Prebetic Zone Research Articles

Magnetotelluric characterization of a salt diapir: a case study on Bicorb–Quesa Diapir (Prebetic Zone, SE Spain)

Abstract: The availability of geophysical data to determine the geometry at depth of a salt diapir and its surrounding structure is essential because of the complexity and changing geometry of these structures. For this purpose, potential field methods are commonly the first steps in subsurface exploration whereas seismic techniques are applied in later stages. However, the magnetotelluric method (MT) may fill the gap between the precision and the time that these two methods entail. To prove the usefulness of the MT method, it has been applied to the Bicorb–Quesa Diapir. This diapir is composed of evaporitic rocks piercing an overburden of carbonate rocks, providing a wide and contrasting range of electrical resistivity values. In this situation, the MT method may provide a coherent and realistic image of the subsurface. The application of the MT method has allowed us to (1) recognize a basement fault with a vertical throw of 1000 m, (2) elucidate the shape of the diapir, which is asymmetrical, with a north wall vertical and south wall dipping southwards, and (3) determine that all of the Late Triassic salt is concentrated in the diapir core. Supplementary material: Original curves and the model adjustments are available at http://www.geolsoc.org.uk/SUP18386 .

Estimation of the tectonic slip-rate from Quaternary lacustrine facies within the intraplate Albacete province (SE of Spain)

The Quaternary lacustrine basin of Cordovilla (CB) represents one of the most active tectonic areas of the Prebetic Zone (Albacete, SE of Spain). The Quaternary sedimentary deposits of this basin are mainly endoreic lacustrine carbonate and alluvial deposits, developed in a semi-arid climate (Pleistocene-present). The basin is a NW–SE-elongated graben bounded by a major right-lateral oblique-fault, the Pozohondo Fault. This fault trends NW–SE, with an approximate trace of 55 km, and is composed of various segments which are identified by fault scarps. In order to establish the slip-rate of the most active segment of the Pozohondo Fault, called the Cordovilla segment, we carried out a detailed study of the affected Quaternary lacustrine deposits. We found that the lacustrine facies could be related to episodic moderate paleoearthquakes. The slip-rate is calculated to be 0.05 and 0.09 mm/yr, using radiometric dating for the vertical offsets of the lacustrine facies. A trenching study at the northern part of the Cordovilla segment revealed two events caused by paleoearthquakes, with the most recent expressed as an oblique-fault off-setting a poorly-developed soil. The magnitude of the last event was greater than 6, using various empirical relationships for the fault displacement and the surface-length rupture. We estimate episodic activity across the Cordovilla segment, to be characterized by moderate-sized paleoearthquakes (M6), which is in agreement with the tectonic context of an intraplate zone of the Iberian plate.

History and evolution of the North-Betic Strait (Prebetic Zone, Betic Cordillera): A narrow, early Tortonian, tidal-dominated, Atlantic–Mediterranean marine passage

The North-Betic Strait, located in the Prebetic Zone (outermost part of the Betic Cordillera), was a narrow seaway connecting the Atlantic Ocean and the Mediterranean Sea through the Betic Cordillera during the Early Late Miocene. Strait facies consist of heterozoan bioclastic carbonates and mixed siliciclastics–carbonates. Local fan-delta deposits (conglomerates) also occur. The North-Betic Strait deposits exhibit huge trough cross-bedded structures. Single troughs are up to 5 m high and point to the east in easternmost outcrops. In central outcrops, they reach up to 15 m in height and some tens of metres in length and point both to the east and to the west. The westernmost outcrops show the largest cross-bedded structures, up to 20 m in height and some hundred of metres in length, all pointing to the west. The large-scale cross-bedding was generated by the migration of very large dunes moved by tides. The largest structures are inferred to have formed at depths of 90 m. Depth estimations indicate that the North-Betic Strait shallowed to the east and deepened to the west. The Miocene evolution towards the formation of the North-Betic Strait can be divided into a series of steps. The palaeogeography evolved from a south-facing platform, marginal to a northeastern relief, to a wide-open marine passage limited by a southern platform and, finally, to a tidal-dominated strait. The closing of the North-Betic Strait is recorded by the presence of lagoonal, silty sediments covered by a stromatolite layer and crowned by a red soil in the westernmost outcrops.

