SUBMARINE CANYON COMPLEXES IN THE MIDDLE EOCENE TORREY SANDSTONE, ARDATH SHALE, AND SCRIPPS FORMATION, CALIFORNIA: ARCHIVES OF COMPLEX GEOLOGICAL HISTORY OF THE CONTINENTAL MARGIN

Submarine canyons are known as long-term geological features, which are main conduits to bypass sediments from shelf to deepwater on continental margins, but also work as large sediment storages. Submarine canyons can potentially record the complex geological history of continental margins. This paper focuses on sedimentary facies, bed geometries as indicators of upper to lower canyon and canyon mouth successions, and the stacking pattern and forcing factors as indicators of the continental margin evolution.

Submarine canyon complexes are exposed along the 5km beach cliff in the Californian middle Eocene forearc basin, as the Torrey Sandstone, the Ardath Shale, and the Scripps Formation, north of La Jolla. The successions, studied by 10 measured sections and photomosaics, exhibit 2 major canyon complexes, 180m thick, based by major erosion surfaces.

The lower complex, the Torrey Sandstone and the Ardath shale, is mainly composed of heterolithic channels. The individual channels are 100-400 m wide and 30-90 m deep. The channel stacking pattern shows cross-cutting amalgamated relationships. This complex is interpreted as the upper submarine canyon succession where bypass and erosional process dominate. The upper canyon complex, the Scripps Formation, is composed of three sections. The lower section contains conglomerate scour fills and planar laminated sandstone with continuous laterally flat erosional surface, which is interpreted as the canyon mouth succession. The middle section consists of sandy channels with basal conglomerate which occur in relatively organized and aggradational stacking pattern. The individual channels are 70-200 m wide and 10-30 m deep. This succession is interpreted as the lower submarine canyon succession. The upper section occurs as heterolithic channels with a cross-cutting pattern which is similar to the lower complex, but the volumetric proportion is considerably smaller.

The overall change in the two complexes exhibits that the upper canyon succession is overlain by the canyon mouth and lower canyon succession. Thus the stacking pattern changes from retrogradational to progradational. This change coincides with the termination of the slab rollback (53-47 Ma), interpreted to control regional subsidence rates surpassing the sediment supply rates during slab rollback.