Sedimentology (2012) 59, 766–794
doi: 10.1111/j.1365-3091.2011.01275.x
Mesophotic coral buildups in a prodelta setting (Late Eocene, southern Pyrenees, Spain): a mixed carbonate–siliciclastic system MICHELE MORSILLI*, FRANCESCA R. BOSELLINI, LUIS POMAR, PAMELA H ALLOCK§, MARC AURELL– and CESARE A. PAPAZZONI *Dipartimento di Scienze della Terra, Universita` di Ferrara, Via G. Saragat 1, 44100 Ferrara, Italy (E-mail:
[email protected]) Dipartimento di Scienze della Terra, Universita` di Modena e Reggio Emilia, Largo S. Eufemia 19, 41100 Modena, Italy Departamento de Ciencies de la Terra, Universitat de les Illes Balears, Ctra. Valldemossa km 7.5, E-07122 Palma de Mallorca, Spain §College of Marine Science, University of South Florida, 140 Seventh Ave. S., St. Petersburg, FL 33701-5016, USA –Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain Associate Editor – John Reijmer ABSTRACT
Lower Priabonian coral bioherms and biostromes, encased in prodelta marls/ clays, occur in the Aı´nsa-Jaca piggyback basin, in the South Central Pyrenean zone. Detailed mapping of lithofacies and bounding surfaces onto photomosaics reveals the architecture of coral buildups. Coral lithosomes occur either isolated or amalgamated in larger buildups. Isolated lithosomes are 1 to 8 m thick and a few hundred metres wide; clay content within coral colonies is significant. Stacked bioherms form low-relief buildups, commonly 20 to 30 m thick, locally up to 50 m. These bioherms are progressively younger to the west, following progradation of the deltaic complex. The lowermost skeletal-rich beds consist of bryozoan floatstone with wackestone to packstone matrix, in which planktonic foraminifera are abundant and light-related organisms absent. Basal coral biostromes, and the base of many bioherms, consist of platy-coral colonies ‘floating’ in a fine-grained matrix rich in branches of red algae. Corals with domal or massive shape, locally mixed with branching corals and phaceloid coral colonies, dominate buildup cores. These corals are surrounded by matrix and lack organic framework. The matrix consists of wackestone to packstone, locally floatstone, with conspicuous red algal and coral fragments, along with bryozoans, planktonic and benthonic foraminifera and locally sponges. Coral rudstone and skeletal packstone, with wackestone to packstone matrix, also occur as wedges abutting the buildup margins. Integrative analysis of rock textures, skeletal components, buildup anatomy and facies architecture clearly reveal that these coral buildups developed in a prodelta setting where shifting of delta lobes or rainfall cycles episodically resulted in water transparency that allowed zooxanthellate coral growth. The bathymetric position of the buildups has been constrained from the light-dependent communities and lithofacies distribution within the buildups. The process-product analysis used here reinforces the hypothesis that zooxanthellate corals thrived in mesophotic conditions at least during the Late Eocene and until the Late Miocene. Comparative analysis with some selected Upper Eocene coral buildups of the north Mediterranean area show 766
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similarities in facies, components and textures, and suggest that they also grew in relatively low light (mesophotic) and low hydrodynamic conditions. Keywords Coral buildups, delta, Eocene, mesophotic corals, mixed systems.
INTRODUCTION Scleractinian-dominated buildups encased in terrigenous successions are common in the geological record. Nevertheless, there is a generalized inclination to view corals and siliciclastics as mutually exclusive (Sanders & Baron-Szabo, 2005). The concept of the modern ‘tropical carbonate factory’ is usually associated with the primarily biologically controlled carbonate production that occurs in warm, well-illuminated, oligotrophic, near-surface waters of the tropics and subtropics (Hallock & Schlager, 1986; Schlager, 2000, 2003; Hallock, 2005). These settings are typically dominated by a range of photosynthetic autotrophs (for example, calcareous green algae) and by organisms with photosynthetic symbionts [for example, corals and larger benthonic foraminifera (LBF)] whose sediment association is known as Photozoan (sensu James, 1997). Evolutionary and palaeoecological reconstructions often assume that coral reefs in the past occupied habitats similar to optimal conditions in the modern, rather than considering the spectrum of environments in which corals can construct reefs, as noted by Hallock & Schlager (1986) and Hallock (1988). Moreover, an important question is exactly how representative Holocene conditions are of other geological periods. Another critical issue to consider, just emerging in the literature, is the extent, ecological importance and evolutionary significance of modern corals and coral reefs in terrigenous-dominated marginal habitats and in oceanic waters at mesophotic depths (Mass et al., 2007; Chan et al., 2009; Lesser et al., 2009; Kahng et al., 2010, and references therein). Potts & Jacobs (2000) proposed that the scleractinian–zooxanthellae symbiosis developed in turbid and mesotrophic to eutrophic habitats, and suggested that a variety of turbid, inshore habitats have been available continuously through geological time. Thus, it is useful to note that in the geological record, as well as at present, corals and coral reefs thriving in episodically or permanently turbid waters, as a result of terrigenous input, are relatively common (Woolfe & Larcombe, 1999; Potts & Jacobs, 2000; Perry, 2005; Sanders & Baron-Szabo, 2005; Perry & Smithers,
2010). Mixed carbonate–siliciclastic settings can differ from the well-lit, oligotrophic settings in a number of important ecological parameters, including turbidity, solar energy, salinity, sedimentation and nutrient availability; temporal environmental variations in these mixed systems are likely to expose corals to a range of physiologically stressful extremes. Potts & Jacobs (2000) postulated that corals adapted to inshore variable environmental conditions would necessarily have broad physiological tolerances and suggested that these early adaptations were readily translated into advantages for corals to succeed in oceanic habitats. Recent reviews by Sanders & Baron-Szabo (2005) and Lokier et al. (2009) discussed the fossil record of coral reefs in mixed systems, noting that there are limited data concerning their detailed structure and composition, depositional patterns and their relationship with the main controlling factors during specific key intervals (Braga et al., 1990; Wilson, 2005; Lokier et al., 2009). The Eocene is usually considered to be a transitional period in the evolution of coral reefs between the Palaeocene, commonly considered a time of slow recovery and reorganization of reef ecosystems after the K/T crisis, and the flourishing of reefs in the Oligocene-Miocene (Perrin, 2002). This epoch is bounded by the Palaeocene– Eocene thermal maximum (PETM) (Zachos et al., 2001) and the Eocene–Oligocene Transition (EOT) (Prothero et al., 2003). During the Eocene, important palaeogeographic and climatic changes (i.e. strengthening of oceanic thermal gradients) were occurring (Hallock et al., 1991; Hallock & Pomar, 2008) that probably influenced the evolution of zooxanthellate corals and their reef building capacity. Eocene reefs have been reported in the literature from the central Tethyan area (Perrin, 2002), and in particular from the circum-Mediterranean regions (Santisteban & Taberner, 1988; Darga, 1990, 1992; Eichenseer & Luterbacher, 1992; Alvarez et al., 1994, 1995; Taberner & Bosence, 1995; Schuster, 1996; Bassi, 1998; Bosellini, 1998; Nebelsick et al., 2005; Lokier et al., 2009). Pochon et al. (2006) hypothesized that extant Symbiodinium zooxanthellae harboured by corals originated during the Early Eocene, some 50 Myr
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ago, and that diversification coincided with periods of global cooling. According to these authors the first significant diversification occurred during Eocene cooling, after the PETM, while the majority of extant lineages have diversified since the Middle Miocene. Using the Pochon et al. (2006) interpretation of Symbiodinium evolution, Pomar & Hallock (2007) postulated that a shallowing of coral habitat, as well as a change in coral-building capacity that occurred during the Late Miocene in the Mediterranean region, was a consequence of a strengthening of the bathymetric thermal gradient induced by global cooling and coeval diversification of zooxanthellae, as well as a possible increase in carbonate saturation associated with progressive isolation of the Mediterranean preceding the Messinian Salinity Crisis. Pomar & Hallock (2007) speculated that the meso-oligophotic zones provided an optimal habitat for zooxanthellate corals in a warm Earth, prior to Late Miocene times. Middle and Upper Eocene reefs are well-represented in northern Spain, where they have been interpreted to occur in a suite of different depositional settings: intrashelf–intraplatform, shelf or platform margin, carbonate ramp, delta and fan-delta systems. These Eocene examples are commonly interpreted to have developed above fair-weather wave base, with a reef physiography largely characterized by patch reefs and rarely by fringing reefs with back-reef to fore-reef facies. The modern reef-like palaeobathymetric zonation in coral composition and growth forms has also been used to build depositional models (Santisteban & Taberner, 1988; Alvarez et al., 1994, 1995; Milla´n et al., 1994; Taberner & Bosence, 1995; Romero et al., 2002; Lokier et al., 2009). The aim of this paper is to propose a model for Upper Eocene coral buildups based on those that developed in a deltaic system in the South Central Pyrenean zone. The palaeoenvironmental interpretation, including turbidity and light penetration, hydrodynamic energy and nutrient availability, is based on the facies architecture and the distribution of both skeletal components and rock textures within the buildups.
