Matías Reolid

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 communities, the growth of which on surrounding muddy bottoms was very limited or impossible. Sedimentation rate, substrate, bathymetry, light, nutrients, oxygenation, and environmental (water) energy determined the eco-sedimentary conditions responsible for the occurrence and composition of spongiolithic facies in the area studied. Throughout the Late Oxfordian and the earliest Kimmeridgian, the progressive increase in the rate of sedimentation, especially influx of siliciclastics, forced the disappearance of sponge bioherms-microbial lithoherms from the Prebetic shelf.

Foraminiferal Assemblages as Palaeoenvironmental Bioindicators in Late Jurassic Epicontinental Platforms: Relation with Trophic Conditions

Foraminiferal assemblages from the neritic environment reveal the palaeoecological impact of nutrient types in relation to shore distance and sedimentary setting. Comparatively proximal siliciclastic settings from the Boreal Domain (Brora section, Eastern Scotland) were dominated by inner-shelf primary production in the water column or in sea bottom, while in relatively seawards mixed carbonate-siliciclastic settings from the Western Tethys (Prebetic, Southern Spain), nutrients mainly derived from the inner-shelf source. In both settings, benthic foraminiferal assemblages increased in diversity and proportion of epifauna from eutrophic to oligotrophic conditions. The proximal setting example (Brora Brick Clay Mb.) corresponds to Callovian offshore shelf deposits with a high primary productivity, bottom accumulation of organic matter, and a reduced sedimentation rate for siliciclastics. Eutrophic conditions favoured some infaunal foraminifera. Lately, inner shelf to shoreface transition areas (Fascally Siltstone Mb.), show higher sedimentation rates and turbidity, reducing euphotic-zone range depths and primary production, and then deposits with a lower organic matter content (high-mesotrophic conditions). This determined less agglutinated infaunal foraminifera content and increasing calcitic and aragonitic epifauna, and calcitic opportunists (i.e., Lenticulina). The comparatively distal setting of the Oxfordian example (Prebetic) corresponds to: (i) outer-shelf areas with lower nutrient input (relative oligotrophy) and organic matter accumulation on comparatively firmer substrates (lumpy lithofacies group) showing dominance of calcitic epifaunal foraminifera, and (ii) mid-shelf areas with a higher sedimentation rate and nutrient influx (low-mesotrophic conditions) favouring potentially deep infaunal foraminifers in comparatively unconsolidated and nutrient-rich substrates controlled by instable redox boundary (marl-limestone rhythmite lithofacies).

Fossil assemblages, lithofacies, taphofacies and interpreting depositional dynamics in the epicontinental Oxfordian of the Prebetic Zone, Betic Cordillera, southern Spain

Abstract The study of six sections within the External Prebetic (Pozo Canada, Fuente Alamo, Chinchilla de Montearagon and Riogazas-Chorro II) and the Internal Prebetic (Navalperal and Rio Segura), comprises the first results of a taphonomic study to be incorporated into the ecostratigraphic interpretation of the Oxfordian in the Prebetic Zone, Betic Cordillera, southern Spain. A taphonomic analysis was made of 9350 specimens (including fragments) of macroinvertebrate fossils: ammonoids (64%), benthic organisms (26%), and belemnoids and nautiloids (10%). The taphonomic features analysed were: (a) state of preservation (ratio of moulds with/without shell and type of mould filling); (b) fossil size; (c) position within the bed; (d) corrasion (see [Brett, C.E., Baird, G.C., Palaios 1 (1986) 207–227]; (e) fragmentation; (f) epibionts and encrustment; (g) disarticulation; (h) uncoupling and (i) deformation. Analysis of the relative significance of these features, and their relation to the lithofacies, allows us to characterise and distinguish three major taphofacies: (a) taphofacies I, characterised by the greater average size of the specimens recorded, higher values of corrasion, encrustation and colonisation, and by the fact that a high proportion of the specimens are in a quasi-horizontal position; (b) taphofacies II, characterised by a higher proportion of smaller specimens and a greater variety of azimuthal orientation, although the quasi-horizontal position is always predominant and corrasion values are low; (c) taphofacies III, with predominantly small specimens, the highest proportion of quasi-horizontal orientation and fragmentation, and no evidence of corrasion. The taphofacies are stratigraphically and palaeogeographically distributed. In interpreting the taphofacies, the exposure time of skeletals is an essential parameter, and is related to the controlling effect of the sedimentation rate. This latter depends on factors such as the tectonic–eustatic dynamics, the relative distance from the shore, and the bottom physiography.

