Subsurface and petroleum geology of the southwestern Santa Clara Valley ("Silicon Valley"), California

Stanley, Richard G.; Jachens, Robert C.; Lillis, Paul G.; McLaughlin, Robert J.; Kvenvolden, Keith A.; Hostettler, Frances D.; McDougall, Kristin; Magoon, Leslie B.

doi:10.3133/pp1663

Cited by 15 publications

(27 citation statements)

References 32 publications

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“…Faults also control the flow of groundwater within the Santa Clara Valley and subdivide the aquifer system into three subregions, with a western Cupertino basin, a central graben-like feature, and the eastern Evergreen basin (Hanson et al, 2004b;Stanley et al, 2002;Langenheim, et al, 2013). The faults identified as part of this study that affect the flow of groundwater include the Silver Creek and Evergreen faults in the eastern part of the valley and the Monte Vista and New Cascade faults in the western part of the valley (Figs.…”

Section: Hydrogeologic Structurementioning

confidence: 98%

Hydrologic framework of the Santa Clara Valley, California

Hanson

2015

Geosphere

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The hydrologic framework of the Santa Clara Valley in northern California was redefined on the basis of new data and a new hydrologic model. The regional groundwater flow systems can be subdivided into upper-aquifer and lower-aquifer systems that form a convergent flow system within a basin bounded by mountains and hills on three sides and discharge to pumping wells and the southern San Francisco Bay. Faults also control the flow of groundwater within the Santa Clara Valley and subdivide the aquifer system into three subregions.After decades of development and groundwater depletion that resulted in substantial land subsidence, Santa Clara Valley Water District (SCVWD) and the local water purveyors have refilled the basin through conservation and importation of water for direct use and artificial recharge. The natural flow system has been altered by extensive development with flow paths toward major well fields. Climate has not only affected the cycles of sedimentation during the glacial periods over the past million years, but interannual to interdecadal climate cycles also have affected the supply and demand components of the natural and anthropogenic inflows and outflows of water in the valley. Streamflow has been affected by development of the aquifer system and regulated flow from reservoirs, as well as conjunctive use of groundwater and surface water. Interaquifer flow through water-supply wells screened across multiple aquifers is an important component to the flow of groundwater and recapture of artificial recharge in the Santa Clara Valley. Wellbore flow and depth-dependent chemical and isotopic data indicate that flow into wells from multiple aquifers, as well as capture of artificial recharge by pumping of water-supply wells, predominantly is occurring in the upper 500 ft (152 m) of the aquifer system. Artificial recharge represents about one-half of the inflow of water into the valley for the period 1970-1999. Most subsidence is occurring below 250 ft (76 m), and most pumpage occurs within the upper-aquifer system between 300 and 650 ft (between 91 and 198 m) below land surface.Overall, the natural quality of most groundwater in the Santa Clara Valley is good. Isotopic data indicate that artificial recharge is occurring throughout the shallower parts of the upper-aquifer system and that recent recharge (less than 50 yr old) occurs throughout most of the basin in the upper-aquifer system, but many of the wells in the center of the basin with deeper well screens do not contain tritium and recent recharge. Age dates indicate that the groundwater in the upper-aquifer system generally is less than 2000 yr old, and groundwater in the lower-aquifer system generally ranges from 16,700 to 39,900 yr old. Depth-dependent sampling indicates that wellbores are the main path for vertical flow between aquifer layers. Isotopic data indicate as much as 60% of water pumped from production wells originated as artificial recharge. Shallow aquifers not only contain more recent recharge but may be more susceptible to anthro...

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Section: Hydrogeologic Structurementioning

confidence: 98%

Hydrologic framework of the Santa Clara Valley, California

Hanson

2015

Geosphere

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“…Regional gravity (Roberts et al, 2004) shows a deep sedimentary basin (the Cupertino Basin) with a steep west side and a more gently sloped east side ( Fig. 6A; Langenheim et al, 1997;Stanley et al, 2002Stanley et al, , 2005. The steep west side is interpreted herein (see below) as a normal or right-lateral oblique normal fault concealed by superposition of the active fault zone (for other possible interpretations, see Stanley et al, 2002).…”

Section: Structural Superposition On the Western Margin Of Santa Clarmentioning

confidence: 99%

“…6A; Langenheim et al, 1997;Stanley et al, 2002Stanley et al, , 2005. The steep west side is interpreted herein (see below) as a normal or right-lateral oblique normal fault concealed by superposition of the active fault zone (for other possible interpretations, see Stanley et al, 2002). Unlike the Evergreen Basin, the Cupertino Basin is not a pullapart basin.…”