GEOLOGICAL SETTING The study area is located in the Aı´nsa-Jaca Basin, a narrow east–west trending piggyback basin in the South Central Pyrenean Zone (Fig. 1A). This basin was induced by the southwards propagation of the southern Pyrenean fold and thrust belt
during the Palaeogene (Milla´n et al., 1994; Mun˜oz et al., 1994; Castelltort et al., 2003; Huyghe et al., 2009). Due to the flexure of the foreland, the main Aı´nsa-Jaca Basin developed and was carried piggyback by the frontal thrusts. Coeval relief created to the east became the source area for extensive sediment discharge and establishment of deltaic systems: the Campodarbe Formation (Heard & Pickering, 2008) or Sobrarbe deltaic complex (Dreyer et al., 1999; Fig. 1B). The deltaic complex prograded westward in the southern part of the Aı´nsa-Jaca Basin (Fig. 1B), progressively overlying the lower-middle Eocene turbiditic systems of the Hecho Group (Mutti et al., 1985; Remacha et al., 2003, 2005). The Aı´nsa-Jaca basin fill, as a whole, comprises deep-marine deposits about 4 km thick, spanning the Early to Middle Eocene (Ypresian to Lutetian) and recording about 10 Myr of deep marine sedimentation (Heard & Pickering, 2008), and more than 2 km of deltaic to alluvial sediments that are Middle to Late Eocene (late Lutetian to Priabonian) in age (Milla´n et al., 1994, 2000; Dreyer et al., 1999; Sztra´kos & Castelltort, 2001; Callot et al., 2009). Contemporaneous to basin infill, a series of thrust ramps and oblique folds with a north– south axis (Sierras Exteriores, also known as External Sierras) started during the Lutetian and progressively propagated westwards until the Bartonian to early Priabonian (Milla´n et al., 1994, 2000; Poblet & Hardy, 1995; Poblet et al., 1998; Dreyer et al., 1999; Castelltort et al., 2003), with variable shortening rates (Huyghe et al., 2009). Thus, the emplacement of the Sierras Exteriores to the south, with an east to west progression of a series of north–south trending folds a few kilometres wide (Fig. 1B and C), exerted control on the distribution of the depositional profile, the thickness of the sedimentary sequences and sub-cycles within the sedimentary sequences through the continuous uplift of the anticlines, and the regional variations of relative sea-level (Milla´n et al., 1994). In the study area, the Eocene stratigraphy is composed of four main formations (subdivisions according to Milla´n et al., 1994; Fig. 2). The Guara Formation, Lutetian in age, is an 800 m thick, shallow-water carbonate ramp succession, rich in nummulitids and alveolinids. It is overlain by the Arguis Formation, an up to one kilometre thick clay and marly succession with intercalated thick carbonate beds, interpreted as prodelta deposits accumulated during latest Lutetian to earliest Priabonian times. The overlaying
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Fig. 1. Geological and stratigraphic settings. (A) Location of the Aı´nsaJaca Basin in the South Central Pyrenean zone; simplified from Dreyer et al. (1999). (B) Palaeogeographic reconstruction of Sobrarbe deltaic complex during late Lutetian (dark blue: Boltan˜a anticline; lighter blues: delta front and prodelta of the Sobrarbe deltaic complex). Series of oblique folds and thrust ramps (A-1 to A-7) started during the Lutetian and progressively propagated westwards until Bartonian to early Priabonian times; based on Milla´n et al. (1994) and Dreyer et al. (1999). (C) Schematic stratigraphic architecture of the four depositional sequences (DS) and sequence boundaries (‘1’ to ‘4’) within the Arguis Formation in the YesteRasal-Pico del Aguila area; simplified from Milla´n et al. (1994); coral buildups interval in red. A-4 to A-7: oblique folds and thrust as in (B).
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Yeste-Arre´s Formation, and the lateral equivalent Belsue´-Atare´s Formation, is an up to 200 m thick sandstone and marlstone succession interpreted as a delta front system, Priabonian pro parte (p.p.) in age. The Campodarbe Formation, Priabonian p.p. to Oligocene in age, is the fourth unit, with a maximum thickness of more than a kilometre. It is composed of alluvial conglomerates, sandstones and clay (Milla´n et al., 1994, 2000; Castelltort et al., 2003). Variations in pollen spectra observed along the eastern side of the Ebro Basin (north-east Spain) indicate a change from middle Bartonian warm conditions (complex mangrove swamps) to a climate with a dry season during the Priabonian (increased variety of herbaceous plants) and to a dry and slightly cooler climate during the Early Oligocene (Cavagnetto & Anado´n, 1996). Postigo Mijarra et al. (2009) have also documented the disappearance of palaeotropical genera through
the second half of the Eocene, including typical mangrove taxa, related to the notable climate changes that culminated in the EOT. Huyghe et al. (2009) suggested that the increased sediment fluxes derived from the Pyrenees during the Priabonian–Rupelian (37 to 30 Ma) are related to a climate-induced increase of erosion near the Eocene–Oligocene boundary. The studied coral buildups, early Priabonian in age, occur within the upper part of the marly Arguis Formation (Fig. 3). In this formation, Milla´n et al. (1994) distinguished four sequences on the basis of flooding surfaces, angular unconformities and onlap geometries (see Fig. 1C), and suggested that their origin might be controlled by regional tectonic pulses. Each sequence, with a clear westward progradational trend, consists of a lower marly interval (outer ramp) and an upper, shallow-water, mixed siliciclastic and carbonate interval (middle to inner ramp). An increase in
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M. Morsilli et al. foraminifera and solitary corals in sequence III; and bioclastic-rich layers interbedded with marls that sharply grade into coral bioherms in sequence IV (Fig. 1C). Biostratigraphic data from the Arguis Formation (Canudo et al., 1988, 1991; Milla´n et al., 1994; Sztra´kos & Castelltort, 2001) provide a late Lutetian to early Bartonian age for sequence I (Guembelitrioides higginsi and the lower part of the Turborotalia cerroazulensis pomeroli planktonic foraminiferal biozones; boundary between NP-16 and NP-17 nannoplankton biozones for the upper limit of the sequence), middle to upper Bartonian age for sequence II (T. pomeroli biozone p.p.), lower part of the early Priabonian for sequence III (Globigerinatheka semiinvoluta biozone p.p.), and the upper part of the early Priabonian for sequence IV. At the top of the Arguis Formation, Canudo et al. (1988) and Papazzoni & Sirotti (1995) reported the presence of nummulitids belonging to the Nummulites fabianii zone (SBZ 19 = early Priabonian, sensu Serra-Kiel et al., 1998).
METHODS
Fig. 2. Lithostratigraphic units of the Aı´nsa-Jaca Basin in the Rasal area (after Milla´n et al., 1994) and major climate trends (after Cavagnetto & Anado´n, 1996).
tectonic subsidence activity resulted in flooding and subsequent deposition of a thick succession of blue marls, while periods of diminished tectonism allowed deposition of carbonates (Milla´n et al., 1994). According to these authors, the skeletal components of the carbonate facies recognized in the studied area (sequences II to IV) mostly consist of bryozoans, with subordinate bivalves, LBF (Nummulites and Operculina) and echinoids in sequence II; pectinids with subordinate echinoids, bryozoans, oysters, benthonic
Relatively continuous exposures between Rasal and Yeste villages (Fig. 3A and B) offer the opportunity to analyse in detail the lithofacies, bedding geometries and facies architecture of a series of coral buildups occurring in the uppermost part of the Arguis Formation (sequence IV of Milla´n et al., 1994). Fieldwork for this study consisted of: (i) mapping and GIS analysis (ArcGIS by ESRI) of coral buildups on orthophotographs (aerial photographs geometrically corrected); (ii) 13 logged stratigraphic sections of the upper part of the Arguis Formation (Fig. 3C), through some selected coral buildups; and (iii) detailed facies mapping (biofacies, lithofacies and bounding surfaces) of two coral buildups onto oblique photomosaics and vector maps with point acquisitions using GPS connected to a mobile device with ArcPad software (ESRI). Estimated error of GPS was less than 5 m during the survey and points added were controlled directly in the field through orthophotographic visualization on the mobile device. A detailed geological map of one coral buildup and adjacent lithofacies near the Yeste locality was carried out with the same
Fig. 3. (A) Simplified geological map over an orthophotograph between the Yeste and Rasal villages (red: coral buildups; yellow lines: prominent sandstone successions in the Yeste-Arre´s Formation). (B) Reconstructed crosssection from (A). (C) Stratigraphic sections (log no.) measured in the Yeste-Rasal area (facies codes as in Table 1). Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology, 59, 766–794
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methods in order to reconstruct the original geometry of this buildup and determine the relationships between the various lithofacies cropping out. Public metadata and orthophotographs available from the SITAR (Sistema de Informacio´n Territorial de Arago´n: http://sitar. aragon.es/) official site of Gobierno de Arago´n were used. Microfacies analyses, including rock textures and skeletal components, were performed using 200 thin sections. The coral facies study consisted of a field survey of the main coral taxa, with identification of growth forms and bidimensional measurement of colony size (maximum diameter and maximum height of corals in growth position). Coral taxa, mainly at the generic level, have been identified directly in the field or through analysis of thin and polished sections.
