Episodic folding inferred from syntectonic carbonate sedimentation: the Santaren anticline, Bahamas foreland

Sedimentary Geology 146 (2002) 11 – 24 www.elsevier.com/locate/sedgeo

Episodic folding inferred from syntectonic carbonate sedimentation: the Santaren anticline, Bahamas foreland Jose´ L. Masaferroa,*, Mayte Bulnesb,1, Josep Pobletb,1, Gregor P. Eberlia, 2 a

Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA b Departamento de Geologı´a, Facultad de Geologı´a, Universidad de Oviedo, C/Arias de Velasco s/n, 33005 Oviedo, Spain Received 29 December 1998; accepted 18 June 2001

Abstract Sedimentation coeval with growth of the Santaren anticline provides an excellent opportunity to study the relationships between sedimentation and anticline uplift through time. The Santaren anticline is a kilometre-scale, NW – SE trending fold offshore of Cuba, in the Bahamas foreland of the Cuban fold and thrust belt. The growth strata associated with this anticline consist of a thick package of carbonate sediments that were deposited without major interruptions from Neogene (and perhaps before) to present day. The excellent seismic resolution and age control of a number of seismic horizons within the growth strata allowed us to define 25 growth beds, each of them representing between 0.1 and 3.2 Ma. An analysis of the thickness of these beds allowed us to determine accurate quantitative values of cumulative decompacted thickness and crestal structural relief at the time of their deposition. In addition, for the same periods, sedimentation and fold uplift rates were calculated. Moreover, some information on relationships between sedimentation and fold uplift rates was inferred from the growth stratal geometry. Growth beds that overlap the fold crest and thin over it indicate that sedimentation rates outpaced fold growth rates during their deposition. Some overlapping beds have constant thickness indicating that no fold uplift occurred during their sedimentation. The rest of the growth beds exhibit onlap/offlap geometries that do not indicate a unique sedimentation/fold uplift rate relationship. Only in those cases in which the geometry of the underlying bed at the end of its deposition is known is it possible to infer a specific sedimentation/fold uplift rate relationship. As a result of this analysis, we have been able to (1) illustrate that the geometry of the growth strata associated with the Santaren anticline results from competition between sedimentation and tectonic fold uplift, (2) document the episodic and non-steady nature of fold growth, and (3) show that short-term rates (at the scale of hundreds of thousands years) provide much insight into the interplay between sedimentation and tectonic fold uplift that control the growth stratal patterns. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Growth stratal geometry; Fold uplift; Episodic folding; Santaren anticline; Bahamas foreland

* Corresponding author. Shell International Exploration and Production B.V.-EPT-HM, SIEP B.V., P.O. Box 60, 2280 AB, Rijswijk, The Netherlands. Fax: +31-70-3113064. E-mail addresses: [email protected] (J.L. Masafer ro); [email protected] (M. Bulnes); [email protected] (J. Poblet); [email protected] (G.P. Eberli). 1 Fax: +34-98-5103103. 2 Fax: +1-305-3614632.

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 0 1 ) 0 0 1 6 3 - 4

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1. Introduction The kinematics of natural examples of fault-related folds can be unraveled by estimating some uplift, shortening and/or limb dip values through time derived from the study of height of growth terraces (Rockwell et al., 1988; Molnar et al., 1994; Nicol et al., 1994), sequential restorations of growth strata (DeCelles et al., 1991; Bloch et al., 1993; Rowan et al., 1993; Verge´s et al., 1996; Ford et al., 1997; Suppe et al., 1997), palaeomagnetism of syntectonic unconformities (Holl and Anastasio, 1993) and analysis of growth stratal patterns (Poblet and Hardy, 1995; Poblet et al., 1998; Schneider et al., 1996; Butler and Lickorish, 1997). In many of these studies, it is assumed that deformation rate between the estimated values for different time instants was continuous through time. Recently, the study of fold kinematics and fold axial surfaces (Shaw and Suppe, 1994), syntectonic unconformities (e.g. Barnes, 1996), throw along fault scarps (Kelson et al., 1996), and terraces and alluvial fan ridges (Mueller and Suppe, 1997) allowed the growth of some natural fault-related folds to be attributed to geologically instantaneous events

