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 excursion 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 documented 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

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

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38° 00′

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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

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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

–122° –121° 45′ Haywar SF Bay Fl

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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 Evergreen 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

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. It was the discovery of this subchron that et al., 2004), La Palma, Canary Islands (Quidelleur and Valet, 1996; Valet et al., confirmed the concept of seafloor spreading and helped lead to the modern 1999), and La Guadeloupe, French West Indies (Carlut et al., 2000). Several theory of plate tectonics (Glen, 1982). Although the approximate duration of of these transitional flows have been dated using the40 Ar/39Ar method (Baksi the Jaramillo has been well known for many years, various attempts have et al., 1992; Singer and Pringle, 1996; Singer et al., 2002, 2005; Coe et al., 2004). been made to accurately date its boundaries. The 40Ar/39Ar dating of transi- The unspiked K-Ar method (Cassignol and Gillot, 1982) was used to date the tional lava flows from Tahiti (Singer et al., 1999) indicates that the lower and La Guadeloupe lava flows (Carlut et al., 2000). Although some of the ages upper boundaries of this subchron are 1050 and 990 ka, respectively. The group near 795 ka, Singer et al. (2002, 2005) argued that these represent ini- other reversal within the latter half of the Matuyama is the Cobb Mountain tial instability of the geodynamo and that the actual polarity reversal occurred Normal Polarity Subchron. Although it was of very brief duration (~10–25 k.y.; ~18 k.y. later at 780 ka, generally considered the best age for this boundary. Mankinen et al., 1978; Clement, 1992), this subchron represents a full reversal The Matuyama Reversed Polarity Chron lasted from 2581 to 780 ka. of the geomagnetic field that has been recorded worldwide both on land and

22 2 VADM (10 Am ) in deep-sea sediments. Re-dating of a rhyolite flow from the type locality by 0 612 40 39 0 the Ar/ Ar method (Turrin et al., 1994) yielded an age of 1186 ± 6 ka for the Mono Lake (32 ka) Cobb Mountain Subchron. Laschamp (41 ka)

Blake 2 (100 ka) Blake 1 (120 ka) Geomagnetic Excursions

Pringle Falls 2 (190 ka) In contrast to geomagnetic reversal boundaries, the use of geomagnetic Pringle Falls 1 (211 ka) excursions for stratigraphic correlation and dating is much more problematic. Excursions are wide departures from the normal geomagnetic field, typically defined as having occurred when a virtual geomagnetic pole (VGP) departs more than 45° from its time-averaged direction at any given locality. They are generally brief, ranging from ~500 yr to perhaps 3–5 k.y. (Gubbins, 1999), and = normal polarity may record a complete polarity reversal, but more often do not. Excursions = reversed polarity may represent extreme variation of normal paleosecular variation, or possibly represent aborted or failed reversals (e.g., Valet et al., 2008). Although unusual Age (ka) = excursions magnetization directions have been found in paleomagnetic records from many areas, some have proven to be due to physical disturbances within sedi- 500 mentary sequences, chemical alteration, remagnetization effects, or other non- geomagnetic processes. Even many probable excursions occur in sedimentary Big Lost (565 ka) sequences where their age cannot be determined directly by any of the absolute dating methods and thus must be estimated indirectly. A single geomagnetic excursion can appear to occur at different stratigraphic levels in separate geologic sequences because of the presence of hiatuses, variable sedimentation rates, or structural complications. It is easy to understand how the number of suspected excursions can proliferate; ~20–30 have been proposed within the Brunhes Epoch, despite the fact that many sedimentary sequences were BRUNHES (780 ka) MATUYAMA deposited too slowly to record any excursions (Roberts and Winklhofer, 2004). For a thorough review of the difficulties in using geomagnetic excursions for geological correlation, see Bol’shakov (2007). Most of the excursions reported worldwide probably will prove to have Kula/Santa Rosa (900 ka) occurred during periods of very low dipole intensity, which typically lasted some tens of thousands of years (e.g., see Guyodo and Valet, 1999). When the strength of the dipole field is weak, non-dipole fields will predominate and (990 ka) 1000 Jaramillo unusual field directions can be expected, albeit generally on a regional rather (1050 ka) than global scale, depending on proximity of the site to a strong non-dipole source. Thus, one should not expect anomalous field directions of relatively brief duration (hundreds of years) in different regions to be entirely synchro- nous or to have the same morphology. As a result, any similarities in geomagnetic behavior over large distances are likely to be coincidental, and attempts Cobb Mtn (1190 ka) at long-range correlation of such excursions should be avoided. Merrill and McFadden (2005) cautioned, based on their analysis of spherical harmonics Figure 3. Part of the late Cenozoic geomagnetic polarity time scale modified to include those of the geomagnetic field, that correlation of excursions be limited to angular geomagnetic excursions considered most likely to be found in the San Francisco Bay region. distances of <30°. Unusual field directions may also occur repeatedly during Ages are from various sources (see text). Column to the left of the polarity scale is a plot of these weak dipole intervals, as non-dipole features wax and wane or drift relative magnetic paleointensity adapted from Valet et al. (2005). Dashed vertical lines on the relative paleointensity curve are the mean values for the Brunhes and the preceding 500 k.y. geographically (Bullard et al., 1950; Yukutake and Tachinaka, 1968; Merrill and (Valet et al., 2005). VADM—virtual axial dipole moment. McElhinny, 1983). Lund et al. (1988), for example, reported four recurrences of

an excursional waveform, each with diminishing amplitudes, apparently ini- led the authors (Denham and Cox, 1971; Denham, 1974) to conclude that the tiated by the Mono Lake excursion ca. 32 ka (calendar years before present). Laschamp had not been recorded at Mono Lake. Extensive sampling of the Negrini et al. (1994) similarly described a repeating waveform near an earlier Wilson Creek Formation of Lajoie (1968) at four additional sites along Wilson excursion. Creek, northwest of Mono Lake, by Liddicoat and Coe (1979) provided more details about the Mono Lake excursion and the period of time immediately preceding it. Liddicoat and Coe (1979) found that the counterclockwise rotation Possible Excursions in the San Francisco Bay Region of the excursion reported by Denham and Cox (1971) was preceded by a larger, clockwise rotation of direction. Other records of what is considered to be the Because of the regional aspect of geomagnetic excursions, the total number Mono Lake excursion have been reported from southern Oregon (Negrini of excursions recorded in any given area is probably limited. Accordingly, in et al., 2000, 2014), Nevada (Liddicoat, 1992), Ocean Drilling Program (ODP) Site Mankinen and Wentworth (2003) discussion was restricted to those excursions 919 from the Irminger Basin (Channell, 2006), and perhaps the island of Hawaii that have been reported from the western North America–eastern Pacific re- (Holt et al., 1996). gion under the assumption that they are the most likely to have been recorded Dating of tephra in the Summer Lake basin, Oregon (Negrini et al., 2000) by sediment of the Santa Clara basin. Thus the Mono Lake, Laschamp, Blake, yielded an age of ca. 29,500–27,300 yr (radiocarbon years) for the Mono Pringle Falls, and Big Lost were considered to be the most likely excursions to Lake excursion. The age of Ash #15 (midpoint of the Mono Lake excursion) have been recorded near San Francisco Bay during the Brunhes Chron. in the Pyramid Lake basin, Nevada (Benson et al., 2003), was determined to be 28,620 ± 300 14C yr B.P. Organic material between 2 intermediate polarity paleomagnetic samples in core 17 from the CCOC drill hole, Santa Clara Valley, Laschamp Excursion California (Mankinen and Wentworth, 2004) yielded an age of 28,090 ± 330 14C yr B.P. (32.8 ± 0.3 ka; calibrated according to Bard, 1998). The Laschamp excursion was one of the earliest found and best docu There have been suggestions that only a single excursion occurred ca. mented, in part because it was recorded in volcanic rocks. Two reversed 40 ka in the Mono Lake basin (e.g., Kent et al., 2002; Guillou et al., 2004; Zim- polarity lava flows were discovered near Laschamp and Olby in the Chaîne merman et al., 2004; Cox et al., 2012; Vazquez and Lidzbarski, 2012). These sug- des Puys volcanic province (France) by Bonhommet and Babkine (1967). Re- gestions are based on new radiocarbon dates, ages determined by the 40Ar/39Ar vised ages obtained using thermoluminescence and 14C methods on sediment and 238U-230Th methods, and interpretation of a paleointensity correlation to a baked by the Laschamp lava flows (Huxtable et al., 1978; Gillot et al., 1979), and global reference curve. Several problems with attributing this excursion to the K-Ar, 39Ar/40Ar, and 230Th/238U methods on the flows (Condomines, 1978; Hall Laschamp have been pointed out (see the discussion by Negrini et al., 2014). and York, 1978; Gillot et al., 1979) indicated that the age of this excursion was Despite the ongoing controversy concerning the identification of the excur- probably in the range 45–40 ka. Excursions occurring at about this time have sion recorded at Wilson Creek, the preponderance of evidence is that the Mono since been reported from many localities worldwide, in on-land sequences and Lake and Laschamp are separate excursions within this general time interval, deep-sea sediments. Extensive new paleomagnetic sampling and age determi- with Mono Lake being ~10 k.y. younger. Two excursions in this time frame were nations from the Chaîne des Puys provide a revised age of 41.3 ± 0.6 ka for the recorded from the Southern Hemisphere in the Auckland volcanic field of New Laschamp excursion (Laj et al., 2014). Zealand (Cassata et al., 2008). Eight basaltic lava flows with anomalous directions have yielded 40Ar/39Ar ages of 39.1 ± 4.1 and 31.6 ± 1.8 ka. It is important that these excursions all seem to have occurred during the same general inter- Mono Lake Excursion val during which dipole intensities were ~40% below average (Mankinen and Champion, 1993; Laj and Kissel, 1999; Teanby et al., 2002; Laj et al., 2002). Following the discovery of the Laschamp excursion, which was initially Although the entire interval containing these two excursions was charac- estimated to have occurred between 20 and 8 k.y. ago (Bonhommet and terized by below normal geomagnetic intensities, two very pronounced drops Zähringer, 1969), Denham and Cox (1971) undertook a paleomagnetic study of in paleointensity were recorded by lava flows in the Chaîne des Puys (Laj et al., the Wilson Creek Formation of Lajoie (1968) near Mono Lake, California (esti 2014). The first yields an age of 41.3 ± 0.6 ka (the Laschamp excursion) and a mated age 30,400–13,300 yr) to search for additional evidence of this excur- second occurs in two flows yielding an age of 34.25 ± 1.2 ka. The older is a sion. Their study showed that a large, rapid, and counterclockwise excursion pooled age based on unspiked K/Ar and 40Ar/39Ar ages. The younger is pre- of paleomagnetic directions occurred at an estimated 24,600 yr ago (Denham cisely determined from the GLOPIS-75 global paleointensity stack (Laj et al., and Cox, 1971; Denham, 1974). Although this anomalous field behavior oc- 2004) and cosmogenic records. Although these latter two flows do not exhibit curred in the same general time frame as that reported for the Laschamp ex- anomalous directions, the paleointensities indicated are much too low for typi- cursion, the fact that a full reversal in direction was not recorded apparently cal secular variation and likely also record the Mono Lake excursion.

