Since their first occurrence in the late Cretaceous, seagrasses have played a major role in carbonate production and sedimentation across shallow‐water and nearshore environments, sustaining a ...prolific carbonate factory and contributing to sediment accumulation through the combination of baffling and trapping effects. Most reported Palaeogene seagrass occurrences developed in oligo−mesotrophic shallow warm‐water habitats and are characterized by distinct associations of small and larger benthic foraminifers adapted to low terrigenous influence. This study describes a number of seagrass episodes interbedded in the Bartonian (middle Eocene) of San Fausto–Lazkua area (Navarra region, North Spain), within a nearshore to inner‐ramp succession that, in spite of being deposited under general transgressive conditions, was highly influenced by terrigenous supply from the adjacent land. Up to twelve different seagrass bed intervals occur interbedded in a cyclical manner with high‐energy nearshore siliciclastics and inner ramp bioclastic carbonates rich in mesophotic−oligophotic foraminifers and heterozoan biota (red algae, echinoderms, bryozoans). Seagrass deposits exhibit typical unsorted textures, abundant bioturbation and moderate to high terrigenous content, and comprise a characteristic skeletal association of epiphytic foraminifers, red algae and, most particularly, of abundant encrusting acervulinids, commonly with distinct hooked and tubular growth forms. This abundance of suspension‐feeders relative to autotrophs and mixotrophs may be indicative of temperate waters, although the taxonomic diversity of the foraminiferal assemblages in both seagrass and non‐seagrass embedding deposits supports the interpretation of shallow, warm‐water conditions. The studied seagrass deposits provide evidence that high siliciclastic supply and associated nutrient input may determine the occurrence of temperate‐like seagrass deposits in warm‐water settings, analogous to extensive heterozoan carbonate production in modern shallow‐tropical environments. Thus, the identification and correct interpretation of past seagrass‐vegetated environments are crucial for reconstructing palaeoecological conditions in ancient shallow‐marine environments. Therefore, in comparison with carbonate‐dominated environments, the mixed terrigenous−carbonate seagrass deposits are volumetrically less important, presenting a more irregular, patchy distribution, and a skeletal assemblage dominated by heterotrophs, regardless of the water temperature.
The carbonate factories, their controlling factors and their palaeoecological and sedimentological signals recorded in sedimentary successions are key elements for understanding the evolution of ...carbonate platform systems. Luis Pomar has dedicated most of his academic life to the study of carbonate rocks and carbonate factories. The idea for this special issue to celebrate Pomar’s career arose during a session at the 34th International Association of Sedimentologists meeting held in Rome, entitled ‘Understanding carbonate factories through palaeoecological and geochemical signals’. The proposal encountered great response among participants, and additional contributions followed an email invitation to other specialists. This issue contains a variety of papers on carbonate sedimentology and carbonate factories. Here, an introduction that contextualizes the papers and key concepts discussed in this thematic issue is provided. It reviews Luis Pomar’s major achievements in carbonate sedimentology and discusses the evolution of the concept of the carbonate factory and the series of palaeoecological and sedimentological signals used to characterize the wide spectrum of carbonate depositional systems found in the natural world.
During the Aptian (Cretaceous), in what is now the South Atlantic, the largest chemogenic (abiotic) carbonate factory so far identified in the Phanerozoic geological record developed as a vast ...hyper‐alkaline lake system. This covered at least 330 000 km2, producing carbonates, locally over 500 m thick, in what are now the offshore Santos and Campos basins (Brazil), and Kwanza Basin (Angola). Current evidence supports the view that almost all of this carbonate was chemogenic in origin, precipitated from hyper‐alkaline, shallow lake waters, probably by evaporation. This unit, best documented from offshore Brazil and known as the Barra Velha Formation (Santos Basin) and the Macabu Formation (Campos Basin), consists of just two basic carbonate components, millimetre to centimetre sized crystal shrubs and spherulites. These are commonly in situ but can also be reworked into a range of detrital facies. Demonstrable microbialites are generally rare. These carbonates are associated with Mg silicates (as clays) which had a profound influence not only on the textural development of the in situ carbonates, but also on their diagenesis. The dissolution of the clays produced much of the porosity in these limestones, which are the hosts for multi‐billion barrel oil fields. The source of the carbonate was most likely from metasomatic alteration of mafic rocks, such as continental flood basalts related to Atlantic opening, with some contribution from much older continental basement. Clear evidence that serpentinization of possible exhumed mantle is lacking but mantle CO2 is likely to have been a critical factor in determining the composition of the fluids from which the carbonates formed and the high alkalinities of the lake waters.
