Sequences, cycles and hiatuses in the Upper Albian-Cenomanian of the Iberian Ranges (Spain): a cyclostratigraphic approach

Sedimentary

Geology ELSEVIER

Sedimentary

Geology

103 (1996) 175-200

Sequences, cycles and hiatuses in the Upper Albian-Cenomanian of the Iberian Ranges (Spain): a cyclostratigraphic approach A. Garcia a, M. Segura b, J.F. Garcia-Hidalgo

b

a Dpto. Estratigrafia, Inst. Geol. Econ. U.C.M.-C.S.I.C., E-28040 Madrid, Spain b Dpto. Geologh, Univ. Alcala’de Henares, E-28871 Alcala’de Henares, Spain Received

22 June 1994; revised version

accepted

12 September

1995

Abstract The Upper Albian-Cenomanian deposits in the Iberian Ranges are composed of a complex alternation of continental (sandstones), coastal (lime sandstones and marls) and marine (carbonates) facies. Facies are arranged mainly in transgressive and deepening-upward sequences and parasequences, but deepening-shallowing-upwards sequences and parasequences also exist. Depositional sequences and parasequences are bounded by subaerial erosive surfaces or ferruginous crusts, and by their correlative conformities. It is evident from the facies alternation that deposition reflects a cyclic process in which is superimposed several orders of cycles of relative sea-level rise and fall. The cycles are preserved as 3rd-order depositional sequences and 4th- and Sth-order discontinuity-bounded parasequences, the latter being the basic blocks of the field record. A cyclostratigraphic model for sedimentation shows that each of the 3rd-order cycles is composed of five 4th-order cycles. A 4th-order cycle is also composed of five Sth-order cycles. This 1:5:5 ratio reflects the maximum number of parasequences observed in the most complete and marine sections. The ratio remains unchanged throughout the basin, except by onlap caused by relative sea-level rises or truncations caused by relative sea-level falls. A detailed correlation of sections separated by more than 200 km shows that thickness differences are caused mainly by parasequence disappearance, not parasequence thinning. Cycles are mainly eustatic in origin, controlled by accommodation space. Increased accommodation generates thick parasequences of wider regional extension; they contain the more marine facies in any section. Only some sequences with a restricted regional distribution are recorded if the accommodation is low. The average duration of 3rd-order cycles was 1.95 Ma (ranging from 1.33 to 2.66 Ma), an average 4th-order cycle was 390 ka (266-533 ka) and a Sth-order cycle was 78 ka (53-106 ka).

1. Introduction Cyclic sedimentary sequences are a common topic in the stratigraphic literature since the works of Sloss et al. (1949) and Sloss (1963). This has

been particularly true from the end of the 7Os, with development of sequence stratigraphy, which renewed interest in cyclicity of the stratigraphic 0037-0738/96/$15.00 0 1996 Elsevier XSDI 0037-0738(95)00109-3

Science

record. Cycles from 1st to 6th order, ranging from several hundred million years to less than ten thousand years, have been recognised, described and their potential origin discussed (Vail et al.,

1977; Van Wagoner et al., 1988; Haq et al., 1988). The origin of these cycles has been controversial. Vail et al. (1977) proposed eustatic causes and they supposed a global extension for the

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cycles to build a global chronostratigraphic chart of sequences (Haq et al., 1988). On the contrary. other authors suggested tectonic, eustatic and sedimentary causes, which may generate the different order of cycles and sequences (Pitman, 1978; Watts et al., 1982; Vail et al., 1YYII. General agreement exists that many eustatic sea-level changes and cycles are related to the astronomical Milankovitch cycles (Schwarzacher, 1993). Cyclic sequences in carbonate environments have been widely recognised (Fischer, 1964; Wilson, 1975; Tucker and Wright, 1990). The common occurrence of cyclic sequences in carbonate strata reflects environmental changes, influenced by changes in relative sea level, which in turn have tectonic, sedimentary or climatic origins. Descriptions of higher-order cycles (4th to 6th) are common in tidal (Goldhammer et al.. 1990; Strasser, 1991) and pelagic environments (Einsele and Ricken, 1991; De Boer, 1991), whereas in the shelf and slope 3rd- to 4th-order cycles are more commonly recognised (Jacquin et al., 1991; Borer and Harris, 1991). Models for the generation of carbonate cycles are computer-generated (Read et al., 1986), or conceptual (Einsele and Bayer, 1991). Rarely is the superimposition of cycles described from field

data (Goldhammer et al., 1990; Borer and Harris, 19921, as emphasis is placed on the description of systems tracts. In this paper we present a model of cyclic sedimentation based primarily on field data of carbonate and mixed terrigenous-carbonate platforms of Upper Albian-Cenomanian age in the Iberian Ranges. The model tries to explain the superimposition of sequences of different order and their numeric relationships. It also explains why some sequences are thicker than others and why some are thin or even missing. Finally, we try to establish the time duration of the cycles. As a model derived from field data its validity is merely local, but we feel that it can be applied with minor modifications to many other basins. In this paper ‘cycle’ is used for the theoretical repetition in time of the processes controlling sedimentation, and ‘sequences’ are the sediments deposited during a cycle. Cycle is used as a timerelated idea and sequence is sediment-related.

2. Geologic setting and facies The Iberian Ranges are an Alpine erogenic belt (Fig. 1) located in the eastern part of the

Cenozoic Ebro Basin

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Fig. 1 Geographic (A) and geologic (B) setting of the eastern Iberian (Figs. 3 to 9) and the stratigraphic cross-sections of Figs. 2 and 3.

Peninsula

50 km

showing

location

of sections

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former Iberian microplate. During Permian to Cretaceous times the region was a site of active sedimentation located between two continental areas, the Hesperian and the Ebro Massifs. Through the time, the basin was either a wide and shallow basin, with deposition eustatically controlled, or a complex basin being tectonically controlled. Therefore, a varied sedimentary infill exists (2000-5000 m thick), consisting of continental to open-marine sediments. During the Upper Albian-Cenomanian, as a result of a complex Tethyan transgression in this basin, several shallow-water platforms were stacked and successively onlapped towards the continent. These platforms are characterized by a mix of carbonates and terrigenous sediments (Figs. 2 and 3). Platform facies have been described by M&s (19811, MelCndez (1983), Garcia et al. (1989b3), Ruiz (1993), Carenas et al. (19941, etc. They can be divided into terrigenous and carbonate facies.

Geology 103 (1996) 175-200

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Terrigenous fucies. (1) Cross-bedded coarsegrained sandstones and conglomerates with thin mudstone intercalations; they have erosive bases, basal coarser lags, plant debris and paleosoils; low-sinuosity fluvial environments. (2) Medium- to fine-grained sandstones and calcareous sandstones and mudstones. Sandstones are massive, cross-bedded, current- and wave-rippled, or burrowed; mudstones are massive or evenly laminated; interpreted as coastal littoral environments with tidal influence. (3) Green marls and mudstones, usually massive. They contain oyster-banks and root-burrowed limestones (wackestones). Restricted, lowenergy, lagoon with mangrove swamps. Marls similar to these have been interpreted as deposited in relation to regressions (Deconinck and Strasser, 1987). Carbonate facies. (1) Algal-laminated dolostones (stromatolites). Tidal flat sediments. (2) Thin-bedded, burrowed, nodular or mas-

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Fig. 2. Downdip cross-section showing the basin architecture based on 3rd-order sequences (A to E) and 4th-order parasequences (Aa to EC). The A-B boundary is conformable seawards (Puerto de1 Remolcador), but landwards it is a type-l sequence boundary with erosion and toplap development (Fuente la Puerca to Puerto de Casas Bajas). The B-C boundary is a type-2 sequence boundary with thick marl sedimentation (Chera marls bed). The C-D sequence boundary is a type-2 sequence boundary seawards, where marl sedimentation took place, but landwards there was erosion and a type-l sequence boundary developed. The D-E sequence boundary is a type-l sequence boundary with platform exhumation and paleokarst development. Main facies and the stratigraphic interval studied in each section of Figs. 3 to 9 are also shown. Facies grade from continental (thick dots) to open marine (blank) through different tidal, littoral and inner shelf facies (thin dots). The age (right) is based on Ammonites and benthic Foraminifera (see also Fig. 10).

