Ramp Deposits Research Articles

Timing and local perturbations to the carbon pool in the lower Mississippian Madison Limestone, Montana and Wyoming

Coupling of carbon isotope stratigraphy with strontium isotope analysis by laser ablation MC-ICP-MS and biostratigraphy of the Mississippian Madison Limestone (southeastern Wyoming to southwestern Montana) defines a chronostratigraphic framework that constrains surface to subsurface high-resolution sequence stratigraphic relations and provides considerable insight into the nature, origin and evolution of facies variability across the Madison ramp during the Early Mississippian. The δ 13C carb values from eight localities distributed across inner to outer ramp facies exhibit an overall rise and subsequent fall throughout the Kinderhookian and lower Osagean succession reaching maximum values of up to + 7.5‰ PDB. This peak in δ 13C values coincides with least radiogenic carbonate 87Sr/ 86Sr values near the Kinderhookian–Osagean transition and is defined as a maximum flooding surface. Several shorter-term fluctuations in δ 13C carb values are superimposed on this longer-term trend and are also recorded in subsurface carbonates of the inner Madison ramp. The δ 13C carb values show no significant isotopic variability between bulk carbonate and microsampled micrite, calcitic brachiopods and marine cements and are generally independent of facies including dolomitized inner ramp and calcareous outer ramp deposits. However, δ 13C carb values at and around 3rd-order maximum flooding surfaces exhibit a progressive increase in values with distance from proximal regions of the Madison ramp towards the Antler foredeep and with deepening sedimentary facies. We interpret the long-term increase of the δ 13C values and coincident decrease of the 87Sr/ 86Sr as a product of increased productivity and preservation of organic matter in ocean basins with decreased continental weathering rates due to flooding of previously exposed land masses during sea-level rise. The subsequent decrease of δ 13C carb values and concomitant increase of the 87Sr/ 86Sr are interpreted as a product of limited productivity and oxidation of organic matter with accelerated continental weathering rates due to exposure of land masses during sea-level falls. The transramp spatial variation of the δ 13C carb is attributed to increased environmental restriction proximal to land, suggesting that facies of the inner ramp were covered by stranded and aged water masses. Long-term fluctuations of the δ 13C carb values on the Madison ramp in western North America correlate with fluctuations documented from lower Mississippian successions in Belgium and the Urals but the δ 13C carb values from the Madison ramp reach significantly higher maximum values (by up to + 2‰). The more positive δ 13C carb values in western North American basins are interpreted to record local processes including water mass restriction, increased nutrient cycling, and enhanced biological pumping, all superimposed on secular seawater δ 13C variations. The increased δ 13C further documents that local perturbations in carbon cycling in epeiric seas and associated foredeeps can significantly modify the mean oceanic carbon signature, thus epeiric-sea marine δ 13C carb values must be considered as approximations of the global carbon pool. Nevertheless, statistically similar and time-equivalent secular variations in seawater δ 13C are recorded throughout the Madison ramp, permitting the use of lower Mississippian δ 13C carb values as a high-resolution chronostratigraphic tool.

High-frequency cycles in Upper-Miocene ramp-temperate carbonates (Sorbas Basin, SE Spain)

Uppermost-Tortonian temperate carbonates occur at the southern margin of the Sorbas Basin (Almeria, SE Spain). These carbonates, included in the Azagador Member, formed in a gentle, shallow-water ramp. Six facies cycles in ramp deposits comprise alternating bivalve-shell concentrations and coralline algal beds. The basic cycle reflects the landward advance, as relative sea level rose, of coralline algal deposits, which were the facies of the outer ramp, over bivalve biostromes, which grew in the shallower areas of the mid-ramp. Biostromes were mainly built by oysters and locally by Isognomon. In many cases, however, the removal of smaller shells by storms left only thin, discontinuous patches of large bivalves as residual remains of the oyster biostromes. Some original cycles might be missing due to complete removal of bivalve shells from the biostromes. The six cycles recognised, therefore, should be considered as the minimum number of original cycles in the Azagador carbonates. The available age constraints suggest these cycles were forced by orbital precession or some higher-frequency process. Lithological cycles forced by precession are characteristic of the basinal deposits laterally equivalent to the Azagador carbonates.

