Walker Formation
Walker Formation
early Miocene to late Eocene
Consists of a diachronous sequence of nonmarine clayey sandstone, siltstone, and claystone, with very minor intercalated pyroclastic deposits, exposed in a fairly continuous, arcuate strip near the southeastern margin of the San Joaquin Valley. Unconformably overlies basement and interfingers laterally (basin-ward/westward) with several marine sedimentary sequences.
Significant age and compositional changes occur from north to south within the formation. Bartow and McDougall (1984) reported compositional changes across the Bakersfield arch: arkosic sandstone, conglomerate, and minor sandy claystone predominate to the south, whereas sandy claystone predominates to the north. Saleeby and others (2016) also reported that the base of the unit is time-transgressive, grading from late Eocene in the north, to early Miocene in the south. For this reason, Saleeby and Saleeby (2019) designate the northern and southern parts of the Walker Formation outcrop belt as “paleo”-Walker and “neo”-Walker, respectively.
Includes strata comprised of kaolinitic claystone and quartzose sandstone, which resemble the Eocene Ione Formation to the north (map unit io; Bartow and McDougall, 1984). Also includes the Bealville fanglomerate, which occurs along Caliente Creek in the southeastern mapped extent, and which Bartow and McDougall (1984) describe as a coarser equivalent to the type Walker Formation, but Saleeby and Saleeby (2019) interpreted as rock avalanche deposits. Thickness varies laterally from 200 to ≥ 900 m (Bartow and McDougall, 1984).
Lowest strata (of “paleo”-Walker) include a 40.10 ± 0.27 Ma reworked pyroclastic fall deposit; a 26.94 ± 0.20 Ma ash bed occurs in upper section (of “paleo”-Walker); uppermost (“neo”-Walker) strata are interbedded with 19.79 ± 0.27 and 19.56 ± 0.30 Ma ash beds; formation is unconformably overlain by the 17.0 ± 0.20 Ma Ilmon Basalt (all ages are Ar-Ar from Saleeby and Saleeby, 2019).
May be regarded as a composite unit which could be subdivided, but extant mapping has not divided it.