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sive limestones (mudstones and wackestones) and dolostones, with Pelecypods, Gastropods, Echinoderms and Foraminifera. Open, low-energy, lagoon sediments. (3) Thin- to medium-bedded, rippled or crossbedded limestones (packstones), sometimes sandy, limestones with Pelecypods, Gastropods and Foraminifera. Open shallow, high-energy, lagoon. (4) Thin-bedded, massive with erosive bases, fossiliferous limestones (packstone-grainstone); these facies are usually thin intercalations in marls (terrigenous facies 3), being composed almost exclusively of Foraminifera. Storm beds in lagoon sediments. (5) Medium- to thick-bedded, cross-stratified, skeletal and oolitic limestones (grainstones, minor packstones); hummocky cross-stratification is also common. Subtidal bars in high-energy open shelf. (6) Fossiliferous (Rudists and Corals) limestones and dolostones (framestones, wackestones, packstones) forming tabular beds with isolated patches, or biostromes. Low-energy open platform. Facies are grouped into three facies belts: (a> continental-littoral terrigenous facies belt, where sandstones are the main lithologies (terrigenous facies 1 and 2); (b) coastal mixed-facies belt; green marls are the main facies (terrigenous faties 3), but containing carbonate and terrigenous intercalations (terrigenous facies 2, and carbonate facies 1 and 2); cc> open-marine carbonate facies belt (carbonate facies 2 to 6) (see Fig. 3). There are several differences between the platforms. Upper Albian platforms were narrow, but their records are thicker, with predominance of detrital and fossiliferous limestones (carbonate facies 4, 5 and 6) (Fig. 2); the mixed-facies belt is narrow or absent; these platforms are interpreted as high-energy, steep, open platforms. On the contrary, Cenomanian platforms were wider, and their records are thinner. Shallower facies dominate (carbonate facies 1,2 and 3); the mixed-facies belt is extensive and well developed (green marls, terrigenous facies 3, are the main facies) (Fig. 2); they are interpreted as low-energy, shallow-water platforms. There are other minor differences between the successive platforms regarding specific

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lithologies and fossil content, which are essential for precise field recognition of each platform.

3. Depositional sequences and parasequences The Upper Albian-Middle Cenomanian sedimentary record, in the Iberian Ranges, was formerly interpreted as part of a transgressive-regressive episode (Mas et al., 1982). It is composed of several transgressive events bounded by stratigraphic discontinuities (Garcia et al., 1977). Later, detailed studies showed that each transgressive event was composed of minor transgressive pulses, bearing similar facies (Garcia et al., 1987, 1989b). Presently, the transgressive-regressive episode, the transgressive events and the minor transgressions are interpreted within a sequence stratigraphic framework as 2nd- to Sth-order depositional sequences (Garcia et al., 1989b, 1993). There exist five 3rd-order sequences, which are composed of 4th- and Sth-order parasequences. Sequence recognition has been based on the lateral and vertical facies distribution (Segura et al., 1983, 1985; Soria et al., 1992; Ruiz, 1993; Gil et al., 1993; Schroeder et al., 1993; etc.), and on the strata1 pattern and systems tracts organisations (Garcia et al., 1987, 1989b, 1991, 1993; Garcia-Hidalgo et al., 1992, 1994). Sequence ages are Upper Albian (A), Vraconian (B), Lower Cenomanian (C), Middle Cenomanian (D) and Upper Cenomanian (E) (Fig. 2). Each sequence was successively extended towards the continent, onlapping underlying sequences and even older deposits landwards. Field recognition of depositional sequences and parasequences is a complex process. It is based on successively detailed description of outcrops and sections. A single section does not present commonly unequivocal evidence of cyclicity and usually several close sections are necessary to reach an optimum knowledge of the depositional sequence arrangement and then also of the cyclicity. Thus, for the interval studied, and in the area shown in Fig. 1 more than 350 sections are found. Sections range in thickness from over 440 m in the east, near the Mediterranean Sea (Puerto de1 Remolcador section, 1 in Fig. 1; Fig.

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Geology 103 (19%)

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Marls Mudstones Wackwbmes Packstones

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A. Garcia et al. /Sedimentay Geology103 (1996) 175-200

41, to 50 m in the west (La Cabrera section, 6 in Fig. 1; Fig. 3). For this paper, seven sections (Fig. 1) have been selected to show sequence and parasequence arrangement and the cyclostratigraphic model inferred. The downdip cross-section (Fig. 2) shows the main group of sedimentary facies, the basin architecture and the sequences and parasequences stacking patterns. Finally, the detailed cross-section (Fig. 3) shows how 5thorder parasequences are recognised, stacked and correlated. Other sections and cross-sections, showing similar detailed correlation up to 5thorder parasequences have been presented by Garcia et al. (1989a, 19931, Carenas et al. (19891, Ruiz et al. (1994), and Segura et al. (1994). 3.1. Fourth-order depositional parasequences At outcrop scale, the basic blocks of the record are the 4th- and Sth-order parasequences (Figs. 3, 4). Parasequences, as described here, are not bounded by marine flooding surfaces (mfs), as standard parasequences (Van Wagoner et al., 1988). Instead they are discontinuity-bounded parasequences (also called simple sequences by Vail et al., 19911, which have been considered ‘the best descriptive working scheme’ (Walker, 1990). Parasequence boundaries are either subaerial erosive surfaces (generating toplap and onlap, usually in relation to sequence boundaries, e.g., top of DC parasequence, Fig. 3) and their correlative conformities traced along the entire basin; or sedimentary break surfaces (ferruginous crusts, burrowed and bored surfaces), which are traceable only in parts of the basin (Db-Dc boundary, Fig. 3). In relation with these surfaces sudden

181

facies changes occur, with the input of elastic wedges tens of kilometres towards the basin (Garcia et al., 1993) (Da basal boundary and Da-Db boundary, Fig. 3). These parasequences range from 25 to 50 m. Where different facies are present they grade from continental-coastal sandstones (Da and Db parasequences at Puente San Pedro, DC parasequence at La Cabrera; Fig. 3) or littoral green marls (Da, Db and DC parasequences at Villalba de la Sierra, Fig. 3) up to tidal and inner shelf facies (sandy/ marly limestones and dolostones, stromatolites and, burrowed limestones and dolostones), and finally to open-marine carbonates (Ba, Bb, Bc, Fig. 4). The vertical facies gradation from terrigenous to carbonates, signifies a rise in the relative sea level; then they are transgressive and deepening-upwards parasequences. Sometimes, however, at the tops of the thicker parasequences open-marine facies grade further upwards to tidal and littoral facies (stromatolites, burrowed dolostones and limestones, and dolomicrites) (top of parasequence Da at Puente Vadil10s and Puente San Pedro, Fig. 3; AC, Cb, Cc, Fig. 4). This is a regressive episode, recorded mainly in basinal areas (Aa and Ab, Fig. 4). Sandstones or marls of the overlying parasequence abruptly rest on the carbonates (Da-Db boundary; Fig. 31, with an erosive surface or a ferruginous crust (Da-Db boundary at Abdnades and Puente San Pedro, Fig. 3). This represents a basinal shift in the facies belts, which can be important. (Note that marine dolomitic sedimentation in a Da parasequence reached the Abinades section and that the subsequent continental terrigenous reached the farther Puente Vadillos section, more than 70 km basinwards in a downdip direction, Fig. 3). The abrupt facies

Fig. 4. Puerto de1 Remolcador section log (see Figs. 1 and 2 for location), showing facies distribution and sequence and parasequence interpretation. Facies grade from tidal and coastal marls to open-marine carbonates. Marls sedimentation (interpreted as condensed sediments) occurred in relation to 3rd-order sequence and 4th-order parasequence boundaries (Chera marls bed at B-C boundary, see also Fig. 2; Aa-Ab, Ab-Ac, etc.). Carbonates are located towards the middle and top of sequences and are usually amalgamated (Aba-Abd, Bee-Bed, Cca-Ccc, etc.). Maximum-marine facies (Rudist-bearing limestones) for each sequence are located at parasequences Aa, Bc, Cc, Db and Eb. Facies and thickness trends allow correlation with cycles (see Fig. 5).

