Introduction

Carbonate platform slope facies typically exhibit a zonation into depth-dependent facies belts (Wilson 1975; McIlreath and James 1978; Schlager 2005). The belts are the product of a combination of several intrinsic factors including the initial bathymetry as well as biological and sedimentary processes (Wright and Burgess 2005). This facies belt model alone, however, cannot explain the complex facies variability observed in carbonate slope outcrops. Calcium carbonate production areas may arise in patchworks or mosaics and along-slope processes may be localized (Wright and Burgess 2005). Many of these mosaic elements are not depth-dependent and can change through time due to the influence of subtle extrinsic factors, i.e. siliciclastic input in the carbonate factory (Wright and Burgess 2005).

Miocene tropical carbonate platform slopes crop out in the Neogene basins of the Betic Cordillera in southeast Spain and were the subject of numerous studies (Esteban 1980; Dabrio et al. 1981, 1985; Riding et al. 1991; Martín and Braga 1994; Braga and Martín 1996; Esteban 1996; Franseen and Goldstein 1996; Cuevas-Castell et al. 2007). In cross section, both the Cariatiz carbonate platform in the Sorbas Basin and the Polopos carbonate platform in the Níjar Basin display a progradational pattern with well-developed clinoform bodies. These clinoform bodies have a downslope facies zonation, from reef-framework blocks and breccia to fine-grained packstone. This facies zonation may change in distinct growth packages depending on available accommodation space ultimately related to sea-level fluctuations (Reolid et al. 2014).

Slope facies and microfacies distribution exhibit a high degree of variability, which is mainly due to the presence of abundant serpulids and siliciclastics. This study addresses the causes and significance of the factors determining this facies distribution in carbonate platform slope clinoform bodies. We discuss how it is a consequence of exposure to water turbulence and hydrographic perturbations in response to phases of enhanced continental runoff.

Geological setting

Carbonate platform slopes crop out in the intramontane Neogene basins of southeast Spain (Fig. 1). Two sections from the Cariatiz carbonate platform, the Barranco de la Mora and Barranco de los Castaños sections, at the northern margin of the Sorbas Basin, and two sections from the Polopos and Níjar carbonate platforms at the northern margin of the Níjar Basin were analyzed (Fig. 1). The Sierra de los Filabres with metamorphic rocks from the Internal Betic Zone borders the E–W-elongated Sorbas Basin to the north, the Sierra Alhamilla and Sierra Cabrera bound it to the south (Fig. 1). There, a Messinian carbonate platform with coral reefs (Fringing Reef Unit, Riding et al. 1991; Braga and Martín 1996) and related basinal silty marls, marls and diatomites from the Upper Abad Member (Martín and Braga 1994) is cut by several ravines, which provide insights into the platform-internal and slope-facies architecture (Fig. 2a). The Sierra Alhamilla and Sierra Cabrera border the NE–SW elongated Níjar Basin to the north, the Neogene volcanic complex Cabo de Gata bounds it to the southeast. A Messinian carbonate platform with coral reefs and associated basinal silty marls from the Upper Abad Member occurs at the northern margin of the basin (Fig. 2b; Dabrio et al. 1981).

Fig. 1
figure 1

A Location map of the study area in SE Spain. B Regional setting of the Neogene basins of SE Spain showing the carbonate platform discussed in this study: Cariatiz (Barranco de la Mora and Barranco de los Castaños sections), Polopos and Níjar

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Fig. 2
figure 2

a Neogene lithostratigraphy of the Sorbas Basin. The rectangle indicates the interval of the succession shown in detail in Fig. 3 (Modified from Reolid et al. (2014)). b Neogene lithostratigraphy of the Polopos area. The rectangle indicates the interval of the succession shown in detail in Fig. 7

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Methods

Carbonate platform slope facies were mapped using panoramic photomosaics of the best-exposed successions. Two hundred samples from different facies were petrographically analyzed to identify microfacies and components. In addition, polished slabs were used to analyze large bioclasts, sedimentary fabrics, and structures. Where possible, the slopes of the Cariatiz carbonate platform were scanned with an Optech Laser Imaging ILRIS 3D terrestrial LIDAR. The LIDAR data were digitally processed using 3D-Reconstructor (Gexcel). Slope dimensions and slope angles in the Barranco de los Castaños section were previously presented by Reolid et al. (2014).