Foraminiferal morphogroups in dysoxic shelf deposits from the Jurassic of Spitsbergen

Analysis of benthic foraminiferal assemblages was performed in Bathonian to Kimmeridgian deposits through a section covering the lower half of the Agardhfjellet Formation in central Spitsbergen. The section consists mainly of organic-rich shales, which contain low-diversity agglutinated assemblages. In this foraminiferal succession five morphogroups were differentiated according to shell architecture (general shape, mode of coiling and number of chambers), integrated with the supposed microhabitat (epifaunal, shallow infaunal and deep infaunal) and feeding strategy (suspension-feeder, herbivore, bacterivore, etc.). The environmental evolution of the analysed section is interpreted by using the stratigraphic distribution of morphogroups, combined with species diversities and sedimentary data, in a sequence stratigraphic framework. The section comprises two depositional sequences, which demonstrate that species diversity and relative frequency of morphogroups are correlative with transgressive–regressive trends controlling depth and oxygenation of the water column. In both sequences, the maximum flooding interval is characterized by increased organic carbon content, dominance of the epifaunal morphogroups and reduced species diversity: features reflecting the increased degree of stagnation separating the transgressive phase from the regressive phase.

The Toarcian oceanic anoxic event in the Western Saharan Atlas, Algeria (North African paleomargin): Role of anoxia and productivity

We used an integrated analysis of benthic foraminiferal assemblages, geochemical proxies (paleoproductivity, redox, and detrital proxies), and C and O stable isotopes to characterize the Toarcian oceanic anoxic event along the North African paleomargin, more precisely in the Saharan Atlas (Algeria). Special emphasis was placed on time intervals previous to the anoxic event and the recovery of normal conditions after this event. The Fisher9s alpha diversity of the foraminiferal assemblages decreased with respect to the end of the Pliensbachian in the polymorphum Zone, with lower values of shallow infaunal forms and an increase in opportunists ( Reinholdella , Lenticulina , and Eoguttulina ). This indicates fluctuations in the oxygenation degree of the infaunal microhabitat in the polymorphum Zone despite the fact that redox proxies do not show any fluctuations, indicating that dysoxic conditions were not reached. The lower part of the levisoni Zone reflects an abrupt paleoenvironmental change, evidenced by the extinction of foraminifera, enrichment in redox-sensitive elements, increase in detrital and paleoproductivity proxies, and fluctuations of δ 13 C and δ 18 O. An anoxic or strongly dysoxic event happened by the polymorphum - levisoni zone boundary in the Western Saharan Atlas, coincident with increasing eolian detrital input and paleoproductivity. The upper part of the levisoni Zone represents a return to normal oxic conditions, and the slow recovery of benthic foraminiferal assemblages dominated by opportunistic forms ( Lenticulina , Eoguttulina , and Reinholdella ) of low diversity. The Toarcian oceanic anoxic event occurred in a generalized transgression that may have favored water stratification and confinement of bottom waters in the subbasins developed in the Atlasic domain. This research demonstrates the potential of the integration of data from foraminiferal assemblages and a wide range of geochemical proxies for understanding anoxic events during the Mesozoic.

