Abstract
AbstractThe 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.
Highlights
The mid-Pliocene warm period was a time of high global temperatures (2 °C to 3 °C above pre-industrial temperatures) and high atmospheric CO2 concentrations (360–400 ppm) (Jansen et al, 2007)
Microscopic alteration was assessed by typical methods (e.g., scanning electron microscopy (SEM) and elemental concentrations), only ∼30% of all specimens were considered sufficiently preserved to be analyzed, and many were chalky to the touch, indicating that preservation is variable throughout the units
Diagenetic alteration of aragonitic specimens within the San Diego Formation could have resulted from groundwater flow from surrounding igneous rocks, pore water in contact with granitic constituents of the inorganic matrix of the formation, and the frequent saturation of sediments due to sea level changes during the Pleistocene
Summary
The mid-Pliocene warm period (ca 3 Ma; Jansen et al, 2007) was a time of high global temperatures (2 °C to 3 °C above pre-industrial temperatures) and high atmospheric CO2 concentrations (360–400 ppm) (Jansen et al, 2007). The sedimentology and fossil content of the Foxen Mudstone and Careaga Sandstone of Santa Barbara County (Woodring and Bramlette, 1950), the Pico Formation of Los Angeles County (Winterer and Durham, 1962; Squires et al, 2006; Squires, 2012), and the San Diego Formation of San Diego County (Hertlein and Grant, 1944; Hertlein and Grant, 1972; Deméré, 1983; Vendrasco et al, 2012) in California have been thoroughly described (Fig. 2) Faunas preserved within these strata are diverse and closely related to the modern southern California biota (Woodring and Bramlette, 1950; Winterer and Durham, 1962; Deméré, 1983; Squires et al., 2006; Powell et al, 2009; Squires, 2012; Vendrasco et al, 2012). The age designation “late Pliocene to early Pleistocene” lacks sufficient resolution to allow the various climatic fluctuations (discussed above, Fig. 1) to be differentiated, further hindering our ability to understand the climatic context of these communities
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