Siliciclastic Shelf Research Articles

Organic geochemistry in a sequence stratigraphic framework. The siliciclastic shelf environment of Cretaceous series, SE France

For a better understanding of siliciclastic shelf environments, correlation between sequence stratigraphy and organic geochemistry is used. Our study is focused on the Cretaceous deposits of Marcoule (Gard, France), particularly on a close-packed siltites layer (200–400 m thick), which is well characterized as a marine flooding facies of a single trangressive–regressive cycle. During the Uppermost Albian and the Lower Cenomanian, the stratigraphic data indicate a change in the depositional environment from offshore to shoreface. Organic geochemistry is used in order to characterize origin and variability of the organic matter in relation to the stratigraphic data. The study is carried out on core samples from 2 drill holes (MAR 203 and MAR 501). Analyses of the aliphatic and aromatic hydrocarbons were performed using GC–MS and focused on biomarker distributions. The biomarkers indicate a contribution of mixed terrestrial and marine organic matter. The changes in molecular signatures are related to variations in the source of organic matter (marine versus terrestrial), preservation conditions (largely influenced by clay and early diagenesis), environmental oxidation-reduction and acidic conditions as well as bioturbation. Various environmental zones, characterized by different molecular signatures, can be distinguished. Resin derived biomarkers can be assigned to higher plant material input and may reflect the evolution and diversity of Gymnospermae versus Angiospermae during the transgressive/regressive cycle. The relative sea-level variations are clearly correlated with the nature and the preservation of the organic matter. For example, the Pr/Ph and Pr/n-C17 ratios as well as the regular steranes distributions underline the maximum flooding surface evidenced by other studies. We observe a good correlation between the organic data and sequence stratigraphy: changes in geochemical signatures reflect the 3rd order depositional cycles.

Quantifying the sequence stratigraphy and drowning mechanisms of atolls using a new 3‐D forward stratigraphic modelling program (CARBONATE 3D)

ABSTRACTThis paper describes a new 3‐D forward numerical model (CARBONATE 3D) that simulates the stratigraphic and sedimentological development of carbonate platforms and mixed carbonate–siliciclastic shelves by simulating the following sedimentary processes: (1) Carbonate shallow, open‐marine production, dependent on water depth, restriction and sediment input; (2) Carbonate shallow, restricted‐marine production, dependent on water restriction; (3) Pelagic sediment production and deposition; (4) Coarse and fine siliciclastic input; (5) Erosion, transport and redeposition of sediment, dependent on currents, slope, depth and restriction as well as sediment grain‐size and composition; (6) Dissolution of subaerially exposed carbonate. In this paper the model is used to investigate the controlling mechanisms on the sequence stratigraphy of isolated carbonate platforms and atolls and to predict distinctive architectural signatures from different drowning mechanisms. Investigation of the mechanisms controlling atoll strata shows that although relative sea‐level is the major control, antecedent topography, environmental setting and early diagenesis have profound influence on what stratigraphic geometries and facies develop. Hence care must be taken if sea‐level curves are interpreted from real stratigraphies. Atoll drowning by fast sea‐level rise, by lowered production and by repeated exposure and fast subsequent sea‐level rises are investigated and different stratigraphic signatures for the respective mechanisms predicted. A fast relative sea‐level rise results in a bucket‐shaped morphology developed prior to drowning and a sharp transition from the platform margin facies to a pelagic cover. Drowning caused by lowered platform margin production is predicted to result in the development of a dome‐shaped, shallow‐water shoal over the whole platform top prior to drowning. Fourth order amplitudes of several tens of metres, typical of ‘icehouse’ settings, cause atoll drowning at subsidence rates where atolls subject to fourth order amplitude of only a few metres, typical of ‘greenhouse’ settings, can keep up with the rising sea‐level. In the resultant strata, vertical facies belts are less well developed but horizontally extensive facies bands are more prominent. High fourth order amplitudes (up to 80 m) without sufficient third order scale subsidence will not lead to drowning, however, as the platform can recover in each fourth order lowstand. These results suggest that atolls might be easier to drown in ‘icehouse’ rather than in ‘greenhouse’ conditions but only in situations with suitably high rates of longer‐term relative sea‐level rise or sufficient lag times.

