Massive marine mudstones containing scattered pebbles crop out at several places in California in strata ranging from Jurassic to Pliocene. Similar rocks, also characterized by unsorted lithology with widely dispersed pebbles, are exposed in Switzerland in Wildflysch strata (Eocene) and in Bavaria in Cenomanian beds.
The California pebbly mudstones are found as lenticular beds in association with slump structures, including slump overfolds. They are interbedded with graywacke, mudstone, conglomerate, and sandstone that were in part laid down by turbidity currents as shown by graded bedding, load casts, current bedding, and other primary structures. Here at places successive stages illustrating the origin of the pebbly mudstones are preserved. Beds of graded conglomerate, laid down on soft water-saturated mud, became unstable, slumped downslope, and mixed pebbles with mud.
Pebbly mudstones originating by this process resemble many rocks designated as tillites. In fact, some glacial episodes may have been introduced into geologic history based on the interpretation of such rocks as tillites without independent supporting evidence. Such “tillites” should therefore be restudied, for they may have originated by slumping and mixing of interbedded gravel and mud penecontemporaneously with deposition.
Small-scale slump structures and mixed rocks, such as those described here, are considered as part of a continuous series which ranges upward from turbidity currents through slumps and large slides to huge tectonic units at the upper extreme. All these phenomena owe their origin to downslope movement under gravity.