EVALUATING A COUPLED GEOMORPHIC-SEDIMENTARY SYSTEM IN AN EXTENSIONAL TECTONIC SETTING: LORETO BASIN, BAJA CALIFORNIA SUR, MEXICO

The sedimentary rocks that are preserved as the infill to continental extensional basins provide a tantalizing record of the evolution of coupled geomorphic-sedimentary systems during rift development. Even though the stratigraphy can be considered to provide the only complete record of external forcing on basin evolution, how the sedimentary effects of this forcing come to be preserved in the stratigraphic record is not well understood. Moreover there are potentially several processes (some coupled) that need to be considered including variations in tectonics, sediment supply, eustacy and climate.

The Pliocene Loreto Basin, Baja California Sur, Mexico, contains an exceptionally well-exposed sequence of sixteen stacked Gilbert deltas deposited over approximately 100Ka. Previous work proposes an explanation of pulsed tectonic subsidence due to episodic earthquake clustering on the basin bounding fault, and demonstrates that eustatic fluctuations cannot explain the timescales of delta progradation and drowning. A climatic control on sediment supply to the basin would imply extreme climatic fluctuations, which are inconsistent with the timing of basin development. Our field observations indicate that the stratigraphy preserved in the Loreto Basin records the response of a sedimentary system either directly, or indirectly, to tectonic forcing. Tectonic forcing controls the rate and extent of footwall uplift, and consequently the source area and the surface processes operating in footwall catchments. It also controls accommodation creation in the hanging wall and thus the transport and depositional processes operating within the basin itself.

Detailed stratigraphic analysis is being used to document and quantify the facies characteristics and architecture of individual progradational packages with the stacked sequence in the Loreto Basin. The aim is to better understand the underlying controls on this system, and the timescales on which they operate.