induced by earthquake-related slip on active faults. The formation and amplification of natural faultrelated folds during well-dated earthquakes during the 1980s and 1990s have also been documented (Yielding et al., 1981; Stein and King, 1984; Klinger and Rockwell, 1989; Lin and Stein, 1989; Phillip et al., 1992; Berberian and Qorashi, 1994). These observations raise the question of whether deformation may occur as a continuous or a discontinuous process. In the latter case, the present-day state of a growth structure might result from a sum of episodes of deformation and non-deformation. In addition, the different deformational/non-deformational events might be geologically instantaneous or steady during a variable period of time (see Fig. 6 in Suppe et al., 1997). In this paper, we show that a detailed analysis of the geometry of syntectonic sediments associated with a natural growth fold, together with good chronostratigraphic control, can be an invaluable tool to understand the interplay between sedimentation and tectonic activity and how they influence the growth stratal patterns, determine short-term variability in sedimentation and fold uplift rates, and

Fig. 1. (a) Map of central Cuba, south Florida and the contiguous area. (b) Simplified structural map of the central part of the Cuban fold-andthrust belt and Bahamas foreland (modified after Echevarrı´a-Rodrı´guez et al., 1991).

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Fig. 2. (a) Unmigrated, time seismic profile across the Santaren Anticline. See Fig. 1b for location. (b) Geological interpretation of unmigrated, time seismic profile showing the syntectonic sediments located in the north limb of the Santaren anticline. Letters C to N correspond to the growth horizons studied in this paper.

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ascertain the continuous or discontinuous nature of fold growth. Unfortunately, in the case of episodic fold growth, growth strata on their own are not able to clarify the reason for the fold’s episodic evolution. The natural example documented in this paper is the Santaren anticline, located offshore of Cuba, in the Bahamas foreland of the Cuban fold and thrust belt (Fig. 1). This growth fold has been used because: (1) the stratigraphic and structural features of the growth strata associated with the Santaren anticline are exceptionally imaged in an available seismic profile; (2) this seismic profile illustrates a complete section of the growth strata, from the oldest beds preserved over the pre-growth units to the youngest beds that form the sea bottom (Fig. 2); (3) the seismic profile was recently migrated and depth converted (Figs. 3 and 8 in Masaferro et al., 1999), which has allowed us to visualize the true growth stratal geometry; and (4) seven new well logs acquired during the Ocean Drilling Project (ODP) Leg 166 (Eberli et al., 1997) farther north (Fig. 1 in Masaferro et al., 1999) provided an accurate chronostratigraphic and lithologic control (Fig. 4 in Masaferro et al., 1999) on the Neogene growth beds involved in the Santaren anticline. Only the northern limb of the anticline will be addressed because: (1) the seismic resolution in this limb is better than in the southern one; (2) the dated growth horizons were correlated from the wells to the northern limb of the anticline using a grid of seismic profiles (Masaferro et al., 1999); and (3) unlike the southern limb, the northern limb includes a well-developed, wide basin that is adjacent to the anticline and filled with flatlying growth beds.

2. Geological setting Cuba is a north/northeast-directed, fold-and-thrust belt that formed during different orogenic phases from the Late Cretaceous to the Eocene and is disrupted by a number of major NE –SW striking faults (see summaries in Echevarrı´a-Rodrı´guez et al., 1991; Draper and Barros, 1994) (Fig. 1). The tectonic structures observed in the cross-sections onshore (e.g. Meyerhoff and Hatten, 1968; Echevarrı´aRodrı´guez et al., 1991; Draper and Barros, 1994; Iturralde-Vinent, 1994) extend north of Cuba into the

offshore area (Bahamas foreland basin) as documented by seismic and well data (e.g. FurrazolaBermu´dez et al., 1964; Meyerhoff and Hatten, 1968; Idris, 1975; Pardo, 1975; Tator and Hatfield, 1975a, 1975b; Ball et al., 1985; Echevarrı´a-Rodrı´guez et al., 1991; Dorobek, 1995; Masaferro and Eberli, 1995). The Santaren anticline, located offshore in the Bahamas foreland basin, is a NW –SE trending anticline parallel to the folds and thrusts seen onshore and in the adjacent offshore area, and it corresponds to the northernmost boundary of folding associated with the Cuban orogen (Fig. 1). This anticline, about 8 km wide and 70 km long, is approximately symmetrical and has a subvertical axial plane and a subhorizontal fold axis (Fig. 2a). Ball et al. (1985) and Echevarrı´a-Rodrı´guez et al. (1991) interpreted the Santaren anticline as a fault-related fold, and Masaferro et al. (1999) concluded that it was a detachment fold involving limb rotation according to the mechanisms proposed by Epard and Groshong (1995), Homza and Wallace (1995) and Poblet and McClay (1996).