LITHOFACIES Six main lithofacies and various sub-lithofacies have been distinguished, including both siliciclastic and carbonate rocks (Table 1). Lithofacies definition is based on rock textures, components, bedding and geometric relationships. Because corals in these buildups did not build rigid frameworks, the Insalaco (1998) ‘growth fabric’ nomenclature is used for facies dominated by in situ and in growth position skeletons of calcifying organisms. The Insalaco (1998) basic growth fabric
descriptors refer to the shape and relative position of the metazoan skeletons, adding categories to adequately describe growth fabrics and preventing problems in interpreting the biological effect from forms, such as the Embry and Klovan framestone, bindstone and bafflestone – framework constructing, binding/encrusting or baffling, respectively. Insalaco (1998) defined the basic growth fabric descriptors according to the dominant growth form: platestone (platy to tabular skeletons), sheetstone (sheet-like and lamellar skeletons), domestone (domal and irregular massive skeletons), pillarstone (branching, rod and tubular solitary forms with relatively restricted lateral growth) and mixstone (not dominated by one growth form and including a variety of growth forms). Nevertheless, the Dunham (1962) boundstone is retained, sensu Embry & Klovan (1971), for autochthonous limestones in which the type of organic binding cannot be recognized. Boundstone was used as a generic term for autochthonous limestones without growth-fabric specific differentiation.
Coral boundstone (CB) The coral boundstone facies consists of scleractinian coral colonies mostly in growth position (Fig. 4). A dense interlocking framework made of closely packed coral colonies has not been recognized. Instead, corals are sparse in the skeletal matrix or/and in clay-marl. Inter-coral skeletal matrix ranges from a few millimetres to a few
Table 1. Main lithofacies and sub-facies description.
CB
SP
Facies
Sub-facies
Description
Coral boundstone (domestone, platestone, pillarstone and mixstone)
CB1
In situ and in growth position corals, with mudstone-wackestone matrix (fine-grained), abundant red algae
CB2
In situ and in growth position corals, with packstone matrix (coarse-grained), skeletal fragments and abundant red algae
SP1
Well to moderate sorted, fine-grained mud-dominated skeletal packstone
SP2
Poorly sorted, coarse grained (locally floatstone) mud-dominated skeletal packstone
SP3
Poorly sorted packstone, with LBF, locally with terrigenous grains
Skeletal packstone (bioclastic calcarenite)
CR
Coral rudstone
Coral and red-algae fragments, LBF, bryozoans, bivalves
BF
Bryozoan floatstone
Bryozoan and serpulid floatstone with wackestone to packstone matrix
CM
Clay to marls
Laminated or bioturbated
S
Sandstones
Siliciclastic sandstones
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D
E
F
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Fig. 4. Coral boundstone (CB) lithofacies. (A) Characteristic nodular aspect is related to clay and fine-grained matrix between corals (log no. 11). Person for scale is ca 1Æ7 m tall. (B) Platy coral of Actinacis in growth position (Yeste area; pen for scale is ca 14 cm long). (C) Coral domestone in plan view (Yeste area; hammer for scale is ca 31 cm long). (D) Phaceloid coral colony in growth position (Caulastrea) (log no. 2; Fig. 3). (E) Intercoral-rudstone matrix with wackestone matrix (CB-1) (‘c’: fragment of poritid coral; ‘g’: Gyroidinella magna). (F) Intercoral-rudstone matrix with packstone matrix (CB-2) (‘c’: coral fragment of Actinacis; ‘v’: Victoriella sp.). Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology, 59, 766–794
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B
Fig. 5. (A) Coral biostrome encased in clay (CM lithofacies; log no. 6, Fig. 3). (B) Single coral bioherm cropping out in the Rasal area (log no. 2, Fig. 3); dashed lines highlight bedding planes, person for scale is ca 1.9 m tall; this coral bioherm is part of a larger buildup (see Figs 3 and 9A; person for scale is ca 1.8 m tall).
centimetres thick, and commonly consists of floatstone/rudstone with fine-grained, poorly sorted wackestone (sub-facies CB1; Fig. 4E), to mud dominated packstone (sensu Lucia, 1995) (sub-facies CB2; Fig. 4F). This lithofacies occurs in nodular biostromal beds (Fig. 5A) (15 cm to a few metres thick) or in small bioherms (Fig. 5B), which may amalgamate to form up to 40 m thick buildups. This lithofacies, although variable, is commonly nodular depending on the amount of claymarl content. Coral biostromes, bioherms and larger buildups are always encased in the blue marly clays. Coral colonies in growth position, or slightly dislocated but still preserved in situ, include the genera: Actinacis, Colpophyllia, Caulastrea, Astrocoenia, Goniopora, Cyathoseris, Astreopora, Siderastrea, Alveopora, Agathiphyllia, Leptoria and Plocophyllia, with various shapes and sizes. Growth forms can be platy (mainly agariciids like Cyathoseris, but also some Actinacis, Goniopora and Astreopora; Fig. 4B), massive (Actinacis, Goniopora, Astreopora, Siderastrea, Colpophyllia, Leptoria, Agathiphyllia; Fig. 4C), thickbranched (Actinacis) or phaceloid-like (mainly Caulastrea; Fig. 4D). According to Insalaco (1998), platestone, domestone, pillarstone and also locally mixstone growth fabrics can be distinguished. The coral platestone is the most distinguishable growth fabric and is dominated by unbound, dispersed, platy colonies as defined by Rosen et al. (2002) by having a cross-section profile with a width/height ratio of 4/1 or more. These colonies can be lamellar or tabular (width/ height ratio in cm: 10 to 20/2 and 100/20) and commonly occur in biostromes 0Æ5 m to a few metres thick and a few hundred metres wide, preferentially in the lower part of the sections
(Fig. 4B). Coral domestone with domal and irregular massive coral skeletons is quite abundant within the coral buildups. In two-dimensional outcrop exposures, the width of domal colonies ranges from about 20 to 70 cm, mostly 40 to 50 cm, with a width/height ratio of about 2/1. Coral colonies are not closely packed; they are sparsely distributed within the rock, surrounded by variable amounts of sediment, and lacking in situ intergrowths characteristic of an organic framework (Fig. 4C). Coral pillarstone occurs in thin layers with phaceloid-like corals (mainly Caulastrea), averaging 60 cm in diameter and up to 45 cm high, or in 1 to 2 m thick monogenericcoral beds characterized by in situ branching Actinacis. Branches of Actinacis are bioeroded and encrusted by polygenic laminar coralline algae and foraminifera; they mostly occur in the middle and upper part of the buildups (Fig. 4D). Together with the coral-growth fabrics described above, coral mixstone can locally be recognized throughout the measured sections. Bioerosion is common; the most important borings are Lithophaga on massive corals, although sponge and worm borings also occur, especially on phaceloid colonies and on fragments of branching corals. Within the matrix, red algae are conspicuous as both laminar and branching forms, and coral and echinoid fragments are abundant. Both macroscopic observation and microfacies analysis revealed fragments of a suite of branching corals that are very rarely preserved in growth position, i.e. Stylophora, Acropora, Pocillopora, Astreopora, Alveopora, Astrocoenia and Bacarella. Fragments of hydrozoans have also been recognized. Bryozoans, bivalves (mainly ostreids), serpulids, ostracods and, locally, chaetetid sponges are common. Planktonic and smaller benthonic foramini-
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Mesophotic coral buildups in a prodelta setting fera are present, as well as, locally, LBF such as Nummulites spp., Heterostegina sp., Asterocyclina sp., Operculina ex gr. alpina. Polygenic laminar red algae and encrusting foraminifera (Miniacina sp., Haddonia sp., Carpenteria sp., Victoriella sp., Fabiania sp., Gyroidinella magna, Acervulina linearis and Gypsina sp.) are common on corals. These coral biostromes and bioherms can be considered to be a ‘close cluster reef’ sensu Riding (2002); skeletons are adjacent, but not in contact, resulting in a matrix-supported fabric. Relatively high matrix/skeleton ratios and low volumes of extra-skeletal early cement characterize this type of bioconstruction. In these reefs, sediment trapping is an important corollary of skeletal growth and cluster reef organisms are considered to be tolerant of loose sediment. The absence of a true framework clearly limits the topographic relief that cluster reefs can attain relative to spatial extent and may permit bedding to develop within the reef.
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the base of some small mounds. Common components are coral fragments (mostly branching and phaceloid corals; Fig. 7A and B), laminar red algae and red-algal fragments, encrusting bryozoans and foraminifera (Gypsina sp., Acervulina linearis, Gyroidinella sp.), gastropods, echinoids, bivalves, nummulitids, miliolids and, locally, abundant rhodoliths.
Bryozoan floatstone (BF) Interbedded and interfingered with the blue clay to marl (lithofacies CM), some carbonate-rich intervals consist of bryozoan-floatstones with wackestone to packstone matrix (Fig. 7C). Bryozoans occur as both whole colonies and fragments (Fig. 7 D), and serpulids and bivalves are also common. Among foraminifera, planktonics are abundant and smaller benthonics may locally be present. The LBF are represented by small Nummulites sp., Spiroclypeus sp. and Pellatispira sp. In this lithofacies coral fragments are rare and red algae absent.
