MOUNT, Jack D. , University of California, Riverside.

MOLLUSCAN EVIDENCE FOR THE AGE OF THE IMPERIAL FORMATION, SOUTHERN CALIFORNIA¹.

The Imperial Formation, named by Hanna (1926) and redefined by Woodring (1931), consists of marine clastic sediments distributed widely in Imperial and Riverside Counties, California. Considerable diversity of opinion as to the age of the formation has existed with determinations ranging from Cretaceous to Pleistocene. On the basis of the following points I believe that the Imperial Formation is Late Pliocene in age using a two-fold division of the Pliocene. 1) The molluscan fauna is decidedly modern in character and is closely allied to the tropical Gulf of California fauna; 73% of the taxa are extant. 2) 96% of the extant species do not have an age range older than Late Pliocene. 3) None of the extinct taxa are restricted elsewhere to strata older than Late Pliocene. 4) The presence of Argopecten mendenhalli (Arnold) and Patinopecten dilleri (Dall) provides a tie with the Late Pliocene of the standard Pacific coast megafaunal chronology. 5) Turritella imperialis, Hanna, previously compared to Early and Middle Miocene forms, is closely allied to T. mariana Dall living in the Gulf of California. 6) When compared to coastal Cenozoic faunas, the Imperial fauna is most similar to that of the San Diego Formation of probable Late Pliocene age.

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¹Reprinted in Bulletin of the Southern California Paleontological Society, vol. 6, no. 6, June 1974, p. 60.

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