Abstract

The outcrops presented here are located on the Pacific coast of Baja California, Mexico. They comprise a 1000-m (3300-ft)-thick part of the upper Rosario Formation, an Upper Cretaceous (late Campanian-early Maastrichtian) slope succession that consists of hemipelagites and gravity-current deposits. In the Canyon San Fernando area, the upper Rosario Formation consists of the fill of a midslope submarine canyon overlain by a genetically related, unconfined channel-levee complex. Deposition covers approximately 1.6 million years, a time scale consistent with a third-order sea level cycle. Paleocurrent directions and paleogeographic reconstructions demonstrate that these channel systems flowed to the south-southwest on a west-southwest-facing continental margin, indicating that channelized transport was obliquely down the slope. The orientation of the channel systems was probably pinned by the location of a fault trending oblique to the slope.

The Canyon San Fernando stratigraphic succession is divided into three stages based on lateral facies relationships. Stage I consists of a submarine-canyon fill that eroded more than 250 m (820 ft) into upper Campanian slope mudstones, and was 500 to 3000 m (1600–9850 ft) wide. The submarine canyon walls dip at 10–25° and exhibit tens of meters (>50 ft) of local topography and undulation; these strongly influenced the behavior of gravity currents. The submarine canyon is filled with single-story, multistory, and multilateral channel bodies that are interpreted variously as erosionally confined and constructionally confined (leveed) channels. Channel bodies range from 2 to 50 m (7–165 ft) thick, 20–400 m (65–1300 ft) wide (perpendicular to paleocurrent direction), and can be correlated for up to 3 km (1.9 mi) in the paleocurrent direction.

Stage II, the middle 600 m (2000 ft) of the succession, consists of a large channel complex that onlapped the aggrading, coeval slope to the east and interfingered with associated aggrading levees to the west. The channel-levee complex climbed upslope (eastward), indicating that sedimentation rates on the western levee were high enough to pin the complex to the slope.

The final 150-m (500-ft)-thick Stage III exhibits levees on both the western and eastern margins of the channel belt, indicating that it was fully aggradational and confined only by its own constructional topography. The complex is overlain by a thick (>10 m [>33 ft]) condensed section, interpreted as the abandonment-phase deposit. Composite channel-body sizes in the upper parts of the channel-levee complex range up to 800 m (2600 ft) wide and 100 m (330 ft) thick.