Late Ordovician brachiopod communities of southeast China

The Upper Ordovician (mid-Ashgill) Xiazhen and Changwu formations in the Zhejiang-Jiangxi border region of southeastern China contain an abundant and diverse suite of brachiopods. A cluster analysis of 59 key samples from 180 collections (over 23 000 specimens) allowed delineation of eight distinct brachiopod communities, herein named the Ectenoglossa minor Community, the Eospirifer praecursor Community, the Ovalospira dichotoma Community, the Antizygospira liquanensis – Sowerbyella sinensis Community, the Kassinella shiyangensis Community, the Altaethyrella zhejiangensis Community, the Tcherskidium jiangshanensis Community and the Foliomena folium Community. These communities occur predominantly in limey, clayey, to silty mudstone facies, similar to Ziegler's Early Silurian brachiopod communities in the Welsh Borderland. On the basis of lithological, faunal, and paleogeographical evidence, the eight communities are interpreted to have distributed along an onshore–offshore gradient from BA1 to BA6 settings on the northeastern margin of the Zhe-Gan Platform. In a broad paleo-embayment and mixed carbonate–siliciclastic shelf setting, the distribution of the eight brachiopod communities appears to have been controlled primarily by water depth because a given brachiopod community can be found in different lithofacies. Substrate types, however, must have played an important role, as some of the communities show preferred lithofacies. This study shows that the ecological zonation and community organization of the Late Ordovician pre-extinction brachiopods were similar to those of Early Silurian post-extinction brachiopods, although pronounced brachiopod provincialism during the Ashgill makes it difficult to directly correlate these communities with coeval brachiopod communities reported from other paleo-plates.

A glacial sequence stratigraphic model for temperate, glaciated continental shelves

Abstract Temperate, glaciated continental shelves are by nature complex basins in that they not only have typical low-latitude siliciclastic processes acting to produce a sedimentary record and depositional architecture, but they also have the consequences of glacial action superimposed. For these settings glacial systems tracts are defined and related to glacial advance and retreat signatures, which can then be evaluated relative to changes in other external forces. We define glacial maximum (GMaST), retreat (GRST), minimum (GMiST) and advance (GAST) systems tracts separated by bounding discon-formities, which are respectively, the grounding-line retreat surface (GRS), the maximum (glacial) retreat surface (MRS) and the glacial advance surface (GAS). Each glacial advance and retreat sequence is bounded by a regionally significant unconformity, a glacial erosion surface (GES), or its equivalent conformity. Parasequence motifs vary across the shelf but include facies dominated by the movement of grounding lines. Facies motifs may have subglacial till above a GES and this is the main facies of the GMaST; however, tills are commonly absent. Although diamictic debrites are often associated with groundingline deposystems, the GRST succession is dominated by sorted deposits of gravel, rubble and poorly sorted sands due to the dominance of glacial meltwater. These deposits commonly have the geometry of banks or wedges/fans. They form the offlap-break at the outer continental shelf and also form a retrogradational stacking of bank systems on the shelf. Banks are capped by glacimarine rhythmites including thin debrites, turbidites, cyclopsams and cyclopels and perhaps iceberg-rafted varvites in fan to sheet-like geometries. Above these are draped sheets of bergstone muds that grade into paraglacial muds. The GMiST occurs with glacial retreat onto land or into fjords. The GMiST is represented in nearshore areas by progradational deposits of paralic systems dominated by deltaic and siliciclastic shelf systems, and in offshore areas by condensed sections. The GAST is represented by the inverse of the GRST facies succession, but is also the most likely interval to be eroded during readvance.

