Thomas C. Brachert

Controls on modern carbonate sedimentation on warm-temperate to arctic coasts, shelves and seamounts in the Northern Hemisphere: Implications for fossil counterparts

SummaryIn contrast to the well studied tropical carbonate environments, interest in non-tropical carbonate deposition was rather low until the basic ideas of theForamol-concept were outlined byLees & Buller (1972). In the following two decades studies on non-tropical carbonate settings evolved as a new and exciting branch of carbonate sedimentology (seeNelson 1988). This is archieved in a great number of publications dealing on temperate carbonate deposits from numerous coastal and open shelf settings on both hemispheres. The existence of wide extended carbonate depositional systems and even reefal frameworks in Subarctic and Arctic seas which are in focus by our research group made it possible to study modern non-tropical carbonate settings along a latitudinal transect from the warm-temperate Mediterranean Sea to the cold Nordic Seas. Because of increasing seasonality in environmental conditions towards high latitudes, the major controls in biogenic carbonate production can be more clearly addressed in these areas. After the initiation of the priority program “Global and regional controlling processes of biogenic sedimentation-evolution of reefs” by the German Science Foundation four years ago, a set of modern case studies were comparatively analysed specifically with regard to their principle controlling processes:(1)Modern and Holocene coralline algal reefs and rhodolith pavements formed in wave-protected shallow waters along the coast of the Brittany and northern Norway. Their finetuned interaction with herbivores resulted in the development of widespread but low-diverse, slowly growing coralline algal frameworks with high competitive value against the rapid-growing phaeophytic communities.(2)The MediterraneanCladocora caespitosa-banks provide an instructive example of non-tropical hermatypic coral framework construction out of the subtropical-tropical coral reef belt.(3)The geometry and environmental controls of several kilometer long coral reefs formed by the azooxanthellateLophelia pertusa andMadrepora oculata are studied in more than 250 m water depth in mid and northern Norway.(4)ModernBryomol-sediments are widely distributed on non-tropical deeper shelf settings. The formational processes converting bryozoan-thickets into huge piles of sand and gravel dunes are recently studied on the outer shelves off northern Brittany and off northern Norway.(5)Arctic sponge-bryozoan buildups on the seamount Vesterisbank in the Greenland Sea and(6)balanid-dominated open shelf carbonates on the Spitsbergen Bank form the Arctic endmembers of modernForamol-deposits. Seasonalice-edge phytoplankton blooms and efficient mechanisms of pelagic-benthic food transfer characterize these depositional settings. Fossil counterparts of each of these modern case studies are discussed in context with their paleoceanographic and environmental settings.

Nearshore, temperate, carbonate depositional systems (lower Tortonian, Agua Amarga Basin, southern Spain): implications for carbonate sequence stratigraphy

Abstract The bryozoan-rich lower Tortonian carbonates of the Agua Amarga Basin in southern Spain (Province of Almeria) provide an example of sediments formed in a nearshore, non-tropical depositional setting. Based on data derived from logging of sections and from field mapping, these lower Tortonian carbonates form a depositional sequence, which is subdivided into several depositional systems. A lowstand systems tract, which consists of volcaniclastic fan deltas and washover deposits, formed on the leeward side of a basement shoal which delimited the basin towards the south. A transgressive systems tract, which is characterised by a landward encroachment of deposits, is represented by submarine bars and dune deposits. A highstand systems tract consists of progradational beach deposits. Owing to strong reworking and re-distribution of the particles by currents and waves, microfacies differentiation of the deposits in poor. These Tortonian non-tropical carbonates display a style of deposition similar to siliciclastics suggesting that sequence stratigraphic concepts derived from tropical carbonates do not apply to non-tropical carbonates.