We present a new locality with at least 880 vertebrate tracks found at the top of a limestone bed from the lower Miocene Tudela Formation (Spain). The trampled surface was formed by artiodactyls that ...crossed a muddy carbonate accumulated under the influence of water level variations in a palustrine environment. The tracks reflect different types of morphological preservation. The well-preserved tracks have tetradactyl digit impressions caused by both manus and pes, and are the type series of a new artiodactyl ichnotaxon, Fustinianapodus arriazui ichnogen. nov. and ichnosp. nov. The rest of the tracks, which are not as well preserved, are didactyl and were classified as undetermined artiodactyl tracks. According to their preservation, morphology, size, arrangement and orientation, we propose that this tracksite is the product of a social behaviour, particularly gregariousness, of a multi-age group of artiodactyls ~19 Ma ago. The morphologic and palaeoecologic data presented here suggest that the trackmakers were a group of anthracotheres with a livelihood similar to current hippos. They crossed, periodically, a fresh water palustrine area along some preferential pathways (trails).
Understanding how carbonate factories influence platform evolution is either based on qualitative conceptual models or quantitative numerical stratigraphic forward models. This study establishes new ...production depth profiles for four Cenozoic carbonate factories and uses two‐dimensional stratigraphic forward models to explore how interactions between sediment production and transport within carbonate systems influence carbonate platform development. Newly established production/depth profiles are used to model photozoan and heterozoan carbonate grain associations, and the associated carbonate producing factories, and results are compared with well‐studied outcrop successions. Sediment production from photozoan and heterozoan grain associations is also equalized, so that the total sediment production is the same but the depth/production profiles retain their distinctly different form. Thus, the effect of the different production profiles can be assessed. Ramps form when sediment diffusional transport rates are high relative to production rates and flat‐top steep‐margin platforms form when sediment diffusional transport rates are low relative to production rates, whether they are photozoan or heterozoan grain associations. The control exerted by sediment production and transport is expressed as a sediment transport–production ratio where transport ratio is a diffusional sediment transport in two‐dimensions and production ratio is the total sediment production rate which is the product of a production profile that varies in depth and laterally. The transport–production ratio is a key control on the evolution and geometry of carbonate platforms. This is the case with different production profiles (both euphotic and oligophotic) and in mixed grain‐size and mixed transport‐rate systems. Carbonate producing factories significantly influence the rate of sediment production, the depth distribution of sediment production (production profiles), as well as the type of grain sizes produced (influencing resistance to erosion). Thus, different types of carbonate grain associations, and the associated carbonate producing factories, can produce the critical differences between carbonate platform geometries.
This paper regards the lower Pleistocene temperate‐water carbonate deposits disconformably overlying an escarpment made up of faulted Cretaceous to Miocene limestones of the Apulia Foreland (southern ...Italy). Study deposits discontinuously crop out along the present‐day eastern Salento sea cliff, and form isolated fan‐shaped bodies, up to 1 km wide and up to 40 to 50 m thick, each of them covering an area of a few square kilometres. The internal arrangement of beds is represented by up to 25° to 30° lobate, seaward dipping clinobeds thinning and onlapping onto a rocky foreslope in the proximal sector and passing to gently inclined to sub‐horizontal strata in the distal sector. Seven facies were distinguished, mainly composed of coarse‐grained skeletal carbonates made up of a heterozoan association including coralline algae, large and small benthic foraminifera, echinoids, molluscs, bryozoans and serpulids. Since clinobeds were formed thanks to hyperconcentrated density flows (grain flows) bypassing the upper part of the inherited escarpment, these skeletal grains represent ex situ deposits whose shallow‐marine factory was located upward (landward) with respect to the bypassed zone, likely in the almost flat area on top of the Salento Peninsula. Clinobeds are often affected by tens of metres wide and long channel‐like structures interpreted as landslide scars. Inside these gullies, contorted beds (slumps) or matrix‐supported intra‐bioclastic floatstone/rudstone (massive deposits) are present. The occurrence of supercritical‐flow structures (for example, backset‐bedded beds) indicates the development of hydraulic jumps along the steep slope of gullies. Since these clinostratified, fan‐shaped carbonate bodies represent carbonate slopes, and that the latter are known as aprons, normally related to linear sourced sediments, an acceptable oxymoron for studied fan‐shaped carbonate bodies is suggested: ‘isolated base‐of‐slope aprons’.
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