SAN ONOFRE STATE BEACH, CA – The warm spring weather in Wooster, OH has us dreaming of the beach rather than finals week. Some lucky Wooster Geologists had a recent beach fix when they visited San Onofre State Beach in southern California during the Keck Symposium. Symposium attendees had two options for the traditional Friday Field Trip: San Onofre Beach or Mecca Hills. It was a difficult choice, but the prospect of spending a warm day on the Pacific coast in April was quite persuasive for this midwest geologist.
The San Mateo sandstone is a well-sorted marine unit with highly rounded grains that was likely deposited in a submarine fan setting. Although the sandstone itself is beautiful, what really caught our eye was the well-exposed Cristianitos Fault and associated deformation.
Underlying the San Mateo sandstone is the mid Miocene Monterey Formation, a layered marine siltstone locally interbedded with volcanic ashes. Slumps occur in the Monterey Formation all along the coast.
Above the marine units is a thin, flat-lying cobble-rich layer. The cobbles were deposited on an ancient marine terrace ~125 ka. They littered the beach, so we had a chance to look at them up-close. What we observed were gorgeous hard-rock cobbles from the Cretaceous Catalina Blueschists and the Peninsular Ranges Batholith.