Results

Carbonate slope facies

The characteristics of the main slope facies of the Messinian fringing reef-slopes are summarized in Tables 1 and 2. Locally, there are facies containing up to 50% siliciclastics (Tables 1, 2). Individual slope bedsets may consist entirely of siliciclastics.

Table 1 Platform-slope facies of Barranco de la Mora section
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Table 2 Platform-slope facies of Polopos section
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Barranco de la Mora section: slope facies distribution

The more than 190-m-long Barranco de la Mora section in the northwestern part of the Cariatiz carbonate platform contains several carbonate platform growth packages (Fig. 3). The slope facies, described in Table 1, occur in 14 sedimentary bodies, including reefal clinoform bodies (CB), serpulid build-ups, lowstand wedges, and siliciclastic-rich bodies (Fig. 3).

Fig. 3
figure 3

a Outcrop panorama of the Barranco de la Mora section in the Cariatiz carbonate platform. b 2D projection of main surfaces derived from a LIDAR data set and traced onto a plane oriented in parallel to the direction of progradation (N170E). The slope facies form 14 sedimentary bodies, including reefal clinoform bodies (CB) with a Halimeda mound (H), serpulid build-ups (SB), lowstand wedges (LW), and siliciclastic-rich bodies (SRB)

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Clinoform bodies

In the Barranco de la Mora section, there are seven clinoform bodies. The maximum thickness of these bodies is 7 m, and the maximum extension in the direction of progradation (~N170E) is 80 m. The slope angles vary from up to 35° at the reef talus to 25° at the proximal slope, and 15° at the distal slope. The facies distribution follows the model presented by Riding et al. (1991). The uppermost facies in these clinoform bodies is the breccia facies (Fig. 4a), with an average thickness of 5 m. Porites sticks corals can be found in the reef-framework blocks. Reef-framework debris size and amount decrease basinward and the lithology gradually changes into a bioclastic floatstone in the proximal slope.

Fig. 4
figure 4

Clinoform body facies in the Barranco de la Mora section. a Breccia facies in the talus slope with large Porites-framework blocks. b Close-up of the Halimeda mound in the proximal slope of CB3 (Fig. 3). c Photomicrograph of the distal-slope bioclastic wackestone with bryozoan fragments (black arrow) and planktonic foraminifera (white arrow) in a silt-sized bioclastic matrix with minor siliciclastics

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In the proximal reef-slope, a 60–100-cm-thick and 3–4-m-wide mound consisting of an accumulation of Halimeda plates occurs within one of the clinoform bodies (CB3 in Fig. 3). The Halimeda plates range in size from 2 to 10 mm in section and make up a rudstone with minor micritic matrix in the remaining pore spaces (Fig. 4b).

The gradual transition from the proximal to the distal slope deposits corresponds to a well-bedded bioclastic wackestone to floatstone facies fining downslope (Fig. 4c). Where present, bioturbation is confined to the finest intervals. Some beds host patches rich in bivalves, mainly from the families Veneridae, Glycymeridae, and Pectinidae.

Serpulid build-ups

These bodies are up to 6 m thick and are up to 70 m wide in slope normal direction. Their uppermost part consists of serpulid boundstone (Fig. 5a, b), while there is a downslope change towards accumulations of serpulid debris (Fig. 5c, d). The serpulid-rich bodies are faintly bedded. Bedding is better-developed downslope, where other bioclasts, especially red algae and bivalves, are common. In these areas, the deposits are marls.