Skeletal packstone (SP) Skeletal packstone occurs in beds of variable thickness (20 cm up to 1Æ5 m), locally amalgamated, mainly on the flanks, and in the upper part of the buildups. Boundaries are sharp to gradual, with a planar to wavy shape. Internally, these beds are disorganized or locally normally graded (fining upward). Even lamination is rarely visible. Three sub-facies can be distinguished on the basis of grain size, sorting and composition: (i) SP1 consists of well to moderately sorted, fine-grained mud-dominated skeletal packstone, rich in red algae, bivalves, echinoids and unrecognizable bioclasts (Fig. 6A); (ii) SP2 consists of coarsegrained, poorly sorted skeletal packstone with fragments of corals, red algae (branching abundant), rare bryozoans, echinoids, miliolids and other foraminifera (Fig. 6B); and (iii) SP3 consists of poorly sorted packstone with LBF (nummulitids), bivalves, echinoids, bryozoans and rare gastropods; other smaller benthonic (Rotaliidae, Gypsina sp., Acervulina linearis, Planorbulina sp. and Pellatispira sp.) and planktonic foraminifera are also present (Fig. 6C).
Coral rudstone (CR) Coral rudstone, with packstone matrix, occurs in thin to thick beds (20 to 50 cm, rarely 1 m) commonly at the flanks and top of the buildups, often associated with SP lithofacies, and locally intercalated with clay-marls (lithofacies CM) at
Blue clay to marls (CM) Blue marls to clays, macrofauna barren, form the bulk of the Arguis Formation; they either occur laminated or structureless (locally bioturbated), or locally contain interbedded centimetre-thick sandstone beds, locally cross-laminated (current or wave structure). Up section, the marly clays are interbedded with marly intervals with abundant skeletal debris.
Sandstones (S) Very well-bedded sandstones, with variable bed thickness (from 5 cm up to 1 m), locally occur interbedded within the blue marly clays (lithofacies CM), but also at the base or lapping onto the flanks of the coral buildups. Sandstone beds are dominantly limeclastic (sensu Bates & Jackson, 1987) with variable amounts of quartz grains. Skeletal grains also occur, with variable composition, in the sandstone beds around the buildups; some beds contain LBF (Nummulites and Amphistegina), bryozoan and coral fragments (Fig. 7F), whereas other beds contain abundant, small unidentifiable bioclasts. Grain sizes range from fine to coarse, and locally very coarse. Graded beds and parallel laminations are common, as are symmetrical ripples at the top of some thin beds (Fig. 7E). Locally asymmetrical ripples and hummocky cross-stratifications are visible. Burrows, although rare, are also present. Sand-
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Fig. 6. Skeletal packstone (SP) microfacies. (A) Moderate to well-sorted fine-grained skeletal packstone. (B) Poorly sorted, coarse-grained skeletal packstone (‘r’: red algal fragments; ‘m’: miliolids). (C) Poorly sorted, coarse-grained skeletal packstone with LBF (‘b’: bivalves; ‘n’: Nummulites ex gr. fabianii).
stone beds progressively increase to the top of the Arguis Formation, above the coral buildups, and become dominant in the overlying Yeste-Arre´s Formation.
FACIES ARCHITECTURE AND DEPOSITIONAL MODEL Carbonate-dominated lithofacies (CB, SP, CR and BF) are organized in a recurrent association
forming buildups, always encased in the siliciclastic blue CM. Estimation of the relative bathymetry for deposition of the lithofacies is crucial for interpretation and building depositional models, but difficult; here, it has been done through analysis of the hydraulic energy inferred from textural analysis and the types of light-dependent skeletal components. In wave-dominated systems, hydraulic competence and light penetration both exponentially decrease with depth, inducing ecological bathymetric gradients that result in nar-
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Fig. 7. (A) Coral rudstone (log no. 1; Fig. 3). Part of pencil shown for scale is ca 7 cm long. (B) Rudstone-matrix consisting of unsorted packstone with abundant coral and red algal fragments (CR lithofacies) (‘r’: red algae; ‘c’: coral). (C) Bryozoan floatstone lithosome (BF lithofacies) encased in blue clay/marls in the Rasal area (log no. 1; Fig. 3; see also Fig. 9A); notebook is 20 cm long. (D) Bryozoan floatstone microfacies (BF lithofacies) (‘b’: Bryozoa). (E) Sandstone beds (S lithofacies) with wave ripples in the Yeste area (log no. 10; Fig. 3) (hammer for scale is ca 31 cm long). (F) Limeclastic sandstone (S lithofacies) with quartz grains and LBF (‘n’: Nummulites spp.; ‘o’: Operculina sp.). Ó 2011 The Authors. Journal compilation Ó 2011 International Association of Sedimentologists, Sedimentology, 59, 766–794
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rower bands of biotic zonation in shallow than in deep water (Kain & Norton, 1990).
Hydrodynamism Evaluating the hydrodynamic-bathymetry gradient is an imprecise science due to the complexity of the processes controlling the wave-base depth. Waves are tied to wind regimes and orientation and fetch of the basin, and wind and tidalinduced currents depend on a complex interaction of basin physiography, palaeoceanographic regime and climate. Furthermore, observations from modern oceans are not a priori applicable to fossil ones (Immenhauser, 2009). Nevertheless, bathymetric estimates for the conditions necessary to form coral buildups can be achieved from indirect arguments. Coral mounds in a delta setting occur interbedded in prodelta clays and, above the mounds, there are typically 10 to 40 m of clay before the first sandstone beds start to appear in the sections (see Fig. 3). The absence of sandstone successions at a similar stratigraphic position to the coral buildups eliminates the possibility that coral buildups are laterally equivalent to active delta lobes, in interlobe settings. Although highly variable, the sand– clay transition occurs in water depths of around 10
to 20 m in many deltas (Table 2). Consequently, a first estimate has to include some of the thickness of the clay succession above the coral mound plus the depth at which the transition between the prodelta clay and the delta-front sand have occurred. The impact of possible high-frequency relative sea-level changes cannot be evaluated due to the outcrop conditions. There are no storm deposits in the clays surrounding the mounds. No sandstone beds with the characteristic thickening and coarsening upwards succession are associated with, or occur around, the coral buildups. Thickening and coarsening upwards sandstone successions start to occur some tens of metres above the coral mounds and are related to the progradation of the delta-front. Only a few isolated and thin sandstone beds, either graded or containing hummocky cross-stratification, occur locally around some buildups. These thin sandstone beds may either reflect density flows induced by storms or hyperpycnal flows triggered by floods (Mutti et al., 2003). Nummulites are scarce, but they locally occur around and above the coral mounds associated with sandstone beds (allochthonous sediments) or in graded beds (turbiditic flows). The allochthony of Nummulites tests indicates that coral buildups were deeper than the loci for Nummulites to thrive.
Table 2. Depth of transition between delta front sand and prodelta muds in some modern river delta systems (from various authors). Locality
Transition depth
References
Indo-Pacific area Mahakam Delta River Yangtze Delta River Mekong Delta River Mekong Delta River Huanghe Delta River Fly Delta River Red Delta River
70 18 18 15 12 50 18
Storms et al., 2005 Hori et al., 2002 Ta et al., 2002 Xue et al., 2010 Chun-ting et al., 1995 Harris et al., 2004 Tanabe et al., 2006
Indian Ocean Ganges-Brahmaputra Indus Delta River
70 m (base of prodelta) 20 m
Michels et al., 1998 Giosan et al., 2006
Atlantic Ocean Orinoco Delta River Amazon Delta River Paraı´ba do Sur Delta River Niger Delta River
10 m up to 40 m 60 m (base of prodelta) 8 to 10 m 18 to 20 m
Van Andel, 1967 Sternberg et al., 1996 Murillo et al., 2009 Allen, 1964
Mediterranean Sea Po Delta River Po Delta River Ombrone Delta River Rhoˆne Delta River Ebro Delta River Ebro Delta River
15 18 20 20 30 11
Fox et al., 2004 Correggiari et al., 2005 Tortora, 1999 Sabatier et al., 2009 Somoza et al., 1998 Guillen & Palanques, 1997
m to to m to m to
(base of prodelta) 25 m 20 m 18 m (base of prodelta) 25 m
m to 25 m m to 25 m to 40 m m
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Fig. 8. Bathymetric estimations for deposition of the different lithofacies. (A) Bathymetric light zones and their lower boundaries based on the presence of in situ photoautotrophs and mixotrophs. Chlorocline is the deepest occurrence of in situ seagrasses and non-dasyclad green algae. Rhodocline is the deepest occurrence of in situ red algae. Due to decreasing surface irradiance with latitude, the lower limit of the photic zones co-varies with turbidity and latitude, being deeper in the tropics/subtropics and becoming shallower to sub-Arctic areas (modified from Liebau, 1984). A mesophotic zone may be distinguished between the euphotic and the oligophotic zones when skeletal components allow for a finer bathymetric/light-penetration differentiation; see also Fig. 12. (B) Bathymetric position of the lithofacies described in this paper with respect to surface-wave base and light penetration; in wavedominated systems, both exponentially decrease with depth; facies codes as in Table 1 (green circle: coral buildups). (C) Proportion of surface light with depth for different extinction coefficients of light, and growth rates of modern corals. Curves of light penetration for different extinction coefficients of light based on Kanwisher & Wainwright (1967), Huston (1985), Hallock & Schlager (1986) and Kahng et al. (2010). Eocene corals occupied the mesophotic bathymetric zone, the zone occupied by modern plate corals. The yellow line indicates the bathymetric estimation used in this paper for the Eocene corals according to light penetration in modern deltaic systems. Note that the horizontal scale is not linear, in order to better show the curves in the lowest-light range.