ABSTRACT: Evolution of the Belize Barrier Reef: from an Isolated Carbonate Platform to a Rimmed Mixed Carbonate Siliciclastic Shelf

ABSTRACT: Evolution of the Belize Barrier Reef: from an Isolated Carbonate Platform to a Rimmed Mixed Carbonate Siliciclastic Shelf

The La Luna formation: chemostratigraphy and organic facies in the Middle Magdalena Basin

A detailed geochemical study and a sequence stratigraphic interpretation have been conducted on a sedimentary sequence of the Upper Cretaceous La Luna Formation, in a section outcropping in the eastern flank of the Middle Magdalena Basin (MMB), Colombia. The goals were to evaluate geochemical variability related to lithofacies and organic facies changes, characterize depositional environment and investigate the possible relationship between geochemical data and sequence stratigraphic cycles. The La Luna Formation is composed of organic-rich sediments of monotonous appearance, with good to excellent potential for oil generation. Most of the bulk, petrographic and biomarker parameters display a relatively narrow range of variation. However, the geochemical variations are sufficient to differentiate organic facies types B, BC and C in the Salada Member, B and D in the Pujamana Member and B in the Galembo Member. Certain biomarker ratios are consistent within the La Luna Formation and are characteristic of its depositional environment, for example, average ratios of diasterane/sterane are lower than 1, Ts/Tm averages are less than 0.33, the C35/C34 hopane ratio is more than 0.92, and oleanane/C30 hopane ratios range from 0.02 to 0.19. Regarding depositional condition indicators, the C35/C34 hopane ratio shows a good positive correlation with HI. This suggests that in carbonate environment changes in this parameter are more strongly related to redox condition than to changes in carbonate content. Regarding the possible relationship between organic matter characteristics and sea level changes, in regressive carbonate shelves during shallow stages, HI tends to increase and TOC tends to decrease, while in regressive siliciclastic shelves, both TOC and HI decrease continuously. Some biomarker ratios (oleanane/C30 hopane, C20/C23 tricyclic, Ts/Tm) increase during base level falls. Regarding δ 13C/12C isotope composition, the aromatic fraction and whole bitumen display an isotopic shift associated to the main deepening event in the section.

Depositional and diagenetic signatures of Late Eocene–Oligocene sediments, South Carolina

Abstract Surficial and near-surface soils of the South Carolina Coastal Plain reflect a variety of lithologies and depositional environments that are difficult to differentiate because of intense leaching and abrupt or laterally inconsistent facies changes. Binocular microscopic examination, scanning electron microscopic/energy dispersive X-ray (SEM/EDX) observations, and X-ray diffraction (XRD) analyses indicate that onshore Late Eocene to Late Oligocene Barnwell Group sediments are transitional facies ranging from high-energy fluvial deposits to offshore siliciclastic shelf sands. Interfingering of the units results in alternation of mineralogic signatures within a low-gradient fluvial/transitional/marine depositional system. Late Eocene and Early Oligocene offshore sediments were deposited in a mixed carbonate–siliciclastic, middle- to outer-shelf environment that was subjected to periods of erosion or non-deposition during transgressive events. Detrital and diagenetic characteristics of the onshore kaolinite-enriched, Late Oligocene Upland Unit sediments reflect deposition in a high- to low-energy fluvial system. Differentiation between these uppermost sediments and the underlying low-energy fluvial deposits of the Late Eocene Tobacco Road Sand is based on distinctive hydroxy-interlayered vermiculite (HIV) signatures. Intervals of HIV-enrichment are coincident with accumulations of carbonaceous material and identified as paleosols; these “soils” are used to infer offshore transgressive periods. Onshore sediments of the Late Eocene Dry Branch Formation contain high concentrations of smectite and flocculated, relatively poorly crystallized kaolinite flakes reflective of marine depositional conditions. At the base of this unit, authigenic Ca-minerals (Ca-zeolites and calcite) and quartz lepispheres (opal-CT) form coatings on and between sand grains. Late Eocene siliceous microfossils that contribute to opal-CT formation are identified in southwestern North Atlantic continental margin shelf, slope and rise deposits and are a valuable correlation tool for onshore sediments.

