1,721,007 research outputs found
Short- and long-term tectonic control on internal architecture and stacking pattern of Pleistocene depositional sequences (Mejillones Formation, northern Chile)
No full textIn the Mejillones Formation, a shallow-marine Pleistocene succession of northern Chile, the cyclic stratigraphic record is the result of the complex interaction of regional uplift, glacio-eustasy, local tectonics, and sediment supply. Stratal geometries, characteristics of sedimentary facies, and nature of sequence-bounding unconformities have been investigated to evaluate the influence of: (i) intrabasinal, short-term normal faulting on both along-strike variations in sequence architecture and genetic complexity of key stratal surfaces; and (ii) long-term regional uplift on sequence stacking pattern. The stratigraphic succession, dissected by small-displacement (few meters) normal faults striking obliquely with respect to the palaeo-shoreline trends, displays systematic variations in sequence architecture and the nature of bounding surfaces across them. Indeed, depending on position with respect to the fault plane, two basic types of internal organisation can be recognised in the examined shallow-marine sequence. Within grabens it consists of a siliciclastic-rich transgressive systems tract (TST), which is bounded beneath by a transgressively modified, Glossifungites-demarcated sequence boundary (SB/RS), overlain by a mollusc-bearing falling-stage systems tract (FSST). The erosional downlap surface that separates the TST from the FSST is the regressive surface of marine erosion (RSME). On the footwall crests the combination of marine regressive erosion, during falls in relative sea-level, and uplift has resulted in complete removal of the sediments of the TST from these sites, leading to the formation of a tectonically enhanced basal unconformity composed of the RSME superimposed onto the previous SB/RS (SB/RS/RSME). The prominent lateral change in component units (systems tracts) and nature of bounding surfaces within the studied sequence is directly related to the presence of normal faults and indicates that fault activity had a major impact on the sequence stratigraphic evolution of the Mejillones Formation, enhancing subsidence within the grabens and promoting unconformities in the horsts.
Overall, the Mejillones Formation records a long-term sea-level fall driven by the contemporaneous regional uplift, punctuated by repeated, high-frequency eustatic sea-level changes. The effect of this superimposition was that glacio-eustatic sequences were displaced progressively downward and basinward and stacked in a distinct downstepping, tectonically enhanced falling-stage sequence set, which reflects basin-wide loss in accommodation space. The sequence set is underlain by a composite RSME that becomes progressively younger basinward and is made up by the lateral and down-dip connection of a series of lower-rank sequence boundaries including hanging-wall SB/RSs and footwall SB/RS/RSMEs of successive sequences
Dermatitis associated with 'hypopodes' in a horse: The first case reported in Italy
No full textThe authors report, for the first time in Italy, a case of dermatitis of 'hypopodes' origin in a horse. The hypopodes are a particular nymphal stage of mites of the suborder Astigmata. The 'hypopus' is non-feeding, lacks a mouth and has a ventral suctorial plate with suckers and conoids for attaching itself to insects as a mode of dispersal. Some of these larval stages can enter into the hair follicles and into the subcutaneous layers causing lesions similar to mange. There are few reports of dermatitis in horses specifically associated with the presence of hypopodes and these have been attributed to an allergic response to the body fluid of crushed hypopial stage. It may be that infestations of these nymphal stages are widespread, but the nature of the condition is not yet clear; therefore, they may be underdiagnosed. (C) 2000 Elsevier Science B.V
Basin physiography and tectonic influence on sequence architecture and stacking pattern: Pleistocene succession of the Canoa Basin (central Ecuador)
No full textFacies, 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 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, 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 and by upward fining shoreface to inner-shelf facies successions above the ravinement (backstepping shelf wedge). 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.
At a multicycle time scale, tectonics influenced the long-term trend of the relative sea-level 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 set
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 text- …