The Eocene Rusayl Formation, Oman, carbonaceous rocks in calcareous shelf sediments: Environment of deposition, alteration and hydrocarbon potential

Paralic carbonaceous series intercalated among calcareous shelf sediments have seldom been investigated. During the early Eocene, calcareous and siliciclastic sediments were deposited on a wide shelf in front of low-reliefed hinterland in the Al Khawd region in NE Oman. The siliciclastic-calcareous sediments originated from strongly reworked debris of the Arabic Shield. The underlying Semail Ophiolite did not act as a direct source of debris but provided some heat to increase the maturity of carbonaceous rocks and modify the isotope signal of the calcareous minerals in the Rusayl Formation. A multidisciplinary approach involving sedimentology, mineralogy, chemistry, coal petrography and paleontology resulted in the establishment of nine stratigraphic lithofacies units and provides the reader with a full picture from deposition of the mixed carbonaceous-calcareous-siliciclastic rocks to the most recent stages of post-depositional alteration of the Paleogene formations. The calcareous Jafnayn Formation (lithofacies unit 1) developed in a subtidal to intertidal regime, influenced episodically by storms. Deepening of the calcareous shelf towards younger series was ground to a halt by paleosols developing on a disconformity (lithofacies unit 11) and heralding the onset of the Rusayl Formation. The stratigraphic lithofacies units III and IV reflect mangrove swamps which from time to time were flooded through washover fans from the open sea. The presence of Spinozonocolpites and the taxon Avicennia, which today belong to a coastal marsh vegetational community, furnish palynological evidence to the idea of extensive mangrove swamps in the Rusayl Formation [El Beialy, S.Y., 1998. Stratigraphic and palaeonenvironmental significance of Eocene palynomorphs from the Rusayl Shale Formation, Al Khawd, northern Oman. Review of Palaeobotany and Palynology 102, 249-258]. During the upper Rusayl Formation (lithofacies units V through VII) algal mats episodically flooded by marine incursions make up a greater deal of the sedimentary record than mangrove swamps. Terra rossa paleosols mark the end of accumulation of organic material (OM) and herald supratidal conditions at the passage of Rusayl Formation into the overlying Seeb Formation. In the subtidal-supratidal cycles of lithofacies unit VIII the terra rossa horizons are thining upwards and become gradually substituted for by deep-water middle ramp sediments of lithofacies unit IX Framboidal pyrite, (ferroan) dolomite with very little siderite are indicative of an early diagenetic alteration stage I under rather moderate temperatures of formation.

Oxfordian ramp system (La Manga Formation) in the Bardas Blancas area (Mendoza Province) Neuquén Basin, Argentina: Facies and depositional sequences

The outcrops of the Oxfordian La Manga Formation at Bardas Blancas, Neuquén Basin, west-central Argentina, allow the recognition of six different depositional facies (A to F) on the basis of sedimentological analysis, taphonomic attributes and microfacies studies. These depositional facies correspond to outer ramp (A), middle ramp (B), inner ramp— oolitic shoal (C), inner ramp margin (patch reef) (D), lagoon deposits (E), and a paleokarst surface (F). Outer ramp deposits which are not completely represented, consist of greyish carbonate beds, where the fabric of the shell beds (gryphaeids) reflects the action of waves and currents. Middle ramp deposits consist of a packstone–grainstone lithofacies indicating the importance of storm processes and is dominated by ooids, intraclasts, pelecypods, echinoderms and gastropods which accumulated on a middle-ramp storm-dominated shoreface. Trace fossils belong to the Skolithos and Cruziana Ichnofacies characterizing the upper, lower and middle shoreface setting respectively. The inner ramp deposits consist of oolitic grainstones and subordinate packstones shoal with a small sponge bioherm at the base. Different types of ooids, peloids and coated grains are abundant, as well as skeletal fragments of molluscs, echinoderms and corals. Lithofacies and microfacies studies suggest a high energy and shallow-water depositional setting. The inner ramp margin deposits consist of reef core facies, fore and back reef facies characterized by a scleractinian community of relatively low generic diversity. The rich associated fauna consists of bivalves, echinoids, serpulids, bryozoans, dasycladacean algae and cyanophytes, as well as foraminifers and ostracods. The growth forms of the corals are indicative of shallow well illuminated water. Both the back and fore reef deposits suggest intensive reworking by storm waves or currents. The lagoon deposits consist of bioclastic and peloidal wackestones as well as bioclastic–intraclastic packstones which accumulated on a lagoon under intermittently agitated water in a shallow subtidal to intertidal settings. A stratiform breccia with both matrix and clasts supported fabrics is interpreted as paleokarst. The clasts are derived from the rocks of oolitic shoal and inner ramp margin (patch reef). The six depositional facies are included into a major organizational framework of three third-order depositional sequences (DS-1, DS-2, DS-3) mainly represented by transgressive and highstand systems tracts stages with sequence boundaries of regional importance. The general depositional evolution is here related to the slow subsidence experienced during the Oxfordian–earliest Kimmeridgian time related to tectonic inversion in the Neuquén Basin. A four step (architectural and sedimentary) schematic model of the response of the platform to sea-level changes is proposed.

