Abstract The Atuba Complex occurs in the eastern region of Paraná State, located in the Southern Ribeira Belt. It is comprised of the Curitiba Terrane, northward of the Luís Alves Terrane, and southward of the Apiaí Terrane. This research was carried out by means of detailed petrography, microtectonics, and brittle and ductile structural analysis. The Atuba Complex is composed of migmatites, felsic granulites, granitoids, quartzites, phyllonites, schists, amphibolites, and cataclasites. The lithotypes are cut by diabase or gabbro dikes, lamprophyre dikes, pseudotachylites, and quartz, epidote, carbonate, or chlorite veins. The migmatites are characterized by two migmatization phases, the first composed of tonalitic leucosomes and the second formed by granitic leucosomes. The stromatic and mylonitized migmatites suggest dislocation creep mechanisms and dynamic recrystallization processes, mainly in quartz and feldspars. During the initial phase, melting reactions took place through dehydration, while in the subsequent phase, melting was facilitated by the presence of water-fluxed. The Atuba Complex comprises two foliations, Sn-1 (Dn-1 phase) and Sn (Dn phase). The Sn-1 surface (compositional banding) presents a ductile and anastomosed aspect with an N45W/55NE attitude, whereas the Sn mylonitic foliation is defined by the presence of mineral flattening or stretching and by the orientation of amphibole and biotite shapes, with a maximum plane of N45E/85NW. The observed faults and fractures strike in the NE-SW and NW-SE main directions. The paleostress study provided the recognition of a NE-SW sigma 1 associated with a NW-SE-striking sigma 3 for the left-lateral strike-slip faults and an E-W compressive SHMAX and an N-S distensive SHMIN for the right-lateral strike-slip faults. State Belt Terrane petrography microtectonics analysis granulites granitoids quartzites phyllonites schists amphibolites cataclasites dikes pseudotachylites epidote carbonate veins phases processes feldspars phase dehydration waterfluxed. waterfluxed water fluxed. fluxed water-fluxed foliations Sn1 Sn- Dn1 Dn (Dn- phase. . compositional banding N45W55NE NWNE N45W 55NE N W NE attitude shapes N45E85NW NENW N45E 85NW E NW N45E/85NW NESW SW NWSE SE directions NWSEstriking striking leftlateral left lateral strikeslip slip EW NS S rightlateral right

Abstract This research aims to obtain tomographic images to interpret the seismic stratigraphic profile around the San Rafael (SR) knickpoint using seismic tomography. The SR waterfall on the Coca River sinkhole in February 2020 caused regressive soil erosion, suggesting that the knickpoint is highly unstable. Seismic tomography for the P-wave velocity model, utilizing data from 14 stations in the permanent Ecuadorian seismic network, produced vertical cross-sections at azimuths of 0°, 45°, 90°, and 80°. These sections reveal geological formations and their transition to volcanic deposits, while a horizontal cross-section illustrates the extent of the volcanic deposits. The tomographic images facilitated the identification and correlation of seismic stratigraphy with the area's geological features. The most unstable materials for erosion lie from 0 km to 1.5 km with velocities of up to 3.5 km/s. The erosive process impacts the population and infrastructure settled on the banks of the Coca River with a high susceptibility to erosion in zone 1, with velocities of 3 km/s and a 10 km extension. The Coca Codo Sinclair Dam (CCSD) is located in zone 2, with velocities of 3.8 km/s extending up to 40 km with medium susceptibility to erosion. (SR 202 Pwave P wave model 1 network crosssections cross 0° 45 45° 90 90° 80 80° deposits crosssection section areas area s features 15 5 1. 35 3. kms extension CCSD (CCSD 2 38 8 4 20 9

