Outcrops of the Upper Cretaceous Rosario Formation along Arroyo San Fernando, Baja California Norte, Mexico, expose a continuous cross-sectional view through a submarine-fan valley-levee complex that is similar in scale to many modern submarine-fan valleys. This unusually complete exposure provides an opportunity to compare an ancient turbidite sequence with modern submarine-fan systems and to test some aspects of submarine-fan facies models.
The Arroyo San Fernando submarine-fan valley fill is 670 m thick and 5.5 to 7.5 km wide perpendicular to paleocurrent direction. The valley fill is bordered by >500-m thickness of levee deposits; these levees aggraded with the valley maintaining positive relief over the surface of the fan. The valley fill consists of coarse-grained channel deposits that alternate with fine-grained interchannel deposits. The channel deposits within the valley fill consist of both single (0.25-2 km wide and 10-50 m thick) and amalgamated (as much as 5 km wide and 45-210 m thick) channel deposits. The levee deposits that border the valley fill consist of interbedded sandstone and mudstone beds that are commonly slumped. These slumps slid both toward and away from the valley axis, indicating that slumping was due to positive relief and not solely to undercutting by channels within the valley. The valley system records an overall aggradation, with very little down-cutting of the valley floor or undercutting of the levee walls by channels.
The Arroyo San Fernando submarine-fan valley has dimensions comparable to those of modern submarine-fan valleys. The channels within it are comparable in size to many inferred ancient submarine-fan valleys described in the literature and are the same size as channels within modern submarine-fan valleys. These data, together with facies relationships, may indicate that many ancient deep-marine deposits interpreted as submarine-fan valley fill only represent channels within a submarine-fan valley.