3. Syntectonic carbonate sedimentation The Santaren anticline includes two stratigraphic units separated by a major unconformity (Ball et al., 1985; Masaferro et al., 1999) (Fig. 2). All these authors agree that the geometry of the sediments of the lower unit indicates that they were deposited prior to fold growth. Masaferro et al. (1999) interpreted the upper unit as growth strata based on the fan-like geometry of the sediments (thinning as they approach the fold limb/crest and decreasing in dip up section) (Fig. 2). The geometry of the sediments of the upper unit resulted mainly from fold growth and not from compaction because of: (1) their quick consolidation due to rapid cementation (Eberli et al., 1997; Friedman, 1998); (2) their low decrease in porosity with depth and the little compaction they underwent (Eberli et al., 1997; Masaferro et al., 1999); and (3) the comparison with a nearby buried platform margin which was tectonically inactive and was onlapped/overlapped by approximately flat-lying upper unit sediments (Ball et al., 1985; Masaferro et al., 1999). Since the objectives of this paper concern the interplay between sedimentation and tectonics,

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and the nature of fold amplification inferred from syntectonic deposits, only the upper unit will be analysed here. 3.1. Depositional setting The Santaren anticline is located in the southern portion of the Santaren Channel, approximately 15 to 25 km away from the southwestern edge of the Great Bahama Bank (Fig. 1b). The Great Bahama Bank is a pure carbonate environment that has been functioning as a carbonate factory since its inception in the Upper Jurassic. Its western margin comprises Miocene to Holocene laterally stacked, prograding sequences that evolved from a lowangle to a steep-sided platform (Eberli and Ginsburg, 1989). Maximum production and export of sediments to the adjacent slopes occurred when the platform top became flooded during sea-level highstands, while platform exposure during low sea level terminated carbonate production and resulted in low sedimentation rates (Eberli et al., 1997). Syntectonic carbonate sedimentation in the vicinity of the Santaren anticline has taken place during the Neogene (and perhaps before) until present day. Because of the distal position of the anticline with respect to the platform margin, the growth strata associated with the anticline were governed mainly by pelagic/hemipelagic sedimentation and were less influenced by platform-derived material (Ball et al., 1985; Eberli et al., 1997). 3.2. Growth stratal architecture The growth sequence involved in the Santaren anticline consists of an alternation of strong, continuous, highly coherent reflections with more transparent reflections (Fig. 2a). These seismic facies were interpreted as mixed pelagic/hemipelagic sediments intermittently interrupted by platform-derived carbonates with various amounts of clay material (Ball et al., 1985; Eberli et al., 1997). The growth strata architecture consists of beds mostly onlapping the pre-growth strata in the anticline limb (lower part of the growth sequence), and beds mostly overlapping the anticline crest (middle/ upper part of the growth sequence) (Fig. 2b). Within both packages (lower and middle/upper part

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of the sequence), some horizons show offlap/onlap architectures with respect to underlying beds. Stratal terminations against the major unconformity and against the underlying growth beds show a general trend of onlapping/offlapping reflections migrating towards the fold crest in the lower/middle part of the sequence and away from it in the upper part of the sequence. A number of beds thin as they approach the anticline limb/crest, whereas others go over the anticline but show no thickness change. Such a geometry may be caused by episodic fold growth (Hardy and Poblet 1994), variations in the local sedimentation rate, sea-level variations and/or erosion by bottom currents (Sheridan, personal communication). The lack of onlap/offlap geometries within equivalent sediments where they onlap/ overlap a tectonically inactive, buried platform margin located a few kilometers to the west –southwest of the Santaren anticline, and the distal position of the Santaren anticline with respect to the margin of the Bahamas platform (Fig. 1b) allowed Masaferro et al. (1999) to discard the sea-level variations and/ or erosion by bottom currents hypotheses. 3.3. Age of main horizons Biostratigraphic indicators collected in a number of ODP Leg 166 boreholes that penetrate a coeval carbonate sequence farther northwest in the Santaren Channel provided precise ages of a number of seismic reflections (Eberli et al., 1997). Masaferro et al. (1999) traced 11 of these reflections to the Santaren anticline using a grid of seismic profiles, and labeled them using letters from C to M (Fig. 2b). The age of these key stratigraphic markers ranges from the lowermost Early Miocene to the uppermost Early Pliocene (Fig. 2b) according to the time scale of Cande and Kent (1992). The age of these stratigraphic markers indicates that sedimentation was fairly continuous throughout the Neogene without major interruptions except for a hiatus of 2.5 Ma within horizon K (Late Miocene), so that the anticline ended up covered under a thick blanket of carbonate sediments. In the time seismic profile illustrated in Fig. 2a, 14 seismic reflections have been interpreted between and in addition to the dated horizons. We have labeled these undated seismic reflections using the letter of the underlying

