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The Great Wisconsinan Ice Age Mudflows Of the Southern California Coast

I. P. Colburn and S. Korm
California State University, Los Angeles, CA, [email protected], [email protected]

During the Wisconsinan Ice Age, southern California experienced coastal British Columbia type climate. Heavy rains generated numerous large mudflows that coursed down the fronts of coastal mountains. The mudflow successions are recognized in the face of the modern sea cliff on the coast at Pacific Palisades, Crystal Cove State Park, Laguna Beach/South Laguna, and San Onofre State Beach.

The mudflow successions range in thickness from 20 feet on the Laguna Beach coast to more than 100 feet at San Onofre. Approximately 50 samples were collected and analyzed for textural and compositional characteristics. Samples were obtained from a July 2008 mudflow at the Mount Whitney Fish Hatchery for comparison purposes.

These successions consist primarily of non-stratified to poorly stratified beds of mudstone, sandy mudstone, and gravelly sandy mudstone; colors range from red brown at San Onofre, brown at Crystal Cove, to dark gray at Pacific Palisades. Gravelly sand intervals are locally present indicating fluvial reworking of the mudflows.

For all localities, except the Pacific Palisades, the mudflows rest on the “old” wave-cut bench that formed during the Sangamon high stand of the sea. At most localities the “old” benches had “old” beach sand, gravel, and mollusk shells on them that were buried by the mudflows.

From the great thickness of the coastal mudflow successions it can be inferred that they probably extended at least a quarter of a mile seaward from the modern-day sea cliff. The distal ends of the mudflows were destroyed and the sea cliff was formed by wave erosion during the post-Wisconsinan eustatic sea level rise. The Fish Hatchery mudflow, which traveled more than 4 miles down a slope that decreased from 5° to 2°, left a mudflow deposit 1 foot thick and serves as a modern analog of the ancient mudflows.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009