w--w---

-4Lq_d--u-

-PlJ

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A. Garcia et al. /Sedimentary Geology 103 (1996) 175-200

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Fig. 5. Fuente de la Puerca section (Valencia) (see Figs. 1 and 2 for location). The cyclicity is clearly shown by the alternation of open-marine carbonates (vertical cliffs) and tidal and coastal marls (gentle slopes). Thicker marls are related to sequence boundaries (A-B, B-C and C-D), where condensation occurred. Thicker carbonates were deposited in the middle of the sequences (Aba, Bc, Cc), where amalgamation was common (Bcb-Bee-Bed). Facies alternation and the location of the maximum-marine facies allow correlation with cycles in the model (see Fig. 5). Thickness trends, mainly in the 5th order, also suggest their relative position within cycles. Car at the bottom (encircled) for scale.

transition associated with erosion suggests a fall in the relative sea level. The most important differences in the facies

are caused by their paleogeographic location. Seawards, the siliciclastics wedge out and parasequences are composed of an alternation of marls

Fig. 5. Cyclicity model (left) and relationships between sequences and parasequences, hiatuses and cycles for the Puerto de1 Remolcador section (right). A 1:5:5 ratio between 3rd:4th:5th_order sequences and parasequences is inferred. Because sequence B is composed of five 4th-order parasequences and parasequences Aa and Da are also composed of five 5th-order parasequences. Therefore a 3rd-order sequence is composed of five 4th-order parasequences and twenty-five Sth-order parasequences (wavelength at the same scale, relative amplitude not at scale). Maximum and minimum for the three orders are supposed to be coincident and the composite curve is the superimposition of the different order curves. Sedimentation occurred in relation to inflection points of rising sea level (cycles 13, 23, 33, 43 and 53). Condensation or erosion occurred in relation to inflection points of falling sea level (cycles 11, 15-21, 25-31, 35-41 and 45-51) where the longer hiatuses originated (missing beats); minor hiatuses are related to 4th-order boundaries. There are also time gaps concerning Sth-order parasequences due to lateral facies differentiation (not showed).

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and carbonates (Villalba de la Sierra, Fig. 3), or even mainly of carbonates (Puerto de1 Remolcador section, Figs. 4 and 5). This facies alternation is clearly reflected in the landscape, because marls usually form gentle semicovered slopes and the limestones form vertical cliffs (Fuente la Puerca section, Fig. 6). On the contrary, landwards the marls and carbonates pinch out. Parasequences are composed of terrigenous grading to dolomitic sandstones and dolostones (Puente San Pedro and Abinades sections, Fig. 31, but farther landwards terrigenous sediments are the only facies (La Cabrera section, Fig. 31, being capped by thicker ferruginous (probably lateritic) crusts and paleoalterations (Ruiz, 1993). 3.2. Fifth-order paraseqwnces A detailed analysis of the facies arrangement in a 4th-order parasequence allows the recognition of minor (5th) order parasequences. Where continental-coastal and marine facies digitate, the Sth-order parasequences (5-10 m thick) can be more clearly distinguished by the shifts in the facies belts. Therefore, between the Abinades and Puente San Pedro sections (Fig. 3) minor sandstone-carbonate alternations in Db parasequence allow differentiation of Dba, Dbb and Dbc parasequences; the same alternation in Da parasequence occurs between the Puente San Pedro and Puente Vadillos sections (Fig. 3) allowing distinction of Dab, Dac, Dad and Dae parasequences. A Sth-order parasequence is also composed of sandstones, marls and carbonates. Sandstones were progradational elastic wedges deposited during lowstand; marls are condensed sediments deposited seawards of the sandstones; finally, carbonates are interpreted as aggradational highstand sediments (Ruiz et al., 1994). The terrigenous-carbonate boundary is a surface of low sedimentary rate, where marls, burrowed surfaces and paleosoils were commonly developed. Parasequences show a lateral facies differentiation from landwards to seawards sections (sandstones predominate landwards and carbonates seawards), reflecting the variation of sedimentary

Geology 103 (1996) 175-200

processes and then the different sites of erosion and sedimentation during parasequence development. Parasequences were originated by cycles of sea-level fall (terrigenous progradation) and rise (aggrading carbonates). Correlation of parasequences is usually tentative landwards, since facies similarity between different parasequences is the norm. Only sometimes, the presence of both minor tidal reworked facies at the top of some parasequences (Ruiz and Segura, 1993), and mudstone intercalations with thicker paleosoils (Ruiz, 1993; Ruiz and Segura, 19931, are interpreted as Sth-order parasequence boundaries. Sandstones wedge out seawards; in these areas, the presence of regional guide beds, such as some marly units and beds with specific lithologies or fossil content, clearly support the correlation. The presence of littoral marl intercalations mark the parasequence boundaries (Dab, Dac, Dae parasequences at the Villalba de la Sierra section, Fig. 3; Baa to Bee parasequences at the Puerto de1 Remolcador section, Fig. 4). In some parasequences the marly intercalations disappear resulting in amalgamation of carbonate parasequences (Aaa-Aab, Aba-Abb-Abe-Abd, BccBed, etc. at Puerto de1 Remolcador section, Fig. 4); in these cases parasequence boundaries are hard to recognise, but they can be differentiated landwards. In other cases, mainly in the Sth-order parasequences related to 3rd- and 4th-order boundaries, the basal marl intervals are thicker, containing very thin carbonate intercalations; in these cases, boundaries are difficult to recognise, being located in a transitional zone related to thin Sth-order stacking parasequences (B-C boundary at the Puerto de1 Remolcador section, Fig. 4, where Bc, Bd and Ca 4th-order parasequences are hardly distinguished, but their 5thorder parasequences are indistinguishable). The number of parasequences, in a 4th-order parasequence, remains constant in different and distant sections. Minor differences in the number of parasequences are related both to the basal onlap (Daa disappearance from the Puerto de1 Remolcador section, Fig. 4, to the Villalba de la Sierra section, Fig. 3; see also fig. 10 in Garcia et

A. Garcia et al. /Sedimentary Geology 103 (1996) 175-200

al., 19931, or to erosion related to 3rd-order sequence boundaries (A-B boundary near the Campo de Arriba section, Fig. 2; D-E boundary, Fig. 3; etc.).

4. The cyclicity in the Upper Albian-Cenomanian: a model for cyclic sedimentary sequences Sequences and parasequences originated by repeated and cyclic changes in relative sea level. The origin of the cycles is thought to be purely eustatic, but tectonics and sedimentation control the final depositional sequence (see below). The presence of 3rd-order sequences and 4thand Sth-order parasequences implies the existence of three different orders of cycles. The record was thus generated by the complex superimposition of these cycles. From the field data a model can be derived to explain the cyclic organization of the record (Fig. 5, left). As a model, following Walker (1984), it is used in the Iberian basin as a norm for comparison, as a framework and a predictor in new areas or where outcrops are poor, and, finally, as a basis for interpretation (explaining thickness differences, facies distribution, the lacking of parasequences, and the temporal relationships between sedimentation and hiatuses). In order to develop a model of the cyclic record it is necessary to study those sections with a complete record. This is usually made on deep basinal facies, where sedimentation is continuous. On platforms, sea-level falls frequently generate missing beats (Goldhammer et al., 1990), and the record is incomplete. Nevertheless, cyclic models can be developed with some restrictions; all values and ratios obtained should be considered as minima. For cyclostratigraphic studies, seaward sections are useful because these are the sections where the maximum number of cycles was recorded. These sections are mainly composed of shelf carbonate facies, and sequence and parasequence distinction may be poor; although they

