Abstract

Sedimentary models that apply to the Middle Miocene succession in Amazonia are controversial. Although tidally-influenced sedimentary deposits have been described from several locations, the identification of brackish-water or marine facies has been hampered by limited outcrop exposure. Also, ichnological data largely have been ignored.

This study focuses on ichnological and sedimentological relationships observed in outcropping strata of the Solimoes Formation (Middle Miocene) along the Acre River in western Brazil and northern Bolivia. The studied strata comprise a fine-grained lower unit that is sharply overlain by dipping, interbedded sands and muds, known as inclined heterolithic stratification (IHS). The IHS is present throughout the length of the outcrop, about 80m. The outcropping strata are interpreted to represent two depositional subenvironments: (1) A lower unit that resulted from sediment accumulation in a shallow, restricted, subaqueous depositional environment. The deposit ultimately became emergent with subsequent paleosol development. (2) An upper unit dominated by marginal marine point-bar deposits that developed in a channel. Trace fossils observed in the upper unit provide evidence that mesohaline waters occupied the channel at the time of sediment accumulation. This is supported most strongly by the presence of Scolicia, a common marine trace fossil, and reburrowed (composite) Ophiomorpha. The resultant ichnofabric represents a response to sedimentary events that demonstrates the IHS beds reflect seasonal or annual cyclicity.

The analysis of the river-exposed outcrop at Boca de Santa Pedro, Brazil, leads to four conclusions: (1) the IHS exposed in the upper portion of this deposit are possibly tidally influenced and almost certainly accumulated in a brackish-water channel; (2) if IHS are bioturbated, their temporal significance can be assessed; (3) seasonal fluctuations in discharge were significant enough to alter depositional and biological processes in this paleochannel, and; (4) brackish-water incursion into Amazonia during the Middle Miocene can be traced as far south as northern Bolivia.

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