Shallow Marine Facies Research Articles

Paleoenvironmental control of species distributions in Neogene invertebrate taxa of the Dominican Republic

Neogene deposits of the northern Dominican Republic contain a diverse fossil assemblage that is especially rich in corals and mollusks. To see if faunal change was concordant or discordant within and among taxa and decipher factors controlling distributions, we compared distributions of coral communities, the gastropod families Strombidae and Marginellidae, and the bivalve family Corbulidae. We also incorporated published ranges for the Cardiidae (Vokes, 1989), Cancellariidae (Jung and Petit, 1990), and the columbellid genus Strombina (Jung, 1986).First and last appearances of individual mollusk species were diachronous among sections. Within the sections, however, first and last appearances of mollusk species tended to coincide. Concordance of species ranges could be caused by unconformities or faults, be an artifact of sampling, or indicate similar responses by species to environmental changes. Neither stratigraphic gaps nor faults appear to correspond to concordant first or last appearances. Although the absence of mollusk taxa generally corresponds to less intense sampling, sampling intensity is highly correlated with the presence of macrofossils and therefore, taxa absence is probably real. First and last appearances do coincide with paleoenvironmental changes such as rapid deepening, introduction of marine conditions, increased intensity of erosional bottom currents, and changes from reefal to sand flat facies.Comparisons among taxa also helped elucidate other distributions patterns. For instance, comparing coral communities to strombid ranges showed that strombid diversity increased in grass flats (inhabited by free living corals) and reefal deposits, indicating similar ecologic preferences to many modern strombid species. Using the coral fauna to distinguish a grass flat community from other shallow marine facies also helped explain corbulid abundances as environmentally induced, with lower numbers in grass flat deposits.Simultaneous comparison of species distributions within diverse taxa can help explain the nature of species occurrences. For several mollusk taxa from the Neogene of the Dominican Republic, the general correspondence in distribution patterns across taxa indicates that paleoenvironmental conditions were controlling species distributions.

Ichnofabric of the Paleozoic Skolithos Ichnofacies and the Nature and Distribution of Skolithos Piperock

Dense concentrations of Skolithos, commonly called piperock, exemplify the characteristic ichnofabric of the Skolithos ichnofacies in Paleozoic strata. Piperock occurs in sandstones and coarse siltstones within all types of shallow marine facies and is not restricted to beach, littoral or intertidal sediments. Piperock, however, is restricted to terrigenous clastic sediments. An analysis of the temporal distribution of piperock confirms the previous observations that piperock is typical of the Cambrian but also demonstrates that piperock occurs throughout the Paleozoic, decreasing in abundance after the Cambrian. This distribution may be a result of large-scale physical or biological factors. Preserved nearshore sandstones may decrease through this time interval. In addition, the decline of piperock corresponds with the Ordovician faunal diversification when bioturbation increases in other environments and, thus, may reflect the appearance of new and better adapted fauna in the nearshore terrigenous clastic setting.

Uppermost Precambrian(?)–Lower Cambrian of mainland Nova Scotia: faunas, depositional environments, and stratigraphic revision

The uppermost Precambrian(?)–Lower Cambrian of the Avalon Zone in the northern Antigonish Highlands is composed of two dissimilar sequences in thrust contact. These include the sandstones and slates of the Doctor's Brook allochthon and the volcanoclastic-rich Malignant Cove authochthon.Lithostratigraphy of the “Black John Formation” (designation abandoned) in the Doctor's Brook allochthon is comparable to the uppermost Precambrian–Lower Cambrian in eastern Placentia Bay, southeastern Newfoundland, and Cape Breton Island. A unified stratigraphic nomenclature is appropriate in these Avalonian areas. The lower part of the “Black John” is an unconformity-bounded depositional sequence with subaerial rift facies (Rencontre Formation, 178+ m), overlying marine siliciclastic mudstones and fine sandstones (Chapel Island Formation, 59 m), and a quartzite cap (Random Formation, 2.05 m). The Chapel Island Formation has the oldest faunas from mainland Nova Scotia (Watsonella crosbyiZone, lower Placentian Series). A post-Random unconformity known in Newfoundland and Cape Breton Island lies at the Random-Bonavista Group (Cuslett Formation) contact in the upper “Black John Formation.”Fossiliferous Lower Cambrian limestones and siliciclastic mudstones previously reported from the Malignant Cove autochthon are actually clasts in basalt pebble-dominated slope deposits of the Arbuckle Brook Formation. These clasts were eroded from shallow-marine facies comparable to those in the Doctor's Brook allochthon during local uplift associated with Middle Cambrian(?) extension and mafic volcanism.Eight species are illustrated from the Placentian and Branchian Series.AnabaritellusMissarzhevsky, 1974, emend. (=SelindeochreaValkov, 1982) is a Lower Cambrian calcareous tube-dwelling metazoan(?) known from tri- through multisulcate conchs that are morphologically intermediate betweenAnabaritesandColeoloides.

