A facies model for an Early Aptian carbonate platform ...

Facies DOI 10.1007/s10347-012-0315-3

ORIGINAL ARTICLE

A facies model for an Early Aptian carbonate platform (Zamaia, Spain) Pedro Angel Fernańdez-Mendiola • Jone Mendicoa Sergio Hernandez • Hugh G. Owen • Joaquıń Garcıá-Monde´jar

•

Received: 15 February 2012 / Accepted: 19 June 2012 Ó Springer-Verlag 2012

Abstract The Cretaceous (Early Aptian, uppermost Bedoulian, Dufrenoyia furcata Zone) Zamaia Formation is a carbonate unit, up to 224 m thick and 1.5 km wide, which formed on a regional coastal sea bordering the continental Iberian craton. A high-resolution, facies-based, stratigraphic framework is presented using facies mapping and verticallog characterization. The depositional succession consists of a shallow estuarine facies of the Ereza Fm overlain by shallow-water rudist limestones (Zamaia Fm) building relief over positive tectonic blocks and separated by an intraplatform depression. The margins of these shallow-water rudist buildups record low-angle transitional slopes toward the adjacent surrounding basins. Syn-depositional faulting is responsible for differential subsidence and creation of highs and lows, and local emplacement of limestone olistoliths and slope breccias. Two main carbonate phases are separated by an intervening siliciclastic-carbonate estuarine episode. The platform carbonates are composed of repetitive swallowingupward cycles, commonly ending with a paleokarstic surface. Depositional systems tracts within sequences are recognized on the basis of facies patterns and are interpreted in terms of variations of relative sea level. Both Zamaia carbonate platform phases were terminated by a relative sea-level fall and karstification, immediately followed by a P. A. Fernańdez-Mendiola J. Mendicoa (&) S. Hernandez J. Garcıá-Monde´jar Dpto. Estratigrafıá y Paleontologıá, Universidad del Paı´s Vasco, Apdo 644, 48080 Bilbao, Spain e-mail: [email protected] P. A. Fernańdez-Mendiola e-mail: [email protected] H. G. Owen Department of Earth Sciences, The Natural History Museum, London, Cromwell Road, London SW7 5BD, UK

relative sea-level rise. This study refines our understanding of the paleogeography and sea-level history in the Early Cretaceous Aptian of the Basque-Cantabrian Basin. The detailed information on biostratigraphy and lithostratigraphy provides a foundation for regional to global correlations. Keywords Early Aptian Carbonate platforms Basque-Cantabrian Basin Stratigraphy Facies analysis Depositional sequences

Introduction The Early Aptian marine sediments record, globally, is characterized by turn-overs in marine floras and faunas (Caron 1985; Coccioni et al. 1992; Erba 1994; Aguado et al. 1997; Mutterlose and Bo¨ckel 1998). These changes are coeval with paleoceanographic events such as marine anoxia (Schlanger and Jenkyns 1976; Arthur et al. 1990), drowning of carbonate platforms (Schlager 1989), volcanic superplumes and intense volcanic degassing with rapid release of methane hydrates (Larson 1991), pelagic biocalcification crises (Erba 1994) and sea-level changes (Hallam 1992), all during a time of greenhouse climate (Larson 1991). An accurate timing of these complex events is needed to improve correlations between them (Erba 1994; Bischoff and Mutterlose 1998). Similarly an understanding of the characteristics of each major carbonate platform developed in the Early Aptian is a prerequisite for the reconstruction of the paleoceanographic changes reported above (e.g., Skelton and Gili 2012). The main aim of this investigation is to present a stratigraphical-sedimentological analysis of a late Early Aptian carbonate succession in the Zamaia Mountains of northern Spain, in order to construct a sedimentary model of how the platform carbonates developed and disappeared. The

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particular feature of this model is the presence of banks of rudists growing in a coastal setting, surrounded by siliciclastic sediment; this arrangement allows an evaluation of the episodic growth and demise of the shallow-water platforms. The Early Cretaceous Zamaia Fm, present to the south of Bilbao in the Bizkaia province of northern Spain (Fig. 1), is a W–E-trending rudist buildup with 200 m average thickness. Paleogeographically, the Zamaia carbonates formed on the edge of a shallow-marine ramp bordering the coastal area of the Iberian craton. This rock-based study generating a stratigraphic model will also help to refine our understanding of Aptian stratigraphy. It will provide clues to an understanding of the causes of the episodic pattern of carbonate platform growth in low paleolatitudes punctuated by periods of crisis linked with oceanic anoxic events (OAEs) (e.g., Dercourt et al. 1993, 2000; Philip et al. 1995; Skelton 2003a). These crises involved changes in platform biota, especially rudists, which are common dwellers of carbonate environments throughout the Tethys Ocean (Masse and Philip 1981; Masse 1989; Ross and Skelton 1993; Fo¨llmi et al. 1994; Scott 1995; Weissert et al. 1998; Steuber and Lo¨ser 2000; Skelton 2003b; Burla et al. 2008, among others).

Methodology Fieldwork with facies mapping was undertaken and three logs were measured to generate the model presented here.

Thin-section studies provided facies characterization. Sequences and their boundaries and maximum flooding surfaces were distinguished in the logged sections. Sequence boundaries were interpreted at significant erosional surfaces above shallowing-upward vertical successions. Maximum flooding surfaces were placed within the deepest-water facies within the sequences. In order to correlate sedimentary units from different sections, the top of the limestones was used as a datum for the cross section. High-resolution stratigraphic analyses were used to interpret the tectono-sedimentary evolution of the sequences deposited in adjacent structural blocks with characteristic subsidence rates.

Previous work The Aptian-Albian carbonates in the Basque-Cantabrian Basin have traditionally been known as the Urgonian Complex (Rat 1959). This is characterized by micritic limestone with rudists, corals, and orbitolinids, and reaches up to 7 km in thickness (Ca´mara 1997). Rat (1959) was the first author to give a brief description of the Zamaia limestones near Bilbao, establishing their parallelism with other limestones in the nearby area. These limestones replace siliciclastic deposits of the Ereza Fm and change laterally to a terrigenous facies towards the Cadagua River (Fig. 2). Garcıá-Monde´jar and Garcıá-Pascual (1982) described in greater detail the limestone outcrops of the Urgonian complex in the central area of the Basque-Cantabrian

BAY OF BISCAY

Tertiary Late Cretaceous Aptian-Albian Jurassic & Early Cretaceous Keuper (diapir) Permian & Triassic

Palaeozoic Main faults

Fig. 1 Geological map with the location of the Zamaia Mountain (w) in the central part of the Basque-Cantabrian Basin (Northern Margin of Iberia south of the Bay of Biscay)

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Facies

Bilbao Peñas Blancas

Arraiz San Roque

Eretza Zaramillo Fault

R

ive r

Zaramillo

Ordaola

Arnotegi

Seberetxe

Zamaia

ad ag ua

Galdakao

Basauri

Pagasarri

C

Borto Fault

Sodupe

Bilbao Anticline axis

Lemoa

Ganekogorta Arrigorriaga

Lower Aptian carbonate platform

Igorre

Igneous dyke

N

Fault Reverse fault

Llodio

Zeberio

Anticline Syncline

Castillo y Elejabeitia Zamaia area location

0 km

1

2

3

Villaro (Areatza)

Fig. 2 Aptian limestone outcrops to the south of Bilbao

Basin. They contributed further, describing the Zamaia Mountain outcrop as the growth of two carbonate banks, separated by a siliciclastic episode and with facies changes to marlstones and siltstones towards the flanks. They also studied other limestones in the nearby area (Ordaola, San Roque, Santa Lucıá) and concluded that they belong to the same episode dated as late Early Aptian. They proposed a stratigraphic framework with diachronism towards the margins of the carbonate banks. The following studies of EVE (1990) established a geological map of the area (1:25,000), which correlated all the Aptian carbonate banks mentioned in the previous works. More recently (Garcıá-Monde´jar et al. 2009a) studied three sections in the San Roque-Bolintxu area equivalent to the Zamaia sections, with thicknesses ranging from 57 to 220 m. The San Roque-Penãscal limestones were dated as upper Bedoulian based on the presence of Orbitolina (Mesorbitolina) parva (Douglass) and Iraqia simplex (Henson). The base and top of this unit are diachronous. Three growth stages of formation have been identified, separated by two short interruptions that show karstification and subsequent drowning, the final drowning being

widespread. Finally, sections of this age have been studied recently in the Basque-Cantabrian Basin in the Aralar Mountains (Garcıá-Monde´jar et al. 2009b), where for the first time in this basin the four classic ammonite Zones of the Early Aptian (e.g., Hancock 1991) were identified.