Development of carbonate platforms on an extensional (rifted) margin: the Valanginian–Albian record of the Prebetic of Alicante (SE Spain)

Within the upper Valanginian to upper Albian deposits of the easternmost part of the Prebetic Zone of the Betic Cordillera (Iberian Peninsula), seven lithostratigraphic formations made up of shallow-water carbonate and carbonate-siliciclastic sediments and of outer-platform hemipelagic sediments have been recognized. These formations were deposited in the most distal part of a platform that developed on the Southern Iberian Continental Palaeomargin. The geodynamic context was a margin affected by extensional or transtensional faults that produced tilted blocks. The interval studied records three major second-order transgressive-regressive facies cycles: (I) A late Valanginian to earliest Aptian cycle, mostly represented by hemipelagic and condensed sedimentation, with the development of a tectonically controlled high without sedimentation that separated two sectors with different sedimentary evolution and that ended with an episode of shallow-water carbonate platform development; (II) An earliest to latest Aptian cycle, with a transgressive phase represented by a retrogradational shallow-water carbonate platform capped by a drowning event leading to hemipelagic sedimentation, which was affected by an anoxic event (OAE 1a); the regressive phase is represented by progradation and aggradation of shallow-water carbonate deposits. Finally (III) a latest Aptian to early-late Albian cycle that records the expansion of mixed platform deposits in the entire area, ending with a phase of shallow-water carbonate platform development. Extensional tectonics leading to spatial and temporal changes in subsidence patterns is envisaged as the main control on sedimentation at a local scale, resulting in notable lateral changes in thickness as the main signature. Tectonics exerted a strong control on the distribution of sedimentary environments only during Cycle I. At a higher order, sea-level fluctuations are responsible for sequential organization, and environmental factors determined shallow-water carbonate platform development and demise, as well as oceanic anoxic events. The relevant continuity of the stratigraphic record in the distal part of the Prebetic platform has led to the recognition of events related to cycle boundaries, which result mainly from a combination of tectonics and sea-level changes.

Stratigraphy of Palaeocene phosphate pelagic stromatolites (Prebetic Zone, SE Spain)

The hemipelagic domain of the ancient southern continental margin of Iberia is home to a strongly condensed pelagic succession (6–15 cm thick) characterized by the presence of phosphate stromatolites. This succession, probably generated in the slope of the continental margin, records a period of some 9 Ma, corresponding to the latest Maastrichtian to Late Thanetian interval. A microstratigraphical analysis allows for characterizing and biostratigraphically dating six successive developmental stages in the succession, which outline the main environmental evolution of the depositional setting. The first of them determined the generation of a submarine hardground during the latest Maastrichtian to earliest Danian interval. The other five are represented by five successive microstratigraphical, unconformity-bounded, genetic units, respectively Early–Middle Danian, Late Danian–Early Selandian, intra-Selandian, Late Selandian–Early Thanetian, and Middle–?Late Thanetian in age. The three oldest units are characterized by the accretion of phosphate stromatolites, favoured by very low rates of pelagic sedimentation and by a microbially mediated extra input of phosphate. The two youngest units are dominated by carbonate deposition, which has always taken place at very low rates. Condensed sedimentation was abruptly interrupted at the end of the Palaeocene (?latest Thanetian), when the condensed succession and its hosting substrate were gravitationally slumped and re-deposited at the base of the slope in the form of a mega-debris flow that can be now observed in Sierra de Aixorta (Alicante, SE Spain). The Aixorta pelagic phosphatic stromatolites are among the youngest ever described, and their existence suggests that the oceanographic conditions necessary for their development prevailed during most of the Palaeocene, but disappeared during the Late Selandian, never to return.