Pachyscalpellum cramptoni: A new genus and species of lepadomorph cirripede from the Cretaceous of northern Hawke's Bay, New Zealand

Abstract Pachyscalpellum cramptoni, a new genus and species of lepadomorph cirripede, is described from the late Cretaceous (Haumurian) of Hawke's Bay, New Zealand. Pachyscalpellum is also recognized from the Santonian-Campanian of the Perth Basin, Western Australia. The genus is unusual in that, although the capitular valves are very much thickened, the tergum demonstrates negative allometric growth along its scutal margin. Pachyscalpellum is known only from shallow water marine facies, and the heavily calcified valves are interpreted as a defence against damage in this high energy environment.

Integrated Analysis of Parasequences in an Oligocene Transgressive Systems Tract: San Joaquin Basin, California

The Vedder-Jewett sands represent a retrogradational parasequence set forming a seismically defined, 1500-ft thick transgressive systems tract (TST) that was deposited in a ramp setting on the eastern margin of the San Joaquin basin. This interval has produced 656.5 MMBO and 221.9 BCFG, largely from fault-bounded traps. Due to basin subsidence, the underlying highstand nonmarine Walker was transgressed and overlain by nonmarine and marine deposits of the Vedder and Jewett sands. Seismic, well-log, and core data indicate that parasequences in the upward-deepening TST are comprised of alluvial, fluvio-deltaic, and shallow-marine facies. Each parasequence is punctuated by marine-transgressive episodes. This TST is manifested as a succession of backstepping and aggradational seismic reflections becoming progressively more transparent upward. Geologically defined parasequences and stratigraphic discontinuities are seismically detectable because of impedance contrasts between marine and nonmarine sediments at marine-flooding surfaces. Reflection amplitude loss coincides with a sandstone-to-shale facies change within a parasequence and, therefore, is a good estimate of lithosome continuity. Parasequences and their bounding surfaces in the TST have diagnostic sedimentologic and biostratigraphic signatures. Low benthonic foraminiferal abundances characterize shallow-marine deposition; abundances increase to a maximum at the seismic downlap surface (maximum flooding surface). Dinoflagellate occurrences accurately record the transgressive episodes, especiallymore » in the predominantly nonmarine lower TST.« less

Predicting the Distribution of Upper Cretaceous Aquifers Using Sea-Level Analysis and Regional Paleogeography, Alabama Coastal Plain

In the inner coastal plain of Alabama, Upper Cretaceous (i.e., Santonian-Maastrichtian) stratigraphic units containing key aquifers dip south at 7m/km; the aquifers consist mainly of porous and permeable barrier-island facies (including upper-shoreface and tidal-pass sands), barrier-related sand facies (including tidal deltas and lagoonal and marine tempestite beds), and shallow-marine sand facies (including offshore bars and conglomerate sandy turbidite tongues). Confining aquitard and aquiclude facies include lagoonal silty clays, shallow-marine glauconitic clays, clayey marls, and marine chalky marls. The gross geometry, thickness, and lateral and vertical distribution of aquifer sands in both the shallow-subsurface and outcrop (i.e., recharge) area is predictable based on regional paleogeographic reconstructions and the regional Late Cretaceous relative sea-level curve. At a local scale, facies maps and shallow-subsurface correlations provide essential data for aquifer exploration and recharge-area protection. For example, in the Eutaw Formation, barrier-island and barrier-related facies developed along a curved east-west striking shoreline; aquifers include shoreline facies, tempestite beds, and turbidite sands. In the younger, northwest-striking Blufftown-Cusseta and Ripley-Providence systems, aquifers are barrier-island and barrier-related (especially tidal-delta) facies.

