Benthic foraminifera and paleobathymetry of the Eocene Llajas Formation, southwestern Santa Susana Mountains, California

The late early to middle Eocene Llajas Formation, southwestern Santa Susana Mountains, California, contains an abundant and diverse benthic foraminiferal fauna. At least 37 genera and 68 species were identified. Most of the• foraminifers are well preserved and unabraded, with little mixing of faunas, suggesting minimal post-mortem transport. However, shallow water faunas found in the slope facies show signs of abrasion and fragmentation. The age of the Llajas Formation, based on benthic Foraminifera, ranges from lower Ulatisian for the lower one third, through upper Ulatisian for the remainder of the formation. The most recently accepted age of the Ulatisian is lower to lower middle Eocene, and my studies support this. Variations in foraminiferal assemblages within the section suggest paleobathymetries ranging from inner neritic to upper bathyal during deposition of the Llajas Formation. Expressed as the number of species per sample, diversity ranges from 4 species per sample from the middle neritic portion of the shallow marine facies to a high of 44 species per sample from the upper bathyal portion of the outer shelf to slope facies. Agglutinated Foraminifera have the greatest abundance and diversity in the upper bathyal portion of the section, whereas porcelaneous types are most abundant and diverse in the middle and outer neritic portion of the section. The hyaline suborder predominates throughout the section. Planktonic foraminifers, diatoms, radiolarians, and calcareous nannofossils all increase in abundance with increasing depth, while ostracodes decrease in abundance with increasing depth. Inner to middle neritic conditions are indicated in the lower and upper portion of the section, indicating transgressive and regressive phases of deposition.