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Serviço Geológico do Brasil - CPRM Universidade Federal do Pará University of Oslo Universidade de São Paulo Universidade Federal do Pará Universidade Federal do Pará Universidade Federal do Pará Universidade Federal do Pará Serviço Geológico do Brasil - CPRM University of Oslo The groundwater from coastal aquifers in the Eastern Amazonia coast has been documented under intense stress by anthropogenic and natural impacts ranging from saltwater intrusion, contaminated effluents by mining industries and agriculture, and the pollution due to the proximity of big cities and secondarily by oil spillage from small boats. Here we characterized the hydrostratigraphy from the coastal aquifers of this region provide information for future research to verifying the resilience of groundwater to both natural and anthropogenic impacts. Based on outcrops and borehole logs, lithological profiles were used to generate cross-sections along Eastern Marajó Graben (EMG) and Bragantina Platform (BP). The groundwater information system (SIAGAS) of the Geological Survey of Brazil (SGB), with headquarters in the Metropolitan Region of Belém, the main city of Eastern Amazonia, involving five districts, contains 2348 registered wells in its database. Samples from five wells of the Groundwater Monitoring Network (RIMAS) of the SGB are being analyzed for palynological and micropaleontological (foraminifera and ostracodes) analysis and provenance studies of heavy minerals, Sm-Nd, and detrital zircon geochronology. The tectonic subdivision of this region in the EMG and BP was not considered in the previous aquiferous system definition, including the Oligocene-Miocene carbonate-siliciclastic rocks, the Pirabas Formation, sandwiched by Cretaceous rocks and the middle Miocene Barreiras Formation. The new proposal indicates that the 120 m-thick Pirabas Formation, generally observed below 50m deep in the wells, is restricted to the BP. Their siliciclastic correlate deposits in the EMG are the Miocene Marajó Formation. The 70 m-thick subsurface succession in EMG, previously considered Barreiras deposits, is also reinterpreted here as the Marajó Formation, which comprises mudrock and fine- to coarse-grained sandstone. The 20 m-thick Barreiras Formation is exposed in the EMG and BP. It does not belong to the Eastern Amazonia coast's aquiferous system, which is now wholly inserted in the Marajó Formation. The upper part of the Marajó Formation has 30 m-thick of estuarine mudrock beds, sealing the underlaid 200 m-thick fine marine deposits with microfossils representing the main interval for the potable water. The hydrostratigraphy proposed here allows us to draw new strategies to manage groundwater exploration on the Eastern Amazonian coast and mitigate actions to protect this aquifer's groundwater resources.

Glaciotectonic features studied in the siliciclastic deposits of Cabeças Formation, Upper Devonian, represent the first evidence of Famennian glaciation in Southeastern Parnaíba Basin, Brazil. Outcrop-based stratigraphic and facies analyses combined with geometric-structural studies of these deposits allowed defining three facies association (FA). They represent the advance-retreat cycle of a glacier. There are: delta front facies association (FA1) composed of massive mudstone, sigmoidal, medium-grained sandstone with cross-bedding and massive conglomerate organized in coarsening- and thickening-upward cycles; subglacial facies association (FA2) with massive, pebbly diamictite (sandstone, mudstone and volcanic pebbles) and deformational features, such as intraformational breccia, clastic dikes and sills of diamictite, folds, thrust and normal faults, sandstone pods and detachment surface; and melt-out delta front facies associations (FA3), which include massive or bedded (sigmoidal cross-bedding or parallel bedding) sandstones. Three depositional phases can be indicated to Cabeças Formation: installation of a delta system (FA1) supplied by uplifted areas in the Southeastern border of the basin; coastal glacier advance causing tangential substrate shearing and erosion (FA1) in the subglacial zone (FA2), thus developing detachment surface, disruption and rotation of sand beds or pods immersed in a diamicton; and retreat of glaciers accompanied by relative sea level-rise, installation of a high-energy melt-out delta (FA3) and unloading due to ice retreat that generates normal faults, mass landslide, folding and injection dykes and sills. The continuous sea-level rise led to the deposition of fine-grained strata of Longá Formation in the offshore/shoreface