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dated horizon and a numerical subscript (Fig. 2b). This has allowed us to define 25 different beds bounded by the interpreted seismic reflections. 3.4. Sedimentation rates In order to estimate sedimentation rates of the growth sequence, we have measured the true thickness of the 25 beds. To do that, we have migrated and depth-converted the seismic profile illustrated in Fig. 2a using the seismic velocities and formulae utilised by Masaferro et al. (1999). The thickness of these 25 beds on the anticline crest and in the basin adjacent to the anticline has been decompacted using the porosity data obtained in borehole 1007 (ODP Leg 166) supplied by Eberli et al. (1997) and the decompaction algorithm proposed by Angevine et al. (1990), and Allen and Allen (1990). The ages of the 14 undated seismic reflections interpreted in this paper have been determined by interpolating the ages of the 11 dated horizons assuming constant sedimentation rates in each time interval. Thus, each bed between two seismic horizons records between 0.1 and 3.2 Ma. The results have been plotted on a cumulative thickness versus time graph for both the sediments deposited on the anticline crest and in the basin adjacent to the anticline (Fig. 3a). This plot shows that sedimentation rates (slope of the lines that connect different horizons) in the basin adjacent to the anticline are slightly greater than on the anticline

crest, and they are not constant through time. Fig. 4c illustrates the sedimentation rates obtained for each bed. The sedimentation rates in this region can be grouped into two short episodes characterized by high sedimentation rates (34.0 cm/ka during the Early Miocene and 29.0 cm/ka during the uppermost Late Miocene) separated by two long episodes of low sedimentation rates (from 2.1 to 8.1 cm/ka). Similar relatively high sedimentation rates were obtained at boreholes 1007, 1003, 1005 and 1006 (ODP Leg 166) (Eberli et al., 1997) for the lower Miocene (approximately equivalent to interval C – D) and upper Miocene/lower Pliocene (approximately equivalent to interval K –M) sediments. Relatively high sedimentation rates were probably the result of a combination between deposition of drift sediments and deposition of platform-derived material that bypass the upper slope and were deposited at the toe-of-slope (Eberli et al., 1997). Relatively low sedimentation rates, such as the ones determined within the intervals E – K and M –N, are within the range of normal pelagic deposition (3– 4 cm/ka) and coincide with the low sedimentation rates determined at borehole 1006 (ODP Leg 166) (Eberli et al., 1997). These authors attributed the sedimentation rate variations to the turning off (low sedimentation rate) and on (high sedimentation rate) of platform production in response to sea level changes and/or the effect of lateral transport of sediments by ocean currents (drift deposits).

Fig. 3. (a) Plot of cumulative decompacted thickness versus time for syntectonic beds deposited on the Santaren anticline crest and in the basin adjacent to the anticline. (b) Plot of the crestal structural relief of the pre-growth strata versus time for the Santaren anticline. The letters C to N correspond to the growth horizons interpreted in Fig. 2b.

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4. Fold uplift 4.1. Calculation procedure The crestal structural relief of a fold is the elevation of a particular stratigraphic horizon in the anticline crest with respect to the ‘‘regional datum’’ defined by the same stratigraphic horizon. The regional datum is the elevation of a particular

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stratigraphic horizon where it is not involved in the tectonic structure (McClay, 1992). Fig. 3b illustrates a plot of the structural relief of the pregrowth strata involved in the Santaren anticline versus time (during growth strata sedimentation). The crestal structural relief of the Santaren anticline at a specific time ‘‘t’’ has been obtained by subtracting the thickness of all the growth beds deposited on the anticline crest prior to time ‘‘t’’ from

Fig. 4. (a) Crestal structural relief versus time graph for horizons C and C1 to illustrate the variability in uplift rates. (b) Crestal structural relief versus time graph for horizons G1, H and H1 to illustrate the variability in uplift rates. (c) Plot of sedimentation and fold uplift rates for each growth bed. The fold uplift rates have been estimated assuming a ‘‘fill to the top’’ approach.

