La Negra Formation
La Negra Formation (Spanish: Formación La Negra) is a geologic formation of Jurassic age, composed chiefly of volcanic and volcaniclastic rocks, located in the Coast Range of northern Chile. The formation originated in marine and continental (terrestrial) conditions, and bears evidence of submarine volcanism as well as large explosive eruptions. The volcanism of La Negra Formation is thought to have lasted for about five million years.
| La Negra Formation | |
|---|---|
| Stratigraphic range: Jurassic | |
View of La Portada, an arch made of bright Miocene sediments resting on darker rocks of La Negra Formation | |
| Type | Geological formation |
| Underlies | Caleta Coloso Formation |
| Overlies | Cerro de Cuevitas Beds Pan de Azúcar Formation Posada de los Hidalgo |
| Thickness | ~7,000 m (23,000 ft) |
| Lithology | |
| Primary | Volcaniclastic rocks, andesite, basaltic andesite, basalt |
| Other | Dacite, hyaloclastite breccia, conglomerate, limestone, marl, peperite, grey breccia |
| Location | |
| Region | Antofagasta Region |
| Country | Chile |
| Type section | |
| Named for | Quebrada La Negra |
| Named by | Floreal García (1967) |
| Thickness at type section | 3,000 m (9,800 ft) |
The formation has a thickness of about 7,000 metres (23,000 ft) with the type section being a 3,000 metres (9,800 ft) thick exposure in Quebrada La Negra next to Antofagasta. La Negra Formation deposited diachronously along a series of intra-arc basins and overlies conformably the formations of Pan de Azúcar and Posada de los Hidalgo. The formation is intruded by plutons of varied composition including gabbro and granite.