Fig. 5
figure 5

Serpulid-rich facies in the Barranco de la Mora section. a Close-up of the surface of the serpulid boundstone of SB2 (Fig. 3). b Photomicrograph of the serpulid boundstone. c Close-up of the serpulid rudstone of SB1 with oyster debris (white arrow) and abundant bryozoan coatings (black arrows). d Photomicrograph of the serpulid rudstone with common fragmentation of the serpulid tubes. e Close-up of a serpulid-red algal nodule floating in a marl matrix from the distal slope of CB5. f Outcrop picture of the distal slope of CB5 with an alternance of thin marl beds and thick nodule-rich beds

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Clinoform body 5 (Fig. 3) contains a mixture of serpulid build-up and reef-related facies including a breccia with large reef-framework blocks to the top. The proximal slope is covered by vegetation and it is not possible to identify the facies. The distal slope consists of an accumulation of serpulid-red algal nodules arranged in 20–30-cm-thick beds locally deformed by the load and dredging effect of large reef-framework blocks. The nodules range in size from 5 to 20 cm and consist of clusters of red algae, serpulids, and minor bryozoans (Fig. 5e). An alternation between thin fine-grained beds and thick nodule-accumulation beds occurs (Fig. 5f). The fine-grained beds consist of marls alternating with sand-grade carbonate bioclasts.

Siliciclastic bodies

Two siliciclastic intervals, each up to 3 m thick, are intercalated into the carbonate platform-slope succession (Fig. 3). The stratigraphically older body (SRB1 in Fig. 3) consists of a conglomerate with up to 5-cm clasts in a sand-sized matrix, which is a mixture of siliciclastic and carbonate grains. Most of the siliciclastic pebbles are encrusted with bryozoans representing the first generation of encrusters followed by serpulids and red algae (Fig. 6). The younger body (SRB2 in Fig. 3) consists of a faintly bedded mixed carbonate-siliciclastic sand with unidentifiable sand-sized bioclasts.

Fig. 6
figure 6

Siliciclastic-rich facies and encrusters of the Barranco de la Mora section. a Close-up of the siliciclastic-rich facies with abundant serpulids. b Photomicrograph of the siliciclastic-rich facies with serpulids (SRB1, Fig. 3). c Close-up of a pebble from the siliciclastic-rich facies encrusted by bryozoans (white arrow) and serpulids. Serpulids are encrusted by red algae (black arrow). d Photomicrograph of a bioclastic floatstone with common siliciclastic and serpulids encrusted by red algae (black arrow). e Photomicrograph of a bioclastic floatstone with common siliciclastics showing a large bioclast encrusted by bryozoans (white arrow) and by red algae (black arrow)

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Lowstand wedges

The platform-slope succession contains two packages of well-bedded bioclastic packstone to rudstone, with a maximum thickness of 8 m and an extension of 40 m (Fig. 3). The bodies thin out upslope.

Polopos section: slope facies distribution

A succession of seven SE-prograding clinoform bodies extending over 300 m provides a good overview of the slope facies arrangement of this carbonate platform (Fig. 7). The slope facies, described in Table 2, occur in seven sedimentary bodies, including reefal clinoform bodies (CB) with intercalated siliciclastic bodies.

Fig. 7
figure 7

a Outcrop panorama of the Polopos section. b Interpretation of the main clinoform bodies (CB), bedding, and facies

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In their uppermost part, the clinoform bodies comprise the reef-framework facies with Porites stick surrounded by thick stromatolithic crusts. The coral colonies form sub-vertical cliffs with an inclination of 80°–60° (Fig. 8a). The thickness of the reef-framework facies commonly varies from 10 to 15 m with a slope normal extension between 15 and 25 m. The reef-framework facies may contain up to 15% siliciclastics ranging from sand-sized (Fig. 8b) to pebble-sized (Fig. 8c). Locally, lagoonal facies overlies the reef framework (Fig. 7).

