Palaeoenvironmental reconstruction of the basal Cretaceous vertebrate bearing beds in the Northern part of the Aquitaine Basin (SW France): sedimentological and geochemical evidence

The Purbeckian facies of the northern margin of the Aquitaine Basin is exposed in the Cherves-de-Cognac section (SW France), considered as basal Cretaceous in age. Two lithological units have been identified. The basal unit (U1) is composed of an alternation of gypsum and finely bedded black dolomitic marlstone, occasionally stromatolitic in nature. It represents mostly hypersaline, dolomitic tidal flat, lagoonal, and sabkha-type environments. The upper unit (U2) consists of variably fossiliferous limestone-marl alternations that contain a diverse fauna and flora. In the upper part of this unit, a 4-m-thick level has yielded a rich vertebrate fauna. The preservational quality of the vertebrate fossils varies within the section. On the basis of faunal, floral, sedimentological and mineralogical information the vertebrate remains accumulated in brackish water environments in which the influence of freshwater has progressively increased towards the top of the section. The close association of environments representing varying degrees of marine and continental influence suggest an estuarine setting.     

The late jurassic ‘Massenkalk fazies’ of Southern Germany: calcareous sand piles rather than organic reefs

Summary Upper Jurassic (Malm δ to ζ1) massive limestones (‘algal-sponge-reefs, sponge-reefs, reef-complexes, reefs, algal-sponge-bioherms, biolithites, Massenkalk, bioherms, Stillwasser-Mudmounds’) were analyzed in the Southern Swabian Alb, the Southern Franconian Alb and in drilling wells in the Molasse basin (Southern Bavaria). This analysis was carried out within the frame of a multidisciplinary DFG-study with the objective of decifering the controls on the development of Upper Jurassic spongiolites, their three-dimensional distribution, their characteristic faunal composition, and the diagenetic trends of the different primary facies. The data base consists of detailed facies mapping in the areas of the Eybtal and the Blautal (1300 samples) as well as comparative studies in the Upper Donautal (Swabian Alb) and the Southern Franconian Alb (400 samples). All together about 500 thin sections were studied. The distribution of the most important components (ooids, intraclasts, peloids, corals, sponges, sponge spicules, cyanobacterial crusts, brachiopods, molluscs, echinoids, bryozoans, serpulids,Terebella, Tubiphytes), and diagenetic features (dolomite, dedolomite, silicification, stylolites, clay flasers, hematite patches) results in a spatial distribution pattern of facies types. The largest part (70 %) of the massive limestones consists of a peloid-lithoclast-ooid sand facies rich in completely or partly micritized ooids. These ooids, especially in beds of the Malm δ to ε, might be the clue to a reinterpretation of the water depth. True biogenic constructions occur (about 30 % of the volume; sponge-algalmudmounds, algal-sponge-boundstones, and brachiopod-algal-sponge-mounds) within and at the margins of this facies and are interpreted as platform sands. The spatial distribution of the buildups in relation to the sand facies was probably controlled by hydrodynamic conditions. In addition, zoned sponge-algal-mounds occur in intraplatform channels and nodular sponge-algal-mudmounds in the marly basin sediments between platform sand areas. Breccias and slumpings in beds older than the Malm ζ have to be reinterpreted. Most of the breccias found originated from the flanks of the sand platforms, reflecting the faunal composition of the algal-sponge-boundstones which stabilized the flanks. Breccias of this composition occur throughout the Malm δ-ζ1 and differ markedly in their composition from the sand facies. The boundary breccia (Malm ε/ζ1) is interpreted as marking a regressive maximum. The increasing growth of buildups, rich in brachiopods in the Malm ζ1, is ascribed to an increase of reef growth at the beginning of a transgression. Detailed facies analyses necessary for the reconstruction of the spatial distribution of different facies types are in progress. Most of the older data on faunal distributions cannot be used for detailed facies analysis because they differentiated only between massive facies and bedded facies. Therefore Upper Jurassic limestones of Southern Germany should be restudied in order to recognize the volumetric importance of sand facies and buildups within massive limestones.     

