Marine influence in Amazonia: Evidence from the Geological Record

Abstract

Marine infl uence in Amazonia during the Miocene is a controversial issue, one about which nearly opposite theories (continental vs marine) have been proposed. Increasing multidisciplinary palaeoenvironmental data sets from various Miocene stratigraphic levels and parts of Amazonia have revealed depositional complexities triggered by repeated, high-frequency base-level changes and a very low depositional gradient. As a result, Early-Middle Miocene strata (Pebas phase) are organized into recurring 3-10 m-thick transgressive-regressive bay-margin successions. An array of evidence indicates some sort of marine infl uence in these deposits. This evidence includes mangrove pollen, foraminifers, rare brackish-water mollusc species, barnacles, common brackish-water trace fossil assemblages and tidal sediments. The ongoing debate now focuses on the degree of saltwater infl uence in the Pebas phase strata. The Late Miocene (Acre phase) stratigraphic record comprises mainly channelized deposits, and fossil faunas are dominated by terrestrial and freshwater vertebrates. The evidence for marginal marine infl uence in these strata is mainly confi ned to inclined heterolithic stratifi cation (IHS)-bearing channels and includes tidal sediments, restricted brackish-water ichnofossil assemblages, and locally teeth of euryhaline sharks and mangrove pollen (Nauta and Madre de Dios Formations). In particular, the tidal signature in these deposits is well developed, pointing to local development of deltaic/estuarine settings. There are also suggestions of marine infl uence on other stratigraphic intervals throughout Amazonia, particularly in Pliocene and Quaternary strata. The allegedly marine conditions have been proposed based on purported global sea-level highstands, the low altitude of the Amazon region and supposed coastal geomorphological features such as deltas. We conclude that these suggestions are misconceptions and that geological data to support such marine settings during the Late Neogene are lacking.

A 60-million-year Cenozoic history of western Amazonian ecosystems in Contamana, eastern Peru

“…An early late Miocene marine incursion is recorded throughout the NPFB and Ucayali Basin ( Fig. 12; Boonstra et al, 2015 and this work), testifying to a single event, perhaps distinct from the later one(s) documented in the SPFB (e.g., Hovikoski et al, 2010). From Pliocene times onward (initiated at~4 Ma; Espurt et al, 2007;Roddaz et al, 2010), the Fitzcarrald Arch was uplifted as a pivotal topographic high at the Western Amazonian scale, which may explain the discrepancies between the Ucayali basin, connected with the NPFB, and the Madre de Dios basin (SPFB).…”

“…Most of these localities represent a series of snapshots in the history of Proto-Amazonia and/or glimpses into the evolutionary history of a given taxonomic group, which are not easily correlated temporally, although synthetic works have begun to pave the way for broader analyses: see Hoorn (1993) and Jaramillo et al (2011) for Neogene and Cenozoic palynostratigraphy, respectively; Wesselingh et al (2006) for Miocene mollusks; Sheppard and Bate (1980), Muñoz-Torres et al (1998), Ramos (2006), Wesselingh and Ramos (2010), or Gross et al (2013Gross et al ( , 2014 for ostracods and/or foraminifers; Lovejoy et al (1998Lovejoy et al ( , 2006, Monsch (1998), Brito and Deynat (2004), Brito et al (2007), and Lundberg et al (2010) for fish evolution and distribution; Negri et al (2010) for mammals and birds; Hovikoski et al (2010), Lundberg et al (2010), Boonstra et al (2015) for sedimentological and paleontological arguments of Miocene marine incursions.…”

Impacts of Cenozoic global cooling, surface uplift, and an inland seaway on South American paleoclimate and precipitation 18O

“…Others have interpreted the sedimentary structures as a fl uvial system with avulsive rivers, megafans, swamps, and lakes (Latrubesse et al, 2010). The confl icting evidence for continental and marine infl uences in the same formation can be reconciled by a scenario in which multiple incursions of brackish water occurred into an otherwise continental environment (Hovikoski et al, 2007;Hovikoski et al, 2010;Wesselingh et al, 2006). In a recent review, Hoorn et al (2010) describe the following three phases of seaway incursions in Amazonia; (1) ca.…”

Could coastal plants in western Amazonia be relicts of past marine incursions?

“…An additional avenue for marine incursion along the Essequibo River (Hovikoski et al., ) (Figure ) is also supported by the distribution of extant plants, as the candidate species reach far beyond the highest level of today's tides, c . 100 km upstream from its mouth (Worts, ).…”