Cretaceous and/or lower Tertiary marine sedimentary rocks crop out on the three major northern Channel Islands. Study of the rocks and their contained foraminiferal assemblages reveals many faunal and lithologic similarities between these three islands.

On Santa Cruz Island foraminiferal assemblages indicate that deposition of Cretaceous and Paleocene strata took place in a shallow-water marine basin which became continuously deeper during Eocene time until lower bathyal depths were present by the end of late Eocene time. Sedimentary structures and textures, as well as lateral thickening of strata, suggest source areas on the north or northeast.

On Santa Rosa Island, where the oldest rocks exposed are of middle and late Eocene ages, foraminiferal assemblages indicate a similar deepening.

On the west, however, foraminifers from Cretaceous and early Tertiary strata of San Miguel Island indicate continuous deep-water conditions and open-ocean circulation during Cretaceous, Paleocene, and Eocene times.

The writer suggests that all three depositional sites were part of a single basin or two closely contiguous basins with a northwest-southeast axial trend.

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