Blake Excursion Herrero-Bervera et al. (1994) later estimated that the excursion must have occurred between ca. 218 and 171 ka based on 40Ar/39Ar ages on a tephra layer Paleomagnetic results from deep-sea cores from the Greater Antilles Outer exposed at a second locality near Pringle Falls and tephra from a correlated Ridge indicated a reversed polarity interval within abyssal brown clay depos- sequence. Using K-Ar, Ar-Ar, and thermoluminescence ages, along with cor- ited during part of the last interglacial period (Smith and Foster, 1969; Denham, relation of tephra layers from various sites within Pleistocene pluvial Lake 1976; Denham et al., 1977). This excursion has been reported as occurring in Chewaucan, Negrini et al. (1994) considered the age of this excursion to most numerous deep-sea sediment cores from different oceans, in loess deposits likely be in the interval from ca. 190 to 180 ka. Several lines of evidence point in China and Germany, and has been tentatively identified in a few volcanic toward deposition of sediment containing both the Pringle Falls and the cor- rocks. Sedimentary sequences recording this excursion typically show two relative Summer Lake II excursion (Negrini et al., 1994) as having occurred reversed polarity intervals separated by a short normal interval. Its existence during oxygen isotope stage 6, compatible with the age ranges reported by has been generally accepted by many, even though its precise age remained Herrero-Bervera et al. (1994) and Negrini et al. (1994). An excursion found elusive. Estimates of its age have been made using biostratigraphy, correlation only in marine and lake sediment cores, the Iceland Basin excursion (Channell with oxygen isotope curves, and with thermoluminescence, fission-track, and et al., 1997), has been found near the marine isotope stage 6-7 boundary and K-Ar dating methods. These estimates generally range from 100 ka to as old as is a likely correlative. Eight North Atlantic sites with age control from oxygen 160 ka for some of the volcanic rocks. It is far from certain, however, that all of isotope data yield an age of 190.2 ± 1.8 ka (Channell, 2014) for this excursion. the supposed correlations to the Blake episode are correct. If sediment at the Cinder cones in the Albuquerque-Belen Basin that record a short excur- original locality is representative of marine oxygen isotope stage 5, as seems sion (Geissman et al., 1990) have yielded whole-rock K-Ar ages of 155 ± 47 ka, likely, the excursion probably can be no older than ca. 130 ka (see summary and a 238U-230Th age of 156 ± 29 ka (Peate et al., 1996). Uncertainties in these by Morrison, 1991). ages made it possible that these rocks could be recording either the Blake or Intermediate polarity directions found in tuffs and lava flows of the Reyk- Pringle Falls excursions. Basalt from the Albuquerque volcanoes and Ash D janes Peninsula, Iceland, by Kristjansson and Gudmundsson (1980), referred from Pringle Falls were redated using the 40Ar/39Ar method (Singer et al., 2008). to as the Skálamaelifell excursion, were initially correlated with the Laschamp Results now indicate that excursions at both localities are the same, yielding a excursion (Levi et al., 1990). However, new 40Ar/39Ar incremental heating experi weighted mean age of 211 ± 13 ka. Thus there seems to be strong evidence for ments on an additional 4 transitional polarity samples from the area yield an two excursions occurring in the same general time interval. As with the Blake age of 91 ± 13 ka (Jicha et al., 2011). Combining these ages with 238U/230Th ages excursions, we refer to these two excursions as Pringle Falls 1 and 2 in Figure 3. from 2 of the lava flows (Jicha et al., 2011), yields a weighted mean age of 94.1 ± 7.8 ka for this excursion. Jicha et al. (2011) considered this excursion to be the Big Lost Excursion younger of 2 excursions in the 125–90 ka interval of low geomagnetic field intensity. Results were obtained from 40Ar/39Ar incremental heating and unspiked Shallow negative (reversed) inclinations were recorded by 2 of 13 lava K-Ar experiments (Singer et al., 2014) on intermediate polarity lava flows from flows sampled in a drill hole from the Snake River Plain, Idaho, by Champion Lipari and Amsterdam Islands, and from New Mexico, USA (the Laguna flow et al. (1981); they determined a whole-rock K-Ar age of 465 ± 50 ka for this of Champion et al., 1988). The Amsterdam Island lava flow yields an age of episode. This reversed episode was found in a second drill hole (Champion 120 ± 12 ka, whereas the Lipari Island and New Mexico flows are 105 ± 1 and et al., 1988), and additional K-Ar determinations revised its age to 565 ± 14 ka. 104 ± 12 ka, respectively. Thus the two Blake excursions seem reasonably well Intermediate polarity magnetization directions also were found at two sites determined at 120 and 100 ka. We refer to these two excursions in Figure 2 as within the informally designated basaltic andesite of Hootman Ranch (Lan- Blake 1 and 2, respectively, whereas Jicha et al. (2011) and Singer et al. (2014) phere et al., 1999) near Mount Lassen, northern California. Based on a 40Ar/39Ar referred to the younger as the post-Blake. Although the Blake excursions seem plateau age of 565 ± 12 ka and an isochron age of 576 ± 12 ka for this flow, well represented globally, they have not been conclusively documented in the along with the age of the underlying Rockland tephra (weighted mean plateau western United States other than at the one locality in New Mexico. age 609 ± 7 ka), Lanphere et al. (2004) considered the andesite of Hootman Ranch to be another record of the Big Lost excursion. Pringle Falls Excursion Late Matuyama Excursion Paleomagnetic records from a sedimentary sequence near Pringle Falls, Oregon, revealed a magnetic excursion that Herrero-Bervera et al. (1989) Within the late Matuyama Chron, intermediate polarity directions from a initially correlated with the Blake excursion. This correlation was based on rhyolite dome (Cerro Santa Rosa I) in the Valles caldera, New Mexico, along the presence of two reversed intervals separated by a short normal interval with a K-Ar age of ca. 900 ka led Doell and Dalrymple (1966) to propose that similar to the pattern described for the Greater Antilles Outer Ridge cores. the rhyolite represented the termination of the Jaramillo Normal Polarity