Abstract The Quadrilátero Ferrífero (QF) is a globally known region for its world-class metallic deposits. Although its Precambrian units have been vastly studied since the 1960s due to economic importance, the Cenozoic geological record represents an upcoming subject of interest. The occurrence of Cenozoic sedimentary deposits has been noticed since the end of the 19th century, but most of them are not yet fully characterized. This paper brings a complete description, mapping, 3D modeling, and simulations of the sedimentary deposit named Bandeira Basin, Serra do Tamanduá, QF. Based on field data, such as mapping and drill core database analysis, three drill cores were selected for further investigation and detailed description. The Bandeira Basin can be characterized and subdivided into three units: Unit A: found only in the southeast area and shows exclusively sediments from the Nova Lima Group; Unit B: intermediate unit that occurs in central and southeast areas, with Rio das Velhas Supergroup as a possible source of the sedimentary material; and Unit C: occurs at the top, and it is essentially colluvial deposits from Minas Supergroup units. The contacts between units are abrupt or erosive, recording successive deposition stages. The basin is thicker on its central and southwest ends and thinner on the east and north, where part of the basement outcrops. Cenozoic tectonic events were probably active in this area, controlling the sedimentary processes, with the NW-SE and NE-SW lineaments reactivated, triggering sediment deposition and migration to the deepest spot of the basin. These sediments were 3D modeled in Leapfrog Geo, the visualization of any given region of interest, in the modeled scale. This methodology allowed an assertive characterization of sedimentary filling and could be applied to other basins. This comprehension is important because these sediments are used indirectly or directly in civil projects. QF (QF worldclass world class s importance interest th century description D modeling Tamanduá data analysis A Group B areas material C top erosive stages north outcrops processes NWSE NW SE NESW NE SW reactivated Geo scale basins projects

Abstract On the southern edge of the Amazonian Craton, the Mirassol d’Oeste and Guia formations constitute the cap carbonate of the Araras Group. Petrography, X-ray diffraction, and quantification of terrigenous material allowed the identification of detrital grains (e.g., mica, quartz, and feldspar) in different proportions and diagenetic features. The sequential leaching of carbonates along a section of the upper portion of the Guia Formation yielded 87Sr/86Sr ratios increasing from 0.7070 to 0.7077, which is considered the primary Sr isotopic signature of seawater. The increasing continental influx of dissolved Sr from freshwater accounts for the increase in 87Sr/86Sr along the section. Such abrupt change toward more radiogenic values is also observed in carbonate layers soon after the Marinoan glaciation, elsewhere in the world. The same samples provided Nd-TDM model ages (1.84–2.54 Ga) comparable with those of the continental crust of the Paleoproterozoic provinces of the Amazonian Craton, which forms the basement of the Araras Group. The ԐNd(635Ma) values between -4.7 and -11.1, probably indicate the presence of Nd from the suspended material transported by continental waters to the ocean where the carbonates precipitated. The random variations of Nd-TDM ages and ԐNd values suggest important and rapid variations in the continental contribution to the post-Marinoan oceans. Craton dOeste d Oeste Group Petrography Xray X ray diffraction e.g., eg e g (e.g. mica quartz feldspar features 87Sr86Sr SrSr 87Sr 86Sr 07070 0 7070 0.707 07077 7077 0.7077 seawater glaciation world NdTDM TDM 1.84–2.54 184254 1 84 2 54 (1.84–2.5 Ga ԐNd635Ma ԐNdMa 635Ma Ma ԐNd(635Ma 4.7 47 4 7 -4. 11.1, 111 11 -11.1 precipitated postMarinoan post oceans e.g. (e.g 0707 707 0.70 1.84–2.5 18425 8 5 (1.84–2. 4. -4 11.1 -11. e.g 070 70 0.7 1.84–2. 1842 (1.84–2 - 11. -11 07 0. 1.84–2 184 (1.84– -1 1.84– 18 (1.84 1.84 (1.8 1.8 (1. 1. (1 (

Abstract The Bauru Supersequence comprises two chronocorrelates groups, namely, Caiuá and Bauru. While substantial efforts have been dedicated to detailed mapping in the eastern portion of the Bauru Basin, the western part, specifically the Caiuá Group, remains unresolved and undivided. The aim of this research was to characterize the occurrence area of the Rio Paraná Formation in the state of Mato Grosso do Sul, which is located in the Midwest region of Brazil, as well as its lithostratigraphic characteristics, and the implications for a better understanding of the paleoclimate and paleogeography of that region during the Cretaceous. The Rio Paraná Formation is constituted by six lithofacies: sandstone with cross-stratification (Sct), sandstone with trough cross-stratification (Sct-t), sandstone with cross-stratification pebbles (Sctp), massive sandstone (Sm), massive sandstone with fragments of basalt (Smb), and sandy mudstone (Fme). It was possible to group three facies associations: large-size eolian dunes, interdunes, and sand sheet deposits. The direction of paleocurrent is southwest, which is similar to those previously recognized in the eastern part of the basin and chronocorrelates basins in Brazil. Based on the results obtained in this research, it was possible to provide evidence for the future paleogeographic reconstruction of the Late Cretaceous in South America. groups namely Basin Group undivided Sul Brazil characteristics lithofacies crossstratification cross stratification Sct, Sct , (Sct) Sctt, Sctt t (Sct-t) Sctp, Sctp (Sctp) Sm, Sm (Sm) Smb, Smb (Smb) Fme. Fme . (Fme) associations largesize large size dunes interdunes deposits southwest America (Sct (Sct-t (Sctp (Sm (Smb (Fme

