DO STRATIGRAPHIC SEQUENCES IN FORELAND BASIN DEPOSITS IMPLY MOUNTAIN BUILDING EPISODES?

The pattern of accumulation of synorogenic sediments in foreland basins adjacent to Cordilleran uplifts indicates that sedimentation has been episodic. Well-defined unconformity-bounded sedimentary packages occur, suggesting extensive intervals of bypass or erosion, and causing the temporal compartmentalization of the sedimentary rock record. Examples from the well-documented Rocky Mountains of Colorado are compared to settings in the Andean foreland basin of Bolivia.

In the Colorado Rocky Mountains, sediment was deposited in a foreland basin on the eastern side of the Front Range during the Late Cretaceous to Early Tertiary Laramide orogeny. These strata are preserved as a pair of unconformity-bounded sequences in the Denver Basin. The first sequence is up to 550 m thick and ranges in age from about 68 to 64 MY. The second sequence is up to 400 m thick and ranges in age from 55MY to perhaps 50MY. On the premise that accommodation is fostered by thrust fault activity and adjacent tectonic downwarping, enhanced by sediment loading, we interpret the observed sedimentary pattern to reflect episodic activity of range-bounding thrust faults.

In the Andean Foreland Basin of Bolivia, a 2000 m thick package of Miocene to Recent sediments has accumulated above a regional erosional surface cut into Upper Mesozoic strata. We suggest that this pattern also reflects episodic growth of the Central Andean orogen and that regional and episodic tectonic changes are reflected in the transition from erosion or non-deposition to times of sediment aggradation.

Studies of modern depositional systems in Bolivia are used to document the geomorphic and stratigraphic character of active and ongoing synorogenic sedimentation. By analogy, we interpret the discontinuously accumulated ancient Denver Basin sediments to reflect episodic mountain building.