Geological setting Regional geological setting The Cretaceous Iberian sub-plate underwent tectonic warping and deformation to form various types of sedimentary basin. Today, the Iberian Craton is bordered to the north by a convergent margin with the Eurasian Plate, forming the fold and thrust belt of the Pyrenees. This craton periodically provided siliciclastic sediments to the Basque-Cantabrian shelf located on the northern border of Iberia. The shelf started life as an intra-cratonic rift in the Triassic and developed into a passive margin in the Cretaceous. This culminated in the active tectonic phase in the Cenozoic (Montadert et al. 1979; Le Pichon et al. 1971; Rat 1959).

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The area studied here is located in the Zamaia Mountains, near Bilbao (Bizkaia province, N Spain) (Fig. 1). Geologically it belongs to the western end of the Pyrenean mountain chain. Structurally, the Zamaia Formation stands on the northern margin of the NW–SE-trending Bilbao Anticline. The Zamaia outcrops are divided by the NW– SE-trending Zaramillo and Borto faults in two blocks: western and eastern (EVE 1990) (Fig. 2). Each block has a distinctive facies development with differences in thickness and stratigraphic development. The regional structure was mainly affected by NW–SE-trending faults parallel to the Bilbao and Villaro lineaments. The present-day structure was developed in response to interplate compressional tectonics in the Bay of Biscay-Pyrenees region. Regional paleogeography The Early Cretaceous is marked by the rifting between the Iberian and Eurasian plates. The Iberian sub-plate started to separate from Eurasia and moved towards the SE. It developed passive margins on the north, west and southeast margins of the sub-plate. The northern margin of the Iberian sub-plate faced the opening Bay of Biscay seaway, a branch between the Atlantic and Neo-Tethys oceans. This branch lay several degrees north of the Equator in subtropical paleolatitudes (30°N according to Gerdes et al. 2010) (Figs. 1, 3). Climate modeling of the Aptian indicates that the region was influenced by winds and waves from the north to southeast (Poulsen et al. 1999) (Fig. 3).

Early Cretaceous intra-shelf basins were created as a result of tectonic movements and Triassic salt migration (e.g., Garcıá-Monde´jar 1990). Rudist banks, such as those seen in the Zamaia area, were deposited on the margins of these intra-shelf basins in the Aptian, on a margin attached to a Hercynian craton to the south (the Iberian Massif) (GarcıáMonde´jar op. cit.). During the Aptian, this platform was located on the northern margin of the Tethys-Atlantic seaway (Fig. 4). During this time, carbonate platforms developed in the central Basque-Cantabrian Basin (Rat 1959). Tectonics played a key role in controlling sedimentation on this platform, leading to rapid lateral facies changes in response to differential basement subsidence. A shallow-marine facies (0–50 m deep) was deposited in these coastal settings in the Zamaia area.

Stratigraphic framework In the Zamaia area, a complete section of late Early Aptian sediments is present with a maximum thickness of 224 m. It consists predominantly of limestones with rudists alternating with and passing laterally into marlstones, siltstones and sandstones. These facies are time-equivalent to the Galdames Formation (Garcıá-Monde´jar and Garcıá-Pascual 1982), which overlies the sandstones of the Ereza Formation and underlie the marly facies of the Bilbao Formation (Fig. 5).

Early Cretaceous (Summer)

80ºN

60º North America

40º

Basque-Cantabrian Basin

Eurasia

Te thys Ocean

20º

0º South America 140ºW

120º

100º

80º

60º

African-Arabian Plate 40º

Fig. 3 Aptian wind pattern (Poulsen 1999) and location of the Basque-Cantabrian Basin (w)

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20º

0º

20º

40ºE

Facies

Basque-Cantabrian Basin

Fig. 4 Global paleoceanography during the Early Cretaceous (120 Ma) (Blakey 2004), showing the approximate location of the BasqueCantabrian Basin (w)

Ente Vasco de la Energıá (EVE 1995) divided the Lower Cretaceous shelf succession into four formations: Weald, Ereza, Galdames, and Bilbao. The Weald consists of continental fluvial–lacustrine deposits spanning the Berriasian to Barremian. The Ereza sandstones and marls and the Galdames/Zamaia limestones span the Early Aptian. The Ereza Fm is here divided into three members: (1) a lower sandy Ganekogorta Mb. with scarce ammonites, (2) a middle siltstone-black shale ammonitebearing Nocedal Mb, and (3) an upper sandy Gongeda Mb. The Ganekogorta and Nocedal sandstones display channels, cross-beds with bidirectional orientation, flaser and lenticular bedding, symmetrical ripples and Skolithos ichnofacies trace fossils, suggesting deposition in nearshore environments influenced by wave and tidal currents. The Bilbao Formation, composed of marls with ammonites, spans the Late Aptian to Early Albian. In the lower part of the Bilbao Fm, ammonites indicate the base of the Late Aptian (martiniodes Zone). The Zamaia limestones contain Palorbitolina lenticularis (Bluemenb.), Iraqia simplex (Henson), Chofatella decipiens (Schlumberger, 1904) and Orbitolina (Mesorbitolina) parva (Douglass). The ammonite species Cheloniceras (Cheloniceras) meyendorffi (D’Orbigny) has been found at the base of the limestone coeval to the Zamaia Fm in Arrigorriaga (see Fig. 2, for location). This indicates a late Early Aptian age (upper Bedoulian), more precisely the upper part of the D. furcata Zone.

Zamaia Formation stratigraphy The outcrops of the carbonate buildups in the Zamaia Mountain area are elongate towards the northwest, and are cut by NW- and W-trending Alpine faults. The Zamaia Fm is subdivided into the lower (MB-1), middle (MB-2) and upper (MB-3) Zamaia members, based on facies and geometries indicative of distinct depositional environments (Figs. 6, 7, 8). Lower member (MB-1) With a thickness of 63–70 m, this member comprises a dominant rudist-coral limestone facies in Zamaia west and east, and grades to siltstones and sandstones in the Zamaia Central A and B sections (Fig. 8). Middle member (MB-2) The middle member ranges from 14 to 30 m in thickness and is mainly composed of siltstones, marlstones and sandstones with subordinate limestones. In the west Zamaia section, it consists of 14 m of marls and marly limestones lacking shallow-water carbonate benthos and containing sponge spicules. In the east Zamaia section, the succession reaches 30 m and is made up of silty marls, marly limestones and sandstones (at the top). Two intervals of carbonate breccia occur at meters 73 and 80 (Fig. 8). The lower one contains a large clast (8 9 2 m) of coral-

123

Age

Formations and members

UPPER APTIAN

Bilbao Fm. Pagomakurre-Gallarta

TST

T- R Cycles 2nd order Hardenbol (1998)

Fig. 5 Synthetic cross section of the Lower Aptian in the central area of the BasqueCantabrian Basin. Hardenbol (1998) transgressive–regressive sequences have been defined

Minor T-R Cycles

Facies

1400

SB

Galdames Fm. Zamaia Fm.

HST

SB

Middle Mb. (Mb-2) Lower Mb. (Mb-1)

800

600

400

Ganekogorta Mb.

Lower Bedoulian

Upper Mb. (Mb-3)

1000

Ereza Fm. Nocedal Mb.

LOWER APTIAN

Upper Bedoulian

HST

Gongeda Mb.

TST

1200

Limestone Marlstone Sandstone 200

Siltstone

123

Regression

CP. Weald

BARREMIAN

Shale 100

Transgression 0m

Facies

A

Road

Zaramillo fault

Middle Member

Upper Member Limestones of the Middle member

Upper Member Middle Member

Lower Member

Zamaia Central A section SC2

Lower Member

Zamaia Central B section SC1

Zamaia West section

Road Borto fault 0m

250

500

Zamaia East section

750

Rudist limestones Carbonate platform

Mixed siliciclastic-carbonate Siltstones, marls and sandstones Coastal clastics

Marlstones Basin

SC1

Mixed siliciclastic-carbonate Marly siltstones (sandstones) Shallow-water platform basin

Gongeda Mb. Sandstones and marlstones Nearshore clastics

Sandstone beds

SC2

Area showed in Fig. 7

B

Sandstone ridge 1

Sandstone ridge 2

Fig. 6 Areal photography (a) and geological map (b) of the Zamaia area

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Facies

S

N Lo we r

(G

on

ge

da

Zamaia Fm.

