Saturday, September 8, 2012

The Rising of South America—Part IV

South of the Chaco foreland basin, the Paranense Sea covered a wide area in northern Argentina and Uruguay. For much of this time the entire South Amrrican continental plate, including Patagonia, suffered tremendous efforts of contraction and extension, which rose and sank the ground intermittently. The land subsidence allowed the Atlantic Ocean to cover the continent during several periods, depositing sediment laden with remains of marine organisms. As shown in the last post, this sediment base covered almost all of the area east of the present-day Andes range except for the Guiana and Brazil highlands.
The last great Atlantic ingression, which invaded most of the present Argentinean territory, deposited fossils which were first found on the banks of the Parana river, over 600 miles north of the northern boundary of Patagonia. That is why the sea that covered Argentina at the time is called the Paranaense sea—a combination of Patagonia and Parana, meaning “natural to the area”—the Parana Sea/Paranense Sea was natural to this area; today it means someone or something belonging to the State of Parana, and is the name of a famous sports team from the area.

On the Atlantic coast of the Patagonian province of Chubut, marine sediments of the Paranaense sea are known as the Puerto Madryn formation (see the last post). They can be easily recognized on the cliffs near the beaches, as a broad, almost horizontal brownish band, often superimposed on lighter-colored sediments. In the surroundings of the city of Puerto Madryn, they can be observed over many miles. The photograph below shows them occupying the top third of Cerro Avanzado, 15 kilometers southeast of the city.
Left: Cerro Avanado Beach, Patagonia Atlantic coast; Right: sea shells hundreds of feet above sea level imbedded in the cliffs that were once submerged
Just to the north is the current country of Paraguay, which lies geologically at the borderzone between several cratons (old and stable part of the continent) or highlands. These basins or lowlands are covered with thick Tertiary sediment and regolith (fragmental and more or less decomposed matter drifted by wind, water or ice from other sources) that was at one time covered by the sea, leaving sedimentarhy basins covered with flood basalt coating that the ocean floor and are now above sea level.
Covering the northern half of Paraguay is the foreland Chaco Basin, a very large depression resulting from lithospheric flexure (bending) area in isostatic equilibrium, or isostasy (equal weight or level where a depression remains lower and stable). This covered a very large area because of the flexural rigidity of the underlying lithosphere that was, in this case, both flexible and thin (younger), the basin received sediment that eroded off the adjacent highlands. This area extended into Argentina and Bolivia where it bordered the Andes thrust front—the area of the present day Andes range. At a deeper level, the Paraguayan Chaco is made up of four sub-basins, the Pirizal, Pilar, Caradaity and Curupaity basins. The Pirizal is made up of younger sediments and borders the Parana Basin across to the east from the present-day Paraguay River.
The Parana Basin is also a large sedimentary basin situated in the central-eastern part of the continent, covering an area of about 580,000 square miles, with the infill of the floor now about 23,000 feet thick (meaning at one time, the ocean covered this area that is now filled with sediment from the sea). Today it is an intracratonic basin, meaning it is surrounded by craton (highlands) or shield; however, at one time it was a gulf, that opened to the southwest.
An overview of the Bolivian Chaco. GB = Grande River basin, GF = Grande megafan, PB = Parapeti River Basin, PF = Parapeti megafan, OB = Otuquis–Tucavara basin, OF = Otuquis–Tucavara megafan. Megafans are fluvial (created by rivers and seas) deposits of extreme variance (strong movement of water), meaning waterways with large fluctuations in discharge and their instability
Time-equivalent marine incursions (similar periods as determined by the presence of a particular palaeoenvironment) have been determined in the Amazon Basin, north of the Paranense Sea and Chaco Basin). Several marine incursions have been determined as widespread in the wetlands of the Amazon Basin, reaching their westernmost extension in the Madre de Dios Basin along the Brazilian-Bolivian border (ending just east of present-day Cusco). A connection between the marine incursions of the Madre de Dios Basin and the northern part of the Chaco Basin would have been along the Andean foothills, with a connection between the marine facies (similar rock formations) of the Chaco Basin and the Paranense Sea most probable, with the seaway passing along the Paraguayan Chaco. Owing to the fact that the marine Yecua facies linked both to the Paranense Sea in the south and the Amazon Basin in the north, with an intracontinental connection between these two regions via the Chaco Basin was most likely.
Left: Map of Patagonia (southern Chile and Argentina); Center: The Paranense Sea covering most of Patagonia; Right: The highlands and lowland basins in Patagonia
South of the Chaco Basin, the Paranense Sea, a marine estuary system reaching from Uruguay to the north eastern part of Argentina, shows a doubtful relation with the Yecua Formation of the Chaco Basin since it is not known at this time whether the Yecua Sea formed in a forebulge or a back-bulge basin. However, new lithology data of the southern Bolivian Yecua Formation within the Chaco Basin indicate depositional environments of floodplain facies and restricted shallow marine facies. Foraminifera indicate that there were marine incursions in the Chaco Basin, and the lithostratigraphic data support the eventual decreasing marine influence within the Bolivian Yecua Formation toward the south.
The point is, all of this suggests, as the maps of previous posts have shown, that much of the South American continent of today was under water, or covered by numerous seas, given separate names by geologists, but basically connected and forming one sea—or actually all part of the Atlantic Ocean, since they emptied eastward through the Amazon Arm and the gulf opening in Patagonia.

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