Section: Structural Superposition On the Western Margin Of Santa Clarmentioning

confidence: 99%

Structural superposition in fault systems bounding Santa Clara Valley, California

et al. 2015

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Santa Clara Valley is bounded on the southwest and northeast by active strike-slip and reverse-oblique faults of the San Andreas fault system. On both sides of the valley, these faults are superposed on older normal and/or right-lateral normal oblique faults. The older faults comprised early components of the San Andreas fault system as it formed in the wake of the northward passage of the Mendocino Triple Junction. On the east side of the valley, the great majority of fault displacement was accommodated by the older faults, which were almost entirely abandoned when the presently active faults became active after ca. 2.5 Ma. On the west side of the valley, the older faults were abandoned earlier, before ca. 8 Ma and probably accumulated only a small amount, if any, of the total right-lateral offset accommodated by the fault zone as a whole. Apparent contradictions in observations of fault offset and the relation of the gravity fi eld to the distribution of dense rocks at the surface are explained by recognition of superposed structures in the Santa Clara Valley region.

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“…During early to middle Miocene time, this landscape further subsided beneath a shallow sea and was deformed locally into deeper on January 29, 2015 geosphere.gsapubs.org Downloaded from depressions such as the Cupertino Basin in the southwestern part of the valley. Miocene strata exposed along the southwest margin of the valley indicate marine deposition at depths of ~150-1500 m (Stanley et al, 2002), and deepening of that basin migrated northward between Gilroy and Cupertino between the early and middle Miocene (McLaughlin et al, 1999;Fig. 4B).…”

Section: Pre-18 Mamentioning

confidence: 99%

“…Driller's logs from a minor oil fi eld in the Los Gatos area indicate a thick (780 m) section of brown shale interpreted to be Monterey Formation; geochemistry from an oil sample from this fi eld indicates derivation from a Monterey Formation source rock generated at a depth of ~2.1-2.7 km (Stanley et al, 2002). Gravity data indicate that the basin is ~3 km deep, 30 km long, and ~10 km wide, with a steep southwest margin and gently sloping northeast margin (Langenheim et al, 1997;Stanley et al, 2002). A seismic-refraction profi le (Catchings et al, 2006) confi rms the presence of a deep basin in the Los Gatos area, with velocities of <5 km/s extending to depths of 3 km.…”

Section: Pre-18 Mamentioning

confidence: 99%

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

et al. 2015

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The late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley illustrates the dynamic nature of the North American-Pacifi c plate boundary and its effect on basin and landscape development. Prior to early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan complex basement structurally interleaved in places with Coast Range ophiolite and Mesozoic Great Valley sequence, and locally overlapped by Paleogene strata. During early to middle Miocene time, this landscape was fl ooded by the sea and was deformed locally into deeper depressions such as the Cupertino Basin in the southwestern part of the valley. Marine deposition during the middle and late Miocene laid down thin deposits in shallow water and thick deeper-water deposits in the Cupertino Basin. During this sedimentation, the San Andreas fault system encroached into the valley, with most offset partitioned onto the San Andreas fault southwest of the valley and the southern Calaveras-Silver Creek-Hayward fault system in the northeastern part of the valley. A 6-km-wide right step between the Hayward and Silver Creek faults formed the 40-km-long Evergreen pullapart basin along the northeastern margin of the valley, leaving a basement ridge between it and the Cupertino Basin. The Silver Creek fault was largely abandoned ca. 2.5 Ma in favor of a compressional left step between the Calaveras and Hayward fault, although some slip continued to at least mid-Quaternary time. Gravity, seismic, stratigraphic, and interferometric synthetic aperture radar (InSAR) data indicate no other major San Andreas system faults within the central block between the present-day range-front faults bounding the valley and the SilverCreek fault. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block), the area rose above sea level, and a regional surface of erosion was carved into the Mesozoic and Tertiary rocks. Alluvial gravels were deposited on this surface along the margins of the valley beginning ca. 4 Ma, but they may not have prograded onto the central block until ca. 1 Ma, because no older equivalents of the Pliocene-Quaternary Santa Clara gravels have been found there. Thus, either the central block was high enough relative to the surrounding areas that Santa Clara gravels were never deposited on it, or any Santa Clara gravels deposited there were stripped away before ca. 1 Ma. Analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of ~0.4 mm/yr since ca. 0.8 Ma, possibly extending north to northern San Francisco Bay. Today, the central block continues to subside, the range-front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley.

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Subsurface and petroleum geology of the southwestern Santa Clara Valley ("Silicon Valley"), California

Cited by 15 publications

References 32 publications

Hydrologic framework of the Santa Clara Valley, California

Hydrologic framework of the Santa Clara Valley, California

Structural superposition in fault systems bounding Santa Clara Valley, California

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

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