Fig. 8
figure 8

a Outcrop picture of the reef-framework facies with vertical Porites tubes and thick stromatolitic crusts from CB3 (Fig. 7). b Outcrop example of fine grained siliciclastics in the gaps among stromatolitic crusts at the base of the reef framework. c Outcrop example of coarse siliciclastic grains among horizontal Porites tubes in the transition from the reef framework to the siliciclastic-coral breccia

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The reef-talus slope facies has an inclination of 25° and consist of a Halimeda breccia with large reef-framework debris (Fig. 9). Halimeda breccia packages are 5–15 m thick and extend between 10 and 70 m in slope normal direction. In the case of CB1 and CB5 (Fig. 7), the talus-slope facies is a siliciclastic-coral breccia with a maximum thickness of 15 and a 70-m extension in slope normal direction. Sandstone to conglomerate beds locally cut across the siliciclastic-coral and the Halimeda breccia (Fig. 9).

Fig. 9
figure 9

a Outcrop picture of large framework blocks from the Halimeda breccia facies of CB3 overlying a thick siliciclastic body (Fig. 7). b Outcrop picture of the bioturbated red algal-bivalve packstone facies mixed with siliciclastics in the distal slope of CB3

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The proximal Polopos carbonate platform slope display angles of 20°–15° with a considerable facies variability. The facies may consist of (1) a siliciclastic-red algal-serpulid rudstone, locally displaying an alternation of floatstone intervals with rudstone beds (Fig. 10a, b); (2) a serpulid rudstone (CB3 and CB7 in Figs. 7, 10c, d); and (3) a siliciclastic-coral rudstone.

Fig. 10
figure 10

Main carbonate producers of the slope facies of the Polopos carbonate platform. a Close-up of a talus-slope accumulation of Halimeda plates. b Photomicrograph of a Halimeda rudstone interval within the Halimeda breccia facies. c Outcrop picture of a serpulid patch in the siliciclastic-red algal-serpulid rudstone. d Photomicrograph of the siliciclastic-red algal-serpulid rudstone showing two serpulid tubes filled in with siliciclastic grains. e Close-up of a conglomeratic interval within the siliciclastic-red algal-serpulid rudstone with serpulid tubes (black arrows) and red algal crust (white arrow). f Photomicrograph of a rhodolith, red algal encrustation around a bioclasts, in the Halimeda breccia facies

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The distal slope has inclinations between 15° and 10° and consists of red algal-bivalve packstone to floatstone that may interfinger with the proximal slope facies in its upper part. The thickness of the red algal-bivalve packstone to floatstone facies decreases downslope from 10 to 5 m. Where the siliciclastic content is high, the red algal-bivalve facies occurs as a siliciclastic-rich packstone to rudstone (CB5 and CB6 in Fig. 7).

In general, the reef-slope facies are rich in siliciclastics. The thickest siliciclastic body is a 2-meter-thick greyish conglomeratic sandstone intercalated between CB2 and CB3 (Figs. 7, 9).

Discussion

Miocene slope facies composition and distribution

The slope-facies composition and distribution result from the interaction of several factors including coral growth, in situ slope carbonate production, rockfalls and sediment gravity flows, hemipelagic rain and reworking of reef-slope facies, as well as siliciclastic input (Reolid et al. 2014). Changes in accommodation space related to sea-level fluctuations shaped the relative impact of these factors (Reolid et al. 2014).

Porites-stromatolite reef framework rims the uppermost part of most of the Miocene tropical carbonate platform slopes in southeastern Spain (Fig. 11). Stromatolites encrusted the Porites colonies, producing an early lithification (Riding et al. 1991). The breakage of the Porites-stromatolite framework at the reef front subsequently produced rock and debris falls (Dabrio et al. 1981; Riding et al. 1991). This process resulted in the accumulation of blocks and debris on the reef-talus slope as described for the Barranco de la Mora and the Polopos sections (Figs. 4, 9, 11). Rockfalls and debris falls triggered sediment flows that spread basinward to the distal reef-slope with decreasing transport capacity (Adams et al. 1998; Reolid et al. 2014). Under conditions of low accommodation space, these rockfalls may export large reef-framework blocks to more distal settings than during conditions of high accommodation space, as recorded in the Barranco de los Castaños section (Fig. 11; Reolid et al. 2014). During lowstand periods, the erosion and reworking of the slope deposits intensified, resulting in the accumulation of bioclastic sands in fan-like bodies, which form the lowstand wedges of the Cariatiz carbonate platform (Fig. 11; Braga and Martín 1996; Cuevas-Castell et al. 2007).