Mi

Me

mb

E sa reza nd sto Fm. ne s)

er

dd

le

Up

pe

Me

mb

rM

Zamaia mine

em

er

be

r

Bil

Zaramillo Fault

ba

oF

m.

Olis

Ere

za

Fm

.

tolit

h

Qu a lan tern ds ary lid e

Limestones with corals and rudists Marlstones Siltstones and sandstones Limestone olistolith Fault Stratification lines in sandstones 0m

50

100

150

Fig. 7 Lateral view of the eastern section of the Zamaia limestones

rudist limestone embedded in truncated underlying marls. In the Zamaia central-A section, this middle member consists of 20 m of dominant rudist-coral limestones with minor marly limestones on top. Two paleokarstic surfaces are located at meters 52 and 68. Upper member (MB-3) The upper member reveals a significant thickness variation from 64 m in the west to 123 m in the east (Fig. 8). It is formed by rudist-coral lime mudstones. It grades to silty marls at various margins (Fig. 6b). Three separate rudistcoral lithosomes are respectively distinguished in the west, central and eastern sectors (Fig. 6b).

Facies analysis Based on lithological characteristics, fossils, textures, and structures, seven facies types were differentiated which represent distinct depositional environments (Table 1; Figs. 9, 10). Three major types of rudist were recognized in the Zamaia buildups. These are requieniid, polyconitid and caprinid rudists. Requieniids are forms that occur attached to the substrate and to other rudists or metazoans (Figs. 9c, 10f). They are the most abundant rudists in Zamaia and can occur anywhere within the carbonate bank facies. Polyconitid rudists are elevated forms that make up a minor component of the Zamaia limestones (Fig. 9e). They occur

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commonly on the bank tops and in shallow lagoons, forming densely packed beds and bioherms. They belong to the newly defined species Polyconites hadriani (P. W. Skelton Personal Communications; Skelton et al. 2010) (Fig. 9e). Caprinid rudists are recumbent forms and occur as a minor component among the requieniids (Fig. 9d). The rudist facies on the Zamaia platforms formed banks within accumulations of mud with skeletons lacking a rigid framework structure. Exceptionally, there occur horizons where the bioherms of rudist-coral-microbialite form boundstones (Fig. 9f). The facies described below are summarized in Table 1. Facies type 1: lime mudstones with requieniid rudists (shallow lagoon) Description: Wackestones and floatstones occur throughout the Zamaia buildup. They are massive to wavy layered beds composed of skeletal peloidal grains with large amounts of lime mud. They contain diverse assemblages of benthic foraminifera. The carbonate succession is mostly composed of lime mudstones with rudists (Fig. 9c–e, g). Subordinate taxa within this facies include branching and massive corals, gastropods, nerineids and echinoderms (Fig. 10f). At the base of the second carbonate unit (upper Member MB-3) and in the upper part of the first carbonate unit (lower Member MB-1) rudists form mound structures (Fig. 9f), in contrast to the more tabular stratiform appearance of strata in the rest of the succession.

120

140

Caprotinid

Boring

Sponge

Nerineid

Wood fragment

Orbitolinid

Monopleurid

Rudists fragment

Echinoderm

Bivalve

Ostreid

Polyconites

100

Ammonite

Branching coral

0m

Sedimentary cycle

Miliolid

Gastropod

20

40

0m

Sequence A (Part)

Massive coral

Requieniid

Silty lamina

Palaeokarst

Intraformational breccia

Sandy lamina

Marly lamina

Siltstone and finegrained sandstone

Subaerial exposure Unconformity

Wavy limestone

SB-1

Maximum Flooding Surface

80

100

Sequence B

SB-2

Subaerial exposure

Zamaia West

Marly limestone

Sandy limestone

Sandy wavy limestone

Calcareous breccia

Marl

Packstone-grainstone

Marlstone

Calcareous sandstone

Lime mudstone

W

500

1000

C4

S1

S1

C1

C1 0m

20

HST

40

TST 60

Co rre lat ive

arker

rbitoli nid m

d&o

Milioli

C2

S1

S1

S2

C1

C1

C1

Co nfo r

bed

rker bed

Polyconite rudist ma

C2 0m

20

40

60

80

100

HST

Palaeokarst

Zamaia Central B

C1

C1

C2

C4

C2 C2 C2 C2 C2 C1 C1

Deepening pulse

C1

C1

C1 C2 C1

C2 C1 C1 C1 C1 C2

Deepening pulse

Zamaia Central A

m

ity

0m

20

40

60

80

100

120

140

160

180

200

220

1500

Zamaia East

C5

C1

C1

C7

C7

C1

C2

C1

C1

C1

C1 C1

C1

1,2 m

C5

C3

C1

C3

C6

C1

C1

C1

C1

C1

5m

LST

2m

LOWER MEMBER (MB-1)

MIDDLE MEMBER (MB-2)

UPPER MEMBER (MB-3)

E

Facies

Fig. 8 Correlation between the western, central, and eastern sections, with their respective sedimentary cycles

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Facies

Micritic limestone with rudists

Micritic limestone with corals

Orbitolinidmiliolid packgrainstone

Marlstone

Limestone breccia

Calcareous sandstone to sandy limestone

Calcareous siltstone

Paleokarst

No.

1

123

2

3

4

5

6

7

8

Terrigenous-filled erosional holes (hollows) in limestone

Locally, oyster beds occur

Silt with abundant mica

Fine-grained sandstone, with abundant micaceous grains to limestones with silt or fine-grained sand

Olistoliths up to 8 m wide 9 1.9 m long

Most outcrops covered by vegetation

Sharp lateral transition from the micritic limestone

Silt and fine-grained sand. Some mica

Wackestone to floatstone. One single bed 0.5 to 2 m thickness

Wackestone with corals (up to 30 cm)

Locally, more silty, with bioclastic debris

Wackestone to packstone with rudists in life position

Description

Table 1 Facies characteristics and depositional environment

Pitted surface

Bi-directional cross lamination and stratification

Erosive surface at the top of underlying marlstone

Mounds, borings in rudists

Mounds, borings in rudists

Sedimentary structures

Micritic

Micritic

Matrix (in rudstone/ floatstone)

None

Ostreids

Very rare coral, rudist, and echinoderms

Ostreids

Facies of blocks same as micritic limestone with rudists

Some small branching corals, terebratulids and echinoderms

Unfossiliferous

Orbitolinids, miliolids, rudists and coral fragments, gastropods and echinoderms

Gastropods, echinoderms, bivalves, rudists and calcareous sponges

Branching and massive corals, gastropods, nerineids and echinoderms Dominant branching and massive corals

Dominant requieniids, local caprotinids, monopleurids, and polyconitids

Fossils

High

Moderate

Moderate/ High

High

Moderate

Moderate

Low

Low/ moderate

Energy

–

15–20 m

10–15 m

40 m

15–40 m

15–30 m

10–15 m

5–10 m

Water depth

Coastal plain exposed

Estuary with tidal influence

Coastal-marine with terrigenous input and tidal influence

Basinal slope

(Intra-platform basin?)

