Calcareous tempestites in pelagic facies (Jurassic, Betic Cordilleras, Southern Spain)

ELSEVIER Sedimentary Geology 109 (1997) 95-109

Sedimentary Geology

Calcareous tempestites in pelagic facies (Jurassic, Betic Cordilleras, Southern Spain)

J.M. Molina a'*, P.A. Ruiz-Ortiz a, J.A. Vera b

a Departamento de Geologfa, Universidad de Jadn, Campus Universitario, Jadn 23071, Spain b Departamento de Estratigraffa y Paleontologia, Facultad de Ciencias, Universidad, Granada 18071, Spain

Received 2 January 1996; accepted 29 July 1996

Abstract

Calcareous tempestite levels interbedded with Ammonitico Rosso and other related pelagic facies have been recognized. The previously described examples of calcareous tempestites in pelagic facies are scarce. The studied outcrops are Middle and Late Jurassic in age and correspond to ancient sediments in the Southern Iberian Continental Paleomargin. These outcrops are now included in a notably deformed geological unit (External Subbetic) in the External Zones of the Betic Cordillera. The calcareous tempestites are calcarenite and calcisiltite beds, grainstone and packstone with peloids and bioclasts (mainly 'filaments' and Saccocoma), showing an internal structure with hummocky cross-stratification. The deposits are thought to be formed by tropical storms and hurricanes and their recurrence intervals have been estimated (200 ka in average). The presence of these calcareous tempestite levels and the symmetrical wave-ripples on the top of the beds are two important arguments in favour of a palaeobathymetric interpretation of related pelagic sediments in the sense that the deposition occurred below, but near to the storm wave base, and that calcareous tempestites are episodic resedimentation, mainly coincident with relative sea-level falls (lowstand phases), in which major storm waves affect the sea bottom.

Keywords." Tempestites; Hummocky cross-stratification; Pelagic; Subbetic; Jurassic; Palaeobathymetry

1. Introduct ion

The l imestones analysed in this paper are calcarenites and calcisi l t i tes of Middle and Late Jurassic age in the External Subbetic outcropping in the South of C6rdoba and JaEn provinces (Fig. 1), Andalucfa, Southern Spain. These beds present general ly an internal succession of structures and a gradation in the texture and composi t ion, s imilar to other storm layers known in the sedimentological literature (e.g.,

* Corresponding author. Fax: +34 53212141.

Aigner, 1985). Although mainly in the last twenty years many papers on sediments and sedimentary rocks generated by storms have been published, the majori ty of the described examples refer to si l iciclas- tic sediments or rocks, and there are fewer publica- tions in which carbonates with this origin predomi- nate. Table 1 presents an extensive selection of the papers related to carbonate tempesti tes including the previous works about the levels studied here (Checa et al., 1983; Molina et al., 1987; Molina, 1987) and others related to analogous palaeogeographic con- text (e.g., Monaco, 1992; Monaco et al., 1994). The

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J.M. Molina et al. ~Sedimentary Geology 109 (1997) 95-109

• Intermediate Domain

~ ' ~ Camarena-Lanchares unit (External Subbetic)

Gaena unit (External Subbetic)

~ Ahillo unit (External Subbetic)

Other Subbetic units

~ Triassic

Syn- and post-orogenic Neogene-Quaternary

• Location of the studied sections, with calcareous tempestite levels.

1.- Cortijo de Veleta. 2.- Puerto Escafio. 3.- Lanchares quarries. 4.- East of Ahillo.

Fig. 1. Geological sketch of the studied outcrops and tectono-stratigraphic units.

96

recognition of these storm deposits in pelagic marine sequences and the analysis of their sedimentological characteristics can be of great importance for the palaeogeographic reconstructions and basin analysis, especially in lithologically monotonous sequences.

