Geometries of reef advance in response to relative sea ...hera.ugr.es/doi/15013352.pdf ·...

Sedimentary Geology 107 (19%) 61-81

Geometries of reef advance in response to relative sea-level in a Messinian (uppermost Miocene) fringing reef

(Cariatiz reef, Sorbas Basin, SE Spain)

J.C. Braga *, J.M. Martin

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Deparramenio de Estratigmfia y Paleontologia, Universidad de Granada, Campus Fuentenueva, 18002 Granada. Spain

Received 2 October 1995; accepted 14 March 1996

Abstract

The pre-evaporitic Messinian sequence in the Sorbas Basin includes two reef units. At the northern margin of the basin, near Cariatiz, the uppermost unit consists of a fringing reef advancing 1150 m towards the basin centre. Reef-facies distribution and geometries in three N-S sections parallel to reef progradation reveal cyclic relative sea-level changes during reef development. Two orders of cyclic@ can be recognised. The lower order (Cl cyclic@) is represented by one cycle and by the beginning of another one that is interrupted in its ascending phase. This cyclicity is modified by higher-frequency cycles (C2 cyclicity), which developed between consecutive wedges of calcamnite beds that onlap an erosive surface on previous deposits and pinch out landwards (inverted wedges). The inverted wedges are thought to represent the lowstand deposits of C2 cycles and no coeval reef growth is recorded. Sea-level rise within C2 cycles results in reef aggradation and aggradation combined with progradation. Reef deposits prograded during the highest sea level, and offlapped during sea-level fall. Relative proportions of aggmding. prograding, and offlapping geometries inside C2 cycles depend on the interference of Cl and C2 cycles. The estimated relative sea-level change in the complete Cl cycle is about 100 m. Sea-level oscillations in C2 cycles have an amplitude of several tens of metres. Biostmtigrapbic and magnetostratigraphic data indicate that the Cariatiz reef developed in less than 0.36 Ma. If eustasy was the majur factor controlling relative sea-level change, this temporal range, together with the observed amplitudes and relative frequencies, would suggest that Cl and C2 cycles may represent short eccentricity and precession cycles, respectively. However, the estimated sea-level changes are greater than the high-frequency eustatic oscillations reported from Upper Miocene deposits from other areas.

Fan-delta siliciclastics locally interfered with reef growth. Main fan-delta activity took place during the high and descending sea-levels of the first Cl cycle but there was no significant elastic influx during the lowstand.

1. Introduction

Sea-level control of reef development and facies architecture has been illustrated in fossil and Re-

*Corresponding author. Fax: +34 58 243203; E-mail: jbraga@ goliat.ugr.es.

cent (Holocene) examples for more than two decades (Mesolella et al., 1969, 1970; James and Ginsburg, 1979; Playford, 1980; Marshall and Davies, 1982; James and Macintyre, 1985). It is also currently accepted that sea level is a major factor controlling the productivity, depositional geometries and sequence stratigraphy of carbonate platforms (Kendall and

0037-0738/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SOO37-0738(96)00019-X

NEOGENE @jsediments

a volcanics

LLi BASEMENT

bo de Gata lm

Fig. I. (A) Regional setting of the Sorbas Basin in SE Spain. (B) Location of Cariatiz and the study area (inset)

Schlager, 1981; Bosellini, 1984; Sarg, 1988; Eberli and Ginsburg, 1989; Pomar and Ward, 1991, 1994).

Some western Mediterranean Upper Miocene reef platforms have been the subject of detailed stratigraphic analyses that reveal the influence of sea-level fluctuations on their depositional architecture (Saint- Martin and Rouchy, 1986; Goldstein and Franseen, 1993, but the most fruitful and precise analyses have been carried out in the exceptionally well-exposed Upper Miocene reef complex of Mallorca. Here, several hierarchically-stacked units reflect the eustatic oscillations of different frequency orders throughout a 20 km platform progradation (Pomar, 1991; Po- mar and Ward, 1991, 1994; Bosence et al., 1994). Stratigraphic relationships of these reef units and their boundary surfaces have led to the development of new concepts in carbonate sequence stratigraphy (Pomar and Ward, 1991, 1994).

In this paper we analyse facies distribution and geometries in an early Messinian reef platform in Cari- atiz, at the northern margin of the Sorbas Basin in southeastern Spain (Fig. 1). This platform constitutes one of the pre-evaporitic Messinian sedimentary units in the Sorbas Basin (Braga and Martin, 1992; Martin and Braga, 1994). The Messinian Cariatiz reef was referred to by Dabrio et al. (1985) and Ott d’Estevou

and Montenat (1990) in their descriptions of the Up- per Miocene stratigraphy and sedimentary evolution of the Sorbas Basin. Riding et al. (1991) analysed the peculiar reef-framework facies of this reef and Braga and Martin (1992), Bosence et al. (1992), and Martin and Braga (1996) reported on the cyclic development of reef advance, but no detailed analysis of geometries has been undertaken up to now.