Mississippian carbonate ramp-to-basin transitions in south-central New Mexico; sequence stratigraphic response to progressively steepening outer-ramp profiles

ABSTRACT The sequence stratigraphic architecture of Mississippian strata from the Sacramento and San Andres Mountains, south-central New Mexico, illustrates the combined effects of relative sea-level change and progressively increasing depositional gradients on sedimentary processes and resultant stratal geometries along the outer parts of a carbonate ramp-to-basin transition. Mississippian strata can be subdivided into three progradational sequences (Sequences 1-3) and a fourth, basin-restricted, onlapping sequence (Sequence 4). Sequences 1-3 contain wackestone- to mudstone-dominated transgressive systems tracts that are overlain by packstone- to grainstone-dominated progradational systems tracts. Progradational systems tracts created distinct ramp-to-basin transitions (outer ramp margin) that are characterized by basinward stratal thinning and a concomitant change to muddy lithofacies. Sequence 4 consists of onlapping, fining-upward stratal packages of peloidal-skeletal grainstone that upward to laminated, spiculitic lime mudstone. The outer-ramp-margin facies of Sequences 1 through 3 gradually steepened the outer ramp profile until the cumulative effects of the steepening caused the depositional gradient to surpass an grade (i.e., the gradient at which deposition and erosion/bypass are about equal). Once the depositional gradient exceeded the equilibrium for the caliber of sediment supplied to the outer ramp margin, widespread submarine erosion and sediment bypass on the outer ramp was initiated, resulting in deposition of basin-restricted strata of Sequence 4. Similar relationships and inferred processes have been recognized in progradational siliciclastic shelf deposits and are collectively described by previous workers as slope readjustment. The Mississippian ramp-to-basin transition in south-central New Mexico illustrates how sediment deposition, dispersal, and erosion responded to evolving depositional gradients and in turn controlled resultant, large-scale stratal geometries.

Lithostratigraphy, sedimentary evolution and sequence stratigraphy of the Upper Proterozoic Lyell Land Group (Eleonore Bay Supergroup) of East and North-East Greenland

The Late Proterozoic Lyell Land Group is an approximately 3 km thick succession of siliciclastic shelf deposits, within the upper part of the Eleonore Bay Supergroup. It is widely exposed in the region between Ardencaple Fjord in the north and Canning Land in the south. In this paper the seven formations named by Sønderholm & Tirsgaard (1993) are formally described. These are from base to top: the Kempe Fjord Formation (400-600 m thick), the Sandertop Formation (200-405 m thick), the Berzelius Bjerg Formation (250-450 m thick), the Kap Alfred Formation (500-640 m thick), the Vibeke Sø Formation (290-325 m thick), the Skjoldungebrae Formation (205-240 m thick) and the Teufelsschloss Formation (35-110 m thick).
 Five facies associations have been recognised. Outer shelf deposits dominated by dark green, brown to dark red mudstones with thin sandstone lenses are mainly found in the Sandertop, Kap Alfred and Skjoldungebræ Formations. Storm- and wave-dominated inner shelf deposits comprising fine-grained sandstones and dark heterolithic mudstones are common in the Sandertop, Kap Alfred, Vibeke Sø and Skjoldungebrae Formations and are also found in southern outcrops of the Teufelsschloss Formation. Tidally influenced shoreface deposits form stacks of laterally extensive sandstone bodies separated by heterolithic mudstones and are only found in the middle part of the Kap Alfred Formation. Storm- and wave-dominated shoreface deposits comprise highly mature, thick and laterally very extensive sandstone bodies of which a few may be traced for distances exceeding 150 km. This association is present in several intervals within all formations of the Lyell Land Group. Tidally dominated coastal plain deposits consist of stacked sandstone sheets forming laterally extensive, multistorey units separated by heterolithic mudstones and sandstones. These sediments form part of the Kempe Fjord and Berzelius Bjerg Formations and are also found in northern outcrops of the Teufelsschloss Formation. Evidence from palaeocurrent data combined with regional lithological variations suggest a consistent general N-S coastline with the basin deepening in an eastward direction. Deflection of geostrophic currents suggest a palaeolatitude on the southern hemisphere.
 The deposits of the Lyell Land Group are subdivided into four, large-scale sequences which overall show the same general sedimentary evolution through time reflecting large-scale, cyclic changes in relative sea-level. The sequences vary in thickness from 400-1000 m and are all readily traceable 300 km parallel and 100 km perpendicular to inferred palaeocoastline. The development of all sequences indicates that major regional translation of facies are related to large-scale forced regressions. Sequence stratigraphic considerations suggest that correlation of formations of the Lyell Land Group with units of the Petermann Bjerg Group some 75 km to the west may be very difficult to carry out.
 Citation: Tirsgaard, H. & Sønderholm, M. 1997: Lithostratigraphy, sedimentary evolution and sequence stratigraphy of the Upper Proterozoic Lyell Land Group (Eleonore Bay Supergroup) of East and North-East Greenland. Geology of Greenland Survey Bulletin 178, 60 pp.

