Abstract

The generally accepted correlations of the Jurassic of southwestern Oregon and California are as follows: Galice and Mariposa, Dothan and Franciscan, Myrtle and Shasta. The Dothan is said to be younger than the Galice. On the other hand it has been stated that the Dillard, the lower part of the original Myrtle, is the equivalent of the Franciscan. Since the Dillard unconformably overlies Dothan and Galice, it is clear that one or the other of these correlations must be erroneous. Evidence is presented to show that the Dothan is older than the Galice and cannot be the same age as the Franciscan. The Dillard, as originally defined, includes Knoxville (Upper Jurassic) and possibly infolded Lower Cretaceous, but the great bulk of the Dillard is to be correlated with the Franciscan. The Myrtle, as mapped and defined, includes beds of both Upper Jurassic and Lower Cretaceous age and at least one important unconformity. The name should be redefined and restricted or abandoned.

The Nevadan orogeny took place in the late Kimmeridgian and uplifted the Sierra Nevada and depressed the belt now occupied by the Coast Ranges. In the geosyncline thus formed the Franciscan and Knoxville were deposited. There is no unconformity between the Franciscan and Knoxville; these groups were deposited during the Portlandian and Tithonian.

At the base of the Lower Cretaceous is a widespread unconformity. The diastrophism which closed the Jurassic is called the Diablan orogeny.

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