transition in the Early Carboniferous. RESUMOEstruturas glaciotectônicas estudadas nos depósitos siliciclásticos da Formação Cabeças do Devoniano Superior representam a primeira evidência da glaciação fameniana na borda Sudeste da Bacia do Parnaíba, no Norte do Brasil. A análise estratigráfica e faciológica em combinação com estudos geométrico-estruturais desses depósitos permitiram definir três associações de fácies (AF) representativas de um ciclo de avanço-recuo de geleira, a saber: associação de fácies de frente deltaica (AF1), composta por pelitos maciços, arenitos finos a médios com estratificação cruzada sigmoidal e conglomerado maciço, organizados em ciclos grano- e estrato-crescentes; associação de fácies subglaciais (AF2), consistindo de diamictitos maciços seixosos (seixos de arenito, pelito e rochas vulcânicas) e feições deformacionais, tais como brechas intraformacionais, sills e diques clásticos de diamictitos, dobras, falhas normais e de cavalgamento, pods de arenitos e plano de descolamento; e associação de fácies de frente deltaica de degelo (AF3), constituída por arenitos maciços ou laminados (estratificação plano-paralela e/ou cruzada sigmoidal), localmente com deformações sin-sedimentares. Três fases deposicionais são indicadas para os depósitos da Formação Cabeças: instalação de um sistema deltaico (AF1), advindo de áreas soerguidas do Sudeste da bacia; avanço de geleiras costeiras, que causa cisalhamento tangencial e erosão do substrato (AF1), em zona subglacial (AF2), desenvolvendo superfície de descolamento, rompimento e rotação de camadas arenosas ou pods imersos em diamicton; e recuo das geleiras acompanhado pelo aumento relativo do nível do mar, pela instalação de um sistema deltaico de degelo de alta energia (AF3) e processos de alívio de pressão que geram falhas normais, escorregamento de massa, dobras e injeções de diques e sills. O aumento contínuo do nível do mar levou à deposição dos sedimentos finos da Formação Longá na zona de transição offshore/shoreface no início do Carbonífero.

Os sedimentos do lago Coari, de ambiente de terra firme esculpido nos depósitos do Plio-Pleistocenos, e o Acará, típico lago de várzea e ambos formados nos sedimentos quaternários da planície de inundação do médio Solimões, no oeste da Amazônia, Brasil, foram estudados para investigar as condições ambientais durante sua formação. Este estudo inclui dados da composição mineralógica, química, isótopos de Pb, palinologia, datações de radiocarbono e a configuração morfológica dos lagos obtida por imagens SRTM. As condições geológica e ambiental dos lagos variam e sugerem que suas evoluções refletem processos autogenéticos em condições de floresta úmida e chuvosa. Embora caulinita, quartz, muscovita, illita e esmectita sejam os principais minerais em ambos os lagos, a geoquímica indica fonte distinta, os sedimentos do lago Acará têm maior concentração de Al2O3, Fe2O3, FeO, CaO, K2O, MgO, Na2O, P2O5, Ba, V, Cu, Ni, Zn, Pb, Sr, Li, Y e La e têm mais Pb radiogênico que os sedimentos do lago Coari. As idades de radiocarbono sugerem que há aproximadamente 10160 anos AP o lago Coari iniciou o desenvolvimento devido a avulsão do rio Solimões, enquanto o lago Acará foi formado devido ao abandono de meandro do rio Solimões e retendo o domínio das gramíneas nas suas praias há aproximadamente 3710 anos AP. The sediments from the Coari lake, a “terra firme” lake sculpted into Plio-Pleistocene deposits, and the Acará lake, a flooding-type lake developed on Quaternary sediments in the floodplain of the mid-Solimões river, in the western Amazônia, Brazil, were studied to investigate the environmental condition of their developing. This study includes mineral composition, geochemistry, Pb isotope, palinology, radiocarbon-age and morphological framework of the lakes obtained from SRTM satellite images. The geological and the environmental conditions in the two lakes are highly variable and suggest that their evolution reflect autogenic processes under humid rainforest condition. Although kaolinite, quartz, muscovite, illite, and smectite are the main minerals in both lakes, the geochemistry indicates distinct source, the Acará lake sediments have higher concentrations of Al2O3, Fe2O3, FeO, CaO, K2O, MgO, Na2O, P2O5, Ba, V, Cu, Ni, Zn, Pb, Sr, Li, Y and La and have more radiogenic Pb than the Coari lake sediments. The radiocarbon ages suggest that at 10160 yr BP the Coari lake started to be developed due to avulsion of the Solimões river, and the Acará lake was formed by the meander abandonment of Solimões river retaining its grass dominated shore at ca. 3710 yr BP.