A Model for Transgressive Black Shales in a Sequence Stratigraphic Framework

Commonly, at times of sea-level rise, basinal organic-rich facies extend their distribution into areas characterized by shallow water deposition. This occurrence of black shales in transgressive systems tracts is a widespread but little understood phenomenon. It has been suggested that such black shales are themselves formed in shallow water, although they are separated from underlying shallow marine facies by a transgressive surface often associated with a coarse lag. Several formative mechanisms have been proposed to explain this distribution, including either restricted circulation in a sea of limited extent or a zone of nearshore, high productivity. However, none of these models predicts the asymmetric distribution of the black shales for they are not found in highstand systems tracts when the shallow marine area was of a similar extent to that of the transgression. A new (expanding puddle) model is proposed in which the water depths of early transgression are considered to be the deepest achieved at any point on the sea-level curve. Sea-level rise is characterized by the formation of sediment-trapping estuaries and infilling of the preceding lowstand drainage system that leads to sediment starvation in the marine realm. The combination of sea-level rise, sediment starvation, and subsidence causes a rapidmore » deepening of the water column to a depth at which organic-rich shales can accumulate (assuming oxygen renewable to the bottom waters is by vertical advection). Black shale deposition will be initiated in the most rapidly subsiding areas and the anoxic puddle of bottom water will progressively expand outwards into basin margin settings.« less

The sedimentological significance of a clastic wedge in the western basin margin of the Triassic Tethys (Iberian Range, Spain)

The Middle Triassic in the NW of the Iberian Basin consists of both carbonate and terrigenous sediments. A new sedimentary model is proposed for the clastic wedge of the Cuesta del Castillo Saudstones and Siltstones (CCSS) Formation (Garcia-Gil, 1990), and its relationship with the Upper Muschelkalk carbonate Formations. The model will take account of tite following characteristics: 1. Low depositional gradient being integrated in a homoclinal carbonate ramp. 2. Combined tectonic, climatic aud eustatic controis. 3. Presence of continental antA shallow marine facies associations. 4. Homogeneity of textural facies. The sedimentation of tite CCSS Formation took place during a relative sea level rise (high stand system tract).

上部白亜系中部蝦夷層群三笠層の浅海堆積相分布と前進性シーケンス

上部白亜系中部蝦夷層群三笠層の浅海堆積相分布と前進性シーケンス

The Lower Kahmah Group of Oman: the carbonate fill of a marginal shelf basin

Abstract Following the rifting and spreading related to the opening of the Tethys Ocean during the Triassic and Early Jurassic, relaxation and subsidence affected its passive margins. In Iran and Oman, such subsidence of the continental margins gave rise to deeper marine, marginal shelf basins, that initially were filled with pelagic sediments, but were progressively infilled by prograding shallow shelf-derived sediments. In Oman the fill sequence is well displayed for study in surface outcrops in the Oman Mountains as well as in subsurface geological and geophysical data. This paper provides an integrated picture of the geometry and facies distribution of the sediments, which filled this marginal shelf basin. The pelagic facies comprises the Rayda Formation and the basin, therefore, is named the Rayda Basin. The prograding, shelf-derived, fill facies comprises the Salil Formation. Surface-type sections of these two formations are described from outcrops in the Oman Mountains. The last stage of the basin fill is represented by a diversified shallow marine facies, referred to as the Habshan Formation, which is defined in Abu Dhabi. This unit prograded over the entire sag basin and almost levelled the pre-existing basin topography in such a way that overlying deposits hardly reflect the original Rayda Basin outline. The Habshan shoal facies is therefore in part coeval with the Rayda and Salil and exhibits a complex interdigitation with those two formations towards the basin margin. The seismostratigraphic expression of the Habshan, Salil and Rayda Formations provides a valuable tool for defining basin outline and mode of infill. A number of seismic lines are presented in relation to well log correlation panels to highlight the complexity of the sedimentary processes affecting these units. The fill of the basin was completed by the Valanginian and the ovelying Lekhwair Formation of Hauterivian to Barremian age represents a renewed marine transgression.

