioeroded surface furthernorth. Since the Cretaceous/Miocene contact at Foz da Fonte gentlydips towards northeast, the bioeroded surface in this location ispositioned lower than in the southwestern end of the cove, a meremeter or so above present day high tide level.

In this exposure, due to the fact that the contact was protectedfrom subaerial erosion, the bioeroded surface is a lot fresher andmore exuberant than at the previously known location, showing adensely bored irregular surface, with several nooks and crannies(Figs. 5a, b, c). The preliminary analysis of the newly discoveredsurface revealed that the most common bioerosion structures in itare the bivalve borings Gastrochaenolites torpedo Kelly andBromley, 1984 and Gastrochaenolites lapidicus Kelly and Bromley,1984. Clionaid sponge borings Entobia sp. are also present. Locally,in some of the crannies, a wealth of well preserved in situ fossils ofepilithic organisms such as barnacles and oysters is visible (Fig. 5d).

In this new exposure along a stretch of some 10 m on the presentday sea shore, the irregular bioeroded surface cuts vertically through

Fig. 2. Geographical and geological setting of the Oura site (southern Portugal). Agesymbols and stage abbreviations after Harland et al. (1989): N1Tor – Miocene,Tortonian, N1Srv – Miocene, Serravalian.

Fig. 2. Localização geográfica e enquadramento geológico do afloramento de Oura(Sul de Portugal). Símbolos de idades e abreviaturas dos andares segundo Harland et

al. (1989): N1Tor – Miocénico, Tortoniano, N1Srv – Miocénico, Serravaliano.

Fig. 3. General view of the Foz da Fonte outcrop (N 38º 27’ 07,39’’ / W 09º 12´ 05,63´´) showing the contact surface between Cretaceous limestones (Albian) and Miocene (middle-upper Burdigalian) terrigenous sediments. Bioerosion structures occur on the planar surface that bounds both rock units. Age symbols and stage abbreviations after Harland et al.(1989): N1Bur- Miocene, Burdigalian; K1Alb- Lower Cretaceous, Albian.

Fig. 3. Vista geral do afloramento de Foz da Fonte (N 38º 27’ 07,39’’ / W 09º 12´ 05,63´´) mostrando a superfície de contato entre os calcários cretácicos (Albia) e os sedimentosterrígenos miocénicos (Burdigaliano médio-superior). As estruturas bioerosivas ocorrem na superfície de contato com as duas unidades estratigráficas. Símbolos de idades eabreviaturas dos andares segundo Harland et al. (1989): N1Bur- Miocénico, Burdigaliano; K1Alb- Cretácico Inferior, Albiano.


Fig. 4. Detailed view of the paleoichnological features of the substrates at Foz da Fonte. a) Distribution of Gastrochaenolites lapidicus (Gl) and G. torpedo (Gt) structures showingphobotaxis behavior. Scale bar: 2 cm. b) Detail of the bioeroded surface with G. torpedo (Gt) and G. lapidicus (Gl). c) Detail of the bioeroded surface presenting Rogerella isp. d)Pycnodonte squarrosa bored by Entobia, Caulostrepsis, and Gastrochaenolites lapidicus and encrusting previous G. lapidicus on the bioeroded surface. Scale bar: 10 cm. e)Cretaceous limestone showing borings of Gastrochaenolites isp. (black arrows) and Ericichnus asgaardi (white arrows). (Hammer scale = 30 cm). f) Surface showingGastrochaenolites and Caulostrepsis isp. Scale bar: 2 cm.

