The Upper Cretaceous Stratigraphy and Paleontology of the Northern Santa Ana Mountains

The Upper Cretaceous deposits of the Santa Ana Mountains in the area between Trabuco and Santa Ana Canyons are divisible into three formations, each of the two upper formations being further divisible into two members. The Trabuco formation at the base has an estimated average thickness of 250 feet. It lies unconformably upon a basement complex made up of metamorphosed Triassic sedimentary rocks, and later intrusive igneous rocks. The Trabuco formation is a red, soft, incoherent, boulder conglomerate, is unfossiliferous, and may be of continental origin. The Trabuco formation is apparently conformably overlain by the Ladd Formation, which has a maximum thickness of at least 1700 feet, and which is divided into the Baker sandstone and conglomerate member below, and the Holz shale member above. The Baker member, approximately 200 feet thick on the average consists of hard, well-cemented, gray, unfossiliferous boulder conglomerates below grading into thick-bedded to shaly highly fossiliferous sandstones above. The Holz shale member has a maximum observed thickness of approximately 1500 feet, is composed principally of silty, micaceous, gray shale. The lower half of the shale is almost unfossiliferous. The upper half yields a prolific molluscan fauna. The Williams formation rests upon the Holz shale with a slight unconformity. It is divided into the Schulz conglomerate and sandstone member below, and the Pleasants sandstone member above. The Schulz member is unfossiliferous, composed of well-worn and rounded pebbles and boulders alternating with arkosic sandstones. It is approximately 200 feet thick. The Pleasants member consists of thin-bedded, rather shaly, light-colored micaceous sandstones, alternating with coarse, calcareous sandstones. It is highly fossiliferous. Two faunal zones are distinguished. The fauna of the Baker member, or Acataeonella oviformis zone of Packard, is not certainly known elsewhere in California, but is probably in part represented in the basal beds of the Rogue River Valley Upper Cretaceous of Oregon. The Glycymeris veatchii zone includes the upper part of the Holz shale member, and all of the Williams formation. It is divisible into three subzones from below upward (a) the subzone of Turritella chicoensis, typical variety, (b) the subzone of Turritella chicoensis, giant variety, (c} the subzone of Metaplacenticeras pacificum. Fossiliferous Cretaceous beds in the Simi Hills and the Santa Monica Mountains of southern California are probably to be correlated with the subzone of Metaplacenticeras pacificum, and to be somewhat younger than the Upper Cretaceous beds at Chico Creek, Butte County. A fauna of approximately one hundred ten species is listed and discussed, a number of new species are described and figured, and new facts concerning the systematic position of a number of previously described fossils are presented.