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can be understood when regional data permit sequence and parasequence correlation. The most complete section for the Upper Albian-Cenomanian in the Iberian basin is the Puerto de1 Remolcador section (Schroeder et al., 1993) (Figs. 2, 4 and 5). In this section, the B depositional sequence (Figs. 4 and 5) is composed of five 4th-order depositional parasequences (Ba to Be, Fig. 5); and, Aa and Da 4th-order parasequences are also composed of another five 5thorder parasequences (Aaa to Aae and Daa to Dae, Fig. 5). In a pure cyclic model this ratio (1:5:5) should be kept constant through time (Fig. 5, left), and it is the basic ratio assumed for the development of the sedimentary record in the basin. That is, any 3rd-order cycle should be composed of five 4th-order cycles, each being composed of another five Sth-order cycles (Fig. 5, centre). Therefore, a 3rd-order cycle is composed of twenty-five Sth-order cycles. The 1:5:5 ratio is in good agreement with the data by other authors in different basins and ages. Thus, Borer and Harris mentioned that 3rd-order cycles in the Permian of the United States are composed of five 4th-order cycles, each composed of four Sth-order cycles (1:5:4 ratio). Goldhammer et al. (19901, in the Triassic of the Alps, mentioned a 1:4 or 1:5 ratio for 4th- and Sth-order cycles. Third-order cycles are numbered 1 to 5, from base to top; 4th- and Sth-order cycles are also correlatively numbered from base to top (12, 13, 14, etc. for the 4th-order, and 111, 112, 113, etc. for the Sth-order) (Fig. 5, centre) Although the model is field-derived, it assumes that relative sea-level oscillations were sinusoidal, with the period and the amplitude being constant through time (as theoretical models, Read et al., 1986; Einsele and Bayer, 1991) (Fig. 5, left). In the model each cycle-order has its own curve, and it is superimposed on the immediate lower order in a simple way (as in fig. 1 of Einsele and Ricken, 1991) (Fig. 5, left). This cyclical pattern has generated asymmetric rock sequences (Fig. 5, right), as also shown by Einsele and Bayer (1991). Cycles of different order are considered approximately in phase to keep the model as simple as possible, although that is a clear oversimplifi-

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cation. Thus, the inflection points of the rising and falling sea level of 3rd-, 4th- and Sth-order cycles are coincident (Fig. 5, left). Accommodation controls how much of a cycle remains as a depositional sequence and parasequence. It is usually considered a function of sea-level fluctuations, subsidence and compaction of underlying sediments (Jervey, 1988). Accommodation also exists for parasequence development; the accommodation rate is, however different. For short intervals (i.e. a 4th- or Sth-order cycle), compaction and subsidence can be considered unimportant and the accommodation space was controlled mainly by the eustasy. On the other hand, for longer intervals (3rd-order cycles), the subsidence and the compaction played an important role in the potential accommodation. The presence of the three different orders of cycles (Fig. 5, left), each having their own potential accommodation, suggests that it was really the superimposition of the different accommodation rates (of 3rd-, 4th- and Sth-order cycles), which generate a variable rate of sediment accommodation and a different type of depositional sequences with different facies and thicknesses. Therefore, the potential accommodation space changed cyclically with time. The higher potential accommodation was located near or at the inflection point of any rising sea-level curve (accommodation should be maximum at cycles 12-13, 22-23, 32-33, 42-43 and 52-53; Fig. 5, centre), where cycles correspond to a eustatic maximum. These are materialized as depositional parasequences throughout the basin, and have the higher probabilities of being preserved (e.g. ten Sth-order cycles, 121-135, are recorded as nine Sth-order parasequences Aaa to Abd; ten cycles, 221-235, are recorded as eight parasequences Baa to Bbd, Fig. 5, centre and right). On the contrary, the lower potential accommodation was located near or in the inflection point of the falling sea-level curves (cycles 11, 15-21, 25-31, 35-41, 45-51, 55; Fig. 5, centre). These cycles correspond to a eustatic minimum, and they were materialized as parasequences in those areas of the basin where accommodation left enough space for sediment accumulation (e.g. ten Sth-order cycles, 251-315, are recorded as two

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parasequences Bea and Caa; ten cycles 351-415 none was recorded, etc.; Fig 5, centre and right). Therefore, there is an important reduction in the number of 4th- and Sth-order parasequences related to the 3rd-order minimum, where hiatuses were longer than usual (Fig. 5, centre and right). Accommodation also controls thicknesses, faties and regional extensions of parasequences (Figs. 2, 3 and 4). Th ere is a cyclic pattern in their thicknesses, facies and regional extension. When accommodation was higher there was more space to be filled up, and thus thick parasequences were formed (Dac-Dad and Dbb-Dbc in Fig. 3; Aa and Ab, Da and Db in Fig. 4); the sea-level rise extended shallow seas over wider areas, and marine intercalations reached farther landwards as well (Bc, Fig. 2; Dad and Dbc, Fig. 3). Amalgamation of sequences (Goldhammer et al., 1990) also occurred (e.g. Dac-Dad and DbbDbc at the Villalba de la Sierra section, Fig. 3, and the Puerto de1 Remolcador section, Fig. 4). On the other hand, when accommodation was lower there was little or even no space to be filled up; parasequences were thin or absent (Dab and Dae parasequences, Fig. 3; Sth-order parasequences in Bd, Be and Ca, Figs. 4 and 5). If recorded, they had a restricted regional extension; marine sediments were located in basinal areas; meanwhile, terrigenous input reached farther basinwards (Dab and Dba parasequences, Fig. 3; Bd, Be and Ca parasequences in Fig. 2). This is an application of Walther’s law to depositional sequence development. Varying subsidence rates caused some differences between the Upper Albian and the Cenomanian sequences and parasequences. The Upper Albian parasequences are thicker than Cenomanian ones, which probably reflects a higher subsidence rate for the Upper Albian. The Upper Albian sequences and parasequences have a narrow regional extension, which also suggests that the area of sedimentation was a site of active subsidence. Finally, the stratigraphic architecture of parasequence shows landwards a complex structure with toplap and erosion between parasequences (Fig. 2), which is not fully understood due to the presence of several monotonous terrigenous units whose stratigraphic relation-

A. Garcia et al. /Sedimentary

ships are not clear. This architecture is probably tectonically induced. Finally, changes in accommodation due to compaction of underlying sediments have not been considered. Nevertheless, the presence of some parasequences whose thickness does not exactly correspond to that supposed by the model, can be interpreted as caused by local changes in the accommodation due mainly to compaction. The higher thickness of D parasequences in the Abinades section (Fig. 3) in relation to their thickness in nearby sections (La Cabrera and Puente Vadillos), is interpreted as due to the compaction of mudstones in the Weald continental facies, underlying those parasequences only in the AbLnades area (Fig. 3).

5. The sedimentary record of the cyclic@ 5.1. Sequences One of the best places in the Iberian Ranges to show and study the cyclicity is the Fuente de la Puerca section (Fig. 6; 2 in Fig. 1; FP in Fig. 2). This section was located basinward and the Upper Albian record is composed of rhythmic alternations of littoral marls and marine limestones (see above for facies description). Sequences and parasequences have a good morphological expression with vertical cliffs of limestones and gentle slopes of marls, usually partially covered by vegetation. Sequence A (lower left, Fig. 6) rests on older Cretaceous sandstones. The Sth-order parasequences show first a thickening-upwards trend and subsequently thin upwards. This boundary is a clear contact in the field, mappable on a regional scale. Facies below and above the contact are also different; the lower limestones are beige sandy skeletal limestones (grainstones-packstones), and the overlying ones are light-grey skeletal limestones (mainly packstones). These are interpreted as two different 4th-order parasequences (Aa and Ab, Figs. 5 and 6). The previous thickness trend suddenly changes to a thicker parasequence, which is considered a different 4th-order parasequence (AC, Figs. 5 and 6).

