1,721,034 research outputs found
TRANSGRESSIVE CHANGE FROM SHOREFACE TO SHELF-INDENTING CHANNEL DEPOSITIONAL SYSTEMS: THE LOWER PLIOCENE SÚA MEMBER OF THE BASAL UPPER ONZOLE FORMATION, ECUADOR.
No full textShelf-indenting canyons and their tributary systems are fairly common constituents of Quaternary shelves of active continental margins, but they have been rarely reported from older successions. As a consequence, facies models for shelf-indenting conduits are noticeably lacking in the geologic literature and far from complete. In this respect, the lower Pliocene Súa Member, northwest Ecuador, is of particular interest in that it is fairly well exposed and gives unprecedented documentation for the stratigraphic organization of a backstepping nearshore clastic wedge that is dissected by shelf-indenting channels and overlain by their turbiditic infill. This succession accumulated along a narrow, active continental margin during tectonically induced transgression and affords a rare opportunity to evaluate the stratigraphic evolution of such systems from an outcrop perspective. A comprehensive facies characterization combined with application of sequence stratigraphic concepts has led to definition of the following physical surfaces and stratal units in ascending order. (i) A regionally extensive erosional surface resulting from the superposition of the wave ravinement surface onto the previous sequence boundary. (ii) A nearshore, sand-prone lithofacies succession comprising a condensed basal shellbed deepening upwards through lower-shoreface bioturbated silty sandstones into inner-shelf sandy mudstones. (iii) Two U-shaped erosional features (turbidite shelf-entrenchment surfaces), interpreted as submarine channels, deeply incised into the subjacent nearshore sediments; shelf-entrenchment surfaces are carved by bypassing sediment-laden turbidite currents in shelfal settings during transgression and headward erosion of shore-connected shelf channels is proposed as the most viable mechanism for their development. In order to address for sediment bypass across the shelf and deep-water sedimentation during transgression, existing sequence stratigraphic models need to be extended and the turbidite shelf-entrenchment surface adequately included. (iv) A thick, fining-upward sedimentary succession laid down within the confines of the channels by high- and low-density turbidity currents and including both bed-load and suspended-load deposits.
This study contributes to extend the existing sequence stratigraphic models, further attesting that shelf-sediment bypass and deep-water sedimentation can take place at sea levels other than lowstand
Evoluzione dell'avanfossa messiniana dell'area laziale-abruzzese-marchigiana
No full textLa geologia del sedimentario nella ricerca di base e nelle sue applicazion
La tendinite del flessore e dell'estensore ulnare del carpo nel tennista
No full textLa tendinite del flessore e dell'estensore ulnare del carpo nel tennista
First record of a warm-water Pliocene shell bed from northern Chile.
No full textWe recognized a few Pliocene D. ponderosa shell beds in different localities of Mejillones Peninsula
[Antofagasta (Chile), 23°S]. To the best of our knowledge, this is the first case study of Pliocene fossil concentrations
formed by a warm-water species so well south (more than 2000 km) of its extant southern biogeographic
limit. In this paper we provide taphonomic features of an impressive shell bed largely dominated
by this infaunal bivalve that outcrops at Cuenca del Tiburon locality. Moreover, we discuss its mode
of formation in the light of particular paleogeographic and environmental conditions allowing the immigration,
survival, growth and reproduction of large populations of this tropical species in northern Chile
Shell concentrations as tools in characterizing sedimentary dynamics at sequence-bounding unconformities: examples from the lower unit of the Canoa Formation (late Pliocene, Ecuador)
No full textThe Canoa Basin in Manabì, Ecuador, contains a mainly marine, clastic sedimentary succession of Late Pliocene and Pleistocene Age (Canoa Formation and Tablazo Formation). A stratigraphic and sedimentologic study of the entire sedimentary succession indicated that sedimentary facies recur in consistent deepening–shallowing transgressive–regressive patterns and that on this basis it can be divided into three different informal units (Clow, Cupp, Tb). The lowest of these units (Clow) is composed of at least four depositional sequences, each bounded by a ravinement surface and containing a basal environmentally time-averaged shell bed accumulated during rapid sea-level rise
under conditions of reduced terrigenous sediment supply (transgressive systems tract). Owing to intense heavy bioturbation, the internal architecture of these shell beds is not recognizable. Each of these shell beds is mantled by poorly fossiliferous sandy and silty shales accumulated during subsequent progradation (highstand and regressive systems tract). Fossil assemblages, accounting for inner/middle shelf
settings, are dominated by infaunal suspension-feeding bivalves (Linga cancellaris, Chione mariae, etc.), most of which require sandy substrate or are ubiquitous. In terms of trophic/life-habit groups, the total succession shows an increase of infaunal suspension-feeder species’ richness from the lowest (C1) to the upper (C4) shell bed, whereas epifaunal taxa decrease in richness. Shell packing density also
decreases from C1 to C4. The reconstruction of the short-term sedimentary dynamics based on taphonomic and paleoecologic observations indicates a slightly progradational staking pattern of depositional sequences, representing high-frequency sixth-order eustatic sea-level fluctuations within a third-order tectonically induced cycle
Long-Term Tectonic Influence on Sequence Architecture and Stacking Pattern Revealed by Hiatal Shell Bed Features: Pleistocene Succession of the Canoa Basin (Central Ecuador)
No full textFossil mollusk association from Isabela Island (Galápagos, Ecuador)