Subchron. With the revised ages (1050–990 ka) for this subchron provided by no certainty as to which of these excursions, if any, the Maui and New Mexico Singer et al. (1999), it is apparent that Cerro Santa Rosa I episode is distinctly excursions can be correlated. To avoid perpetuating a dubious correlation, we younger than the Jaramillo and represents an excursion during the latter part propose that the name Kamikatsura be restricted to the southwest Pacific re- of the Matuyama Reversed Polarity Chron (Singer and Brown, 2002). The cur- gion. Because the excursion on Maui was recorded by lava flows of the Kula rently accepted age for this excursion, determined by the 40Ar/39Ar method Formation (MacDonald, 1978), we informally refer to the excursions seen on (Singer and Brown, 2002), is 936 ± 8 ka. Singer et al. (1999) also concluded that Maui and in New Mexico as the Kula–Santa Rosa excursion, in order to keep the Santa Rosa excursion is separate from and ~37 k.y. older than an excursion the designation regional in scope and to allow a distinction between the two in reported from Haleakala caldera on Maui, Hawaii. Because Cerro Santa Rosa I the event that multiple excursions occurred at nearly the same time. is an intrusion, however, there is no stratigraphic evidence from this locality to show that two separate excursions occurred in the same general time interval. Tentative Excursions A thorough sampling of the Haleakala sequence (Maui; Coe et al., 2004) revealed 16 intermediate polarity lava flows that were bracketed by reversed We emphasize the possibility that additional, as yet undetected anoma- polarity units, confirming an excursion in the late Matuyama Chron prior to lous magnetic field behavior may have been recorded by rocks of the western the Brunhes-Matuyama boundary. A mean 40Ar/39Ar age of 900.4 ± 4.7 ka was United States, although we cannot predict with any degree of certainty where determined for this excursion, which Coe et al. (2004) referred to as the Kami- new anomalous directions are likely to be found. One possibility for an addi- katsura event. Only one excursion was recorded in this sequence even though tional excursion is a reversed polarity lava flow from the Snake River Plain (site the sequence clearly overlaps the time interval when Cerro Santa Rosa I was sr40; Tauxe et al., 2004) that yielded a 40Ar/39Ar age of 0.39 ± 0.04 Ma. Because emplaced, as evidenced by ages ranging from 961 to 915 ka (Coe et al., 2004) of the low to very low paleointensities occurring at about this same time (e.g., for the reversed polarity lava flows preceding the excursion. Although there see the VADM curve shown in Fig. 3), Tauxe et al. (2004) suggested the pos- is convincing evidence for anomalous geomagnetic field behavior, currently sibility that site sr40 may have recorded a previously unreported excursion. available data are not able to distinguish whether a single excursion ca. 900 ka Evidence for three excursions in the age range of our interest (Fig. 3) was re- is represented, or if 2 occurred very close in time, in a fashion similar to the ported by Michalk et al. (2013) from the trans-Mexican volcanic belt. These may Mono Lake and Laschamp excursions. Preliminary data from the Cascade correspond to the Big Lost, the Kula–Santa Rosa, and a previously unreported Range (Lanphere et al., 1997), if confirmed, may indicate that this excursion excursion from western North America ca. 670 ka. However, errors for all ages was recorded by lava flows at Mounts Baker and Hood. (reported at 1s) are large, and those excursions are considered tentative. It is unfortunate that the best documented excursion in this interval (Coe et al., 2004) was called the Kamikatsura, because that name is based on a tenta tive correlation to a single normal polarity lava flow that underlies an undated GEOLOGIC SUMMARY reversed polarity flow in southwest Japan (Hirooka and Kawai, 1967). The recalculated K-Ar age (Mankinen and Dalrymple, 1979) for that normal polarity The Quaternary section beneath the Santa Clara Valley accumulated in a lava is 0.85 ± 0.03 Ma. This age was later correlated with anomalous directions subsiding basin (the Santa Clara basin) as alluvial deposits supplied primarily recorded by sediments of the Osaka Group, which are associated with the from the Santa Cruz Mountains to the west (Fig. 1). This section was described Kamikat sura Tuff and occur between the Jaramillo and the Brunhes-Matuyama in detail in Wentworth et al. (2015). The basin is between the San Andreas and boundary (Maenaka, 1983). Further study of the Osaka Group (Takatsugi and Hayward-Calaveras faults, and its western and eastern margins are marked by Hyodo, 1995) shows a second, younger excursion overlying the Azuki Tuff, re- reverse and thrust faults. All sediment sampled by the 6 new drill holes is allu- ferred to as the Takatsuki excursion. Neither the Takatsuki nor the Kamikat- vial (Wentworth et al., 2015), as is the section sampled by a 305 m well (SUNY) sura excursions have been directly dated; instead, their ages are estimated by drilled in the 1960s (Meade, 1967). Estuarine deposits are present farther north extrapolation of sedimentation rates for Holocene marine clay as determined around and beneath San Francisco Bay. The alluvial section (Fig. 4) consists of by radiocarbon dating. Takatsugi and Hyodo (1995) estimated that the Takat- an upper cyclic sequence ~300 m thick and a lower, finer grained sequence as suki excursion occurred ca. 0.85 Ma and that the Kamikatsura must be a few thick as 150 m or more, all overlying an unconformity on bedrock having more tens of thousands of years older, perhaps ca. 0.87 Ma; they also suggested than 365 m of relief. That unconformity is cut across bedrock of the Meso that multiple excursions or regional reversals may have occurred during the zoic Franciscan Complex and Coast Range Ophiolite beneath the center of the period 0.87–0.84 Ma. Only a single excursion at about that time was recorded basin and Miocene fill of the buried Cupertino basin to the west; east of the in marine sediment collected from the Philippine Sea (Horng et al., 2002) that Silver Creek fault it is underlain by late Cenozoic fill of the buried Evergreen was dated by astronomically tuning the oxygen isotope record from that pis- basin. No age-diagnostic fossils have been found in the section that might help ton core, with a resulting age of 925–920 ka. It is clear that there were 2, and in its subdivision, and no radiometric ages are available beyond several 14C perhaps 3, excursions ca. 900 ka recorded in the southwest Pacific. There is dates in the shallow section.

Cycle Age, ka CCOC (Mankinen and Wentworth, 2003) was used to help constrain that 0 correlation. 1 18 2 In the deeper parts of the basin the cyclic section is underlain by a largely fine-grained unit (Fig. 4) that reaches a thickness >150 m near the Silver Creek 140 3 fault. This section is known from four deep wells that range southeast across 252 the basin from MOFT, near San Francisco Bay, through GUAD and an unusu- 100 4 ally deep water well (CRIT) to EVGR beyond the Silver Creek fault (Fig. 2). It 341 is separated from the overlying cyclic section by a prominent unconformity 5 Figure 4. Stratigraphic diagram described in Wentworth et al. (2010) (see following discussion of the mid- showing subdivision of the 433 Quaternary unconformity). That unconformity marks a period of uplift and ero- Quaternary sedimentary fill 6 of the Santa Clara basin (after sion that separates the upper and lower sedimentary sections into two sepa 200 536 Wentworth et al., 2015). Ages rate basin-forming episodes. 7 in thousands of years (ka) The entire Quaternary section in the basin postdates the Pliocene to early Upper cyclic sequence except where noted. Ages of 630 cycle boundaries are from cor- Pleistocene gravels that are exposed at the margins of the basin (Fig. 2); the 8 relation with the marine oxy distinctive clast lithologies of those gravels are not present in any of the new Quaternary 718 gen isotope record (Wentworth wells, three of which (MGCY, WLLO, and GUAD) reach the underlying bedrock 300 950 and Tinsley, 2005); estimates of ages of the mid-Quaternary (Andersen et al., 2005; Wentworth et al., 2015). The sedimentary layering of Thickness, in meters and basal unconformities are this section in the interior of the basin is undeformed, except in a negative from Wentworth et al. (2015). flower structure along the Silver Creek fault (Fig. 2; Wentworth et al., 2010). Bedrock includes early Pleisto- cene deposits in the Evergreen At well EVGR, the lower part of the cyclic section is thinned, apparently due 1.5 Ma basin. to gradual uplift above a presumed underlying thrust (see description of the 400 Evergreen seismic reflection line [EG of Fig. 2] in Williams et al., 2006).