An integrated environment analysis—lithofacies, chemofacies, biofacies—of the Oligocene calcareous-siliciclastic shelf deposits in northern Germany

During the Oligocene monotonous fine-grained siliciclastic and calcareous shelf sediments of the Septaria Clay Fm. were deposited in the North German Basin. The uniform grain size spectrum and the homogeneous outward appearance of these sediments forces environmental analysis to be based on a joint investigation of the inorganic and organic facies. The transgression of the Rupelian sea caused tidal sand sheets to form off the mouth of estuaries. Upon deepening of the basin the benthonic oxygen content of the bottom water diminished and inner shelf muds were deposited. Following this early transgression, a conspicuous change may be recognized when a siliciclastic shelf was replaced by a calcareous-siliciclastic shelf upon shoaling of the basin and warming of the seawater. These outer shelf conditions persisted until fine-grained clastic sediments became more and more dominant during the early Late Oligocene. Their depositional environment may be described as shallow lagoon. In the succeeding formation of the Oligocene tidal bars and estuarine sands are characteristic. The uppermost formation penetrated by the drill hole in the Gartow area contains estuarine channel sands. In conclusion, the Oligocene sedimentary record reflects a symmetrical facies pattern showing calcareous-siliciclastic cyclothems in the central section and siliciclastic shelf deposits below and above, reflecting the cyclical nature of the Oligocene sea in a dominantly tidally-influenced depositional environment. The use of clay minerals, heavy minerals, carbonates, phosphates and sulphides together with dinocysts, nannofossils and foraminifera has proved to be a successful tool in palaeoecological and palaeoenvironmental analyses.

Influences of depositional environment and diagenesis on geophysical log response in the South Carolina Coastal Plain: effects of sedimentary fabric and mineralogy

Interpretations of depositional environments and hydrologic units are made routinely from the study of geophysical well logs. Spontaneous potential (SP) and resistivity logs can be used as indicators of textural parameters. Gamma-ray logs denote lithologic zones based on the presence of radioactive material, particularly in fine-grained sediments. On the South Carolina Coastal Plain, surficial fluvially derived cobbles, sands and clays are characterized by very low radioactivity. Weathered paleosurfaces enriched in hydroxy-interlayered vermiculite (HIV) and associated mica produce higher gamma-ray responses at discrete intervals. The underlying sequence of siliciclastic shelf sands, deposited under conditions of eustatic sea-level rise, has high concentrations of smectite and low gamma-ray signatures. Small-scale variability of gamma radiation reflects temporal changes of sub-environments; higher values are recorded in finer-grained, organic-rich nearshore sediments. Calcareous Ecocene sediments are characterized by highly variable signals on the gamma-ray, SP and resistivity logs. Phospharic material, smectite and disseminated organic matter produced very high gamma-ray values; corresponding low SP readings are a function of low macro- to microscale porosity resulting from the fine grain size and authigenic pore-filling cement. Intervals that resemble massively bedded chalks at the macroscale are characterized by low gamma-ray and resistivity values and high SP readings. High SP values result from microposity preserved in biogenic tests rather than intergranular porosity that would be expected for a permeable sand. These observations indicate the importance of incorporating sedimentologic techniques with well-log data for a comprehensive evaluation of subsurface lithologic units.