The main aims of this paper are: ( l) to establish the recognition, and to characterize carbonate tern-

pestites from the External Subbetic Jurassic; (2) to estimate their recurrence intervals; (3) to provide a genetic interpretation and to deduce their palaeogeographic significance. In the latter case this concerns especially those implications related to the palaeobathymetry, for this sector and time, in the Southern Iberian continental palaeomargin. The presence of

J.M. Molina et al. ~Sedimentary Geology 109 (1997) 95-109

Table 1 Some selected examples of calcareous tempestites

97

Author(s) Age Location

Haines (1988) Proterozoic South Australia Chuanmao et al. (1993) Middle-Upper Cambrian Helan Mountains (NW China) Markello and Read ( 1981 ) Upper Cambrian Virginia Lee and Kim (1992) Lower Ordovician South Korea Pratt and James (1986) Lower Ordovician Western Newfoundland Brookfield and Brett (1988) Middle Ordovician South of Ontario Kreisa ( 1981 ) Middle-Upper Ordovician Virginia Jennette and Pryor (1993) Upper Ordovician Ohio and Kentucky Sami and Desrochers (1992) Early Silurian Anticosti Island, Canada Kelling and Mullin (1975) Carboniferous Morocco Faulkner (1988) Early Carboniferous Gower (Wales) Wright (1986) Lower Carboniferous South Wales Wu (1982) Lower Carboniferous South Wales Handford (1986) Mississippian Arkansas Ball ( 1971 ) Pennsylvanian Oklahoma McKie (1994) Late Permian England Galli (1989) Triassic Italy Aigner (1979, 1982a, 1984, 1985) Middle Triassic South German Basin Aigner et al. (1978) Middle Triassic South German Basin Calvet and Tucker (1988) Middle Triassic Catalonia Duringer (1984) Middle Triassic East of France Hagdorn (1982) Middle Triassic SW Germany Trrrk (1993a,b) Middle Triassic Southern Hungary Monaco (1992) Toarcian Umbria-Marche area (Italy) Monaco et al. (1994) Toarcian Umbria-Marche area (Italy) Molina (1987) Middle and Upper Jurassic Subbetic (Spain) Molina et al. (1987) Middle and Upper Jurassic Subbetic (Spain) Ager (1974, 1993) Upper Jurassic Moroccan High Atlas Checa et al. (1983) Upper Jurassic Subbetic (Spain) Specht and Brenner (1979) Upper Jurassic Wyoming Prave and Duke (1990) Upper Cretaceous Behobia-St. Juan de Luz Aigner (1982b, 1983) Middle Eocene Gyza Pyramids Plateau Aigner (1985) Quaternary South of Florida Ball (1967) Quaternary Florida and Bahamas Ball et al. (1967) Quaternary South of Florida Gagan et al. (1988, 1990) Quaternary Great Barrier, Australia Hine (1977) Quaternary Lily Bank, Bahamas Wanless et al. (1988a,b) Quaternary Caicos, British West Indies

Some general works that include significant and detailed allusions to carbonate tempestites: Aigner (1985), Burchette and Wright (1992), Einsele (1992), Einsele et al. (1991), Ein~le and Seilacher (1982, 1991), Kreisa and Bambach (1982), Seilacher and Aigner (1991), Shinn (1983) and Tucker and Wright (1990).

these calcareous tempestite levels is an important argument to establish a clear shallow-marine origin for these pelagic sediments.

2. Geological setting

The Jurassic calciclastic levels studied in this paper correspond to sedimentary rocks deposited in

the South Iberian Continental margin, which was the westernmost part of the European margin of the Tethys. The External Zones of the Betic Cordilleras were the materials deposited on this margin from Triassic to Lower Miocene, and later forming a com- plex of thrust sheets detached from their Palaeozoic basement because of the Tertiary Alpine tectonics. The External Zones have been subdivided into sev-

98 .LM. Molina et al./Sedimenta¢ T Geolog.~ 109 (1997) ~5-109

eral tectonic ensembles that normally coincide with the palaeogeographic domains of the palaeomargin (Vera, 1988). The External Subbetic, made up of several tectono-stratigraphic units, is one of these domains and it is characterized by pelagic facies from the Upper Jurassic, although in some units the pelagic facies started in the Upper Liassic: the Mid- dle Jurassic shows pelagic facies in some units while in others shallow-marine, oolitic carbonate facies appear (Molina, 1987).

