Facies associations, sequence stratigraphy and timing of the earliest Jurassic peak transgression in central Spain (Iberian Range): correlation with other Lower Jurassic sections.
The precise timing and definition of the facies associations of the transgressive-regressive (TR) cycles is of primary importance to perform high-resolution correlations based on sequence stratigraphy among the different subbasins. Occurrence of ammonoids in the Jurassic deposits represents a conventional tool for bio(chrono)stratigraph ical dating and allows excellent basis for correlation with other areas.
In the Iberian Range, which is a northwest trending folded and thrusted belt located in central and eastern Spain (Fig. 1), the Triassic-Jurassic transition occurs within a mainly evaporitic thick succession (the Lecera Formation) or its dissolved equivalent unit (the Cortes de Tajuna Formation) (Gomez et al., 2007). This represents the regressive portion of the TR Upper Triassic-Lower Jurassic Cycle 1 (LJ-1) defined by Gomez and Goy (2005) (Fig. 2). The first Early Jurassic transgression resulted in the installation of a very shallow extensive carbonate platform, on which subtidal to intertidal and supratidal environments have been recognized. The initial extensive carbonate platform was breaked-up into several blocks due to extensional syndepositional tectonics around the Sinemurian-Pliensbachian transition. In the northeastern part of the area, movements along the Montalban Fault (Fig. 3) caused subsidence of hanging wall blocks located to the northeast of the fault, and the generation of enough accommodation space as to allow better development of the Sinemurian-Pliensbachian TR facies Cycle LJ-2. Ammonoids-bearing external platform facies, represented by the Rio Palomar and the Almonacid de la Cuba Fms were deposited northeast of the Montalban Fault, whereas peritidal carbonate deposition continued southwest of the fault (Figs. 2, 3). The transgressive peak of the Cycle LJ-2 (Gomez and Goy, 2005) has been dated northeast of the Montalban Fault as Lower Pliensbachian Jamesoni Zone on the basis of the ammonoids and brachiopods found in the deposits of the Almonacid de la Cuba Fm (Sequeiros et al 1978; Comas-Rengifo, 1985; Comas-Rengifo et al., 1997, 1999). However, in the peritidal facies of the Cuevas Labradas Fm, deposited southwest of the Montalban Fault, the known record of the Sinemurian-Lower Pliensbachian ammonoids and brachiopods remains very poor (Goy et al., 1976) and in the southern part of the Iberian Range no ammonoids allowing dating of the Cuevas Labradas Fm were previously found.
The aim of this work is to report the results of the work performed in the sequence and cycle stratigraphy of the Early Jurassic shallow platform to peritidal carbonates of the Cuevas Labradas Fm, which includes the analysis of the facies associations and the finding of ammonoids that allowed dating of the peak transgression of Cycle LJ-2 in this part of the Iberian platform system. The synchronism of some of these key stratigraphical intervals is also analyzed through the correlations with other sections located in Spain as well as in Western Europe and in the Tethys domains.
[FIGURE 1 OMITTED]
2. Stratigraphical succession
The studied section is located in the Barranco de la Hoz creek, situated near the town of Sarrion, south of Teruel (Fig. 1b). The stratigraphical succession (Fig. 4) represents the upper part of the Cuevas Labradas Fm and the base of the Barahona Fm. The succession, which is mainly constituted by carbonates with interbedded marls, is representative of the LJ2-2 facies Cycle (Fig. 2). On the basis of the facies associations, which have been based on field observations, four units (A to D) have been distinguished. These units constitute well differentiated facies belts which can be mapped along most of the southtern part of the Iberian Range.
2.1. Unit A
Unit A is formed by mudstone to wackestone limestones which are organized in aggradational muddy thickeningand shallowing-upward sequences. The unit can contain layers of bioclastic packstone limestones showing a sharp erosional base, internal hummocky cross-lamination and bioclastic rills (Figs. 4, 5A). This unit was deposited in a restricted shallow platform affected by the storms.
2.2. Unit B
Unit B is composed of shallowing- (B1) and deepening-upward (B2) sequences. The shallowing-upward sequences (B1 in Fig. 5) are composed of thickening- and coarsening-upward sequences showing a marly unit at the base, which grades upward to nodular mudstone and bioclastic wackestone limestones. The upper part of the sequence is constituted by intraclastic packstone to grainstone limestones showing internal hummocky cross-lamination. Geometries are tabular to lenticular and the base is frequently marked by an erosive surface.
The deepening-upward sequences (B2 in Fig. 5) are composed of thinning- and fining-upward sequences showing at the base amalgamated packstone to grainstone limestones with hummocky cross-lamination. The middle part is constituted by wackestone-packstone limestones which can contain layers of packstone-grainstone limestones and interbedded marls that predominate in the up per part of the sequence.