Zechstein reservoirs in The Netherlands

Abstract Thick, shallow-water gypsum banks, formed during Zechstein times at the site of regional palaeohighs along the margins of the Southern Permian Basin, acted as foundations for shallow-water carbonate platforms. The carbonate platform margins, in particular their northern to northeastern windward margins, were characterized by deposition of grainstones dominated by aragonitic ooids, pisoids, intraclasts and bioclasts, while muddy, stromatolitic carbonates and nodular anhydrites accumulated on tidal flats and in lagoons behind the platform margins. Usually the platforms are surrounded by thick slope deposits consisting of carbonate mudstone and redeposited intervals, but locally the platforms deepened gradually and changed, without intervening slope deposits, into a shallow-marine facies devoid of ooids. The slope aprons grade basinwards into thin successions of finely laminated carbonate. Porosity creation in Zechstein carbonates was due to meteoric-water leaching and to subsurface leaching by carbon dioxyde-enriched formation waters during burial. Porosity destruction was mainly caused by anhydrite pore plugging and subsurface calcitization. The best reservoir properties are found in the unrestricted platform facies and in parts of the slope facies. Fractures are important in Zechstein carbonates; their presence is a prerequisite for high production rates in gasfield wells.

Depositional Facies of Hydrocarbon Reservoirs of Upper Cherokee Group, Anadarko Basin: ABSTRACT

The Desmoinesian upper Cherokee Group sequence in the Anadarko basin is the subsurface equivalent of the Cabaniss Group of eastern Oklahoma. This sequence includes the Pink limestone, Skinner sandstone, Verdigris limestone, and Prue sandstone intervals. The upper Skinner sandstone, which has not been well documented, is an important hydrocarbon-producing reservoir in the Anadarko basin. The Skinner sandstone is represented by channel, delta-front-prodelta, and shallow marine facies. Channel facies consist of a primary elongate trend extending 40 mi southeast-northwest across Custer and Roger Mills Counties, Oklahoma. Several small secondary channels trending northeast-southwest were also observed. Active channel-fill sequences in the primary trend exceed 100 ft in thickness and represent the major producing reservoir of the upper Skinner sandstone. Delta-front-prodelta sequences are dominated by shale and interbedded sandstone-shale units. Shallow marine facies consist of massive coarsening-upward units that reach 300 ft in thickness. This facies belt is broad and slightly elongated, approximately 12 mi wide by 20 mi long, and trends northeast-southwest somewhat normal to channel facies orientation. Lithologically, the upper Skinner channel sandstone is feldspathic litharenite with abundant feldspar and quartz overgrowth. Both primary and secondary porosity were observed in the upper Skinner sandstone. Secondary porosity evolved mainly from dissolution ofmore » feldspar and lithic fragments. However, extensive cementation in the shallow marine facies has reduced porosity to negligible amounts and consequently reduced reservoir quality.« less

Sedimentology of a Late Cenozoic collisional sequence: the Misis Complex, Adana, southern Turkey