The studied outcrops are located in three different tectono-stratigraphic units belonging to the External Subbetic. These three units are composed by Tri- assic-Miocene sedimentary rocks showing a nappe structure: (a) the Camarena-Lanchares unit is lowest tectonically in the External Subbetic; (b) the Gaena unit overthrusts the Camarena-Lanchares unit: and (c) the Ahillo unit outcrops in a tectonic window below the Triassic materials (Fig. 1). The Gaena and Ahillo units present similar Jurassic stratigraphic sections (Fig. 2) with pelagic facies considered as deposited in a pelagic swell or pelagic carbonate platform (Molina, 1987: Vera et al., 1988). The Ca- marena-Lanchares unit differs mainly in the Middle Jurassic (Dogger) materials that are composed of a thick sequence of oolitic limestones (Camarcna Fro.) contrasting with the other two, in which the Dogger shows pelagic facies (Molina, 1987).

The analysed levels are of Middle Jurassic age in the Gaena and Camarena-Lanchares units and they are of Late Jurassic age in the Ahillo unit. Specifically, in the Gaena unit they appear in Aale- nian-lower Bajocian limestones with chert (Veleta Fro.) and in upper Bajocian-middle Bathonian nodular limestones (Upper Ammonitico Rosso Fro.). In the Camarena-Lanchares unit the studied levels are in Bajocian-Bathonian oolitic limestones (Camarena Fm.). Calcarenitic and calcisiltite levels also appear in the Upper Jurassic (Kimmeridgian-Tithonian) of the Ahillo unit interbedded in nodular limestones (Upper Rosso Ammonitico Fro.). As an important aspect in this last unit it is necessary to emphasize that minor discordances and lateral wedges, slumps and breccias appear locally in the Upper Jurassic rocks.

We have studied in detail two stratigraphic sections from the Gaena unit (Cortijo Veleta and Puerto Escafio sections, Fig. 1, nos. 1 and 2) and another from the Ahillo unit, located jus! to the east of Abillo

mountain (Ahino section, Fig. 1, no. 4). The detailed logs of these sections are represented in Fig. 2. We have selected these sections not only because they have a good development of the tempestite beds, but mainly for their high content of ammonites which has allowed us to establish their precise biostratigraphy. Also we have made observations in outcrops of the Lanchares quarries (Camarena-Lanchares unit) (Fig. 1, no. 3), although here we have not obtained the same biostratigraphic accuracy because of the lack of characteristic macrofossils.

3. Caleiclastic levels

The studied calciclastic levels have a thickness between 5 and I(X) cm. They arc composed of grainstone and packstone with peloids and bioclasts (mainly "filaments' or Saccocoma), with a fine-sand or silt size grain, although more locally coarser sand or rudites, with the same microfacies, appear. They have an internal organization in which the ideal sequence consists of three divisions described next and represented in Fig. 3A.

3.1. l)ivision 1

It is composed of calcarenite or calcisiltite in which there are, in complete sequences, from bottom to top: horizontal or slightly wavy parallel lamination, hummocky cross-stratification and low-angle cross-lamination (maximum 15 °) related to symmetrical ripples (Fig. 3A, division 1, a, b, c, and d; Figs. 3B, 4 and 5). A similar sequence has been recognized in different examples of distal carbonate tempestites (Kreisa, 1981; Aigner, 1985; Monaco, 1992). The bottom may be (Fig. 3A, e-h): (e) erosive, with breccias and intraclasts, also wavy lamination as a result of the filling and adjustment to the bottom irregularities or clasts can locally appear: (f) planar, without distinguishable irregularities; (g) stylolitie, separating grainstone or packstone facies superposed to mudstone or wackestone facies; (h) with load structures, which appear as irregular protuberances. with a relief of some centimetres and associated with tlame structures. Locally the bottom shows trace fossils attributable to Thalassinoides (Fig. 3B).