Cyclic relative sea-level changes controlled reef development and depositional architecture during reef-platform advance from the margin towards the basin centre. The resulting geometries differ in some aspects from those described in previously reported examples. The deposits related to the lowest sea level in the higher-order cycles appear to be especially significant for the understanding of reef-platform development. Fan-delta siliciclastics occur intercalated in reef carbonates. The relative sea level at which terrigenous sedimentation took place can be inferred from the stratigraphic relationships of carbonates and siliciclastics.

2. Regional setting

The Sorbas Basin is a narrow, E-W elongate basin within the Betic Cordillera in southeastern

J.C. Braga, J.M. Martin/Sedimentary Geology 107 (19%) 61-81 63

N-S 200 Ill I

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Fig. 2. Neogene lithostratigraphy of the Sorbas Basin (after Martin and Braga, 1994).

Spain (Fig. 1). The reliefs bounding the basin and its basement are made up of metamorphic rocks from the Internal Betic Zone. The Neogene basin filling consists of several units separated by unconformities (Fig. 2). The lowermost sediments are red conglomerates and sands that can be attributed to the Middle Miocene (Serravallian?). At the north- em margin, the overlying upper Tortonian unit consists of platform carbonates including some coral patch reefs, which change laterally to conglomerates in localised areas. To the south the conglomerates grade to a mixture of conglomerates, turbidite sandstones, and silty marls deposited in submarine fans (Kleverlaan, 1987, 1989). Calcarenites and bioclas-

tic sandstones (Azagador Member of Ruegg, 1964) overlie all these materials. In some areas at both margins of the basin, conglomerates are mixed with these temperate carbonates composed of fragments of bryozoans, coralline algae, molluscs, and macro- foraminifers and lack hermatypic corals and green algae (Braga and Martin, 1992; Mart&t and Braga, 1994). The platform deposits grade upwards and laterally into fine-grained calcarenites and marls. The planktonic foraminiferal assemblages record the Tortonian-Messinian boundary in the centre of the basin near the base of the marls (Sierro et al., 1993). The lowest Messinian reef unit lies on top of the temperate carbonates and marls. At the margins this unit

64 J.C. Braga, J.M. Martin/Sedimentary Geology 107 (1996) 61-81

consists of platform carbonates locally mixed with siliciclastic sediments. Coral and algal bioherms occur within these deposits (Martin and Braga, 1990; Braga and Martin, 1992, 1993; Braga et al., 1993, 1996). Basinwards they give way to silty marls and marls with diatomites that include reef blocks and breccias shed from the bioherms on the platform margin. The next unit, analysed in detail below, is made up of fringing reefs that prograded to the centre of the basin. The top of the fringing reefs is marked by an erosional surface with signs of sub- aerial exposure. Evaporites (selenitic gypsum) lie on this unconformity surface in the central parts of the basin. Overlying the evaporites are sands and silts that onlap the margins and grade to coarser siliciclastics mixed with oolites, stromatolites and thrombolites with some intercalated coral patch-reefs (Terminal Complex of Riding et al., 1991) (Martin et al., 1993). These shelf and slope deposits lie directly on the erosional surface at the top of the fringing reef. Fluvial red conglomerates, sands and silts, and lacustrine limestones (Zorreras Member of Ruegg, 1964), probably Pliocene in age, and a thin Pliocene marine unit constitute the rest of the Neogene basin succession.

The uppermost Tortonian and Messinian record of the Sorbas Basin has been interpreted as being the result of combined tectonic activity and eustatic changes (Martin and Braga, 1996). Two depositional sequences can be recognised (Martin and Braga, 1994, 1996). The lower one, probably representing the TB3.3 cycle of Haq et al. (1987), includes the temperate carbonates as the lowstand systems tract and the bioherm and fringing reef units as the transgressive and highstand systems tracts, respectively. Therefore, the unit analysed in this paper, the fringing reef unit, is considered a highstand systems tract of a third-order cycle. The upper sequence begins with the evaporites as the lowstand, and continues with the Terminal Complex and the Pliocene continental deposits, respectively representing the transgressive and the highstand systems tracts of a cycle that can be correlated with the TB3.4 of Haq et al. (1987).

3. Methods

The study of the reef system architecture has been made by detailed mapping of lithologies and reef

facies. Conventional surface mapping (Fig. 3) has been combined with facies mapping on panoramic photomosaics of the best-exposed sections. The results have been transfened to the topographical base of a 1 : 10,000 scale map. Reef-facies geometries and relationships have been analysed in three composite sections, cropping out at the sides of the two main ravines in the area. They have been projected onto N-S lines, approximately parallel to the direction of reef progradation. These sections are named Bar- ranco de 10s Castafios East (BCE), Barranco de 10s Castaiios West (BCW), and Barranco de la Mora (BCM) (Fig. 3).