Uniformitarianism as a guide to rocky-shore ecosystems in the geological record

Literature on Pleistocene and Neogene rocky shores is reviewed from a uniformitarian perspective to assess the fidelity of physical and biological information entrained in the geological record through present-day processes. Coverage by latitude spans tropical to subarctic shores. Variations in paleotopography are illustrated by unconformities between sedimentary accumulations and rocks of igneous or non-igneous origin. Former rocky shores occur across a range of geographic scales that include (i) spot localities, (ii) embayments with continuous transition from exposed outer to protected inner shores, and (iii) entire islands with windward and leeward shores. Differences in rate and magnitude of sea-level change affect rocky-shore physiography. Terrace deposits characterize the Pleistocene, but ramp deposits typify older Neogene shores. On average, species with hard parts account for about 50% of the modern rocky intertidal fauna on a regional basis, regardless of latitude. Biodiversity is highest in mid-latitudes. Fixed organisms with encrusting, wedging, or boring habits (corals, barnacles, oysters, other bivalves, and vermetid gastropods) show high potential for fossilization in growth position. Mobile organisms that cling to rocks (gastropods, crabs, and echinoids) suffer post-mortem transport but may be immured within neptunian dikes or interstices among cobbles and boulders. At best, localized fossil deposits from the Pleistocene reflect <30% of the biodiversity of skeletonized organisms tabulated from modern rocky shores on a regional basis. Uniformitarianism provides little insight into the diversity of soft-bodied organisms (excluding stromatolites) that joined the ecosystem in Precambrian times and the extent to which soft-bodied organisms dominated shore life through later times.

Ovummuridae (calcareous microfossils) from a Late Devonian ramp: their distribution, preservation potential, and paleoecological significance

Enigmatic calcareous microfossils that belong to the Ovummuridae Munnecke, Servais, and Vachard, 2000 are present in ten cores through the upper part of the Escarpment Formation and Alexandra Formation in the Hay River area of the Northwest Territories, Canada. These Late Devonian (Frasnian) siliciclastic, mixed carbonate–siliciclastic, and carbonate ramp deposits accumulated on the western margin of Laurussia. Finding ovummurids in these deposits is significant because it provides the first formal documentation that these microorganisms existed in the Devonian and confirms that the stratigraphic range of the family extends from the Lower Silurian to Upper Permian. The microfossils, which are of unknown biological affinity, include Minourella gotlandica, previously known only from Silurian strata, Ovummurus duoportius, and Minourella cameroni sp. nov. The preservation potential of these microfossils was markedly enhanced by thick cement overgrowths, akin to the syntaxial overgrowths that are common on Paleozoic crinoid fragments. The distribution of ovummurids in these ramp deposits is significant because it demonstrates that ovummurids were capable of inhabiting turbid, likely nutrient-enriched marine environments, with significant siliciclastic influx, that were unsuitable for most other carbonate-secreting organisms in the Paleozoic.

A foraminiferal assemblage as a bioevent marker of the main Ladinian transgressive stage in the Betic Cordillera, southern Spain

The Betic Cordillera comprises the mountain range in the southern Iberian Peninsula that extends from Cadiz to Alicante, which displays typical features of alpine cordilleras. The Betic Cordillera includes two large geological tectonic domains, namely an External Zone, and an Internal Zone. The External Zone is essentially composed of epicontinental Triassic rocks that consist of the Buntsandstein, Muschelkalk and Keuper facies. The Buntsandstein facies rarely occur in outcrops, whereas the Keuper stratigraphic successions are truncated. The Muschelkalk facies frequently displays complete successions that are very useful for interpreting the Ladinian stage palaeogeography. The Muschelkalk facies consists of a lower carbonate member formed by three thick, massive limestone beds with intervals of thin-bedded marly limestone, and an upper marly member with intercalation of distinct beds of limestones and marls, where tempestites predominate. The first member has been interpreted as a transgressive systems track in which subtidal and ramp deposits can be identified. The second member is related to a highstand systems track, and consists of marine-platform and coastal deposits, which correspond to a shallowing-upward sequence. We describe and interpret a distinct bioclastic limestone bed with high concentrations of Involutinidae benthic foraminifers, that has been found in the Muschelkalk of several distant sections. This unusual bed is just above the third bed of massive Muschelkalk limestone, and consists of a bioclastic wackestone–packstone, up to 8 cm thick, with many Involutinidae of Ladinian age. The most abundant species are: Lamelliconus gr. biconvexus– ventroplanus, Lamelliconus cordevolicus and Triadodiscus eomesozoicus. These foraminiferal concentrations form a well-defined marker bed that may be related to a bioevent. The appearance of these foraminiferal taxa, which are also very common in the Triassic Alpine realm, coincides with the maximum flooding interval of a transgressive sequence. We surmise that these Tethyan foraminifers moved into newly developed coastal zones during maximum transgression and, at times, produced prolific shallow-water communities, which were reworked by storm currents that caused their unusual concentrations in deeper water. These concentrations thus generated the marker beds of foraminiferal packstones that can be found over distant outcrops in the region. Similar involutinid foraminiferal assemblages occur in the upper Muschelkalk of the Iberian Ranges and Catalan Coastal Ranges, as well as in the upper part of the transgressive systems track of the UAA-2.2 third-order cycle identified in the Muschelkalk of the Pyrenees Basin. Thus, this foraminiferal assemblage can be considered as an extensive bioevent marker related to an important transgressive stage of Ladinian age.

