Tidal-channel deposits on a delta plain from the Upper Miocene Nauta Formation, Maranon Foreland Sub-basin, Peru

Miocene siliciclastic sediments of the Maranon Foreland Sub-basin in Peru record the sedimentary response to regional marine incursions into Amazonia. Contrary to previous interpretations, the Late Miocene Nauta Formation provides evidence of the last known marine incursion before the current Amazonia river basin became established. Sedimentological, ichnological and palynological data from well-exposed outcrops along a ca 100 km road transect suggest that the Nauta Formation represents a shallow, marginal-marine channel complex dominated by tidal channels developed in the inactive, brackish-water portions of a delta plain. The main facies associations are: FA1 - slightly bioturbated mud-draped trough cross-stratified sand; FA2 - locally, pervasively bioturbated inclined heterolithic stratification (IHS); and FA3 - moderately bioturbated horizontally bedded sand-mud couplets. These identify subtidal compound dunes, tidal point bars and shallow subtidal to intertidal flats, respectively. Bi-seasonal depositional cycles are ascribed to the abundant metre- to decimetre-scale sand-mud couplets that are found mainly in the IHS association: semi-monthly to daily tidal rhythmicity is inferred from centimetre- and millimetre-scale couplets in the mud-dominated parts of the decimetre-scale couplets. The ichnology of the deposits is consistent with brackish depositional conditions; the presence of Laminites, a variant of Scolicia, attests to episodic normal marine conditions. Trace fossil suites are assigned to the Skolithos, Cruziana and mixed Skolithos-Cruziana ichnofacies. Pollen assemblages related to mangrove environments (e.g. Retitricolporites sp., Zonocostites sp., Psilatricolporites maculosus, Retitricolpites simplex) support a brackish-water setting. Uplift of the Merida Andes to the North and the consequent closure of the Proto-Caribbean connection, and the onset of the transcontinental Amazon drainage, constrain the deposition of the Nauta sediments with around 10 to 8 Ma, probably contemporaneous to similar marine incursions identified in the Cuenca (Ecuador), Acre (Brazil) and Madre de Dios (Southern Peru) (sub)basins, and along the Chaco-Paranan corridor across Bolivia, Paraguay and Argentina.