10. Cretaceous Stratigraphy and Paleo-Facies Maps of Northwestern South America

This paper presents 13 stratigraphic cross sections and 12 paleo-facies maps, spanning the Cretaceous, and summarizes the Cretaceous geological history surrounding northwestern South America during this period. The work outlines a regional sequence stratigraphic framework for the Cretaceous, evolving within the context of the complex geological history and interaction of at least three tectonic plates. Cretaceous rocks, including local uppermost Jurassic and Paleocene deposits, form a mega-sequence bounded by regional unconformities that are locally angular. On a broad scale, Cretaceous rocks represent a major transgressive-regressive cycle with the maximum flooding surface close to the Cenomanian-Turonian boundary (MFS 8), corresponding to the maximum Cretaceous, and even Mesozoic, eustatic level. Superimposed on this large-scale trend are several smaller transgressive-regressive cycles, suggesting an oscillating relative tectono-eustatic level. These minor cycles correspond to the several proposed stratigraphic sequences.Cretaceous sedimentary history can be summarized in four episodes: (1) Berriasian to Early Aptian: Sedimentation was restricted to rapidly subsiding extensional basins where great thickness of sediment accumulated. Sedimentation started in continental environments followed by a marine transgression. Marine shelves developed with carbonate- dominated sedimentation in Venezuela and northeastern Colombia, and mud-dominated sedimentation in Colombia, while in Peru sands and muds were deposited in shallow marine to deltaic environments. During the Aptian, a tectonically induced unconformity resulted from the closure and accretion of the Quebradagrande oceanic margin basin to western Colombia and Ecuador, or alternatively from the change from active rifting to regional thermal subsidence. (2) Late Aptian to Cenomanian: Regional thermal subsidence resulted in an increase of in the area of marine sedimentation via the coalescence of isolated basins into a regional basin extending along the continental margin from Venezuela to Peru. Marine shelf sedimentation was mud- dominated in Venezuela and Colombia, and carbonate-dominated in Peru. (3) Cenomanian to Santonian: During the latest Cenomanian-Early Turonian, sea level reached its maximum. Sedimentation was controlled by eustatic changes. Coincident with maximum Cretaceous flooding, anoxic events favored accumulation of organic matter at the sea bottom resulting in the best petroleum source rocks in northern South America. From Venezuela to Ecuador, pelagic shale and pelagic fine-grained limestone sedimentation prevailed. During the Coniacian-Santonian in Venezuela and Colombia, marine upwelling favored development of siliceous plankton and chert deposition. In Peru, marine shelf sedimentation was mud-dominated. (4) Campanian to Early Paleocene: Marine regression and a general shallowing of sedimentary environments occurred throughout northwestern South America, coeval with the appearance of compressional deformation events, which started earlier in Peru. During the Late Campanian, initial collision of the Caribbean Plate with northern Colombia generated a tectonically induced unconformity extending into western Venezuela. During the Late Maastrichtian and Paleocene, regional marine regression resulted in continental sedimentation extending from Venezuela to Ecuador. In Venezuela, the prevailing passive margin changed to an active margin with obduction of oceanic terranes of Caribbean affinity during the Maastrichtian and Paleocene. In Colombia, Campanian and Maastrichtian collision of the Caribbean Plate resulted in accretion of oceanic fragments to the continental margin and generated uplift of the Central Cordillera and its northern prolongation in the Lower Magdalena Valley (Plato-San Jorge area) and the Sierra Nevada de Santa Marta. Uplift of the Cordillera Real in Ecuador and the sub-Andean zone including the western part of the Oriente basin also occurred. Campanian uplift ofthe Peruvian Andes provided a source of detrital sediments that accumulated as continental and alluvial fan deposits in active synclines. During the Paleocene, most of the basins began to compartmentalize due to active deformation.