SEMINAR - John Warme

Interpretation and Significance of Cretaceous and Eocene Coastal Outcrops, San Diego County

John E. Warme
Professor Emeritus, Colorado School of Mines, Golden, Colorado

April 16th, 2008

The Upper Cretaceous and Eocene formations that crop out along seacliffs and landward in San Diego County have been studied and reinterpreted several times over the last century. By the 1970's they were regarded as prime examples of lateral facies relationships from continental to deep marine, then reinterpreted as pioneering examples of sedimentary intervals separated by significant surfaces that formed discrete unconformity-bounded intervals, or sequences, of the evolving concept of Sequence Stratigraphy.

Cretaceous examples include the Point Loma thin bedded turbidites and mudstones and overlying Cabrillo Formation conglomerates that are still validly argued as genetically-related lateral facies, or conversely, completely separate sequences; these units require more analyses.

In contrast, the uppermost lower Eocene to upper Eocene series exhibit superb examples where unconformity surfaces that separate genetically unrelated intervals define stratigraphic "Sequences". However, such surfaces do not in every case follow the prior definitions of formation boundaries. One of several examples is the unconformity the cuts across the shallow-water to intertidal beds of the Delmar Formation and Torrey Sandstone and is overlain by a sandy and conglomeratic facies of the Ardath "Shale", which is interpreted as the floor of a fossil submarine canyon. Because petrographically similar sandstones occur above and below this surface, the beach-cliff outcrop was everywhere mapped as Torrey. Attention to bedding thicknesses and physical and biogenic sedimentary structures reveals that the sandstones below and above the newly recognized sequence boundary have very different character.

Of special interest is the variegated-fill channel system contained within the boundaries of this "Eocene Torrey Submarine Canyon"; the system is proposed as a submarine canyon facies model. Moreover, the unconformity that defines the boundaries of the canyon is the same age as bounding surfaces that contain canyons on several other continents, implying that they all formed during the same eustatic fall in sea level at about 49.5 million years ago, and all were the conduits that delivered coarse-grained sediments to coeval deep-basin submarine fans.