Lithospheric Flexure and Related Base-level Stratigraphic Cycles in Continental Foreland Basins: An Example from the Putumayo Basin, Northern Andes

First-order lithospheric flexure, in response to discrete tectonic and sedimentary loads, controls basin-scale, base-level cycles in upstream deposits of retroarc continental foreland basins, in the absence of dynamic topography. A depositional sequence in this type of basin may be defined as a sedimentary succession formed during the adjustment of the fluvial systems to the equilibrium stage at or near base level. This sequence spans two accommodation episodes. The initial episode with high rates of subsidence corresponds to a thrust-loading period near the hinterland end of the elastic plate and may be identified by regional seismic-reflector onlap shifts from the foreland toward the hinterland. The second episode deepens and enhances the foreland flexure under the weight of the new sediment and may be identified seismically by a continuous onlap migration toward the foreland. A depositional cycle ends either when a period of nondeposition dominates the basin because the fluvial system attains the graded stage at or near-base level or with the reactivation of thrusting activity, initiating a new cycle 'No deposition' has been changed to 'nondeposition. ' Base level appears to control the top of the sedimentation boundary, but can adjust passively, during vertical movements caused by the regional isostatic compensation of the elastic plate. Regional paraconformities appear to represent periods of equilibrium of the depositional profile when the fluvial systems reach a graded stage and total tectonic quiescence. Seismic and well data from the northern-Andes, continental retroarc Putumayo foreland basin, in Colombia, are used to test the model for flexure-induced, base-level cycles. Results suggest that onlap seismic facies migrating toward the foreland predominate during sediment-controlled flexural periods (on average, 77% of the total deflection), whereas onlap shifts from the foreland toward the hinterland mark those periods when thrust belt loads dominate flexure (on average, 23% of the total deflection). The seismostratigraphic record exhibits as much as eight flexure-controlled Cenozoic sequences in the basin, in correspondence with an equal number of tectonic reactivation episodes. Geodynamically, the Putumayo foreland basin has been modeled to encompass a total added lithospheric deflection of as much as 450 km (279 mi) wide. During the Oligocene, the maximum subsidence rates reached approximately 150 m/m.y. (492 ft/m.y.) and the maximum width of the effective tectonic load (a discrete part of the thrust belt) affecting the plate reached a value of approximately 30 km (20 mi). The geometry of the effective thrust belt and the wavelength of the lithospheric deflection modeled in this article preclude the need to invoke dynamic topography as a downward force acting on the plate and creating extra accommodation in the basin. The effective elastic thickness of the plate is 30 10 km (19 6 mi; and has not changed apparently since the early Paleocene). Each loading event, for instance, tectonic and sediment loads, produces a corresponding forebulge whose location and dimensions change concomitant with the evolution of the basin.