Light penetration Light penetration is modulated by factors affecting water transparency (for example, terrigenous
discharge, nutrient input, riverine dissolved organic matter and humic substances, etc.). The lower limit of the light-penetration zones (photic zones) also co-varies with latitude (Fig. 8A),
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Fig. 9. Buildup architecture; facies codes as in Table 1. (A) Architecture of a large, complex buildup near Rasal (see Fig. 3 for location). Perspective deformation has been partially corrected and only the accessible parts have been mapped (the scale is approximate and not constant). Note the lateral heterogeneities that prevent lateral correlation of log sections. (B) Panoramic view of a coral buildup near Yeste (see Fig. 3 for location; the scale is approximate and not constant). (C) Detailed facies map of the coral buildup cropping out in (B); lithofacies boundaries have been mapped with GPS survey. (D) Reconstructed 2D geometry of the buildup from the geological map above (scale 2:1). Note the flank asymmetry; skeletal packstones are more abundant in the eastern side and coral rudstones occur associated with sandstone beds in the western flank. This buildup is encased in clay (CM lithofacies) and sandstone beds locally lap onto the flanks.
being deeper in the tropics–sub-tropics and becoming shallower with decreasing surface irradiance and an increase in the angle of incidence of light at higher latitudes (Liebau, 1984; Lu¨ning, 1990). Depth distribution patterns of green and red algae are one of the best indicators of ancient depositional depths because of their strong light dependence (Flu¨gel, 2004). Although the three macro-algal groups (green, brown and red) have chlorophyll, with absorption peaks in the blue and red range, the typical colour differences of the green, brown and red algae are due to accessory pigments with maximum and differential absorption in the green part of the spectrum (Lu¨ning, 1990). Because of phylogenetic chromatic adaptation, red algae commonly prevail at greater depths than green algae and seagrasses. Nevertheless, members of the three algal groups occur over a broad range of depths and light intensities, as other pigments complement the absorption spectra of chlorophyll, red algae being particularly effective in this regard (Lu¨ning, 1990; Lee, 1999). Untransported dasyclad green algae point to very shallow depths (a few tens of centimetres to a few metres), but codiacean green algae have a broader depth range. Halimeda bioherms, for example, can be found as deep as 60 m in areas of high water transparency (Hallock, 2001 and references therein). Thus, the range of modern seagrasses and, with caution due to some chromatic adaptations, non-dasyclad green algae can be used to define the euphotic zone. The lowest normal limit of perennial seaweeds (usually laminar red algae) occurs at a depth that receives irradiance equivalent to about 0Æ05 to 0Æ1% of the surface irradiance; Rhodophyceae grow at greater depths than other algae because they can better utilize the blue light that penetrates furthest into water (Lee, 1999). Additionally, strategies for life of deep-water algae include slow growth (which minimizes respiration) and resistance to grazing pressure, as important prerequisites for longevity of the thallus. The growth form of laminar coralline algae is represented in
all regions as the only conspicuous algal growth form left at the lower algal limit and in the inner parts of sea caves (Lu¨ning, 1990). Thus, the rhodocline (sensu Liebau, 1984), as the deeper occurrence of in situ red algae (Fig. 8), is adequate to define the lower limit of the oligophotic zone. Consequently, the distinction between euphotic (good light and, in open seas, commonly high wave energy), mesophotic (sufficient light for coral growth, commonly below wave action), oligophotic (sufficient light for coralline red algal growth) and aphotic (absence of sufficient light for photosynthesis) zones becomes a useful tool in analyzing carbonate platforms and evaluating genetic processes. For mixotrophs, such as corals and LBF, some degree of uncertainty about their bathymetric position may derive from either the absence of modern counterparts (as in some LBF), the evolution of the algal partners (LaJeunesse, 2005; Pochon et al., 2006) and, as in corals, the plasticity in changing symbiont clades (Baker et al., 2004; Fautin & Buddemeier, 2004; Rowan, 2004; Chen et al., 2005; Stanley, 2006). Nevertheless, the irradiance requirements for modern coral growth overall (Yentsch et al., 2002) and for growth of the three most common morphologies, branching, domal and platy (Hallock & Schlager, 1986, and references therein), are quite wellknown. Moreover, morphological adaptations for even partial dependence on photosynthesis in low light (i.e. thin plates for maximum light capture) are so completely different from morphological adaptation for exclusively heterotrophic feeding on plankton (i.e. upright branching or net-like morphologies as in octocorals or bryozoans), that the transition from mesophoticmixotrophy to aphotic-heterotrophy can be quite abrupt.
Bathymetric estimation of the lithofacies accumulation Bryozoan floatstones, with wackestone to packstone matrix, mostly occur as biostromes with
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variable thickness (commonly 0Æ5 to 2 m, locally up to 3 to 4 m) and discrete bioherms encased in clay, stratigraphically below the coral buildups (Fig. 7C). This lithofacies contains abundant filter-feeding organisms (heterotrophs), while photic-related components (autotrophs and mixotrophs) are absent. Planktonic foraminifera are common and small benthonic foraminifera very scarce. When the bryozoan floatstones occur in bioherms they laterally pass to clay. These characteristics indicate deposition in the aphotic zone below storm wave base (Fig. 8B). The coral boundstone lithofacies with platestone growth fabric mostly occur as biostromal layers (from 10 to 20 cm up to 2 m) interbedded with clay, but they are also common at the base of some buildups. Platy corals (for example, Cyathoseris) are surrounded by lime-mud (wackestone) and silty-clay matrix, in which laminar red algae and planktonic foraminifera are abundant, and bryozoan fragments and the encrusting foraminifer Miniacina spp. are common. The presence of a fine-grained wackestone matrix suggests deposition below wave base, but the presence of laminar red algae, along with platy corals, places production of platestone fabric in the meso-oligophotic zone, above the rhodocline (Fig. 8B). The domestone growth fabric mostly occurs in bioherms up to a few metres thick (see Fig. 5B), which may stack to form up to 40 m thick complex asymmetrical buildups (Fig. 9), but also occurs as 50 cm to a few metres thick laterally extensive biostromes (see Fig. 5A); both bioherms and biostromes are interbedded with clay. Direct observation of the total extension of these biostromes is limited due to outcrop continuity, but it is on the scale of a few metres to several hundred metres. In these buildups, coral-rudstone and skeletal-packstone lithofacies always occur as wedges abutting the buildups on their margins (Fig. 9C and D). Thin coral-pillarstone layers also occur locally, but these are preferentially located on top of some simple bioherms. Within the coral boundstone, the matrix commonly consists of fine-grained, poorly sorted wackestone (CB1) to mud-dominated packstone (CB2), with variable proportions of coral fragments, indicating a dominantly low-energy environment of deposition, but hit by episodic high-energy events. Smaller benthonic foraminifera are common and planktonic foraminifera also occur. Photic-independent filter feeders are also common. In some outcrops, Acanthochaetetes, a demosponge that seems to be characteristic of reduced light conditions in cryptic or fore-reef settings (Reitner & Engeser,
1987), is also present. Among photic-related organisms, red algae are abundant as both laminar and branching forms, green algae are absent, and LBF such as Heterostegina, Operculina and some discoidal Nummulites occur within the matrix. These compositional characteristics indicate that deposition of the coral domestone and pillarstone growth fabrics was shallower than the platestone, although still within the mesophotic zone.
Depositional model The spatial distribution of these carbonate lithosomes within the blue clay-marls of the Arguis Formation, along with their textural characteristics and skeletal composition, suggest a depositional model (Figs 10 and 11). The Arguis Formation corresponds to the pro-delta clays, overlain by the delta front sandstones (YesteArre´s Formation). In this deltaic complex, carbonate factories produced and accumulated sediments in the mesophotic to aphotic zones, in clay-dominated depositional environments below the wave-base level, between or some distance from active delta lobes and distributaries. Carbonate sediments produced in the shallowwater euphotic zone, like green algae or seagrassrelated foraminifera, have not been recognized within the coral buildups. In the mesophotic zone, below the chlorocline (sensu Liebau, 1984), corals lived and grew along with red algae (see Fig. 8). In the upper part, corals were able to form small bioherms that coalesced forming buildups (Fig. 10). In this setting, episodic high-hydrodynamic pulses provided the energy to rework skeletal sediments and to produce the coral rudstone and skeletal packstone wedges (see also Fig. 9). As irradiance decreased with depth, platy corals developed, along with laminar red algae and bryozoans. The carbonate factory was not as efficient here as it was in the upper part, and biostromal layers instead of bioherms resulted. The absence of reworked lithofacies in this zone indicates that production/deposition occurred in a quiet environment. Deeper, in the aphotic zone, carbonate production relied on heterozoans only. Orientation and outcrop quality does not allow for documentation of the continuity/discontinuity of the mesophotic, oligophotic and aphotic carbonate factories (Fig. 11). The facies changes observed could be related to some combination of depth and changes in water transparency associated with migrating delta lobes or with climate cycles in which more
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Fig. 10. Sketches showing the origin of the coral buildups. In the mesophotic zone, corals, along with red algae, formed small bioherms (see Figs 5 and 9A). Episodic high hydrodynamic pulses shed skeletal fragments off the buildup and produced the rudstone and packstone wedges on the flanks. Larger coral buildups, as seen in Figs 3 and 9, resulted from coalescence and vertical stacking of single bioherms. CB: coral boundstone; CR: coral rudstone; SP: skeletal packstone; CM: clay and marls; S: sandstone.