On the top of this division in the Ahillo section symmetrical oscillation ripples with rounded and

99

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J.M. Mo/ina et al . / Sedimentary Geology 109 (1997) 95-109

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JURASSIC AHILLO UNIT

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~ Limestones

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• Marly "Ammonitico Rosso"

Slightly nodular limestones

Calcareous "Ammonitico Rosso"

• Marly limestones

[ ~ Cherty limestones

~ Calcarenites and calcisiltites

• Most important storm layers Fig. 2. Jurassic sections of Gaena and Ahillo units (modified from Molina, 1987; Molina et al., 1987). Biozones: O = Opalinum-

Murchisonae; H = Humphr ies ianum; L = Leptosphinctes; A = Annulatum; P = Parkinsoni; B = Beckeri; Y = Hybonotum; A =

Albert inum; V = Verruciferum; R = Richteri; AB = Admirandum-Biruncinatum; U = Burckardticeras.

straight crests are present• Their wave lengths (L) are between 30 and 70 cm and their heights (H) from 2 to 6 cm. The ripple indices (L/H) are between 7.1 and 30 and their symmetry index is very near to 1. In accordance with their structure they show foreset laminae dipping in only one direction and wavy parallel lamination in the upper part, as seen in Figs. 4 and 5. The ripple crests have a constant direction N 150°E, perpendicular to parting lineation (Fig. 3B). It is notable that the directions of the ripple crests generated by storms, according to different authors (cf., Komar et al., 1972; Aigner, 1982a,b; Cheel and Leckie, 1993) have a tendency towards being parallel or slightly oblique to the ancient coastline. In

other outcrops these ripples do not appear and the transition to the overlying division II is gradual.

In the Middle Jurassic of the Gaena unit there also appear very peculiar hummocky morphologies with cross-stratification (Fig. 6), their foreset dips have been measured resulting in an approximated direction N30°E, which would indicate palaeocurrents to N 120°E.

The average grain size in this division I is between 0.05 and 0.2 mm, but can reach up to 30 cm, in the Camarena Fm. The lamination, with laminae from 0.5 to 2 mm thick is marked by alternative laminae of peloidal levels (peloidal grainstone) and others bioclastic ('filaments' or Saccocoma packstone).

100 J.M. Molina et al./Sedimentary" Geology 109 ¢ 1997) 95-109

A IDEAL SEQUENCE I ~ a 2O t o

Division

........ _.1"= ~ ; ~ ng linaatiOn ~ g

Erosional/sharp surface Thalassinoides

C SEQUENCE TYPES

X o

r-

I

4'

_J Bioturbation

Fig. 3. Ideal sequence and other types of sequences, of the calcareous tempestites, observed in the analysed outcrops. (A) Ideal sequence of the carbonate storm layers, with three divisions (I, II, III), slightly modified from Molina et al. (1987). Explanation in the text. Key a, b, c, d: variation in the internal structures of the I division (a = horizontal lamination: b = low-angle cross and ondulate laminations: c, d: hummocky cross-stratification). Key c. f, g, h: types of boundaries at the base of the sequences (e = erosive, with breccias and intraclasts; f = planar; g = stylolitic; h = load and flame structures). (B) Internal structure in the I division (L = wave length: H = height), with indication of the orientation of the wave ripples and the parting lineation, fC) Proposed model of the sequence types according to their proximality and bioturbation degree (modified from Molina et al., 1987).

