ALBERT

Description: Five widespread upper Cenozoic tephra layers that are found within continental sediments of the western United States have been correlated with tephra layers in marine sediments in the Humboldt and Ventura basins of coastal California by similarities in major-and trace-element abundances; four of these layers have also been identified in deep-ocean sediments at DSDP sites 34, 36, 173, and 470 in the northeastern Pacific Ocean. These layers, erupted from vents in the Yellowstone National Park area of Wyoming and Idaho (Y), the Cascade Range of the Pacific Northwest (C), and the Long Valley area, California (L), are the Huckleberry Ridge ash bed (2.0 Ma, Y), Rio Dell ash bed (ca. 1.5 Ma, C), Bishop ash bed (0.74 Ma, L), Lava Creek B ash bed (0.62 Ma, Y), and Loleta ash bed (ca. 0.4 Ma, C). The isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments. For example, the widespread Bishop ash bed is correlated from proximal localities near Bishop in east-central California, where it is interbedded with volcanic and glacial deposits, to lacustrine beds near Tecopa, southeastern California, to deformed on-shore marine strata near Ventura, southwestern California, to deep-ocean sediments at site 470 in the eastern Pacific Ocean west of northern Mexico. The correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control. Relative age variations of as much as 0.5 m.y. exist between marine biostratigraphic datums [for example, highest occurrence level of Discoaster brouweri and Calcidiscus tropicus (= C. macintyrei)], as determined from sedimentation rate curves derived from other age control available at each of several sites. These discrepancies may be due to several factors, among which are (1) diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds, (2) upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western United States and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums, (3) dissolution of microfossils or selective diagenesis of some taxa, (4) lack of precision in isotopic age calibration of these datums, (5) errors in isotopic ages of tephra beds, and (6) large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums. Correlation of tephra layers between on-land marine and deep-ocean deposits indicates that some biostratigraphic datums (diatom and calcareous nannofossil) may be truly time transgressive because at some sites, they are found above and, at other sites, below the same tephra layers.

Description: Evidence for submarine hydrothermal activity along oceanic spreading centers has been accumulated from a large and diverse body of geochemical and geophysical data (e.g., Boström and Peterson, 1966; Wolery and Sleep, 1976; Humphris and G. Thompson, 1978), and the alteration of oceanic rocks by seawater has been the subject of intense study for the last few years. Previous studies of Deep Sea Drilling Project (DSDP) samples have found little evidence of high-ternperature hydrothermal activity in the upper part of the oceanic crust. Instead, most of the alteration described appears to be the result of low-temperature interchange between basalt and seawater under ambient conditions on the seafloor. This type of alteration is most pronounced along fractures and open channelways and involves the growth of smectite, carbonate, and occasionally pyrite and phillipsite. Olivine crystals and interstitial glassy material within the basalts are frequently replaced by smectites and more rarely, carbonate, but the plagioclase feldspars and clinopyroxenes are normally unaffected. The resulting chemical changes usually involve notable increases in H2O, Fe2O3, K2O, Rb, Cs, and U and smaller increases in total iron and Sr. TiO2, Al2O3, P2O5, Nb, Zr, and Y are usually considered to be little affected by alteration (e.g., Bass, 1976; Robinson et al., 1977). Several workers (e.g., Pritchard et al., 1979) have noticed an increase in the degree of alteration with age.