UNRAVELING ANCIENT ECOSYSTEMS USING MODERN ANALOGS: DENVER BASIN AND BOLIVIA

The tenet that the present is the key to the past is as valid today as when first proposed. We illustrate the concept with a direct comparison between the modern synorogenic succession accumulating today in the Andean foreland basin in eastern Bolivia and the ancient synorogenic succession that accumulated in the Denver Basin during the Laramide Orogeny.

The Andean foreland basin near Santa Cruz, Bolivia, is characterized by very dynamic and extensive alluvial and fluvial distributary fan systems. A distinct rainfall gradient exists, decreasing from north to south along the Bolivian Andes. This allows a comparative study of distributary fluvial systems' responses to climate and vegetation variability. Northern systems, characterized by perennial rainfall are comprised of superimposed complexes of meander belts, and cut-off ox bow lakes are common. The southern area is characterized by broad distributary fan systems with upper sand-rich reaches giving way to fine sand- and mud-rich distributary plains, and here avulsions and major crevasse splay events are regular events.

Satellite imagery, dating from 1975 to present, illustrates frequent avulsions and crevasse splay developments on the low-gradient Rio Grande fluvial distributary fan. Ground observations and auger data obtained in 2005 provide ground control for a set of shallow cross sections constructed on the Rio Grande distributary fan. This data set allows us to characterize detailed sedimentation patterns, extract recent sedimentation rates, and extrapolate preservation potential of landscape elements. These patterns are compared directly to the observed stratigraphic architecture of Laramide fluvial deposits of the Denver Basin. Our reconstructions of the ancient landscapes of the Denver Basin are refined based on our observations in Bolivia. This includes the buried Wildcat Mountain distributary fan complex and the more distal lignite-bearing strata of the Denver Basin, now interpreted to represent long-lived back-levee swamp and paludal settings.