Paleomagnetic Record Determined in Cores from Deep Research Wells in the Quaternary Santa Clara Basin, California GEOSPHERE; V
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Research Paper THEMED ISSUE: A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara (“Silicon”) Valley GEOSPHERE Paleomagnetic record determined in cores from deep research wells in the Quaternary Santa Clara basin, California GEOSPHERE; v. 12, no. 1 Edward A. Mankinen and Carl M. Wentworth U.S. Geological Survey, 345 Middlefield Road, MS 937, Menlo Park, California 94025, USA doi:10.1130/GES01217.1 11 figures; 1 table; 1 supplemental file ABSTRACT short for this 210 k.y. part of the Matuyama Chron, during which several times CORRESPONDENCE: emank@ usgs .gov that thickness of section probably should have accumulated. This observation Paleomagnetic study of cores from six deep wells provides an indepen- indicates that a significant unconformity should be present in that short sec- CITATION: Mankinen, E.A., and Wentworth, C.M., dent temporal framework for much of the alluvial stratigraphy of the Qua- tion between the Jaramillo Subchron and the Brunhes-Matuyama boundary. 2016, Paleomagnetic record determined in cores from deep research wells in the Quaternary Santa ternary basin beneath the Santa Clara Valley. This stratigraphy consists of 8 Deeper cores in two wells (GUAD and EVGR) all have normal polarity and Clara basin, California: Geosphere, v. 12, no. 1, upward-fining cycles in the upper 300 m of section and an underlying 150 m or seem to represent much of the Jaramillo Subchron, although no base for that p. 35–57, doi:10.1130/GES01217.1. more of largely fine-grained sediment. The eight cycles have been correlated subchron was found. The resultant minimum rate of sedimentation for this with the marine oxygen isotope record, thus providing one means of dating lower section beneath the unconformity is 170 cm/k.y. Received 18 June 2015 the section. The section has also proved to contain a rich paleomagnetic record The Mono Lake (ca. 32 ka), Pringle Falls (ca. 210 ka), and Big Lost (ca. Revision received 4 November 2015 Accepted 4 December 2015 despite the intermittent sedimentation characteristic of alluvial environments. 565 ka) geomagnetic excursions all seem to be represented in the Santa Clara Published online 7 January 2016 Each well was designed to reach a depth of ~300 m, although 2 were ter- Valley wells. Possible correlations to the Laschamp (ca. 40 ka) and Blake (ca. minated at shallower depth where bedrock was encountered and one (GUAD) 110 ka) excursions are also noted. Three additional excursions that have appar- was deepened to bedrock at 407.2 m. Cores were taken at intermittent inter- ently not been previously reported from western North America occur within vals in most of the wells, composing ~20%–25% of their depths. In GUAD cycle 6 (between 536 and 433 ka), near the base of cycle 5 (after 433 ka), and an attempt was made to core the entire upper 300 m, with core recovery of near the middle of cycle 2 (before ca. 75 ka). 201.8 m (67%). The paleomagnetic framework ranges from the 32 ka Mono Lake ex- cursion near the top of the second sedimentary cycle to below the 780 ka INTRODUCTION Brunhes-Matuyama geomagnetic reversal beneath the eighth cycle. These ages nicely fit those assigned to the section based on correlation with the Stratigraphic and paleomagnetic study of the Quaternary alluvial fill be- marine oxygen isotope record. Several episodes of anomalous magnetic incli- neath Santa Clara Valley (California, USA; Figs. 1 and 2) has been made possi- nations were also found within the cyclic section in some of the wells. Some ble by the drilling and partial coring of six deep wells in a collaborative effort of the episodes of anomalous magnetic inclinations are only separated by between the U.S. Geological Survey and the Santa Clara Valley Water District. short normal intervals in a pattern similar to that described for some well- The immediate purpose of the drilling was for long-term monitoring by the docu mented excursions. We consider that a geomagnetic excursion was likely water district of groundwater levels and chemistry, but geologic study of the only if the anomalous inclinations were found at approximately the same wells and cores was also possible. The total depth of each of the wells was stratigraphic position in more than one drill hole. A deeper time constraint is projected to be ~300 m, although drilling was terminated in 2 wells where bed- provided by the upper boundary (990 ka) of the Jaramillo Normal Polarity Sub- rock was encountered. One well along Guadalupe Creek (GUAD, Fig. 2) was chron recognized at a depth of 302 m in one deeply penetrating well (GUAD). extended deeper in an effort to reach the bedrock reflection evident in a nearby Approximately 100 m of normal Jaramillo section is