Abstract The sedimentary basins in the Continental Rift of Southeastern Brazil (CRSB) provide an opportunity for ichnological analyses due to its well-known facies relationships and paleoenvironmental interpretations. This study reports invertebrate trace fossils from the Neogene fluvial deposits (Floriano Formation) and applies integrated ichnological and facies analyses to reconstruct the environmental conditions recorded in this lithostratigraphic unit. Fieldworks revealed a rich ichnofauna in the Floriano Formation, whose facies associations have been interpreted as indicative of meandering river environments. Four sedimentary facies were identified, including bioturbated sandstone, horizontally stratified sandstone, laminated sandstone, and mudstone. Six ichnogenera were identified, Arenicolites, Beaconites, Camborygma, Palaeophycus, Skolithos, and Taenidium. Those ichnogenera occur as Skolithos, Taenidium, and Beaconites ichnocoenoses. The Skolithos ichnocoenosis is primarily represented within sand bar deposits, indicating arthropod colonization in abandoned channels. The Taenidium ichnocoenosis, which is observed within mudstone layers and presents Camborygma burrows, suggests transitional environments between aquatic and subaerial conditions, locally with high water table levels. The Beaconites ichnocoenosis indicates changes in substrate consistency associated with progressive desiccation and extended exposure time. Therefore, the identified ichnocoenoses, combined with the analysis of physical sedimentary structures, suggest generally humid conditions during colonization by invertebrates of the meandering river deposits of the Floriano Formation. CRSB (CRSB wellknown well known interpretations Formation unit sandstone Arenicolites Palaeophycus ichnocoenoses channels burrows levels time Therefore structures

Abstract The final consolidation of southwestern Gondwana during the Ediacaran–Cambrian resulted in the formation of a large landmass originally surrounded by the newborn Panthalassic Ocean. The Río de la Plata Craton is one of the last pieces that complete the geotectonic puzzle along the austral part of this supercontinent. However, the sedimentary record corresponding to the interval between the consolidation of SW Gondwana until the initial deposition in a large Ordovician cratonic basin is apparently missing. In this context, the Ediacaran–Cambrian epiclastic shallow-marine ramp of the Cerro Negro Formation, the uppermost unit of La Providencia Group, Tandilia System, is envisaged as the first known stratigraphic record of the hidden pericratonic basins of Gondwana. Geochronological and provenance data, coupled with robust stratigraphic and sedimentologic background, support that this unit keeps more tectono-sedimentary affinity with the Ordovician Balcarce Formation than the Neoproterozoic Sierras Bayas Group. The presence of similar Neoproterozoic source areas for both units, the absence of any metamorphism and deformation related to the Brasiliano cycle, and shallow marine deposition strongly suggest that the paleogeography and physiography of the original southernmost continental platform of Gondwana were similar to the Ordovician ones. Thus, the Cerro Negro Formation appears as the missing link between the ultimate stages of the Brasiliano Orogenic Cycle and the final stabilization of the continental platform from the SW part of Gondwana. EdiacaranCambrian Ediacaran Cambrian Ocean supercontinent However context shallowmarine Group System data background tectonosedimentary tectono units cycle ones Thus