Fig. 4. Vista detalhada das caraterísticas paleoicnológicas dos substratos em Foz da Fonte. a) Distribuição das estruturas Gastrochaenolites lapidicus (Gl) e G. torpedo (Gt)mostrando comportamento fobotáxico. Escala: 2 cm. b) Detalhe da superfície bioerosionada ostentando G. torpedo (Gt) e G. lapidicus (Gl). c) Detalhe da superfície bioerosionadaapresentando Rogerella isp. d) Pycnodonte squarrosa perfurada por Entobia, Caulostrepsis, e Gastrochaenolites lapidicus e encrustando a superficie perfurada por G. lapidicus.Escala: 10 cm. e) Superfície calcárea cretácica com perfurações de Gastrochaenolites isp. (setas pretas) e Ericichnus asgaardi (setas brancas). (Escala martelo = 30 cm). f) Superfíciemostrando Gastrochaenolites e Caulostrepsis isp. Escala: 2 cm.


several beds of the Lower Cretaceous rocky substrate clearly showingthe relief of the ancient rocky shore face in this location in earlyMiocene times.

3.2 Megasurface of oura (Albufeira, Southern Portugal)

The bioeroded surface of Oura corresponds to a sub-horizontaltransgressive surface cutting through an emerged and karstifiedcarbonate relief (Fig. 6). This surface was identified as an intra-

Table 1. Information on Portuguese Miocene sites displaying bioerosion structures inrocky paleoshores. Abbreviations and symbols used in the table: S – Surface exposure;* – Age symbols and stage abbreviations after Harland et al. (1989): K1Alb – LowerCretaceous, Albian; N1Bur – Lower Miocene, Burdigalian; N1Srv – Lower Miocene,Serravalian; N3Tor – Middle Neogene, Tortonian.

Tabela 1. Síntese da informação dos afloramentos Neogénicos Portugueseses analisadoscom evidencias de estruturas bioerosivas. Abreviaturas e símbolos usados na Tabela:S – Superfície exposta; * – Símbolos de idades e abreviaturas de períodos depois deHarland et al. (1989): K1Alb – Cretácico inferior, Albiense; N1Bur –Miocénicoinferior, Burdigaliense; N1Svr –Miocénico inferior, Serravaliense; N3Tor –Neogénicomédio, Tortoniense.

ouTCRoPS

SubSTRATE

Portugal

Foz da FonteN1 Bur

OuraN3 Tor

Age (*) K1 Alb N1 Srv

Lithology Limestone Limestone

Exposure type S S

Surface preservation Regular Regular

IChNoTAxA

Gastrochaenolites isp. ●

G. torpedo Kellyand Bromley

● ●

G. turbinatus Kellyand Bromley

●

G. lapidicus Kellyand Bromley

●

G. ornatus Kellyand Bromley

●

Entobia isp. ●

Caulostrepsis isp. ●

Trypanites weise

Mägdefrau●

Rogerella isp. ●

Circolites kotoucencis

Mikuláš●

Ericichnus asgaadi

Santos and Mayoral●

EPIlIThobIoNTS

Pycnodonte squarrosa ●

Barnacles ●

Colonial corals ● Fig. 5. a) General view of the new Foz da Fonte northern outcrop (N 38º 27´20,07´´ /W 09º 11´ 55,84´´) showing the bioeroded surface recently exposed by marine erosion.b) View of abundant G. lapidicus in cross-section in the same locality. c) Cross-section(detail of Figure 5b) of the Cretaceous limestones surface with G. lapidicus filled byoverlying Miocene sediments. d) Fossils of oysters and barnacles in living position onthe Cretaceous limestone surface.

Fig. 5. a) Vista geral do novo afloramento no extremo norte da Foz da Fonte (N 38º27´20,07´´ / W 09º 11´ 55,84´´) mostrando a superfície bioerosionada posta a descobertopor eventos de erosão costeira. b) Vista de numeros G. lapidicus em seção transversalna mesma superfície. c) Corte transversal (detalhe da Figura 5b) da superfície calcáriacretácica com G. lapidicus preenchidos com sedimentos miocénicos suprajacentes. d)Fósseis de ostreídeos e cirrípedes em posição de vida sobre a superficie calcáriaCretácica.


Miocene (early Tortonian) marine abrasion platform by Domènechet al. (1999).

This surface presents an impoverished ichnoassemblage ofbioerosion structures composed exclusively by truncatedGastrochae nolites (G. torpedo and G. turbinatus) (Fig. 7a). Theichno assemblage is equivalent to the Gastrochaenolites-Entobia

ichnoassemblage described in Gibert et al. (1998) and corresponds

to the Entobia ichnofacies of Bromley and Asgaard (1993), encom -passing shallow-marine boring assemblages produced on hard,lithified surfaces associated with rocky shores (Cachão et al., 2009).