4. Reef facies

Facies in the fringing reef at Cariatiz were described by Riding et al. (1991), although they re- stricted themselves to the last episodes of reef development. The following facies can be differentiated in a single stage of reef growth from the basin to the reef crest (Fig. 4).

(1) Distal slope (lowermost slope). Silty marls and calcisiltites-calcarenites (packstones) intercalat- ing with basinal marls and diatomites.

(2) Proximal slope (middle slope). Calcarenites and calcirudites (packstones to rudstones). Recog- nizable bioclasts are mainly coralline algae, serpulids and molluscs. Hulimeda bioclasts are also very abun- dant locally.

(3) Reef-talus slope (uppermost slope). Frame- work blocks and coral breccias with Halimeda plates, bivalves, serpulids, and coralline algae. Some laminar Porites colonies encrusting bioclasts also occur.

(4) Reef framework, including three vertical di- visions. (a) A lower Pinnacle Zone. Stick Porites connected by laminar growths form colonies in the shape of inverted cones 2-3 m high. Coral colonies are grouped in pinnacles up to 15 m in height separated by areas with reef debris. Porites skeletons are coated by thin coralline algal-foraminiferal crusts and thick stromatolitic crusts. A bioclastic micritic matrix fills the spaces between the resulting frame. (b) A Thicket Zone similar in composition but with more laterally continuous coral growth. (c) A Reef Crest characterised by laminar horizontal to irregularly contorted Porites colonies. Vertical stick

J.C. Bmga, J.M. Martin/Sedimentary Geology 107 (19%) 61-81 65

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Fig. 3. G~ologkal map of the study area showing locations of sections: Barranco de 10s Castaikx East (BCE), Baanco de 10s Castafios West (BCW), Barranco de la Mom (BCM).

66 J.C. Braga, J.M Martin /Sedimentary Geology 107 (1996) 61-81

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colonies are very scarce. Coral skeletons are coated by thick stromatolitic crusts that constitute most of the framework (75% or more). The matrix filling the voids is mainly rudstone. From the bottom of the Pinnacle Zone to the top of the Reef Crest, the Reef Framework is about 20 m thick in the sites described by Riding et al. (1991).

(5) Lagoon. Not described by Riding et al. (1991). Calcarenites to calcirudites (packstones to floatstones) with coral fragments, coralline algae, foraminifers and molluscs, mainly gastropods. Porites heads or thick-branched colonies are common near the reef framework. Siliciclastic grains are locally mixed with the carbonates.

In a single episode of reef development the horizontal lagoonal beds grade to the south (basinwards) to the subvertical reef framework, which in turn gives way downwards to the poorly-bedded, steeply dipping talus-slope breccias and then to better-bedded, more gently dipping sediments in the proximal and distal slope. Slope deposits thin basinwards from proximal to distal positions very rapidly (Fig. 4).

Two other types of deposit occur associated to the reef system at Cariatiz.

(1) Inverted wedges. Well-bedded, onlapping, wedge-like units that pinch out landwards (Fig. 5A) and consist of packstones to rudstones with fragments of molluscs, mainly oysters and pectinids, bryozoans, red algae, serpulids and scarce coral fragments. Locally they show erosive lower surfaces, bar morphologies and cross-bedding directed landwards (Fig. 5B).

(2) Fan-delta deposits. In the central part of the study area, siliciclastic deposits in which debris-flow conglomerates alternate with sandstones interfere with reef carbonates (Fig. 3). Individual conglomerate lobate bodies are a few hundreds of metres wide and some hundreds of metres long. Clasts, up to 1 m in size, are mainly made up of quartzites, micaschists, marbles and amphibolites from the pre- Neogene basement cropping out at Sierra de Filabres to the north. The clasts are supported by a micro- conglomeratic or sandy matrix. These deposits represent the middle fan facies of a fan delta prograding southward from the northern basin margin. The conglomerates and sandstones grade basinwards into alternating sandstones and siltstones constituting the outer fan facies. Coral growth took place on the con-

J.C. Braga, J.M. Martih/Sedimentary Geology 107 (1996) 61-81 67

Fig. 5. Inverted wedges in the BCM section. (A) The inverted wedge (I W) pinches out to the upper right (narthwards) between talus-slope breccias (b). Note the onlapping geometry of beds inside the wedge. Talus breccias pass downslope into well-bedded calcarenites (c). (B) Inverted wedge showing erosive lower surface (white arrows) on proximal-slope calcarenites (c), and cross-bedding (black arrowhead) dipping north.

glomerates and sandstones, probably during inactive p&xls of the fan delta or on temporarily inactive areas of the fan. The conglomerates locally incorpo- rate reef blocks and smaller reef-derived fragments formed by the destruction of previous reef deposits by new debris flows. In general, reefs associated with fan deltas are common in the Upper Miocene in the Neogene basins of SE Spain (Martin et al., 1989; Braga et al., 1990).

