Marine influence in western Amazonia during the late Miocene

Abstract

The dynamic environments of western Amazonia during the Neogene (23 to 2.58 million years ago [Ma]) included continental-scale wetlands that were episodically influenced by marine incursions originating from the Caribbean. The nature and frequency of such events is highly debated, with a general consensus on short events during the early (23 to 16) and middle (16 to 11.6 Ma) Miocene. However, scattered evidence for more events up until the late Miocene (11.6 to 5.3 Ma) exists. Based on a palynological analysis of borehole 1-AS-9-AM from the Solimões Formation (western Amazonia, Brazil), we provide further support for late Miocene marine influence in the region. Comparisons with other sites in the Solimões Basin indicate an age of earliest Tortonian (~11–10 Ma) for an assemblage with foraminiferal linings, acritarchs and dinocysts, totalling ~17.3% of the palynological count ( n = 374 palynomorphs) at 34.10 m of depth. This is the topmost sample with a significant amount of marine indicators and its age is consistent even when different biostratigraphic interpretations are appreciated. Our data and comparisons also show that a larger proportion of sediments of the Solimões Formation should be placed within the late middle to early late Miocene. Our reconstruction of the Solimões deposits in the early Tortonian indicates intermintent low salinity regimes and a persistent connection with the paleo-Orinoco drainage until the closure of this connection in the late Miocene to Pliocene times. • New pollen data is presented from a borehole (1-AS-9-AM) in western Amazonia. • Graphic correlation was applied with two nearby sites, ages found from middle to late Miocene. • A thicker late Miocene section than previously interpreted is suggested. • A marine assemblage (~17% of the palynological count) at 34 m is composed of foraminiferal linings, dinocysts and acritarchs. • Regional comparisons support an early Tortonian episode of marine influence in a wide geographical area in western Amazonia.