Deep-water stromatolites andFrutexites Maslov from the early and Middle Jurassic of S-Germany and Austria

SummaryDespite extensive discussions during the last 20 years stromatolites are still used by many geologists as unequivocal indicators of very shallow-water conditions. We investigated four stratigraphic units from the Lower and Middle Jurassic of southern Germany (Posidonien-Schiefer, Amaltheen-Ton) and of the Northern Calcareous Alps (Adneter Kalk, Klauskalk), which were formerly interpreted as shallow marine sediments by some authors due to the occurrence of stromatolites. Our interpretations of the macro-, micro- and ultrafacies of these sediments are not compatible with shallow-water settings. We therefore propose a deep-marine, aphotic origin of these stromatolites.Former interpretations of the Posidonien-Schiefer as a shallow-water deposit are mainly based on the occurrence of stromatolites. We favour the model of a temporarily stagnant, deep, aphotic basin for these planktonrich sediments. Particles resembling ooids, but lying within mudstones cannot be taken as evidence for shallow agitated water. They either formed within the mud or are allochthonous.The deep-water setting of the red limestone of the Alpine Early and Middle Jurassic is indicated by a lack of platform-typical components like coated grains and phototrophic benthos and by shells of plankton and nekton forming a major part of the sediment. Stromatolites occur on the steep slope of a drowned Rhaetian reef with an estimated relief of 50–100 m and immediately below and within radiolarian limestones, deposited below the aragonite compensation depth (ACD).The aphotic stromatolites show some morphological differences to their shallow water counterparts. In all of our sections they occurred during intervals of reduced sedimentation. They form only thin horizons and probably grew very slowly. Mineralizations by Fe−Mn oxides and phosphate are very common. The presence of a microbial film is evident from binding of sedimentary particles, but the nature of the microbes is not known. Growth habits within the very distinct environments of red limestone and black shales show some common features, but also clear differences.The microproblematicumFrutexitesMaslov is a very common component in deep-water stromatolites, but may also itself form small crusts or dendrolites. It occurs in two different forms. Opaque, slender forms with indistinct outlines probably grew within the weakly lithified sediment. Thicker, transparent forms with well defined outlines are found in cavities and probably also grew on the seafloor. Well preserved specimens display an internal fabric of radially arranged fibres of Fe−Mn oxides and calcite. It is suggested that calcite or aragonite were one original mineralogy ofFrutexites, which was later replaced by Fe−Mn oxides or phosphate.It is not certain whetherFrutexites is an organic, biomineralized structure or an inorganic mineralization, but the variable mineralogy and growth forms in different environments point to an organic origin. But even if organic, the occurrence in cryptic habitats and negative phototactic growth-directions make it clear thatFrutexites was not phototrophic.

Laminar Micrite Crusts and Associated Foreslope Processes, Red Sea

ABSTRACT Forereef slopes in the Red Sea of Sudan exhibit a uniform biozonation that is independent from the topography of the slopes. Below - 120 m, ledges protrude horizontally from sleep cliffs of barrier reefs and atolls as well as from patches of in situ lithified slope sediment on inclined fringing reef slopes. Free surfaces and cavities within these ledges are partly covered by laminar micrite crusts of 7-20 mm thickness. The ledges are formed by an organic framework of living azooxanthellate corals, bryozoans, serpulids and fossil red algae. They are affected by repeated episodes of boring, infilling, and cementation which obliterate much of the original fabric. Concomitant cementation and boring result in asymmetric cement linings which often show geopetal fabrics. The laminar m crite crusts, however, show no significant traces of bioerosion, which are a typical feature of fossil deep-water stromatolites. Two types of lamination have been observed: 1) vertically stacked, irregular, anastomosing laminate with ragged outlines and little lateral persistence which are often accentuated by iron staining, and 2) 0.5-mm-thick laminae of light-grey micrite, rich in fine bioclasts, showing smooth, thin coatings of dark homogenous micrite, which are weakly fluorescent in ultraviolet light. No algal films are preserved, but micritic tubes 10-30 µm in diameter, often bifurcated, form a dense network on the upper surface of the crusts by overgrowing and binding particles. Associated aragonitic rosettes 30-40 µm in diameter may represent calcified coccoid algae or bacteria. If true, these organisms suggest a formation of the latter type of crust by biogenic processes not related to the photic zone. Fenestral porosity and Frutexites textures are absent. Radiocarbon dates from corals within the ledges provide ages of 10,500 YBP. From the time-related decrease in oxygen isotope ratios (versus PDB) of boring micrite (18O = +3.72), micrite crusts (18O = +1.69), and unlithified mud (18O = +0.47) within the youngest voids of the hard substrates, we conclude that crust formation must have taken place in shallower water when sea level was lower during the Early Holocene rise of sea level.