5. Reef geometries

Vertical shifts of reef facies are evidence for cyclical relative sea-level changes along with reef advance (Figs. 6-8). The total progradation of the Cariatiz reef system from the northern margin was at least 1150 m. Facies distribution and geometries at the BCE section (Figs. 6-8) reveal cyclic@ at two different levels.

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70 J.C. Braga. J.M. Martin/Sedimentaty Geology 107 (1996) 6/-X1

Fig. 8. Panoramic view of the southernmost part of the BCE section from the west. f = reef framework; b = talus-slope breccias; c = slope calcirudites-calcarenites; s = coarse siliciclastics; bl = isolated talus blocks.

5.1. Cl cyclic&y

At the northern part of the section, talus-slope fa- ties crop out in progressively higher positions up to an area (segment A, Fig. 6A) where only proximal- slope calcarenites are preserved. Southwards, talus- slope breccias crop out again and shift downwards as the reef system advances to the south, reaching their lowest position at point B (Fig. 6A). Framework facies occur on top of some of the slope breccias following their downwards shift (Fig. 6B). From point B, talus-slope and framework facies move upwards again up to the end of the reef outcrop (Fig. 6A, 6B). This shift of facies can be interpreted as the result of relative sea-level change involving an up-down cycle, and the beginning of another one interrupted in its ascending phase (Fig. 6A, 6B) (Braga and Martin, 1992; Martin and Braga, 1996). This is the lowest-order cyclicity recognised in the reef system (Cl cycles).

Lagoon deposits overlie talus-slope breccias and proximal-slope calcarenites in the two northern thirds of the BCE section (Figs. 6A and 7A, 7B).

The surface below the lagoonal deposits is karstified and locally encrusted by iron deposits. The karst cavities are filled by breccias made up of reef clasts in an iron oxide and carbonate matrix. This facies relationship implies partial emergence and erosion- karstification of reef deposits during the low sea- level stage of the first cycle (C1.l) and subsequent deposition of lagoonal facies on top of the erosion surface during the sea-level rise stage of the following cycle (C1.2). Lagoon beds onlap the karst surface at the northernmost part of the section, revealing that flooding of the surface progressed as sea-level rose.

5.2. C2 cyclicity

The Cl cycles are modified by a higher-frequency cyclicity that is clearly marked by reef-facies distribution between two consecutive inverted wedges. This pattern is very evident in the southern half of the BCE section (Fig. 6B). There, talus-slope breccias lying on top of a precise inverted wedge shift upwards and then downwards again below the next

J.C. Bmga, J.M. Martin/Sedimentary Geology IO7 (1996) 61-81 71

inverted wedge. Preserved framework facies follow the same trends (Fig. 6B).

These C2 cycles can be interpreted as the result of relative sea-level oscillations of a higher frequency than those controlling the Cl cycles. Inverted wedges mark the beginning of the cycles and are the deposits thought to have formed at the lowest sea level in each C2 cycle.

Reef aggradation and aggradation combined with progradation took place during sea-level rise in the C2 cycles. Reef deposits prograded during the highest sea level and offlapped during sea-level fall (Fig. 9). However, the relative proportions of aggrading, prograding and offlapping geometries inside C2 cycles are controlled by the interference of Cl and C2 cycles. In cycle C2.5 and C2.6, which took place during the descending phase of cycle Cl. 1, progradational and offlapping geometries are better represented than aggradational ones (Fig. 6B). In contrast, the best represented geometries in cycle C2.7, which took place in the earlier ascending phase of cycle C1.2, are aggradation and aggradation plus progradation while offlap progradation is comparatively reduced (Figs. 6B, 8).

In cycle C2.7, relative sea-level rise caused aggradation of the reef framework. This aggradation took place through thickening of the pinnacle zone, which reaches 36 m in thickness at this point (Figs. 6B, 7C), i.e. almost three times the thickness it has at the southernmost end of the reef, at the outcrop described by Riding et al. (1991). This indicates that sea-level rise was initially very rapid and then reef growth kept pace with the rise, resulting in the development and aggradation of only the deeper reef framework facies of the pinnacle zone (keep-up reef in the sense of James and Macintyre, 1985). When the sea-level rise decreased or stopped, the reef growth approached the sea surface and shal- lower facies (thicket and reef crest facies) formed at this point (catch-up phase of the reef in the sense of James and Macintyre, 1985).

The downslope extent of coral blocks and breccias, i.e. the distance from the bottom of the coeval reef framework to the base of the talus-slope facies changes from point to point (Figs. 6, 8, 10). This may be due to several causes. Different rates of reef production and destruction and differences in ac- commodation space can lead to different downslope

spreading of these deposits. In some cases, clearly exemplified in section BCM (Fig. lOA), breccia beds extending much farther down than the surrounding middle slope facies suggest single episodes of mass- flow that extended unusually far downslope. This means that the position of the base of the talus-slope breccias cannot be used for precise palaeodepth es- timations, as the vertical distance between the latter and the base of the coeval framework is not constant and, therefore, cannot be used to estimate accurately sea-level position.