Benthic foraminiferal morphogroups of mid to outer shelf environments of the Late Jurassic (Prebetic Zone, southern Spain): Characterization of biofacies and environmental significance

Benthic foraminiferal assemblages belonging to a mixed carbonate–siliciclastic shelf succession of the Oxfordian (Upper Jurassic) in the Prebetic, southern Spain, were analyzed. The faunal data, obtained on thin sections of strongly lithified sediments, allowed the detailed differentiation of a foraminiferal morphogroups system to be applied to the interpretation of paleoenvironmental conditions. Eleven morphogroups were differentiated according to shell composition, general morphology, number of chambers and mode of coiling. Paleoenvironmental analysis combines these features with inferred life-style (epifaunal, shallow infaunal and deep infaunal) and feeding strategy (suspension-feeder, deposit-feeder, herbivores, etc.) of the foraminifera. Analogies and differences regarding previously established morphogroup systems could be outlined accordingly. In the Prebetic Oxfordian, distribution trends of foraminiferal morphogroups allow for a comparison of mid to outer shelf environments occupying the South-Iberian paleomargin, indicating that nutrient supply was the most important factor controlling morphogroup development. The mid shelf settings are dominated by epifaunal active herbivores–phytodetritivores, in shallow to deep substrate position of the redox boundary. The outer shelf is dominated by agglutinated and infaunal detritivore-bacterial scavenger morphogroups revealing a deep substrate position of the redox boundary. Particularly favorable trophic conditions in mid shelf environments are related with higher phytodetritus input from more proximal and shallower shelf areas.

Taphonomy of fossil macro-invertebrate assemblages as a tool for ecostratigraphic interpretation in Upper Jurassic shelf deposits (Prebetic Zone, southern Spain)

Composition and taphonomy of macro-invertebrate fossil assemblages, together with facies analysis, have been approached in order to interpret shifting paleoenvironmental conditions in the External Prebetic (S-SE Spain) during the early Late Jurassic (Middle Oxfordian). In oolitic and spongiolitic limestones, the size of fossil remains, mode of preservation, within-bed position, corrasion, fragmentation, epibiont and biogenic encrustation, disarticulation and uncoupling, allow recognition of two taphofacies, respectively. Identified ecostratigraphic events and trends accord with rapid flooding under high-energy conditions related to ecospace enlargement for cephalopods and then the persistence of lower energy, long-lasting exposure of skeletals and higher sedimentary rates. The paleoenvironmental interpretation is consistent with neritic environments shifting from shallow carbonate to hemipelagic sedimentation and enlarging of shelf ecospace for marine invertebrates.

Palaeoenvironmental implications of ferruginous deposits related to a Middle–Upper Jurassic discontinuity (Prebetic Zone, Betic Cordillera, Southern Spain)

