Combustibles sólidos (asfaltita). XVIII Congreso Geológico Argentino, Neuquén, Relatorio
Résumé
Where the Neuquén Basin abuts the Andes, hundreds of veins of solid hydrocarbon (asphaltite) crop out. Many of these veins were mined in the past. By consensus, the bitumen has resulted from maturation of organic-rich shale, especially the Vaca Muerta Fm. of late Jurassic age. To explain the maturation, recent authors have invoked regional subsidence, whereas early investigators invoked magmatic activity. In Neuquén Province, the veins are mostly sub-vertical dykes. They tend to be straight and continuous, crosscutting regional structures and all strata, from Jurassic to Palaeocene. Almost all of the dykes lie within 50 km of Tromen volcano, although two are close to Auca Mahuida. On both volcanoes, volcanic products are of late Pliocene to Pleistocene age. Although regionally the bitumen dykes track the current direction of tectonic compression (ENE), locally they radiate outward from the volcanoes. The thicknesses are greatest where the host rocks are the most resistant to fracturing. Many of the dykes occur near reactivated basement faults, especially at the foot of Tromen. Here the bitumen is highgrade, whereas elsewhere it tends to be lower-grade. Also near basement faults, bitumen dykes pass upward into surface caprocks of hydrothermal calcrete. Some dykes or their wallrocks contain hydrothermal minerals. A few contain fragments of Vaca Muerta shale. We infer forceful expulsion of source rock. Finally, some dykes splay upward near the current land surface. We conclude that the bitumen dykes of Neuquén province formed during Pliocene to Pleistocene volcanic activity.