First evidence of marine influence in the Cretaceous of the Amazonas Basin, Brazil

Abstract

An integrated investigation emphasizing sedimentological and ichnological studies of Cretaceous deposits of the Alter do Chão Formation, exposed in the western Amazonas Basin, was undertaken with the aim of determining depositional environments. Four facies associations attributed to upper shoreface, foreshore, delta mouth bar, and lower/middle shoreface-prodelta depositional environments are recognized. The upper shoreface deposits were deposited by storm flows. They are interbedded with highly bioturbated sandstones displaying Thalassinoides, Planolites and Diplocraterion traces. The foreshore deposits, which are coarser-grained than the shoreface strata, are characterized by tabular sandstones displaying planar or trough cross-lamination/stratification, wavy/flaser lamination, and parallel lamination. These strata also contain an abundance of trace fossils. The delta mouth bar deposits comprise upward-coarsening beds displaying a lobed geometry. The lower/middle shoreface-prodelta settings consist of well-stratified, very fine-grained sandstones and mudstones deposited mostly by storm action. A wavedominated delta system that prograded into a marine-influenced basin is supported for the study area. Therefore, in contrast to previous interpretations, it seems that a widespread Cretaceous transgression resulted in the submergence of large continental areas in the north of Brazil, affecting sediment deposition even in the innermost portions of the intracratonic Amazonas Basin.

Middle Turonian trace fossils from the Bystrzyca and Długopole sandstones

“…Lob sters and crabs are also pro posed (Frey et al, 1984;Myrow, 1995;Bromley, 1996;Gingras et al, 2002;De, 2005;Rossetti and Netto, 2006). Cerianthid sea anem o nes, acorn worms and fish are also spec u lated as the pos si ble tracemakers of Thalassinoides (Myrow, 1995).…”

Análise faciológica da Formação Alter do Chão (Cretáceo, Bacia do Amazonas), próximo à cidade de Óbidos, Pará, Brasil

“…Na borda norte da bacia, próximo a Manaus, foram caracterizadas, em seções sísmicas, feições evidentes de corte e preenchimento de canais na base dessa formação (Costa 2002). Nesta mesma região, um sistema deltaico, dominado por onda e com infl uência marinha, tem sido proposto para os depósitos da Formação Alter do Chão (Rossetti & Netto 2006).…”

“…Esses depósitos são constituídos, principalmente, de arenitos bem estruturados, além de conglomerados e pelitos limitados no topo por um paleossolo laterítico de idade paleógena (Boulangé & Carvalho 1997). Estudos enfocando aspectos sedimentológicos e paleontológicos nos depósitos cretáceos da bacia do Amazonas foram abordados, de maneira detalhada, pela primeira vez na ilha de Careiro, próximo de Manaus (Rossetti & Netto 2006). Esse estudo atribuiu, naquele lugar, infl uência marinha à Formação Alter do Chão, fato este observado pela primeira vez na literatura, uma vez que tais depó-sitos tinham sua gênese relacionada com ambiente fl ú-vio-deltáico-lacustre (Kistler 1954, Caputo et al 1972, Wanderley Filho 1991, Cunha et al 1994, Dino et al 1999, Nogueira et al 1999.…”

Trace fossils from the Permian Teresina Formation at Cerro Caveiras (S Brazil)

“…The virtual absence of vertical burrows and the common occurrence of monospecific colonization of Planolites, a common suite in non-marine environments, suggest that such fluctuation would be extreme, leading to the establishment of freshwater conditions and to the maintenance of these conditions by long periods (e.g. Bann & Fielding, 2004;Melchor, 2004;MacEachern et al, 2005;Rossetti & Netto, 2006;Gani et al, 2007;Buatois et al, 2008;Carmona et al, 2009).…”