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Overlying (centre, Fig. 6) sequence A-B boundary is a thin marl interval. Parasequence recognition is difficult. It probably represents a lowstand condensed section. Farther basinwards (Puerto de1 Remolcador, Fig. 41, the presence of limestone intercalations allows parasequence differentiation and correlation. Sequence B is composed of four parasequences. Parasequence Bb has three Sth-order parasequences with a thickening-upwards trend. At the top there is a light grey, Rudist-bearing limestone bank (top of Bbc, Fig. 61, which is a regional correlation bed, bearing the more-marine facies in the parasequence. This bed is interpreted as deposited during the maximum transgression, and it was followed by a fast regression representing a Bb-Bc boundary. The Bc parasequence is composed of four Sth-order parasequences (Bca to Bed); Bca and Bcb are composed of transgressive inner shelf detrital limestones (pa&stones) and, Bee and Bed are composed of outer shelf, grey, Rudist-bearing limestones. The maximum of the transgression is located in the Rudist-bearing facies at the top of the Bee parasequence, which also reflects the maximum of potential accommodation for the entire 3rdorder sequence. On the other hand, the Bed parasequence is mainly regressive. Amalgamation of Bcb, Bee and Bed exists (sensu Goldhammer et al., 1990). It was caused because the relative fall in the Sth-order sea level was not sufficient to compensate the relative rises of both 3rd- and 4th-orders (see also the Bee-Bed amalgamation in Fig. 4). The thick overlying marls (Fig. 6) are composed of green and yellow marls and siltstones. The marls contain four limestone intercalations; each composed of nodular limestones and mangrove paleosoils. Marls and limestones are interpreted as 4th-order parasequences, but having thicknesses in the Sth-order parasequence range. The change of the thickness trend, at the middle of the marls, suggests that the 3rd-order boundary should be located there. At the top of the section (upper right, Fig. 6) pervasive dolomitization occurs and obscures faties relationships. Here Sth-order parasequences are not as clearly shown as below and the thicker

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Fig. 7. Campo de Arriba section (Valencia) (see Figs. 1 and 2 for location). View is restricted to the Upper Albian below the Chera marls (see Fig. 2), sequences A and B. It was located more landwards than Fuente de la Puerca. Parasequences Aa and Ab can be of pax ‘asequence clearly identified because they show the same facies and colour as in the Fuente la Puerca section. Disappearance AC is due to toplap erosion (see Fig. 2), whereas parasequence Ba was not deposited due to the basal onlap of se‘quence B. Amalgamation of Bc parasequences is also clearly shown; they show similar characteristics as at Fuente la Puerca (COImpare with Fig. 6). There is a clear reduction in the number of Sth-order parasequences as compared to Fuente la Puerca. Thickness differences with Fuente de la Puerca are mainly due to this reduction, and not to a mere thinning of the 4th-order par: Man in circle for scale.

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marl-carbonate alternations are interpreted as 4th-order parasequences. The accommodation was higher in A and B sequences, probably because of a higher subsidence rate. It is there where the parasequence was rapidly reduced in thickness landwards. Thus, in the Campo de Arriba section (near Tuejar, Valencia, Fig. 7; see Figs. 1 and 2 for location), the main differences with the Fuente de la Puerca section lie in the Upper Albian, just the portion shown in Fig. 7. Sequence A is composed of two 4th-order parasequences, separated by a clear contact; these are Aa and Ab parasequences, bearing similar facies as in the Fuente la Puerca section. In Campo de Arriba, however, these 4th-order

Fig. 8. Puerto de Casas Bajas section Cretaceous sandstones; conformably sequence B is dramatically reduced reduction is due to disappearance of the sea-level fall at the top (Bed).

(Cuenca) overlying compared sequence

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parasequences are composed of merely three Sth-order parasequences. They are also thinner, reflecting both the onlap at the base and the smaller accommodation space that was available towards the continent. The marls at the A-B sequence boundary in the Fuente la Puerca section have disappeared due to erosion. Sequence B is composed of three 4th-order parasequences (the top parasequence, Bd, is not shown in Fig. 5). The top of the Bb parasequence is the Rudist-bearing limestone bed (the regional guide bed mentioned above). Parasequence Bb is composed of two Sth-order parasequences (Bbc and Bbd); Bba disappearance from Fuente la Puerca is related to the basal onlap. Parasequence Bc is still composed of

(see Figs. 1 and 2 for location). The Upper Albian is thin (12 m) and rests on older are the Chera marls and the rest of the Cenomanian. Sequence A is absent and to Fuente la Puerca (Fig. 6) and Campo de Arriba (Fig. 7). The main thickness A and Sth-order parasequences of Bc, both caused by the basal onlap (Bca) and to

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four Sth-order parasequences, but having smaller thicknesses. The presence of the same parasequences in the Fuente de la Puerca section suggests that the accommodation was higher in parasequence Bc. This was probably due to the higher relative sea-level rise at the middle of cycle 2. Amalgamation of parasequences Bcb, Bee and Bed also occurred here. Farther landwards, the number of Upper Albian parasequences in the outcrops is dramatically reduced. In the Puerto de Casas Bajas sec-

Geology 103 (1996) 175-200

tion (Cuenca, Fig. 8; see Figs. 1 and 2 for location), the Upper Albian thickness is just 12 m. At the outcrop, only sequence B has been identified. This sequence rests on continental and coastal sandstones and marls (lower left, Fig. 8). It could correspond either to deposits of older pre-Upper Albian sequences (the usual interpretation), or they may be interpreted as the lateral equivalents of sequence A, containing continental facies. The absence of well-exposed outcrops makes an accurate interpretation impossible.

Fig. 9. Ibdes section (Zaragoza) (see Figs. 1 and 2 for location). Continental terrigenous and open-marine carbonates alternated in the Cenomanian. Sequences and parasequences grade mainly from basal continental terrigenous to marine carbonates at top. Their boundaries reflect seawards shifts in the facies belts (see also Fig. 3). B-C boundary is‘an erosive surface originated by a relative sea-level fall (Chera marls bed is absent). Sequence C is composed of a single 4th-order parasequence (Cc), Ca and Cb disappearance is due to the sequence’s basal onlap. Sequence D has three parasequences with the same thickness trend throughout the Iberian basin (compare with Figs. 3 and 4). The presence of an Ichtyosarcolithes bed at Dbc top allows for regional correlation. Differences with Puerto de1 Remolcador are minor, two Sth-order parasequences (Daa and Dae), in 200 km. An erosive surface separates sequences D and E (a type-l sequence boundary). Sequence E represents the connection between Tethys and Atlantic margins in the Iberian microplate and from this sequence Ammonites finds are common, allowing a Cenomanian-Turonian boundary location (see Fig. 10). The Ibdes section can be easily correlated to the Puente San Pedro section (Fig. 3); observe also thickness trends and facies relationships. Car in circle for scale.

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Two parasequences (Bb and Bc) are clearly recognised due to the presence of a guide bed at the top of the Bbd parasequence, as in the Fuente de la Puerca and the Campo de Arriba sections

Age

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(Fig. 8). Parasequence Bb is composed here of two Sth-order parasequences, the same as in Campo de Arriba; meanwhile parasequence Bc is composed of only two (Bcb and Bee) (Fig. 8). The

Sedimentary sequences and parasequences

Paleontological IWCOlti

TURONIAN Mammites rlabokks Vascocemtktae

@j Metoiaxetas

gestinianum

CENOMANIAN @J Calyctxeras m&u/am Van Hinte (1976) 6.0 Ma Hartand et al. (1962) 6.0 Ma Odin et al. (1962) 4.0 Ma Kent 8 Gradatein (1965) 6.5 Ma Gradatein et al. (1994) 56Ma

Pnpkosnticwrsshispanicurn a

Primitive Olbifdri~s

ALBIAN

Fig. 10. Location of Cenomanian boundaries based on Ammonites (Cenomanian-Turonian boundary; see also Segura et al., 1993a, b) and benthic Foraminifera Wbian-Cenomanian; see also Schroeder et al., 1994) on an Iberian parasequence scale. Cenomanian length is the base for the estimation of the duration of the cycles (see Table 1). Parasequence Cb corresponds to cycle 32 (Fig. 5); therefore, the Cenomanian lasted from cycle 32 to the basal cycle of the overlying sequence (not studied here), and that is fifteen 4th-order cycles.