Record of climatic change in neritic carbonates: turnover in biogenic associations and depositional modes (Late Miocene, southern Spain)

In order to evaluate the geological record of climatic change in neritic carbonates, we studied Late Miocene rock outcrops in southern Spain. Six episodes of reef growth are documented (Burdigalian to Messinian) in Neogene basins of the Betic Cordillera, which were located close to the margin of the global reef belt. The reefs are characterized by various zooxanthellate corals which decrease in diversity with time, andHalimeda; the youngest reefs of the latest Messinian are characterized by the dominance of the genusPorites. Late Miocene coral reefs and reef-rimmed platforms alternate over time with non-reefal carbonate ramps characterized by skeletal calcirudites or with gypsum such as that formed during the Messinian salinity crisis. The calcirudites lack reef corals, calcified green algae and extensive marine cement, but exhibit skeletal components described from both modern and fossil nontropical carbonates. These include bryozoans, mollusks, foraminifers, echinoderms and minor balanids, as well as coralline algae of a bryomol association. The presence of some larger foraminifers indicates high temperatures, close to the lower temperature threshold of the reef assemblage. Sea level lowstands and highstands are documented by wedges of bryomol carbonate and chlorozoan patch reefs or prograding platforms. Thus, temperate climate depositional modes correspond to relatively low sea levels, and warm-water modes to high sea levels. The Neogene infill of the Agua Amarga and Sorbas basins documents two of these cycles. Other climate/sea-level cycles (including Messinian gypsum in the cool water depositional mode) are well established in adjacent Neogene basins in southern Spain. This type of composite sequence seems to occur only along the margin of the global reef belt and indicates an oscillatory latitudinal movement of the margin, which is associated with global climatic change. The analysis of turnover in neritic depositional carbonate systems may therefore be considered a sensitive tool for reconstructing climatic change from the fossil record. However, warm-water modes and temperate-water modes of carbonate sedimentation and diagenesis differ significantly. For this reason the interpretation of composite system sequences by sequence stratigraphy requires an extended concept. The particular type of mixed bryomolchlorozoan depositional sequence also bears some potential for drowning, because sea level rise may be faster than the net production rate of temperate carbonate systems.

Shallow burial diagenesis of skeletal carbonates: selective loss of aragonite shell material (Miocene to Recent, Queensland Plateau and Queensland Trough, NE Australia) — implications for shallow cool-water carbonates

In burial environments, carbonate sediments undergo mineralogical stabilization and increasing lithification with depth. As yet, however, little knowledge exists with respect to the corresponding effects on fossil preservation and taphonomic modification of the original sediment composition. Countings of particles (>63 μm) in Miocene to Recent periplatform sediments (ODP Leg 133, NE Australia) exhibit a clear trend of reduction of skeletal aragonite downcore. Low- and high-Mg-calcite grains occur in a continuous order of magnitude over the studied interval (<600 m sub-bottom depth). Original microtextures are retained in high-Mg-calcite biota, although converted to low-Mg-calcite. Thus, the conversion to low-Mg-calcite appears to occur without introducing a significant quantitative bias. Aragonite skeletons (pelagic gastropods), however, exhibit a successive exposure of deeper crystal layers and a chalky preservation with burial depth, which we interpret to result from dissolution. Hints for the originally more numerous existence of aragonite biota exist in soft sediments and chalks by the presence of internal moulds, shells replaced by microspar, and mouldic porosity in early cemented hardgrounds. In deep sections barren of aragonite, the number of casts and replaced shells remains unchanged and is insignificant as compared to aragonitic biota probably originally present within the sediment (<2% in ooze/chalk vs. 30–50% of grains in modern periplatform sediments). Therefore, palaeontological information must be significantly biased through selective removal of aragonite. Rates of preservation and destruction depend on external factors during sediment accretion (sedimentation rates, clay content, total organic carbon content) and rates of fluid flow within the sediment. These observations are relevant with respect to the diagenetic potential and patterns of fossil preservation in little cemented calcitic cool-water carbonates, because they may originally have contained more aragonite biota as important constituents of an ecosystem than is commonly suspected, and calcite/aragonite ratios in ancient carbonate sediments may not necessarily reflect original input signals (climate or sea level).

Lowstand carbonates, highstand sandstones?