Abstract Mass movements naturally occurring along slopes, especially after heavy rains, cause heavy losses of materials and life and affect highways, pipelines, and bridges, among other structures. Among the most common mass movements, landslides and subsequent mass flows especially stand out. Remote sensing and geoprocessing techniques are useful tools for geological and geotechnical analysis, here applied to characterization of mass movements from the analysis of the influence of slopes, lineaments or fractures, and geological-geotechnical at the junction of the Curral homocline and the Moeda syncline, Quadrilátero Ferrífero, Southeast Brazil. The area consists of Archean granitic-gneissic basement and the greenstone belt sequence of the Rio das Velhas Supergroup with Proterozoic metasedimentary rocks of the Minas Supergroup covering these two older successions. Based on satellite images, a preliminary map of lineaments was created, and fractures were grouped according to their direction before being rasterized. Each lineament group was associated with a previously known geological and/or structural feature. Geological units mapped in the area were numerically classified according to their mass movement susceptibility and related geotechnical units. Using a Shuttle Radar Topography Mission (SRTM) image, a terrain slope raster was generated. The objective was to obtain a raster indicating low and high probabilities of mass movements. Four sets of lineaments were identified: the first set associated with the northeast strike stratification of the rocks of the Moeda Formation; the second set related to eastward-trending warp axis lineations; the third set related to the southeast trend of stretch lineaments or mafic dykes directions; and the fourth set evidently associated with the watershed pattern of the Curral Range crest, perpendicular to the strike of the rocks in the area. Simulations with different combinations of parameters produced eight scenarios were generated were constructed to classify the area as to its susceptibility to rupture through changes in slope, lithotype, and lineament density; the first four (1 to 4) were classified by using the slope percentage raster without interval reclassification, so its values in the cartographic algebra were not grouped. The last for (5 to 8), consider the slope percentage classified. This methodology successfully combines mathematical models for predicting mass movements and determining areas in the urban landscape most susceptible to these phenomena. As such, it comprises a useful tool for government planning of preventative actions for areas of high risk. slopes rains highways pipelines bridges structures out geologicalgeotechnical syncline Ferrífero Brazil graniticgneissic granitic gneissic successions images created rasterized andor feature SRTM (SRTM image identified Formation eastwardtrending eastward trending lineations directions crest lithotype density 1 ( 4 reclassification 5 8, 8 , 8) phenomena such risk

Abstract The Rio dos Remédios Group comprises a supracrustal sequence that occupies the base of the Espinhaço Supergroup, São Francisco Craton, Brazil. Its basal formation, Novo Horizonte, crops out in the Paramirim region mainly as metavolcanic rocks that represent one of the fewer occurrences of blue quartz phenocrysts in South America. Their mineralogy consists of quartz and K-feldspar phenocrysts, whereas biotite, muscovite, fluorite, allanite, chlorite, sericite, zircon, and opaque phases occur immersed in a quartz-feldspar-rich groundmass. Such heterogeneous composition is also supported by x-ray diffraction and chemical data. Electron probe microanalysis in some samples revealed the presence of two distinct groups of biotite (magmatic and neoformed), in addition to the presence of iron-rich white mica and almost pure orthoclase feldspar. Our data suggest that the studied metavolcanic rocks have maintained their magmatic characteristics, which were progressively overprinted by hydrothermal fluids and ductile-to-brittle deformation. The magmatic mineralogy is akin to strongly peraluminous and alkaline magmas, common in anorogenic settings – a fertile site for the origin of blue quartz-bearing rocks worldwide. Supergroup Craton Brazil formation Horizonte America Kfeldspar K feldspar muscovite fluorite allanite chlorite sericite zircon quartzfeldsparrich rich groundmass xray x ray neoformed, neoformed , neoformed) ironrich iron characteristics ductiletobrittle ductile brittle deformation magmas quartzbearing bearing worldwide