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disappearance of parasequence Bca landwards is interpreted as the result of basal onlap of Bc parasequence. Comparing the three sections (Figs. 6, 7 and 81, it can be clearly observed that the differences in thickness between sections is due more to the number of Sth-order parasequences present in any section, than to a mere thinning landwards of the 4th- or Sth-order parasequences, which, however, also occurs. Sequences and parasequences are also present in the terrigenous facies and a similar pattern can be extended to these sediments. Lateral transition from carbonates to terrigenous facies is not clearly observed in the Upper Albian, because of the lack of well-exposed outcrops. It is difficult to establish accurate correlations towards coastal and continental areas for this interval; however, it is well exposed and clearly seen for the Cenomanian (sequences C, D and E, Fig. 2). Sequence C is composed of marl-carbonate parasequences in the open shelf areas (Puerto de1 Remolcador and Fuente de la Puerca, Figs. 4 and 6). Landwards, however, terrigenous wedges predominate and parasequences are here composed of sandstone-marl-carbonate facies. In the Ibdes section (Zaragoza, Fig. 9; see Figs 1 and 2 for location) cyclicity in mixed and terrigenous facies does not have such a good morphological expression as in carbonate facies. The cyclicity can still be observed, mainly 3rd order, when carbonate intercalations are considered. In Fig. 9 there are four 3rd-order sequences (B to El. The sandstones at the base (lower left, Fig. 9) are interpreted as lateral equivalents of the limestones in sequence B (see Fig. 2). They are unconformably overlain by sequence C. This sequence is composed of sandstones at the base and a thick carbonate bank at the top. Regional correlation allows its interpretation as a single ilth-order parasequence (Cc); the disappearance of Ca and Cb landwards (Fig. 2) is related to basal onlap of sequence C. The 4th-order parasequence is composed of three Sth-order parasequences that present a thickening-upwards trend. There is an important facies difference as compared with the Fuente de la Puerca section (Fig.

Geology 103 (1996) 175-200

61, where it is composed of marls and carbonates. The C-D sequence boundary is located 60 cm above the carbonate top. The boundary is also clearly marked by the basinward shift in the overlying elastic wedge (see also Fig. 3). Sequence D is composed of three 4th-order parasequences (Da, Db and DC), which are the same throughout the basin (Figs. 3, 4, 9 and 101, even maintaining their relative thickness, but with an overall thinning-upwards trend. Each parasequence is composed of basal sandstones and top carbonates; sandstones predominate in the basal parasequence and carbonates predominate in the top one, where no sandstones were deposited. The maximum-marine facies are located near the top of parasequence Db in a Rudist-bearing limestone bed (Ichtyosarcolithes bed), which is another regional marker bed used for wide regional correlations (Mojica and Wiedmann, 1977). The marl-sandstones alternations and the presence of very thin carbonate intercalations, define Sth-order parasequences (Fig. 9). There are three Sth-order parasequences in each 4thorder parasequence, except in the top one (DC) which is only composed of a single Sth-order parasequence. The Sth-order parasequences present a thickening-upwards trend and the moremarine facies are located in the top carbonates, both allowing interpretation of Sth-order parasequences as the basal parasequences (Daa-DabDac and Dba-Dbb-Dbc) in the 4th-order one, with an upward increase in the accommodation space. At the top of parasequence DC there is a conformable but clear contact with local paleokarst development (Maestrazgo area), there is also a change in the sedimentary trend. This is the D-E sequence boundary, which reflects an important fall of relative sea-level with platform exhumation and karst development. Sequence E is composed of three 4th-order parasequences (upper right, Fig. 9). It represents the maximum marine transgression in the basin for the interval considered, and an open-marine platform covered much of the eastern Iberian microplate connecting their Tethyan and Atlantic margins. This sequence, with the same facies, rests over Jurassic sediments and its lateral conti-

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nental terrigenous facies sometimes even rest on the Hercynian basement. Parasequences are thin (Ea)-thick (Eb)-thin (EC), and maximum-marine facies are located at parasequence Eb. Parasequence Eb contains Rudist-bearing limestones at the Puerto de1 Remolcador section (Fig. 41, and it is the section’s top due to faulting. Regression at the top of the sequence was forced by the northwestwards microplate tilting. The following transgression had an Atlantic origin and flooded the basin from the north and northwest, opposing the previous transgressions that were Tethyan and flooded the basin from the southeast (Garcia et al., 1993; Segura et al., 1993). 5.2. Cycles The model as a basis for interpretation allows the assignment of cycles to preserved parasequences (Fig. 5, right). Thick parasequences, usually amalgamated, bearing marine carbonate faties were located in or close to the eustatic maximum of cycles (+ in Fig. 5, left). Thin parasequences, usually condensed parasequences, bearing a high ratio of continental coastal sandy facies were located in relation to eustatic minimum of cycles (-in Fig. 5, left). Parasequences Aa, Ab and AC (Fig. 6) are considered cycles 13, 14 and 15 (Figs. 5 and 61, because they have a thinning-upwards trend. The maximum-marine facies of the entire sequence are located in parasequence Aa (Fig. 41, which supports such interpretation. The only Sth-order parasequence in parasequence AC is considered Act, cycle 153 because it represents the maximum accommodation for the 4th-order cycle. Sequence B contains five parasequences (Ba to Be, Fig. 51, which correspond to cycles 21 to 25. Parasequence Ba onlaps and disappears landwards (Fig. 6). Parasequence Bb (cycle 22) is composed of three Sth-order parasequences. Their thickening-upwards trend and the presence of maximum-marine facies at the top suggest that they correspond to cycles 221 to 223. The amalgamation of facies and the thickness trend suggest that Sth-order parasequences of Bc correspond to

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.cycles 231 to 234. Besides, cycle 233, in the inflection point of the 3rd-order cycle, is the only parasequence of the entire sequence containing small coralline patches in life position (Puerto de1 Remplcador section, Fig. 4). At the B-C boundary, preserved cycles are considered the 243, 253, 313 and 323 (parasequences Bdc, Bet, Cat and Cbc, Figs. 5 and 6), because those cycles have the highest possibilities for materialization and preservation as parasequences, being located in their 4th-order maxima. Other different Sth-order cycles may also be recorded in the marls. Nevertheless, they are difficult to separate as depositional parasequences, because they are very thin and too homogeneous to be distinguished and correlated to nearby sections. Moreover, the thickening-upward trend of the Ca, Cb and Cc parasequences is interpreted as caused by the increased accommodation towards the middle of cycle 3. They correspond to cycles 31, 32 and 33. Cc parasequences correspond to cycles 331, 332 and 333 (parasequences Cca, Ccb and Ccc, Fig. 6), because accommodation (thickness) increases towards the middle of cycle 33; the presence of Rudist limestones in parasequence Ccc (Fig. 4) also supports such interpretation. The presence of a Rudist bed (Ichtyo.surcolithes bed) in a wide area of the Iberian basin (Fig. 3) is commonly interpreted as maximum transgressive in sequence D. It corresponds to parasequence Dbc, and it is interpreted as cycle 433, at the inflection point of the 3rd-order cycle, where accommodation is higher. It is supposed that the rest of parasequences correspond to cycles 42 (Da), 43 (Db) and 44 (DC), respectively. Parasequence Da (cycle 42) is composed of five Sth-order cycles (421 to 425). As parasequence Dbc is assigned to cycle 433, underlying parasequences Dba and Dbb correspond to cycles 431 and 432. Finally, the DC parasequence consists of a single cycle, being interpreted as 443, at the middle of 4th-order cycle. Finally, sequence E corresponds to cycle 5. Thickness relationships thin-thick-thin for 4thorder parasequences and the presence of maximum-marine facies at parasequence Eb suggest that they may be respectively interpreted as cy-

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cles 52, 53 and 54, at the middle of the entire cycle 5 (Fig. 5). 5.3. Hiatuses The stratigraphic record is incomplete (Ager, 1981); there are more time gaps in the sediments. There are several causes for this. For example, erosion is a common process, either linked to tectonic uplift or, as subaerial erosion, linked to relative sea-level fall. Eustatic sea-level changes result in no sedimentation on the platforms when relative sea-level falls enough to expose the platform, generating missing beats (Goldhammer et al., 1990). The relative sea-level rise usually causes parasequence onlap; therefore, time can be recorded in some areas and not recorded in other areas, generating diachronous sequence boundaries. Finally, time is not equally distributed in the sediments: the development of a thin paleosoil in a marly unit usually takes more time than the sedimentation of a thick subtidal bar. If purely tectonic causes are excluded, the different sedimentary causes can be related with the order of the cycles. The Sth-order parasequences show an homogeneous thickness and strong facies differentiation. The time of cycle preserved as sediments in Sth-order parasequences may be proportionally longer than in 4rd-order parasequences and 3rd-order sequences. Through time sedimentation took place in different areas (Ruiz et al., 1994); open platform areas record mainly highstand carbonate deposits (Fig. 31, whereas continental areas record mainly lowstand terrigenous sediments (Fig. 3). There is a time gap in relation to parasequence boundaries mainly landwards due to erosion; seawards the presence of condensed marl deposits suggests minor time gaps. Besides the latter causes, 4th-order cycles may also have time gaps due to the absence of 5thorder cycles in their boundaries. Cycle disappearance, however, is greatest in relation to the 3rdorder cycle boundaries. These are missing beats (Goldhammer et al., 1990) in relation to relative sea-level falls. The relative sea-level fall is usually more important landwards where platform exhumation and erosion are common; seawards, the