This study is concerned with Miocene sequences formed within part of the Cukurova Basin of southern Turkey, a major downwarp created during the early stages of collision between the Afro-Arabian plate and the Tauride-Anatolian microplate assemblage. We report here the results of a combined stratigraphical, sedimentological, structure and geochemical study of the Misis Complex, a structurally elevated segment of Cukurova basin-fill. This analysis has demonstrated the structurally imbricated nature of the Misis sequences and the tectonic juxtaposition of originally well-separated coeval facies. The initial discernible phase of basin-filling (pre-Burdigalian) was marked by deep marine conditions and marginal tectonic instability (probably related to a late phase of Tauride nappe advance). The subsequent phase of crustal extension (Burdigalian/Early Tortonian) produced expansion of the Cukurova basin limits and an upwards-shoaling succession, in which sediment transport towards the southwest gradually became dominant, as a result of earlier suturing and uplift in northeasterly sectors of the collision zone. The Miocene arenites of the Misis Complex are petrographically diverse (with common siliciclastic and carbonate detrital admixtures) and mineralogically immature and their position in petrotectonic fields indicates a compound provenance from both recycled orogen and magmatic arc sources. A noteworthy petrographic feature is the compositional similarity of arenites from both shallow and deep marine facies. The general succession of Neogene facies observed in the Cukurova Basin and Misis Complex is consistent with their evolution within a perisutural foreland basin, but this interpretation is complicated by petrographic and geochemical features attributed to remnants of an older magmatic arc. Later stages of basin-filling are also marked by the local occurrence of “anomalous” palaeoenvironmental associations (including olistostromic units) attributed to contemporaneous intra-basinal tectonism.

Cenozoic palaeogeographic development of northern Tunisia, with special reference to the stratigraphic record in the miocene trough

Cenozoic deposits are present in five northeast-trending provinces of northern Tunisia, from northwest to southeast: the Numidian belt, the western (Brjaoua) and eastern (Bou Sefra) parts of the Miocene Trough, the Fortuna Basin, and the Eastern Platform. The critical Bou Sefra sequence is an essentially complete fossiliferous Miocene succession. These Cenozoic deposits record six episodes: (1) Widespread Paleocene and Eocene Tethyan transgression flooded the entire region. (2) During Oligocene time the inner part of the northwestern cratonic margin was uplifted and eroded while a pelagic facies filled a northeast-trending precursor of the Fortuna Basin. At the same time a shallow marine facies prevailed to the east, south, and west. (3) Increased Early Miocene deformation in the Fortuna Basin area led to accumulation of thick, conglomeratic deltaic to alluvial sandstone which graded laterally into shallow marine deposits on the Eastern Platform. Neritic sediments also accumulated along much of the Miocene Trough to the west, and later spread eastward over the Fortuna Formation. Farther northwest a thick succession of sandy Numidian turbidites was deposited along the outer continental margin of north Africa. (4) In Middle Miocene time Numidian nappes were emplaced in the northwest while deformation and local uplift within the Miocene Trough was accompanied first by marine marl and mud and then by accumulation of a muddy flyschoid facies that graded into neritic to paralic deposits farther east. (5) During late Miocene time uplift and erosion prevailed in the far northwest while a shallow marine facies in the Miocene Trough was succeeded by evaporite-bearing neritic to paralic deposits before the end of the Miocene Epoch. Neritic to paralic facies dominated the eastern part of the Fortuna Basin area and the Eastern Platform throughout this episode. (6) Intense deformation at the end of Miocene time produced uplifts in the northwest and along the Tunisian Atlas flanked by coarse-grained nonmarine Pliocene deposits, as well as local subsidence and marine basin fill in the Bizerte area in the northeast and on Cap Bon Peninsula to the east.

Middle Cenozoic Depositional, Tectonic, and Sea Level History of Southern San Joaquin Basin, California