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the thickness of the same growth beds in the basin adjacent to the anticline (Fig. 12 in Masaferro et al., 1999). In the case of horizons deposited at time ‘‘t’’ that do not extend over the fold crest because they onlap/offlap underlying beds, the crestal structural relief estimated using the procedure described above are minimum values. In the case of horizons that onlap pre-growth strata, maximum possible crestal structural relief values have been obtained by subtracting the structural relief at present day of horizons deposited at time ‘‘t’’ from the crestal structural relief at present day measured using the pre-growth strata. In the case of horizons that offlap underlying growth strata, the maximum crestal structural relief corresponds to the value of the crestal structural relief measured for the overlying growth horizon deposited at ‘‘t + 1’’ time. The thicknesses used to perform the calculations are not the present-day thicknesses, but the decompacted thickness in the case of the youngest growth bed analysed and the partially decompacted thickness in the case of the underlying growth beds. Our method to estimate the crestal structural relief of a fold uses the thickness of the growth strata in areas where they are flat-lying, i.e. fold crest and basins adjacent to the anticline. These data can be measured in sections both perpendicular and oblique to the anticline trend and are not influenced by the orientation of the cross-sections with respect to the anticline. This procedure assumes that the growth beds were deposited horizontally, that their variations in thickness after deposition were due exclusively to compaction, and that no shear occurred on the pre-growth/ growth surface. The results achieved using this technique may include some uncertainties such as errors related to the decompaction procedure, errors related to the occurrence of palaeodepositional slopes, and errors related to possible tectonic thickness variations during fold growth. However, all these uncertainties are hardly quantifiable. Error bars associated with these values have been omitted in Fig. 3b for clarity. Knowing the crestal structural relief of the Santaren anticline at different times makes it possible to estimate fold uplift and fold uplift rates. The fold uplift during deposition of a particular growth bed equals the crestal structural relief for the horizon

located at the top of this particular growth bed minus the crestal structural relief for the horizon located at the base of this growth bed. The fold uplift rate takes into account the lapse of time between deposition of the base and top horizons. When dealing with beds that do not continue over the fold crest, it has not been possible to determine precisely the fold uplift and fold uplift rate, because of the infinite possible intermediate paths between the maximum and minimum values of crestal structural relief. There are two main different situations in which fold uplift and fold uplift rates for the Santaren anticline could not be estimated precisely. (1) In the case of two consecutive growth horizons that onlap the pre-growth strata (e.g. bed C – C1), the fold uplift and fold uplift rates are (Fig. 4a): (a) minimum when the maximum crestal structural relief for the onlapping beds is considered; (b) high when we consider minimum crestal structural relief values for the older onlapping bed and high crestal structural relief values for the younger onlapping bed; and (c) intermediate when we consider cases between the two end members described in (a) and (b). An intermediate fold uplift rate is obtained when considering the minimum crestal structural relief for each of the onlapping beds (‘‘fill to the top’’ approach, i.e. the depositional surface is flat so that no topography above/below the sea bottom is formed, see Hardy and Poblet, 1995). (2) In the case of an onlapping/offlapping growth bed located in between overlapping growth beds three different situations occur. To illustrate this situation beds G1 –H and H –H1 will be used (Fig. 4b): (a) when the maximum crestal structural relief for the offlapping horizon H is considered, fold uplift and fold uplift rates are maximum for bed G1 – H and minimum (equal to zero) for bed H –H1; (b) when the minimum crestal structural relief for horizon H is considered, fold uplift and fold uplift rates are minimum for bed G1 – H and maximum for bed H – H1 (‘‘fill to the top’’ approach); and (c) an intermediate situation results for cases between the end members described in (a) and (b). Fig. 4c illustrates the fold uplift rates obtained for all the growth horizons involved in the Santaren anticline. The values corresponding to growth beds that do not extend over the fold crest have been calculated using the ‘‘fill to the top’’ approach.