Lower fine-grained unit As an accumulation of alluvial sediment, the Santa Clara section contains numerous breaks in sedimentation, both at the unconformable base of each

y of the sedimentary cycles and scattered throughout the section, as indicated Bedrock by numerous relict soils evident in the well cores (Wentworth et al., 2015). The formation of each of those soils required exposure of the ground surface for periods of tens to thousands of years. Thus the sedimentary section is rife with mostly pre- Quaternar hiatuses of varying duration, and was sampled by cores scattered through the section that amounted to ~20% of the depth of 5 of the new wells and 66% of the upper 305 m of GUAD. The upper, cyclic section contains eight upward-fining cycles of inter The time significance of the different parts of the sedimentary cycles varies. layered gravel, sand, silt, and clay that, because of the upward fining, can each The cycle boundaries have been assigned specific ages. The duration of the be subdivided into a fine top and a coarse bottom (Fig. 5; Wentworth et al., cycle boundary unconformities is probably relatively short, based on carbon 2015). The cycles and their fine and coarse subdivisions are laterally contin- ages near the base of cycle 1 (Fig. 5) and the lack of well-developed soils be- uous and extend throughout the basin, and thus can be used in correlating neath those boundaries (Wentworth et al., 2015). between wells. In Wentworth and Tinsley (2005), a climate-driven process to Wentworth et al. (2015) subdivided the cycles into laterally continuous account for the cyclicity was proposed. An anomalous departure from this coarse bottoms and fine tops, with the bottoms consisting of one to several generally fining-upward pattern within the cycles occurs in the top of cycle 2, coarse layers that persist across the basin, above which other less continu- where an upper coarse interval (C2a) within the cycle can be mapped around ous coarse layers come and go. The resistivity curves of Figure 5 indicate this the basin (Wentworth et al., 2015). coarse-fine variation in the section, with the peaks representing coarse layers The bases of the eight sedimentary cycles are unconformities that have and the troughs representing fine layers (the coarse-fine textural boundary been correlated with sea-level lowstands indicated by the marine oxygen is placed at fine sand). For each cycle, the top and bottom boundaries are isotope record (Wentworth and Tinsley, 2005; Wentworth et al., 2015), which of essentially uniform age and limited duration, and according to the model yields estimates of the ages of the unconformities that range back to 718 ka of Wentworth and Tinsley (2005), the laterally persistent basal coarse layers in stage 18 at the base of cycle 8 (see Figs. 4 and 5). The early discovery of represent relatively brief periods of time, also of essentially uniform age. The the Brunhes-Matuyama boundary just beneath the base of cycle 8 in well interiors of the cycles are probably less regular, marked by various pulses of

Cycle GUAD CCOC STPK age, ka MGCY WLLO EVGR 0 1 14.2 ka 18 12.4 ka 58 ka 32.8 ka 2 135 50 3

252

100 4 341

150

433 6

200

Depth, in meters 536

250 Miocene serpentinite sandstone 630

8 718 300

Explanation Example Well 350 Column cycle resistivity log 12.4 ka boundary C2a - Coarse 14C age, Cal yrs fine top top of cycle 2

400 coarseness curve coarse base Franciscan

Figure 5. Stratigraphic details of wells with cores in the Santa Clara Valley. Bases of sedimentary cycles are marked by green lines, with cycle numbers (black) and boundary ages in thousands of years (green) in central column; coarse basal intervals are in orange and coarse top of cycle 2 (C2a) is in green; resistivity logs are in black (16 inch normal, scale 0–75 ohm-m), and coarseness curves are in red (scale 0%–0100%). Carbon ages are recalculated according to Bard (1998). Modified from Figures 11, 14, and 15 of Wentworth et al. (2015).

rapid supply scattered both in time and space, separated by hiatuses of differ- A total of 515 cylindrical samples, ~18 cm3 in volume, and 90 additional ing duration, and all superimposed on a generally declining rate of accumu- 7 cm3 samples were taken for paleomagnetic study from the split cores. Paleo- lation, which would deny simple linear interpolation of ages between cycle magnetic sampling was generally confined to the finest grained portions of the boundaries. We can thus make fairly confident inter-well correlations between core, ranging to fine-grained sand and silty sand. The cores were oriented only similar positions near the cycle boundaries and within the basal coarse layers, with respect to stratigraphic top, thus permitting magnetic inclination to be but must be more cautious for the cycle interiors. Correlation within a cycle determined, but not declination (azimuth). Samples were taken near the center between positions high in one well and low in another should be avoided; of the split core to avoid any deformation along the core margins that could however, because the age of the boundary within a cycle between its coarse deflect the magnetic inclination. Noting where biscuits occurred also was im- bottom and fine top may differ from well to well and coarse layers in one well portant because it determined in which cores relative differences in direction may grade laterally to fine layers in an adjacent well, the situation near the between samples could not be used to interpret geomagnetic behavior. In the interior coarse-fine boundary is less clear. That boundary is a guide, but cannot absence of biscuits, relative directional variations can be determined because be taken as a strict time boundary, and thus correlations that cross it or are all samples were taken from the same face of the split core. Deformation at bis- otherwise not quite parallel to it could be reasonable. cuit boundaries or similar zones could also destroy part of the magnetic signal or otherwise significantly affect the paleomagnetic inclinations. Natural remanent magnetization (NRM) of each sample was measured METHODS using a super-conducting magnetometer housed in a magnetically shielded room. Progressive alternating-field (AF) demagnetization experiments on the Cores from the wells were obtained with a Christensen 94 mm wireline large 18 cm3 specimens were performed using a modified three-axis tumbling core barrel attached behind a donut-shaped drill bit within a clockwise-rotating demagnetizer (Doell and Cox, 1967). Doell and Cox recognized that this type of drill stem (looking down the hole). As the drill bit advanced downward, the demagnetizer could impart a spurious component of magnetization along its 6.2-cm-diameter core was pushed up into a rigid plastic tube (liner) within the innermost rotation axis. This rotational remanent magnetization (RRM; Wil- core barrel. Upon wireline recovery, this liner was cut to fit the actual length of son and Lomax, 1972) is particularly prevalent in sediments with low magnetic the core and capped at each end. Recovered lengths ranged from quite short stability. To eliminate the effects of RRM, samples were demagnetized twice to rarely the full 152 cm of a typical coring advance (push). Core axes were at each increment of AF above the point where unsystematic behavior was vertical and the shallow end of each core was marked at the time of recovery. first suspected (generally 20 mT or higher). For the second of these demagne- Compass orientation could not be determined. The core and cylindrical plastic tization pairs, the long axis of the cylindrical sample was reversed 180° with liner were kept undisturbed until they were split longitudinally in the labora- respect to the innermost tumbler axis and the data were averaged using the tory with a mechanical knife and then a stiff wire. One half of the core was method of Hillhouse (1977a). The smaller, 7 cm3 specimens were AF demagne- described and sampled, and the other half reserved as an archive. Cores were tized sequentially (i.e., not inverted after each demagnetization step) using an numbered sequentially downward and their position in the hole recorded as in-line nested Helmholtz coil set mounted above the superconducting magne- depth to the top of the core (in feet), with the position of a sample within the tometer (Kirschvink et al., 2008). core recorded as depth within the core (in centimeters). The cores were typically intact and internally undisturbed, but downward drag associated with insertion of the core into the liner during the push oc- DATA AND ANALYSIS curred along the perimeter in some cores. Other, more extreme disturbances involving injection of drilling mud and other processes were clearly evident in The geometric mean NRM intensity for 375 large 18 cm3 samples from places. Some fine-grained intervals in the cores contain thin horizontal lam- wells CCOC, GUAD, MGCY, STPK, and WLLO (Fig. 2) is 18.4 mA/m (5.8–58.3 ina dragged downward at the margins that separate and define apparently mA/m = range of 1 standard deviation). Samples taken below ~225 m in the intact blocks of sediment typically 1–2 cm thick. Some of these laminae can WLLO well were not used in the calculation of this mean because of the pres- be shown to truncate primary features, and thus were imposed after depo- ence of abundant coarse (and strongly magnetic) serpentine clasts (Oze et al., sition. Structures like this were routinely recognized and called “biscuits” in 2003). Similarly, a sedimentary source area for the EVGR well that differs from the Deep Sea Drilling Program (e.g., see Shipboard Scientific Party, 1995), and that for most of the drill holes in the center of the valley results in a somewhat are observed there only in cores taken during rotary drilling. Although the higher (28.7 mA/m) mean NRM intensity. The Great Valley sequence on the core barrel rides on bearings to isolate it from the rotating drill stem, some east side of the Santa Clara Valley is the principal source for the Evergreen torque apparently can still be applied. Care was taken during paleomagnetic section, whereas the southeastern Santa Cruz Mountains on the west side sampling to avoid all recognizable core deformation, including the interfaces of the valley are the primary source for most of the basin (Wentworth et al., between biscuits. 2015). Progressive AF demagnetization of representative samples from cores

throughout the basin revealed similar response to the experimental treatment excursion. To infer that an excursion may have occurred, we first look for for all sampled grain sizes (Fig. 6). Anomalous components of magnetization multiple anomalous inclinations, often with large serially correlated swings were rare and, where present, were generally removed by AF of 10 mT or lower. in relative direction within a core where no evidence of internal deformation Based on behavior during stepwise demagnetization, all remaining specimens is apparent. More problematic are intervals that show large, systematic direc- were demagnetized at a minimum of three AF values (5, 10, 15 mT) to confirm tional changes without associated anomalous inclinations. In any event, only if their stability and direction. The median destructive field, the strength of the the anomalous inclinations and/or directions were found at approximately the AF required to reduce the remanent intensity to half of its initial value, is typ- same stratigraphic interval in more than one drill hole did we consider that a ically between 10 and 30 mT in these samples. These values are consistent geomagnetic excursion was likely. with titanomagnetite being the magnetic carrier in these sediments. Repre- sentative magnetic inclinations for each sample were determined by fitting least squares lines (Kirschvink, 1980) to three or more vector endpoints of the INTERPRETATION magnetic component isolated during demagnetization, and these are shown in Figure 7 and listed in Supplemental Tables 1–6 in the Supplemental File1. Of A summary of the results from the six drill holes is given in Figure 9; nor- the 605 samples obtained, only one proved to be magnetically unstable. Maxi mal magnetic polarities are shown in blue, reversed polarities in red, and ex-

