Paleomagnetic record determined in cores from deep research wells in the Quaternary Santa Clara basin, California

Abstract

Paleomagnetic study of cores from six deep wells provides an independent temporal framework for much of the alluvial stratigraphy of the Quaternary basin beneath the Santa Clara Valley. This stratigraphy consists of 8 upward-fining cycles in the upper 300 m of section and an underlying 150 m or more of largely fine-grained sediment. The eight cycles have been correlated with the marine oxygen isotope record, thus providing one means of dating the section. The section has also proved to contain a rich paleomagnetic record despite the intermittent sedimentation characteristic of alluvial environments. Each well was designed to reach a depth of ~300 m, although 2 were terminated at shallower depth where bedrock was encountered and one (GUAD) was deepened to bedrock at 407.2 m. Cores were taken at intermittent intervals in most of the wells, composing ~20%-25% of their depths. In GUAD an attempt was made to core the entire upper 300 m, with core recovery of 201.8 m (67%). The paleomagnetic framework ranges from the 32 ka Mono Lake excursion near the top of the second sedimentary cycle to below the 780 ka Brunhes-Matuyama geomagnetic reversal beneath the eighth cycle. These ages nicely fit those assigned to the section based on correlation with the marine oxygen isotope record. Several episodes of anomalous magnetic inclinations were also found within the cyclic section in some of the wells. Some of the episodes of anomalous magnetic inclinations are only separated by short normal intervals in a pattern similar to that described for some welldocu mented excursions. We consider that a geomagnetic excursion was likely only if the anomalous inclinations were found at approximately the same stratigraphic position in more than one drill hole. A deeper time constraint is provided by the upper boundary (990 ka) of the Jaramillo Normal Polarity Subchron recognized at a depth of 302 m in one deeply penetrating well (GUAD). Approximately 100 m of normal Jaramillo section is evident below that in wells GUAD and EVGR. The reversal that we identify as the 780 ka Brunhes-Matuyama boundary, found at depths of 291-303 m in three wells, indicates an average rate of deposition in this upper section of ~37 cm/k.y. In GUAD, the top of the underlying normally polarized section, which we assign to the upper part of the Jaramillo Normal Polarity Subchron, was found between 301.8 and 304.5 m. The resultant 10 m of reversed polarity section above the Jaramillo seems anomalously short for this 210 k.y. part of the Matuyama Chron, during which several times that thickness of section probably should have accumulated. This observation indicates that a significant unconformity should be present in that short section between the Jaramillo Subchron and the Brunhes-Matuyama boundary. Deeper cores in two wells (GUAD and EVGR) all have normal polarity and seem to represent much of the Jaramillo Subchron, although no base for that subchron was found. The resultant minimum rate of sedimentation for this lower section beneath the unconformity is 170 cm/k....

An assessment of uncertainties in VS profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

“…2.3.1 Site 1: Guadalupe River, San Jose, CA, USA Site 1 overlaps with the location of drill hole 006S001W26Q001 (longitude − 121.93505°, latitude 37.37770°) (Wentworth et al 2015;Mankine and Wentworth 2016). Wentworth et al (2015) and Mankine and Wentworth (2016) interpreted data from the drill hole and showed that a 407-m-thick strata of Quaternary sediments overlies a bedrock of late-Mesozoic Franciscan Group consisting of metamorphic sandstones.…”

“…2.3.1 Site 1: Guadalupe River, San Jose, CA, USA Site 1 overlaps with the location of drill hole 006S001W26Q001 (longitude − 121.93505°, latitude 37.37770°) (Wentworth et al 2015;Mankine and Wentworth 2016). Wentworth et al (2015) and Mankine and Wentworth (2016) interpreted data from the drill hole and showed that a 407-m-thick strata of Quaternary sediments overlies a bedrock of late-Mesozoic Franciscan Group consisting of metamorphic sandstones. The upper 300 m of sediments shows eight cycles of coarse through fine sediments; a coarse unit of gravels at base of cycle 3 (depth range 103-116 m) serves as a marker separating slower V S above and faster V S below, as seen on the shear-wave borehole log of Fig.…”