5.3. Signi$cance of the inverted wedges

The inverted wedges limiting the C2 cycles pinch out upwards and landwards but do not change laterally upslope to any reef facies (Fig. 5A). Upwards, the time equivalents of these units are erosional or non-depositional surfaces. Basinw~ds they parallel the reef-slope beds, as far as can be observed at the outcrops (IW2 in Fig. lOA, IW.5 and IW.7 in Fig. 6), or pinch out between the reef-slope calcarenites (IW.l in Fig. lOA). Their stratigraphic setting at the lowest position of reef materials of two consecutive C2 cycles suggests that they represent the lowstand deposits of the C2 cyclicity (see inverted wedges IW.5 and IW.6 in Fig. 6 and Fig. 9). Fragments of Porites included in the calcarenites-calcirudites of the inverted wedges indicate destruction of former or coeval reefs. Nevertheless, if reef growth took place at the time of formation of inverted wedges, it was very reduced since coral fragments represent only a small volume of these deposits and no coeval reef sediments are preserved. As mentioned above, their lower surfaces are erosive on the previous talus-slope breccias and proximal-slope calcarenites-calcirudites (Fig. 5B). In some cases they show internal cross-bedding directed landwards (to the N) and bar morphologies indicating sediment reworking and transport (Fig. 5B). Although these latter deposits presumably correspond to shallow, shore-face coastal bars, there is no definite sedimentary evidence on the precise formation depth of inverted wedges.

The inverted wedges display the same morphol- ogy and accretion pattern as the onlupping lowstand fans described by SchIager (1989), which are fed by downslope transport of sediment while previ-

72 J.C. Braga, J.M. Martin/Sedimentary Geology 107 (1996) 61-X/

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Fig. 9. Model of reef-advance geometries in C2 cycles. Inverted wedges mark the beginning of cycles and are the deposits formed at

lowest sea level. Reef aggradation and aggradation combined with progradation took place during sea-level rise. Lagoon beds onlapped the eroded and karstified previous deposits. Reefs prograded during highest sea level and offlapped during sea-level fall, at which time reef deposits from former phases began to erode. The relative proportions of aggrading,

prograding and offlapping geometries inside C2

cycles were controlled by the interference of Cl and C2 cycles.

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ous shelf deposits are exposed at lowstand of sea level.

Sedimentary bodies similar in geometry to these inverted wedges but representing a different spatial and temporal scale have been described in the Great Bahama Bank by Eberli et al. (1994) as ‘onlapping packages’ that constitute the base of the seismic sequences. These onlapping packages, however, are several kilometres long and tens of metres thick, and record large stratigraphic intervals (they occur in five seismic sequences from the Pliocene to the Recent).

In contrast, the lowstand sea level in the Miocene Mallorca platform is represented by prograding reef- framework and slope facies thinning towards the basin, which constitute the lowstand systems tract defined by Pomar and Ward (1991, 1994).

5.4. Depositional wedges

Framework and slope facies inside C2 cycles are arranged in depositional wedges thinning downslope and basinwards. The clarity of the wedge definition changes from place to place throughout the studied sections. Although they are sometimes well-defined with distinct lower and upper limiting surfaces (e.g. last wedge in C2.7 cycle, Figs. 6, 8), in most cases no evident surface delimits wedges inside talus-slope breccias and/or the reef framework and it is even difficult to distinguish them as separate sets of proximal-slope beds. These wedges may represent phases of reef growth but no cyclic arrangement of facies can be recognised inside them. In any given wedge, framework facies may ascend, descend, or remain at the same height according to the pattern imposed by the C2 cyclicity (Fig. 9>, but they do not show any internal oscillation that could be related to a higher-frequency cyclicity during wedge deposition.

The number of recognisable wedges in each C2 cycle varies from four to seven in the best-exposed, analysable parts of the studied sections.

Similar depositional wedges and/or sigmoids (so called when they grade landwards to horizontal lagoon beds) have been described in the Mallorca Late Miocene reefs (Pomar, 1991; Pomar and Ward, 1991, 1994; Bosence et al., 1994). There, they are considered to be the highest resolution units of reef development cyclicity, bounded by erosion surfaces.

They reflect cycles of seventh order equivalent to the fourth-order magnitude of sea-level change rec- ognizable in the Mallorca platform (Pomar, 1991: Bosence et al., 1994). Modelling of platform development suggests time periods of one to several tens of ka for such sigmoids, with a good matching of the model to the outcrop geometries when time periods of 5 ka and sea-level oscillations of 4 m are considered (Bosence et al., 1994).

Despite the geometrical similarities to the Mal- lorca examples, wedges in the Cariatiz reef show neither evident internal cyclicity nor regularity in their distribution nor well-defined boundary surfaces. They may represent more or less distinct phases of reef growth, but no significant sea-level oscillation is involved in their development, which is only controlled by sea-level changes related to C2 cycles (Fig. 9).