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4.2. Fold uplift and fold uplift rates The most conspicuous feature of the crestal structural relief versus time graph (Fig. 3b) is that the average slope of the curve is very gentle. This demonstrates that, in general, growth of the Santaren anticline took place very slowly during the Neogene. The curve can be divided in two portions. In the first portion (lowermost Early Miocene), which reflects the initial growth stages (onlap predominates), the slope may be steep or gentle depending on the crestal structural relief values used (maximum or minimum values). In the second portion (from middle Early Miocene to present day), which reflects the later growth stages (overlap predominates), the slope is gentle. The same two episodes may be distinguished for fold uplift rates if a ‘‘fill to the top’’ approach is considered (Fig. 4c). The first episode comprises the lowermost Early Miocene and the rates estimated are 34.0 cm/ka. The second episode ranges from middle Early Miocene to present day and the rates rarely exceeded 8.1 cm/ka. There are a number of events in which the fold uplift rates were 0 cm/ka and there is a local event during the uppermost Late Miocene in which the fold uplift rate was 29.0 cm/ka.

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The geometry of the variable thickness beds may result from sedimentation that either was or was not coeval with tectonic uplift. Thus, a bed may thin because it covers a previously formed fold which was inactive at the time of sedimentation. In this situation, no tectonic uplift occurred during sedimentation. Alternatively, a bed may thin because the fold was being formed during sedimentation. In this case, sedimentation was coeval with fold amplification. A bed may also thin because the fold formed partially before deposition, but was still active during deposition. In this case, sedimentation was also coeval with fold amplification. Fig. 5 schematically illustrates beds with different geometries observed within the growth unit associated

5. Relationships between tectonics and sedimentation 5.1. Inferences on the relationships between sedimentation and fold uplift rates from the geometry of the growth beds The upper stratigraphic unit involved in the Santaren anticline has been interpreted as a syntectonic sequence coeval with fold growth. In this section, we analyse the geometry of single growth beds instead of all the growth strata as a whole, because this provides detailed information on tectonics/sedimentation relationships. There are two main types of beds within the upper stratigraphic unit (Fig. 2b): those that have constant thickness and those that thin towards the fold limb/crest. No tectonic uplift took place during the deposition of the constant thickness beds, and this is why we propose the term ‘‘atectonic beds’’ to refer to them.

Fig. 5. Conceptual diagram that illustrates four different bed geometries observed within the growth strata associated with the Santaren anticline. (a) Onlap over the major unconformity and thickness change. (b) Overlap over the fold crest and thickness change. (c) Onlap/offlap over underlying growth beds and thickness change. (d) Overlap over the fold crest and constant thickness.

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with the Santaren anticline. In Fig. 5a, a bed onlaps the unconformity that separates the pre-growth and growth strata, and thins as it approaches the fold limb. If folding was previous to sedimentation (i.e. the fold uplift rate during sedimentation was equal to zero), then the bed is ‘‘atectonic’’ and the sedimentation rate was greater than the fold uplift rate. If folding commenced at the same time as sedimentation, then the sedimentation rate was equal (in the case of a ‘‘fill to the top’’ approach) or less than the fold uplift rate. If folding was partially previous to and partially simultaneous with sedimentation, then it is not possible to precisely determine the relationships between the sedimentation and fold uplift rates, unless the bed ‘‘fill to the top’’. In this case, the sedimentation rate was greater than the fold uplift rate. Beds onlapping the unconformity that separates the pre-growth and growth strata, and thinning as they approach the fold limb occurred mainly during early growth stages of the Santaren anticline (beds below horizon C1) (Fig. 2b). A second situation occurs when a bed overlaps the fold crest and thins over it (Fig. 5b). Irrespective of the chronological relationships between sedimentation and fold uplift, the sedimentation rate in this case was greater than the fold uplift rate and no bathymetric relief developed. This situation occurred repeatedly during most stages of Santaren anticline growth exce- pt for the initial stages (beds D –E, F – G, H –H1, H1 – I, K1 –K2, L3 – L4, L4 – M, M– M1 and M4 – N in the basin adjacent to the anticline) (Fig. 2b). The third situation occurs when a bed onlaps/ offlaps an underlying growth bed and thins towards the anticline limb/crest (Fig. 5c). Relationships between sedimentation and fold uplift rates during the deposition of an onlapping/offlapping bed are equivalent to the ones described in the first case of a bed that onlaps the major unconformity (Fig. 5a). If sedimentation is coeval with fold uplift, the closer the onlap/offlap point to the anticline crest, the less the difference between sedimentation and fold uplift rates, and vice versa. Beds that onlap/offlap previous growth beds occur at several points during amplification of the Santaren anticline: within the package in which onlaps over the major unconformity predominate (bed C1 – D) and within the package in which overlaps predominate (beds E – F, G1 – H, I–