[FIGURE 2 OMITTED]
This unit B contains Polymorphites sp., Uptonia cf. jamesoni (SOW.) and Uptonia cf. angusta (QUENST.) that characterize the upper part of the Jamesoni Zone of the lowermost Pliensbachian. The exclusive presence of adults, with the absence of juvenile forms, suggests that these parts of the platform were not colonized by ammonoids and that the found shells arrived to the studied area by drift, derived from more external parts of the platform, coinciding with the overall deepening conditions.
2.3. Unit C
Unit C is composed of shallowing-upward sequences of "muddy" type in the lower part of the unit and "grainy" type in the upper part. Marls are very scarce even at the base of the sequences. The "muddy" thickening- and shallowing-upward type of sequences, are frequently constituted by lime mudstones to wackestones, occasionally bioturbated, which can contain rills of bioclastic wackestone to packstone limestones with internal hummocky cross-lamination and lime mudstones at the top (C1 in Fig. 5). These bioclastic layers, which can contain a bioclastic lag at their base (Aigner, 1982) can be amalgamated and are interpreted as tempestitic deposits. The lower part of unit C was deposited in a shallow external restricted platform frequently affected by the storms.
The sequences of "grainy" type, corresponding to the upper part of unit C (C2 in Fig. 5), are composed of bioclastic and intraclastic packstone to grainstone limestones showing planar and through-type cross-bedding. These deposits mainly represent wave-dominated shoals located at the shallow portions of the platform, on which beach environments including the shoreface and foreshore subenvironments, can be recognized. These "grainy" type of deposits represent the progradation of the high energy belt of the shallow platform over the lower energy restricted platform located at the front of the bioclastic shoals, during the regressive part of the cycle.
2.4. Unit D
Unit D is constituted by the classical peritidal shallowing-upward sequences (D in Fig. 5). The lower part is constituted by thin marls and nodular lime mudstones which can contain thin and lenticular layers of carbonate breccias. These breccias contain clasts of limestones representatives of the different peritidal sub-environments, such as lime-mudstones, bioclastic wackestones and mudstones with algal laminations. The marls are commonly absent, and the base of the sequences is constituted by thick-bedded bioclastic wackestone to packstone limestones containing mainly unfragmented gastropods and bivalves. The middle part of the sequence is composed of lime mudstones with algal laminations and the upper part of algal laminations occasionally with fenestral porosity and mud-cracks. The top of the sequence is sometimes constituted by a pebble breccia of similar composition to the one included in the marls, containing fragments of the described lithologies.
The depositional environments represented in this unit are ranging from the shallow restricted subtidal environments of the internal platform (lagoon) to the intertidal and supratidal environments. The breccia located at the top of the sequences is interpreted as tempestitic deposits accumulated in the supratidal portion of the ramp, which was reworked into the basal lag of the overlying sequence at the onset of the following stratigraphic cycle.
3. Discussion of the results
The superposition of sequences shown in figure 5 illustrates the transgressive-regressive parts of Cycle LJ2-2. The transgressive phase is represented by unit A and sequences of type B1, which culminates in the peak transgression represented by the sequences of type B2. The base of the transgressive deposits does not outcrop in the section, but the regressive part of the cycle is well represented by units C and D.
[FIGURE 3 OMITTED]
The succession of shallowing- and deepening-upward sequences plotted against the recognized depositional environments is shown in figure 6. The definition of parasequences of Van Wagoner et al. (1990) only included shallowing-upward sequences. However several authors recognized the presence of transgressive deposits, suggesting the possibility of redefinition of the parasequence concept to include those deposits (Arnott, 1995). On the other hand, Myers and Milton (1996) pointed out that shallow marine sediments are commonly arranged into regular upward-coarsening units with an upward-shoaling facies succession, separated by much thinner units representing a deepening-upward facies succession. These shallowing- and deepening-upward sequences have also been described in many areas of the Jurassic deposits of Spain (e.g.; Gomez, 1991; Gomez and Fernandez-Lopez, 1994, 2006; Gomez and Goy, 2000, 2005).
For some authors (Guillocheau, 1995), the high frequency sequences can be modified by the superposition of several signals that record the different variations of sea-level. This effect, which is named "sequence or stratigraphic distortion", implies that in a long period of time of relative sea-level fall, a more asymmetrical pattern of the high frequency sequences can be preserved and the record can be restricted to the progradational units. On the contrary, a high frequency sequence formed during a long period of time of sea-level rise, tends to generate a symmetrical cyclical pattern. In our case, a part of unit B at the Barranco de la Hoz section shows an arrangement consisting of some alternating deepening-upward and shallowing-upward sequences (cf. Guillocheau, 1995). Unit B corresponds to a maximum in the relative sea-level rise representing a maximum in the generation of accommodation space and consequently to the low frequency retrogradational phase. Units A, C, D and sequences B1 have a shallowing-upward trend corresponding to the low frequency aggradational and progradational phases.