Fig. 11
figure 11

Comparison of Miocene tropical platform slopes from SE Spain including: a Barranco de los Castaños and b Barranco de la Mora sections in Cariatiz carbonate platform (Sorbas Basin); and the sections of the c Níjar carbonate platform and d the Polopos carbonate platform slopes (Níjar Basin)

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Halimeda plates are the main facies component in Miocene carbonate platform slopes of southeastern Spain (Mankiewicz 1988; Martín and Braga 1990; Riding et al. 1991; Braga and Martín 1996; Reolid et al. 2014). Halimeda plates either accumulated in situ as small mound-like structures as in Barranco de la Mora section (Fig. 4b) or were exported downslope by sediment flows, forming parautochtonous to allochtonous accumulations along the slope as in the Polopos section (Fig. 10). Mankiewicz (1988) described similar Halimeda-rich facies and related them to upwelling conditions. Reolid et al. (2014) proposed that upwelling of nutrient-rich waters during sea-level rise and highstand stages promoted the flourishing of Halimeda in the Barranco de los Castaños section. The decreasing accommodation space with continued sea-level fall promoted the mixture of water masses and consequently the interruption of upwelling the absence of Halimeda algae (Reolid et al. 2014). Under such conditions, bioclastic packstone to rudstone formed with common bivalves, gastropods, echinoids, red algae, serpulids, and benthic foraminifera as described in the Barranco de los Castaños section (Reolid et al. 2014) and in the Barranco de la Mora and Polopos section (Fig. 11). Turbidity currents by episodic river discharge are an additional factor that may affect the substrate colonization by Halimeda algae and their development (Mankiewicz 1988).

Silt and marl in the distal reef-slope (Fig. 11) indicate relatively quiet-water conditions (Sánchez-Almazo et al. 2001, 2007). The extension of distal slope facies may diminish in clinoform bodies deposited during sea-level fall and lowstand (Fig. 11).

Slope facies variations: serpulid build-ups and siliciclastics

The carbonate platform slopes have a higher degree of facies variability in comparison to other Miocene carbonate platform slopes from southeastern Spain (Fig. 11). This facies variability depends on the intercalation of serpulid-rich and siliciclastic-rich facies in the typical slope sediments.

Facies variability related to siliciclastics

During the Neogene, carbonate platform growth in southeastern Spain was coeval with fan-delta sedimentation (Braga et al. 1990), and therefore these platforms underwent episodic input of coarse-grained sands and conglomerates (Fig. 11; Braga et al. 1996). Siliciclastic shedding in the Sorbas Basin started during the early Messinian, related to the uplift of the Sierra de los Filabres and Sierra Alhamilla (Braga et al. 2003). In the Polopos area, the siliciclastic input was triggered by the synsedimentary tectonic activity of the reverse Polopos fault, which delimits the carbonate platform to the northwest (Pedrera et al. 2012; Giaconia et al. 2012). In addition to the tectonic activity, the intensification of weathering may control the siliciclastic influx, which varied independently of sea-level changes (Braga and Martín 1996).

In the Barranco de la Mora area, two siliciclastic-rich fan delta bodies (Fig. 3; Table 1) contain a conglomerate in the most proximal part (SRB1 in Fig. 3), and sands in the distal part (SRB2 in Fig. 3). In the Polopos area the influence of the continental discharge is greater than in the Barranco de la Mora area with a complete range of siliciclastic-rich facies from pure fan delta deposits (Siliciclastics in Fig. 7; Table 2) to mixed facies where the fan delta mixed with the talus-slope breccia, siliciclastic-coral breccia, or mixed with the different proximal-slope facies, such as siliciclastic-coral floatstone, siliciclastic-red algal-serpulid rudstone, and siliciclastic-rich packstone to rudstone (Fig. 7; Table 2).