Chemostratigraphic and Sequence Stratigraphic Constraints on Vendian-Cambrian Basin Dynamics, Northeast Siberian Craton

$\delta^{13}$ chemostratigraphic and sequence stratigraphic data from terminal Vendian and earliest Cambrian strata of northeast Siberia are used to constrain the history of regional basin development. These boundary strata consist of 300 to 500 m of latest Vendian age carbonate ramp deposits and <100 m of Nemakit Daldyn age siliciclastic shelf deposits; the two successions are separated by a regional paleokarst unconformity. Six $\delta^{13}C$ chemostratigraphic profiles of the carbonates exhibit patterns of secular variation similar to those in coeval strata, with allowances for local diagenetic and tectonic effects. Isotopic profiles are used to recognize time lines (chemo chrons) within individual depositional sequences and to illustrate the geometry of stratigraphie units, and variation in subsidence and stratigraphie completeness across the basin. The Vendian Khorbosuonka Group and correlative deposits are divided into 3 depositional sequences that record sedimentation from ca. 555 to 543 Ma in a ge...

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In recent years, enormous progress has been made in our understanding of the geology, geophysics and sedimentology of continental shelves, largely due to the development and application of new concepts such as sequence stratigraphy and of new and powerful investigation techniques. The papers in this volume illustrate this progress, and the detail and multi-disciplinary nature of current research performed around the world on modern shelves and on shelf deposits in the geological record. The book is organized around four major themes: stratigraphy and sedimentary geology of siliciclastic shelves modern siliciclastic shelves: architecture, sea level, tectonics and sediment supply nearshore and coastal environments new techniques in continental shelf research It contains a number of overview papers, new data and an introduction to new techniques, and provides a unique collection of papers at the forefront of a rapidly developing subject. This volume will be of interest to research workers in the fields of marine geology, sedimentary geology and stratigraphy.

Reading Pleistocene eustasy in a tectonically active siliciclastic shelf setting (Crotone peninsula, southern Italy)

Transgressive-regressive cycles are recognized in early to middle Pleistocene siliciclastic shelf sediments that crop out in the tectonically active Crotone basin (southern Italy). Chronological constraints from calcareous nannofossil biostratigraphy and magnetostratigraphy indicate that these cycles are synchronous with oxygen isotope stages 19 to 33 and hence with global sea-level fluctuations. A major facies break in the succession correlates with oxygen isotope stages 22–24, corresponding to the major mid-Pleistocene climatic shift. These findings indicate that Pleistocene eustatic oscillations can be recognized in the lithostratigraphy of a tectonically active siliciclastic shelf setting. By integrating biomagnetostratigraphy with physical stratigraphy, a highly resolved chronology can be developed for these settings similar to that available for deep-sea sediments.

210Pb and 14C as Indicators of Callianassid Bioturbation in Coral Reef Sediment

ABSTRACT Callianassid shrimps are the dominant endofauna of reef-top sediments on John Brewer Reef, a small platform reef in the central section of Australia's Great Barrier Reef Province. Sediment reworking by these shrimps was investigated using the 210Pb radioisotope in conjunction with 14C radiometry and textural data. 210Pb analyses of cores from two reef-top sites revealed a classical tripartite subdivision characteristic of depositional settings in which sediment reworking dominates over sediment accumulation: (1) A vertical surface mixed layer (SML) extending to a depth of 50 cm beneath the sediment surface and representing a zone of intensive callianassid reworking; (2) An intermediate region of radioa tive attenuation in which excess 210Pb activity decreases logarithmically to a depth of slightly more than one meter beneath the sediment surface, and interpreted as representing a region utilized less frequently by Callianassa for coarse grain storage and predator avoidance; (3) A basal region, beyond callianassid influence, of consistently low background levels of 210Pb activity. Sediment turnover rates in excess of 120 cm2 yr-1 in the SML and up to 32 cm2 yr-1 in the region of radioactive attenuation were high compared to those typical of siliciclastic continental shelf and deep sea deposits, emphasizing the intensity of callianassid reworking in shallow coral reef environments. 210Pb geochronology is unsuitable for determining sediment accumulation rates in such settings, because the disruptive impact of Callianassa gives rise to unacceptably high figures. The depth of callianassid influence established by 210Pb radiometry corresponds to points of inflection, indicating the bases of vertical surficial sections, in 14C subsurface age profiles for the two John Brewer Reef sites Inflection points in other 14C profiles may similarly indicate sediment reworking depths at locations across the reef where 210Pb data were not obtained.