J, J –K, K2 –K3, L –L1 and M2 – M3 in the basin adjacent to the anticline) (Fig. 2b). The fourth situation consists of a bed that overlaps the fold crest with no thickness changes (Fig. 5d). This indicates that the sediments passively covered a flat depositional surface with no bathymetric relief during a period of tectonic quiescence. The sedimentation rate was necessarily greater than the fold uplift rate, because no fold uplift took place during the deposition of this bed. This type of situation occurs at several points within growth of the Santaren anticline (beds G – G1, K – K1, K3 – L, L1 – L2, L2 – L3, M1 – M2 and M3 –M4 in the basin adjacent to the anticline) (Fig. 2b). These beds are ‘‘atectonic’’, but also posttectonic with respect to underlying growth sediments, and pre-tectonic with respect to overlying growth sediments. 5.2. Tectono-sedimentary history of the Santaren anticline The syndepositional growth history of the Santaren anticline can be divided into four tectono-sedimentary episodes based on the fold uplift rates, sedimentary rates and onlap/overlap geometry of the growth strata. (1) The first episode initiated by 23.7 Ma (and perhaps before) and lasted until time between 23.2 and 19.2 Ma (lowermost Early Miocene). The minimum duration of this event was slightly greater than 0.5 Ma. The beginning of this episode is based artificially on the age of the oldest dated horizon. The depositional geometry in the underlying undated strata is consistent with the geometry of the beds included in this episode (Fig. 2b); therefore, the rates of fold uplift and deposition could have been similarly since the onset of the lower unit sediments. The maximum sedimentation rates recorded during fold amplification were attained in this period (34.0 cm/ka) (Fig. 4c). Growth beds onlapping the fold limb and thinning as they approach the fold limb predominated during this period (beds below C1), although a local offlap occurred (bed C1 –D) (Fig. 2b). Since the precise age of the beginning of fold growth is unknown, the attitude of the onlapping beds does not uniquely indicate the relationship between sedimentation and fold uplift rates. Nevertheless, taking the onlapping beds as a whole, the occurrence of onlaps and offlaps, and the steeper dip

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of the oldest onlapping beds with respect to the youngest onlapping beds (Fig. 2b), indicates that a certain amount of fold amplification took place during this period. (2) The second episode initiated between 23.2 and 19.2 Ma, and lasted until 8.7 Ma (middle Early Miocene – lowermost Late Miocene). The duration of this event was 10.5 Ma or slightly greater. Sedimentation rates varied from 2.1 to 8.0 cm/ka (Fig. 4c). Deposition of the first growth sediments that overlap the fold crest took place during this period. Most beds deposited during this episode overlap the fold crest and thin over it (beds D – E, F– G, H – H1 and H1 – I) (Fig. 2b). This means that sedimentation rates outpaced fold uplift rates. However, within this overlapping package some beds onlap/offlap and thin towards the fold limb/crest (beds E – F, G1 – H, I– J and J –K). This may indicate that during the sedimentation of each offlapping bed, or just before, the fold uplift rate was higher than the sedimentation rate. There also is a bed (G – G1) that overlaps the fold crest but has constant thickness, which indicates that no fold growth occurred during its sedimentation. (3) From 8.7 to 6.2 Ma (middle Late Miocene), a hiatus occurred within horizon K, so that no sedimentation was recorded (Fig. 2b, 3a). This interruption lasted 2.5 Ma. Since bed K – K1, deposited above the hiatus, overlaps the fold crest and maintains constant thickness, this implies that no fold uplift took place during the hiatus. (4) The last episode lasted from 6.2 Ma to present day (uppermost Late Miocene – Quaternary). Sedimentation rates were high during the earliest part of this event (29.0 cm/ka), intermediate during the middle part of this event (8.1 cm/ka) and low during the latest part of this event (4.8 cm/ka) (Fig. 4c). The large number of overlapping, constant thickness beds deposited during this episode (beds K –K1, K3 – L, L1 – L2, L2 – L3, M1 – M2 and M3 – M4) (Fig. 2b) implies that there were many lapses within this episode during which no tectonic uplift occurred. Many beds overlap and thin over the fold crest (beds K1 –K2, L3 – L4, L4 –M, M – M1 and M4 –N), which means that fold uplift rates were lower than sedimentation rates during many lapses of time. Some beds offlap underlying beds (beds K2 – K3, L– L1 and M2 – M3). During sedimentation of these beds or just