Tectonically controlled late cretaceous terrestrial to neritic deposition (Northern Calcareous Alps, Tyrol, Austria)

Summary The Turonian to Santonian terrestrial to neritic succession (Lower Gosau Subgroup) in the Northern Calcareous Alps of the eastern part of the Tyrol, Austria, provides an example for deposition on a compartmentalized, narrow, microtidal to low-mesotidal, wave-dominated, mixed siliciclastic-carbonate shelf. The shelf was situated in front of a mainland with a relatively high, articulated relief, and underwent distinct changes in facies architecture mainly as a result of tectonism. The investigated succession was deposited above a deeply incised, articulated truncation surface that formed when the Eo-Alpine orogen, including the area of the future Northern Calcareous Alps, was uplifted and subaerially eroded. Distinct facies associations were deposited from (1) alluvial fans and fan deltas, (2) rivers, (3) siliciclastic lagoonal to freshwater marsh environments, (4) areally/temporally limited carbonate lagoons, (5) transgressive shores, (6) siliciclastic shelf environments, and (7) an aggrading carbonate shelf. During the Turonian to Coniacian, the combination of high rates of both subsidence and sediment accumulation, and a narrow shelf that was compartmentalized with respect to (a) morphology of the substratum, (b) fluviatile input of siliciclastics and contemporaneous input of carbonate clasts from fan deltas, (c) deposition of shallow-water carbonates, and (d) water energy and-depth gave rise to an exceptionally wide spectrum of facies as a distinguishing feature of the succession. With the exception of facies association 7, which formed only once, depositional sequences in the Turonian to Coniacian interval contain all of the facies associations 1 to 6. During Turonian to Coniacian times, the shelf was microtidal to low-mesotidal, and was dominated by waves, storm waves and storm-induced currents. In vegetated marshes, schizohaline to freshwater marl lakes existed. Transgressions occurred onto fan deltas and in association with estuaries, or in association with gravelly to rocky shores. The transgressive successions, including successions deposited from transgressive rocky carbonate shores, are overlain by regressive successions of shelf carbonates or shelf siliciclastics. Deposition of shallow-water carbonates generally occurred within lagoons and over short intervals of time. A „catch-up” succession of shelf carbonates about 100 m thick accumulated only in an area protected from siliciclastic input. In its preserved parts, the Turonian to Coniacian succession does not record deposition adjacent to major active faults. Lateral changes in thickness result mainly from onlap onto the articulated basal truncation surface. Subsidence most probably was controlled by major detachment faults outside the outcrop area, and/or was distributed over a wide area in association with secondary faults above the major detachments. During Coniacian to Early Santonian times, both the older substratum and the overlying Turonian-Coniacian succession were subaerially exposed, faulted and deeply eroded. The following Early Santonian transgression ensued with rocky carbonate shores ahead of a sandy, narrow shoreface-inner shelf environment and a deeper shelf with intermittentlydysaerobic mud. The transgression was associated with the influx of cooler and/or nutrient-rich waters, and heralds an overall deepening. Still during the Early Santonian, the deepening was interrupted by another phase of subaerial exposure. Subsequently, a short phase of shelf deposition was terminated by deepening into bathyal depths.     

A seaward prograding siliciclastic sequence from upper tidal flats to salt marsh facies (Southern North Sea)

Summary The uppermost part of the seaward prograding sequence in quartz sandy areas of the southern North Sea coast starts at the base with tidal flat layers, often strongly perturbated by burrowing endobenthic animals. This section is overlain by microbial mats intercalated by tempestites, and in part bioturbated by shaped burrows. The top consists of salt marsh facies strongly penetrated by roots. The seaward prograding sequence is characterized by numerous vertical changes in small-scale facies. The facies development and changes are caused by both organisms (halophytes, mat building microbes, benthic animals) and primary physical processes (erosion, transport and sedimentation, or sedimentation alone). The small-scale facies includes (i) bioturbation structures, (ii) biogenic growth structures caused by microbial mats, (iii) subaerial and subaqueous tempestites. Typical is the sudden breakdown of the biocoenosis by fast sedimentation due to tempestites or wind-blown sand sheets. Typical is also the renewal of the former biocoenosis in the new top section. Such sequences may be preserved by wind-blown sand sheets or by washover fans. The sequence is thus an important and unique tool for the identification of fossilized sequences of the intertidal-supratidal transition. It is also a useful mark for estimating the mean sea level and the level of the lower supratidal horizon within vertical sections. Finally the sequence indicates progress in island development governed by the interaction of biological components with physical processes.