The sedimentary facies, sediment dynamics and sequence architecture of modern high-energy shelves in the mid and high latitudes are largely governed by wave abrasion processes. Cool-water carbonates may form there, if the influx and/or net accretion of siliciclastics is kept at a minimum. Little dilution of the carbonate produced in situ is generally promoted by a wide “epicontinental” shelf, subdued topography of the adjacent mainland, the predominance of limestone outcrops, and an arid climate. The aforementioned requirements are rarely met, and thus will automatically lead to the formation of mixed siliciclastic–cool-water carbonates. Such an example is found in the Early to Mid-Miocene Lagos-Portimao Formation (Algarve, S-Portugal), which formed on a narrow high-energy shelf of the Atlantic Ocean that was bounded by a mountain range. The sediments of the formation consist of fossiliferous sandstone (FS), shell beds, and rhodolith blankets. Along strike, the stratification of the formation is monotonous for tens of kilometres and well exposed in coastal cliffs, whereas no outcrops of dip sections exist. The bulk skeletal composition of the sediments is typical for the warm-temperate climatic zone: various endo- and epibenthic bivalves, bryozoans, coralline algae, echinoderms, gastropods, and large foraminifers (Heterostegina). In some very rare beds, a few isolated, not framework-forming specimens of zooxanthellate corals (Porites, Tarbellastrea) indicate temporally elevated surface water temperatures close to the lower threshold of the coral reef ecosystem. In sandstones, the fauna is well preserved and burrowing bivalves are commonly found in life position. In limestone beds, the state of preservation of the grains ranges from intact to disintegrated and abraded specimens. We infer an accumulation of the shell beds through winnowing of fine materials (siliciclastic sand and carbonate mud) at wave abrasion depth and concentration of calcareous skeletons associated with the subsequent attraction of new epibiota in a complex shell bed. The vertical alternation of fossiliferous sandstone and shell beds, and in-phase variations of the “Photo Index” (photic biota vs. bryozoans) and “Bryozoan Index” (bivalves vs. bryozoans) is envisaged to document variations of water depth (and sea level). Sandstone units built up when wave abrasion depth (WAD) rose above the sea floor during TST (and early HST), whereas the shell beds formed during LST when the WAD for sand intersected with the sea floor. Clastic sediments were probably brought on the outer shelf during early transgression, and by longshore currents. Sea-level signatures inferred in the mixed siliciclastic–cool-water carbonate shelf setting of S-Portugal therefore significantly deviate from conventional concepts of carbonate sequence stratigraphy, which were developed for flat-topped platforms. Successful interpretations of ancient mixed sequences must therefore take into consideration the processes of production, concentration and accretion of the carbonate sediments.

Climatic signatures in shallow-water carbonates: high-resolution stratigraphic markers in structurally controlled carbonate buildups (Late Miocene, southern Spain)

Abstract Neogene intramontane basins of the Betic Cordillera (SE Spain) have a complex tectonic and sedimentary history. In the eastern Almeria province movements along the Carboneras strike-slip fault system (N50°E) and its conjugated faults (N140°E) controlled the distributions of depocenters and stratigraphic architectures of Late Miocene sediments. We have chosen the Agua Amarga area (1) to exemplify the potential of climatic signatures of biogenic associations in shallow-water carbonates for high-resolution chronostratigraphic correlations and (2) to describe stratigraphic architectures and facies distributions in a setting subject to synsedimentary tectonics. The southwestern margin (La Higuera) is formed by a gentle ramp dipping in a N50°E direction. Late Tortonian and Messinian deep and shallow-water carbonates divide into eight depositional units. Neptunian autoclastic breccias, neptunian dikes, and submarine biogenic pavements (unit H 1) document rapid flooding of the basin during a phase of intense tectonic dilatation (Late Tortonian). Units H 2–7 (Late Tortonian–Early Messinian) form wedges which dip and thicken in a basinward direction. Little lateral facies change and internal stratifications parallel with the basal surfaces imply deposition on a subhorizontal substratum. The upper bounding surfaces are erosional and formed by truncation of tilted strata. From wedge to wedge, the angle of dip decreases upsection and is near zero in between the last units which have parallel bounding surfaces and rest subhorizontally (unit H 7–8). This pattern results from cumulative rotational uplift which decreased upward, but does not necessarily exclude vertical displacements of the youngest units. The southeastern margin of the basin is mostly eroded but may have trended N50°E as indicated by the distribution of slumped reef blocks. A relict of the margin is preserved at Mesa de Roldan and exhibits steep clinoforms dipping towards the NW (N300°E) and W (N250°E). Synsedimentary tectonics are documented by nine shallow-marine depositional units (unit R 1: conglomerates associated with neptunian dikes; units R 2–9: skeletal carbonates and reefs) bounded by deep erosion surfaces and/or truncation of individual units along a Miocene fault. At both flanks, the shallow-water biofacies record an identical trend of climatic warming and therefore bracket a similar amount of time. Temperate conditions prevailed during the latest Tortonian and earliest Messinian, whereas tropical temperatures existed during the Early and Late Messinian and perhaps during the Tortonian/Messinian transition. The number of depositional units reflecting these climatic periods of time, and the stratal geometries, however, differ at both sites and within the basin, which implies spatial variations in the generation of accomodation space on independent fault blocks. These data also suggest that there cannot be any universal sequence stratigraphic model for Late Miocene shallow-water carbonates of SE Spain, and that the downstepping geometries observed in many other Early Messinian reefs of the Betic Cordillera reflect tectonic uplift.