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relative sea-level fall may not be enough to expose the platform and minor sedimentary breaks occur in relation to lags due to carbonate factory shift. The A-B sequence boundary is located seawards, in a marl unit (Figs. 5 and 61, being a type-2 sequence boundary; landwards, erosion took place with parasequence disappearance and toplap development (type-l sequence boundary). Reduction in the number of parasequences linked to this boundary is not very important because a high subsidence rate existed in the basin. In the more marine area (the Puerto de1 Remolcador section, Figs. 4 and 5), the sedimentary record is rather complete with seven 5thorder parasequences missing. Landwards, the number of cycles recorded as parasequences decreases rapidly; in the Fuente de la Puerca section (Fig. 6) the entire record has been diminished and nine parasequences were missing (corresponding to cycles 144 to 212, Fig. 5). Farther landwards, in Campo de Arriba, the presence of erosion makes the record shorter with eighteen parasequences being absent (cycles 134 to 221, Fig. 5), including three complete 4th-order cycles not recorded in the sedimentary record. The B-C sequence boundary is a type-2 sequence boundary. Sea-level fall was not enough to expose the platform and subsidence was lower than for the A-B boundary. Sedimentation was more continuous and condensation occurred, parasequences being hardly to recognise at the boundary (Fig. 6). There are sediments of all the parasequences, with at least one Sth-order parasequence represented (Figs. 5 and 6). At the sequence boundary only four cycles were missed (254 to 312, Fig. 5), and all the 4th-order parasequences were recorded in contrast with the underlying boundary. The C-D boundary is either a type-2 sequence boundary, in those areas (Maestrazgo central) where marl sedimentation took place. The marls are interpreted as regressive deposits deposited in relation to the sea-level fall. On the other hand, in the rest of the basin there was erosion and a type-l sequence boundary was developed, as happens in the Puerto de1 Remolcador section (Fig. 4). Here three 4th-order cycles (34, 35 and

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41, Fig. 5), and seventeen Sth-order cycles are always missing (Fig. 5). These differences between areas with marl sedimentation and areas of erosion are interpreted as due to the different subsidence rates. Higher subsidence rates, which kept pace with sea-level fall, cause marl sedimentation and condensation. On the other hand, lower subsidence rates, less than sea-level fall, caused erosion. Finally, the D-E boundary is another type-l sequence boundary. The relative sea-level fall exposed the entire platform (paleokarst development), and the subsidence rate was the lowest. Between two and three complete 4th-order cycles (44, 45 and 51, Fig. 5) may be absent.

sequence boundaries, and quantitative methods cannot be used with a poor biostratigraphic record. Finally, biostratigraphic scales based on different organisms or made for different faunistic provinces have not been fully matched. Recently, a new method has been developed: the correlation with cycle charts such as the standard cycle chart of Haq et al. (1988). This chart is based on the global correlation of cycles, which are thought to be mainly eustatic and with global extension. This method has also several problems regarding the global extension of the cycles and their world-wide correlation (Miall, 1992). Cycles charts may be useful when used according to paleontological and other available data.

6. Age model of the cyclic@

6.1. Biostra tigraphy

The duration of the sedimentary record can be estimated if the record is considered cyclic, and then any cycle had a constant length. Two sets of data are necessary: the total number of cycles, recorded or not, and the total time involved in the record. The total number of cycles is obtained based on the available stratigraphic data. It is the Puerto de1 Remolcador section (Figs. 4 and 5) which presents the highest number of cycles. From this section a 1:5:5 rate has been inferred (see above), but this is a minimum value, because underestimation of the number of cycles may occur. The Upper Albian-Cenomanian record of the Iberian basin is, thus, composed of 5 3rd-order cycles (all recorded as depositional sequences), 25 4th-order cycles (16 recorded as depositional parasequences in the Puerto de1 Remolcador section) and 125 Sth-order cycles (47 recorded) (Figs. 4 and 5). It is concluded that only 37.6% of the total Sth-order parasequences are present. The total time involved in sedimentation is sometimes problematic to obtain since absolute ages are not available (i.e. in the Upper AlbianCenomanian of the Iberian basin). Time is alternatively obtained from biostratigraphic data; this, however, may also be problematic. Fossils can be found or not due o several causes, their record can also be diachronous as

The paleontological record of the Upper Albian-Cenomanian in the Iberian Ranges is composed of Pelecypods, mainly Oysters in the marls and Rudists in the limestones, with minor Gastropods, Corals and Foraminifera. Ammonite founds are scarce in the Iberian basin. Upper Albian ammonites (Proplacenticeras hispanicum, Mas and Wiedmann, 1980, in parasequence Bd) are Tethyan endemic species with no clear correlation to the standard Ammonite scales. Ammonites with chronostratigraphic value (Mojica and W ie d mann, 1977) have been found from sequence E, where Calycoceras naoiculare (found in parasequence Eb) and Metoicoceras geslinianum (found in parasequence EC), suggest an Upper Cenomanian age (Fig. 10). The presence of ammonites was common in the hemipelagic marls of the overlying sequence (not studied here), where the presence of Vascoceratids and Mammites nodosoides allows the location of the Cenomanian-Turonian boundary (Segura et al., 1993a, b) (Fig. 10). Benthic Foraminifera are very commonly found throughout the Iberian basin and they have been widely used for biostratigraphic purposes (Peybernes, 1976; Fourcade and Garcia, 1982; Canerot, 1982; Calonge, 1989). Although benthic Foraminifera lack the precision of Cephalopods, they allow a stage and substage division of the

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record (Schroeder and Neumann, 1985; Schroeder et al., 1993). The Upper Albian-Cenomanian boundary is located at the Ca-Cb boundary. Parasequence Ca has an uppermost AIbian age by the appearance of primitive Alveolinids (Fig. 10). In parasequence Cc the appearance of Praealveolinids is coincident with the first Cenomanian Orbitolinids (Orbitolina concava), and its age is Cenomanian. In parasequence Cb there are no unequivocal data, but its Alveolinid and Orbitolinid faunas suggest a Cenomanian age, being the Albian-Cenomanian boundary at its base. Based on these data, the entire sequences A and B, and the first 4th-order parasequence of sequence C are of Upper Albian age (Fig. 10). The rest of sequence C, sequences D and E, and the basal 4th-order parasequence in the overlying sequence (Fig. 10) are of Cenomanian age. 6.2. Cycle charts Correlation with the standard cycle chart of Haq et al. (1988) is also possible based on different arguments. Cycle UZA 2.4 (Haq et al., 1988) contains the biozones of Calycoceras naviculare and Metoicoceras geslinianum, as sequence E. The Cenomanian-Turonian boundary is placed by Haq et al. (1988) in the overlying cycle (the UZA 2.5), which contains the Mammites nodosoides biozone. Cycle UZA 2.3 is of Cenomanian age (Lower-Middle Cenomanian, Haq et al., 1988); the Albian-Cenomanian boundary is placed at cycle UZA 2.2 (Haq et al., 1988). In the Iberian basin sequence D has a Cenomanian age and the Albian-Cenomanian boundary is located close at the base of sequence C. Sequences C and D can then be correlated with cycles UZA 2.2 and 2.3, respectively. Sequences A and B are of Upper Albian age. Available fossil data suggest that only Upper Albian sediments were recorded (Schroeder et al., 1993). In the cycles chart of Haq et al. (1988) there are two Upper Albian cycles, the UZA 1.5 and 2.1. A tentative correlation of sequences A and B with cycles UZA 1.5 and 2.1, respectively, is suggested. Although these correlations are tentative, there