3.2. Division II

It is made up of grey micritic limestones, with parallel lamination in the bottom level and with bioturbation in the upper part. The grain size is ho- mogeneous although locally normal grading appears in the bottom with thin levels of very fine sand and silt. The lamination is generally very faint and is de- fined by laminae 0.3 to 0.8 mm thick with different content in small bioclasts. In the upper part (20 cm at maximum) trace fossils appear: vertical and inclined burrows (Chondrites, Teichichnus), others branched parallel to the stratification (Planolites) and feeding marks (Fodichnia) belonging to the Cruziana ichnofacies. These burrows are mainly filled by red micrite, making their recognition easy. The transition to the upper division (division III) is gradual and marked by an increase in bioturbation and for the appearance of the nodular aspect.

3.3. Division II1

It is composed of nodular limestones or marly limestones with red or yellow colour (fossiliferous wackestone or mudstone), locally with abundant ammonites. Strongly bioturbated, they are often made of grey micritic limestone nodules in a red or purple marly matrix.

The upper part of division II and division III can be more or less developed depending on the importance of the bioturbation. Sequences can also appear which are almost completely bioturbated (Figs. 3C and 5A).

4. D i s c u s s i o n

In the Upper-Middle Jurassic rocks from the Sub- betic different kind of facies appear deposited in very diverse palaeogeographical contexts linked to a passive continental margin divided, according to

J.M. Molina et al./Sedimentary Geology 109 (1997) 95-109 101

Fig. 4. Different aspects of the tempestites in the field. (A) Symmetrical ripples on the top of the division I (east of Ahillo section, Ahillo unit). (B) Symmetrical ripple (division 1) with foreset laminae dipping in only one direction (east of Ahillo section, Ahillo unit). (C) Wavy parallel lamination in the upper part of division I (east of Ahillo ,section, Ahillo unit).

the differential subsidence, after an important rifting phase, in troughs and swells (Vera, 1981, 1988).

The Middle Jurassic rocks from Camarena-Lan- chares unit (Camarena Fm.) show a shallowing- upward sequence ending with emergence at the

end of the Middle Jurassic. There are limestones deposited in a shallow platform environment and intercalated between carbonate pelagic sediments (Molina et al., 1985; Ruiz-Ortiz et al., 1985; Molina, 1987). In the Middle Jurassic of the Gaena unit we

102

. ~ .

J.M. Molina et al. /Sedimentat 3'

A

Fig. 5. Different aspects of tempestites in the field. (A) Paral- lel horizontal lamination, bioturbated in the upper part of the bed (Puerto Escafio section, Gaena unit). (B), (C), Hummocky cross-stratilication in division I (east of Ahillo section, Ahillo unit).

Geology 109 (I997) 95-109

can observe the transition from cherty limestones to nodular limestones indicating probably a bathymet- ric change to a shallower depth. In any case during the Middle Jurassic this area was a pelagic swell in the continental margin according to the palaeogeographical reconstructions proposed in other papers (e.g., Vera et al., 1988). The Upper Jurassic (Maim) of the Ahillo unit was deposited also in a pelagic swell with a very low rate of sedimentation and subsidence (Molina, 1987).

The genesis and sedimentological features of the caicarenitic and calcisiltitic levels studied can be in- ferred from the interpretation of the ideal sequence. We will consider their three divisions (I, II and III), which originated in different time and hydrodynamic conditions and sedimentation rates. The divisions I and II were properly linked to storms (peak storms and waning of the storms' energy) with higher sedimentation rate. The features mainly from division I, with hummocky cross-stratification and symmetrical wave-ripples, are the common criteria of storm layers, according to numerous papers (e.g., Aigner, 1982a,b, 1985; Duke, 1985; Einsele and Seilacher, 1991; Cheel and Leckie, 1993). Division III repre- sents the sediment deposited and/or disturbed under quiet conditions (post- or pre-storm with low sedimentation rates).