Supplemental Table 1. Paleomagnetic data from the Coyote Creek Outdoor Classroom drill hole mum angular deviations (Kirschvink, 1980) are generally <5° and indicative cursions in yellow. Numbers to the right of each excursion are the numbers for

Core Sample Depth InclinationPolarity IM.A.D.Intensity of well-defined inclinations. Where larger deviations occurred, many of the the cores in which the excursions occur. The purple shading in the individual (meters) (degrees)(degrees) (degrees)(mA/m) samples were from intervals of anomalous inclination and thus consistent with drill-hole logs are indicative of intervals composed of gravel or other material 2 0E4024.03 53.7N-3.0 0.89.05 4 0E4037.14 48.8N-7.9 2.528.6 5 0E4048.70 50.0N-6.7 0.924.1 having been magnetized during weak geomagnetic fields. too coarse for paleomagnetic sampling. The upper part of the Quaternary sec- 6 0E4059.62 56.6N-0.1 1.191.3 7 0E406 11.06 55.5N-1.2 1.730.2 Absolute values of measured magnetic inclination were compared with the tion has been subdivided into eight upward-fining alluvial sequences that can 8 0E407 13.08 60.6N3.91.1 17.8 15 0E408 23.17 60.8N4.11.6 20.7 inclination that would be produced at the sampling site (56.7°) by a geocen- be correlated with the major climate-driven oscillations evident in the marine 16 0E409 24.53 46.3N-10.43.2 14.2 17 0E431 25.21 19.9I-36.86.6 22.7 tric axial dipole (GAD). The GAD model is a fundamental concept in paleo- oxygen isotope record (Wentworth and Tinsley, 2005; Wentworth et al., 2010, 17 0E410 25.24 12.9I-43.84.6 102 17 0E432 25.32 38.4N-18.31.7 58.2 magnetism wherein the best approximation of the time-averaged magnetic 2015). If each of the eight cycles represents sediment accumulated during a 18 0E433 26.39 54.9N-1.8 1.617.5 18 0E411 26.64 61.1N4.41.7 9.53 19 0E412 28.65 46.4N-10.31.2 14.0 field is one that would be produced by a single magnetic dipole at the center particular glacial cycle, then each cycle represents a distinct interval of time. 20 0E413 30.04 59.3N2.62.5 20.0 21 2E555 31.54 52.1N-4.6 2.89.11 of the Earth and aligned with the rotational axis. Results of the comparisons No age-diagnostic fossils have been recovered from these drill holes, but the 21 0E472 31.62 47.8N-8.9 1.84.88 21 0E414 31.66 26.7I-30.01.0 12.9 (DI) are given in the Supplemental File. Inclination-only statistics of McFadden correlation with the dated marine isotope record provides a means of esti- 21 0E473 31.70 47.3N-9.4 0.79.64 21 2E556 31.95 37.9N-18.81.7 56.5 and Reid (1982) are given in Table 1. Inclination results from all samples were mating the age of each of the cycles (Fig. 5). Green lines in the figure denote 22 0E415 32.68 54.3N-2.4 1.913.2 23 0E416 34.74 54.8N-1.9 1.737.9 24 0E417 35.74 46.4N-10.31.6 19.1 combined and are shown in Figure 8. Of those samples, 52% were found to boundaries of the climate cycles and the cycles are numbered to the left of 25 0E418 37.08 61.9N5.23.1 22.7 26 0E419 39.02 53.7N-3.0 1.210.5 have inclinations within ±5° of the expected value, and 75% within ±10°. Incli- MGCY. Correlations based on the magnetostratigraphy from each drill hole are 27 0E420 39.49 55.0N-1.7 1.332.6 28 0E421 40.22 57.5N0.80.8 14.4 nation flattening due to compaction that is sometimes found in sedimentary discussed below. 29 0E422 70.97 57.3N0.61.1 11.4 30 0E423 72.20 49.0N-7.7 3.74.56 sequences (e.g., see Deamer and Kodama, 1990; Kodama, 2012) is not a factor 33 0E424 77.26 60.5N3.80.7 20.4 34 0E490 78.15 72.9N16.2 2.015.7 in the Santa Clara Valley drill holes. 34 0E425 78.28 56.9N0.21.9 24.5 Brunhes-Matuyama Boundary 34 0E491 78.44 61.8N5.11.4 25.2 34 0E488 78.70 64.8N8.10.8 36.2 The relatively small value for the concentration factor k (Fisher, 1953) in all 34 0E485 78.75 60.1N3.40.9 35.9 34 0E489 78.93 58.8N2.10.7 37.5 drill holes (Table 1) is entirely consistent for sample populations that have av- More than 10 m of reversed polarity sediment occurs below 291 m in the 34 0E426 79.08 64.3N7.60.9 32.6 35 0E486 79.67 60.0N3.31.7 39.6 eraged geomagnetic secular variation. Because k increases as the distribution GUAD drill hole. This interval is much thicker than those considered to be 35 0E427 79.82 58.8N2.10.9 41.1 of sample directions becomes more concentrated, values of 100 or more could possible excursions (described below) indicating that the upper part of the

1Supplemental File. Seven tables summarizing paleo- signal that the sediment has been remagnetized. We conclude, from the stabil- Matuyama Chron is represented. The first fully reversed polarity sample at magnetic data in samples from cores obtained from ity of magnetization (as evidenced by the representative examples in Fig. 6), 291.1 m in the GUAD well is taken to be the Brunhes-Matuyama boundary. deep research wells in the Santa Clara Valley, Califor- inclinations that agree with the expected direction for the region (Fig. 8), and Eight normal, reversed, and intermediate polarity samples occur over a 1.6 m nia. Records in the tables are identified by a unique paleomagnetic sample identifier, the number of the the statistical parameters given here (Table 1), that the sediment obtained from span near the base of the CCOC well (beginning at 305.1 m). This can be best core from which the sample was taken, and the depth the Santa Clara Valley drill holes provides an accurate recording of the geo- characterized as a mixed interval. A similar mixed interval occurs near the base of the sample in the well. Measured values include magnetic field. of the STPK well, beginning at 303.0 m. Even though there is not a direct transi- magnetic inclination, polarity, deviation of inclination The data in Figure 8 confirm our earlier suggestion (Mankinen and Went- tion from reversed to normal polarity in the CCOC and STPK drill holes, mixed from that expected at the sample locality, maximum angular deviation from principal component analysis worth, 2003) that inclinations deviating more than 20°–30° from the expected polarities are commonly found near polarity transitions in sedimentary se- (Kirschvink, 1980), and intensity of magnetization. field are anomalous with respect to the rest of the sample population. Accord- quences because of variations in the magnetization lock-in process (e.g., Okada Each table also contains the drill hole location. Please ingly, we tentatively assign an intermediate polarity to those samples that and Niitsuma, 1989; van Hoof et al., 1993), overprinting of weak magnetizations visit http://dx .doi.org/10 .1130/GES01217.S1 or the full-text article on www.gsapubs.org to view the Sup- have inclinations <30° or >80°. We emphasize, however, that a single inter- acquired during a polarity transition (Coe and Liddicoat, 1994), and complexities plemental File. mediate polarity sample does not, by itself, indicate a geomagnetic polarity in the reversal process (Mankinen et al., 1985; Bogue and Merrill, 1992).