Facies variability related to build-ups

Large serpulid build-ups, comparable to those from the Barranco de la Mora section, require low-energy settings which allow larval settlement (ten Howe and van den Hurk 1993; Qian 1999). Modern serpulid build-ups occur exclusively in low-energy settings such as Albufera of Menorca, in the Balearic Islands (Fornós et al. 1997) and the Galway Bay in Ireland (Bosence 1973). The area of the Barranco de la Mora section was probably a quiet environment with marls and limestones deposited in calm waters of the shallow distal slope (Sánchez-Almazo et al. 2001, 2007) between 30 and 40 m (Fig. 3). A low-energy environment is congruent with the restricted occurrence of the Halimeda plates in CB3 that remained mostly in situ and were rarely transported downslope in contrast to other carbonate slopes in the area (Fig. 11; Reolid et al. 2014). The patches of bivalves in the distal slope deposits represent allochthonous assemblages, eventually exported downslope from the proximal slope as is the case in the Níjar platform slope (Jiménez and Braga 1993). Occasional events of increased hydrodynamic energy, however, in this part of the carbonate platform is indicated by the presence of centimetre-sized terrigenous clasts.

The average hydrodynamic energy at the distal slope with packstones to rudstones was higher in the area of Polopos. The siliciclastic input in Polopos was also more significant with frequent terrigenous clasts throughout the section. Such conditions may explain why serpulids, although abundant in the Polopos slopes did not form build-ups there but patches similar to those described in other Messinian carbonate platform slopes (Sola et al. 2013; Reolid et al. 2014).

Salinity appears as another factor controlling serpulid buildup. In Menorca, low salinity apparently favors serpulids relative to other organisms (Fornós et al. 1997). Serpulid build-ups in low-salinity settings also grew in the Miocene of the Styrian Basin in Austria (Friebe 1994). We propose that fresh water influx in the Barranco de la Mora area lowered salinity weakening the reef and favored the development of opportunistic and pioneer organisms such as serpulids after episodes of enhanced continental runoff. Porites is among the coral genera that are least sensitive to salinity changes (Moberg et al. 1997; Manzello and Liman 2003), but its survivorship capacity decreases with the duration of low-salinity conditions (Manzello and Liman 2003). Elsewhere, serpulids act as pioneer organisms during repopulation of shallow-water zones under stress conditions (ten Howe and van den Hurk 1993; Climaco et al. 1997; Rosso and Sanfilippo 2005; Schlögl et al. 2008). In the more turbulent area of Polopos, the input of fresh water was probably reduced and therefore less favorable for serpulid build-ups.

After terrestrial discharges salinity normalized and Porites progressively recovered its role as main reef builder in the Barranco de la Mora section. CB5 represents a transitional stage from a serpulid to a Porites-dominated clinoform body (Fig. 3). The serpulid-red algal nodules occurring in the distal slope of CB5 (Fig. 5e) are probably fragments of serpulid build-ups, in this case with red algae, reworked by the same gravity-driven processes that exported reef-framework blocks to the distal slope. The hydrodynamic conditions of the basin became more energetic as deduced from the alternation of thin fine-grained beds and thick nodule-accumulation beds (Fig. 5f), which caused the decline of large serpulid build-ups.

Facies belts or facies mosaic?

Previous studies on western Mediterranean Miocene reefs described the facies distribution as belts (Pomar 1991; Pomar and Ward 1994; Dabrio et al. 1981; Franseen and Mankiewicz 1991; Riding et al. 1991; Braga and Martín 1996; Cuevas-Castell et al. 2007). Facies belts result from the combination of controlling factors such as the initial bathymetry as well as biological and sedimentary processes (Wright and Burgess 2005). The Miocene tropical carbonate platform slopes of Almería show a downslope facies zonation, i.e., facies belts, by the basinward decrease in grain size, from reef-framework blocks and breccia to fine-grained packstone (Fig. 11).