More than eight distinct or composite main mechanisms have been proposed for sediment supply, erosion, transport and deposition of sediments during storm events (see for instance, Aigner, 1985, p. 42; Cheel and Leckie, 1993, pp. 114-119). Among the main mechanisms in which the storm waves and currents generated by storms erode, transport and deposit sediments, we can consider: (a) storm surge ebb; (b) wind forced or geostrophic currents; (c) gra- dient currents; and (d) turbidity currents generated from storms. In one way or another the sediment is deposited from a suspension sediment cloud remobi- lized by storm waves. In the maximum energy periods breccias and intraclasts can also originate, that made up locally the bottom of the sequence (division I).

It is interesting to point out that though sometimes grading is not observed, the hydrodynamic behaviour of the planar bioclasts and peloids, that are the most important components in the analysed materials, is very different, according to the concept of 'hydraulic equivalence' that takes account of particle density,


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Mega-scale Hummocky Morphology

Fig. 6. Sketch and photography of an interesting outcrop in the Cortijo Veleta section (locahty 1 m r~g. 1, tJaena umt), m wmcn we can see a typical mega-scale hummocky morphology and cross-stratification. The wave length of the ripple is about 12 m and the height is close to 1 m.

104 J.M. Molina et al./Sedimentar 3" Geology 109 (1997) 95-I09

volume and shape, and it is an important point to consider in any analysis of sediment transport and deposition. Mainly the 'filaments' and also the planar fragments of Saccocoma have a high surface area/volume ratio and for this reason they can behave as would particles of a smaller size than the real, by reference to the deduced mechanisms of transport and deposition. From the sediment (peloids and bioclasts) cloud in suspension the peloids would be deposited first, followed by the 'filaments' resulting in parallel lamination by the millimetric alternation of these kind of particles.

The symmetrical ripples with straight crests show the action of oscillatory fluxes at least during the last stages of the division I deposition. These ripples must be formed in a depth less than 200 m, that is the maximum depth obtained by the oscillatory currents produced by storm waves (cf. Komar et al., 1972; Kreisa, 1981). The presence of Cruziana ichnofacies in division II and the Thalassinoides. locally in division I, described before, are also an argument in favour of a similar palaeobathymetric interpretation for those sediments.

The lack of biogenic structures in division I and in the lower part of division II is explained as the result of a momentary high rate of sedimentation and the absence of benthic organisms. The high currents velocities and linked erosion, and also a high rate of sedimentation annihilate and remove the populations of epibenthic organisms and surficial infauna in extensive areas. In this way the absence of biogenic structures can be due more to the low temporal density of population than to the high sedimentation rate, although both phenomena could be related.

From precise biostratigraphic dating, using ammonites, we have estimated the recurrence interval of storms capable of forming tempestites by dividing the duration of the studied sections by the number of storm beds. Although many errors are inherent in this method, we have not found consistency with the results of other works, with recurrence intervals between 400 and 15,000 years (e.g., Cheel and Leckie, 1993). More specifically, we have calculated the recurrence interval of the events in the Bajo- cian (Leptosphinctes and Parkinsoni biozones) and in the lower Tithonian (Verruciferum biozone) (Fig. 2). Considering the total duration of the Bajocian

as 6 m.y. (Hallam, 1984; Harland et al., 1990; Kent and Gradstein, 1985; Gradstein et al., 1994) and considering for every one of the six biozones an approximately equal length of time, the average duration of each biozone is 1 m.y. and so the frequency of events would be approx. 1 every 200,000 years. For the Verruciferum biozone (lower Tithonian) a frequency of 1 event every 103,000 years results (Checa et al., 1983).