Jurassic Fe-Mn macro-oncoids from pelagic swells of the External Subbetic (Spain): evidences of microbial origin

Ferromanganesiferous macro-oncoids are distinctive from the External Subbetic Zone (Betic Cordillera, SE Spain) in relation to a major heterochronic unconformity, with a Middle Bathonian-Lower Oxfordian minimum hiatus and a Lowest Bathonian-Lowest Kimmeridgian maximum hiatus. The Fe-Mn macro-oncoids (43 mm mean-size) consist of microbial laminae with planar and arborescent to dendrolitic morphologies. Under petrographic microscopy, the planar morphologies are made up by condensed fibrillar meshworks whereas the dendrolitic ones are similar to Frutexites. Alternation between these two types of laminae reveals a rhythmic growth in the Fe-Mn macro-oncoids. Bacterial and fungal filaments are observed in SEM analyses as microbial mats constituted by a disperse web of filaments exhibiting a branching tube-like morphology with diameters ranging between 2 and 10 µm. Aggregates of coccoid-shaped forms are also registered by SEM analyses. Taxonomical approximation of the microbiota is complex, though in the thin section the condensed fibrillar meshworks look like cyanobacteria, and in SEM images the morphology of the filaments resembles fungal hyphae and green algae, whereas coccoids are assigned to cyanobacteria. The precipitation of Fe-Mn is related to the chemoorganotrophic behaviour of the benthic microbial communities, probably corresponding to the fungal mats and other chemosynthetic microbes. Inorganic precipitation mechanisms are regarded as insufficient for the accumulation of a significant amount of MnO. An efficient precipitation of Mn from natural water largely depended on the presence of Mn-oxidizing microorganisms. Sediment-starved zones of pelagic swells of the External Subbetic, located in the deep euphotic zone, were the best places for microbially mediated authigenesis.

TAPHONOMIC FEATURES OF LENTICULINA AS A TOOL FOR PALEOENVIRONMENTAL INTERPRETATION OF MIDSHELF DEPOSITS OF THE UPPER JURASSIC (PREBETIC ZONE, SOUTHERN SPAIN)

Abstract Taphonomic analysis of Lenticulina from the Upper Jurassic foraminiferal assemblages of midshelf environments was carried out in a marl-limestone rhythmite of the Prebetic. Size, fragmentation-corrasion, microboring, encrustations, recrystallization, mineralization, sedimentary infillings, and cementation of tests were analyzed by thin section. Encrustations, microborings, and dominance of micritic infillings in the tests are indicative of long exposure on the sea bottom related to low-sedimentation rates. In contrast, the frequency of sparitic cements and reelaborated tests is indicative of higher-sedimentation rates, which are also increased by the input of sediment winnowed from shallower areas. The stratigraphic distribution of these taphonomic features reveals trends in sedimentation rates and sediment input that may be interpreted according to the sequence stratigraphy proposals for the Prebetic. During transgression, increasing relative distance from shore produces a lower-sedimentation rate and prolonged exposure of the fossil remains. This results in increased amounts of microborings, encrustations, and micritic infillings, while fragmentation and the proportion of the reelaborated tests of Lenticulina decrease. Rapid burial is related to increasing sediment input and sedimentation rates and can be associated with aggradation and progradation during the development of a highstand systems tract and a shelf margin wedge. A higher-sedimentation rate explains the scarcity of encrustations and microborings and the abundance of sparitic cements. A higher allochthony of Lenticulina is indicated by the locally high values of fragmentation and reelaborated tests. This taphonomic analysis is useful in monotonous epicontinental lithofacies as marl-limestone rhythmites can reveal features indicative of depositional developments that can be fitted to sequence stratigraphic models.