Moreover, the biocalcarenite sediments where the contact wascarved also presents bioturbation structures assigned to Bichordites

(previous assigned to Laminites in Cachão et al., 2009) andThalassinoides, which are well exposed on the bioeroded surface. On

Fig. 6. General view of the bioeroded megasurface of Oura (N 37º 05´ 01,22´´ / W 08 13´ 49,20’’) showing the contact surface between the carbonate and the siliciclastic Miocenesequences. Age symbols and stage abbreviations after Harland et al. (1989): N1Srv - Miocene, Serravalian, N3Tor - Miocene, Tortonian.

Fig. 6. Vista geral da megassuperfície bioerosionada de Oura (N 37º 05´ 01,22´´ / W 08 13´ 49,20’’) mostrando o contato entre as sequências miocénicas carbonatada e siliciclástica.Símbolos de idades e abreviaturas dos andares segundo Harland et al. (1989): N1Srv – Miocénico, Serravaliano, N3Tor – Miocénico, Tortoniano.

Fig. 7. Detailed view of paleoichnological features of the substrate at Oura. a) Spatial distribution of Gastrochaenolites torpedo and G. turbinatus. Scale bar: 4 cm. b) Detail of thebioeroded surface showing Thalassinoides isp. filled with well-cemented grey fine sandstone and bioeroded by Gastrochaenolites. Scale bar: 10 cm. c) Bichordites isp. Scale bar:10 cm. d) General view of the siliciclastic unit of the Cacela Formation with Gyrolithes isp. and Thalassinoides isp. Scale bar: 10 cm.

Fig. 7. Vista detalhada das caraterísticas paleoicnológicas do substrato em Oura. a) Distribuição espacial de Gastrochaenolites torpedo e G. turbinatus. Escala: 4 cm. b) Detalheda superfície mostrando Thalassinoides isp. preenchidos por sedimento arenoso fino bem consolidado e bioerosionado por Gastrochaenolites. Escala: 10 cm. c) Bichordites isp.Escala: 10 cm. d) Vista geral da unidade siliciclástica da Formação de Cacela com Gyrolithes isp. e Thalassinoides isp. Escala: 10 cm.


the well-cemented top surface of the biocalcarenites, the outcroppingThalassinoides are filled with grey fine sandstone and their verticalshafts are truncated by erosion (Fig. 7b). The Bichordites present anactive bioclastic infilling, displaying the typical meniscate shape (Fig.6c). The marine fine sandstones of the Cacela Formation that overlaythe Oura surface show an abundant ichnoassemblage, mainlycomposed of Gyrolithes and Thalassinoides bioturbation structures(Cachão et al., 2009) (Fig. 7d). The information available on theseoccurrences is summarized in Table 1.

4. Paleoenvironmental interpretation

The bioerosion features described herein represent traces producedon former rocky shores by an array of animals adapted to life inshallow marine environments and the intertidal zone. In the case ofFoz da Fonte the ichnological succession seems to record repeatedevents of bioerosion (Santos et al., 2010).

The environments represented (ancient rocky shores) are the samefor all the study sites. The abundance of Gastrochaenolites andEntobia on the investigated ichnoassemblages is interpreted asindicating a very shallow marine environment with a low or null rateof sedimentation (Bromley and Asgaard, 1993; Gibert et al., 1998).