6. Sequence stratigraphy

Recently, Pomar and Ward (1991, 1994) developed a new model of sequence stratigraphy for prograding reef platforms in response to the diffi- culties of applying the classical siliciclastic sequence stratigraphy (Vail et al., 1977, 1991; Van Wagoner et al., 1988) and Sarg’s (1988) carbonate sequence stratigraphy to such depositional contexts. In their model, based upon the stratigraphy of the Upper Miocene reef platform of Mallorca, i.e. established for deposits close in space and time to the Cariatiz reef, Pomar and Ward (1991, 1994) recognised depositional sequences stacked in different magnitudes of accretionary units. Each depositional sequence consists of a lowstand systems tract, an aggrading systems tract, a highstand systems tract and a downstepping-offlapping systems tract, the latter similar to the forced regressive systems tract in Hunt and Tucker (1992, 1995).

The depositional geometries of the Cariatiz reef, especially those of the C2 cycles, display an over- all similarity to the sequence model in Pomar and Ward (1991) and Pomar and Ward (1994). How- ever, there are two main differences impeding the complete application of the Mallorca model to our example. (1) The absence of a lowstand systems tract composed of prograding reef deposits. In the Cariatiz reef, sea-level lowstands are represented


by inverted wedges with no recorded coeval coral growth. (2) The absence of a well-defined downlap surface bounding the offlapping reef deposits at their base. Offlapping depositional wedges neither con- verge in a single and distinct surface nor merge in a condensed section as is the case in Mallorca. The surfaces underlying the lowstand inverted wedges can be considered boundary surfaces of depositional sequences but no internal surfaces precisely delim- iting systems tracts are distinct in the reef complex studied (Fig. 9).

7. Sea-level oscillation amplitudes

Assuming that onlapping lagoonal beds were originally horizontal, the reef system of Cariatiz is now tilted 3” to the southwest (the strike of the plane is N126E). To estimate relative sea-level oscillation amplitudes, the sections have been counter-tilted to return the lagoonal beds to the horizontal. However, as no very precise measurements of sea-level oscillations can be made due to uncertainties in the precise palaeobathymetric significance of reef facies and to the lack of preservation of reef facies in the outcrop, similar results tend to be obtained from unrestored sections. The restored BCE section will be termed RBCE (Fig. 11A).

7.1. Cl cyclicity amplitude

In segment A of section RBCE, only proximal- slope calcarenites-calcirudites crop out (Fig. 11A). Talus-slope breccias must have been higher up from these proximal-slope facies before being eroded. Therefore, the highest present-day height of this calcarenite-calcirudite outcrop is the deepest possible point of talus-slope breccia deposition in segment A. The lowest outcrop of talus-slope facies is the bottom of the barranco at point B. From the highest point in segment A to point B there is a difference of 100 m, i.e. relative sea-level change from the highest level to the lowest level in Cl.1 cycle is at least 100 m (Fig. 11). As discussed above, this is only a rough estimation since the position of the base of talus-slope breccias cannot be used for very precise, metre-scale, palaeodepth reconstructions.

7.2. C2 cyclicity amplitude

In cycle C2.7 of the RBCE, the bottom of the framework rises 31 m from the lowest point at the beginning of reef development in the cycle to its highest outcrop, then it descends 17 m to the end of the cycle (Fig. 11B). The bottom of the talus-slope facies in the same cycle rises at least 41 m and then falls 28 m (Fig. 11B). In the previous C2 cycle, cycle C2.6, the base of talus-slope breccias rises 29 m and then falls 52 m (Fig. 11B). As mentioned above, none of these reef facies can pinpoint the sea-level position at their deposition time, but their vertical shifts allow us to estimate relative sea-level oscillations of some tens of metres for the C2 cycles. Only the reef crest can be used on a metre scale as a reference level to calculate sea-level oscillations. Unfortunately, the reef crest is only preserved in a few discontinuous outcrops from the latest stages of reef development.

8. Temporal range

Magnetostratigraphic and biostratigraphic work in sections of the Sorbas Basin centre reveals some chronological datums in the marls, silty marls and diatomites laterally equivalent to the reefs that developed at the basin margins. Magnetostratigraphic and biostratigraphic data from Gautier et al. (1994) indicate marl deposition ended at 5. 7 Ma, applying the geomagnetic polarity time scale from Baski (1993). In addition, Sierro et al. (1993) and Gautier et al. (1994) recognised in the Sorbas Basin a planktonic foraminifer event characterised by the coiling change from mostly sinistral to predominately dex- tral in Neogloboquadrina acostaensis. (PF-Event 4 in Sierro et al., 1993). This event occurs just below the beginning of the normal polarity interval 3An.ln (magnetostratigraphic scale from Cande and Kent, 1992), at approximately 6.06 Ma on Baski’s scale (1993). The same planktonic foraminifer event has been recorded in sections BCE and BCW in the middle part of the bioherm unit, which immediately underlies the fringing reef unit studied (Figs. 2, 6, 10). The upper half of the bioherm unit and the whole fringing reef unit must therefore have been deposited within an interval of about 0.36 Ma on Baski’s time scale (1993), between this planktonic

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foraminifer event and the end of the deposition of basinal marls laterally equivalent to the reefs.

