WARME, JOHN, Colorado School of Mines, Golden, CO
ABSTRACT: Active Margin Sequences and Submarine Canyon Facies Models
Superb lower and middle Eocene forearc facies in northern San Diego County, California, exhibit three stratigraphic sequences that exhibit shoreline and shallow marine facies (Delmar/Torrey sequence) unconformably overlain by submarine canyon deposits (Ardath and Scripps sequences). Delmar/Torrey facies include fan-delta, lagoon, barrier beach, and marine shelf environments, backed by alluvial-fan complexes. This sequence contains suites of sedimentary structures, fossils, and trace fossils characteristic of the now-familiar proximal facies models.
In contrast, the large scale of submarine canyons and their sedimentary fills has inhibited development of applied models for them. The study area provides rare, continuous exposures that can be applied to subsurface exploration and development. They show that the Delmar/Torrey sequence is separated from the overlying Ardath submarine canyon facies by a major sequence boundary and facies shift. This unconformity represents the floor and margins of the fossil "Eocene Torrey Submarine Canyon." The submarine canyon facies model includes a basal massive sandstone, overlain by crosscutting marine channels with unpredictable heterolithic fill. The channels contain active-channel conglomerates, diverse sandstones and siltstones, and abandoned-channel mudstones, collectively termed "variegated fill." A second sequence-bounding unconformity caps these channels, overlain by the conglomeratic Scripps sequence.
Surprisingly, age dating of the lower canyon boundary and the canyon fill strongly suggests that their development was driven primarily by eustasy, even though the regional setting was a tectonically active forearc.
AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures