Part three of this series on San Diego geo-tourism heads inland to the stunning badlands and canyons of Anza-Borrego Desert State Park, where world-class outcrops in Lycium Wash reveal rapid basin evolution amid active tectonics! Legend Tim Demko guided us through a fantastic field trip there with Applied Stratigraphix. I'll bring you along in this edition so you can see the highlights!
Molly Turko
Structural Geologist
Devon Energy
Anza-Borrego Desert State Park and Its Geology
Geologists viewing large bedform. Photo courtesy of Molly Turko.
These latest-Miocene ( roughly 6.3–5.3 Ma) exposures feature a spectacular deepwater supercritical fan of the Lycium Member, built almost entirely by large migrating bedforms (cyclic steps and antidunes) deposited by high-energy, Froude-supercritical turbidity currents.
Early records: Sourced from steep alluvial fans and fan deltas along the Peninsular Ranges, these deposits record the earliest marine inundation of the Fish Creek-Vallecito Basin by the Gulf of California, before being capped by megabreccias as the basin continued to evolve.
More geologic and tectonic history:
Anza-Borrego sits in the complex, oblique boundary between the Pacific and North American plates.
The Salton Trough formed as a transtensional pull-apart basin system due to dextral strike-slip motion with an extensional component along the West Salton Detachment Fault and related structures. Rapid subsidence created deep basins that filled quickly with sediment.
This transtension, active since roughly 7–5 Ma with major acceleration in the Pleistocene, drove basin inversion: Early subsidence flipped to uplift, faulting, and tilting, exposing thick sedimentary sections for rock nerds to explore today!
Fun fact: The park also preserves one of the thickest Cenozoic records in the region (more than 20,000 feet in some places), uplifted rapidly by ongoing earthquakes and fault slip.
Geologists in the field at Anza-Borrego State Park. Photo Courtesy of Molly Turko.
Unlike the Eocene submarine fans we saw on the coast, Anza-Borrego showcases a spectacular latest-Miocene supercritical fan (a deepwater fan built almost entirely by large migrating bedforms rather than classic sheet sands or channels) in the Lycium Member. After hiking about 1 mile into Lycium Wash you will discover this large, world-class exposure.
Look for:
Antidunes: Large, upstream-migrating waveform bedforms that form under Froude-supercritical flows. In the outcrop, they appear as sets of dozens of conformably aggrading sigmoidal (S-shaped) turbidite beds that build distinctive wave-like geometries meters high and tens to hundreds of meters long.
Rip-up clasts: Dark, angular-to-rounded fragments of seafloor mudstone torn up by erosive turbidity currents. They look like “chocolate chips” floating within sandstone beds, often concentrated at bed bases or imbricated.
Compensational stacking of large-scale bedforms: Successive bedsets that onlap and shingle laterally over previous deposits as the giant antidunes and cyclic steps migrate upslope, producing abrupt facies changes over just tens of meters.
Megabreccias As you hike through Lycium Wash, you may notice enormous boulders—some the size of buses—suspended high in the canyon walls. These impressive features are known as megabreccias.
Definition: A megabreccia is a type of sedimentary deposit consisting of large, angular rock fragments (often over a meter across, and sometimes much bigger) embedded in a finer-grained matrix.
These are classic mass-transport deposits, formed when huge volumes of rock and debris suddenly slid or flowed downslope.
The upper megabreccias in Lycium Wash formed as a subaqueous sturzstrom (catastrophic, long-runout debris flow/landslide) from the collapse of unstable basin-margin basement rocks. It was emplaced as a chaotic mass-transport deposit that eroded, deformed, and overrode the underlying Lycium Member turbidites.
The megabreccias consist primarily of metamorphic clasts (biotite gneiss, schist, marble) and plutonic igneous clasts (granodiorite, tonalite, granite, pegmatite).
Some megabreccias show jigsaw-puzzle fabric (clasts fit back together like a broken puzzle) and minimal mixing, preserving source rock zones even after flowing kilometers underwater.
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