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before, the fold uplift rates were greater than the sedimentation rates. Since the older the beds, the greater the sedimentation rate during this period, the greatest fold uplift rate took place during or just before deposition of bed K2 – K3 (Fig. 4c). 5.3. Other considerations From the sedimentation and fold uplift graphs and the geometry of the reflectors, it is difficult to establish a fully calibrated sequence of behaviour of the anticline. The evolution of the Santaren anticline consists of cycles that involved tectonically active periods separated by interruptions in which the tectonic activity fell to zero. The duration of either event may be very variable. The transition from no tectonic activity to tectonic activity and vice versa may be gradual (constant thickness, overlapping bed-variable thickness, overlapping bed) or abrupt (constant thickness, overlapping bed-variable thickness, offlapping bed). The tectonic climax reached during the active tectonic periods may be very intense or smooth. The patterns of the growth strata associated with the Santaren anticline have been analysed as a function of the local sedimentation rate and fold uplift rate (which includes sediment compaction, subsidence due to syntectonic sedimentary load, fold uplift due to tectonic shortening and subsidence due to fold uplift). However, they may be also influenced by the regional subsidence of the foreland basin (e.g. Doglioni and Prosser, 1997). Constraining regional-scale tectonic controls on the syntectonic sedimentation is difficult because we have considered only a single anticline. Modelling such relationships requires more complex, interactive algorithms and more regional data which are not available and are beyond the scope of this study.

6. Conclusions The geometrical analysis of the Neogene– Quaternary syntectonic carbonate sequence located in the northern limb of the Santaren anticline (Bahamas foreland, Cuban fold and thrust belt), together with the chronostratigraphic control on a number of seismic growth horizons, allowed us to understand the

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influence of sedimentation and local tectonics in defining growth stratal geometries, and to define short-term variability in sedimentation and fold uplift rates. Growth strata associated with the Santaren anticline exhibit different types of geometries: onlap, offlap, overlap, variable and constant thickness. These geometries appear to be a result of competition between sedimentation and tectonic fold uplift. Neither sedimentation nor tectonic fold uplift on their own dictates the final growth stratal geometry. The growth stratal geometry coupled with the relationships between sedimentation and fold uplift rates enabled us to document the episodic nature of the growth of Santaren anticline. Thus, the amplification of the Santaren anticline was a discontinuous process characterized by several tectonic uplift pulses of different duration and intensity interrupted by periods of variable duration in which no fold growth occurred. Carbonate sediments appear to be a good record of fold kinematics due to the ability of the carbonate system to produce sediments independently from tectonism. In many analyses of growth structures, best-fit curves have been used to describe the long-term rates of parameters such as the sedimentation and deformation rates. However, we must keep in mind that they are only approximations, and that both sedimentation and deformation may be discontinuous and instantaneous processes produced at variable rates. In this sense, short-term rates (at the scale of hundreds of thousand years) provide more insight on the interplay between sedimentation and fold uplift that control the growth stratal patterns.

Acknowledgements We wish to thank Mark T. Harris, Wesley K. Wallace and the editors of this volume (Mariano Marzo, Josep A. Mun˜oz and Jaume Verge´s) for their helpful comments and suggestions. Financial support by projects AMB98-1012-CO2-O2 (Actividad sismotecto´nica, estructura litosfe´rica y modelos de deformacio´n varisca y alpina en el NO de la Penı´nsula Ibe´rica) and PB98-1557 (Mecanismos de plegamiento: teorı´a y aplicaciones en geologı´a econo´mica y regional) funded by DGICYT (Ministry for Education and Culture,

Spain), and by Accio´n Integrada Hispano-Brita´nica HB1999-0038 (Cinema´tica de pliegues y estructuras menores asociadas a cabalgamientos a partir del estudio de materiales sintecto´nicos y de su modelizacio´n) funded by the Spanish Ministry for Education and Culture and the British Council is gratefully acknowledged.

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