Dilce de Fátima Rossetti

Events of sediment deformation and mass failure in Upper Cretaceous estuarine deposits (Cametá Basin, northern Brazil) as evidence for seismic activity

Abstract Event beds recording Late Cretaceous synsedimentary seismicity occur in the Cameta Subbasin, northern Brazil. The deposits consist of nearly 40 m of kaolinized, fluvio-estuarine deposits. The lower fluvial (tidal-influenced) complex, which carries a high-quality commercial kaolin, includes genetically related channel fill, floodplain, crevasse splay, and mud plug deposits. The upper estuarine deposits contain trace fossils indicating marine-influenced settings (Thalassinoides, Ophiomorpha, Teichichnus, Diplocraterion, Skolithos, Cylindrichnus, Rhizocorallium, and Planolites), and include heterolithic, sandy, and bioturbated units, interpreted as tidal-flat, storm-overwash, and low-energy estuarine bay deposits, respectively. In the upper portion of the estuarine lithosomes, four successive events (events 1–4) of synsedimentary brittle and ductile deformation and large-scale sediment failure, anomalous in estuarine settings, were recognized and attributed to earthquakes. Event 1 is recorded by heterolithic and bioturbated deposits with vertical to subvertical fractures that disappear into underlying undeformed deposits. Event 2 is recorded by thin, sharply bounded sections composed of heterolithic mudstone, and very fine-grained sandstone that shows convolute folds and pillow-like structures. Event 3 is documented by intraformational conglomerate and breccia, and minor mudstones and sandstones, with abundant trace fossils (Skolithos and Thalassinoides). Event 4 is characterized by vertical to subvertical fractures and faults with homogenized sandstone intrusions along their planes, mostly developed in the upper parts of the deposits corresponding to Event 3. A detailed analysis of facies relationships, combined with consistent orientations of brittle deformation structures and the regional tectonics of the Cameta subbasin, suggests earthquakes as the triggering mechanism for sediment deformation and large-scale mass failure.

Internal architecture of mixed tide- and storm-influenced deposits: an example from the Alcântara Formation, northern Brazil

Abstract The uppermost portion of the Itapecuru Group is exposed in the eastern margin of the Sao Luis Basin, northern Brazil, where it consists of two units: the Alcantara Formation (Cenomanian) and the Cujupe Formation (Late Cretaceous-early Tertiary? [Rossetti, D.F., Truckenbrodt, W., 1997. Revisao estratigrdfica para os depositos do Albiano-Terciario Inferior (?) na Bacia de Sao Luis (MA), norte do Brasil. Bol. Mus. Paraense Emilio Goeldi (Ser. Cienc. Terra), in press]). The Alcântara Formation, which contains the large-scale structures discussed in this paper, consists of deposits attributed to mid- to upper-shoreface, foreshore, tidal channel, and lagoon/washover environments attributed to a regressive, barred shoreline. Several types of large-scale cross bedding (i.e., simple foreset, compound, mixed, undulatory, and intricately bounded) were recognized in the shoreface facies association. These structures are interpreted to record the interaction of storm and tidal processes. The storm influence is suggested by a combination of factors, mostly including: (a) the genetic association with other storm-generated sedimentary structures (i.e., swaley cross stratification and undulating parallel lamination with internal truncations); (b) the deposition on prominent surfaces formed by storm erosion, which are defined by large-scale, either symmetrical or asymmetrical scours arranged in a regular, repeating pattern; (c) the sedimentary features formed under combined (unidirectional and oscillatory) flow processes (e.g., compound/mixed bedding with superimposed either swaley cross sets or complexly truncating cross sets with highly undulating boundaries; large-scale, undulatory and intricately bounded cross beddings); and (d) the lateral change in structural styles within short distances, which records frequent modification from asymmetrical to symmetrical/nearly symmetrical bedform profiles (more likely to occur under storm-generated combined flows). The tidal signature is locally recognized by regularly spaced, thick/thin sandstone bundles defined by reactivation surfaces and/or mud drapes, which are attributed to tidal (ebb/flood) cycles. The analysis of paleocurrent distribution suggests that vigorous, southwestward-oriented storm flows interacted with local tidal currents on the shoreface to promote the landward transport of significant volumes of sand, which resulted in the large-scale cross stratification described in this paper. In addition, a secondary, southeastward-directed (oblique- to shore-parallel) combined flow would have periodically interacted with the main flow. The origin of this oblique- to shore-parallel flow is attributed to either the refraction of the main storm waves as they approached the paleocoastline or the interference of a separate storm episode, which competed with the major, landward-moving one. This complex storm flow pattern can be related to past penecontemporaneous seismic activity (i.e., tsunamis), as suggested by the paleogeography of the study area during the Cenomanian, combined with the structural history of the Sao Luis Basin and the sedimentary features recognized in the Alcantara Formation.