The facies distribution of the Miocene platform slope was usually assumed static in architectural and stratigraphical analyses (Braga and Martín 1996; Cuevas-Castell et al. 2007). Reolid et al. (2014), however, showed that slope facies arrays are not static, because changes in accommodation space control the relative impact of processes affecting the slope (i.e., carbonate production and gravity flows) as well as their intensity, and, in this respect, the extent and occurrence of facies belts (Fig. 11; Reolid et al. 2014). Facies patterns depended on changes in the accommodation space that control the intensity of gravity-driven processes along the slope margin and changes in the hydrography conditions of the basin (Fig. 12). The gravity-driven processes are usually triggered by collapse of the reef front (Riding et al. 1991; Reolid et al. 2014), and they may vary in occurrence and intensity along the platform edge (Fig. 12; Playton et al. 2010). These collapses also changed through time in response to sea-level fluctuations (Reolid et al. 2014). The variability associated with gravity-driven processes may result in the reduction or obliteration of some facies belts (Fig. 12). Sea-level fluctuations may also affect the facies variability, as in the case of Halimeda-rich facies in the Cariatiz carbonate platform that abundantly occur during relative highstands and disappear during sea-level falls and lowstands, replaced by bioclastic packstone to rudstone with common red-algae (Reolid et al. 2014).

Fig. 12
figure 12

Model of the facies variability in the Miocene tropical carbonate platform slopes from SE Spain (Fig. 11). A Slope facies distribution under conditions of increased accommodation space. Facies display a downslope decrease of grain size, from reef-framework blocks and breccia to fine-grained sediments at the distal slope, and an extensive development of Halimeda algae in areas with upwelling (highstand conditions; Reolid et al. 2014). B Facies distribution along a slope with episodic continental input. The continental influx changes the hydrographical conditions of the basin causing a poor development or absence of Halimeda growths. These conditions are suitable for the opportunistic serpulids that may form build-ups under quiet hydrodynamic conditions. C Facies distribution in slopes forming under conditions of decreased accommodation space affected by intensive rock falls and gravity flows. The extensive distribution of reef-framework debris may reduce or suppress some facies belts (Reolid et al. 2014)

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Reolid et al. (2014) linked the dynamic behavior of slope facies belts to sea-level fluctuations. This approach, however, does not fully explain the apparently random facies distribution of the Polopos and Cariatiz carbonate platform slopes (Fig. 11). Wright and Burgess (2005), drawing on Wilkinson et al. (1999), proposed the concept of facies mosaics to characterize slopes with an arrangement of lithological elements lacking significant linear trends but showing some statistically significant relationship between element size and frequency. The processes determining the facies distribution in a mosaic could be derived for each patch of sediment (Wright and Burgess 2005), but collecting such information is difficult to impossible. Another study advocates an intermediate position between the facies belts and mosaics to explain the patchy distribution of coral framework in the northern Red Sea, and the Gulfs of Suez and Aqaba (Riegl and Piller 2000).

This work shows that the occurrence of terrestrial influx and variations of the hydrodynamic energy of the carbonate platform slopes lead to local changes of the dominant facies assemblage (Fig. 11). Such changes are independent of the accommodation space and may occur at any sea-level position (Fig. 12). Although the occurrence of continental input in the carbonate platform slopes seems to be random, the facies association related to the input is predictable (Fig. 12). We propose a banded mosaic that considers the stochastic phenomena along the slope (i.e., occurrence, of restricted areas or local terrestrial influx) but also allows predicting the depth zonation related to biological and gravity-driven processes, ultimately related to sea-level changes.

Conclusions

Miocene tropical carbonate platform slopes show classical reef-slope facies distribution but also an unexpected abundance of serpulid-rich facies, locally forming build-ups. We propose that the extensive occurrence of the serpulid-rich facies is related to recolonization of the substrate by the pioneer serpulids after hydrographical changes in the environment related to intense continental runoff. The development of large serpulid build-ups correlates to quiet hydrodynamic conditions of the slope in restricted areas of the carbonate platforms. The interplay of sea-level changes and continental supply control the facies variability laterally and along the slope, as well as through the progradation of the carbonate platform. We suggest a new model that explains facies variability in the context of intrinsic and extrinsic factors.