No full textIn this paper a fossiliferous assemblage coming from Puerto Villamil (Isabela, Galápagos) is examined. The sandy sediments chiefly contain a molluscan association besides arthropod pincers, spicules of siliceous sponges, sea urchin spines, barnacle plates, rare foraminifers and abraded fragments of corals and briozoa. The mollusk assemblage represents a subtidal community associated with coral reefs and is dominated by Panamic forms. The modern aspect of this fauna and the radiometric ages till now proposed suggest a Pleistocene age for the Villamil deposits, although the presence of the extinct species Pegophysema spherica, for the first time reported from Isabela, might indicate a Pliocene age
Paleontological and sedimentological observations on the Canoa Formation (Manabí Basin, Ecuador)
No full textBasin physiography and tectonic influence on sequence architecture and stacking pattern: Pleistocene succession of the Canoa Basin (central Ecuador)
No full textcies, shell bed features, and sequence stratigraphic framework for the shallow-marine Pleistocene upper Canoa and Tablazo Formations are presented, based on outcrop data from the southern coast of Cabo San Lorenzo, Ecuador. Sediments of this succession exhibit a distinct cyclic pattern, consisting of a stack of eight depositional sequences (cyclothems) likely developed under the main control of orbitally induced sea-level changes. As a rule, within the studied interval an idealized cyclothem is composed of a transgressive systems tract (TST) and a highstand systems tract (HST), whereas deposits attributable to the lowstand and falling-stage systems tracts are not present. Transgressive lithosomes may be defined by estuarine deposits interposed between the sequence boundary and the ravinement surface (back-barrier wedge) and by upward fining shoreface to inner-shelf facies successions above the ravinement (backstepping shelf wedge). Separated by an expanded siliciclastic core, hiatal shell concentrations occur at the base (onlap shell beds) and the top (backlap shell beds) of the transgressive shelf wedges, and some occur at the base of highstand systems tracts (downlap shell beds). On the basis of sedimentary facies, geometry, taphonomy, and paleoecology of shell beds, and the nature of the transition between siliciclastic and mollusk-bearing sediments, cyclothems were classified into two main types that show dependence upon paleoshoreline morphological configuration: sheltered (in the upper Canoa Formation) and exposed (in the Tablazo Formation). Notwithstanding the different synsedimentary tectonic and climatic regimes, the Ecuadorian cyclothems share basic patterns of condensation and facies assemblages with other roughly coeval cyclothemic successions around the world. This suggests that (1) hiatal shell bed development is not just a temperate-latitude phenomenon; (2) a global process, such as glacio-eustatic sea-level change, is the primary mechanism of control for the general architecture of sequences; and (3) specific paleogeographic settings play an important role by determining the taphonomic and paleoecologic characteristics of key shell beds, the nature of their contacts with the encasing sediments, and the type of the component set of facies. At a multicycle time scale, tectonics influenced the long-term trend of the relative sealevel changes and consequently the large-scale stratigraphic organization. Owing to the continued tectonic uplift of the area, successive high-frequency depositional sequences are nested to form a longer-order falling-stage sequence se
Stratigraphic evolution from shoreface to shelf-indenting channel depositional systems during transgression: Insights from the lower Pliocene Súa Member of the basal Upper Onzole Formation, Borbón Basin, northwest Ecuador
No full textShelf-indenting canyons and their tributary systems are fairly common constituents of Quaternary shelves of active continental margins, but they have been rarely reported from older successions. Recognition of these prominent geomorphologic features in the ancient record has important implications not only for a proper understanding of shoreface-to-shelf depositional systems, but also from a petroleum exploration standpoint as they represent efficient conduits for moving coarse-grained river- and nearshore-borne sediments to the adjacent slope even during periods of relative rise in sea level. Coastal exposures of the lower Pliocene Súa Member in the surroundings of Súa (northwest Ecuador), preserve the unusual juxtaposition of incising submarine channels onto nearshore deposits. This succession accumulated along a narrow, active continental margin during tectonically induced transgression and affords a rare opportunity to evaluate the stratigraphic evolution of such systems from an outcrop perspective. A comprehensive facies characterization combined with application of sequence stratigraphic concepts has led to definition of the following physical surfaces and stratal units in ascending order. (i) A polygenetic, regionally extensive erosional surface resulting from the superposition of the wave ravinement surface onto the previous subaerial sequence boundary (SB/wRS). (ii) A nearshore, sand-prone lithofacies succession comprising a condensed basal shellbed deepening upwards through lower-shoreface bioturbated silty sandstones, into inner shelf sandy mudstones. (iii) Two steep, U-shaped erosional features (turbidite shelf-entrenchment surfaces), interpreted as shelf channels, deeply incised into the subjacent nearshore sediments and marking an abrupt deepening of facies. (iv) A thick, fining-upward sedimentary succession laid down within the confines of the channels by high- and low-density turbidity currents and including both bed-load (traction) and suspended-load deposits; the overall fining- and thinning-upward character exhibited by the infill of these channels is thought to reflect decreasing flow energies and is consistent with the gradual cut-off of clastic influx to their upper reaches in response to progressive detachment from an adjacent coastal source during relative rise in sea level. Based on detailed analysis of facies and a sequence stratigraphic interpretation of outcrop data, this study contributes to extend the existing sequence stratigraphic schemes, further attesting that shelf-sediment bypass and deep-water sedimentation can take place at sea levels other than lowstan
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