The Middle–Upper Jurassic boundary in the westernmost Tethyan basins is marked by a discontinuity. A thin iron crust with ferruginous ooids and pisoids and an overlying ferruginous oolitic limestone lithofacies occur in a genetic relationship to this discontinuity with a reduced thickness (< 50 cm) and very local distribution in the Prebetic Zone (Betic Cordillera). The ferruginous coated grains are subdivided into two types. Type A ooids are characterised by thin, regular lamination in concentric layers enclosing a nucleus; they are dominant in the top of the iron crust (100% of the ferruginous ooids) and in the ferruginous oolitic limestone (82%). Type B ooids typically have thick, irregular lamination in a few discontinuous concentric layers enclosing a variable nucleus including bioclasts and foraminifera; they are exclusive to the ferruginous oolitic limestone (18% of the ferruginous ooids). The bulk chemical composition varies between 80% Fe 2O 3 by weight in the iron crust and 67% by weight in the coated grains. In the ferruginous ooids, the contents in SiO 2 (5.4%), Al 2O 3 (6.5%), P 2O 5 (3.6%), and CaO (4.7%) are higher than in the crust. Trace elements (V, Cr, Co, Ni, Zn, Y, Mo, and Pb) in both the crust and ooids show enriched values compared with the bulk composition of the upper continental crust. The mineral composition of the iron crust and ooids is primarily goethite, with small amounts of Al-hydroxide (bohemite) and apatite, whereas hematite is identified only in the iron crust. The Type A ooids are interpreted as having an origin related to the iron crust. Since there is no evidence to support a marine genesis for the iron crust, the possibility of a subaerial origin is presented here. The crust has characteristics (chemical and mineralogical composition) similar to those of ferruginous pisolitic plinthite (highly weathered redoximorphic soil), and goethite shows an Al-substitution range (5–10 mol%) that indicates pedogenic conditions. Soil processes under periodic hydrous conditions are suggested; groundwater soils with hydrous conditions are congruent with the formation of the Type A ferruginous ooids and pisoids. In this situation, a coastal plain with periodically flooded soils would be the likeliest scenario. Callovian shallow carbonate shelf was possibly emerged and weathered, followed by marine sedimentation during the Middle Oxfordian, associated with major flooding of the Prebetic shelf and the erosion of ferruginous pisolitic plinthite. The first marine deposit was ferruginous oolitic limestones. Fragments of iron crust and Type A ferruginous ooids were reworked and incorporated into the marine sediments. A second phase of ferruginous ooids (Type B) with clear marine features developed, benefiting from iron-rich microenvironments due to the redistribution from iron crust fragments and Type A ferruginous ooids.

K‐feldspar alteration to gel material and crystallization of glauconitic peloids with berthierine in Cretaceous marine sediments—sedimentary implications (Prebetic Zone, Betic Cordillera, SE Spain)

AbstractGlauconitic peloids from a Hauterivian condensed level in a hemipelagic unit of the Internal Prebetic (Los Villares Formation, eastern Betic Cordillera) have been studied by X‐ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) and high‐resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM). The sediments forming the condensed level are characterized by abundant spherical to ovoid green glauconite peloids with radial cracks. Quartz, feldspar and muscovite are also abundant, whereas calcium phosphate is rarely detected. XRD analysis of the peloids reveals glauconite and small amounts of berthierine. SEM and HRTEM data show feldspar dissolution features, a Si–Al‐rich gel‐like substance filling K‐feldspar micropores and interlayering of well‐crystallized glauconite and berthierine packets. The last stage of the glauconitization process resulted in conversion of the smectitic precursor. Sedimentary and mineralogical features indicate an autochthonous origin for the glauconite. The depositional environment was a distal, hemipelagic ramp on the Southern Iberian Continental Palaeomargin. Low sedimentation rates lead to sediment condensation in a general transgressive context. The margin was affected by extensional tectonics, creating tilted blocks, resulting in lateral facies changes. The dissolution of K‐feldspars probably occurred after their deposition in the marine environment but predating the glauconitization. An influx of meteoric water is therefore required, probably related to subsurface fluxes from adjacent emergent areas (the higher parts of tilted blocks). Copyright © 2007 John Wiley &amp; Sons, Ltd.

Recovery of marine primary producers after the Cretaceous–Tertiary mass extinction: Paleocene calcareous red algae from the Iberian Peninsula