A paleoenvironmental standard section for Early Ilerdian tropical carbonate factories (Corbieres, France; Pyrenees, Spain)

Early Ilerdian (Early Eocene, Shallow Benthic Zones 5 and 6) carbonate systems of the Pyrenees shelf were deposited after a time of severe climatic (‘Paleocene–Eocene Thermal Maximum, PETM’) and phylogenetic (‘Larger Foraminifer Turnover’) changes. They reflect the radiation of nummulitid, alveolinid, and orbitolitid larger foraminifera after remarkable biotic changes at the end of the Paleocene, and announce their subsequent flourishing in the Middle Eocene. A paleoenvironmental model for tropical carbonate environments of this particular time interval is provided herein. During the Early Ilerdian, the inner and middle ramp deposits from Minerve, Campo and Serraduy revealed the end-member of a tropical carbonate factory with carbonate production dominated by the end-members of biotically (photo-autotrophic skeletal) controlled and biotically induced carbonate precipitation. Inner platform environments are dominated by alveolinids and in part by orbitolitids, middle platform environments are dominated by nummulitids. Corals are present, but they do not form reefs, which is a typical feature for the Eocene. Nummulite shoal complexes, which are well-known from the Middle Eocene are also absent during the studied Early Ilerdian interval, which may reflect the early evolutionary stage of this group.

Sediment production and dispersal on foraminifera‐dominated early Tertiary ramps: the Eocene El Garia Formation, Tunisia

AbstractLarger benthic foraminifera Nummulites are common within Eocene, circum‐Tethyan limestones. Despite their importance as sediment producers, contradictions in the literature constrain current understanding about the location of the Nummulites‘factory’. The El Garia Fm was deposited on a ramp with localized palaeohighs, and whilst some authors suggested that the locus of Nummulites production was in shallow water across the palaeohighs, others concluded that production was significantly reduced over these palaeohighs, and concentrated in the surrounding deeper (30–60 m) water. There are also marked dissimilarities between recent models in terms of the continuity, correlation and resolution of depositional sequences. To assess these models, we integrate studies of the architecture and geometry of the El Garia Fm with detailed taphonomic, biometric, biofabric and palaeoecological characterization of Nummulites tests. We conclude that the highest rates of sediment production occurred in euphotic water over the palaeohighs and in nearshore environments. Nummulites on the palaeohighs were transported into the surrounding deeper water by oceanic and storm currents that swept the platform top, producing a nummulitic sediment package that thickened and became increasingly fine‐grained and fragmented into outer ramp environments. This transport exerted a major control on development of the ramp‐like geometries often seen at outcrop. Our findings question the validity of a recent sequence stratigraphic model that identifies decimetre‐scale Milankovitch cycles, even in largely allochthonous, ‘bio‐retextured’, mid/outer ramp sediments. Our findings also suggest that the thin packages of El Garia Fm on the palaeohighs, which have previously been interpreted as condensed sections that can be correlated with thicker, more distal accumulations, actually represent remnants of the sediment that was produced on the highs and ‘exported’ into the basin.

Mixed sediment deposition in a retro-arc foreland basin: Lower Ellis Group (M. Jurassic), Wyoming and Montana, U.S.A.

The “lower” Ellis Group (M. Jurassic) of northern Wyoming and southern Montana affords an excellent opportunity to examine the influence of tectonics, sea-level change, and incipient topography on facies dynamics and the evolution of mixed sediment ramp deposits. The Sawtooth, Piper, and Gypsum Spring formations (Bajocian to Callovian) represent sedimentation along the forebulge of a retro-arc foreland basin. The “lower” Ellis Group records deposition during two transgressive–regressive cycles, (1) a Bajocian-age cycle dominated by evaporites and red shales, and (2) a Bathonian-age cycle characterized by carbonates, evaporites, and red shales. These cycles are capped by a Callovian-age cycle distinguished by carbonates and red shales that is represented by the “lower” Sundance and Rierdon Formations. Transgressive episodes favored intensified chemical sediment production resulting in thick units deposited in subtidal to peritidal environments. Regressive periods are characterized by supratidal redbed progradation and subsequent shallowing–upward cycles. The depositional cycles in the lower Ellis Group developed due the interplay between sea-level change and tectonic subsidence related to the evolution of a retro-arc foreland basin. Differential subsidence before, during, and after deposition created paleohighs that locally influenced accommodation space and, thereby, complicated depositional and erosional patterns. This paper provides a regional framework for further analysis of the depositional history of the lower Ellis Group by addressing the stratigraphic relationships between the Sawtooth, Piper, and Gypsum Spring Formations.

Production from Muschelkalk carbonates (Triassic, NE Netherlands): unique play or overlooked opportunity?