Up Up Up

STPK, Core 52 CCOC, Core 77 depth: 219.30 m. lithology: silt CCOC, Core 4 depth: 303.70 m. depth: 7.14 m. lithology: sand lithology: silt Axis = 39.5 mA/m

30 mT 40 mT Axis = 16.1 mA/m 250 mT 20 mT 20 mT

Axis = 31.5 mA/m

5 mT 5 mT Down Down Down NRM NRM NRM

Up Up Up

NRM EVGR, Core 48 WLLO1, Core 37 depth: 239.72 m. Figure 6. Orthogonal projection of rema- 15 mT depth: 238.72 m. lithology: silty clay Axis = 13.9 mA/m nence vector endpoints during alternating lithology: sandy, silty clay field demagnetization showing behavior 30 mT of representative lithologies from various drill holes. Open circles are projections GUAD, Core 30 150 mT into the vertical plane. Solid circles are 250 mT projections into a horizontal plane whose depth: 47.38 m. Axis = 294 mA/m axes are arbitrary because the cores were lithology: clay not azimuthally oriented. 100 mT NRM Axis = 5.16 mA/m

Down NRM Down Down

100 mT Up Up Up

MGCY, Core 51 GUAD, Core 121 EVGR, Core 55 depth: 150.61 m. depth: 185.63 m. 200 mT lithology: silty clay depth: 306.93 m. lithology: clayey silt lithology: silty clay

Axis = 125 mA/m 20 mT

NRM 10 mT 150 mT

Axis = 11.3 mA/m

Axis = 17.3 mA/m

Down Down NRM NRM Down

GUAD CCOC STPK MGCY WLLO1 WLLO2 EVGR –90 0900–90 90 –90 0900–90 90 –90–090090 90 –900 90 0 9 4 3 5 9 14 17 20 22 21 30

43 17 21 56

100 64

49 34 97 50 51 52 52

39 121 41 65 66 200 138 69 69 52 70 65 71 72 72 152 78 Depth, in meters 160 75

79 81

193 192 300 199 78 68 69 200 78

Figure 7. Plots of inclination against depth for paleomagnetic samples from all cored wells in the Santa Clara Valley. Core numbers are given for those defining geomagnetic excursions or polarity reversals. Orange shading denotes excursions defined solely by large, serially correlated changes in direction within an individual core (see text and Figs. 10 and 11). Purple shading denotes bedrock. Green shading denotes inclination values we consider to be in the normal range for our locality (see text).

400 203

Reversed polarity samples occurring at about the same depth in the CCOC, sition-subsidence rate of ~37 cm/k.y. is indicated and may be expected almost GUAD, and STPK drill holes indicate that the same reversal boundary (the basin wide. The southeastern Evergreen basin (including at well EVGR) is an Brunhes-Matuyama) has been recorded. In Figure 9, the relative positions of exception; structural compression seems to be offsetting some of the basin the base of cycle 8 (ca. 718 ka) and the proposed Brunhes-Matuyama boundary subsidence, leading to a thinner cyclic section (see Figs. 4 and 9). are mutually consistent. If our interpretation that the Brunhes-Matuyama has The Brunhes-Matuyama boundary was detected only in those three drill been recorded in these 3 drill holes is correct, a long-term post–780 ka depo- holes. In MGCY and WLLO, bedrock was encountered before that stratigraphic

TABLE 1. INCLINATION-ONLY STATISTICS FOR INDIVIDUAL DRILL 160 HOLES FROM THE SANTA CLARA VALLEY, CALIFORNIA Expected inclination o Mean Angular Concentration = 56.7

Drill inclination Number of S.D. α95 factor hole (°) samples (°) (°) (κ) Measured inclination: o mean = 57.3 CCOC 55.4 97 17.4 2.921.7 o EVGR 59.2 73 14.7 2.730.4 log mean = 55.2 GUAD 58.4 252 17.4 2.321.8 MGCY 59.1 55 15.2 3.228.5 STPK 56.0 78 16.3 2.924.7 120 WLLO 57.2 49 10.0 2.165.9 Note: Inclinations calculated using McFadden and Reid (1982); S.D.—standard

deviation; α95—95% conﬁdence cone about average direction; κ—concentration factor of Fisher (1953).

level was reached. Reversed polarities occur near the bottom of the MGCY Figure 8. Histogram showing deviation drill hole (~253–240.8 m), but the material there is Miocene bedrock (contain- Number of Samples 80 of magnetic inclination (ΔI) from that ex- ing ca. 8 Ma diatoms; L. White, San Francisco State University, 2004, written pected at the latitude of the Santa Clara commun.). In well WLLO, serpentinite bedrock was encountered at a depth of Valley drill holes. 245 m. In well EVGR, the Brunhes-Matuyama was not detected, although the well extends far below the base of cycle 8. No core was taken in the interval just below the base of cycle 8, where that boundary would be expected.

Jaramillo Subchron 40 In the GUAD well, 3 m of normal polarity sediment occur below the reversed polarity interval representing the upper part of the Matuyama. This interval is thicker than any of the suspected excursions, and we interpret it to represent the upper part of the Jaramillo Subchron. The mixed interval near the top (cores 199–200) is similar to the mixed polarities at the Brunhes-Matuyama boundary. The top of the Jaramillo Subchron occurs in the top of the lower fine-grained unit (Fig. 4), which is deeply penetrated by two of the cored wells, GUAD and EVGR (Fig. 5). Only one core located just above the underlying 0 -60 -40 -20 0 +20 +40 bedrock was taken in that deeper interval in GUAD, but several were taken in ∆I (degrees) EVGR that span the lower 72 m of that well (Fig. 9). All of these samples have normal polarity, and thus should represent more of the Jaramillo Subchron. It is not clear where its bottom is; no underlying reversed polarities were found, and it could predate the bottom of the lower fine-grained unit. These relations tion of 170 cm/k.y. That is a minimum for the Jaramillo here, because this are consistent with the conclusion (see geologic summary discussion) that the assumes that the base of the subchron occurs at the deep normal sample in lower fine-grained unit postdates the Pliocene and early Pleistocene gravels GUAD and not earlier. This rate is much higher than that obtained for the upper that are exposed around the edges of the basin. cyclic section (37 cm/k.y.). The upper Matuyama reversed interval is not recognized in EVGR, probably because the relevant section just below the base of cycle 8 was not cored. The Mid-Quaternary Unconformity greatest thickness of normal polarity section in these two wells is controlled by the deep sample from GUAD, which indicates a minimum of 102 m of section The 10-m-thick reversed interval representing the upper Matuyama Chron that we would assign to the Jaramillo Subchron, which was 60 k.y. long. Those seems much too thin for its 210 k.y. duration (990–780 ka), given the long-term dimensions yield a sedimentation rate for most of the lower fine-grained sec- average rates of accumulation determined for the sections above and below

Cycle GUAD CCOC STPK MGCY WLLO1 WLLO2 EVGR 0 0 1 Mono Lake (32 ka) 9 Laschamp (41 ka) 4 3 5 14 17 21 Blake 2 (100 ka) 2 20-22 Blake 1 (120 ka) 30 50

Pringle Falls 2 (190 ka) 43 3 Pringle Falls 1 (211 ka) 17 21 56

100 64 4

34 49-51 5 97 150

Age (ka) ) 52 39 rs

41 e t 500 6 121

65-66 me (

200 Big Lost (565 ka) 138 } 69 Depth 52 } 71-72 7 65 152 160 250 8 serpentinite Miocene ? ? sandstone BRUNHES (780 ka) MATUYAMA 300

Kula/Santa Rosa EXPLANATION (900 ka) = too coarse to sample coarse top of 350 cycle 2(C2a) = normal polarity coarse basal (990 ka) 1000 = reversed polarity interval Jaramillo = excursion bedrock (1050 ka) = directional swings ? ? 400

Franciscan Figure 9. Correlation of the paleomagnetic inclination records from drill holes cored in the Santa Clara Valley (see Fig. 7). graywacke Green lines are sequence stratigraphic boundaries of Wentworth et al. (2015). Dark lines within each column represent cores taken and numbers to the right signify the cores in which the excursions were recorded. Cobb Mtn (1190 ka)

that interval (37 and 170 cm/k.y.); those rates would produce thicknesses for (5.2 m in 62 k.y.) is thus a relatively low 8.4 cm/k.y. The model of Wentworth and that interval of 77.7 and 357 m, respectively. The long-term rate for the cyclic Tinsley (2005) calls for just such lower accumulation rates in the fine tops of section may not apply locally; a local rate estimated for the interval between the sedimentary cycles, and this interval below the base of cycle 8 is, in effect, the base of cycle 8 (718 ka) and the Brunhes-Matuyama boundary (780 ka) is the fine top of a cycle 9. Applying this rate to the upper Matuyama reversed quite different. The thickness of that interval (see Fig. 9) in the three wells in interval would yield a thickness of 17.6 m. Although this is closer to the actual which we can define it (GUAD, CCOC, and STPK) averages 5.2 m. The local rate 10 m thickness of the interval, that interval still seems too thin for its duration,

