Steady Sedimentation and Lithologic Completeness, Bermejo Basin, Argentina

Abstract

Late Miocene strata of the Bermejo basin, Argentina, (11-7 Ma) yield a magnetic polarity record of unusually high fidelity. Three sections, spaced along-strike over 20 km, are firmly correlated with the magnetic polarity time scale. All three sections demonstrate high accumulation rates (up to 0.88 mm/yr) and include an interval of normal polarity (extending from 7.84 to 7.81 Ma) that has yet to be resolved on the magnetic polarity time scale. The "polarity completeness" demonstrated by Bermejo strata is a result of high accumulation rates as well as environmental factors. Strata were deposited in an arid setting in which unconfined flows were the primary transporters of sediment. Shallow flow depths led to low basal shear stresses, and thus to little potential for erosion; these interpretations are corroborated by field observations. Steady sedimentation in the Bermejo basin is expressed by the low variance short-term accumulation rates have from the average long-term accumulation rate. Uniformity of sedimentation in the Bermejo basin is expressed by identical accumulation histories, even though time-transgressive formational boundaries and texture indicate that the depositional environments were not identical at all locations. These observations suggest that a change in accumulation rate does not necessarily produce a change in depositional environment and, conversely, a change in depositional environment is not necessarily accompanied by a change in accumulation rate.