The Triassic Muschelkalk carbonates are a unique play in NW Europe, producing hydrocarbons onshore The Netherlands. This paper aims at a comprehensive description of these relatively unknown reservoirs and speculates on additional play potential outside of the current production areas. The Muschelkalk Formation consists of partially porous Lower and Upper Muschelkalk carbonates separated by tight Middle Muschelkalk evaporites. The 100 m thick Lower Muschelkalk holds 1.9 BCM gas in the De Wijk Field. The reservoir is a limestone–dolomite–marl succession producing from several decimetres thick calcisilitic dolo-mudstones encased in tight lime-mudstones. The reservoir shows a stratified architecture composed of 16 cycles with thin but laterally continuous porous units. Good reservoir quality facies are concentrated in the upper third of these cycles. Porosities vary from 6 to 27%, (average 12%) and permeabilities range from 0.01 to 32 mD (average 0.06 mD). The reservoir rocks represent inner ramp sediments of a storm-dominated epeiric ramp. The 50 m thick Upper Muschelkalk contains 0.8 BCM gas in the Coevorden Muschelkalk Field. The reservoir consists of several decimetres thick peloidal-oolitic dolo-grain to packstones of good permeability interbedded with lower permeability dolo-mudstones to wackestones. Porosity varies from 5 to 29% (average 15%) and permeability ranges from 0.01 to 57 mD (average 1 mD). The permeable dolo-grainstones occur chiefly around peak transgression and early regression of the Upper Muschelkalk. The producing Upper Muschelkalk deposits represent the inner ramp section of a storm-dominated homoclinal ramp. Numerous gas shows in the Lower and Upper Muschelkalk, scattered over the onshore territory of The Netherlands, suggest possible missed pay and upside potential. The highest reservoir potential occurs in grainy dolomitic inner ramp deposits. These are likely to be located close to the existing fields in the NE Netherlands. Additionally, currently unrecognized potential could be present in southern Germany and eastern Poland.

Microfacies and Paleoenvironment of Sinjar Formation (Paleocene-Early Eocene), Sinjar Area, Northern Ir

Micofacies analysis and faunal evidence from the stratigraphic sequence of Sinjar Limestone Formation (Paleocene-Early Eocene) integrate three units accumulated in different zones on an isolated platform with bank-ramp depositional profile. These include: (1) a bank-rim buildup composed of red algae with subordinate coral boundstones, and interior bank dominated by nummulitic shoal-bar grainstone, (2) a backbank lagoon-bay of finer-grained carbonates punctuated by miliolids-alveolinids shoal bar and green algae, and (3) forebank rudstones-grainstones of red algal-coral and Nummulites. Paleoecological deduction point to a warm-temperate climate with colder periods during the overgrowth of red-algal bank which had not acted as effective barrier to water circulation. The genesis of the Sinjar carbonate bank can be expressed by alveolinids, miliolids and Nummulites as inner-middle ramp deposits of euphotic-mesophotic zone comprising foram shoals-lagoon-bay behind the domain of coralgal bank in meso-oligophotic zone.

Characterizing structural and lithologic controls on deep-seated landsliding: Implications for topographic relief and landscape evolution in the Oregon Coast Range, USA

In mountainous areas, landslides regulate temporal variations in sediment production and may suppress simple linkages between topographic development and tectonic forcing. Rates and mechanisms of mass wasting depend on lithology, bedrock structure, and climatic and tectonic setting. These factors tend to vary signifi cantly in active tectonic regions, thus clouding our ability to predict how landsliding modulates topographic development over human and geological time scales. Here, we use a novel DEM-based technique to document the distribution of large landslides in the Oregon Coast Range (OCR) and quantify how they affect topographic relief. We developed an automated algorithm that exploits the distinctive topographic signature (specifi cally the relationship between curvature and gradient) of large landslides to map their distribution within the gently folded Tyee Formation (Eocene deltaicsubmarine ramp sediments). In contrast to steep and highly dissected terrain frequently identifi ed as characteristic of the OCR (which exhibits steep, planar sideslopes and highly curved, low-gradient ridgetops and valleys), terrain prone to large landslides tends to have low values of both drainage density and curvature and gradient values that cluster between 0.16 and 0.44. The distribution of failure-dominated terrain in our 10,000 km2 study area is infl uenced by systematic variations in sedimentary facies and bedrock structure. The fraction of terrain altered by large landslides (>0.1 km2 ) varies from 5% in the sand-rich (delta-slope and proximal ramp facies) southern section of our study area to ~25% in the north (distal ramp facies), coincident with an increase in the thickness of siltstone beds and a decrease in the sandstone:siltstone ratio. Local relief declines progressively northward, suggesting that deep-seated landsliding is sensitive to the thickness and frequency of low-shearstrength siltstone beds and may serve to limit topographic development in the OCR. Structural controls are superimposed on facies-related variations as deep-seated landslides are frequently found on slopes whose downslope aspect corresponds to the bedrock dip direction. For 1516 strike and dip measurements in our study area, we calculated the fraction of proximal terrain (<2.5 km) altered by deep-seated landsliding. In the sand-rich southern region, the proportion of proximal slide-dominated terrain increases modestly with bedrock dip. In the silt-rich northern region, terrain altered by deep-seated landsliding is pervasive, and an increase in dip from 0° to 16° corresponds to a change in the fraction of slide-prone terrain from 10% to 28%. Our technique for mapping large landslides has utility for hazard analysis and land management. Over million-year time scales, the progradational character of the Tyee Formation suggests that continued uplift and exhumation of the OCR should result in a southward propagation of slide-prone, silt-rich distal facies. As a result, deep-seated landsliding will become increasingly prominent, and topographic relief in the central and southern OCR will progressively decline. Whereas spatial variability in climatic or tectonic forcing is often invoked to explain systematic variations in topographic development, our results emphasize the importance of structural and intraformation lithologic controls on landsliding. As such, analyses linking surface processes, climate, tectonics, and landscapes should be couched in the context of diverse geologic and topographic data.