According to Santos et al. (2010), the sequence of eventsleading to the formation of the rocky surface and its later bioerosionat Foz da Fonte are reconstructed as shown in Figure 8 herein:

a. Cenomanian/Turonian to Miocene erosion of the emergedCretaceous sedimentary sequence and formation of the limestonerocky surface (Figs. 8.1-2).

b. In Miocene Burdigalian times, an initial shallow-water phaseduring marine flooding led to the colonization of submergedrockground by sponges (producing Entobia), sipunculid worms(producing Trypanites) and a dense population of boring bivalvesproducing Gastrochaenolites torpedo (probably Lithophaga sp.bivalves) and G. lapidicus structures (Fig. 8.3). The erosion andbioerosion of the surface continued, leading to the obliteration ofupper half of the previous bioerosion structures (Fig. 8.4).

c. With the development of the transgression, water depthincreases, resulting in the inhibition of the G. torpedo producers andtriggering the colonization by encrusting organisms such asPycnodonte bivalves, balanomorph crustaceans and colonial corals(Fig. 8.5).

d. Abrasion and bioerosion continues smoothing the bioerodedsurface, and tearing away some right valves of the Pycnodonte

shells (Fig. 8.6). In the external part of the platform (seaward)shallow Caulostrepsis worm borings and Rogerella acrothoracicancirriped borings were produced.

The boring activity was terminated by the burial of the rockysurface under sediment in a shallow marine environment. The overlyingterrigenous sediment was deposited rapidly, forming a homogeneouslayer completely sealing the underlying bioeroded surface.

On the other hand, in what concerns to the Oura megasurface, thesequence of events leading to its formation are discussed in Cachãoet al. (2009) and are reconstructed as shown in Figure 9 herein:

a. Sedimentation and bioturbation (Fig. 9.1-2): Sedimentationof the Lagos Biocalcarenite beds (Lagos-Portimão Formation),colonisation of the marine bottom by excavating organisms, mainlysea-urchins and crustaceans producing Bichordites isp. andThalassinoides isp. (Fig. 9.1-2) in a shallow marine environment,followed by further sedimentation and infilling of the Thalassinoides

burrows.b. Erosion (subaerial and marine) and bioerosion (Fig. 9.3-5):

The formation of the large karst sinkholes implies emersion andsubaerial exposure and erosion (Fig. 9.3). Later the karstifiedcoastline is invaded and cut by a transgressive sea, originating amarine erosional platform. The carbonate surface of the marine

erosion platform is colonised by bioeroding organisms in a littoralto shallow-marine environment (Fig. 9.4-5).

c. Sedimentation and bioturbation (Fig. 9.6): Accumulation of thesiliciclastic sediments of the Cacela Fábrica middle member of theCacela Formation, colonisation of the marine bottom by excavatingorganisms, mainly crustaceans producing Gyrolithes isp. andThalassinoides isp. (Fig. 9.5-6) in a shallow-marine environment.

5. Tectonic and stratigraphic implications

Bioeroded rocky paleoshores enhance discontinuities in stratigraphicsurfaces characterizing the beginning of new sedimentary sequencesduring transgressive events. The recognition of the Foz da Fontebioeroded transgressive surface is a crucial evidence for establishingthe presence of a vertical tectonic disturbance in this region prior tothe middle Burdigalian, associated to the emplacement of a saltdome and a volcanic sill formed earlier, probably already duringPalaeogene times. The bioeroded surface culminates a significantlylong stratigraphic gap (hiatus) spanning the interval from the

Fig. 8. Sequence of formation and palaeoenvironmental interpretation diagram of the Fozda Fonte bioeroded surface (Central West Portugal). Age symbols and stage abbreviationsafter Harland et al. (1989): K1Alb – Lower Cretaceous, Albian; K –Cretaceous, N1Bur– Lower Miocene, Burdigalian. Bioerosion structures: a – Rogerella isp.; b –Caulostrepsis isp.; Encrusting organisms: 1 – Barnacles; 2 – Colonial corals; 3 – Oysters.

Fig. 8. Sequência de formação e diagrama paleoambiental da superficie bioerosionadada Foz da Fonte (Centro Oeste de Portugal). Símbolos de idades e abreviaturas dosandares segundo Harland et al. (1989): K1Alb – Cretácico Inferior, Albiano; K –Cretácico, N1Bur – Miocénico, Burdigaliano. Estruturas de bioerosão: a – Rogerella

isp.; b – Caulostrepsis isp.; Organismos encrustantes: 1 – Cirrípedes; 2 – Coraiscoloniais; 3 – Ostreídeos.