humid conditions resulted in higher watershed and coastal vegetation cover that reduced sediment influx. Yentsch et al. (2002) estimated that zooxanthellate corals require at least 1% of surface light for significant growth. For example, if average water transparency was typical for deltaic waters, with an extinction coefficient of light (ECL) for photosynthetically active radiation of 0Æ3 to 0Æ4, aphotic assemblages would be expected at depths below 15 m (Fig. 8C). If average water transparency increased such that ECL decreased to 0Æ25, some platy coral growth could occur to a depth of nearly 20 m. If average water transparency improved to near oceanic conditions (ECL of 0Æ18), platy coral growth could be expected to about 25 m. Clay/marl lithofacies always occur both around and inside the coral buildups, being the background sedimentation of the overall deltaic setting (see Fig. 9). Inside the Arguis Formation, sandstone beds occur locally interbedded in the blue clay-marl unit, as very thin beds, locally cross-laminated (symmetrical ripples and rare hummocky cross-stratification). In these sandstone beds, parallel lamination, normal gradation and, locally, wave ripples, suggest that sedimentation occurred as episodic events related to storm-induced density flows or triggered by floods, via hyperpycnal flows (Mutti et al., 2003). These events, in any case, did not suffocate the coral bioherms when they were still active. Tens of metres above the buildups, however, the sandstone beds (Yeste-Arre´s Formation through to Belsue´-Atare´s Formation) increase and thicken up section as the result of the delta progradation and storm activity, suggesting that coral mounds developed below the storm-wave base, but were episodically agitated by exceptional events.
DISCUSSION Despite the usefulness of the uniformitarian approach, many authors have argued against its indiscriminate application in carbonate sedimentology. Similarly, many have documented that some depositional environments, as well as atmosphere, sea water chemical composition and relationships with carbonate-producing organisms were unique at times during the Phanerozoic (Sandberg, 1983; Morse et al., 1997; Stanley & Hardie, 1998; Schlager, 2005; Wright & Burgess, 2005; Berner, 2006; Pomar & Hallock, 2008; Kiessling, 2009). The data presented in this paper demonstrate the need for careful and critical application of uniformitarianism. Previous interpretations of Eocene coral buildups reported below, except in Bassi (1998), have been based on the ‘modern’ shallow-water, well-lit (euphotic/oligotrophic) coral-reef paradigm, and ‘barrier reef-lagoon’ and ‘patch reef’ models have commonly been applied. To avoid a possible uniformitarianism bias, analysis has been based on: (i) observations of the anatomy of the coral buildups; (ii) mapping of the facies architecture within the buildups; (iii) analysis of the geometric relationship between the buildups and the encasing facies; (iv) consideration of the skeletal components and their ecological requirements, particularly light for autotrophs and food requirements for heterotrophs; and (v) rock textures, particularly the texture of the matrix, as indicative of hydrodynamic energy. From these analyses, it becomes evident that the euphotic/oligotrophic coral-reef model cannot be applied to the Eocene coral buildups. Contrarily, the Yeste-Rasal developed in clay-
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Fig. 11. Depositional model. Coral buildups developed below the base of wave action in a prodelta setting (see Fig. 3). Coral domestone growth fabrics developed in the upper part of the mesophotic zone. In this setting, episodic strong hydrodynamic events hitting the buildups provided the energy to produce coral rudstone and skeletal packstone wedges. Coral platestone developed in the lower part of the mesophotic zone and bryozoan floatstone in the aphotic zone.
dominated prodelta settings, in low-light conditions and below wave base. These corals did not form rigid, wave-resistant framework structures, but they dwelt on sediment that was either derived from carbonate production on the mound or from settling of clay brought by deltaic hypopycnal flows, producing ‘close cluster reefs’ (sensu Riding, 2002).
Other Eocene examples from the northern Mediterranean region Igualada-Vic area, NE Spain Bartonian to Priabonian coral buildups occur abundantly in the Igualada-Vic area, on the eastern margin of the South Pyrenean foreland basin (Busquets et al., 1985; Santisteban & Taberner,
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Mesophotic coral buildups in a prodelta setting 1988; Alvarez et al., 1994; Hendry et al., 1999; Romero et al., 2002). Besides scleractinian corals, red algae and LBF are conspicuous in these buildups, which developed on prograding shallow-marine siliciclastic systems, prior to widespread accumulation of continental deposits. The Igualada-Vic coral buildups share a series of similarities with the Yeste-Rasal buildups: (i) they occur associated with deltaic systems; (ii) they have a characteristic nodular aspect; (iii) an interlocking framework made of closely packed coral colonies has been not recognized; (iv) individual corals are embedded either in clayey or in welllithified silty-packstone matrix; (v) in addition to corals, articulated and laminar red algae, bryozoans, bivalves, echinoids, LBF and planktonic foraminifera are common skeletal components; (vi) green algae are absent, except in the Calders buildup where Hendry et al. (1999) have reported fragmented dasycladacean algae; and (vii) lithofacies likeness is noteworthy, particularly when compared with the descriptions provided by Alvarez et al. (1994; Table 3). Concerning the composition of the coral fauna, the base of the succession is characterized in both areas (Igualada-Vic and Yeste-Rasal) by a platy coral assemblage dominated by the agariciid coral Cyathoseris, whereas the bulk of the coral boundstone is analogously characterized by abundant Actinacis and phaceloid corals such as Caulastrea and cf. Cereiphyllia. Previous interpretations of the Igualada-Vic coralgal buildups, despite the fact that geometries are not clearly visible, have been based on the modern-reef model, i.e. Caribbean-like reefs (Busquets et al., 1985; Santisteban & Taberner, 1988; Alvarez et al., 1994; Hendry et al., 1999; Romero et al., 2002). This paper proposes, however, that the coralgal buildups in the Igualada-Vic region of the South Pyrenean foreland basin developed in a similar setting as the buildups occurring in the Yeste-Rasal area of the Aı´nsa-Jaca piggyback basin; similarities between buildups of these two areas are: (i) the biotic associations are similar; (ii) rock textures
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within the buildups indicate a low-hydrodynamic energy setting; (iii) the exclusive occurrence of red algae as in situ autotrophs (the occurrence of fragmented green algae in the Calders buildup indicates they are ex situ) points out that in both areas coral buildups grew in the mesophotic zone between the chlorocline and the rhodocline sensu Liebau (1984); and (iv) in both areas, buildups occur associated with deltaic systems.
The Nago Limestone, northern Italy The Bartonian to Priabonian Nago Limestone, northern Italy, consists of marly limestones and red-algal/coral limestones organized in a shallowing upward succession (Luciani et al., 1988; Luciani, 1989). In this limestone unit, Bosellini (1998) distinguished two main groups of facies: (i) micrite-dominated textures, with a low degree of colony fragmentation; and (ii) mounded massive coral beds flanked by skeletal packstone–grainstone. In the micrite-dominated lithofacies, Bosellini (1998) differentiated marls to nodular marly wackestone at the base of the shallowing upward sequence, followed by wackestone/packstone layers. The nodular marly wackestone contains rhodoliths and laminar red algae, small foraminifera, LBF, and subordinate bryozoans, echinoids, small bivalves, oysters, serpulids, together with some ostracods and planktonic foraminifera. Locally, platy coral colonies occur in growth position. The wackestone/packstone layers, locally mounded, are rich in rhodoliths and laminar red algae, LBF and platy corals, with subordinate bryozoans, serpulids, pectinids and echinoid fragments. Massive beds containing amalgamated coral mounds with ‘cluster reef structure’ sensu Riding (2002) characterize the top of the succession. Corals are sediment-supported; close skeletons are not in contact but surrounded by sediment matrix without cavities. The matrix consists of packstone and locally coral floatstone, with abundant fragments of corals, red algae (mastophoroid
Table 3. Comparison between coral-buildup facies described from the Igualada-Vic area (Alvarez et al., 1994) and from the Yeste-Rasal area (this study). Igualada – Vic area (Alvarez et al., 1994)
Yeste – Rasal area (this study)
(a) In situ lamellar corals in lime-mud wackestone (b) Encrusting and branching red algae, and corals, in wackestone matrix (c) Skeletal packstone-rudstone (d) Nodular wackestone with in situ corals (e) Mixed carbonate–siliciclastic packstone, with red algae crusts
Coral platestone (sub-facies of CB) Coral boundstone (CB) Coral rudstone (CR) Coral boundstone (CB) Skeletal packstone (SP)
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dominated), LBF, bryozoans, echinoids and rhodoliths. Low coral diversity, low coral cover and platy coral morphotypes (dominated by the coral genus Cyathoseris) characterize the deepest coral facies. The highest coral diversity occurs in the core of the mounds, with a decreased coral diversity, encrusting morphotypes and coral breccia occurring at the top. Flank facies consist of coarsegrained, bioturbated packstone/grainstone, locally rudstone, with LBF, rhodoliths and red algal debris, and subordinate fragments of molluscs, echinoids, bryozoans and corals (Bosellini, 1998). Bassi (1998), focusing on the red algae and LBF composition, distinguished five litho/biofacies in the Nago Limestone: (i) rhodolith pavement; (ii) rhodolith mound wackestone/packstone with laminar corals; (iii) laminar and branching red algal rudstone; (iv) coral-algal boundstone; and (v) algal/Discocyclina packstone. These lithofacies are not dissimilar to those described by Bosellini (1998): the ‘coral-algal boundstone’ lithofacies is surrounded at the flanks by the ‘algal crust-branch rudstone’, while the ‘rhodolith pavement’, the ‘rhodolith mound wackestone/ packstone’ and the ‘algal crust-branch rudstone’ correspond to the group of micrite-dominated lithofacies of Bosellini (1998). Bosellini (1998) interpreted the Nago Limestone as the product of a reef system largely characterized by patch reefs with a palaeobathymetric facies zonation in which forereef to shallow-water shelf-edge reef facies are commonly associated with changes in coral composition and morphotypes. Bassi (1998), however, envisioned a prograding carbonate ramp to explain the distribution of the coralline-algal facies; in this ramp, Mastophoroideae dominated in shallow-water environments while Melobesioideae and Sporolithaceae dominated deeper-water facies, with coral-algal buildups developing in mid-ramp settings.