Basin

Open-marine carbonate platform

Shallow-water carbonate platform

Shallow-water carbonate platform

Environment

Fig. 9l–n

Figs. 9h, i, k, 10c, d, g

Fig. 9o, p

Figs. 9b, f, n–p, 10h

Figs.9g, 10d, e

Figs. 9h, j, 10a, i

Figs. 9c– g, 10f

Figures

Facies

Facies

Interpretation: These wackestone-packstone to floatstone with dominant requieniid rudists were formed in a shallow lagoonal environment in shallow photic water depths (e.g., Masse and Philip 1981). The fine grain-size indicates low-energy conditions. Facies type 2: lime mudstones with corals (open-marine lagoon) Description: Coral floatstone occurs at the base and top of the lower carbonate unit (lower Member MB-1) as a 5-mthick unit that has both platy and branching corals and some massive head corals, in a wackestone-mudstone matrix (Fig. 9h, j). The corals range from 3 to 50 cm in diameter. Gastropods, echinoderms, bivalves and calcareous sponges are also present as subordinate fossils (Fig. 10a, i). Marly laminae are locally more abundant than in the requieniid facies, which gives these limestones a slightly wavy character. In the upper part of the first carbonate unit (lower member (MB-1), massive coral-head assemblages form carbonate mounds (Fig. 9h). Corals locally occur independently of the rudists, otherwise both are found together in the same biotope. Interpretation: Coral lime mudstones usually form in slightly deeper water than the rudist facies, in relatively low-energy lagoonal settings or foreslopes flanking rudist buildups (e.g., Masse 1992; Gili et al. 1995; Johnson and Kaufman 2001; Scott 1990; Skelton and Gili 2012). The coral facies formed along the flanks of rudist buildups but in slightly deeper waters. Facies type 3: orbitolinid-miliolid pack-grainstones (shallow lagoon) These packstones and grainstones occur in the middle part of the upper carbonate Unit (MB-3) interbedded with requieniid lime mudstones (Fig. 9g). The thickness of the miliolid-orbitolinid facies varies from 0.5 m in the western section to 2 m in the eastern one. They are composed of fine sand-sized, moderately sorted, skeletal miliolid and orbitolinid grains with variable amounts of mud (Fig. 10e). Fragments of rudists, corals, gastropods and echinoderms are minor components. P. lenticularis, I. simplex, O. (M.) parva and Ch. decipiens indicate a latest Early Aptian age (Fig. 10d, e) (e.g., Masse 1995; Garcıá-Monde´jar et al. 2009a, b; Skelton and Gili 2012). Interpretation: Orbitolinid-miliolid grainstones formed in a moderate to high energy environment in a platform interior (Scott 1981; Husinec et al. 2000; Hartshorne 1989). Facies type 4: marlstones (Intra-shelf basin) Marlstones have been found in the Zamaia area, particularly in the intermediate unit (MB-2) and in the lateral

facies transition between both limestone units (MB-1 and MB-2). The exposures are rather scarce due to the vegetation cover in the area (Figs. 9b, n–p, 10h). These marlstones are silty, show some bioturbation and contain benthic forams, rare ostreids, brachiopods, echinoderms and bivalves. Interpretation: This facies was deposited in an intrashelf basin adjacent to rudist buildups with fine-grained terrigenous input (Fig. 9a). Facies type 5: limestone breccia (slope) This facies has been found only at two levels within the intermediate marly unit of the eastern section (MB-2). It is made up of large olistoliths up to 1.9 m high and 8 m long of lime mudstone with requieniids and corals, within the marlstones (Fig. 9p). The lower contact with the underlying marlstone of both levels is a structured surface. Interpretation: This facies was deposited on a carbonate slope adjacent to a carbonate margin as a major debris-flow deposit. Facies type 6, 7: calcareous siltstones and sandstones (estuarine basin) Fine-grained siltstones and sandstones occur in the intermediate mixed carbonate-siliciclastic unit of the Zamaia Formation (MB-2) (Fig. 9a). These are generally quite micaceous and contain ostreids (Figs. 9k, 10b). They show cross-bedding and ripple-lamination (Fig. 9k), and are commonly bioturbated. The measured directions of the structures point to asymmetric bidirectional paleocurrents, in which the westward current (N257°E) is dominant. Locally, sandy limestones with fragments of corals, rudists and echinoderms occur at the margins of the Zamaia upper member, as lateral transitions of platform rudist limestones (Figs. 9a, 10d, g). Interpretation: These terrigenous facies were formed in narrow seaways between carbonate banks. The seaways were filled with land-derived sediments brought to the seashore and transported by waves and tidal wave currents in coastal areas. In the geological record, there are various types of mixed siliciclastic-carbonate cycles. Sea-level changes and availability of terrigenous material are the major controls (see Mount 1984; Doyle and Roberts 1988; Tucker 2003). Mixed lithology cycles are more typical of icehouse periods. At these times of high-amplitude sea-level falls, terrigenous debris is supplied in abundance to shelves and basins, and with successive sea-level rises and flooding of coastal plains, carbonates are extensively deposited (Tucker 2003). In areas with locally active vertical tectonism and tropical latitudes similar cycles can be formed

123

Facies

W

E Middle member Middle member sandstone sandstone crest-2 (SC2) crest-1 (SC2) Upper Member

Marly laminae Lower Member

Middle Member limestone

A

Limestone

Sandstone

Marlstone-Siltstone Marsltone

B

D

C

E

10 cm

(Tucker op. cit.). Terrigenous sediments usually have a detrimental effect on carbonate production, affecting the carbonate-secreting organisms in several ways. Turbidity by fine-clastic sediment reduces light penetration and affects feeding mechanisms. Sudden influxes of mud can

123

F bury organisms and an increase in nutrient levels accompanying terrigenous input can lead to the flourishing of eutrophic communities at the expense of metazoan reefs (e.g., Doyle and Roberts 1988; Tucker 2003; Flu¨gel 2010). In the Mahakam delta of Indonesia coral patch reefs are

Facies b Fig. 9 a Zamaia 1 and Zamaia 2 limestone units separated by an intervening unit (poorly exposed) of siliciclastic sediments. b Shallowing-upward cycle: marlstone overlain by coral limestone. Upper Zamaia Member (eastern section, 125–135 m). c Rudist (requienid) lime mudstone. Zamaia Lower Member (eastern section). d Caprinidrequienid wackestone. e Polyconitid floatstone. Zamaia Upper Member (eastern section, 209 m). f Requienid carbonate mounds, at the base of Zamaia Upper Member. Limestone breccias are intercalated in the marlstone succession below (eastern section). g Orbitolinid-miliolid packstone-grainstone (wavy fabric) overlain by requieniid wackestone. Upper Zamaia member (eastern section, 160 m). h Deepening-upwards unit on top of Zamaia 1, punctuated by paleokarst (A. Rudist-coral limestone; B. Coral limestone; C. Oyster beds). Top of Zamaia Lower Member. i Oyster facies. Top of Zamaia Lower Member. j Coral limestone. Base of Zamaia Upper Member (eastern section, 100 m). k Calcareous siltstonesandstone, with bimodal cross-bedding (A) and cross-lamination (B). Zamaia Lower Member (central section, 23 m). l Paleokarst cavities filled with quartz sandstone (bed 2.5 m thick). Top of Zamaia Lower Member (western section, 58.5 m). m Sandstone filling dissolution cavities, forming parallel laminae (bed 3 m thick). Zamaia Middle Member (central section, 50 m). n Paleokarst (Pk) on top of the Zamaia Upper Member limestones (eastern section, 223 m). o Marlstone (M) with debris bed (DB), composed of broken rudist and coral debris. Zamaia Lower Member (eastern section, 64 m). p Outcrop photo (A) and drawing (B) of the limestone olistolith, up to 8 m long, within the Zamaia Middle Member marlstones (eastern section, 73 m)

able to grow surrounded by terrigenous mud (Wilson and Lokier 2002). The reefs form in shallow-water (\10 m) since light penetration is reduced by the turbidity from terrigenous mud. Ancient reefs growing on fan deltas have also been described in the Tertiary of Spain (Santisteban and Taberner 1988; Braga et al. 1990). Several oyster beds occur within both sandstone and limestone facies (Figs. 9i, 10c). At the base of the western section, they are found within the first limestone facies, just above the Gongeda sandstones and siltstones. Another oyster-rich level has been found in the transition from lime mudstones with corals and rudists to sandstones of the intermediate unit (MB-2). Finally, oyster beds have also been found in some levels of this intermediate unit. The oyster facies tend to occur associated with environments of intermittent water turbidity. The mixed carbonate-terrigenous sedimentation, the bimodal paleocurrents and the turbid water associated oysters likely suggest estuarinetype environments (oysters blanket the estuary floors where they use their foot secretions for attachment). Oysters tend to flourish in the brackish waters of estuaries (Nichols et al. 1991; Hudson 1963; Pufahl and James 2006). Facies type 8: paleokarst facies Irregular, thin sandy beds occur within the Zamaia lower member MB-1. In the eastern section, one horizon appears near the bottom of the section and several other levels occur with horizontal sandy laminae (2 cm) in the last few meters of MB-1. In the western section several horizons

occur with both horizontal and vertical irregular cavities filled with fine-grained sandstones and siltstones (Fig. 9l). Several terrigenous-filled irregular surfaces occur very close together and the fill of sand reaches up to 20 cm. There is also a similar facies within limestones of the middle member MB-2 (Fig. 9m). At the top of Zamaia upper member limestone MB-3 in the eastern section, there is an irregular topography with topographic depressions, erosional surfaces up to 0.5 m deep, filled with marlstone of the overlying unit; these are interpreted as karstic dissolution surfaces (meter-scale dissolution holes and cavities) (Fig. 9n). Sandy horizontal and vertical laminae have been found in the lime mudstones down to 8 m. Intraclast breccias and irregular topography are also found in the same horizon of the western section.

