Integrated Geophysical Investigations of the Pre-Andean Basins in Peru and Bolivia—A Search for Depocenters Concealed beneath a Foreland Basin

ABSTRACT The Andean foreland basin formed throughout the Cenozoic in a retro-arc setting in front of the advancing orogen. A 2500 km (1553 mi) long segment of this basin system passes through eastern Peru and Bolivia and comprises, from north to south, the Marañón, Ucayali, Madre de Dios, Beni, and Chaco Basins. The Andean foreland basin contains substantially thick units of Cenozoic sediments, which overlie Mesozoic and Paleozoic successions and Precambrian crystalline basement. In the deeper parts of the foreland basin, no wells have penetrated the full, pre-Andean sedimentary section and the sheer thickness of the sediments makes it difficult to seismically image crystalline basement in some areas. Thus, the thickness of the pre-Andean sediments and the existence of basins that pre-date the Andean orogeny are partly obscured. Areally extensive gravity and magnetic data sets have been used to build a structural and tectonic framework for the area. Gravity and magnetic 2-D forward modeling and 3-D inverse gravity modeling, constrained by seismic interpretation and well data, enabled base Cretaceous and top crystalline basement horizons to be derived. This approach allowed lateral extrapolation of the detailed but localized seismic interpretation into areas without seismic coverage, and it also extended this interpretation by including the depth to top crystalline basement. The results of this analysis indicate the presence of large pre-Cretaceous depocenters underlying the Andean foreland basin. These include a major depocenter extending from the central Marañón Basin north-northeastward across the Iquitos Arch, two depocenters underlying the Madre de Dios Basin and four depocenters beneath the Beni/Chaco Basins.