Biology Reference
In-Depth Information
the mountains reach suffi cient heights to reverse the fl ow of the rivers from
west into the Pacifi c Ocean to east into the Atlantic Ocean? (c) When did
the mountains begin to cast a rain shadow to the west that contributed to
the formation of the Atacama Desert?
In the Late Cretaceous, marine deposits from the Pacifi c Ocean were
being laid down in the upper Amazon Basin and formed units like the Río
Acre Formation of western Brazil and eastern Peru. By 90 Ma, the Atlantic
Ocean Basin had opened to the latitude of northeastern South America,
and Cretaceous sediments from the Atlantic were deposited in the lower
Amazon Basin to form units like the Marajó Formation. At 65 Ma, tsunami
deposits near Recife, Brazil, associated with an iridium anomaly nearly sev-
enty times the concentration in adjacent strata, record the asteroid impact at
Chicxulub, Mexico. By the beginning of the middle Miocene, the basin was
also receiving sediments eroded from the rising Andes Mountains. Cores
from the basin reveal that the landscape consisted of lowland swamps, la-
goons, and meandering rivers (Hoorn and Vonhof 2006). There were pe-
riodic incursions from the Caribbean Sea through the Maracaibo Basin,
as shown by fossil faunas containing marine fi sh, and by freshwater fi sh
derived from marine ancestors (Albert et al. 2006; Lovejoy et al. 2006). By
the middle Miocene, about 15 Ma, the Andes Mountains had reached suf-
fi cient heights to tilt the lowlands eastward, and the Orinoco River began
depositing sediments along the Venezuelan coast at Delta Amacuro, and the
Amazon River deposited its fi rst sediments into the Atlantic Ocean at Ilha
de Marajó in Brazil. In the late Miocene, the Maracaibo River was dammed
by uplift of the Cordillera Mérida extension of the Cordillera Oriental to
form Lake Maracaibo. There are Amazonian paleofaunas from Peru dated
at 3 Ma, and these mark the last marine incursion into the basin. After
that time, the Amazon Basin continued a more subdued physical evolution
through deposition of sediments from the highlands, some further tilting of
the basin fl oor, and changes in secondary drainage patterns. The vegetation
was affected by these processes, and especially by the intensifying fl uctua-
tions in climate as the ice ages approached and altered precipitation, tem-
perature, and water tables in response to sea level changes. These changes
ushered in the latter phases of rain forest history that are revealed in the
fossil record to be dynamic even in the lowland basins at these tropical
latitudes. This is in contrast to the view held only a few decades ago that the
climates and biota of the tropics were stable and unchanging (chap. 9).
The Northern Andes by about 15 Ma had reached suffi cient heights that
as winds off the Pacifi c Ocean begin to rise along the western slopes they
lost moisture, creating one of the wettest regions of the world—the Colom-
Search WWH ::




Custom Search