The Eisenrichterstein carbonate ramp, Germany The Priabonian coral-bearing Eisenrichterstein limestone complex near Reichenhall, Southern Bavaria, also contains coral buildups lacking a coral framestone texture (Darga, 1990). In this lower Priabonian carbonate-ramp example, shorerelated conglomerates and pebbly sandstones, at the base of the succession, are overlain by finegrained muddy calcarenites with abundant smaller benthonic foraminifera and molluscs. In these calcarenites terrestrial plant remains occur along with LBF, solitary corals and a few colonial corals. Preservation of burrowing structures and
delicate moulds, along with the presence of characteristic seagrass-dwelling bivalves and foraminifera, support the existence of large areas covered by seagrasses. The overlying ‘detrital reef limestone’ consists, at the base, of coralforaminiferal rudstone with abundant red algae (Peyssonneliaceae and Corallinaceae) with micrite-dominated matrix, containing small ‘reef limestone’ patches. Reef limestone consists of very densely packed spherical and branching corals, rarely frame-building, which grew in lowenergy conditions episodically hit by high-energy events. Foraminiferal packstone/grainstone, Discocyclina wackestone, mud-dominated rhodolithic intervals and coral-fragment wackestone also occur associated with the ‘reef-limestone’ buildups. The coral buildups and associated facies interfinger basinward with marls containing coral debris, small benthonic and planktonic foraminifera, as well as some flat Discocyclina and Heterostegina. Darga (1990) envisages the depositional setting as a low-angle carbonate ramp with some coral buildups, in which almost all of the facies types (except the near-beach), accumulated in a low-energy environment only intermittently hit by short-lasting agitated episodes.
Comparative analysis Comparative analysis of the coral buildups in Yeste-Rasal, with those described in the IgualadaVic area in Spain, the Nago Limestone in Italy and the Eisenrichterstein limestone in Germany, shows many similarities in terms of lithofacies, components and spatial facies relationships. Coral bioherms are discrete or amalgamated, there are associated packstone and rudstone-floatstone flanks, red algae are abundant, and the matrix is mud-rich to mud-dominated. In terms of bathymetric interpretation, the dominance of red algae and the scarcity of green algae among photoautotrophs, suggest that the coral-algal buildups developed in the mesophotic to oligophotic zone, which is consistent with the low hydrodynamic energy recorded by the abundance of mud occurring in the matrix. In the Eisenrichterstein limestone, photoautotrophs are not described in detail but rock textures also suggest that buildups grew in a low-energy environment associated with Discocyclina, suggesting mesophotic conditions, whereas euphotic seagrass-dominated areas occurred in shallower nearshore areas. Differences, however, also exist: the Nago Limestone and Eisenrichterstein limestone are carbonate-dominated platforms, with minor siliciclastic input (marly limestones in the deeper
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Mesophotic coral buildups in a prodelta setting lithofacies), whereas the Yeste-Rasal and Igualada-Vic buildups developed on prodelta clays. Other differences, possibly related to the siliciclastic versus carbonate-dominated settings, are the dimension, hydrodynamic energy and density of corals. Whereas Yeste-Rasal buildups developed on an active prograding delta, the Nago buildups occur in a narrower margin of the Lessini Shelf (Bosellini, 1989). The narrower margin of this shelf might explain the higher hydrodynamic energy and may have influenced the higher coral density in the upper part of the buildups. More intense bioerosion and encrustation, abundance of bryozoans, and fewer LBF in Yeste-Rasal than in Nago may also be considered as result of higher nutrient availability. Despite the differences in siliciclastic influx and concomitant nutrient delivery between these examples, almost ‘pure’ carbonate platforms in Nago and Eisenrichterstein and prodelta clays in Yeste-Rasal and Igualada-Vic, the Eocene coral buildups analysed here apparently developed under low-energy mesophotic conditions. Carbonate sediments characteristic of the euphotic zone (for example, seagrass epiphytic organisms) have only been reported from the transgressive succession of the Eisenrichterstein limestone, occurring between the ‘detrital reef limestone’ and the shore-related conglomerates and sandstones (Darga, 1990). In the Nago Limestone, although there are scarce green algae (Bassi, 1998), shallow-water, euphotic lithofacies have not been described. In the buildups occurring within prodelta clays, green algae and seagrassrelated components are absent, except for a few fragmented dasycladacean algae in the Calders buildup (Hendry et al., 1999); this suggests that, like in the modern, the euphotic carbonate factory could not develop in shallow-water areas dominated by high terrigenous supply.
Coral buildups in terrigenous-dominated systems In a seminal paper, Mount (1984) analysed the mechanisms to explain the spectrum of ‘mixed’ sediment composition despite the inhibiting effect that siliciclastic material has on carbonatesecreting organisms. This author emphasized that the allochemical constituents of mixed sediments consist of both coralgal and mostly foramol sediment associations. Mount (1984) explained the predominance of foramol constituents as a result of the effects of turbidity, substrate instability and the clogging of filter-feeding mechanisms associ-
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ated with a siliciclastic influx. Woolfe & Larcombe (1999) suggested that sediments affect coral growth and survival physically in three main ways: (i) sediment accumulation may reduce coral performance through excessive use of energy in activating sediment rejection mechanisms; (ii) high levels of suspended sediment in the water column will decrease light levels, such that photosynthesis will be limited (reduction of the photic zone) or halted altogether; and (iii) the soft tissues of the coral may be damaged through abrasion or impact by sediment particles. When reviewing fossil turbid-water buildups, Sanders & Baron-Szabo (2005) noted that scleractinian-dominated turbid-water buildups are common and suggested that, below some threshold, some taxa cope well with terrigenous turbidity and sedimentation, acclimating by increased heterotrophy. The Eocene examples from the Yeste-Rasal and Igualada-Vic areas analysed here confirm the conclusions of Sanders & Baron-Szabo (2005) regarding the common attributes of coral-buildups occurring in terrigenous-dominated systems: coral lithosomes are discrete and do not stack into reef complexes, cluster to segment fabrics (sensu Riding, 2002) predominate, frame pores with marine cement are scarce or absent, a carbonate slope is absent, and colonial corals resilient to sediment input commonly are massive to platy forms. Coral growth on modern coral patch reefs in relatively turbid waters is restricted to relatively shallow depths (Titlyanov & Latypov, 1991; Wilson & Lokier, 2002; Hallock, 2005; Wilson, 2005, 2008). Turbidity in the Eocene Aı´nsa-Jaca Basin may have been associated with hyperpycnal clay-carrying flows that must have been ephemeral during phases of reef growth. The fact that coral buildups occur in separate clusters within the progradational prodelta clays of the Arguis Formation (see Fig. 3) suggest either phases of delta lobes switching or alternating cycles of increased terrigenous delivery and delta progradation with periods of carbonate accretion, when it is likely that the humid terrestrial environment was well vegetated and mangrove dominated shorelines captured most of the terrigenous sediments.
