Conceptual Models of Forearc Stratigraphy and Outcrop Analogs from Baja California

William R. Morris, Maurice A. Cucci, Scot W. Krueger

Forearc deposits provide examples of stratigraphic cycles where relative sea level change that controls stratigraphic evolution, is dominated by tectonism rather than eustasy. Each stratigraphic sequence is characterized by short-lived uplift events related to changes in the underlying subduction system (plate reorganizations, ridge subduction, etc.). These sequences constitute the response to tectonic cycles 5 Ma or more in duration. Deposition of the idealized sequence is initiated with a short-lived uplift event, creating an unconformity, subsequently followed by renewed subsidence and ending with the next uplift event/unconformity. The stratigraphic succession is characterized by fluvial/alluvial deposits followed by a transition from brackish to open or even deep mar ne sediments.

The Rosario embayment, Baja California, a Late Cretaceous to Paleogene Peninsular Ranges forearc basin complex, provides excellent examples of tectonic cycles. In Early Campanian time, tectonism resulted in the downdropping of basal fluvial deposits to bathyal water depths. Shortening in early to late Campanian time resulted in the uplift of the basin from bathyal water depths to a subaerial environment. In Late Campanian time, extension within the forearc basin resulted in subsidence via down-to-the-west normal faults along the eastern margin of the basin. The final tectonic activity occurred in the Paleocene with shortening of the basin as documented by folding and uplifting of the eastern and western margins of the Rosario embayment.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995