On the reconciliation of biostratigraphy and strontium isotope stratigraphy of three southern Californian Plio-Pleistocene formations

Abstract

The San Diego Formation, Pico Formation, Careaga Sandstone, and Foxen Mudstone of southern California are thought to be late Pliocene to early Pleistocene; however, numerical ages have not been determined. Following assessment of diagenetic alteration via multiple methods including scanning electron microscopy (SEM), X-ray diffraction (XRD), and minor elemental concentrations, we attempted to use strontium isotope stratigraphy to assign numerical ages. Using aragonitic fossils, we obtained ages of 2.0−1.85 Ma for the Careaga Sandstone and 2.0−1.75 Ma for the uppermost Foxen Mudstone, consistent with biostratigraphic work suggesting a Gelasian age for the Careaga Sandstone. Isotope ratios for aragonitic and calcitic fossils from the Pico Formation were poorly constrained, with the exception of one bed yielding ages of 5.1−4.3 Ma. Isotope ratios from the San Diego Formation were also inconsistent within beds, with the exception of two isolated outcrops that yielded ages of 5.0−4.5 Ma and 4.5−2.8 Ma, respectively. The age estimates for the Pico and San Diego Formations are older than most ages inferred from biostratigraphy. Noting that some aragonitic specimens from the San Diego Formation yielded isotope ratios indicative of ages as old as 19.4 Ma, we propose that some outcrops have been affected by diagenesis caused by groundwater flow through proximal granitic rocks and input from detrital sediment. Although we recommend that strontium isotope results for the Pico and San Diego Formations be interpreted with caution, the ages of the uppermost Foxen Mudstone and Careaga Sandstone can be confidently placed within the early Pleistocene.

Reconstructing oyster paleocommunity structure over the last 3.6 million years: A southern California case study

“…Although the numerical age of formations in southern California that span the Plio-Pleistocene boundary remains uncertain due to how the data were originally collected and current refinement of the biostratigraphy, we broke the data down into four-time bins: Holocene (0-11,700 yBP), late Pleistocene (11,700-126,000 yBP), middle Pleistocene (126,000-781,000 yBP), and late Pliocene (~2.6-3.6 Ma) (Fig. 2) (Woodring and Bramlette 1946, Yerkes et al 1965, Bowersox 2005, Buczek et al 2020.…”

“…Lithostratigraphic correlations of rock units within the Los Angeles Basin and San Diego Embayment spanning the Holocene through the Pliocene (modified fromBuczek et al 2020). The age of Pico, Fernando, Niguel and San Diego Formations is heavily debated with estimates ranging from early Pliocene to early Pleistocene.…”

“…Microfossil biostratigraphy analyses result in conflicting interpretations. However, recent work suggests that these formations range into the Gelasian stage of the early Pleistocene Epoch(Buczek et al 2020).…”

Checklist of California Paleogene–Neogene marine Mollusca since Keen and Bentson (1944)

“…Groves (1991a, b) reported also that the lower Saugus [=Pico] in the Santa Susana Mountains (Valencia area) is Plio-Pleistocene age. Buczek et al (2020) reported that the Pico Fm. in the Valencia area might range into the early Pleistocene.…”

“…The Careaga Sandstone, which crops out in the Santa Marina area of Santa Barbara County in southern California, has been historically regarded as of late Pliocene age. Based on strontium-isotope data (Buczek et al 2020) and the "shifting" of the Pliocene-Pleistocene boundary as dictated by the International Commission of Stratigraphy (see a series of online charts initiated by Cohen et al 2009), the age of the Careaga Sandstone and its rich molluscan content has now been assigned an early Pleistocene age. Such an age would normally exclude any molluscan fauna from being included in this current checklist.…”