The Eocene to lower Miocene fill of the southern San Joaquin basin contains three complete depositional sequences--the Tejon, San Emigdio, and Pleito--each of which corresponds to a formal formation. The Tejon sequence (lower to middle Eocene) is marine and incorporates nearshore, shelf, slope, and basinal deposits. The San Emigdio (upper Eocene to lower Oligocene) and Pleito (upper Oligocene) sequences intertongue eastward with alluvial-fan deposits of the Tecuya Formation. The lower part of the San Emigdio sequence was deposited by a westward-prograding Gilbert-type delta. The upper part of the San Emigdio sequence and lower part of the Pleito sequence were deposited by a system of shelf fan-deltas that prograded at least 10 km to the west. The middle and upper parts of the Pleito sequence were deposited by a slope fan-delta in relatively deep (hundreds of meters) water. Regional transgression during the early Eocene initiated deposition in the southern San Joaquin basin. The lower San Emigdio Gilbert-type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 Ma. Relative highstand deposits in the San Emigdio and Pleito Formations consist of widespread progradational shallow-marine and nonmarine facies. The Eocene to early Miocene tectonic history of the southern San Joaquin basin included three distinct periods of increasingly intense activity. Rapid early to middle Eocene subsidence of the basin was associated with the emplacement of the Salinian block. Late Eocene to early Oligocene uplift and subsidence occurred in two discrete pulses, and a seismically(?) generated submarine-slumping episode triggered the change from shelf fan-deltas to slope fan-deltas. During the late Oligocene to early Miocene, the southern San Joaquin basin was disrupted by major uplift, proximal volcanism, and syndepositional faulting, as the Mendocino triple junction migrated past the study area, and San Andreas fault-related tectonism was initiated. In general, local tectonic events, rather than eustatic ea level events, seem to have exerted the predominant control on middle Cenozoic sedimentation in the southern San Joaquin basin.

The shipway limestone of gower: Sedimentation on a storm‐dominated early carboniferous ramp

AbstractA detailed sedimentary study of the Lower Carboniferous (Courceyan) Shipway Limestone Formation at Three Cliffs Bay on the Gower Peninsula (South Wales) has shown that the bioclastic limestones represent a storm‐dominated sequence that contains the storm‐related sedimentary structure hummocky cross‐stratification (HCS). Conformably overlying the Shipway Limestone is a cross‐stratified oolitic sandbody with evidence of subaerial exposure. Six sedimentary lithofacies are identified in these two formations which record a distal to proximal, shallowing‐upward trend that passes from beneath mean wave‐base to above fairweather wave‐base.The shallow marine facies model constructed by Wu (1982) from his study of the Lower Carboniferous limestone sequences of South Wales is re‐evaluated. Modifications proposed for the model include the addition of two distal tempestite facies and a proximal oolite sand body. The Shipway Limestone and Brofiscin Oolite record the first major, basin‐wide, shallowing‐upward phase of the Lower Carboniferous in South Wales.

THE PETROLEUM POTENTIAL OF THE GUNNED AH BASIN AND OVERLYING SURAT BASIN SEQUENCE, NEW SOUTH WALES

The Permo-Triassic Gunnedah Basin has good potential for the discovery of commercial petroleum. Gas shows have been reported from the Porcupine-Watermark, Black Jack and Digby Formations, and from the basal sandstone of the Purlawaugh Formation in the overlying Surat Basin sequence. Gas flowed on drill stem test from the Porcupine-Watermark Formation in the Wilga Park No. 1 discovery well although the find was sub-commercial. An oil show was observed in Lower Permian volcanics, and oil staining has been observed in the Pilliga Sandstone in several wells. The origin of oil staining in the Pilliga Sandstone is unknown, however, and may have been the result of diesel contamination during drilling operations.Structural style within the basin sequence is characterised by north-south and north-north-west/south- south-east trending anticlines which formed in response to periodic compressive and left lateral strike-slip movements along the main Hunter Mooki Thrust Fault. These anticlines are attractive exploration targets.Westerly-derived quartz-rich sandstones occur at several stratigraphic levels within the Black Jack Formation and within the upper Digby Formation. Sandstones of the western bed-load fluvial system (lower Black Jack Formation) are most prospective with thick sections (up to 8 m) giving permeabilities from several hundred to several thousand millidarcies. Marine reworked easterly-derived sandstones up to 12 m thick in the Black Jack and Watermark Formations have minor reservoir potential with permeabilities in the order of tens of millidarcies. All potential reservoirs within the sequence are considered to be adequately sealed. Regionally extensive shaly units deposited either by marine incursion or lacustrine inundation overlie most reservoir horizons; remaining reservoirs are capped by intraformational shales.Organic petrology and geochemistry indicate the best potential source rocks within the Gunnedah Basin are floodplain, lacustrine and shallow marine facies of the Purlawaugh, Napperby, Watermark, Maules Creek and Goonbri Formations. The shallow marine Arkarula Sandstone Member within the Black Jack Formation also has significant potential for oil generation. Vitrinite reflectance, liptinite auto-fluorescence and TAI values indicate Lower Permian sediments are marginally mature to mature for oil generation. Combining the data on source quality and quantity with thermal maturity, the Permian sediments - in particular the Watermark Formation - have the best potential for generating oil.

