Chapter 4 Review of the Neogene Biostratigraphy and Stratigraphy of the Los Angeles Basin and Implications for Basin Evolution
The Los Angeles basin is one of the Neogene basins along the Califomia continental margin that was formed by extension related to complex wrench-fault mechanisms. These extensional mech- anisms caused the pull-apart tectonics that resulted in rapid deepening of many of the continental margin basins in the late Oligocene to early Miocene.
For the last 60 years, the biostratigraphic framework for correlation between the oil fields of the Los Angeles basin has been based on the benthic foraminiferal zones and divisions first published by Wissler (1943,1958). These biostratigraphic units are used to correlate the upper middle Miocene to upper Pliocene clastic reservoirs across the basin. The benthic foraminiferal zones of Wissler (1943, 1958) were correlated to the Miocene benthic foraminiferal zones of Kleinpell (1938, 1980) and the Pliocene- Pleistocene stages of Natland (1952).
With the utilization of other microfossil disciplines (e.g., siliceous and calcareous planktonic microfossils), a more refined biostra- tigraphic scheme has been developed. Correlation of plankton biostratigraphies with the radiometric time scale has in tum allowed correlation and calibration of benthic foraminiferal zonations. Application of this new biostratigraphic- chronostratigraphic scheme can now be used to constrain the timing and magnitude of tectonic and depositional events marking basin development.
Benthic foraminiferal assemblages from three stratigraphic sections, located around the margins of the Los Angeles basin, were correlated to siliceous microfossils, calcareous nannofossils, planktonic foraminifers, and radiometric dates to determine the age relationships of the different benthic foraminiferal zonations.
Three regional cross sections were constructed based on the thickness of the Neogene sedimentary package and based on the chronostratigraphic relationships between the benthic foraminif- eral zones and the other fossil groups. These regional cross sections illustrate the shifting of the central basin depocenter through the late Neogene. The pre-late Miocene deposits are thickest along the basin margin, especially in the northwestern part of the central block, the Puente Hills, and the Capistrano embayment. The sediments that were deposited during the late Miocene through the early Pleistocene were deposited in the central trough, bounded by the Newport−Inglewood, Santa Monica-Raymond Hill, and the Whittier fault systems.
Unconformities and hiatuses are associated with each tectonic block. Some of these unconformities are local events related to structural growth or fault movement. The hiatuses are regional events caused by changes in ocean chemistry or velocity of bottom currents.
The unconformities that occur in the late Miocene to early Pliocene are related to local structural growth in the Palos Verdes Hills area and the Anaheim nose area of the southeastern portion of the basin. Other unconformities that occur in the late Miocene are related to movement along the Newport−Inglewood fault and the Whittier fault.
In the late middle Miocene a hiatus occurs within the Monterey Formation in the Palos Verdes Hills and the Newport Bay area. This hiatus is thought to be related to climatic and oceanic events associated with continental and oceanic glaciation.
Figures & Tables
“The most distinctive characteristic of the Los Angeles basin“The most distinctive characteristic of the Los Angeles basin is its structural relief and complexity in relation to its age and size” (Yerkes et aI., 1965, p. AI6); however, its very complexity caused no small amount of discussion in designing and naming this volume of the AAPG World Petroleum Basin Memoirs. (See the Foreword for a discussion of the scope of these memoirs.) The series coordinators decided early that the Los Angeles basin should be included in the World Petroleum Basins project because of its interesting geology and importance as a hydrocarbon producer. Initially, the Los Angeles basin was considered for a convergent-margin volume, presumably in recognition of the late-stage shortening that has taken place in the Los Angeles region of southern California. There is little doubt, however, that the Los Angeles basin has formed and deformed within the evolving San Andreas transform system (Atwater, 1970, 1989; Campbell and Yerkes, 1976; Blake et al., 1978; Engebretson et al., 1985; Wright, this volume). There is also little doubt among those who have worked in the area that the initial subsidence of the Neogene Los Angeles basin was caused by extension (Yeats, 1968; Crowell, 1974, 1976, 1987; Wright, this volume). The series coordinators decided, therefore, that to portray the Los Angeles basin as a model for basins formed in convergent-margin settings would be misleading.
The title of this volume, Active Margin Basins, is a compromise, but, like many compromises, this title falls short of completely describing its subject