evident below that in seismic reflection profile (not shown; Williams et al., 2002), and bedrock was wells GUAD and EVGR. reached at a depth of 407.2 m. Sediment cores were taken at intermittent in- The reversal that we identify as the 780 ka Brunhes-Matuyama boundary, tervals throughout 5 of the wells, composing ~20%–25% of the total depth of found at depths of 291–303 m in three wells, indicates an average rate of depo- each, whereas in GUAD most of the upper 300 m was cored with recovery of sition in this upper section of ~37 cm/k.y. In GUAD, the top of the underlying 201.8 m of core (67% recovery). All the section sampled for this study is alluvial, normally polarized section, which we assign to the upper part of the Jaramillo and thus involved intermittent rather than continuous sedimentation, as indi- For permission to copy, contact Copyright Normal Polarity Subchron, was found between 301.8 and 304.5 m. The resul- cated by soils scattered through the cores. Despite the intermittent deposition Permissions, GSA, or [email protected]. tant 10 m of reversed polarity section above the Jaramillo seems anomalously and intermittent coring, which guaranteed incomplete sampling through the © 2016 Geological Society of America GEOSPHERE | Volume 12 | Number 1 Mankinen and Wentworth | Paleomagnetic record, Santa Clara Valley, California Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/1/35/4091906/35.pdf 35 by guest on 03 October 2021 Research Paper 122° 30′ 122° 00′ 121° 30′ 38° 00′ C a l i f o r n i a San Hayward F. Livermore Francisco San Valley Francisco Bay Figure 1. Map showing location of the Santa Clara Valley in the southern San 37° 30′ San Andreas Francisco Bay region, California. Alluvial lowlands (yellow) are distinguished from Santa Area of Diablo Rang bedrock uplands (green). Red dot shows gure 2 location of Dumbarton well; green dot (in- F. Clara set) shows location of Manix well. Principal Valley faults are shown in black (after Wentworth Pacific et al., 2015). F.—fault; Mtns—mountains. Ocean Santa Cruz Mtns Cala e v eras F. 37° 00′ 25 Km Monterey Bay time represented by the section, a very rich paleomagnetic record has been GEOMAGNETIC FRAMEWORK obtained for the Santa Clara basin. The first well drilled under the program (CCOC), begun in September 2000 Geomagnetic Polarity Time Scale and located along Coyote Creek, was described in Hanson et al. (2002), Manki- nen and Wentworth (2003, 2004), and Newhouse et al. (2004). Stratigraphic The pattern of reversals of the Earth’s magnetic field during the past 5 m.y. studies of the basin were reported by Wentworth et al. (2005, 2010, 2015), the is well established (e.g., see Baksi, 1995; Gradstein et al., 2004); consequently, third being a detailed presentation of the physical stratigraphy and subdivision ages of the reversal boundaries are often used to provide accurate time lines of the upper 300 m of section into 8 repetitive sedimentary cycles separated by for geologic correlation. We are concerned here with only the youngest re- unconformities. Here we review the geomagnetic framework and the geologic versals (Fig. 3), because the sedimentary section penetrated by the new wells context of the drill holes and then describe the paleomagnetic results, which postdates earliest Quaternary time (Wentworth et al., 2015). The ages of the provide chronological information and a separate means of correlation among youngest reversal boundaries are very well constrained because lava flows the several wells. erupted during the various transitions have been found and radiometrically GEOSPHERE | Volume 12 | Number 1 Mankinen and Wentworth | Paleomagnetic record, Santa Clara Valley, California Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/1/35/4091906/35.pdf 36 by guest on 03 October 2021 Research Paper –122° –121° 45′ Haywar SF Bay Fl t d 5 Km Diablo Range Ca MOFT Ever Silve laveras Flt gr r een SUNY Creek 37° 22.5′ GUAD Ba EG sin CRIT Figure 2. Map of the Santa Clara Valley Cupe CCOC showing groundwater monitoring wells Fault in their geologic context. Red circles— Basinrt STPK ino EVGR cored wells; green circles—wells not cored. WLLO Ever green seismic reflection line is from Wentworth et al. (2010). Areal geology is from Wentworth et al. (1998), Brabb et al. Santa Cruz Mtns MGCY (1998), and Knudsen et al. (2000). SF—San Francisco; Flt—fault. San 37° 15′ Andreas Flt CN Explanation Holocene Bay mud Fault, dotted where concealed Holocene alluvium Wells new, cored Late Pleistocene alluvium EG pre-existing Early Pleistocene and Pliocene gravels Evergreen seismic reflection profile Pre-Quaternary bedrock dated. The Brunhes-Matuyama boundary has been found in Chile (Brown The major time marker within the latter half of the Matuyama Chron is the et al., 1994), Tahiti (Chauvin et al., 1990), Maui, Hawaii (Baksi et al., 1992; Coe Jaramillo Normal Polarity Subchron.