Miocene stratigraphy and structure of the East Bay Hills, California

ABSTRACT The structure and stratigraphy of the Miocene formations east of San Francisco Bay have been described in multiple studies for over a century. We integrated the results of past investigations and provide new data that improve understanding of formation age, the timing of deformation, and the amount of dextral displacement on selected faults. New geologic mapping and better age control show that formations previously inferred to be separate units of different ages are correlative, and new names are proposed for these units. Miocene structures associated with the development of the San Andreas transform system exerted significant control on Miocene deposition in the East Bay area. The developing structure created five distinct stratigraphic sections that are differentiated on the basis of differences in the stratigraphic sequence, lithology, and age. The stratigraphic changes are attributed to significant dextral displacement, syndepositional faulting, and distal interfingering of sediment from tectonically elevated source areas. New stratigraphic evaluations and age control show that prior to ca. 6 Ma, the developing fault system created local tectonically induced uplift as well as spatially restricted subbasins. Regional folding did not occur until after 6 Ma. Past evaluations have inferred significant dextral displacement on some of the faults in the East Bay. The spatial relationships between unique conglomerate clasts and known source areas, as well as the distribution of well-dated and unique tuffs, suggest that dextral displacement on some faults in the East Bay is less than previously reported.

Interaction of extensional, contractional, and strike-slip elements at Mount Diablo and the surrounding eastern Coast Ranges, San Francisco Bay area, California: A model-based analysis

ABSTRACT This study presents three regional cross sections, a structural map analysis, and a schematic map restoration. The sections are constrained by surface geology and petroleum wells and were developed using model-based methods to be consistent with the regional tectonic context and balancing concepts. Together, these products depict the geometry and kinematics of the major fault systems. Insights from this research include the following. Franciscan complex blueschist-facies rocks in the Mount Diablo region were unroofed west of their current location and subsequently thrust beneath the Great Valley sequence in the mid-Eocene. East Bay structures are complicated by overprinting of Neogene compression and dextral strike-slip motion on a Paleogene graben system. Net lateral displacement between the Hayward fault and the Central Valley varies from 26 km toward 341° to 29 km toward 010° in the southern and northern East Bay Hills, respectively. Uplift above a wedge thrust generates the principal Neogene structural high, which extends from Vallejo through Mount Diablo to the Altamont Ridge. Anomalous structural relief at Mount Diablo is due to strike-parallel thrusting on the crest of a fault-propagation fold formed on the west-verging roof thrust. Uplift that exposes the Coast Range ophiolite in the East Bay Hills is formed by oblique thrusting generated by slip transfer at the northern termination of the Calaveras fault. The Paleogene extensional fault system likely extends farther west than previously documented. An east-dipping branch of that system may underlie the Walnut Creek Valley. Three-dimensional restoration should be applied to constrain geologic frameworks to be used for seismic velocity modeling.

Supplemental Material: Miocene stratigraphy and structure of the East Bay Hills, California

Supplemental Materials: 1: Correlation of the Monterey and San Pablo Groups in the East Bay; 2: Ages and correlations of the Contra Costa and San Pablo; 3: The Indian Valley member of the Moraga Formation; 4: Map distribution of the Upper San Pablo Group and Lower Orinda member and inferred displacement across the Divide Ridge fault zone; 5: Analytical data for radiometric dates; and 6: Plate 1: Geologic map of Miocene rocks of the East Bay.

Supplemental Material: Miocene stratigraphy and structure of the East Bay Hills, California

Supplemental Materials: 1: Correlation of the Monterey and San Pablo Groups in the East Bay; 2: Ages and correlations of the Contra Costa and San Pablo; 3: The Indian Valley member of the Moraga Formation; 4: Map distribution of the Upper San Pablo Group and Lower Orinda member and inferred displacement across the Divide Ridge fault zone; 5: Analytical data for radiometric dates; and 6: Plate 1: Geologic map of Miocene rocks of the East Bay.

Supplemental Material: Miocene stratigraphy and structure of the East Bay Hills, California

Supplemental Materials: 1: Correlation of the Monterey and San Pablo Groups in the East Bay; 2: Ages and correlations of the Contra Costa and San Pablo; 3: The Indian Valley member of the Moraga Formation; 4: Map distribution of the Upper San Pablo Group and Lower Orinda member and inferred displacement across the Divide Ridge fault zone; 5: Analytical data for radiometric dates; and 6: Plate 1: Geologic map of Miocene rocks of the East Bay.

Supplemental Material: Miocene stratigraphy and structure of the East Bay Hills, California

Supplemental Materials: 1: Correlation of the Monterey and San Pablo Groups in the East Bay; 2: Ages and correlations of the Contra Costa and San Pablo; 3: The Indian Valley member of the Moraga Formation; 4: Map distribution of the Upper San Pablo Group and Lower Orinda member and inferred displacement across the Divide Ridge fault zone; 5: Analytical data for radiometric dates; and 6: Plate 1: Geologic map of Miocene rocks of the East Bay.

Supplemental Material: Miocene stratigraphy and structure of the East Bay Hills, California

Supplemental Materials: 1: Correlation of the Monterey and San Pablo Groups in the East Bay; 2: Ages and correlations of the Contra Costa and San Pablo; 3: The Indian Valley member of the Moraga Formation; 4: Map distribution of the Upper San Pablo Group and Lower Orinda member and inferred displacement across the Divide Ridge fault zone; 5: Analytical data for radiometric dates; and 6: Plate 1: Geologic map of Miocene rocks of the East Bay.

Supplemental Material: Miocene stratigraphy and structure of the East Bay Hills, California

Supplemental Materials: 1: Correlation of the Monterey and San Pablo Groups in the East Bay; 2: Ages and correlations of the Contra Costa and San Pablo; 3: The Indian Valley member of the Moraga Formation; 4: Map distribution of the Upper San Pablo Group and Lower Orinda member and inferred displacement across the Divide Ridge fault zone; 5: Analytical data for radiometric dates; and 6: Plate 1: Geologic map of Miocene rocks of the East Bay.

Supplemental Material: Miocene stratigraphy and structure of the East Bay Hills, California

Supplemental Materials: 1: Correlation of the Monterey and San Pablo Groups in the East Bay; 2: Ages and correlations of the Contra Costa and San Pablo; 3: The Indian Valley member of the Moraga Formation; 4: Map distribution of the Upper San Pablo Group and Lower Orinda member and inferred displacement across the Divide Ridge fault zone; 5: Analytical data for radiometric dates; and 6: Plate 1: Geologic map of Miocene rocks of the East Bay.