Abstract Central Brazil hosts Paleo-Mesoproterozoic A-type granitic suites related to the Goiás Tin Province (GTP) that contain Sn (±W, Nb-Ta, REE) mineralization associated with greisen, veins, and small pegmatite bodies. The Pedra Branca granite massif (1.77–1.74 Ga) is the main representative of GTP, marked by important cassiterite (±wolframite) contents. The cassiterite contains SnO2= 96–100 wt. %, with the sum FeOtotal, TiO2, WO3, Ta2O5, Nb2O5, In2O3, and UO2 content below 4 wt.%, while wolframite contains WO3= 71.5–74.5 wt.%, FeOtotal= 14.3–17.4 wt.% and MnO = 6.3–9.9 wt.%, as well as Sn, Ca, Ti, Ta, Nb, Pb, In, and U as trace elements. The δ18O data on the quartz-cassiterite pair (quartz = 9.4–10.4‰, cassiterite = 2.6–2.9 ‰) from greisen reveal a magmatic-hydrothermal signature with calculated crystallization temperatures between 410 and 485°C. However, during the Neoproterozoic Brasiliano/Pan-African Orogeny (800–500 Ma), all lithologies and ore sites were subjected to flattening, fragmentation, and mylonitic deformation. Fluid inclusion data revealed the presence of low-salinity aqueous solutions with homogenization temperatures between 215 and 100°C related to Neoproterozoic deformation. Finally, during the Phanerozoic, prolonged erosive produced Sn (±W)-rich alluvium around the Pedra Branca granitic massif. PaleoMesoproterozoic Paleo Mesoproterozoic Atype A type GTP (GTP ±W, W (±W NbTa, NbTa Nb Ta Nb-Ta REE veins bodies 1.77–1.74 177174 1 77 74 (1.77–1.7 Ga ±wolframite (±wolframite contents SnO2 SnO 96100 96 100 96–10 wt % FeOtotal TiO2 TiO WO3 WO Ta2O5 TaO O Nb2O5 NbO In2O3 InO In UO 715745 71 5 71.5–74. 143174 14 3 17 14.3–17. 6399 6 9 6.3–9. Ca Ti Pb elements δO δ quartzcassiterite quartz 94104 10 9.4–10.4‰ 2629 2 2.6–2. ‰ magmatichydrothermal magmatic hydrothermal 41 485C C 485 485°C However Brasiliano/PanAfrican BrasilianoPanAfrican Brasiliano/Pan African Brasiliano Pan 800–500 800500 800 500 (800–50 Ma, Ma , Ma) flattening fragmentation deformation lowsalinity low salinity 21 100C Finally Phanerozoic ±Wrich Wrich ±W rich 1.77–1.7 17717 7 (1.77–1. 9610 96–1 Ta2O Nb2O In2O 71574 71.5–74 14317 14.3–17 639 6.3–9 9410 9.4–10.4 262 2.6–2 48 PanAfrican BrasilianoPan 800–50 80050 80 50 (800–5 1.77–1. 1771 (1.77–1 961 96– 7157 71.5–7 1431 14.3–1 63 6.3– 941 9.4–10. 26 2.6– 800–5 8005 8 (800– 1.77–1 177 (1.77– 715 71.5– 143 14.3– 6.3 94 9.4–10 2.6 800– (800 1.77– (1.77 71.5 14.3 6. 9.4–1 2. (80 1.77 (1.7 71. 14. 9.4– (8 1.7 (1. 9.4 ( 1. (1 9.

Abstract The Geological Strength Index (GSI) system is the basis of parameters used in the Hoek-Brown failure criterion for rock mass strength estimation. The author tested this system and here suggests a modified GSI called Geological Strength Index-slope (GSIslope). The modified system combines two different existing approaches: the GSI system and Slope Mass Rating (SMR). The purpose of GSIslope is to allow engineering geologists to quickly evaluate the stability of natural and excavated slopes or open-pit mining in the field. GSIslope is computed by subtracting a constant value of 10 and the multiplication of adjustment factors for discontinuity orientation and slope (F1, F2, and F3, based on the parallelism of discontinuity and slope, discontinuity dip angle, and the difference between the inclination angle of discontinuity and slope) from GSI, and adding field groundwater rating to it. Modified curves are also proposed in this work to determine the accurate ratings of the adjustment factors. The results of this work are compared to the values obtained from equations of continuous-SMR and SMR-value itself for both the adjustment factors and GSIslope values. The comparison showed that the proposed curves and GSIslope equation are valid and easy to use for estimating the adjustment factors’ ratings and GSIslope value. (GSI HoekBrown Hoek Brown estimation Indexslope GSIslope. . (GSIslope) approaches SMR. SMR (SMR) openpit open pit 1 F1, F1 F (F1 F2 F3 it continuousSMR continuous SMRvalue (GSIslope (SMR (F

Abstract Sinter overlying the first quartz andesite flow at the base of the Serra Geral Group is a surface manifestation of intense hydrothermal processes operating in the Paraná Basin during the Early Cretaceous. The coarse quartz sinter from western Rio Grande do Sul state was studied in satellite images, field surveying, optical petrography, scanning electron microscopy, and electron probe microanalyses, including backscattered electron images and chemical analyses of rocks. Quartz forms large crystals (10 cm) because it was either deposited in a dilute aqueous solution or recrystallized from fine-grained sinter. Well-crystallized chamosite — an iron aluminosilicate (Fe-chlorite) — occurs in quartz crystals, partly associated with fractures. The composition of chamosite is akin to that in ore deposit associations. Noble metals in two sinter samples are present in concentrations of 0.1 ppm Ag and 15 ppb Au. The contents of Ba, Bi, Cu, Mo, S, and W are low but are significant. The present description of sinter quartz signals the presence of a major paleo-hotspring field in the Fronteira Oeste Rift, Rio Grande do Sul, Brazil, that mertis further study to fully characterize the extent and metallogenetic endowment (Au-Ag-Cu) of the epithermal province. Cretaceous surveying petrography microscopy microanalyses rocks 10 (1 cm finegrained fine grained Wellcrystallized Well crystallized Fechlorite Fe chlorite (Fe-chlorite fractures associations 01 0 1 0. Au Ba Bi Cu Mo S significant paleohotspring paleo hotspring Rift Brazil AuAgCu (Au-Ag-Cu province (