Sediment cyclicity Cycles ranging in scale from 0.5 to 10 m defined by marine-flooding surfaces are widely recognized in the Zamaia Fm outcrops, and can be referred to as parasequences as defined by Van Wagoner et al. (1988) and redefined by Spence and Tucker (2007). Ten types of cycle are identified (Fig. 11; Table 2): S1 and S2, and C1 to C8 (Figs. 8, 9). S1 and S2 are dominantly siliciclastic or mixed carbonate-siliciclastic and C1 to C8 are dominantly carbonate. All cycles but one exhibit a shallowing-upward facies pattern; the C8-type cycle has a deepening-upward trend. S1 cycles are composed of two facies: a lower siltstone succeeded by an upper sandy limestone with quartz sand grains and a lime mud matrix with scattered ostreids. These upward-increasing energy cycles are broadly regressive in nature and are interpreted as shallowing upward, but they did not aggrade into intertidal-supratidal facies. In this sense, they are similar to the keep-up cycles of Soreghan and Dickinson (1994). Two S1-type cycles (average thickness 20 m) are recognized in the Zamaia Central section (Fig. 8). There is one S2 type cycle, 8 m thick, and this is composed of siltstones passing up into coral limestones with bedding-parallel quartz sand laminae. A paleokarstic surface caps the unit. This cycle, occurring in the Zamaia Central A section (Fig. 8), is regressive and shows upward increasing energy and diversity of organisms. C1 cycle type is the most common of all cycles (32 cycles, average thickness 10 m). It consists of benthic foraminiferal wavy-bedded limestones with discontinuous mm-thin marl laminae, and no rudists; this is succeeded by rudist wackestones with requieniids. The cycles are regressive and shallow up within the subtidal domain. Similar shallowing-upward cycles are described in

123

Facies

C

Requieniid wackestone

B A

Be

dd

G

ing

pla

ne

Orbitolinid-miliolid packstone

H

5 cm

I

K.a

J

K.b

Fig. 9 continued

Go´mez-Pe´rez et al. (1998). Requieniid rudist wackestones indicate stable seafloor conditions, weak bottom currents, and low sedimentation rates (Ross and Skelton 1993). C2 cycles are the second most common cycle (12 cycles). Average cycle thickness is 4 m. It begins with coral limestones with argillaceous laminae succeeded by rudist

123

wackestones (requieniid dominated), ending with a paleokarstic surface, locally filled with sandstones and siltstones. These cycles are regressive, building up to sea-level, and culminate in subaerial exposure. However, they do not record the final phase of high-energy waters above wavebase, since sediments of the shoreface are not preserved.

Facies

Sandstone fill

50 cm

M

L

M DB Pk cavity filling

M

O

N

Onlapping marls

Minor limestone breccias

Olistolith 2m

P.a

Limestone olistolith

Marl

Wavy limestone

Dark marl

Coral

P.b

Fig. 9 continued

Cycle C3 is a variation of cycles C1 and C2, with a basal marlstone facies succeeded by a coral wackestone with argillaceous laminae and finally requieniid rudist wackestone. Cycle 4 is a variation of cycles S1 and C2. It starts with a marlstone basal member followed by rudist requieniid limestones with paleokarst at the top. Sandstone-filled

pipes and bedding-plane parallel sandstone laminae are present. Cycle 5 is a variation of cycles C3 and S1, with a basal marlstone unit succeeded by a calcareous sandstone facies, overlain in turn by coral limestones with wavy argillaceous laminae.

123

Facies

C

Oy

Oy C 3 mm

3 mm

A

3 mm

B

C

M O Bra Fr

O

Br O

O O

E 3 mm

D

3 mm

E

3 mm

F

Fig. 10 a Coral (C) packstone. Zamaia Middle Member (eastern section). b Chaetetid pack-grainstone with broken rudist shells. Zamaia Upper Member (western section). c Oyster (Oy) sandy packgrainstone. Zamaia Lower Member (western section). d Sandy packgrainstone with bryozoans (Br) and orbitolinids (o). Zamaia Middle Member (eastern section). e Orbitolinid (O)–miliolid (M) packstone. Zamaia Upper Member (eastern section, 160 m). f Rudist packstone with abundant angular shell fragments (Fr), brachiopod (Bra) and

echinoid spine (E). Zamia Lower Member (eastern section, 62.7 m). g Calcareous sandstone. Top of Zamaia Upper Member (eastern section, 223 m). h Marlstone. Bilbao Fm. i Coral lime mudstone (Cm) with calcareous siltstone (Csi), karstic fills and bryozoans (Br). Zamaia Upper Member (eastern section, 213 m). j Rudist-coral wackestone breccia (lithoclast, Li) in a sandy pack-grainstone matrix with bryozoans and oysters (Oy). Top of Zamaia Lower Member (eastern section)

All three cycles C3, C4, and C5 suggest shallowing up and cycle C4 culminates with subaerial exposure.

Cycle C6 starts with packstone of miliolids, orbitolinids, brachiopods and branching corals and is succeeded by requieniid rudist and coral wackestone. This vertical

123

Facies Fig. 10 continued

3 mm

G

3 mm

Csi

H

Li

Cm Br

Br

Oy Br 3 mm

evolution has been interpreted elsewhere as a shallowingupward trend (e.g., Go´mez-Pe´rez et al. 1998). C7 starts with marlstone succeeded by limestone debris with olistoliths, and C8 starts with karstified requieniid wackestone overlain by marlstone. This is the only cycle that suggests a deepening-upward trend and is recorded in

I

3 mm

J

the middle and upper part of the Zamaia section as two distinct deepening episodes (Fig. 8). Although individual cycles may not be traceable from section to section (Fig. 8), there is a suggestion that trends in cycle thickness are broadly correlatable. Cycles tend to become thicker from west to east (Fig. 8), suggesting a

123

Facies

2m

S1

Siltstone

S2 2m

Sandy (quartz) limestone with ostreids

Karst surface Coral limestone (wackestone) with sandy laminae

Rudist limestone (wackestone)

C2

2m

Marlstone

Limestone olistolith 2m

C7

C6 2m

2m

Coral limestones (wackestone) with marly laminae Marl Coral limestone (wackestone) with marly laminae Calcareous sandstone

C5

C4 2m

Rudist limestone (wackestone)

C3

2m

Wavy limestone with thin millimetric marl laminae (wackestone)

Marlstone

Karst surface (quartz sand) Rudist limestone (wackestone) Coral limestone (wackestone) with marly laminae

Karst surface Rudist limestone (wackestone) with sandy laminae at top (locally rare corals) Marlstone

Rudist and coral limestone (wackestone) Packstone: miliolids, orbitolinids, gastropods, branching corals

Marlstone

C8 2m

2m

C1

Siltstone

Karstified limestone

Fig. 11 Small-scale cycle-types recognized in the Aptian of the Zamaia sections

higher rate of accommodation space created in this direction. This trend is also expressed by the greater number of cycles that end with subaerial exposure in the western Zamaia, interpreted as an area of relatively lower subsidence. Shallowing-upward cycles are the basic building block of the Zamaia Formation, followed by a flooding surface indicative of the beginning of the next parasequence (Fig. 8). The lowermost part of each cycle has marlstone, argillaceous limestone, wavy limestone, siltstone or coral limestone with marly laminae. The corresponding upper parts are more pure carbonate, encompassing wackestone with rudists. This upper part of the cycle is locally (cycles C2 and C4) capped by a subaerial exposure karstic surface. The vertical evolution suggests decreasing influence of terrigenous mud and silt. Each cycle represents deposition in progressively shallower water as sediments build up to sea-surface level. Stratigraphic sequences on platforms where carbonates have been deposited in progressively shallower water are

123

common. These sequences develop where the rate of carbonate deposition exceeds the rate at which the receiving basin sinks, so that the sediment surface repeatedly rises towards the water surface (James 1979; Wilson 1975; Anderson and Goodwin 1980). The accumulation of sets of shallowing-upward cycles requires repeated local transgressions. The cause of the transgressions may be tectonic activity or eustatic sea-level changes resulting from glaciation or autogenic processes such as tidal-flat progradation or tidal-island migration (see recent reviews in Bosence et al. 2009 and Tucker and Garland 2010). Cyclic sedimentation in the Zamaia Formation was most likely affected by vertical tectonic movements during deposition (or intermittent subsidence), in relation with the North Iberian rifted continental margin. There is much evidence that tectonic movements modified cyclic signatures, and that differential subsidence on fault blocks gave rise to condensed sequences; tectonism clearly influenced Aptian platform development (Garcıá-Monde´jar 1990).