Facies analysis of the Codó Formation (Late Aptian) in the Grajaú Area, Southern São Luís-Grajaú Basin

Facies descriptions of the Codo Formation in the Grajau area are provided for the first time, and its sedimentary characteristics compared to those from the Codo area to allow paleoenvironmental reconstructions. Deposits in the Grajau area include evaporites, limestones and argillites bearing features indicative of a shallow, low energy, subaqueous, saline environment exposed to meteoric and/or capillary conditions. Floodingevaporative concentration-desiccation cycles suggest a saline pan complex surrounded by extensive evaporitic mudflats. The location of the system, whether coastal or inland, is a matter open for debate. However, the later hypothesis is favored considering: 1. Sr isotopic data, with values higher than those expected for Late Aptian marine waters; 2. calcitic composition of limestones (instead of dolomitic and/or magnesitic as expected in coastal settings); and 3. presence of continental ostracods and lack of marine fauna. This interpretation is consistent with that proposed for Upper Aptian deposits of the Codo area, but the depositional system there was one dominated by more stable, well-stratified, anoxic waters and evaporite precipitation in central lacustrine areas, while in the Grajau area the salt pan was more oxygenated and ephemeral, with salt precipitation mainly in marginal areas or along surrounding mudflats.

Delineating shallow Neogene deformation structures in northeastern Pará State using Ground Penetrating Radar

A caracterizacao geologica de depositos neogenos ocorrentes em sub-superficie rasa no nordeste do Estado do Para, usando Radar de Penetracao no Solo (GPR), revelou a presenca de falhas normais e reversas, bem como dobras, ainda nao documentadas em estudos de campo previos. As falhas sao identificadas por reflexoes inclinadas que cortam bruscamente reflexoes vizinhas, causando frequentes deslocamentos de camadas. As dobras sao reconhecidas por reflexoes fortemente ondulantes, configurando feicoes concavas e convexas que medem ate 50 m de amplitude e 80 a 90 m de profundidade. Estas estruturas deformacionais desenvolvem-se, principalmente, em depositos de idade miocenica, embora algumas falhas possam estender-se, tambem, a depositos mais jovens. Embora as secoes de GPR estudadas mostrem varias difracoes causadas por arvores, umidade do solo, e objetos enterrados, as estruturas registradas aqui nao podem ser explicadas por nenhum destes ruidos. A analise detalhada das secoes de GPR revelou que as feicoes deformacionais estudadas estao relacionadas com distorcoes de camadas causadas por deformacoes ruptil e ductil. Embora falhas e dobras nao sejam amplamente registradas em depositos neogenos da Zona Bragantina, os dados aqui apresentados sao consistentes com modelos estruturais, que propoem uma evolucao complexa incluindo movimentacoes transcorrentes para esta area, do Mioceno ate o presente.

ESTRATIGRAFIA DA SUCESSÃO SEDIMENTAR PÓS- BARREIRAS (ZONA BRAGANTINA, PARÁ) COM BASE EM RADAR DE PENETRAÇÃO NO SOLO.

