Monterey Shale and similar-age marine units
Monterey Shale and similar-age marine units
late to early Miocene
Thin-bedded siliceous to semi-siliceous marine shale occurring in southern and southwestern map area, cropping out at lower elevations of the San Emigdio Mountains, and along the southeastern San Joaquin Valley / foothills transition. In eastern to southeastern extent, locally contains or interfingers with nonmarine sandstone (Kellogg and others, 2008). Also locally includes thin dolomitic intercalations and a basal marine sandstone (Kellogg and others, 2008; Chapman and Saleeby, 2012). Attains a stratigraphic thickness of ~2,100 m in the Temblor Range west of map area, but is significantly thinner within map area (Nilsen and others, 1973). Conformably(?) overlies Temblor Formation (e.g., Kellogg and others, 2008).
Based on diatom biochronology, Barron (2022) interpreted an age range of ~16.2 to ~5.5 Ma (late Relizian to Delmontian) for his San Joaquin Valley section of the Monterey Shale, which is subdivided (from stratigraphically lowest to highest) into the Gould Shale, Devilwater Shale, McDonald Shale, Antelope Shale, and Reef Ridge Shale Members. However, not all of these members crop out within the map area. The Gould Shale crops out within the map area (Kellogg and others, 2008; Nilsen and others, 1973) and ranges in age from ~16.2 to ~14.5 Ma (Relizian to Luisian) based on diatom biochronology (Barron, 2022). Knott and others (2022) also reported a ~15.77 Ma tephra interstratified with Monterey Shale sediments within the map area, which presumably belong to the Gould Shale. Kellogg and others (2008) reported a section bearing Mohnian foraminifera, which represents the McDonald Shale and/or the Antelope Shale which are, respectively, ~13.4 to ~11.9 Ma (Luisian to early Mohnian) and ~11.9 to ~7.1 Ma (early to late Mohnian, possibly to early Delmontian) based on diatom biochronology (Barron, 2022). This indicates that the Devilwater Shale, and possibly the McDonald Shale, are absent in outcrops, which indicates a ~1.1 m.y. or greater hiatus in the exposed section.
Eastern extent of unit, cropping out along the eastern side of the southern San Joaquin Valley, includes occurrences of several other early to late Miocene marine units mapped and described by Bartow (1984) from stratigraphically lowest to highest as: Freeman Silt and Jewett Sand, undifferentiated marine sandstone, Olcese Sand, Round Mountain Silt, undifferentiated marine siltstone and sandstone, and undifferentiated marine sandstone and conglomerate. Collectively, these units predominantly consist of fine- to coarse-grained sandstone, sandy siltstone, siltstone, silty claystone, claystone, and subordinate coarse- to very coarse-grained pebbly sandstone and pebble conglomerate, and diatomaceous shale.