Paleoenvironments and growth of early Neoproterozoic calcimicrobial reefs: platformal Little Dal Group, northwestern Canada

Although the calcimicrobial reefs of the basinal Little Dal Group (<1080 Ma; >780 Ma) are well-described, little is known about related shallow-water facies to the southeast. Study of these platformal carbonates shows that they represent inner, mid- and shallow outer ramp zones of a homoclinal, storm-influenced ramp. Seven depositional sequences are recognized and correlated with reef-bearing strata in the Little Dal basin and in the Shaler Supergroup of Victoria Island. Reef distribution is distinctly depth partitioned across the Little Dal ramp. Inner and mid-ramp carbonates contain meter-scale mounds, patch reefs, and biostromes composed of fine-grained stromatolites including Tungussia, Baicalia, and Jurusania. These reefs are strikingly similar to those reported from coeval shallow-water strata in the Shaler Supergroup of Victoria Island and the Bitter Springs Formation of Australia. Multi-storey stromatolitic biostrome/bioherm complexes greater than 100 m in height, seem to be restricted to the mid-ramp zone of the Little Dal ramp and probably reflect high rates of subsidence in the Mackenzie Basin. Calcimicrobial reefs occur only in the deepest water strata of the study area, associated with outer ramp sediments. These reefs contain calcified cyanobacterial filaments in varying degrees of preservation, and are petrographically indistinguishable from the kilometre scale basinal calcimicrobial reefs to the northwest. The association of calcimicrobial reefs with deep, low-energy environments suggests initial establishment of reef-building calcimicrobes in deep-water benthic environments.

Eustatic and palaeogeographic control of the western Aurès Upper Cretaceous sedimentation (Algeria)

Abstract The Upper Cretaceous sedimentation in the Aurès Mountains occurred in a subsident basin delimited to the south by the Saharan platform and by the Preatlasic high zone to the north. In these series 4 transgressive-regressive megasequences are distinguished, the first one (I) in the late Albian-Cenomanian, the second one (II) in the Turonian, the third one (III) in the Coniacian - Santonian and the fourth one (IV) in the Campanian - Maastrichtian. Each megasequence is made up of three or four sequences, which correspond to third order cycles identified in the eustatic chart of Haq et al. [1987]. In late Albian and lower Turonian periods, during the deposition of basal sequences of the megasequences I and II, the reactivation of basement faults in the Aurès basin occurs consecutively to distension phases, resulting in the formation of rotated blocks. At the same time high eustatic levels are reached according to the global eustatic curve. These processes control the drowning of the carbonate platforms pre-existing to these sequences, and deep ramps progressively form on the tilted block tops. This sedimentary setting generates in the late Albian and lower Turonian series anoxic sequences made up of calcareous and shaly transgressive pelagic intervals. The succeeding sequences lack pelagic facies and are composed of alternate marls / carbonate beds deposited on a homoclinal ramp, indicating a gradual development of shallow open marine conditions, which became progressively restricted upwards. Toward the top of these megasequences, lagunal muds and isolated rudists mounds, surrounded by bioclastic and ooid / pellet banks occur. The Coniacian-Santonian and Campanian-Maastrichtian megasequences are characterized by a shallow ramp sedimentation, essentially marly during the Coniacian, Santonian and Campanian periods, interlayered with some bioclastic / ooid carbonate banks and upwards by sequences mostly homogeneous. The Maastrichtian platform carbonates are composed of bioclastic / ooid sands and were deposited in a ramp-barrier-bank system. Some sequences in the Campanian-Maastrichtian megasequence are condensed or absent due to the accommodation reduction related to a weak subsidence rate period.