There is a lack of general agreement on the hierarchical order of the sequences, genetic units or shallowing/deepening sequences. For some authors (e.g. Myers and Milton, 1996; Haq et al., 1988) the parasequences are cycles of 4th order, but other authors do not assign a hierarchical order to the high frequency sequences (Guillocheau, 1995) or to the genetic units (Homewood et al, 1992). Goldhammer et al. (1990, 1993) distinguished high frequency cycles of 4th and 5th order within the 3rd order sequences. Fernandez-Lopez (1997, 2004) assigns a 5th order to the sequences and recognizes that they can be grouped into sets of sequences of a higher order, which correspond to the 4th order stratigraphical cycles. If the shallowing- and deepening-upward sequences are considered as 5th order sequences or cycles, and the 1 to 5 Ma Cycles (like Cycle LJ2-2) are considered 3rd order cycles, it looks clear that the intermediate cycles should be considered as 4th order cycles.
This 4th order cyclicity has been recognized in the studied section, as shown in figure 6. As the lowermost part of the 3rd order Cycle LJ2-2 does not outcrop in this area, the 4th order cycles have been provisionally named from the lower to the upper part of the studied section as LJ2-2a to LJ2-2g. These cycles also show the transgressive-regressive trend at a lower scale than the 3rd order cycles. The transgressive peak or deepening maximum is located in the ammonite-bearing marly facies of unit B and the shallowest facies correspond to the peritidal deposits of unit D.
The transgressive maximum of the 4th order cycles coincide with major accumulations of marly facies while in the regressive phases calcareous facies are predominant. In unit D, where marly facies are virtually absent and following the criteria established by Goldhammer et al. (1993), the transgressive peaks of the 4th order cycles coincide with the bases of the thickest shallowing-upward sequences (highest accommodation space) and the deepest facies, while the regressive maximums are located at the tops of the shallowing-upward sequences with the thinner (less accommodation space) and the shallowest facies.
Following the Ogg (2004) Jurassic time scale, a duration of about 2.6 Myr is estimated for the 3rd order Cycle LJ2-2 (from the base of the Pliensbachian at 189.6Ma to the top of the Davoei Zone at 187.0Ma). The duration of the 4th order cycles cannot be established here due to the lack of a high resolution stratigraphy, but if it is supposed that Cycle LJ2-2 is constituted by 7 cycles of 4th order and considering that the sedimentary control in the basin is mostly allocyclic, a possible time span of about 0.4Myr for these cycles can tentatively be estimated. This figure is nearly coincident with one of the principal modes of the Milankovitch orbital eccentricity cycles (404 kyr) which seem to have remained relatively stable over much of the Phanerozoic (Hinnov, 2004).
3.2. Comparison with other Lower Jurassic sections
Comparison of the obtained results with other Lower Jurassic sections in Iberia, other areas of Europe and the global cycles proposed by Haq et al. (1988) (Fig. 7) reveals some interesting similarities that can be used as correlation criteria between the different basins. Referring the 2nd order cycles, the boundary between cycles LJ-1 and LJ-2, which represents the onset of the first Jurassic transgression, looks to be nearly synchronous, within a reasonable margin of error, in the Iberian Range (Gomez and Goy, 2005), in the Basque-Cantabrian Basin of Northern Spain (Quesada et al. ,2005; Rosales et al. , 2006), in the Boreal province (Graciansky, de et al. , 1998; Jacquin and de Graciansky, 1998), in the UK (Hesselbo and Jenkyns, 1993) and looks to coincide with the boundaries between cycles 3.1 and 3.2 of Haq et al. (1988).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The boundary between 3rd order cycle LJ2-1 and cycle LJ2-2 looks to be another good correlation criterion in Iberia. This boundary coincides with the limit between sequences 2 and 3 of the Basque-Cantabrian Basin in Northern Spain (Quesada et al. , 2005; Rosales et al. , 2006) and with the onset of the sequence SP in the Lusitanian Basin of Portugal (Duarte et al., 2004; Duarte, 2006) and the boundary between cycles 4 and 5 in the UK (Hesselbo and Jenkyns, 1993).