Kontinuierliche und diskontinuierliche Sedimentation im süddeutschen Oberjura (unteres Kimmeridge; Ludwag/Oberfranken, Nördliche Frankenalb)

ZusammenfassungIm Neuen Steinbruch von Ludwag nordöstlich von Bamberg/Oberfranken sind Schwammriffdolomite des Malm beta aufgeschlossen, die ein erhebliches Relief geschaffen haben. In Depressionen dieser Morphologie blieben riffnahe Bankkalke und Mergel des Weißjura gamma bis tiefsten? delta von der Erosion verschont, während ihre Massenkalkäquivalente weitgehend abgetragen wurden.Die stratigraphische Einstufung der einzelnen Schichtglieder erfolgte mittels bio- und lithostratigraphischer Daten. Mikrofaziestypen wurden vorwiegend nach der Textur definiert. Sie gehen auf kontiniuerliche und diskontinuerliche Prozesse der Gesteinsbildung zurück. Erstere liefen an der N-Flanke eines untersuchten Buildups ab; auch die Massenkalke selbst gehören zu dieser Gruppe. Für die Schwammergel wird der Begriff “Pseudobafflestone” vorgeschlagen, da trotz des umfangreichen Schwammwachstums das Relief der Bioherme im Liegenden nivelliert wird und ein Sedimentfangen der Poriferen somit unwahrscheinlich erscheint. Die Biohermkalke bestehen neben Schwämmen und Algen bis zu 50% aus Algen-Pelletkrusten, während sie in den Pseudobafflesotones nahezu vollständig fehlen. Eben dieser Unterschied scheint für das morphologische Absterben der Bioherme verantwortlich zu sein. Die Mergelsedimentation wird mit regressiven Phasen in Verbindng gebracht.Die diskontinierlich gebildeten Faziestypen (Eventstones) finden sich im Malm gamma 2 an der S-Flanke von Mounds bzw. im gamma 3 überall in Wannen-bereichen. Aus dem Vorherrschen der Mikritsedimentation wird auf lange Phasen ruhiger Sedimentations-bedingungen geschlossen, die nur sproradisch von Stürmen unterbrochen wurden.Aus Brekzien des Malm gamma 3 und? delta, mit in ihren Komponenten “eingefrorenen” Diagenese- und Mikrofaziesmerkmalen zu schließen, war der Ablagerungraum der heute total dolomitisierten und/oder abgetragenen, äquivalenten Algen-Schwamm-Bioherme reicher gegliedert als in den älteren Schichten.Die Schuttströme können liefergebietsnah 14 m Mächtigkeit erreichen, verlieren mit dem Transport aber schnell an Mächtigkeit. Suspensionstromablagerungen am Top dieser Dickbänke greifen weit in die neritische Fazies vor, wo sie mikritische Mergelkalke bilden.Die Erhaltungsform von Kieselschwämmen und Sediment ist räumlich verschieden. Diese “Diagenese-merkmals-Assozationen” werden mit LONGMANs Schema diagenetischer environments (1980) erklärt und daraus für den gesamten Malm gamma ein zeitweiliges Trockenfallen der Bioherme abgeleitet.Die Bioherme entstanden auf Schwellen. Innerhalb dieser bildetetn sich kleine Spezialbecken, Intermound-Wannen, aus. Strömungen und die Form der Schwellen steuerten Geometrie und Lagebeziehungen der Mounds.Die Genese der “sekundären Bankung” (SCHIRMER, 1981) und kegeliger Absonderungsflächen an Buildups beruht auf räumlich verschiedenen Kompaktionsbeträgen, bei gleicher Kompaktibilität, der Platynota-Schichten.SummaryIn the Ludwag-quarry (NE of Bamberg, S-Germany) a suquence of dolomitized sponge-algal-buildups, related intermound-limestones and marls is exposed.Facies-analysis and biostratigraphic data indicate to an Upper Oxfordian age of the deeper stages of mound development. In the Lowermost Kimmeridgian biohermgrowth is interrupted by the sedimentation of marls very densely packed with sponges, locally called “Schwamm-mergel”. Because these organisms were not transported, the term floatstone (DUNHAM 1962) cannot be applied. The sponges are deprived of algal-pellet crusts, whereas in boundstones these crust make up 50% of the rock. At the same time the mud mound topography is successively levelled producing reduced bed-thicknesses above the buildups. This means that sponges did not act as “bafflers”. We have named this construction pseudobafflestone (morphological extinction of buildups and sediments densely packed with potential reef builders).Later in the Lower Kimmeridgian carbonate content increases again, giving rise to a renewed bioherm growth and rearrangement. This is somewhat different from conditions in the late Oxfordian, where the bioherms exhibit spatial relation to palaeocurrent layers and trends of the “Würgau-Görau-Reeftract”.At the southern flanks of this younger biochem generation, which are exposed to the Bay of Ludwag, irregular beds of graded Packstones are formed, consisting mainly of micritic intraclasts. The intermound limestones at the northern bioherm flanks are made up of well bedded limestones consisting of ungraded packstones grading laterally into filament-rich micrites. This implies long phases of mud sedimentation and episodical reworking by storms. In the late Lower Kimmeridgian (and Middle Kimmeridgian?) the intermound zone and the Bay of Ludwag is made up of breccias. These debris flow deposits grade laterally into turbidites forming limestone beds of the neritic facies. The clasts can be relatet to several Microfacies Types of massive and bedded limestones. Reworking allows recognition of early diagenetic features such as solution cavities in boundstones and different types of sponge preservation. This and solution coronas around echinoderm grains provides some evidence for subaerial exposure. Dolomites seem mostly to be of late diagenetic origin.