Facies Table 2 Facies cycle types and interpretation Cycle

Type

Average thickness (m)

No cycles

Facies association Lower

Upper

Vertical tendency

Interpretation

S1

Mixed carbonatesiliciclastic

20

2

Siltstone

Sandy limestone

Shallowing without subaerial exposure

Regressive

S2

Mixed carbonatesiliciclastic Carbonatedominated

8

1

Siltstone

Coral limestone (quartz sand laminae)

Shallowing ending with karstification

Regressive

10

32

Wavy-bedded limestone (mm marl laminae)

Rudist wackestone


Regressive

Coral limestone (mm marl laminae)

Rudist wackestone


Regressive


Regressive


Regressive

Coral limestone (argillaceous laminae)


Regressive

C1

C2

Carbonatedominated

4

12

C3

Carbonatedominated

20

2

Marlstone

C4

Carbonatedominated

2

Marlstone

C5

Carbonatedominated

24

2

Marlstone

C6

Carbonatedominated

3

1

Miliolidorbitolinid packstone

Rudist-coral wackestone


Regressive

C7

Carbonatedominated

6

1

Marlstone

Limestone olistolith (rudistcoral wackestone)

Shallowing from intraplatform basin to foreslope

Regressive

C8

Carbonatedominated

6

2

Rudist wackestone topped by paleokarst

Marlstone

Deepening after karstification

Transgressive

5.5

Coral wackestone (mm marl laminae)

Rudist wackestone

Rudist wackestone (paleokarst with quartz sand filling) Calcareous sandstone

Stratigraphic model Field mapping and careful stratigraphic correlation of sections presented in Figs. 6 and 8 provide the basis for the depositional model of Fig. 12. In this model local sequence boundaries are identified by evidence of exposure or unconformity development. Maximum flooding surfaces were identified by the abrupt onset of a fine-grained, lowenergy, deeper-water marly facies. Syn-sedimentary topography on the Zamaia platform resulted in a differentiation of facies, with elevated rudist biotopes and marginal gentle slopes into adjacent basins. The sequence stratigraphic analysis provided a way to reconstruct the evolution of the Zamaia platform. Two main sequences (the lower one incomplete) have been deduced (Fig. 8). Several drastic vertical and lateral facies changes represent rapid lateral shifts in depositional environments. The Zamaia lower member MB-1 has limestones capped by a significant paleokarstic surface, which marks the

temporary demise of the initial phase of carbonate platform growth in the upper part of the Dufrenoyia furcata Zone (Cheloniceras meyendorffi Subzone). The limestones of this member constitute the upper part of Sequence A (Fig. 8) and are interpreted as highstand deposits. The lower part of this sequence, encompassing the Gongeda sandstones, is not the object of the present study, but preliminary data point to a transgressive systems tract below the Gongeda sandstones, based on the occurrence of ammonite layers related to marine flooding episodes. The Zamaia lower Member developed in two separate locations (Figs. 13, 14) and contracted in area as the buildups grew vertically. The Zamaia middle member (MB-2) represents a renewed episode of siliciclastic input to the basin linked to a general deepening phase. MB-2 is subdivided into two distinct packages. Package-1 forms a wedge-shaped body onlapping a slightly inclined surface, and consists of marlmarly limestone, sandy limestone and debris-flow deposits with limestone olistoliths, interpreted as the lowstand systems tract of the Zamaia Sequence B (Figs. 8, 12) (lowstand

123

Facies LATE APTIAN

EARLY APTIAN furcata Zone (p.p.)

BILBAO Fm.

TST

LST

SB

H ST

HST EREZA Fm. GONGEDA MEMBER

ZAMAIA Fm. MIDDLE MEMBER

UPPER MEMBER

LOWER MEMBER

500

Borto Fault

Clinoforms

Palaeokarst

Oyster beds Sandy limestone



Marlstones

Carbonate mounds

Missing outcrop

1000

UNITS

TST

Datum

E

SYSTEM TRACTS

SB

furcata Zone (meyendorffi Subzone)

1500

AGE

Zaramillo Fault

Fig. 12 Diagram showing the correlation of the three studied sections (west, central, and east) and their spatial and temporal distribution

0m

100 0m

Missing outcrop

Limestone breccia

Orbitolinid-miliolid calcarenite

Rudist micritic limestone

50

100

150

200

W

Coral limestone

Mixed siliciclastic-carbonate facies association

123

Facies

E

10th Stage

W

Platform drowning stage Earliest Late Aptian widespread drowning in the area.

Cadagua

Borto Fault Zaramillo Fault

9th Stage

Platform karstification stage End of carbonate platform development in the Zamaia area.

Cadagua


8th Stage

Carbonate platform and intra-platform trough stage New intra-platform trough development in the eastern side. Thicker limestone sequences towards the East, related to maintained tilting.

Cadagua

Borto Fault

7th Stage

Zaramillo Fault Carbonate mound stage Widespread carbonate platform development, except in the Cadagua and Borto seaways. Eastward tilting continues.

Cadagua

Borto Fault

6th Stage

Zaramillo Fault Isolated platform stage Marginal carbonate central platform growth. Continuing eastward tilting and dominant marlstone and limestone deposition.

Cadagua

Borto Fault

5th Stage

Zaramillo Fault Limestone breccia stage Increasing eastward tilting and deposition of olistoliths on the newly formed eastern slope.

Cadagua


4th Stage

Karstification stage End of Zamaia 1 limestones due to subaereal exposure.

Zaramillo Fault

Borto Fault

Cadagua 3rd Stage

Two domain carbonate platform growth stage Eastward tilting: increasing subsidence towards the east. Progressive narrowing-upwards of platforms and intervening estuarine facies in central and marginal seaways. Multiple karst surfaces at successive horizons, filled with estuarine sandstones (more abundant in W-platform)

Zaramillo Fault

Borto Fault Terrigenous seaway

2nd Stage

Cadagua

Early carbonate platform development stage Two carbonate banks with narrow intervening seaways in the Borto-Zaramillo fault-line zones. (Lower Aptian - furcata)

Borto Fault

Zaramillo Fault

1rst Stage

Cadagua

Widespread estuarine stage Top Ereza Fm. (Lower Aptian - Upper Bedoulian)

Cadagua Micritic limestone


Paleokarst

Carbonate Mound

Marlstone


Limestone breccia

Oyster Beds

Fig. 13 Diagram showing the main stages in the development of the three Zamaia Members, as well as the final part of the Ereza Fm (Gongeda Fm), and the initial part of the Bilbao Fm marlstone, just above the Zamaia Fm

123

Facies W N S E

Terrigenous passageway

Rudist limestone: Carbonate banks

A Lower Member formation stage Basin

Mixed siliciclastic-carbonate: Estuarine Slope Basin Olistolith Siltstones, sandstones and marlstones Mixed siliciclastic-carbonate: Estuarine