Calcareous red algae (corallines, ‘solenoporaceans’ and peyssonneliaceans) are major contributors to Paleocene platform deposits in the Pyrenean Basin (N Spain) and Prebetic Zone (Betic Cordillera, SE Spain). The earliest Danian–middle Thanetian time interval in the Pyrenean Basin is represented by four depositional sequences, the Ma-Da (earliest Danian), Da-1 (early Danian), Da-2 (late Danian), and Th-1 (Selandian–mid Thanetian) sequences, from bottom to top. This precise chronostratigraphic framework allows an accurate assessment of the recovery of these calcareous algal groups after the Cretaceous–Tertiary (K/T) mass extinction. Calcareous algae are recorded shortly after the K/T boundary (∼ 0.2–0.3 Ma after the K/T), although the richness of identifiable species remains low during the Danian: 9 species in the earliest Danian, 9 species in the early Danian, and 11 species in the late Danian. In addition, no change in taxonomic composition is observed in this interval since approximately the same algal species occur in all three Danian sequences. The same low species richness (11 species) and species composition is observed in the Danian algal assemblages of the Betic Cordillera. However, this low level of diversification changes radically in the Selandian–mid Thanetian interval, when 29 species are recorded. Together with the increase in species richness, the taxonomic composition also changes, since 19 species of corallines are recorded for the first time in the Th-1 sequence. Therefore, it can be deduced that species diversity recovery due to a significant radiation of corallines was delayed ∼ 6 Ma after the end-Cretaceous event. The recovery of corallines during the Paleocene shows a characteristic depth-dependent pattern. Throughout the Danian–middle Thanetian interval, representatives of the family Hapalidiaceae subfamily Melobesioideae and plants of the family Sporolithaceae largely dominate the algal assemblages in the deep-water, outer-platform settings, as they do in present-day seas. Members of these two groups are also the most abundant coralline algae in the Paleocene reef and back-reef paleoenvironments, indicating that the shallowest and best-illuminated areas were occupied by algal groups that in modern tropical seas mainly occur in relatively deep habitats. This suggests that all platform habitats were re-occupied after the end-Cretaceous event by the groups that selectively survived the extinction. Only after a time interval of about 6 Ma did the habitat distribution of coralline groups change to its modern patterns. The subfamilies most abundant in present-day shallow tropical waters, that is, Mastophoroideae and Lithophylloideae, for the first time account for more than 1% of the algal assemblages in the Thanetian and even then they remain very minor components. These shallow-water coralline groups were affected more intensively by the K/T extinction event, accounting for the delay in their recovery in their preferred shallow-water habitats. Shallow-water lithophylloids and geniculate corallines have not been found in Danian sequences.

Event stratigraphy of the upper Cretaceous to lower Eocene hemipelagic sequences of the Prebetic Zone (SE Spain): Record of the onset of tectonic convergence in a passive continental margin

In the Eastern Prebetic Zone of SE Spain, upper Cretaceous to lower Eocene hemipelagic sequences deposited in the ancient southern passive continental margin of Iberia are well exposed. The long-term stratigraphy of these sequences is punctuated by six regional tectonic events, which induced rapid changes in palaeogeography and regional tectonics. Each event configured a new genetic scenario for sedimentation, which lasted until the next tectonic reorganization and, in turn, controlled the deposition of an event bounded stratigraphic unit (EBSU). The ages of these events, determined chronobiostratigraphically, are: intra-Coniacian, late Santonian, “mid” Maastrichtian, latest Maastrichtian–earliest Danian, late Thanetian and intra-Ypresian. All the events, but the first, are interpreted here as the result of contractional tectonic pulses, related to changes in intraplate stresses during the onset of African–European convergence. Through correlations with both adjacent shallow marine carbonates (Prebetic platform) and basin pelagites (Subbetic), along with other basins of Iberia and North Africa, the long-term evolution of the continental margin is integrated within the geodynamic framework of the western Tethys.

Approaching trophic structure in Late Jurassic neritic shelves: A western Tethys example from southern Iberia