Ground penetrating radar (GPR) is a geophysical method that consists in the emission of electromagnetic waves of high frequency (between 10-2500 Mhz). This equipment has been increasingly applied for mapping of physical properties and chemical compositions of rocks at shallow depth, as well as for stratigraphic and facies analysis. The application of this methodology in Brazil is still reduced to a few studies, mostly emphasizing soil stratigraphy and eolian paleoenvironments. However, previous studies undertaken in northern Brazil have demonstrated that, in combination with traditional studies using outcrop data, the GPR might be a powerful additional tool to help correlating sedimentary successions and reconstruct depositional environments throughout the Neogene in the Bragantina Zone. In this paper, we will document a pioneer investigation applying a SYR-2 GPR system (Geophysical Survey Systems Inc.) in the Praia do Atalaia area, Salinopolis, northern Brazil, which allowed us to characterize, for the first time, facies and stratigraphy of the deposits known informally as Pos-Barreiras Sediments (Pliocene and younger). The equipment was operated using a 200 Mhz monostatic antenna in continuous mode. The acquired sections were processed using the RADANWIN software in order to increase the reflection resolution and thus allow a refined interpretation of the sections. Based on this procedure, it was noticed that the studied sedimentary succession overlies unconformably the Miocene basement, represented by the Pirabas/Barreiras formations. Three stratigraphic units were also distinguished. The lowermost Unit 1 consists of an interval up to 6 m thick dominated by poorly-defined, low amplitude reflections, which intergrade with medium-scale, tangential oblique and hummocky reflections. This unit revealed to be discontinuous throughout the studied radar sections, occurring mainly where the Miocene basement is depressed. The middle Unit 2 is circa 9 m thick and mostly includes large-scale oblique reflections varying in style from parallel, tangential, sigmoidal to sigmoidal-complex. The two latter ones might reach up to 7 m thick. A strong, high amplitude and laterally continuous reflection marks the top of Unit 2, forming a bounding surface having local depressions of up to 40 m in relief. The uppermost Unit 3 corresponds to an interval ranging from 3.5 to 9 m thick, which is dominated by hummocky reflections, followed by medium-scale oblique, parallel to sub-parallel and cut and fill reflections. The GPR data allows the interpretation that the Pos-Barreiras Sediments were unconformably deposited over the Miocene deposits, as revealed by the mapping of an erosional bounding surface at the base of Unit 1. In addition, the analysis of internal reflection configurations and reflection geometries led us to propose that the Pos-Barreiras Sediments is more variable in terms of facies than previously thought, including eolian deposits (coastal dunes), as well as beach ridge, tidal flat, channel and mangrove deposits. Furthermore, the mapping of the three stratigraphic units described above is important to unravel the complexity of sedimentation versus erosion during the latest Neogene in northern Brazil. The conclusions obtained from the GPR data lead to suggest a drop in relative sea level after the end of the Miocene-lower Pliocene in the Bragantina Zone, which was followed by a transgression and deposition of Unit 1 along embayments and adjacent eolian dunes and/or beach ridges. Renewed phase of relative sea level drop took place, which led to erosion of the uppermost portions of Unit 1, forming a bounding surface that is locally cut by fluvial channel scouring. Large-scale eolian dunes laterally intergraded with tidal flat, filling the depressions formed by channel erosion during a subsequent transgressive phase. At least one more drop in relative sea level appears to have taken place in the study area, as revealed by the discontinuity surface between units 2 and 3, which is in turn mantled by modern sedimentation along mangroves, tidal channels and coastal dune environments.

Molar-Tooth Carbonates: Shallow Subtidal Facies of the Mid- to Late Proterozoic: Discussion

From field work in northern Canada and many other examples documented in the literature, James et al. (1998) highlighted the potential use of molar-tooth structures (i.e., a series of spar-filled cracks) as a stratigraphic indicator for the Mesoproterozoic and Neoproterozoic. These authors also stated that all known occurrences where molar-tooth structures are volumetrically significant are from shallow-water carbonates. Although these are comprehensive statements, considering that most molar-tooth structures reported so far are from shallow-water Mesoproterozoic and Neoproterozoic deposits, I am concerned about such proposed temporal and environmental constrictions, which are not supported by the field data, summarized in the following.nnDetailed field work in the Grajau Basin, northern Brazil, revealed that structures similar to the ones reported by James et al. (1998) are volumetrically widespread in the Codo Formation. This is a lacustrine lithostratigraphic succession formed during the early stages of the South Atlantic rifting in the Aptian (e.g., Petri and Fulfaro 1983). The sedimentary features described here occur in lime mudstone to grainstone, ostracodal grainstone, and crystalline limestone interbedded with shales strongly affected by penecontemporaneous deformation, as indicated by a variety of complex convolute folds, small faults, fissures, and bed collapses. The molar-tooth structures occur throughout these beds, although they substantially increase in volume downward, where evidence for soft-sediment deformation diminishes. These …