The large‐scale dynamics of grain‐size variation in alluvial basins, 2: Application to syntectonic conglomerate

摘要

ABSTRACTThe concept of‘syntectonic’ conglomerate is based on the idea that gravel progradation is mainly generated by an increase in tectonic uplift and erosion of a source area with attendant increase in sediment flux supplied to a basin. However, other mechanisms, such as changes in basin subsidence rates, sorting of supplied sediment, and capability of transporting streams, can also lead to progradation and be difficult to distinguish from a syntectonic origin. Here we use our previously developed model to help understand the origin of gravel progradation in three Neogene alluvial basins ‐ the Bermejo Basin of Argentina, the Himalayan Foreland Basin, and the San Pedro Basin of southern Arizona ‐ all of which have available high‐resolution magnetostratigraphy. Interpretation of the origin of gravel progradation in these basins begins with calculation of basin equilibrium time, which is the time‐scale required for the streams to reach a steady‐state profile, assuming constant conditions.We then compare the time‐scale of the observed changes in the basin with the equilibrium time to determine if and how the model can be applied to the stratigraphic record. Most of the changes we have studied occur on time scales longer than the equilibrium time (‘slow variations’), in which case the key to interpretation is the relationship between overall grain‐size change and sedimentation rate in vertical sections.Of the three examples studied only one, the Bermejo Basin, is consistent with the traditional model of syntectonic progradation. Overall progradation in the two other basins is most consistent with a long‐term reduction in basin subsidence rates. In addition, short‐term variation in diffusivity or sediment flux, probably climatically driven, is the most likely control of small‐scale progradation of gravel tongues in the San Pedro Basin. These results, along with observations from other basins, suggest that subsidence is clearly an important control on clastic progradation on ‘slow’ time scales (i.e. generally a million years or more). If subsidence rates are directly linked to tectonic events, then subsidence‐driven progradation marks times of tectonic quiescence and is clearly not syntectonic in the traditional sense.These examples show that the model can be useful in interpreting the rock record, particularly when combined with other traditional basin‐analysis techniques. In particular, our results can be used to help discriminate between clastic progradation due to tectonic origin and progradation resulting from