Serpulid-Frutexites assemblage from shadow-cryptic environments in Jurassic marine caves, Betic Cordillera, southern Spain

Abstract In the Middle–Upper Jurassic boundary of the External Subbetic, there are abundant discontinuities with neptunian dikes and sills composed of Callovian–lower Tithonian deposits. In cavities developed on a slope with escarpments, cryptobiontic communities were preserved in life position. These cavities were excavated beneath a hardground covering the upper surface of Bathonian oolitic limestones deposited in very shallow pelagic carbonate platforms. The biogenic crusts are composed mainly of serpulids and Frutexites—laminated dendrolitic microstructures—and, secondarily, by sessile foraminifera. The serpulids were pioneer organisms during colonization of the walls of small cavities in stressed shadow-cryptic environments. Serpulid aggregates then grew downward from the top walls of the cavities. The colonization of serpulid tubes was mainly after the death of organisms, first by microborers, secondly by Frutexites, and later by sessile foraminifera. The preferential colonization of these cryptic environments by serpulids can be interpreted as due to possible photophobic behavior and or the possibility of avoiding space competition or predation.

TAPHONOMY OF AMMONITE ASSEMBLAGES FROM THE MIDDLE-UPPER OXFORDIAN (TRANSVERSARIUM? - BIFURCATUS ZONES) IN THE INTERNAL PREBETIC (BETIC CORDILLERA, SOUTHERN SPAIN): TAPHONIC POPULATIONS AND TAPHOFACIES TO SUPPORT ECOSTRATIGRAPHIC INTERPRETATIONS

The taphonomic analysis conducted on ammonoid assemblages has proven useful for palaeobiological and palaeoenvironmental reconstructions in upper Middle-to-lower Upper Oxfordian epicontinental deposits of the Internal Prebetic (southern Spain). Taphonic populations close to type I indicate the proximity, even coincidence, of life areas for neritic ammonites (i.e. parautochthonous assemblages). The relationships among preservation mode, shell size, within-bed position, corrasion, fragmentation, and epibionts (encrustment included), with lumpy-oncolitic and condensed & bioclasts-rich lumpy-oncolitic limestones allows us to typify two taphofacies. Taphofacies I shows higher values of mean shell size, corrasion, epibionts and encrustments, as well as a high number of specimens in quasi-horizontal settlement. The opposite characteristics serve to identify Taphofacies II. Taphonomic features, taphofacies and lithofacies combine in coherence with progressing third-order transgressive-to-highstand system-tract conditions and the resulting ecostratigraphic trends, which are registered by fluctuations in ammonite assemblages.

Toarcian ammonitico rosso facies from the South Iberian Paleomargin (Betic Cordillera, southern Spain): paleoenvironmental reconstruction

The Toarcian ammonitico rosso facies were widespread in the Mediterranean Tethys (between 15 and 30°N latitude) in the North Gondwana Paleomargin (Apulian promontory and North African Margin) and southern Iberian Paleomargin (Betic Cordillera). These facies were associated with epi-oceanic slopes of a sedimentary swell-trough system related to the extensional phase of continental rifting. In the Median Subbetic (southern Iberian Paleomargin), ammonitico rosso facies show a progressive change through the Toarcian on the hemipelagic swells after the fragmentation of a carbonate platform. During the latest Pliensbachian to the Bifrons Zone (middle Toarcian), sedimentation was dominated by epi-oceanic limestone and marl with a high influence of neighboring shallow-water environments represented by common turbidite–tempestite beds (with foraminifera and ooids). Microfossils and trace fossils provide no evidence of oxygen-restricted conditions. In the Gradata Zone (middle Toarcian), the ammonitico rosso facies appear (red nodular limestone and marly-limestone rich in the trace fossils Phycodes, Planolites, Thalassinoides, and Chondrites). Progressively more pelagic conditions and a restricted influence of emergent lands and carbonate platforms are reflected by the reduced input of turbidite–tempestite beds and increase of ammonitellas and radiolaria. A sea-level fall affected the hemipelagic swell during the middle–late Toarcian and favored sediment-winnowing by currents, with subsequent nodulation. The combined action of burrowing, compaction, and dissolution controlled nodulation, which ranges from diffuse nodules to sharp-edged nodules. The sedimentation rate conditioned the time available for nodule growth, the migration of the Ca2+ and HCO3− precipitation horizon, and the degree of nodulation (from horizons with diffuse-edged nodules to semi-continuous to continuous layers formed by the coalescence of sharp-edged nodules).