The palaeoenvironmental conditions and trophic structure of a mid-outer neritic biota (microfossils, mainly forams, and macroinvertebrate assemblages) have been approached in middle Oxfordian–lowermost Kimmeridgian deposits from the Prebetic Zone (Betic Cordillera) in south-eastern Spain. According to relationships between fossil assemblages and lithofacies, a general seaward trend is identified which displays decreasing sedimentation rates and nutrient inputs, but increasing substrate consistency and presumably depth. Midshelf, terrigenous-rich deposits in the External Prebetic relate to the highest sedimentation rates and nutrient availability. These two parameters correlate with the highest content in vagile-benthic, calcareous perforate, epifaunal forams, as well as with potentially deep infaunal forams and infaunal macroinvertebrates. Outer-shelf lumpy deposits in the Internal Prebetic show the lowest sedimentation rates and nutrient availability and the highest records for macro–micro nektonics and planktics. In contrast, vagile-benthic, calcareous perforate epifaunal and potentially deep infaunal forams are scarcer in the midshelf environments. Colonial encrusting forams, benthic microbial communities and sessile benthic macro-invertebrates increase from the middle to outer shelf. Trophic-analysis structuring through the integration of benthic microbial communities, foraminiferal and macroinvertebrate fossil assemblages makes it possible to interpret: (a) a trophic-level frame composed of producers and primary and secondary consumers; (b) a main trophic-group differentiation in suspension-feeders, detritus-feeders, browsers, grazers, carnivores and scavengers; (c) a preliminary approach to food-chain structure supported by suspension-feeders, deposit-feeders and predators (active prey-selection carnivores); and (d) a food-pyramid model, which takes into account both recorded fossils and envisaged —i.e., ecologically inferred-organisms.

Timing of canyon-fed turbidite deposition in a rifted basin: The Early Cretaceous turbidite complex of the Cerrajón Formation (Subbetic, Southern Spain)

The Subbetic Cerrajón Formation of latest Hauterivian–earliest Late Albian age is made up of turbidite sandstones and calcarenites intercalated in marly-limestones and marls. It can be defined as a turbidite complex with three distinct intervals where turbidites are recorded: latest Hauterivian–Barremian p.p., Late Aptian p.p. and Middle to middle-late Albian age. The distribution of four biochronostratigraphic units of these ages throughout the studied outcrops is shown. The main hiatuses, latest Barremian–early Late Aptian and latest Aptian–early Late Albian, mainly the latter, have been identified in many other outcrops of the Southern Tethys, and are related to tectonics and associated palaeogeographic and palaeoceanographic changes. Platform environments, as the source area of the clastics, the feeding submarine canyon, base-of-slope channels and related basinal lobes or sheet systems, have been identified and can be traced laterally along more than 80 km of discontinuous outcrops aligned in a downcurrent direction. The system could have extended more than 150 km in length. The mean palaeocurrent direction fits well with the present alignment of the outcrops and, after corrections for tectonics rotation, with the current palaeogeographic model of the Southern Iberian Continental Palaeomargin (SICP). This conclusion largely supports previous interpretations of a longitudinal pattern for the distribution of clastics during the Cretaceous in that part of the SICP. The ages of the main Cretaceous events deduced from the study of the Cerrajón Formation correlate quite well with the sequence sets and subsidence history of the Prebetic Zone, where the Cretaceous platform environments adjacent to the turbidite basin are recorded. Moreover, the main unconformities dated, such as the onset and the end of turbidite sedimentation, show good correlation with the sequence boundaries described for the European basins. The analysis in time and space of the turbidite distribution and its correlation with the Prebetic carbonate platforms enables us to outline some interesting conclusion that might be a suitable subject for future research.

Microbial encrustations from the Middle Oxfordian-earliest Kimmeridgian lithofacies in the Prebetic Zone (Betic Cordillera, southern Spain): characterization, distribution and controlling factors

Two main types of microbial encrustation were identified in Middle Oxfordian to lowermost Kimmeridgian deposits in the Prebetic Zone (southern Spain), showing existing relationships between skeletal content, fabric and morphology of these organosedimentary structures. Laminated planar and concentric encrustations relate to peloidal fabrics (mainly constituted of microbes = microbial laminated fabrics s. str. and microbial oncoids s. str.), as well as to dense microbial fabrics periodically colonized by encrusting foraminifera (microbial laminated fabrics with nubeculariids and microbial oncoids with nubeculariids). Sedimentation rates, substrate stability and grain size, as well as illumination, influenced microbial growth pattern as major controlling factors in low-energy conditions, and forced palaeogeographic and stratigraphic patterns of distribution. Significant encrustation was identified in terrigenous-poor lithofacies from the middle (Transversarium-Bifurcatus zones) to the outer (Transversarium-Bimammatum zones) shelf in the Prebetic Zone. Rare-to-absent encrustation characterized terrigenous-rich deposits (Bimammatum and Planula zones) in the area.

