For this 100th issue of GeoLifestyle, I would like to show off some of the coolest world-class deep-water outcrops I have ever seen, courtesy of a recent field trip with Applied Stratigraphix led by legendary Tim Demko! This edition will be the first in a three-part series on the San Diego area. Let’s take a look.
Molly Turko
Structural Geologist, Devon Energy
San Diego Area's Epic Geological Tale
An example of San Diego geology. Courtesy of Molly Turko.
The San Diego area boasts a unique geological history.
Along the coast of San Diego, exposed strata are primarily Eocene deep-marine deposits (approximately 50–65 Ma). These sandstones and siltstones record submarine fan and slope systems shed westward into a forearc basin along the active continental margin.
During deposition, the Farallon Plate was subducting beneath North America, driving basin subsidence, sediment supply from an uplifting magmatic arc, and gravity-driven transport into deep water.
Later Neogene transtension associated with Pacific–North American plate reorganization uplifted and faulted these deposits.
Now, that is an epic geologic story!
What is a submarine fan and slope system?
A submarine fan is a deep-marine sediment system in which gravity-driven turbidity currents carry sediment from the continental slope to the basin floor.
Proximal areas feature channel-levee complexes with thick, amalgamated coarse-grained sands from confined flows.
Downfan flows spread into lobes with sheet-like sands showing better lateral continuity.
Frontal fringes have thinner-bedded, finer-grained turbidites with ripple lamination.
The slope includes canyons, gullies, and mass-transport deposits, dominated by erosion and bypass.
Recognizing these textures and architectures in logs, core, and seismic data is key to predicting spatial distribution, guiding target selection, geosteering, production expectations, and anticipating heterogeneity/reservoir quality changes.
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Observe deep-water channel lithofacies and stratal architecture. Keep in mind that channels can be filled with anything, and it’s not always sand!
Differentiate channel margin and thalweg depositional settings. The thalweg (channel axis or deepest part) typically shows the coarsest, thickest, most-amalgamated sandstones or conglomerates from high-energy, erosive flows.
Channel margins feature finer-grained, thinner-bedded sands/silts, often with more mud drapes or overbank/levee deposits from waning or spilling flows.
Learn what a bypass is. Spoiler alert: It's an erosional surface where whatever caused it has “bypassed” and moved along. This was new to me!
Discuss porosity/permeability, geometry, and connectivity of deep-water channel systems. The discussions with your reservoir engineer can run wild!
Trip tips:
Fuel your hunger or grab a drink on the San Clemente Pier at Fisherman’s Restaurant and Bar.
If you park at San Clemente State Beach Parking, look for a cool wave-like structure in the rocks across from the restrooms. This is soft-sediment deposition.
Watch out for trains along this stretch of the beach. This is part of the MetroLink that goes into Los Angeles.
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Torrey Pines State Natural Reserve. Courtesy of Molly Turko.
Torrey Pines State Natural Reserve is named for the rare Pinus torreyana (tree), with roots tracing back to prehistoric times when a cooler, wetter climate supported a larger forest.
If you park in the state natural reserve lot and walk the beach moving north, you'll observe:
A sharp transition from shallow tidal deposits (Del Mar Formation) to deep-water deposits (Torrey Sandstone), reflecting a major transgression likely due to tectonics. This occurs right after crossing the prominent rocky point.
Classic tidal signatures like cyclic spring-neap bundles, oyster hash, and rip-up clasts before the transition.
Prominent deep-water channel where the Torrey Sandstone incises into the underlying tidal Del Mar Formation, featuring large-scale cross-bedding with upflow backsets from supercritical flows. This is located after the rocky point.
Trip tips:
The area is best explored during low tide and may be inaccessible during high tide.
Black's Beach is about a quarter mile walk to the north and is renowned among experienced surfers for having some of the most powerful, consistent waves in Southern California, driven by a deep submarine canyon.
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