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Zusammenfassung Die Verlandungssequenz sandiger Küsten der Nordsee besteht aus einer Abfolge, die von Wattenschichten mit überwiegend ungestalteter Verwühlung im Liegenden über Mikrobenmatten, in die Tempestite mit vorwiegend vertikal angelegten G?ngen eingeschaltet sind, zu supralitoralen Salzwiesen mit starker Durchwurzelung führt. Sie wird durch zahlreiche, oft nur zentimeterm?chtige Fazieswechsel charakterisiert. An der Entwicklung und am Wechsel der Fazies sind Organismen (benthische Fauna, mattenbildende Mikroorganismen und Halophyten) sowie prim?r physikalische, subaquatische wie subaerische Transport-und Sedimentationsprozesse beteiligt. Bezeichnend für die Verlandungssequenz sind wiederholte Zusammenbrüche der lokalen Bioz?nosen durch eingeschaltete Tempestite mit nachfolgender Entwicklung der gleichen Bioz?nose bei hinreichender sediment?rer Ruhezeit. Die beschriebene Sequenz ist ein diagnostisches Mittel zur Identifizierung flachmariner Verlandungsabfolgen. Sie erlaubt, die H?he der mittleren Tidehochwasserlinie festzulegen. Solche Sequenzen k?nnen durch Windsanddecken oder sturmflutbedingte Durchbruchsf?cher überdeckt und erhalten werden.

     

Facies of sunken early cretaceous atoll reefs and their capping Late Albian drowning succession (Northwestern Pacific)

Summary Since first described in detail byHamilton (1956), the causes and timing of the drowning of several hundred guyots in the northwestern Pacific is a puzzling question. Thus, the northwestern Pacific is one of the key areas in deciphering the demise of flat-topped platforms throughout the earth’s history. Based on older paleontological data and the newly found shallow-water benthic foraminifera, the atoll reefs probably had a major period of vertical aggradation during the Barremian and the Aptian into the Late Albian depending on the stage of atoll development (type of guyot). New sedimentologic and stratigraphic data suggest a strong fall in sea level, leading to karstification and the formation of lowstand fringing reefs, prior to an even rapid rise of greater amplitude in the Late AlbianRotalipora appenninica zone ultimately causing drowning. After climatic relaxation, a sea level rise led to the final formation of small barrier reefs, rimming the top of many guyots in the Japanese Group, the Wake Group and the Mid-Pacific Mountains. They can be interpreted as “give-up” structures indicating a final shallow-water carbonate production on top of the atolls during drowning. The facies of the syn- and post-drowning sediments on the guyot tops are strikingly similar even when vast distances apart. This and the biostratigraphic data suggest a synchronous drowning of many seamounts investigated up to now. Biotic composition and facies of the final Albian reefs are very similar to Albian caprinid-dominated reefs in the Caribbean region, indicating comparable environmental controls. In the case of the northwestern Pacific guyots, the simultaneous demise of reefs could be due to a short-term cooling event in the Late Albian, connected with a strong regressive-transgressive cycle with an amplitude of about 180 m. This event is also known from the Tethys and the Atlantic. Climatic disturbances triggering short-term cooling and inducing a high amplitude regressive-transgressive sea level cycle, might be responsible not only for the Late Albian event, but also perhaps for other reef drownings throughout the earth’s history.     

Platform margins, microbial/serpulids bioconstructions and slope-to-basin sediments in the Upper Triassic of the ‘Verbicaro Unit’ (Lucania and Calabria, southern Italy)

Summary  The Upper Triassic carbonates of the area comprised between Maratea (Lucania) and Praia a Mare (Calabria) have been studied. They have been grouped into six facies assemblages which, in turn, define two depositional systems.