Cretaceous to the Miocene during which break-up of the Cretaceouslimestone beds and karstification occurred (Santos et al., 2010).

The intra-Miocene bioeroded surface of Oura is particularlyimportant since it reveals a discontinuity that is usually overlooked,because the sedimentary sequence above it is not marked by a basalconglomerate. The Oura surface marks a 3 Ma hiatus during whichimportant tectonic activity in the Bethic-Rif chain took place withgeneralized uplift and karstification of the carbonate middle MioceneLagos-Portimão Formation. The subsequent anti-clockwise rotationof the Guadalquivir Basin allowed marine sedimentation to resumealong Algarve’s coastal onshore, corresponding to the “3rd Cycle” ofthe Marine Neogene of Portugal (Cachão and Silva, 2000).

6. Conclusions

The main conclusions of this study can be summarized as follows:The borings present in both studied sites correspond to an

ichnoassemblage produced in a very shallow marine rocky substratewith a low sedimentation rate, and related to sedimentary omissionor starvation;

The dominant trace fossil is the characteristic bivalve boringGastrochaenolites;

The trace fossil assemblages may be characterized as correspondingto the Entobia subichnofacies of the Trypanites ichnofacies;

From a tectonic point of view, the recognition of the Foz daFonte and the Oura bioeroded transgressive surfaces are decisive toconfirm the existence of two stratigraphic gaps in the Portuguesegeologic record.

Acknowledgements

Financial support was received by the Junta de Andalucía (Spanishgovernment) to the Research Group RNM276. The authors aregrateful to the reviewer Francisco J. Rodríguez-Tovar (Universityof Granada, Spain) for his suggestions and comments of the textwhich improved the manuscript.This is a publication of the CEI-Cambio Publication Series.

References

Antunes, M. T., 1984. Orla Algarvia. In: Oliveira J.T. (ed) Carta Geológica

de Portugal, 1/200000, Serviços Geológicos de Portugal, Lisboa,Notícia Explicativa, 7: 60–64.

Boucot, J., 1981. Principles of benthic marine palaeoecology. AcademicPress, New York, 463.

Bromley, R. G., Asgaard, U., 1993. Endolithic community replacement ona Pliocene rocky coast. Ichnos, 2: 93–116.

Cachão, M., Silva, C. M. da, 1992. Neogene paleogeographic evolution ofAlgarve Bassin (Southern Portugal): a two step model. Preliminary data.Gaia, 4: 39–42

Cachão, M., Silva, C. M. da, 2000. The three main depositional cycles ofthe Neogene of Portugal. Ciências da Terra, 14: 303–312.

Cachão, M., Silva, C. M. da, Santos, A., Domèmech, R., Martinell, J. andMayoral, E., 2009. The bioeroded megasurface of Oura (Algarve, southPortugal): implications for the Neogene stratigraphy and tectonicevolution of southwest Iberia. Facies, 55: 213–225.

Cachão, M., Redweik, P., Barreira, E., Dinis, J., Catita, C., Silva, C. M. da,Santos, A., Mayoral., E. and Wilfried, L., 2011. Photogrammetric andspatial analysis of a bioeroded Early Miocene rocky shore, westernPortugal. Facies, 57: 417–429.

Choffat, P., 1903-04. L’Infralias et le Sinémurien du Portugal. Comunicações

da Comissão do Serviço Geológico de Portugal, 5: 49–114.Domènech, R., Silva, C. M. da, Cachão, M. and Martinell, J., 1999. Una

megasuperfície bioerosionada en Oura (Albufeira): Implicationes parala evolución sedimentaria del Mioceno de Algarve (S de Portugal).Temas Geol.-Min. ITGE, 26: 226–230.

Frey, R. W. and Seilacher, A., 1980. Uniformity in marine invertebrateichnology. Lethaia 13: 183–207.

Gibert, J. M. de, Martinell, J., Domènech, R., 1998. Entobia ichnofacies infossil rocky shores, lower Pliocene, northwestern Mediterranean.Palaios, 13: 476–487.