A tectonic influence in the relative sea-level oscillations recorded in the Cariatiz reef cannot be discarded. In fact, angular unconformities and inter- ruptions of sedimentary cycles indicate that tectonic activity did indeed affect the Miocene infilling of the Sorbas Basin (Martin and Braga, 1996). In the hypothetical case that tectonic influence was minor or absent and eustasy was the major factor controlling the relative sea-level changes in the fringing reef, this time interval of 0.36 Ma suggests that the Cl cyclicity is inside the spectrum of Milankovitch cyclicity.

The Cl cycles must be of higher frequency than the long eccentricity cycles of ca. 400 ka and may correspond to the short eccentricity cycles of ca. 100 ka. The six C2 cycles occurring in the Cl.1 cycle may represent the precession cyclicity of about 20 ka (ranging from 14 ka to 28 ka, Einsele and Ricken, 1991). The estimated 100 m sea-level oscillation of the C 1.1 cycle is similar to the amplitude of sea-level oscillation for the short eccentricity cycles in the Pleistocene (Chappell and Shackleton, 1986; Boreen and James, 1993). The amplitude of a few tens of metres estimated for the C2 cycles is similar as well to the sea-level changes related to the precession cyclicity in the last 125 ka (Chappell and Shackleton, 1986; Boreen and James, 1993).

Sea-level oscillations of about 100 m have been recognised in the Upper Miocene reef platform in Mallorca but they occur in fourth-order depositional sequences, involving more than 5 km of reef progradation (Pomar, 1991; Pomar and Ward, 1994), and interpreted as having been deposited in periods longer than 100 ka (Bosence et al., 1994). Computer modelling of the Mallorca reef platform suggests that 100 ka cyclicity has a major influence on platform development but sea-level changes in this time interval would be 70 m at most (Bosence et al., 1994), which is the maximum sea-level change measured in fifth-order depositional sequences there (Pomar and Ward, 1994). The computer model proposed for the Mallorca reef does not reflect any clear pattern related to cycles of about 20 ka, whilst higher- frequency cycles of 5 ka seem to have a strong effect on the platform architecture (Bosence et al., 1994).

lOO-ka cycles (related to modulation of precession cycles by eccentricity) and precession cycles are well recorded in the Late Miocene in the Atlantic basins of Morocco (Hodell et al., 1994; Benson et al., 1995). However, palaeoceanographic analyses based upon oxygen isotopic data from sites in the Atlantic, Pacific and Indian oceans (Woodruff et al., 1981; Savin et al., 1985; Isem et al., 1993) indicate that high-frequency sea-level changes in the Late Miocene were less pronounced than in the Pleistocene. In summary, the cyclic relative sea-level changes in the Cariatiz reef are greater than to be expected for glacioeustatism in the Late Miocene according to data from other regions. Therefore, the exclusively eustatic origin of the reported sea-level changes remains debatable, although the estimated time intervals and amplitudes of the two orders of cycles recognised show similarities to those from Late Pleistocene glacioeustatic oscillations.

9. Silfciclasties and relative sea-level chaug+zs

Fan-delta, coarse-grained siliciclastics interfere with reef carbonates in the central part of the study area (Fig. 3). The geometry of these siliciclastic deposits and their relationships with reef materials are best exposed along the Barranco de 10s Cas- tafios West section (BCW) (Fig. 10B). Middle-fan conglomerates and sandstones prograded southwards on outer-fan sandstones and siltstones. Coral growth probably took place on the conglomerates and sandstones at times of arrested delta activity or laterally to the coeval more active delta areas. Reefs growing on top of fan-delta deposits developed talus-slope breccias and slope calcirudites-calcarenites similar to those of fringing reefs outside the fan-delta. Although no in situ reef-framework is preserved, framework blocks incorporated into the talus-slope breccias show the same framework facies already described for the fringing reefs. Conglomerate influxes destroyed previously developed reefs, incorporating reef clasts of various sizes.

Stratigraphic relationships between reef deposits and coarse siliciclastics in the BCW section indicate that the first coarse-terrigenous influxes took place at the highest relative sea level of cycle C 1.1 (Fig. 1OB). Most coarse siliciclastics were subse- quently deposited in the descending sea-level phase

78 J.C. Braga, J.M. Martin/Sedimentuq Geology 107 (1996) 61L~l

of C 1.1 but stopped before the lowest sea level was reached (Fig. IOB). Only a small, isolated, chan- nelised conglomerate body occurs slightly before the lowest point of cycle C 1.1. A similar conglomerate body occurs as well in the upshifting phase of cycle Cl.2 (Fig. 10B).