Role of climate in partial drowning of the Queensland Plateau carbonate platform (northeastern Australia)

Abstract Ocean Drilling Program Leg 133 core data, samples, and geophysical data (logs and seismics) were analyzed to document late Miocene-early Pliocene partial drowning of the Queensland Plateau carbonate platform off Northeast Australia. The modern plateau consists of a mosaic of pinnacle reefs and larger (10 × 50 km) reefs representing relicts of early to middle Miocene buildups. Late Miocene-early Pliocene floatstones, packstones, and mudstones, rich in the larger benthic foraminifers Lepidocyclina and Cycloclypeus, drilled in a transect of sites across the drowned margin of a middle Miocene buildup show that the late Pliocene partial drowning of the platform was preceded by 4 Myr of neritic carbonate deposition without any reefs. The carbonate factory was unable to aggrade to sea level during this period as indicated by the lack of any shallowing trend in the succession. Monitoring of the Miocene to Recent neritic shedding pattern on the windward and leeward side of a pinnacle reef on the Queensland Plateau supports this interpretation. Shedding on the leeward side of the reef, which records periods of active reef growth, played a minor role during the late Miocene, increased during the Pliocene, and reached very high values during the late Pliocene and Pleistocene. Shedding on the windward side of the reef, which was active during lowstands of relative sea level, occurred during the late middle-early late Miocene and during the Pliocene-Quaternary. The data presented here, when combined with paleoceanographic data points to low surface water temperatures (17°–19°C) as a major factor which suppressed reef growth during the late Miocene-early Pliocene period.