Evaluation of methods for retrieving foraminifera from indurated carbonates: application to the Jurassic spongiolithic limestone lithofacies of the Prebetic Zone (South Spain)

Two methods for retrieving foraminifera from strongly lithified carbonates (Amine-O and cold-disaggregation with acetic acid) were tested on the same sample of Oxfordian spongiolithic limestone (Prebetic Zone, SE Spain) and compared with thin section analysis. Differences between the methods concern: 1) weight of sieved residues after disaggregation; 2) foraminifera/gram ratio; 3) preservation features of the tests; 4) foraminiferal assemblage compositions. The results obtained allow us to conclude that accurate paleoecological and taxonomical analysis of indurated carbonates requires the combined use of thin sections and disaggegration treatment of the samples.

Microboring and taphonomy in Middle Oxfordian to lowermost Kimmeridgian (Upper Jurassic) from the Prebetic Zone (southern Iberia)

Microboring was investigated through thin section analysis of Oxfordian rocks from the Prebetic Zone (Betic Cordillera, southern Spain). Microboring description and distribution were analysed, as was their correlation with traits such as the type and size of bioclasts and microtaphonomic features (encrustation and fragmentation). Single and branching microboring are the two major morphological groups differentiated. Microborers were not identified but by their patterns of distribution in the bioclasts and in the environmental setting are assumed to be phototrophic. Microboring only affected bioclasts, with higher values of microboring index ( MB i) registered in thicker ammonoid shells and serpulids, and lower ones in ostracods, foraminifera and small bivalves, gastropods and echinoderms. A very close, direct relationship exists between bioclast size and microboring abundance, as well as between the microboring ( MB i) and encrustation indexes ( E i) used. In contrast, a clear relationship between MB i and the fragmentation index ( F i) has not been proved. Microboring activity has been interpreted to be related to both bioclast-size and time exposure of bioclasts on the seafloor, the latter resulting directly proportional to the rate of sedimentation in low energy environments. The studied features ( MB i, F i, E i and bioclast mean size) were analysed taking into account the stratigraphic intervals investigated (I=Transversarium Zone to lower Bifurcatus Zone, II=upper Bifurcatus Zone, III=Bimammatum Zone, and IV=Planula Zone p. p.), and the trends registered were interpreted as a final consequence of relative sea-level fluctuations. Stratigraphic intervals I and IV, characterised by higher values in MB i and E i, were related to lower sedimentary rates during increasing distality in a context of relative sea-level rise, which is consistent with the development of Transgressive System Tract (TST) conditions. Stratigraphic intervals II and III, showing lower values in MB i and E i, are related to higher sedimentation rates and decreasing distance from shore during relative sea-level falls, which are in accordance with developing Highstand System Tract (HST) and Shelf Margin Wedge (SMW) conditions. Data obtained and interpretations agree with the general ecostratigraphic context in the studied area previously proposed by the authors.

Microboring and taphonomy in Middle Oxfordian to lowermost Kimmeridgian (Upper Jurassic) from the Prebetic Zone (southern Iberia)