5. Palaeogeographic considerations

As can be seen in Fig. 7, the palaeolatitude (20°N) estimated from this region during the Late Jurassic (Ogg et al., 1984), the most recent palaeogeographic reconstructions made for the Jurassic of the Tethys (Dercourt et al., 1986, 1993; Cecca et al., 1993; Fourcade et al., 1993) and also the related palaeo- climatic aspects (Marsaglia and Klein, 1983; Duke, 1985) suggest that strong tropical storms or hurricanes might be responsible for the erosion and redeposition, more than winter storms. Also con- finned by palaeoclimate modelling using General Endulation Models (Price et al., 1995). As stated by Marsaglia and Klein (1983) the storms generated at present by hurricanes are produced between 5 ° and 10°N over the open ocean, and when they impinge on the continent, they affect the eastern margins of the continental areas between latitudes 20 ° and 35°N, with an average of 28°N.

According to the distality-proximality trends (Aigner, 1982a,b, 1985; Monaco, 1992) the analysed tempestites are mainly of distal type, except for some examples in the Middle Jurassic oolitic limestones of the Camarena-Lanchares unit, in which are included breccias and important erosive channels, that can be considered as proximal. This trend or degree of proximality is based mainly on the decreasing effects of storms with increasing water depth and distance from shore. Because of this, a systematic decrease in grain size and in thickness of the beds must be expected the further we go from the coast. So, these changes can be used as a proximality trend according to Aigner (1985). However, over an irregular bottom with marked troughs and swells, as in the whole Subbetic, these simple regional models are difficult to apply. In the present study we can establish a gradation from major to minor proximality trends from

J.M. Molina et el./Sedimentary Geology 109 (1997) 95-109 105

~ Platforms and epicontinental seas

[ ] Pelagic facies (above CCD)

~ P.~_..1 Present coastal lines

Emerged lands (without deposition or with continental deposition)

Fluviodeltaic and margino-litoral deposition

~ Pelagic facies (below CCD)

Palaeolatitude

~ A c t i v e spreading Important faults Fig. 7. Palaeogeographic reconstructions for the western Tethys during the Late Jurassic (early Kimmeridgian, 146--144 Me) after Cecca et el. (1993). The asterisk shows the location of the studied outcrops. Key of the localities: A = Apulia; ACP = Apennine carbonate platform; AM = Armorican Massif; At = Atlas; B = Brianqonais; D m = Dalmatia; Dr = Drama; G = Gavrovo; l = Ionian furrow; IM = Iberian Massif; K = Kabilia; L = Lombard; LB = Ligurian basin; LBM = London Brabant Massif; LT = Lagonero Trough; MC = Massif Central; MM = Moroccan Meseta; MP = Maghrebian Platform; P = Parnasos; PO ---- Pindos-OIonos; RM = Rhenish Massif; SP = Serbo-Pelagonian; Sub = Subbetic; Ta = Tetra; TeT = Tellian Trough; Ts = Transylvanids; Urn = Umbria-Marches Basin.

the ou tc rops o f the C a m a r e n a - L a n c h a r e s unit to the

Midd l e Jurass ic (Dogger ) o f the G a e n a unit, whe reas

the U p p e r Jurass ic o f the Ahi l lo unit wou ld p resen t a

distal t rend, but i n t e rmed ia t e b e t w e e n both.

The ob ta ined r ecur rence intervals d e s c r i b e d be-

fore are exces s ive ly longer by c o m p a r i s o n wi th the

ob ta ined data by o the r authors in anc ien t s to rm

depos i t s (e.g., 1200-3100 years , Kreisa , 1981 and

106 J.M. Molina et al./Sedimentary Geology 109 ¢1997J g5-10c~

Aigner, 1982a,b; 400-2000 years, Goldring and Lan- genstrassen, 1979; 12,500-18,750 years, Brenchley et al., 1979) and also with the proper frequency of the storms and hurricanes in nature. However, it is necessary to take into account various important factors that can appear to lengthen the measured time interval between the events such as described below.

