PEAKALL, JEFF, BEN KNELLER and BILL McCAFFREY, University of Leeds, UK

Abstract: A Classic Submarine Canyon Analogue Revisited: High-Resolution Architecture of the La Jolla Group, San Diego

The Black's Beach section of the Early to Middle Eocene La Jolla Group is a magnificently exposed and largely untectonised example of an ancient submarine canyon fill. This classic section has been interpreted in terms of a submarine canyon model which has remained largely unmodified since the late 1970's.We have undertaken detailed architectural analysis in terms of both facies and geometries using photomontages compiled from helicopter flights. This has provided an adequate base for high-resolution architectural mapping of the 6.9 km by 100 m section. Lithofacies and erosional surfaces were superimposed onto these templates, and supplemented by detailed logging through incised ephemeral stream sections.The resulting digitised architecture maps allow quantitative assessment of the geometries, abundances and distributions of individual facies.

The facies analysis permits the recognition of hitherto undescribed or under-recognised processes that control the fill in the Black's Beach section, and which may be more generally applicable to submarine canyon reservoirs. The interaction of flow with topography can be clearly demonstrated at a number of levels and scales within the fill, suggesting that spatially non-uniform turbidity current models can be applied usefully to submarine canyon deposits. Additionally, prominent slide units, on the order of 10's to 100's of metres, testify to the importance of post-depositional mass movement processes within the finer-grained canyon deposits. Evidence is presented for the importance of tidally influenced currents in controlling some sedimentation in the canyon fill. Existing interpretations of high sinuosity meandering channels and braided channels are reinterpreted in the light of the new dataset and through comparison with studies of modern channel systems.

There is considerable debate as to the variation in age along the Black's Beach transect. Both the benthic foraminifera and nannoplankton show an apparent "younging" trend from the canyon edge in the north to a more central canyon position in the south, however, the magnitude of this variation varies between the nannoplankton and the forams. Lohmar (1978) interpreted the foraminifera data as being predominantly facies, not age related, whilst the nannoplankton data have been interpreted purely in terms of age (May, 1982). However, the presence of large numbers of slide sheets may complicate the age relationships.A comprehensive dating program is being undertaken using nannoplankton, and subsampling successive slide sheets, in order to improve both the age relationships and subsequent reconstructions of the submarine canyon.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England