Abstract Microboring was investigated through thin section analysis of Oxfordian rocks from the Prebetic Zone (Betic Cordillera, southern Spain). Microboring description and distribution were analysed, as was their correlation with traits such as the type and size of bioclasts and microtaphonomic features (encrustation and fragmentation). Single and branching microboring are the two major morphological groups differentiated. Microborers were not identified but by their patterns of distribution in the bioclasts and in the environmental setting are assumed to be phototrophic. Microboring only affected bioclasts, with higher values of microboring index (MBi) registered in thicker ammonoid shells and serpulids, and lower ones in ostracods, foraminifera and small bivalves, gastropods and echinoderms. A very close, direct relationship exists between bioclast size and microboring abundance, as well as between the microboring (MBi) and encrustation indexes (Ei) used. In contrast, a clear relationship between MBi and the fragmentation index (Fi) has not been proved. Microboring activity has been interpreted to be related to both bioclast-size and time exposure of bioclasts on the seafloor, the latter resulting directly proportional to the rate of sedimentation in low energy environments. The studied features (MBi, Fi, Ei and bioclast mean size) were analysed taking into account the stratigraphic intervals investigated (I=Transversarium Zone to lower Bifurcatus Zone, II=upper Bifurcatus Zone, III=Bimammatum Zone, and IV=Planula Zone p.p.), and the trends registered were interpreted as a final consequence of relative sea-level fluctuations. Stratigraphic intervals I and IV, characterised by higher values in MBi and Ei, were related to lower sedimentary rates during increasing distality in a context of relative sea-level rise, which is consistent with the development of Transgressive System Tract (TST) conditions. Stratigraphic intervals II and III, showing lower values in MBi and Ei, are related to higher sedimentation rates and decreasing distance from shore during relative sea-level falls, which are in accordance with developing Highstand System Tract (HST) and Shelf Margin Wedge (SMW) conditions. Data obtained and interpretations agree with the general ecostratigraphic context in the studied area previously proposed by the authors.

Latest Santonian to latest Maastrichtian planktic foraminifera and biostratigraphy of the hemipelagic successions of the Prebetic Zone (Murcia and Alicante provinces, south-east Spain)

This paper presents the first detailed biostratigraphic analysis of the uppermost Santonian through uppermost Maastrichtian hemipelagic carbonate successions of south-east Spain based mainly on the stratigraphic distributions of planktic foraminifera. For the time interval studied, seven biozones of planktic foraminifera have been recognized. From oldest to youngest these are: the uppermost part of the Dicarinella asymetrica Zone, and the Globotruncanita elevata, Globotruncana ventricosa, Globotruncanita calcarata, Globotruncana falsostuarti, Gansserina gansseri and Abathomphalus mayaroensis zones. The biostratigraphic succession obtained from the sections studied in the Prebetic area is compared with those from sections in Tercis (France) and Kalaat Senan (Tunisia).

Evaluation of methods for retrieving foraminifera from indurated carbonates: application to the Jurassic spongiolithic limestone lithofacies of the Prebetic Zone (South Spain)

Two methods for retrieving foraminifera from strongly lithified carbonates (Amine-O and cold-disaggregation with acetic acid) were tested on the same sample of Oxfordian spongiolithic limestone (Prebetic Zone, SE Spain) and compared with thin section analysis. Differences between the methods concern: 1) weight of sieved residues after disaggregation; 2) foraminifera/gram ratio; 3) preservation features of the tests; 4) foraminiferal assemblage compositions. The results obtained allow us to conclude that accurate paleoecological and taxonomical analysis of indurated carbonates requires the combined use of thin sections and disaggegration treatment of the samples.

A Late Jurassic Carbonate Ramp Colonized by Sponges and Benthic Microbial Communities (External Prebetic, Southern Spain)

Abstract During the Middle to Late Oxfordian (locally ranging into the earliest Kimmeridgian), the epicontinental shelf that persisted in the southeastern paleomargin of Iberia (Prebetic Zone) was largely colonized by siliceous sponges (Dictyida, Lychniskida, and Lithistida, in descending order of abundance). Spongiolithic lithofacies (sponge-rich deposits in which buildups are rare and small) represent a marine environment extensively colonized by sponges in the form of large sponge meadows showing associated epibenthic organisms (brachiopods, bivalves, and echinoderms). Marl-limestone rhythmites represent muddy bottoms colonized by scarce endobenthos (bivalves and irregular echinoids). However, the occurrence of patchy sponge bioherms provided a favorable substrate for epibionts and microbial lithoherms (microbialites). The record of meter-scale buildups showing sponge bioherm-microbial lithoherm in marl-limestone rhythmites resulted from the preferred colonization of sponge patches by benthic microbial...