A sedimentary breccia included in the middle Miocene strata of the upper plate of the Malibu Bowl detachment fault provides important evidence concerning the source area of the plate. The breccia contains large clasts derived from identifiable older strata, particularly the Vaqueros Formation (lower Miocene) and the Sespe Formation (upper Eocene to lower Miocene). This breccia was first described in 1958 by R. C. Newton in an unpublished M.A. thesis at the University of California, Los Angeles; he mapped its distribution and recognized its probable landslide origin.

The breccia overlies marine shale of middle Miocene age and is overlain by submarine fragmental basalt or andesite, also of middle Miocene age. There is no evidence of subaerial erosion at the contacts. The breccia was deposited continuously across an area of 1.5 to 2 sq mi; it is locally as thick as 350 ft. In addition to the clasts of Sespe and Vaqueros sandstone, the breccia contains clasts of volcanic rock and shale that probably were derived from earlier middle Miocene deposits. The clasts generally are not mixed; those of Sespe predominate in one area, Vaqueros in another, and volcanics in yet another. The matrix in some places is coarse sand (apparently reworked from Sespe and Vaqueros sandstone), but in many areas altered basalt(?) predominates. The local volcanic matrix indic tes that some volcanism was contemporaneous with the formation of the breccia.

The breccia very probably represents a landslide or series of landslides that originated on a steep slope, slid into an adjacent marine basin, came to rest on middle Miocene sediments, and was buried by later middle Miocene volcanic and sedimentary deposits. Although the Sespe and Vaqueros Formations now are exposed in the central Santa Monica Mountains, they probably were covered by early middle Miocene sedimentary and volcanic strata at the time the breccia was deposited. The nearest area where the Sespe and Vaqueros Formations are known to have been exposed at the approximate time of breccia deposition is about 10 mi north in the Simi Hills, suggesting the direction and general order of magnitude of the displacement of the Malibu Bowl fault.

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