Javier Guerrero

Andean tectonics as a cause for changing drainage patterns in Miocene northern South America

NewdatafromNeogenestratainnorthernSouthAmericasuggestthatMiocenetectonisminthenortheasternAndeswasresponsible for the genesis of the Amazon River and changes in the drainage patterns of other major rivers such as the Magdalena and the Orinoco. Here we present a new model for the paleogeographic evolution of northern South America during the Miocene. In the early Miocene, a large part of the drainage of northwest Amazonia was directed northward along the paleo‐Orinoco river system to a deltainLakeMaracaibo.UpliftoftheEasternCordillerainthelate middle Miocene caused thefirst development of the Amazon River; however, no connection with the Atlantic was established, and the Amazon fed the paleo‐Orinoco river system, which drained toward the Caribbean. Substantial Andean uplift in the late Miocene resultedinmajorchangesinpaleogeography:theOrinocochangedits course, the Amazon established a connection to the Atlantic, causing the drowning of carbonate platforms, and the Amazon-Caribbeanconnectionwasclosed.Thusthedrainageandpaleogeography ofnorthernSouthAmericaintheMiocenewerestronglycontrolled by tectonic movements in the northeastern Andes.

Terrestrial record of methane hydrate dissociation in the Early Cretaceous

Reconstruction of changing C isotopic composition of Early Cretaceous atmospheric CO2 from fossilized C3 vascular land-plant tissue revealed a brief and striking negative excursion (D 25‰) in atmospheric d 13 C, followed by a rapid positive compensation (D 15‰) during the Aptian (ca. 117 Ma). Mass-balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion; this would also result in an increased CO2:O2 mixing ratio as O2 is consumed during CH4 oxidation to CO2, spurring the exponential phase of angiosperm biogeographic expansion.

The nature of Miocene Amazonian epicontinental embayment: High-frequency shifts of the low-gradient coastline

A sedimentological and ichnological data set that covers the Lower-Upper Miocene sedimentary series of western Amazonian foreland basin indicates that widespread, restricted marine ingressions shaped western Amazonia throughout the Miocene. The late Lower–early Upper Miocene sedimentary series (Pebas Formation) consists of stacked, 3- to 10-m-thick, tidally influenced, brackish to freshwater, bay-margin sequences. The overlying Upper Miocene (“post-Pebas”) strata bear tidally influenced, low-salinity, channel deposits that are interbedded with continental deposits. The data suggest that several tens of high-frequency ingressions reached the basin during the Miocene. The ingressions were shallow and restricted, and were interspersed with rapid progradation. Along with the prograding shorelines, the continental environments—swamps, lagoons, floodplains and forests—constrained the extent of the marginal marine embayment. Consequently, the Miocene marginal marine and continental strata are closely interbedded throughout the basin. These results refine the recent depositional models for Miocene Amazonia, and challenge the theory that marine ingressions shaped the area only during one brief time interval (late Middle-early Late Miocene) during the epoch. Much of recent literature has documented fossils of mangrove pollen, brackish-euryhaline fish and brackish-water ostracods, brackish-water trace fossil assemblages, and tidal deposits from various Miocene stratigraphic levels. Commonly, these data sets are collected from the same outcrops as those for which data sets imply freshwater conditions. We propose that these seemingly contrasting data sets can be unified, if the repetitive nature of the ingressions is considered, and all the paleoenvironmental data are presented in a detailed lithological and stratigraphical context.