Transition from a Passive Continental Margin to an Active Margin Documented by Time-Facies Profiles and Geohistory Diagrams: ABSTRACT

The Upper Cretaceous to Neogene sediments in the northern part of the external zone of the Betic Cordilleras (southeast Spain) reflect the evolution of a passive continental margin into an active margin. Time-facies profiles and geohistory diagrams were constructed to identify and date tectonic events and sea level changes in the sedimentary record. During Late Cretaceous to middle Eocene time, parallel-trending shallow marine facies belts at the edge of a slowly subsiding basin evidence a passive continental margin setting. The period from middle Eocene to early Miocene is rather poorly documented. After initial shallowing the whole area emerged and continental conditions prevailed from the late Eocene to early Miocene. The subareal exposure may have been caused by the compound effect of the worldwide Oligocene sea level drop and overall tectonic uplift. Miocene sediments reflect the compressional tectonic regime of an active margin. The parallel facies belts were disrupted and the area was segmented into several tectonic blocks, each displaying an individual sedimentary record due to differential vertical movement. During middle to late Miocene time a major compressional phase generated northeast-trending folds and activated strike-slip and thrust faults. This tectonic phase led to a highly differentiated topography, resulting in the developmentmore » of local depocenters - each with a unique tectonic and depositional record.« less

Fans and fan deltas – precursors to the Armorican Quartzite (Ordovician) in western Iberia

AbstractThe Armorican Quartzite (Lower Ordovician) is a very extensive sandstone body found throughout a large part of Iberia and Brittany; similar quartzites are present in north Africa and elsewhere. In Iberia it generally lies unconformably on a thick, folded, late Precambrian to Cambrian turbidite sequence (the Complexo Xisto-Grauváquico (CXG)), but there are some places where the quartzite has a conformable relationship with the underlying CXG. Where conformable the whole succession shows varied facies sequences including: (i) a regressive mud–sand sequence as a precursor to the main quartzite development, (ii) submarine fan sediments of the CXG passing up into slope and then shallow marine facies, or (iii) fluvially influenced breccio-conglomerates of a marginal fan delta passing up into shallow marine sediments.The inferred development of alluvial fans associated with fan deltas prograding into shallow marine environments implies steep slopes and the likelihood that faulting controlled some of the uplift. The mozaic pattern of differential uplift and subsidence throughout the region suggests that the CXG was affected by local block movements rather than by regional folding.The presence of upstanding blocks during the initial sedimentation of the Armorican Quartzite suggests that the quartz sand might have been derived from multiple sources dispersed throughout the area rather than from sources along a single shoreline at the margin of the depositional area.

Mixed-water dolomitization in a transgressive beach-ridge system, Eocene Catalan Basin, NE Spain

Summary Dolomitization occurs at the top of alluvial fan deposits in the SE margin of the Eocene Catalan Basin, NE Spain. The dolomitized rocks occur just below erosion terraces which show a step-like disposition on the continental sediments and on which shallow marine facies (beach-ridge systems and reefs) are developed. Petrography shows that diagenesis in the continental sediments mainly originated under meteoric water influence, while in beach-ridge systems and reef carbonates there is evidence of early diagenesis in a marine environment. All the nearshore sediments (from alluvial fan to reefs) record a distinct geochemical pattern where a progressively more marine influence parallels the regional trend in facies across the ancient shoreline. Petrographic and geochemical data suggest that mixing of meteoric and marine waters in the sediments representing the continental-to-marine transition caused dolomitization at the top of the alluvial fan deposits. Unlike most mixing-zone dolomites, in this case mixing took place when shallow marine sedimentation took place on erosional surfaces causing the inflow of marine waters that mixed with meteoric porewaters.