Ladinian carbonates of the Cabo Cope Unit (Betic Cordillera, SE Spain): a tethys-maláguide palaeogeographic gateway

The Cabo Cope Unit, which outcrops east of Aguilas (Murcia), belongs to the Malaguide tectonic Complex (Betic Internal Zone) and displays stratigraphic characteristics of particular interest, including Triassic bioclastic carbonate beds which are not common in the Malaguide units. Biostratigrafic fossils have been found in these beds and may correlate with Triassic alpine biofacies. Alpine fauna fossils only appeared in those palaeogeographic units of the Internal Zone of the Cordillera referred to as Alpujarride units, while the influence of the Sephardic faunal province is evident in almost all the cordillera. For these reasons it is noteworthy that new alpine fauna fossils have been found in an Internal Zone unit in which relevant fossils rarely appear. The Triassic succession of the unit studied in this paper can be subdivided into two members: a lower one, which is clastic and contains thick gypsum beds, and an upper one, consisting of carbonate rocks. The lower member has been interpreted as a fluvial-coastal deposit. The upper member is interpreted as a sequence of carbonate ramp deposits. This ramp evolved into a shallow platform with tidal flats typical of a coastal zone. The bivalve fossilsDaonella cf.lommeli (Wissmann) and “Posidonia” sp. have been found in the carbonate member, along with the conodontSephardiella mungoensis (Diebel). These fossils are of the Late Ladinian age and have been found only in this outcroup of the Betic Cordillera. The presence of this fossil assemblage, which belongs to the alpine faunal province, indicates a connection during the Late Ladinian between the Tethys sea and this area of the Malaguide palaeogeographic domain. The palaeogeographic location of the Cabo Cope Unit during the Middle Triassic was at the south-easternmost part of the Betic Basin, implying that the connection between the Tethys and the Betic Basin was established in the easternmost domains of the basin.

Evidence of synsedimentary tectonics in the Northern Tabas Block, East-Central Iran: The Callovian (Middle Jurassic) Sikhor Formation

The Sikhor Formation (new) is a predominantly siliciclastic sediment package intercalated between the marly-silty Baghamshah Formation (below) and the calcareous Esfandiar Limestone and Qal'eh Dokhtar Limestone formations (above). All stratigraphic evidence points to an Early Callovian age of the formation. The Sikhor Formation is restricted to the southern and central Shotori Mountains and consists of two members: The Kuh-e-Neygu Member (new) is composed of fluvialdeltaic conglomerates, sandstones, and siltstones grading into marly silt of the Baghamshah Formation. The overlying Majd Member (new) is characterised by mixed siliciclastic-carbonate sediments that record the interfingering of carbonate ramp sediments with fluvialdeltaic sands and silts. Evidence of erosional truncation of the underlying Baghamshah Formation and confinement of the siliciclastic sediment to a comparatively narrow, NNW-SSE elongated strip suggest that the formation had its origin in the asymmetric uplift of a westdipping tilted fault block in the southern Shotori Mountains that shed its sediment predominantly in a northern and eastern direction. After erosional levelling, the former uplifted areas were overgrown by the highly productive Esfandiar Carbonate Platform. The Sikhor Formation thus is evidence of an extensional tectonic pulse in the early Callovian and underlines that this area of the Tabas Block was a tectonically highly unstable area during most of the Jurassic.

Sedimentological and palaeohydrological responses to tectonics and climate in a small, closed, lacustrine system: Oligocene As Pontes Basin (Spain)

ABSTRACT A small, closed, lacustrine system developed during the restraining overstep stages of the Oligocene As Pontes strike‐slip basin (Spain). The increase in basin accommodation and the headward spread of the drainage, which increased the water input, triggered a change from shallow, holomictic to deeper, meromictic conditions. The lower, shallow, lacustrine assemblage consists of mudstone–carbonate cycles recording lacustrine–palustrine ramp deposition in a saline lake. High Sr content in some early diagenetic calcites suggests that aragonite and calcite made up the primary carbonate muds. Early dolomitization took place together with widespread pedogenic activity. The upper, deep, freshwater, lacustrine assemblage includes bundles of carbonate–clay rhythmites and fine‐grained turbidite beds. Primary calcite and diagenetic siderite make up the carbonate laminae. The Mg content of the primary carbonates records variations in Mg/Ca ratios in lacustrine waters. δ18O and δ13C covariance trends in calcite reinforce closed drainage conditions. δ18O data indicate that the lake system changed rapidly from short‐lived isotopically light periods (i.e. from seasonal to pluriannual) to longer steady‐state periods of heavier δ18O (i.e. from pluriannual to millennial). The small δ13C changes in the covariant trends were caused by dilute inflow, changing the contributions of dissolved organic carbon in the system and/or internal variations in lacustrine organic productivity and recycling. In both shallow and deep carbonate facies, sulphate reduction and methanogenesis may account, respectively, for the larger negative and positive δ13C shifts recorded in the early diagenetic carbonates (calcite, dolomite and siderite). The lacustrine system was very susceptible to high‐frequency, climatically forced water balance variations. These climatic oscillations interfered with the low‐frequency tectonic and morphological changes in the basin catchment. This resulted in the superposition of high‐order depositional, mineralogical and geochemical cycles and rhythms on the lower order lacustrine infill sequence.