(a) If the deposition occurred sufficiently near to the storm base level (pelagic swell or external platform or ramp) the cyclic changes of the sea level could put the marine bottom below this base level, without being affected by the storm waves during long periods of time. Hallam (1978, 1981) for the Jurassic has proposed oscillation cycles of the sea level with a similar duration. Division I would repre- sent the relict sedimentation of the storm-dominated lowstand, and divisions II and III transgressive and highstand phases. From this point of view, each sequence would be the recording of high-frequency sea-level fluctuations.

(b) It is difficult to separate all the condensed events in composite beds. The condensation phenomena and cannibalism described by Seilacher (1982) can be very important and our perception at present would be the final result of numerous erosive and depositional episodes. On the other hand, from these episodes we could only recognize the most intense or strong ones, because of this trend to destroy the imprint of the weaker and intermediate events in energetic terms.

(c) The difficult preservation of sedimentary structures, mainly in thin layers, with a low preservation potential, that can become bioturbated and homog- enized completely by the effects of post-event bioturbation. As important datum, we have found only a few calcarenitic sequences with less than 5 em of thickness, probably because they have not been preserved.

(d) In the end, this excessively long period of time could also be explained if the removal of the mass of water which had eroded and deposited the sediment were not the result of atmospheric climatic factors but from other seismic or tectonic origins. The observed small local discordances, slumps and breccias, cited before, would be consistent with this hypothesis. Such processes can be produced below storm wave base.

6. Conclusions

As has been exposed in other papers (Molina et al., 1983; Vera et al., 1984; Molina et al., 1985: Ruiz- Ortiz et al., 1985; Molina, 1987), the sedimentary environments of the analysed sequences are carbonate external platform or ramp for the Middle Juras- sic of the Gaena unit, shallow platform (sub-inter- tidal) for the Middle Jurassic of the Camarena-Lan- chares unit and pelagic swell for the Upper Jurassic in the Ahillo unit. The presence of storm layers in these sequences allow us to make estimations about the palaeobathymetry of these depositional environments. The sequences of the Middle Jurassic of Gaena unit and the Upper Jurassic of the Ahillo unit, where most distal tempestites appear, would have been deposited below the wave base level, being affected only by the most important storm waves and hurricanes. We estimate their bathymetry, by comparison with modern environments, to have ranged from the fair- weather wave base to 200 m. The ichnofacies present are also consistent with such an interpretation.

The extensive period of time between different sequences is considered to be mainly caused by the bathymetry, already too deep to be affected by many storms, and by bioturbation, which would destroy the sedimentary fabric. In the proposed model, an important consideration is the relation between tempestite levels or sequences and high-frequency sea-level fluctuations.

In the Middle Jurassic (oolitic limestones of the Camarena Fm.) of the Camarena-Lancharcs unit the most proximal tempestites appear, being the shallow- est analysed sequence is an argument in favour of the palaeogeographic interpretation of these Subbetic areas (see Vera et al., 1988; Molina et al., 1995), as shallow pelagic facies (tar away from the continent but not necessarily with great depth) and admitting the local and temporal emersions of the higher part of the tilted fault blocks.

The progressive opening of the Tethys towards the west during the Middle and Upper Jurassic, the location of the depositional environment in the whole of the continental margin where the pelagic swell would be nearer the land, and their location in the southeastern border of the Iberian Plate, are factors which would have facilitated the presence of calcareous tempestite levels in the analysed sections.


Acknowledgements

The authors sincerely thank T. Aigner and A. Desrochers for reviewing this paper and their con- structive suggestions, improving the original manu- script. We also thank B. Sellwood for his editorial advice and comments. Elizabeth Adams is greatly acknowledged for improving the English text. This study forms part of the results obtained in Research Project PB-93-1150 which are financed by the 'Di- reccirn General de lnvestigaci6n Cientffica y Trc- nica' for which we are grateful. We thank also the 'Junta de Andalucfa' for their collaboration in our research projects.

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Calcareous tempestites in pelagic facies (Jurassic, Betic Cordilleras, Southern Spain)