Abstract Seismic data, borehole image logs, and conventional well logs were used to investigate the distribution and characteristics of paleokarst features in the Aptian carbonates of the Barra Velha Formation in a pilot area of the Santos Basin, Brazil. Multiple seismic attributes were used to enhance details on the seismic data and highlight key seismic parameters including strata deformation and geometry, continuity of seismic events, and fault patterns. The study found that karst structures are controlled by faults and fractures along structural highs, which served as a conduit for the flow of dynamic fluids that dissolved the carbonate materials. Several closed, circular depressions and bright spots identified in the northeastern portion of the study area represent possible sinkhole structures. Epigenic and hypogenic processes due to the action of meteoric water, hydrothermal activity, and intra-formation acidity along regional unconformities in the Barremian-Aptian may have been responsible for the dissolution. Limitations of this study are related to the difficulty of integrating multiple datasets with various scales. However, the higher confidence for the occurrence of the karst features is provided by borehole images at the sub-seismic scale. The findings of this study hold significant relevance for the strategic planning of energy development and carbon sequestration initiatives in the Brazilian continental margins, thereby aiding in informed decision-making. Basin Brazil geometry events patterns highs materials closed water activity intraformation intra formation BarremianAptian Barremian dissolution scales However subseismic sub scale margins decisionmaking. decisionmaking decision making. making decision-making

Abstract Benthic foraminifera, ostracods and pteropods are reliable paleoenvironmental indicators in Quaternary deposits. However, in the Ceará Basin, on the Brazilian Equatorial Margin, these microfossils are poorly studied. This paper investigates environmental changes during the Pleistocene–Holocene transition in the Icaraí subbasin based on micropaleontological analysis of the core ANP 1011. Seventy-four taxa of benthic foraminifera, represented predominantly by Globocassidulina, Uvigerina, Pyrgo and Melonis, have been identified. The ostracod assemblages are composed mainly by the families Macrocyprididae, Cytheruridae, Trachyleberididae, Pontocyprididae and Krithidae, of which the genus Krithe was the most abundant. The composition of the ostracod assemblages identified in this study area differs somewhat from other regions of the Brazilian Margin. The assemblages of foraminifera and ostracod characterize a typical bathyal paleoenvironment. The occurrence of pteropods and dominance of epifaunal foraminifera taxa, mainly Pyrgo sp. and Miliolinella sp. in the lower portion of the core (Pleistocene), indicates higher phytodetritus input and oxygen concentration. A conspicuous environmental change was observed in the upper portion of the core, which corresponds to the Holocene, where the increase of infaunal foraminifera (e.g., Uvigerina, Globocassidulina and Melonis) suggests reduction in the organic matter input and, probably, increased bacterial density and depletion in dissolved oxygen in the sediment. deposits However Basin Margin studied PleistoceneHolocene Pleistocene Holocene 1011 Seventyfour Seventy four Uvigerina Melonis Macrocyprididae Cytheruridae Trachyleberididae Krithidae abundant paleoenvironment sp Pleistocene, , (Pleistocene) concentration e.g., eg e g (e.g. probably sediment 101 (Pleistocene e.g. (e.g 10 e.g 1

Abstract Disarticulated shells of three bivalve mollusk species (Anomalocardia brasiliana, Codakia orbicularis, and Divaricella quadrisulcata) were experimentally tested in laboratory flumes to determine the threshold of motion and final orientation of the valves. A total of 150 current flow experiments were conducted on single shells resting on a fixed sand bed. This study demonstrated that shells in the convex-up position are more resistant to flow when the umbo is pointing downstream rather than upstream. Moreover, species with higher frontal areas were more likely to be entrained at lower flow velocities. Results of dimensionless shear stress exhibited values far below the threshold of grains movement for beds of uniform roughness (Shields curve). It was observed that circular shells in convex-up positions were mostly orientated with the umbo pointing downstream. Conversely, elliptical shells in convex-up position tended to align their longer axis parallel to the flow with the posterior side of the valve pointing downstream. These results are not only directly applicable in interpretations of incipient shell motions and in paleocurrent analyses from field and sample data, but also support construction of accurate geological models. Anomalocardia brasiliana orbicularis quadrisulcata valves 15 bed convexup convex up upstream Moreover velocities Shields curve. curve . curve) Conversely data models 1