The Araruama lagoon, covering about 220 km2, is located at the northeastern region of the Rio de Janeiro State, at the Araruama, Sao Pedro d' Aldeia, Cabo Frio and Arraial do Cabo Countyes.
Aspects of the extern and intern morphology suggests that the evolution of the lagoon is related to sea transgressions and regressions during the Quaternary.
Pre-Flandrian continental deposits are cropping out on the southern border of the lagoon conforming also its sandy bottom, which are remainder of an intense erosion caused by two fluvial system flowing from the northwestern (Palmital Hill) and northern areas (Mirim and Sapiatiba Hills).
During the Flandrian transgression (14.000 to 7.000 years BP) the sea had invaded the former small sedimentary basin from the east and south areas, favouring the gradual deposition of sands and interbedded sandy coquinas. At the end of the Flandrian transgression (7.000 years BP), regressive events would have produced the Cabo Frio plain, the beach ridges and the intern marine barriers, forming a number of smalls lagoons into the last two morphologicals units.
The lowering of the sea level, related to the regression, continuous till reaching the negative 25 meters level (in 6.500 years BP) and the sedimentary basin of the actual lagoon become exposed to a new erosion process. Mainly, as a consequence of an increase of humidity related to heavy rains, the fluvial system coming from the northwest was reactivated, producing a strong erosion of the lagoon sediments along an east-west oriented channel showing a canyon like morphology.
A new transgression of the sea, known as Dunkerquian, started at 6000 years BP, reaching its maximun altitude of 2 meters (in 4.500 years BP), was followed by a regressive event at 3.500 years BP. These transgressive and regressive action of the sea would have produced the deposition of sandy sediments as external barriers and the closure of the connection of the canyon with the sea. The barriers are those known as Restinga de Massambaba and Cabo Frio, that caused the formation of the Araruama lagoon with more than 15 meters depth and linkage with the sea throughout the Itarujú 2-4 m depth tide-channel. At the bottom of main channel of the remainder canyon, modern laggonal organic muds have been detected.