JURASSIC TO EARLY CRETACEOUS TECTONOSTRATIGRAPHIC EVOLUTION OF EASTERN ECUADOR

In this contribution we reconstruct the tectono sedimentary evolution of the Oriente Basin during the Jurassic to early Cretaceous, based on sedimentological, stratigraphic and geochronological data from the Chapiza and Hollin formations.

During the early Jurassic the Piuntza volcanic arc developed in eastern Ecuador, and it was partly coeval with deposition of marine sediments of the Santiago Formation. During the middle Jurassic crustal scale extension affected large areas of the subandean basins of northwestern South America. This extension was synchronous with the deposition of continental series of the Chapiza Formation. Volcanic rocks of the Misahualli Formation were deposited on top of the Chapiza Formation and are dated at ~160 Ma. During this period the arc migrated to the west in response to changes in the geodynamics of the subducting slab. Extension ended by the Early Cretaceous, and was followed by a period of compression, related to the accelerated opening of the South Atlantic.

Quartz-rich sediments of the Aptian to Albian Hollín Formation are the main oil reservoir of the Oriente Basin of eastern Ecuador. Seismic data and stratigraphic correlations indicate that the Hollin Formation was deposited on top of pre-Cretaceous basement that was deformed during the Early Cretaceous. Sedimentary rocks at the base of the Hollin Formation yielded 220 to 150 Ma detrital zircon ages derived from the erosion of Triassic anatectites and the exhumed Piuntza and Misahualli arcs. U-Pb zircon ages from volcanoclastic rocks underlying the Hollin Formation, provide a maximum stratigraphic age of 116 Ma for the base of this formation. Proterozoic U-Pb detrital zircon ages for braided channels of the Hollin Formation indicate that the sediments were partially derived from the erosion of the westernmost part of the Amazon Craton. In addition, detrital zircon ages ranging between 450 to 600 Ma suggest also an Andean source supplying detrital material to the Hollin Formation. The location of the detrital sources is consistent with paleocurrents data that suggests a southeast to northwest transport direction, forming a large river system confined to the west by the Andes and to the east by the Amazon Craton. The upper part of the Hollin Formation recorded an Albian marine transgression retrograding from estuarine to shelf deposits. According to the regional stratigraphic correlations and the proposed paleogeography, we suggest that the Albian transgression may have originated in the north and advanced in southward direction.