Cenozoic margin development and erosion of the Barents Sea: Core evidence from southwest of Bjørnøya

Five stratigraphic boreholes at the western Barents Sea margin reveal an evolution from a Palaeocene normal shelf setting to a complex Late Cenozoic offshore-marginal marine system influenced by mass flows, and finally a glacially-influenced shelf environment. Palaeocene epeiric sea sedimentation was sourced by erosion at the Stappen High. Earliest Eocene extension associated with volcanism was followed by rapid subsidence of the Vestbakken volcanic province, which became filled by sediments from an uplifted Stappen High. By the Middle Eocene, siliciclastic shelf sediments were deposited and the infill had reached a balance with subsidence. Tectonic movements at this time may have caused local uplift, which restricted the ventilation of local basins west of the Stappen High, but marine conditions existed in the area until Early Oligocene times, when subaerial exposure was caused by tectonic uplift and probably also a fall in eustatic sea-level. Pronounced Oligocene and Pliocene unconformities reflect major events of uplift and erosion at the margin, and abundant fossil reworking indicates erosion of the uplifted Cretaceous and Tertiary strata. The Oligocene erosion had its associated depocentre to the south and west of the study area, and was less important regionally than the Pliocene-Pleistocene phase. The Pliocene uplift first led to local erosion along the margin. Subsequent subsidence may initially have had a thermal component. This was followed by extensive downwarping under the load of largescale deposition sourced by regional erosion in the Barents Shelf region. An Eocene volcanic phase was probably related to the initiation of seafloor spreading. Isotopically dated volcaniclastic intervals in one of the cores give evidence of a Late Pliocene volcanic phase. This event was associated with local tectonic movements, which may be part of a regional uplift in the western-northwestern part of the Barents Sea. Glaciations caused by the Neogene climatic deterioration became enhanced by uplift, and this led to extensive regional glacial erosion. The main depocentre was offset to the south by ice flow curving around the uplifted area.

Storm features of siliciclastic shelf sedimentation in the mid-Cretaceous epeiric seaway of northern Australia

Late Aptian to Early Cenomanian strata were deposited in a large epeiric seaway which covered much of the northern Australian landmass during mid-Cretaceous time. The seaway stretched from the Carpentaria Basin in the north to the Eromanga Basin in the south. During this period siliciclastic sediments accumulated on a gently sloping epeiric shelf, which formed the western margin of the Carpentaria Basin. The shelf maintained a relatively similar configuration throughout the mid-Cretaceous with water depths rarely exceeding 50 m, and the seafloor was apparently within the influence of storm waves over much of the area. The shelf was storm- and wave-dominated, and most early formed or fair-weather sedimentary structures were destroyed or extensively modified during episodic storms. Storm features include hummocky cross-stratification, shelly lags, erosional conglomeratic lags, gutter casts, lenticular micro-hummocky sand beds, megaripples, characteristic low-diversity and opportunistic ichnofacies, various styles of wave-ripple lamination, and poorly preserved event beds. Most storm-influenced units are preserved as coarsening- and shallowing-upwards facies successions, which aggraded during high relative sea level and prograded during falling relative sea level. Some of these successions were subsequently eroded during further cyclic fluctuations of relative sea level.