especially considering that some part of it probably accumulated at the much 2004) strongly indicates that a true geomagnetic excursion has been recorded. higher rate determined for the underlying normally polarized section. This anomalous interval occurs ~2.5 m below the base of postglacial alluvial An unconformity below the Brunhes-Matuyama boundary seems indi- cycle 1 (within cycle C2a) in this drill hole and contains organic fragments (bark cated. The mixed polarities at both the Brunhes-Matuyama boundary and and/or woody twiglets or roots) within black clayey silt between the interme- the top of the Jaramillo indicate that these are true polarity transitions, which diate polarity paleomagnetic samples taken. These fragments yielded an age requires that the unconformity be entirely within the upper Matuyama inter- of 28,090 ± 330 14C yr B.P. (uncalibrated years), which, when using the Bard val. That unconformity thus forms the base of the overlying cyclic section, ac- (1998) approximation to correct for atmospheric 14C concentrations through counts for the absence of the lower part of a cycle 9, and should separate the time, converts to a calendar age of 32,840 yr ago (Mankinen and Wentworth, high accumulation rate of the underlying section from the lower rate above. 2004). The uncorrected age compares well with the 28,620 ± 300 14C yr B.P. de- Some considerable, but unspecifiable, thickness of missing section, including termined by Benson et al. (2003) for a volcanic tephra layer (Ash #15) near the the lower part of cycle 9, implies a considerable hiatus that could occupy much midpoint of the Mono Lake excursion at the type locality. The close agreement of the 210 k.y. upper Matuyama interval. in ages between both studies clearly indicates that the anomalous inclinations The fact that the Kula–Santa Rosa excursion (Fig. 3) was not found within in core 17 are a record of the Mono Lake excursion. Excursional directions the reversed section is further, albeit just permissive, evidence for this conclu- were encountered in GUAD core 14, STPK core 4, and EVGR core 9. Large sion. In Wentworth et al. (2010, 2015) other evidence for an unconformity just within-core directional swings were found in WLLO1 core 3 and WLLO2 core 5 below the base of cycle 8 was described, and the unconformity was placed (Fig. 10). All occur within cycle 2a and are thus most likely correlative with the within the upper Matuyama reversed interval. The broad extent of the evi- Mono Lake excursion in CCOC (Fig. 9). dence, ranging from well MOFT through GUAD to the Evergreen seismic re- At a depth of 9–11 m below the Mono Lake excursion in GUAD, large clock- flection line (Fig. 2), together with the considerable hiatus involved, indicates wise swings in the magnetic vector, without corresponding anomalous inclina- that this unconformity is a basin-wide event. tions, occur in GUAD cores 20–22 (Fig. 10). The total angular distance covered by the magnetic vector in cores 22 and 21 is ~103° over a thickness of ~1 m, and ~71° in core 20. This behavior seems reminiscent of the recurring waveforms Excursions reported by Lund et al. (1988). We suggest correlation of this behavior with the Laschamp excursion. If this is correct, then the expression of that excursion Some of the difficulties in attempting to use geomagnetic excursions for in our region is subtle and could easily be missed. The feature that Holt et al. geological correlation were mentioned here. Assigning ages within the Santa (1996) correlated with the Laschamp excursion in a drill hole on Hawaii also is Clara section is not straightforward, because the only direct dating available near the limit of normal geomagnetic secular variation at that locality. is from several radiocarbon samples that are limited to the upper 25 m of the A second interval with an anomalous inclination in CCOC was found in core section. However, the stratigraphic subdivision of the upper 300 m of the sec- 21 (depth range 30.78–32.27 m). Although the inclinations here only reach 30° tion into 8 upward-fining cycles provides lateral correlations across the basin, shallower than expected, and are thus exactly at the limit that we use for defin- and correlation of those cycles with the marine oxygen isotope record pro- ing an intermediate polarity, a large clockwise swing of the total magnetic vec- vides estimates of age. Being able to confine any given excursion to within tor (Mankinen and Wentworth, 2003, 2004) strongly suggests that an excursion the age range of one of those cycles greatly improves the chances of a cor- was recorded. Because this anomalous interval is only 6.4 m below the Mono rect identification. Subdivision of each cycle into a fine top and coarse bottom Lake excursion, in Mankinen and Wentworth (2003, 2004) the possibility that it (Wentworth et al., 2015) offers further constraint, as those subdivisions are also could represent either the Laschamp or another excursion within the geomag- approximately equivalent between wells. The coarse bottoms probably accu- netic intensity low between ca. 50 and 15 ka was considered (see Mankinen mulated relatively quickly relative to the fine tops, as indicated by the sedimen- and Champion 1993; Laj et al., 2004). Such a correlation would, however, cross tary model proposed in Wentworth and Tinsley (2005), although more detailed the base of the upper coarse interval (C2a) in sedimentary cycle 2 (defined by rates are uncertain and considerable variation within short sections is likely. Wentworth et al., 2015), and that boundary should probably be considered an approximate time surface, like the cycle boundaries. A more likely correlative of CCOC core 21 would be GUAD core 30, which Mono Lake–Laschamp Excursions (Cycle 2) also occurs within the upper fine-grained interval of cycle 2. Hillhouse (1977b) conducted a paleomagnetic study of a sediment core taken at south end of the The youngest anomalous magnetic inclinations were initially encoun- Dumbarton Bridge (well DUMB in Wentworth et al., 2015). He found a sample tered in core 17, taken from the CCOC drill hole along Coyote Creek in central (77C-107.0) with an anomalous inclination of –2.7° at a depth of 42.67 m, also San Jose (Figs. 2 and 9). A large counterclockwise swing of the total mag- within the upper fine-grained interval of cycle 2. Although simple linear inter- netic vector over a depth range of 0.88 m (Mankinen and Wentworth, 2003, polation of ages between cycle boundaries is probably not valid, the ca. 75 ka

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WLLO1 core 3 depth = depth = depth = depth = 16.00 m. 15.70 m. 15.30 m. 16.51 m. depth = depth = 15.80 m. 15.39 m. depth = depth = 15.55 m. 16.42 m.

60° 30° 30° 60° depth = 16.32 m. WLLO2 core 5 depth = 15.43 m.

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Figure 10. Directional changes of the magnetic vector in samples from WLLO1 depth = and WLLO2 cores probably represent- depth = 34.23 m. ing the Mono Lake excursion, and GUAD 32.59 m. depth = 32.35 m. GUAD core 21 cores 22–20, possibly correlative to the Laschamp excursion (see text). Solid dots depth = are directions on lower hemisphere of an depth = GUAD core 20 32.15 m. equal area projection. Because cores from 33.91 m. the drill holes are not azimuthally ori- 30° 60° 30° 60° ented, each plot was rotated an arbitrary depth = depth = amount about a vertical axis so that the 31.89 m. depth = 33.23 m. depth = beginning of the excursion is near the ex- 31.77 m. 33.38 m. pected normal polarity direction (star) for the area.

depth = 33.57 m. North

depth = 35.62 m.

depth = depth = 35.42 m. 35.82 m. depth = 35.24 m. 30° 60°

depth = depth = 35.06m. GUAD core 22 34.80 m. depth = 34.92 m.

midpoint age of cycle 2 provides a guide. Given the high early and low later 17 (total ~148°) and EVGR core 21 (total ~88°), both in the coarse-grained inter- sedimentation rates within a cycle (Wentworth and Tinsley, 2005), the ages of val, are most likely correlative with GUAD core 64. The occurrence of similar these events should be somewhat older than that midpoint age. That would behavior in three drill holes at depths of ~99 m (GUAD), ~73.5 m (STPK), and be too old for Laschamp and probably somewhat too young for Blake 2, and ~78.0 m (EVGR) at the same approximate level in cycle 3 (Fig. 9) is consistent would thus leave the GUAD 20–22—CCOC 21 event without known correlative. with this episode being representative of one of the Pringle Falls excursions, likely Pringle Falls 1. GUAD core 56 may represent Pringle Falls 2.