Are modern mesophotic corals the counterparts for Eocene corals? Light-dependent zooxanthellate corals (z-corals), azooxanthellate scleractinian corals, macroalgae and sponges dominate modern ‘mesophotic coral
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Fig. 12. Comparison between different bathymetric zones defined from light penetration. Light intensity decreases along a depth gradient. The transition from ‘euphotic’ to ‘oligophotic’ represents the depths where carbonate producers requiring substantial solar energy (for example, mound-shaped zooxanthellate coral and abundant calcareous green algae) are replaced by low-light biotas typically dominated by coralline algae. When skeletal components reveal a finer bathymetric/light-penetration differentiation (for example, corals of primarily platy morphologies), an intermediate ‘mesophotic’ zone can also be defined. The depth-limit of this mesophotic zone will vary with water transparency and adaptations of the mesophotic biota.
ecosystems’. Such coral communities occur in the deepest half of the photic zone, from 40 m down to over 150 m in very clear tropical and subtropical oceanic waters (see Kahng et al., 2010, and references therein). Developments in scientific diving and underwater robotics have sparked the recent interest in these deep reef communities (http://www.mesophotic.org/). This mesophotic zone of biologists is consistent with the mesophotic environments described in this paper (Fig. 12). The z-corals in this zone exhibit specific photo-acclimatization strategies to enhance photosynthesis in low-light conditions (Mass et al., 2007; Chan et al., 2009; Lesser et al., 2009): many, not all, z-corals occurring here are endemic to these depths; they develop plate-like morphology to capture as much light as possible; they may create ‘light traps’ like conical knobs; and some corals have specialized fluorescent pigments or zooxanthellae with higher chlorophyll concentrations. Corals can additionally shift from being primarily autotrophic to a greater dependence on hetero-
trophy for their energy requirements as light decreases with increasing depth (Muscatine et al., 1989; Mass et al., 2007; Alamaru et al., 2009; Chan et al., 2009; Lesser et al., 2009). Some authors have also suggested that some corals utilize heterotrophy as a response to high turbidity (Anthony & Fabricius, 2000; Larcombe et al., 2001). The coral-shape zonation in the Yeste-Rasal area, and by extension in the Igualada-Vic, the Nago, and the Eisenrichterstein areas, implies that these Eocene corals were photodependent. The sediment composition and texture, along with the stratigraphic context, further indicate that these corals thrived in environments with limited hydrodynamic activity in the mesophotic zone. Some degree of heterotrophic reliance, as in modern mesophotic corals, can plausibly be inferred. In low-light settings, these Eocene corals were able to form buildups. Elevation of the sea floor increases feeding competence for suspension feeders as it provides a way to develop fully turbulent flow (Fig. 13A) and increases the impinging efficiency of currents carrying pico-
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Mesophotic coral buildups in a prodelta setting A
B
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lops use local topographic highs (like polychaete worms and large semi-endobenthonic bivalves) to gain elevation above the sea floor to increase feeding efficiency (McKinney et al., 2007). Additionally, a sharp gradient in nutrients and a maximum in fluorescent pigment concentration, representing the maximum concentration of chlorophyll (i.e. in phytoplankton) and its degradation product phaeophytin, are often co-located with the base of the seasonal pycnocline (Fig. 13B), commonly in the lower part of the photic zone (Steele & Yentsch, 1960; Anderson, 1969; Hallock et al., 1991), where nutrient-rich (deeper) water enters the nutrient-depleted photic zone through the mechanism of upwelling or internal waves (Hallock & Pomar, 2008).
‘Local/regional’ versus general causes for Eocene corals to be mesophotic
Fig. 13. (A) Accretion of a bioherm promotes turbulent flow and increases the impinging efficiency of currents carrying plankton, thereby favouring suspension feeders. (B) A maximum in fluorescence, representing peak chlorophyll and phaeophytin (degrading chlorophyll) concentrations, typically occurs at the nutricline, which also coincides with the top of pycnocline. The nutricline generally begins in the lower part of the photic zone, where nutrient-rich (deeper) water can mix upward via upwelling or internal waves, and where available light energy, not nutrients, limits primary production.
plankton and phytoplankton (Bilger & Atkinson, 1992; Hearn et al., 2001; Ribes et al., 2003; Pomar & Hallock, 2008). This strategy has been documented from the modern Adriatic Sea, where mobile suspension-feeding ophiuroids and scal-
A fundamental question is whether the apparent concentration of corals in mesophotic habitats was associated with ‘local/regional’ factors or if this habitat preference was related to a more general constraint. Coral-zooxanthellae symbiosis provided the potential for scleractinian corals to contribute prolifically to reef building. Based on skeletal stable isotopes and organic matrix analyses, Stanley & Swart (1995), Muscatine et al. (2005) and Stanley (2006) postulated that coralzooxanthellae symbiosis had started by the Late Triassic. Palliani & Riding (2000) also suggested a Late Triassic co-evolution of corals and zooxanthellae, based on the coincidence of increasing diversity of the dinoflagellate-cyst family Suessiacea, which are closely related to zooxanthellae, with those of reef-building scleractinians. The co-evolution of anthozoans and dinoflagellates was necessary for the symbiotic association to succeed, and probably occurred several times, probably associated with the wax and wane of zcorals throughout the Mesozoic–Cenozoic (Veron, 1995). Veron (1995) suggested that the evolutionary flexibility to reduce or discard their dependence on symbiosis has provided at least some corals with an eco-physiological adaptation allowing them to survive major extinction events. This symbiotic association required the evolution of: (i) mechanisms to supply inorganic carbon and other nutrients required for zooxanthellae photosynthesis to the algae within the host; (ii) biochemical sunscreens (commonly produced by the symbiont and translocated to the host); (iii) antioxidant defenses to protect against overproduction of photosynthetic oxygen; and (iv)
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special mechanisms (unusual in metazoans) to absorb and sequester inorganic nitrogen in order to thrive in oligotrophic waters (Furla et al., 2005; Stanley, 2006). Lineages of Symbiodinium zooxanthellae harboured by modern corals may have originated in the Early Eocene, with subsequent divergence in two pulses, during the Eocene and since the Middle Miocene, coinciding with periods of global cooling (Pochon et al., 2006). Pomar & Hallock (2007, 2008) suggested that an upward shift in coral buildups to euphotic, shallow-water settings was probably related to the post-Middle Miocene diversification of extant Symbiodinium zooxanthellae. Until the Late Miocene, corals did not form large wave-resistant structures in shallow-water areas in the Mediterranean province, but they formed, along with red algae, discrete buildups in mesophotic conditions. The habitat expansion into the euphotic zone, which greatly increased calcification rates, might have been a consequence of the strengthening of temperature gradients that induced a bathymetric and latitudinal reduction of the coral ‘temperature window’, combined with the global decline in atmospheric CO2 (Pomar & Hallock, 2007, 2008). High irradiance levels near the sea surface supports higher photosynthesis rates, which promote hypercalcification, but also increase the risk of widespread photo-oxidative stress, which can induce mass bleaching during periods of calm and hot weather (Shick et al., 1996; Kleypas et al., 1999, 2001). In addition, because higher pCO2 concentrations elevate the risk of photooxidative stress (Wooldridge, 2009), the Oligocene–Miocene expansion of zooxanthellate corals into high-light environments probably corresponded to the long-term decline in atmospheric CO2 to Neogene concentrations that favoured aragonite hypercalcification (see Pomar & Hallock, 2008, and references therein). That expansion into euphotic habitats also was likely to be associated with acquisition of sunscreens and enhanced antioxidant mechanisms. The key question thus is still open and in need of further research: were Eocene corals, with early clades of Symbiodinium zooxanthellae, restricted to the mesophotic zone? The similarities between coral buildups and associated biota occurring in both terrigenous and carbonate-dominated depositional settings suggest that it was a general characteristic rather than a local effect induced by turbidity or an inhibiting effect of high siliciclastic input in the shallow euphotic zone. The results presented here suggest that corals were restricted
to the mesophotic zone (see Fig. 8). This suggestion also agrees with Potts & Jacobs (2000), who proposed that the origin of scleractinian–zooxanthellae symbiosis occurred in turbid habitats and postulated that the broader physiological tolerances of corals adapted to variable environmental conditions provided adaptations for corals to succeed in oceanic habitats.
CONCLUSIONS Integrative analyses of facies mapping on orthophotographs, detailed buildup-facies architecture (biofacies, lithofacies and bounding surfaces) mapped onto oblique photomosaics and section logs, along with consideration of the ecological requirements derived from the skeletal components and hydrodynamic-energy inferred from the rock textures, has been demonstrated to be a successful technique in building realistic depositional models. In the Aı´nsa-Jaca Basin, corals produced biostromes and bioherms in low-light, sub-wave base, mud-dominated deltaic settings. The bathymetric position of the buildups has been constrained from the biotic associations, particularly light-dependent communities, and lithofacies distribution within the buildups. Critical application of uniformitarianism, through a process-product oriented type of analysis, reinforces the hypothesis that zooxanthellate corals thriving in mesophotic conditions may have been the norm rather than the exception in greenhouse seas, as well as during the transitional state toward the icehouse. This study also supports the idea that modern tropical, high light, shallowwater coral reefs may be an adaptation to a cooling Earth.
ACKNOWLEDGEMENTS Assistance in the field, as well as the suggestions and discussions about foraminifera and coral faunas, of Giulia Silvestri and Guillem MateuVicens are gratefully acknowledged. We also thank Sam Purkis for his comments about mesophotic corals and Adelaide Mastrandrea for identification of sponge specimens. We are grateful to Aitor Payros and Christian Scheibner for their constructive comments and to Associate Editor John Reijmer for his valuable advice. This work has been funded by Spanish project MECDGI CGL2005-00537/BTE, CGL2009-13254 and CGL2008-01237/BTE.
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Manuscript received 19 July 2010; revision accepted 8 June 2011
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