Borto Fault

Ea

Requienid

stw ard Tilt in

B Middle Member

g

formation stage

Main palaeocurrent direction

Zaramillo Fault

Terrigenous passageways

Borto Fault

Ea

stw ard Tilt in

C Upper Member

g

formation stage Zaramillo Fault

Fig. 14 3-D reconstruction for the three main stages of the Zamaia platform. Each stage corresponds to the formation of one of the members: A. Lower Member; B. Middle Member; C. Upper Member

wedge sensu Van Wagoner et al. (1988), or forced regressive wedge sensu Hunt and Tucker 1992; Catuneanu et al. 2009, 2011). Package-2 consists of marl and siltstonesandstone (20 m thick) interpreted as the transgressive systems tracts of Sequence B (Fig. 8). An isolated carbonate platform developed in the central area within this tract. The Zamaia upper member (MB-3) represents the succeeding highstand systems tract of sequence B (Fig. 8), in turn capped by an erosional unconformity interpreted as a sequence boundary (SB-2). This boundary reflects subaerial

123

exposure and paleokarst development causing the final demise of the Lower Aptian carbonate platform. The Zamaia upper member (Fig. 12 MB-3 stage) developed in three separate areas forming three different banks. Each of these banks displays a narrowing-upward trend with a progressive restriction in the area of the buildups. The thicker limestones towards the east are a reflection of synsedimentary differential subsidence (Fig. 12). The depositional history of the Zamaia buildups is summarized in Fig. 13. In stage 1, Early Aptian (Upper

Facies

Bedoulian, probably lower furcata Zone), terrigenous sedimentation was dominant in a coastal area subject to wave and tidal influence in a large estuarine embayment (Stage 1, Fig. 13). Following a marine transgression, carbonate platform sedimentation ensued. Two rudist carbonate banks developed (Zaramillo and Cadagua) with a narrow sea-way between them marked by further terrigenous sedimentation (the Borto passage (Stage 2, Fig. 13). The carbonate banks continued to grow cyclically (Stage 3, Fig. 13). The western bank was subjected to several phases of subaerial exposure. As a consequence, carbonate production stopped abruptly until a new transgression allowed it to start up again and reach sea-level, producing a shallowing-upward facies trend. The eastern carbonate bank developed fewer paleokarstic surfaces and this is interpreted as a result of a more continuous pattern of subsidence. This is in accordance with the upward narrowing and areal restriction of the eastern carbonate bank (Stage 3, Fig. 13). In stage 4, the Zamaia limestones of the lower member stopped growing as a consequence of subaerial exposure and both eastern and western areas were karstified. A sudden pulse of tilting towards the east affected the area, so that the western part remained exposed whereas the eastern part flooded. Terrigenous sandy sediments filled the karstic cavities on the elevated block and the interbank eastern areas, and limestone debris with olistoliths slumped down the low-angle slope just created (Stage 5, Fig. 13). A transgressive phase with marl deposition then invaded the whole area (Stage 6, Fig. 13). In the central area only, an isolated platform developed in a slightly less subsident area, probably linked to early movement on the Borto fault. The partial cessation of terrigenous sedimentation permitted the commencement of the second widespread carbonate phase (Zamaia upper Member), except in the central, perhaps more tectonically subsiding zone of Borto. Rudist carbonate mound development is widespread suggesting that the former tilted topography had been compensated by sedimentation (Stage 7, Fig. 13). The two carbonate banks continued to grow upward, while narrowing in area. The eastern bank is subdivided in two sub-banks separated by a narrow passageway of terrigenous sediment. The eastern bank grew thicker than its western counterpart, suggesting that subsidence was significantly stronger in the eastern block (Stage 8, Fig. 13). At the top of the Early Aptian, a major phase of karstification ended carbonate platform development in the Zamaia area (Stage 9, Fig. 13). A subsequent relative sealevel rise resulted in widespread flooding of this Early Aptian carbonate platform at the beginning of the Late Aptian (Stage 10, Fig. 13). Therefore there is not much of a time gap, as uppermost furcata Zone ammonites are succeeded by martinioides Zone ammonites.

Syn-depositional tectonic activity The relatively uniform thickness of the lower part of the Zamaia Formation across the region suggests approximately constant subsidence rates. Thereafter, the thickness of the sequences and their facies distribution suggest syndepositional tectonic activity. As a result of this, the area to the east of the Borto alignment underwent more extensive down-warping than the western area. The intervening Borto fault is the boundary between these two blocks (Figs. 6, 12). In addition, the overall wedge-shaped geometry of the depositional sequences, thinning from east to west in Fig. 12, points to early movement on the Zaramillo fault. Similar examples of lithosome thinning away from tectonic alignments and depositional facies changes across faults are reported in the Aptian of the BasqueCantabrian Basin (e.g., Garcıá-Monde´jar et al. 2009b), in the Albian of the Basque-Cantabrian Basin (e.g., GarcıáMonde´jar and Fernańdez-Mendiola 1993) and elsewhere (e.g., Al-Ghamdi and Read 2010; Burchette 1988; Williams et al. 2011; Dorobek 1995, 2008; Chen et al. 2001; Ruiz-Ortiz et al. 2004).

Paleoclimate and eustasy The late Early Aptian was a period characterized by warm climates and there is a record of latest Bedoulian thermal instability, with several phases of cooling as in the Dufrenoyia furcata ammonite Zone (Kuhnt et al. 1998; Peropadre et al. 2011; Skelton and Gili 2012). The absence of ooids and evaporites in the carbonate-dominated Zamaia Fm, the abundance of siliciclastic deposits (marl, siltstone, and sandstone) in the adjacent interbuildup areas and the presence of paleokarst surfaces indicate a humid climate during deposition. The Cretaceous period has long been considered a warm, greenhouse climate. However, several studies favor a Cretaceous with intervals of global cooling (Frakes et al. 1995; Johnson and Kaufman 1996; Frakes 1999; Stoll and Schrag 2000). Very cold conditions affected Australia and high latitude regions in the Aptian, with winter freezing of lakes and some glacier development (Kemper 1987; De Lurio and Frakes 1999; Alley and Frakes 2003; Price and Nunn 2010). High-frequency, moderateamplitude sea-level changes (tens of meters) driven by Milankovich rhythms, have been recognized in Shu’aiba sequences in the Middle East in a period with some ice at the poles (Read 1998). Ro¨hl and Ogg (1998) also interpreted high-frequency sea-level changes based on sequence stratigraphy of the Pacific Ocean guyots. The problem with Pacific guyots is that they are tectonically active and this could have also played a significant role in the sedimentation patterns. Six sea-level fall events are placed

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respectively by Ro¨hl and Ogg (op. cit.) in the Early Aptian at 121, 120.5, 119.8, 119.5, 118.9 and 118.1 Ma. The first one corresponds to the Barremian-Aptian boundary and the last one to the Early/Late Aptian boundary and this could be correlated to the top of Zamaia Formation and likely to part of Shu’aiba Formation, too (Immenhauser et al. 2001; Gre´selle and Pittet 2005; Granier et al. 2011; Granier and Busnardo 2012; Rameil et al. 2012). Ogg and Ogg (2006) in the Early Cretaceous revised time-scale identified four global sea-level falls in the Early Aptian at 125.0, 124.6, 124.0 and 121.0 Ma. The first sequence boundary at 125.0 Ma corresponds to the Barremian-Aptian boundary and the 121.0 Ma sequence boundary corresponds to the Ap4 at the Bedoulian-Gargasian boundary (Early to Middle Aptian boundary). This last sequence boundary would be represented by the topmost Zamaia paleokarst surface immediately followed by a significant transgression at the furcata-martinioides boundary. Granier and Busnardo (2012) also recognized a Shu’aibaian maximum flooding surface at the Bedoulian-Gargasian boundary, which is in agreement with the top Zamaia flooding event.