Harland, W. B., Armstrong, R. L., Craig, L. E., Smith, A. G., Smith, D. G.,1989. A geologic time scale. Cambridge Univ Press, Cambridge, 179.

MacEachern, J. A., Pemberton, S. G., Gingras, M. K. and Bann, K. L., 2007.The ichnofacies Paradigm: Afifty-year restropective. In: Miller III W.(Ed.), Trace Fossils. Concepts, Problems, Prospects, Elsevier,Amsterdam, 52–77.

Fig. 9. Sequence of formation and palaeoenvironmental interpretation diagram of theOura bioeroded megasurface (Algarve, southern Portugal). Age symbols and stageabbreviations after Harland et al. (1989): N1Srv - Miocene, Serravalian, N3Tor -Miocene, Tortonian. 1 – Crustaceans, producers of Thalassinoides; 2 – Pectinids andother bivalves; 3 – Echinoids, producers of Bichordites.

Fig. 9. Sequência de formação e diagrama paleoambiental da megassuperficiebioerosionada de Oura (Sul de Portugal). Símbolos de idades e abreviaturas dos andaressegundo Harland et al. (1989): N1Srv – Miocénico, Serravaliano, N3Tor – Miocénico,Tortoniano. 1 – Crustáceos, produtores de Thalassinoides; 2 – Pectinídeos e outrosbivalves; 3 – Equinóides, produtores de Bichordites.


Manupella, G., Antunes M. T., Pais, J., Ármalo, M. M. and Rey, J., 1999.Carta Geológica de Portugal (Notícia Explicativa) 1:50 000, Folha 38-B Setúbal. Instituto Geológico e Mineiro, Portugal, 143.

Pais, J., 1982. O Miocénico do litoral Sul Português. Ensaio de Síntese. PhDthesis, University of Lisbon, Lisbon, 47 (unpublished).

Santos, A., Mayoral, E., Silva, C. M. da, Cachão, M., Domènech, R. andMartinell, J., 2008. Trace fossil assemblages on Miocene rockyshoresof southern Iberia. 431–450. In: Wisshak M., Tapanila L. (Eds.), Current

Developments in Bioerosion. Erlangen Earth Conference Series.Springer-Verlag, Heildelberg, 499.

Santos, A., Mayoral, E., Silva, C. M. da, Cachão, M. and Kullberg J. C.,2010. Trypanites ichnofacies: Palaeoenvironmental and tectonicimplications. A case study from the Miocene disconformity at Foz daFonte (Lower Tagus Basin, Portugal). Palaeogeography, Palaeoclimatology,

Palaeoecology, 292: 35–43.Santos, A., Mayoral, E., Johnson, M. E., Gudveig Baarli, B., Cachão, M.,

Silva, C. M. da and Ledesma-Vázquez, J., 2012. Extreme habitat

adaptation by boring bivalves on volcanically active paleoshores fromNorth Atlantic Macaronesia. Facies, 58: 325–338.

Santos, A., Mayoral, E., Dumont, C. P., Silva, C. M. da, Ávila, S. P., GudveigBaarli, B., Cachão, M., Johnson, M. E and Ramalho, R. S., 2015. Roleof environmental change in rock-boring echinoid trace fossils.Palaeogeography, Palaeoclimatology, Palaeoecology, 432: 1–14.

Silva, C. M. da, Cachão, M., Martinell, J. and Domènech, R., 1995.Estruturas bioerosivas como indicadores de paleolitorais rochosos. Oexemplo do Miocénico da Foz da Fonte (Sesimbra, Portugal). DadosPreliminares. Memórias, Museu e Laboratório Mineralógico e

Geológico da Universidade do Porto, 4: 133–137.Silva, C. M. da, Cachão, M., Martinell, J. and Domènech, R., 1999.

Bioerosional evidence of rocky palaeoshores in the Neogene ofPortugal. Bulletin of the Geological Society of Denmark, 45: 156–160.

two remarkable examples of portuguese neogene bioeroded … - two remarkable examples of... ·...

Documents