Lithostratigraphic correlation at various spatial scales in the Livermore Basin at the Lawrence Livermore National Laboratory, California, U.S.A.

摘要

The stratigraphy of the tectonically active Livermore Basin, California is controlled by local tectonics which produce spatial scale variability within the stratigraphy. The sediments at the Lawrence Livermore National Laboratory (LLNL) form three stacked fluvial fan successions beneath the site. The sediment below 37 m was deposited by the Arroyo Seco, an intermittent stream. The sediment between 19 m depth and 37 m was deposited by the Arroyo Las Positas, another intermittent stream. The sediment above 19 m was deposited by the Arroyo Seco. These provenance study data, plus geomorphic evidence, including knowledge that the Arroyo Seco entered the basin through the Las Positas Fault uplift and an abandoned alluvial fan is present south of the uplift, suggests that the Arroyo Seco was captured and redirected into the basin.;With the assumption that large-scale units (approximately 20 m thick) existed beneath LLNL, facies correlations from 202 well cores taken from LLNL was attempted. The cores were described in detail, resulting in the recognition of seven facies, including a paleosol facies. Correlation attempts using the paleosols as marker units were unsuccessful at LLNL because neither the paleosols nor any of the other units were laterally continuous beyond 6 m to 9 m. Very few wells at LLNL are spaced this closely, and the remaining well spacing at LLNL exceeded the lateral spatial resolution required for correlatability. Since facies were not correlatable, major- and trace-element geochemical data were employed as a possible tool for correlating stratigraphic units. The elemental data, elemental ratios, and weathering indices were plotted against depth and inspected for patterns that might point to a unique geochemical signature for one or more of the individual units. Geochemical plots did not result in identification of individual units that were correlatable between wells. Also, the plots did not indicate a difference in geochemistry between the three stack fluvial fan successions. Although the provenance study indicated a difference in the lithology of detritus, the detritus was isochemical because both local source areas had the same ancestral Sierran-Klamath arc source.;A new Index of Paleosol Development (IPD) was developed for this research to compare variability in the development of individual paleosols. The IPD allows for a quantitative assessment of the strength of paleosol developmental features. IPD values within a well core are used to calculate an average value for the entire core: the Core Index of Paleosol Development (CIPD). Well cores in the northwestern region of LLNL had the highest IPD and CIPD values, indicating strongly developed paleosols and/or comparatively more paleosols per core. The IPD and CIPD values in the central western region were lower, suggestive of weakly developed paleosols and/or few paleosols in that region. This variability in paleosol development across the study area reflects the relative surface stability on which the soils formed. The northwestern region of LLNL aligns with the northernmost anticline of the Springtown anticlines that plunge beneath the site. The syndepositional formation of this anticline provided a relatively stable surface allowing for more strongly developed soils, and thus, paleosols with higher IPD values. The central western region aligns with the plunging syncline of the Springtown anticline pair. The formation of the syncline provided a relatively less stable surface on which aggradation inhibited soil development via frequent burial. At LLNL, large scale features (e.g., stacked fan successions and the plunging Springtown anticlines) are correlatable, while smaller scale features (e.g., facies and sedimentary units thinner than the stacked fan successions) are not correlatable.