Aptian environmental change and carbonate platform development: the Zamaia significance The Aptian stage was a time of significant environmental changes. They include: (a) plume-related volcanism, perturbations of the global carbon cycle with a global negative excursion of d13C possibly enhanced by massive release of methane (Jahren et al. 2001; Beerling et al. 2002; Jenkyns 2003; Renard et al. 2009), (b) a major oceanic anoxic event 1a (OAE 1a) (Mehay et al. 2009; Tejada et al. 2009 Fo¨llmi 2012), (c) pelagic biocalcification crises (Erba 1994; Erba et al. 2010), (d) episodic growth and demise of carbonate platforms, with turn-over of shallow-marine biotas (Masse 1989; Skelton 2003a, b) and (e) extreme climatic fluctuations (Hay 1995; De Lurio and Frakes 1999; Erbacher et al. 1996; Mutterlose and Bo¨ckel 1998; Premoli Silva and Sliter 1999; Larson 1991; Larson and Erba 1999; Hesselbo et al. 2000; Kemper 1987, 1995; Weissert 2000; Jahren et al. 2001; Berner 1991; Haq et al. 1988; Hardenbol et al. 1998; Ruffell and Worden 2000; Steuber and Rauch 2005; Dumitrescu et al. 2006; Ando et al. 2008). Within the Aptian, a faunal extinction event is dated to ca. 116 or 117 million years ago, termed the mid-Aptian extinction event by Masse (1989). It is classified as a minor extinction event and is most significantly detected among marine rather than terrestrial faunas. Nonetheless, the Aptian Extinction Event is an episode of importance, and deserves a higher status among other minor events (Masse 1989). The Aptian event may have been causally connected with the

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Rahjamal Traps volcanic episode in the Bengal region of India, associated with Kerguelen ‘‘hot spot’’ volcanic activity. To establish the nature of the interactions of the processes involved in the environmental changes of the Early Aptian, a detailed temporal and spatial knowledge of the pattern of change is required. Aptian carbonate platforms are extensive in the Tethyan subtropics and respond to environmental and oceanographic changing conditions with episodic growth modes. The growth and demise of carbonate platforms in the Cretaceous reveal an important crisis event in the mid-Aptian (Skelton 2003a) (Fig. 15). This coincides with the demise of the Zamaia buildups, which were analyzed and dated with orbitolinids, rudists and ammonites. The Galdames Formation described by Garcıá-Mondejar and Garcıá Pascual (1982), equivalent to the Zamaia Fm, was originally considered a synchronous shallow-marine carbonate platform unit. Nevertheless, careful work in the Aralar region of North Spain (Garcıá-Monde´jar et al. 2009b), and more recent work in Zamaia, revealed at least three phases of carbonate platform development (Fernańdez-Mendiola et al. 2010) (Fig. 15b: Madotz (Abrevadero Mb), Sarastarri Fm and Zamaia Fm). The first phase of Lower Bedoulian age spans the oglanlensis Zone and a part of the lower weissi Zone. Its principal representative is the Madotz platform of Aralar (Millań et al. 2011) and this can be correlated with: (1) the lower Orbitolina beds of Martin-Closas and Wang (2008) in the Subalpine Chains and Jura Mountains, (2) the Xert Fm in the Maestrat platform of Iberia (Bover-Arnal et al. 2010), (3) the Ponta Alta Member in Portugal (Burla et al. 2008), and (4) the Upper Schratenkalk of Switzerland (Fo¨llmi et al. 2007). The second phase corresponds to the early Late Bedoulian (Late deshayesi-furcata transition Zone) carbonate platform, and includes the Sarastarri limestones of Aralar (Spain) (Garcıá-Monde´jar et al. 2009b), the top of the Mont Ventoux-Languedoc sections in France (Masse et al. 2001), the Praia da Lagoa Member in Portugal (Burla et al. 2008), and the top of the Cupido Fm in Mexico (Longoria and Monreal 1991). The Shu’aiba Fm in the Middle East also displays a condensed section within the furcata Zone. Granier and Busnardo (2012) interpreted the later as a condensed HST bearing ammonites: Gargasiceras sp., Cheloniceras sp. and Pseudohaploceras liptoviense. These ammonites are assigned to the furcata Zone and are correlatable with furcata Zone ammonites from the Lareo Fm in Aralar (Spain) (Garcıá-Monde´jar et al. 2009a, b) and with the Dufrenoyia justinae ammonite Zone of Mexico (Barragań 2001). The base of this episode corresponds to the ‘‘couches superieures a` orbitolines’’-upper Orbitolina beds (Arnaud-Vanneau et al. 2008).

Facies

Fig. 15 a Temporal distribution of the Zamaia carbonate banks compared to the distribution of carbonate platforms in Europe and America during the Cretaceous (Skelton 2003a). b Stratigraphic framework of the Zamaia carbonate platform in the latest Early

Aptian and two other carbonate platform growth phases in the Basque-Cantabrian region: the Early Bedoulian Madotz (Abrevadero) platform and the early Late Bedoulian Sarastarri platform

The third carbonate platform phase of the Early Aptian is of late Dufrenoyia furcata Zone age and includes the Zamaia limestones, which are correlatable with part of the Villarroya de los Pinares Fm in Maestrat (NE Spain, Bover-Arnal et al. 2010), with the top of reservoir 1A of the Shu’aiba limestones (Granier and Busnardo 2012).

Rameil et al. (2012) also reported a coincident timing for the top of the Shu’aiba Fm. The record of three carbonate platform phases in the Basque-Cantabrian Basin reflects a punctuated development style with growth phases ending with subaerial exposure followed by marine flooding in all three episodes.

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Deciphering individual histories of platforms and their chronostratigraphic time-window is crucial in the understanding of local, regional and global factors governing their appearance, development and demise. In this respect the Zamaia platform is highly significant. Recent work by Skelton and Gili (2012) attempted to establish the timing of the episodes of carbonate platform growth and demise in the Tethyan Early Aptian. They established two phases of carbonate platform demise in the mid-Early Aptian and top Early Aptian. The first demise affected most northern Tethyan and New World platforms. This first phase is linked to global carbon cycle perturbations, although causal relationships remain contentious. The recovery of Tethyan carbonate platforms in the late Early Aptian formed Caprinid-rich margins in central and southern Tethys, together with more calcite-rich rudists in northern Tethys around Iberia. We emphasize that this late Early Aptian carbonate platform of North Iberia (Basque-Cantabrian Basin) developed in two phases. The first phase corresponds to the Sarastarri platform (Garcıá-Monde´jar et al. 2009a, b; Millań et al. 2009) dated to the deshayesi-furcata transition Zone and overlain by transgressive outer platform-basin shales of the furcata Zone sensu stricto (Lareo Fm). This Sarastarri phase coincides with the uppermost Lower Aptian transgression reported in Mexico and Maestrazgo (eastern Iberia) (Moreno-Bedmar et al. 2012). Nevertheless, a second phase of carbonate platform growth in the latest furcata Zone corresponds to the Zamaia platform described here. The top of this second carbonate platform phase is assigned to the top of sequence Ap3 of Hardenbol et al. (1998), at the Early to Late Aptian limit (furcatamartinioides boundary). Therefore, the late Early Aptian carbonate platform of North Iberia developed in a step-like mode providing the potential for prospective high-resolution global correlation.

Conclusions The Early Aptian (late Dufrenoyia furcata ammonite Zone) in the Zamaia Mountain region of Northern Spain is represented by a complex rudist platform, formed on a structural high with surrounding intrashelf basins. The close interplay of siliciclastic and carbonate sedimentation allowed the recognition of a complex carbonate buildup architecture, with carbonate banks hundreds of meters wide separated by terrigenous passageways. Seven major facies types have been distinguished: (1) lime mudstone with requieniid rudists, (2) lime mudstone with corals, (3) orbitolinid-miliolid pack-grainstone, (4) marlstone, (5) limestone breccia, (6) calcareous siltstone and sandstone and (7) paleokarst facies. These facies are mostly arranged into meter-scale parasequences, most of which are

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shallowing upward. The Zamaia buildup is composed of a western and an eastern block separated by an intraplatform depression, formed by syn-sedimentary tectonic movements. The commencement of Zamaia deposition was associated with a relative sea-level rise, which pushed back the land-derived terrigenous input. A pulse of relative sealevel fall interrupted the uniform development of the carbonate platform. Back-tilting resulted in re-sedimentation of earlier deposits on the eastern slope. The termination of Zamaia deposition was associated with a new pulse of relative sea-level fall that caused the last Early Aptian unconformity on the top of the Zamaia Formation. Opensea terrigenous marls were deposited during a subsequent rise of sea level and also infiltrated karstic cavities within the Zamaia limestone beneath the unconformity. Acknowledgments This project was supported by the Spanish Science and Innovation Ministry project CGL2009-11308. It was also supported by PhD grant BFI09.122 from the Basque Country Government. We thank M. Tucker and two anonymous reviewers for their constructive criticism and valuable suggestions, which helped us to improve the manuscript.

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