The Messinian reef complex of the Salento Peninsula (southern Italy): Stratigraphy, facies and paleoenvironmental interpretation

Summary An integrated study of the early Messinian reef complex cropping out along the eastern coast of the Salento Peninsula (southern Italy), including stratigraphy, facies analysis and paleoecological aspects, is here presented. Fourteen facies types belonging to three main facies associations (back reef and shelf, shelf-edge, slope) have been recognized. They document a wide spectrum of depositional environments, reef building organisms and growth fabrics, in response to depth and other environmental factors in different parts of the reef complex. The biotic structure of the reef is also described and discussed in detail. It consists of different types of reef building organisms and of their bioconstructions (mainlyPorites coral reefs,Halimeda bioherms and vermetidmicrobial “trottoirs”), that differ in composition and structure according to their position on the shelf edge-toslope profile. Results indicate that the reef complex of the Salento Peninsula has strong similarities with the typical early Messinian reefs of the Mediterranean region. However, the recognition of some peculiar features, i.e. the remarkable occurrence ofHalimeda bioherms and of vermetid-microbial “trottoirs”, gives new insights for better understanding reef patterns and development of the reef belt during the Late Miocene in the Mediterranean.     

Facies belts and communities of the arctic Vesterisbanken Seamount (Central Greenland Sea)

Summary The Arctic Vesterisbanken Seamount, situated far offshore in the central Greenland Sea, provides a unique facility for studing modern cold water siliceous carbonate deposits. A nearly year round sea ice cover, which retreats on average only during two months, and a rather constant temperature and salinity structure of the water column characterize the Arctic conditions of the area. Despite predominantly oligotrophic conditions with a pronounced food supply from the pelagic realm only during the ice-free season, the seamount is covered extensively by extended sponge-bryozoan constructions. Three distinct facies belts reveal a pronounced depth zonation which depends on variations in downslope food transfer and which is specifically effective due to the development of aTaylor current regime over the seamount: i) the crest facies from the summit at −133m to −260 m, ii) the shallow slope facies from −260 m to −400 m, iii) the deep slope facies from −400 m down to the abyssal plain at about— 3.000 m. Different biogenic structures and communities are found within these facies belts, including widely extended biogenic mats, sponge bryozoan-serpulid buildups with mounds, hedges, spurs and flatcake-like structures, bryozoan thickets and sponge-crinoid mounds. Depth zonation, internal structure and controlling parameters in the formation of these biogenic structures are discussed in the context of their significance as a modern end member of the Foramol facies and their implication for the fossil record. In addition, the younger volcanic and hydrothermal history of the seamount is presented with special reference to its bearing on Holocene biogenic colonization patterns.     

Late Ordovician–Early Silurian facies development and environmental changes in the Subpolar Urals

The continuous Upper Ashgill–Sheinwoodian carbonate succession in the most eastern Kozhym River area in the Subpolar Urals comprises the Yaptikshor (Rawtheyan), Kamennaya baba (Hirnantian), Ruchej and Manyuku (Llandovery–?Sheinwoodian) formations. The facies of the deep subtidal Yaptikshor Fm. mark an abrupt sea‐level rise following emergence of the Bad’ya reef (Rawtheyan). Carbonate breccias at the base of the Kamennaya baba Fm. correlate with the beginning of the Hirnantian glaciation and change upwards towards the Ordovician–Silurian boundary with the development of light‐grey massive boundstone/packstone shoal deposits. An abrupt change in facies to the Rhuddanian–Aeronian Ruchej Fm. continental slope environment marked the start of a long‐term sea‐level rise. The uppermost Aeronian–?Sheinwoodian is represented by submarine canyon carbonate conglobreccias of the Manyuku Fm. unconformably underlying the Balban’yu reef. The rapid facies changes at the base of the Hirnantian and at the Ordovician–Silurian boundary were of global eustatic origin. In contrast, the abrupt changes in the Rawtheyan and the formation of the Manyuku Fm. conglobreccias were of local or regional origin associated with tectonics. They were followed by the start of a regional transgression (Yaptikshor Fm.) and a global transgression marked by the initiation of the Balban’yu Reef in the Sheinwoodian.