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are other data that support such correlation. The total number of cycles in the Haq et al. (1988) cycles chart and sequences in the Iberian basin are identical. There is also a clear agreement between type of cycle boundaries and sequence boundaries in the Iberian basin. Cycle boundaries UZA 1.5-2.1 and 2.3-2.4 are considered important sea-level falls with development of lowstand wedges (Haq et al., 1988); in the Iberian basin A-B and D-E sequence boundaries (Figs. 2, 3 and 5) correspond to important sea-level falls with development of type-l sequence boundaries. On the other hand, UZA 2.1-2.2 and 2.2-2.3 cycle boundaries are minor sea-level falls with development of shelf margin wedges (Haq et al., 1988); the B-C sequence boundary and, at some localities, also the C-D sequence boundary are located at thick marl units (Figs. 2 and 3) with paleosoil development, currently interpreted as lateral equivalents of SMW (Garcia et al., 1993). 6.3. Age model To obtain the duration of the different cycles, the total number of cycles is divided by the total time represented in the sediments. For time calculations the Cenomanian length is used. The duration of this stage has been variously estimated at between 4 Ma (Haq et al., 1988) and 8 Ma (Van Hinte, 1976). Recent studies, however, constrain the Cenomanian length to 5.2-6.5 Ma (Kent and Gradstein, 1985; Obradovich, 1993; Gradstein et al., 1994; see Table 1). From biostratigraphic and cycle data, the Cenomanian in the Iberian basin extends from the base of parasequence Cb to the basal parasequence in the overlying Cenomanian-Turonian sequence (see above) (Fig. 10). These are five 4th-order parasequences. If the 1:5:5 ratio is applied, another ten 4th-order parasequences of sequences D and E are of Cenomanian age. Thus, during Cenomanian time, in the Iberian basin 15 4th-order parasequences were deposited. This corresponds to 75 Sth-order parasequences (15 x 5). The 3rd-order cycle ages can be calculated from the 1:5:5 ratio, simply multiplying the duration of a 4th-order cycle by five (the number of 4th-order cycles in a 3rd-order one) (Table 1).

A. Garcia et al. /Sedimentary Geology 103 (19%) 175-200 Table 1 Estimation of duration of the different cycles in the Iberian basin based on chrono- and biostratigraphic data for the Cenomanian stage and on the cycle ages of the Haq et al. (19881 scale Source

Cenomanian ages Van Hinte (19761 Haq et al. (19881 Harland et al. (19821 Kent and Gradstein (19851 Obradovich (19931 Gradstein et al. (19941 Data average Cycle ages Haq et al. (19881

Duration (Ma)

Iberian cycle periodicities 3rd (31

4th (151

5th (751

8.0 4.0 6.0

2.66Ma 1.33 Ma 2.00Ma

533 ka 266 ka 400 ka

106 ka 53 ka 80 ka

6.5

2.16Ma

433 ka

86 ka

5.2 5.4

1.73 Ma 1.80Ma

346 ka 360 ka

69 ka 72 ka

5.85

1.95 Ma

390ka

78 ka

6.0

1.2 Ma

240 ka

48 ka

The presence of fifteen 4th-order cycles during Cenomanian times has been inferred from cyclostratigraphic data; then, the 1:5:5 ratio between 3rd:4th:5th order cycles is applied to obtain the total number of cycles in the 3rd (31 and 5th (751 orders. A correlation of Iberian cycles with cycles on the chart of Haq et al. (19881 is suggested (see text); the five 3rd-order cycles in the Iberian basin are correlated to cycles UZA 1.5 to 2.4, which have a duration of 6 Ma (99 Ma for the base of UZA 1.5 to 93 Ma for the top of UZA 2.4, see fig. 15 of Haq et al., 1988). Cycle periodicities are obtained dividing duration by the total number of cycles (in brackets).

When extreme values are considered, 3rd-order cycles range from 1.33 to 2.66 Ma, 4th-order cycles range from 266 to 533 ka, and Sth-order cycles range from 53 to 106 ka (Table 1). All these values are within the range usually accepted for these cycles (see Miall, 1984, and also the 1990 edition). Data averages are 1.95 Ma (3rd order), 390 ka (4th order) and 78 ka (5th order), which are close to more recent data (Kent and Gradstein, 1985; Gradstein et al., 1994). Cycles within the range presented above have been described by different authors. Schwarzacher (1993) mentioned the presence of a cycle duration of 2 Ma in the Cretaceous with a main orbital component; it must be pointed out that the regularity of the Upper Albian-Turonian cy-

197

cles in his fig. 10-6 showed just the interval considered here. There is also a regularity inferred from sequence duration (see above) of the Haq et al. (1988) scale, with a mean of 1.2 Ma in the interval considered here. Finally, Fischer (1991) mentioned cycles of 1.3 Ma with an orbital forcing (eccentricity). Higher-order cycles have been widely described. Beach and Ginsburg (1978) recorded cycles of 213 ka in the Pliocene-Pleistocene of the Bahamas, much greater than those described here. Heckel(19861, however, shows the presence of cycles in the Carboniferous of 235-393 ka composed of minor cycles of 44-118 ka, which Riegel (1991) supposed to be eustatically controlled in good agreement with our data. Borer and Harris considered the existence of cycles in the US Permian of 400 ka and 100 ka; a similar estimation was made by Goldhammer et al. (1990) for Alpine Triassic cycles. These cycles were caused by cyclic fluctuations of the relative sea level. The final causes for the eustatic fluctuations are a complex problem. There have been many publications on sedimentary cyclicity (Einsele and Seilacher, 1982; Bayer and Seilacher, 1985; etc.) and many and different causes have been envisaged for its origin (Fisher, 1982; Berger et al., 1984; etc.). In a general way, tectonic and eustatic controls have been proposed as the main explanations. In the sedimentary successions climatic events are recorded by eustatic fluctuations, whose rhythms are usually related to Milankovitch cycles, depending on their scale. The close correlation of the average data (1.95 Ma, 390 ka and 78 ka) with the eccentricity data (2.03 Ma, 412.8 ka and 94.9 ka) of Schwarzacher (1993) stands out. The astronomical origin implies that cycles like these can be distinguished in many different basins, but they may have been recorded or not, depending on the sedimentary characteristics of the basin.

7. Conclusions

The sedimentary record of the Upper Albian-Cenomanian in the Iberian Ranges can

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be described as originating from the superimposition of cycles with different length and amplitude. Cycles control facies and thicknesses, and in any section the record of facies and thicknesses varies, closely following a cyclic pattern. The superimposition of three different cycle orders (3rd to 5th) generated a cyclic variation in accommodation space, which controlled final sequence deposition. Thus, when the three orders are in phase with high relative sea-level, thick parasequences, usually amalgamated, are generated. These sequences contain more marine faties and have a wider regional extension. On the other hand, when the three orders are in phase but with low relative sea level, missed beats are the reason that many cycles are not recorded as parasequences but are seen as major 3rd-order sequence boundaries. This is an application of Walther’s law to depositional sequence development. Differences in the local record between marine and coastal areas are mainly due to the absence of 4th- and Sth-order parasequences rather than to the thinning of the parasequences, which also occurs. The number of parasequences usually remains unchanged in the lateral transition from marine to coastal and continental areas. Parasequence disappearance farther landwards is mainly caused by onlap relationships, and, sometimes, but to a lesser extent, by toplaps. Cycles are mainly eustatic and may reflect an astronomical forcing of relative sea level, Their durations can be related to eccentricity cycles (in the Milankovitch band).

Acknowledgements

The funds for the realization of this work were provided by the DGICYT project PB 90.0086. The first original manuscript was highly improved by the comments of G. Evans of the Imperial College. Further comments by P. de Boer, J. Canerot and other unknown referees made us think and reelaborate many of our previous ideas. Constructive critical reading and suggestions by A.D. Miall led to the final version of this manuscript.

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