In section BCE, lateral to the main fan-delta deposition area, a conglomerate lens crops out at the bottom of the most conspicuous downshift of reef facies in cycle Cl.1 (Fig. 6), but conglomerates are absent when reef facies reach their lowest position. They also occur at the highest relative sea-level of cycle C2.6 and again at the bottom of the facies downshift finishing cycle C2.7 (Fig. 6). All this suggests that the occurrence of conglomerates is not related to any particular relative sea-level position within C2 cycles.

Coarse terrigenous deposition took place at the highest and descending sea levels of the major recog- nizable sea-level cyclicity (Cl cycles) but decreased before the end of the sea-level fall and was absent at the lowest sea level. Therefore, relative sea-level position alone did not control the activity of the fan delta, which was inactive at the maximum lowstand. Tectonics as well as climatic changes may have had a major role in switching the coarse siliciclastic discharge on and off.

10. Summary

The Messinian fringing reef at Cariatiz advanced 1150 m from the northern margin to the centre of the Sorbas Basin. Reef facies consist of a subvertical Porites-stromatolite reef framework that gives way downslope to talus-slope poorly-bedded, steeply-dipping breccias, which in turn change to proximal-slope calcirudites/calcarenites and gently dipping calcisiltites/calcarenites and silty marls in the distal-slope (Riding et al., 1991). Lagoon calcarenites developed landwards from the reef framework during some episodes of reef growth. Slope deposits thin basinwards from proximal to distal positions very rapidly (Fig. 4).

Two other types of deposits occur in the Cari- atiz reef system. (1) Inverted wedges consisting of well-bedded packstones to rudstones, which onlap an erosive surface on previous deposits and pinch out landwards. No coeval coral growths have been

preserved. Locally they show bar morphologies and cross-bedding directed landwards (Fig. 5). (2) Fan- delta conglomerates and sandstones which interfere with reef carbonates in the central part of the reef outcrop (Fig. 3).

Reef-facies distribution and geometries reveal that relative sea-level oscillations took place throughout reef advance. Two orders of relative sea-level cyclic change can be recognised. The lower order (Cl cycles) is represented by one cycle and by the beginning of another one that is interrupted in its ascending phase (Fig. 6).

These Cl cycles are modified by a higher- frequency cyclicity (C2 cycles). C2 cycles develop between consecutive inverted wedges, which represent the lowstand deposits (Figs. 6, 8, 10A). Reef aggradation and aggradation combined with progradation prevailed during sea-level rise in the C2 cycles. Progradation and offlapping progradation took place during the highest sea level and sea-level fall (Fig. 9). The relative proportion of aggrading versus prograding and offlapping geometries inside C2 cycles depends on the interference of C 1 and C2 cycles (Fig. 6).

Locally, framework and slope facies inside C2 cycles are arranged in well-defined depositional wedges thinning downslope and basinwards. In most cases, however, no clearly distinct wedges can be de- limited inside slope deposits and/or the reef framework. These wedges may represent phases of reef growth, but no cyclic arrangement of facies can be recognised inside them.

Uncertainties in the palaeobathymetric significance of reef. facies prevent metre-scale measurements of sea-level oscillations, but rough estima- tions of sea-level change can be made from differences in altitude of talus-slope and framework facies throughout reef development. The estimated sea-level change in cycle Cl. 1 is about 100 m and the amplitude of C2 cycles is several tens of metres (Fig. 11B).

Biostratigraphic data and their correlation with the bio and magnetostratigraphy in the basin centre indicate that the Cariatiz fringing reef formed in less than 0.36 Ma. Although the influence of tectonics in the recorded sea-level changes cannot be discarded, if eustasy is considered to be the major factor controlling sea-level oscillations, this temporal

J.C. Braga, J.M. Martin/Sedimentary Geology 107 (1996) 61-81 19

range suggests that the cyclicity in the Cariatiz reef is inside the spectrum of Milankovitch cycles. Ampli- tude of Cl cyclicity is similar to that of Pleistocene sea-level oscillations related to short eccentricity cycles. The frequency and amplitudes of C2 cycles are similar to the Pleistocene oscillations related to Earth precession. However, these sea-level changes are greater than the high-frequency eustatic oscillations recorded in Upper Miocene deposits from other areas.

Fan-delta deposition took place at the highest and descending sea levels within the Cl.1 cycle but decreased before the end of the sea-level fall (Fig. IOB). The fan delta was inactive at the maximum lowstand, i.e. the lowest base level did not promote a higher coarse siliciclastic discharge. Tec- tonics and/or climatic changes may have been major factors controlling fan-delta activity beyond relative sea-level position.

Acknowledgements

This work was supported by DGICYT (Spain) Project PB93-1113. We thank Francisco Femindez for the photographs and Chris Laurin for correct- ing the English text. We are grateful to T. Geel, C. Kendall, T. Roep and M. Tucker for helpful com- ments on the paper.

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