Stratigraphic evidence for the Ross orogeny in the Ellsworth Mountains, West Antarctica: Implication for the evolution of the paleo-Pacific margin of Gondwana

Early Paleozoic crustal deformation has been recognized along the paleo-Pacific margin of Gondwana, including southern Africa (Saldanian tectonism) and the Transantarctic Mountains of East Antarctica (Ross orogeny). Previously, no stratigraphic evidence for the Ross orogeny had been recognized within the Ellsworth Mountains of West Antarctica, believed to have been contiguous with the Transantarctic Mountains prior to the Mesozoic breakup of Gondwana. A disconformity between Middle to Upper Cambrian Heritage Group and Upper Cambrian to Devonian Crashsite Group strata indicates that the Ross orogeny did affect the Ellsworth Mountains, providing additional evidence that these rocks were once part of East Antarctica. Fluvial conglomerates overlying platform carbonates of the upper Heritage Group were derived, in part, from plutonic, volcanic, and metamorphic terranes in East Antarctica. Elsewhere in the Ellsworth Mountains, the disconformity is expressed by an abrupt change from shallow carbonate-platform deposits to siliciclastic shelf deposits. Similarities in lower Paleozoic stratigraphy between the Ellsworth Mountains, parts of the Pensacola Mountains (East Antarctica), and eastern South Africa imply that these regions were inboard of the zone of deformation that affected the proto-Pacific margin of Gondwana during the Ross orogeny and Saldanian, tectonism

CALIZAS ESTROMATOLÍTICAS Y OOLÍTICAS Y DEFINICIÓN DE LA FORMACIÓN ARROYO DE LA PEDRERA (?VENDIANO, URUGUAY )

The Arroyo de la Pedrera Formation is defined as a new Vendian lithostratigraphic unit of Uruguay. The depositional systems are raade up of: (a) a shallow siliciclastic shelf formed by orthoquarzites and mudstones, grouped in the Cerros San Francisco Member; and (b) peritidal carbonates, which define the Cerro Victoria Member, whose depositional systems extends from the shallow subtidal to the supratidal. They are recognized by oolites, stromatolitic associations and scarce trace fossils. An intermitent subaerial exposition is recognized by tepee-structures, balite pseudomorphs and other features. The sediments are arranged in a shallowing - upward sequence, interrupted by tempestites, developed on a stable shelf, located on the margin of the Rio de la Plata Craton. The age of the formation is inferred by stratigraphic relationships and trace fossils. Deposition took place in a warm water environment, implying low paleolatitudes.

An iron‐bearing wave‐dominated siliciclastic shelf: Facies analysis and palaeogeographic implications (Silurian‐Lower Devonian Sierra Grande Formation, Southern Argentina)

AbstractThe Sierra Grande Formation (Silurian‐Early Devonian) consists of quartz arenites associated with clast supported conglomerates, mudstones, shales and ironstones. Eight sedimentary facies are recognized: cross‐stratified and massive sandstone, plane bedded sandstone, ripple laminated sandstone, interstratified sandstone and mudstone, laminated mudstone and shale, oolitic ironstone, massive conglomerate and sheet conglomerate lags. These facies are interpreted as shallow marine deposits, ranging from foreshore to inner platform environments. Facies associations, based on vertical relationships among lithofacies, suggest several depositional zones: (a) beach to upper shoreface, with abundant plane bedded and massive bioturbated sandstones; (b) upper shoreface to breaker zone, characterized by multistorey cross‐stratified and massive sandstone bodies interpreted as subtidal longshore‐flow induced sand bars; (c) subtidal, nearshore tidal sand bars, consisting of upward fining sandstone sequences; (d) lower shoreface zone, dominated by ripple laminated sandstone, associated with cross‐stratified and horizontal laminated sandstone, formed by translatory and oscillatory flows; and (e) transitional nearshore‐offshore and inner platform zones, with heterolithic and pelitic successions, and oolitic ironstone horizons. Tidal currents, fair weather waves and storm events interacted during the deposition of the Sierra Grande Formation. However, the relevant features of the siliciclastics suggest that fair weather and storm waves were the most important mechanisms in sediment accumulation.The Silurian‐Lower Devonian platform was part of a continental interior sag located between southern South America and southern Africa. The Sierra Grande Formation was deposited during a second order sea level rise, in which a shallow epeiric sea flooded a deeply weathered low relief continent.