The Purus Arch () is a

basement high In geology, a basement high is a portion of the basement in a sedimentary basin that is higher than its surroundings. Commonly, structures referred to as basement highs are hidden by the sedimentary fill of the basin. Usually basement highs are e ...

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

that makes up the modern western boundary of the Amazon sedimentary basin. The Purus Arch is thought to be a former

graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

Middle Proterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic), ...

age that was inversed in the

Late Proterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era and succeeded b ...

. Although not obviously noticeable, today the Purus Arch crosses the Amazon River near the mouth of the

Purus River The Purus River (Portuguese: ''Rio Purus''; Spanish: ''Río Purús'') is a tributary of the Amazon River in South America. Its drainage basin is , and the mean annual discharge is . The river shares its name with the Alto Purús National Park a ...

. Following the water breach of the arch about 8–10 million years ago, the western (cf. Pebas Mega-wetland) and eastern parts of the present-day

Amazon basin The Amazon basin is the part of South America drained by the Amazon River and its tributary, tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries ...

became a unified system, which played an important role in the

biogeography Biogeography is the study of the species distribution, distribution of species and ecosystems in geography, geographic space and through evolutionary history of life, geological time. Organisms and biological community (ecology), communities o ...

of many aquatic organisms in the region.

References

Basement highs Geology of Brazil Mesoproterozoic rifts and grabens {{struct-geology-stub