This is a ring structure identified in 1971 by side-scanning radar and lying close to large northwest-trending fractures. It comprises an oval ring some 10 km in diameter and forming a depression open to the west, and a central ridge of 6x2 km running north-south.
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A roughly circular structure 2.5 km in diameter occurs in dense rain forest. It is situated on a very large fault trending northwest-southeast, extending south into Brazil and lying on the eastern edge of the Roraima structure.
Located 5 km northeast of La Mariscala, Cerro Vicheo is a syenite plug approximately 1 km in diameter with a system of radial trachyte dykes extending up to 7 km distance. Sodic ferromagnesian minerals are present in the trachytes but no further petrographic details have been obtained.
This complex of alkaline rocks forms a zone some 30 km from north to south and up to 5 km east-west. Little detailed information is available but it includes quartz syenites, microsyenites and trachytes containing sodic amphiboles.
Pan de Azucar is an upstanding plug which consists of a central nordmarkite 2 km in diameter, surrounded by a ring of syenite. A smaller 2 km diameter intrusion lies 1 km to the northwest. A geological sketch map is given by Bossi and Fernandez (1963, Fig. 3).
A breccia zone up to 15-30 m thick and extending for 1 km along the length of a northwest-southeast-trending fault is filled with carbonate rocks and has an aureole of 'carbonatization' extending outwards for up to 200 m.
The Mitu Group, which is of Permian and/or Triassic age, is widespread in southern and central Peru. It consists of coarse clastic sediments and volcanic rocks, of which a small but unknown proportion are alkaline.
Two large nepheline syenite intrusions cover 100 km2. They lie within volcanics of the Mitu Group, which they metamorphose. Two much smaller masses occur some 20 km to the south.
At Pucallpa three small plugs are intruded into Cretaceous Vivian sandstones and are aligned northeast-southwest. The plugs comprise phonolite and alkali trachyte.
Acahay forms an annular ridge rising 300 m above the surrounding plain and has a deep central basin. Milton and Eckel (1959) distinguish on air photographs numerous small cones to the east and northeast, as well as dykes.