Big Lost Excursion (Cycle C7) Other Possible Excursions A reversed polarity interval was found in the MGCY drill hole in the upper part of cycle 7 that extends over at least 0.6 m from the top of core 72 to the Reversed polarity in GUAD core 43 (66.8 m) defines an excursion occurring bottom of core 71 (depth range 218.7–219.3 m). This interval is overlain by in the coarse-grained lower interval near the base of cycle 2 (Fig. 9). This could more than 1.6 m of normal polarity (top of core 71 to core 70), and is followed represent the Blake 1 excursion; however, cores from this stratigraphic level by another reversed polarity sample at 215.6 m in core 69 (Fig. 7). Intermediate were not obtained from any of the other drill holes, making it impossible to test polarity samples were similarly found high in cycle 7 in core 152 (depth of for the presence of similar events elsewhere in the basin. 230.2 m) in the GUAD drill hole. The anomalous behavior seen in all these drill Fully reversed and intermediate polarity samples were recovered in the holes and samples is probably also represented in core 65 (~223.5 m) of the coarse bottom of cycle 5 over 1.4 m (at ~150.5 m depth) in cores 49B to 51 in CCOC well and core 39 (~177.6 m) in EVGR. None of the samples from these the MGCY drill hole (Fig. 7). The mixed polarities here are suggestive of an last two cores have inclinations that are in the intermediate polarity range that excursion with anomalous directions separated by normal polarity. A possible we have tentatively defined, but 2 samples in CCOC core 65 are at the upper correlative in this same part of cycle 5 is the ~75° declination swing (Fig. 11) in and lower limits (30.6° and 79.0°) of the presumed normal range, and one is EVGR core 34 (~142 m). Another intermediate polarity interval was recovered near the upper limit (77.9°) in EVGR. Large directional swings are also seen from GUAD core 97 (~150 m). Core 97 occurs in the upper fine-grained inter- in these two cores (Fig. 11). The same level was not cored in the STPK well. val high in cycle 5 and thus probably does not represent the same behavior Anomalous directions in three other wells at approximately the same level as seen in the MGCY and EVGR wells. The location of these excursion or excur- the MGCY reversed polarity interval leave little doubt that an excursion was sions within cycle 5 do not correspond to any excursion thus far reported from recorded. Anomalous behavior in the GUAD, CCOC, MGCY, and EVGR drill the western U.S. Inasmuch as MGCY cores 49B to 51 occur above the 414 ka holes at 230, 223, 215–219, and 178 m, respectively, occur in the fine-grained cycle6-cycle5 boundary, they might be correlative with the 390 ± 4 ka reversed interval near the upper boundary of cycle 7 (536 ka) and are thus consistent polarity flow from the Snake River Plain (Tauxe et al., 2004). with the 565 ± 22 ka age (Lanphere et al., 2004) of the Big Lost excursion. The GUAD core 121 (185.6 m) and MGCY core 52 (153 m) are from the fine- occurrence of two anomalous directions near the top of cycle 7 in the MGCY grained interval near the top of cycle 6 and are possibly correlative. From this (~219 and 216 m) drill hole may indicate that more than one excursion may same interval in well DUMB, 3 samples with anomalously shallow inclinations have also occurred in a short time interval around the Big Lost excursion, such (9.4°, 8.2°, and –2.7°) occur between 169.37 and 173.93m (Hillhouse 1977b). as noted for other excursions described here. One sample with a normal inclination (57.2°) occurs between the first two anomalous samples at 173.24 m. In the lower coarse-grained interval of cycle 6, intermediate to reversed polarity found in MGCY cores 65 and 66 (depth Pringle Falls Excursion (Cycle 3) range 189.3–191.1 m), intermediate polarity in STPK core 41 (depth of 186 m) and in GUAD core 138 probably record the same behavior. The indications An intermediate polarity inclination was found in GUAD core 56 (87.0– here are that two separate excursions occurred during cycle 6, between 531 87.9 m) within the fine-grained interval of cycle 3. Stratigraphically lower, in the and 414 ka (Fig. 4). Neither corresponds to any excursion thus far reported coarse-grained interval, is an ~115° counterclockwise swing of direction (Fig. from the western North American region. Reports of excursions between 500 11) in GUAD core 64 (99.0–100.1 m). The estimated difference in age between and 400 ka are rare worldwide, with the possible exceptions of the West Eifel the anomalous directions in cores 64 and 56 is 32 k.y. using our Pleistocene (Schnepp and Hradetzky, 1994) and Calabrian Ridge 2 (Langereis et al., 1997) average sedimentation rate, or 42 k.y. using a linear interpolation between the excursions, and excursions 11A and 13A in ODP Leg 172 cores from the west- boundaries of cycle 3. Although these two estimates show a somewhat larger ern North Atlantic Ocean (ODP Leg 17 Scientific Party et al., 1998). These excur- time gap between the two Pringle Falls episodes than we show in Figure 3, we sions have yielded ages between 515 and 510 using either the 40Ar/39Ar method suggest that this behavior represents their occurrence in the San Francisco Bay or an astronomically tuned oxygen isotope record. We are not proposing a cor- region. Clockwise swings in relative direction (Fig. 11) recorded in STPK core relation to those excursions, but suggest that they may be indicative of a low

North

depth = 223.77 m. depth = EVGR Core 39 177.63 m. CCOC Core 65 depth = depth = 177.53 m. depth = 177.71 m. 223.74 m. 30° 60° depth = 223.38 m. 60° 30° depth = depth = 177.57 m. 223.69 m. depth = depth = 222.95 m. depth = 222.89 m. 177.60 m.

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STPK core 17 Figure 11. Directional changes of the mag- GUAD core 64 depth = depth = netic vector in samples from CCOC core 73.11 m. depth = 98.99 m. 74.01 m. 65 and EVGR core 39 are interpreted as depth = representing the Big Lost excursion (see depth = 73.00 m. text). Directional changes in GUAD core 100.12 m. depth = depth = 64, STPK core 17, and EVGR core 21 are 73.25 m. 73.79 m. considered to represent the Pringle Falls 1 30° 60° 30° 60° depth = excursion. EVGR core 34 is probably cor- 99.22 m. depth = relative with reversed and intermediate 73.48 m. samples from MGCY cores 49B to 51, and depth = perhaps a reversed polarity lava flow from depth = 99.66 m. the Snake River Plain (see text). See Figure 73.65 m. 10 for additional explanation.

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depth = 78.32 m. depth = EVGR core 21 depth = 142.69 m. depth = 78.12 m. 142.12 m. depth = depth = depth = 78.07 m. 142.90 m. depth = 78.24 m. depth = EVGR core 34 77.64 m. depth = 77.87 m. 142.07 m. 30° 60° 60° depth = 30° 77.75 m.

depth = 142.02 m.

geomagnetic intensity at about that time, and one that may have been long western North America–eastern Pacific region. Some comparisons can be lasting. Note the two pronounced dips in the strength of the geomagnetic field made between the Manix and Santa Clara Valley core records. Reheis et al. between ca. 500 and 450 ka (Fig. 3). Our results from these three cores could (2012) considered their youngest excursion (excursion 3b) to represent the fall within such an interval and represent excursions occurring ca. 490 ka that Laschamp excursion. Because our analysis indicates the Mono Lake excursion have not been previously documented from the western U.S. to be better expressed than the Laschamp in our region, excursion 3b may instead be the Mono Lake excursion. The Reheis et al. (2012) age model cor- relates their excursion 5b to one of the Blake excursions, and thus is a possible DISCUSSION AND CONCLUSIONS correlative to GUAD cores 30 or 43. Reheis et al. (2012) did not make a correlation for excursion 7a, but estimated its age to be ca. 190 ka. This estimate We obtained 608 paleomagnetic samples from cores taken in 6 research fits well with the age of the Pringle Falls 2 excursion (Fig. 9). The Reheis et al. drill holes in the Santa Clara Valley. Stability of magnetization, inclinations that (2012) excursion 7b could then be Pringle Falls 1; the oldest excursion they agree with the expected direction for the region, and statistical parameters (an- identify (excursion 11a) is estimated to have occurred ca. 400 ka. This assign- gular standard deviation, concentration parameter) of the paleomagnetic data ment fits well with our data from MGCY cores 49B to 51 and the possible cor- all indicate that the sediment obtained from the Santa Clara Valley drill holes relation with the 390 ± 4 ka reversed polarity flow from the Snake River Plain. provides an accurate recording of the geomagnetic field. Paleomagnetic re- We find no counterparts to the Reheis et al. (2012) excursions 8a and 9a or 9b sults show a reversal boundary below the base of cycle 8 in the GUAD, CCOC, in the Santa Clara Valley wells. and STPK wells (depths ranging from 291 to 303 m), which we interpret as A mid-Quaternary unconformity occurs beneath the base of cycle 8 representing the 780 ka Brunhes-Matuyama boundary. A long-term deposi- throughout the basin that separates the cyclic upper section from the lower tion-subsidence rate of ~37 cm/k.y. for the cyclic section in the central basin is fine-grained unit. That lower unit appears to include much of the Jaramillo indicated using this boundary as reference. Subchron; all samples from that section below the upper Matuyama reversed The paleomagnetic record between 301.7 and 304.5 m in the GUAD drill hole section in the two cored wells that penetrate it have normal polarity. The base is indicative of a significant unconformity between the Jaramillo Subchron and of the Jaramillo was not reached, and may be older than the base of the fine- the Brunhes-Matuyama boundary. This unconformity must underlie the entire grained unit, which is on a bedrock unconformity, as observed in GUAD. A Santa Clara Valley and thus merge with the bedrock unconformity underlying minimum sedimentation rate of 170 cm/k.y. for the known lower interval is the entire basin where it rises toward the basin margins and cuts out the lower required, a rate much higher than the ~37 cm/k.y. average for the upper sec- sedimentary cycles, including in the cored holes WLLO and MGCY. tion. The two sections are independent, however, as they are separated by that Intervals recording large swings of within-core direction, some including mid-Quaternary unconformity. anomalous magnetic inclinations, were found in all the wells. Both clockwise and counterclockwise swings were observed, providing evidence that this be- ACKNOWLEDGMENTS havior is not an artifact of the coring process. Anomalous inclination intervals This study is part of a collaborative effort with the Santa Clara Valley Water District. Primary core that occur within the same part of a sedimentary cycle in more than one re- descriptions were made by J.C. Tinsley (see Wentworth et al., 2015). Constructive reviews by search well strongly indicate that true geomagnetic excursions were recorded. R. Graymer, J. Hillhouse, two anonymous journal reviewers, and guest associate editor V. Langen Some of the anomalous intervals are separated by short normal intervals simi heim are appreciated. lar to the pattern described for some well-documented excursions elsewhere.

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