Depositional history of pre-Devonian strata and timing of Ross orogenic tectonism in the central Transantarctic Mountains, Antarctica

AbstractA combination of field mapping, detailed sedimentology, carbon isotope chemostratigraphy, and new paleontological finds provides a significantly improved understanding of the depositional and tectonic history of uppermost Neoproterozoic and lower Paleozoic strata of the central Trans ant arc tic Mountains. On the basis of these data, we suggest revision of the existing stratigraphy, including introduction of new formations, as follows. The oldest rocks appear to record late Neoproterozoic deposition across a narrow marine margin underlain by Precambrian basement. Siliciclastic deposits of the Neoproterozoic Beardmore Group—here restricted to the Cobham Formation and those rocks of the Goldie Formation that contain no detrital components younger than ca. 600 Ma—occupied an inboard zone to the west. They consist of shallow-marine deposits of an uncertain tectonic setting, although it was likely a rift to passive margin. Most rocks previously mapped as Goldie Formation are in fact Cambrian in age or younger, and we reassign them to the Starshot Formation of the Byrd Group; this change reduces the exposed area of the Goldie Formation to a small fraction of its previous extent. The basal unit of the Byrd Group—the predominantly carbonate ramp deposits of the Shackleton Limestone—rest with presumed unconformity on the restricted Goldie Formation. Paleontological data and carbon isotope stratigraphy indicate that the Lower Cambrian Shackleton Limestone ranges from lower Atdabanian through upper Botomian.This study presents the first description of a depositional contact between the Shackleton Limestone and overlying clastic units of the upper Byrd Group. This carbonate-to-clastic transition is of critical importance because it records a profound shift in the tectonic and depositional history of the region, namely from relatively passive sedimentation to active uplift and erosion associated with the Ross orogeny. The uppermost Shackleton Limestone is capped by a set of archaeocyathan bioherms with up to 40 m of relief above the seafloor. A widespread phosphatic crust on the bio herms records the onset of orogenesis and drowning of the carbonate ramp. A newly defined transitional unit, the Holyoake Formation, rests above this surface. It consists of black shale followed by mixed nodular carbonate and shale that fill in between, and just barely above, the tallest of the bioherms. This formation grades upward into trilobite- and hyolithid-bearing calcareous siltstone of the Starshot Formation and alluvial-fan deposits of the Douglas Conglomerate. Trilobite fauna from the lowermost siltstone deposits of the Starshot Formation date the onset of this transition as being late Botomian.The abrupt transition from the Shackleton Limestone to a large-scale, upward-coarsening siliciclastic succession records deepening of the outer platform and then deposition of an eastward-prograding molassic wedge. The various formations of the upper Byrd Group show general stratigraphic and age equivalence, such that coarse-grained alluvial-fan deposits of the Douglas Conglomerate are proximal equivalents of the marginal-marine to shelf deposits of the Starshot Formation. Paleocurrents and facies distributions from these units indicate consistent west (or southwest) to east (or northeast) transport of sediment. Although the exact structural geometry is unknown, development of imbricate thrust sheets in the west likely caused depression of the inner margin and rapid drowning of the Shackleton Formation carbonate ramp. This tectonic activity also caused uplift of the inboard units and their underlying basement, unroofing, and widespread deposition of a thick, coarse clastic wedge. Continued deformation in the Early Ordovician (younger than 480 Ma) in turn affected these synorogenic deposits, causing folding and thrust repetition of all pre- Devonian units.

Jens Munk Archipelago: Ordovician‐Silurian Islands in the Churchill Area of the Hudson Bay Lowlands, Northern Manitoba

The name “Jens Munk Archipelago” is proposed for a cluster of small monadnocks that rise a maximum of 45 m above present sea level on the shores of Hudson Bay in the Churchill area of northern Manitoba. Resistant Precambrian quartzite formed shoals and islands during the Late Ordovician and Early Silurian periods. Outcrop control in the present tundra environment of the Hudson Bay Lowlands is spotty at best, but the quartzitic monadnocks are fringed by Upper Ordovician and Lower Silurian ramp deposits, mainly carbonate in composition. Five isolated outliers of Ordovician‐Silurian strata are described, and the location of four other potential outliers is indicated. Ramp dip at any one site generally ranges from 10° to 15°, and the surviving ramps on opposite flanks of paleoislands suggest preservation of near synsedimentary conditions. Extensive conglomerates of reworked quartzite are most abundant on a northern exposure within the islands. Ripple marks and oriented cephalopod debris are related to a vigorous long‐shore current generated by northeasterly trade winds that emanated from a subtropical high‐pressure cell in the Northern Hemisphere. A paleogeographic reconstruction links the Jens Munk Archipelago of Manitoba to a larger landmass with a Precambrian quartzite core centered over northern Saskatchewan. The general paleoecology of the Jens Munk Archipelago conforms to patterns of windward and leeward facies preserved on other paleoislands of various ages.