Another good correlation criterion seems to be the peak transgression corresponding to cycles LJ-2 and LJ2-2 identified in this work at the Lower Pliensbachian Jamesoni Zone. Peak transgressions of this age have been reported in the Boreal province (Graciansky, de et al. , 1998) and in the UK (Hesselbo and Jenkyns, 1993), but this age do not coincide with the timing reported in the Basque-Cantabrian Basin, in the Lusitanian Basin, in the Tethyan domain and in the Haq et al. (1988) cycles.
The boundary between LJ-2 and LJ-3 also looks to be a good element for correlation between the different basins. Ages around the Early and Late Pliensbachian boundary are reported in most of the considered areas, including the Haq et al. (1988) cycles. The only exception is the Tethyan domain on which the onset of this transgressive interval seems to be older. The LJ3-1 transgressive peak seems to have an age corresponding to the Upper Pliensbachian Margaritatus Biochron in the Basque-Cantabrian Basin, in the Lusitanian Basin and in the Tethyan domain, representing another key interval for correlation purposes.
[FIGURE 6 OMITTED]
Facies analysis and sequence stratigraphy of the Early Jurassic shallow platform to peritidal carbonates of the Cuevas Labradas Fm studied in the Barranco de la Hoz section allowed the differentiation of 4 units (A to D) which constitute well differentiated facies belts along the southtern part of the Iberian Range.
Unit A, deposited in a restricted platform, is organized in aggradational muddy thickening- and shallowing-upward sequences. Unit B is composed of shallowing- and deepening-upward sequences deposited in an external restricted platform environment. This unit contains Polymorphites sp., Uptonia cf. jamesoni (SOW.) and Uptonia cf. angusta (QUENST.) that characterize the upper part of the Jamesoni Zone of the Lower Pliensbachian. Shells of ammonoids arrived to the studied area by drift from more open-marine parts of the platform, coinciding with the overall deepening conditions. Unit C is composed of shallowing-upward sequences of "muddy" type in the lower part, deposited in the proximal part of the shallow restricted external platform, frequently affected by the storms and "grainy" type in the upper portion, representing wave-dominated bioclastic shoals, located at the shallow portions of the platform. Unit D is constituted by peritidal shallowing-upward sequences representing depositional environments ranging from shallow restricted subtidal environments of internal platform (lagoon) to intertidal and supratidal environments.
The 5th order cycles have been recognized and grouped into seven 4th order cycles, which constitute the 3rd order Cycle LJ2-2. The transgressive peak or maximum deepening is located in the ammonite-bearing marly facies of unit B. The duration of the 3rd order Cycle LJ2-2 is estimated as 2.6 Myr, and the possible average duration of the 4th order cycles as 0.4Myr.
Comparison of the obtained results with other Lower Jurassic sections in Iberia, Europe and the global cycles shows that the boundary between cycles LJ-1 and LJ-2, which represents the onset of the first Jurassic transgression seems to be nearly synchronous, as recorded in Northern Spain, in the Boreal realm, in the UK and in the global cycles. The boundary between cycles LJ2-1 and LJ2-2 looks to be nearly synchronous in Northern Spain, in western Portugal and in the UK.
[FIGURE 7 OMITTED]
The peak transgression identified in this work at the Lower Pliensbachian Jamesoni Zone has been recorded at a similar age at the Boreal domain and at the UK. The boundary between cycles LJ-2 and LJ-3 seems to be good criterion for correlation between the different palaeogeographycal domains, as similar ages have been assigned in Central and Northern Spain, in the Boreal Domain and in the global cycles. The uppermost correlation level here considered is the peak transgression of Cycle LJ3-1, which has been observed with a similar age in Central and Northern Spain, in western Portugal and in the Tethyan domain.
We gratefully acknowledge the valuable comments and constructive reviews made by Dr. S. Fernandez-Lopez and Dr. L.V. Duarte who remarkably contributed to the improvement of the text and figures. This work has been supported by the Spanish Ministerio de Educacion y Ciencia, research project CGL2005-01765/BTE and is a contribution to the Comunidad de Madrid-UCM research groups Mesozoic Biotic Processes (910431) and Sedimentary Basin Analysis (910429).
Received: 21/07/08 / Accepted: 17/11/08
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J. E. Cortes (1), J. J. Gomez (1), A. Goy (2)
(1) Departamento de Estratigrafia, Facultad de Ciencias Geologicas (UCM) and Instituto de Geologia Economica (CSIC-UCM). 28040Madrid. Spain, firstname.lastname@example.org
(2) Departamento de Paleontologia, Facultad de Ciencias Geologicas (UCM) and Instituto de Geologia Economica (CSIC-UCM). 28040Madrid. Spain, email@example.com
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|Author:||Cortes, J.E.; Gomez, J.J.; Goy, A.|
|Publication:||Journal of Iberian Geology|
|Date:||Jan 1, 2009|
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