Biostratigraphic and lithologic correlations of two 

Sonoma County Water Agency pilot wells with the 

type Wilson Grove Formation, Sonoma County, central 

California 

By Charles L. Powell, II1 Robert J. McLaughlin1 and Elmira Wan1

2006 

U.S. Geological Survey Open-File Report 2006–1196 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, SECRETARY 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Menlo Park, California 2006 

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Suggested citation: 

Powell, C.L., II, McLaughlin, R.J. and Wan, Elmira, 2006, Biostratigraphic and lithologic correlations 

of two Sonoma County Water Agency Pilot Wells with the type Wilson Grove Formation, Sonoma 

County, central California. U.S. Geological Survey Open-File Report 2006-1196, p. 37 p. [available 

on the World Wide Web at URL http://pubs.usgs.gov/of/2006/1196/ ]. 

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endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

2
Contents 

Abstract ............................................................................................................................................................................4 

Introduction .....................................................................................................................................................................4 

Fossil occurrences in pilot wells .................................................................................................................................6 

Mollusks .......................................................................................................................................................................6 

Microfossils .................................................................................................................................................................7 

Tephrochronology ..........................................................................................................................................................8 

Well stratigraphy and regional stratigraphic relationships....................................................................................9 

Late Pliocene and Quaternary non-marine deposits ...........................................................................................9 

Late Miocene and Pliocene marine and non-marine deposits ..........................................................................9 

Acknowledgements......................................................................................................................................................12 

References Cited ..........................................................................................................................................................12

Figures 

1. Index map showing the location of the Wilson Grove Formation (type locality), River Road at 

Trenton fossil locality, Sonoma County Water Agency Occidental Road and Sebastopol Road pilot 

wells, Sonoma County, central California ..................................................................................................................5 

2. Benthic foraminifer Nonionella basispinata (Cushman and Moyer) recovered from between 380-

390 ft (116-119 m) core interval in the Occidental Road pilot well .........................................................................8 

3. Diagram showing correlations between Sonoma County Water Agency Occidental Road and 

Sebastopol Road pilot wells, Sonoma County, central California based on mollusks (red) and 

lithology (blue)...............................................................................................................................................................10 

Tables 

1. Faunal list of mollusks identified at Order level or below, in the Sonoma County Water Agency 

Occidental Road pilot well, Sonoma County, central California. ...........................................................................7

Appendix 

1. Macrofossil list from Sonoma County Water Agency Occidental Road and Sebastopol Road pilot 

wells, Sonoma County, central California ................................................................................................................15

2. Lithologic logs of Sonoma County Water Agency Wells...................................................................................27

A. Occidental Road Pilot well ................................................................................................................................27

B. Sebastopol Road Pilot well................................................................................................................................31

3
Biostratigraphic and lithologic correlations of two 

Sonoma County Water Agency pilot wells with the 

type Wilson Grove Formation, Sonoma County, central 

California 

By Charles L. Powell, II1 Robert J. McLaughlin1 and Elmira Wan1

Abstract 

Small mollusk faunas characteristic of the uppermost part of the Wilson Grove Formation at 

Wilson Grove and along River Road at Trenton (Pliocene) were encountered in Sonoma County 

Water Agency pilot wells at Occidental Road well field between 320-500 ft (98-152 m), depth, and 

in the Sebastopol Road pilot well field between 560-570 ft (171-174 m), depth. These mollusks 

support correlations between the two wells made on lithologic grounds. 

A benthic foraminifer was recovered from between 380-390 ft (116-119 m), depth, in the 

Sonoma County Water Agency Occidental Road pilot well. Though an isolated specimen, the 

presence of this well-preserved foraminifer supports the environmental interpretation of less than 

100 m on the continental shelf indicated by the molluscan assemblages at this site. 

For this marine stratigraphic interval of the Wilson Grove Formation, we suggest a 

relatively narrow age range of 5.3 (Miocene-Pliocene boundary) to ~ 4.5 Ma based on the 

stratigraphic relations of correlative marine strata around the upland margin of Santa Rosa plain 

and correlative strata in the Santa Cruz area, although an age between 5.3 and ~ 2.8 Ma cannot be 

discounted. 

Introduction 

The Occidental Road and Sebastopol Road pilot well sites are situated along the west side 

of the Santa Rosa plain which overlies the moderately deep (~2.5 km) Cotati sedimentary basin 

filled with Miocene to Quaternary marine, estuarine and fluvial-lacustrine deposits (Figure 1). 

These strata are, in turn, intercalated with the Tolay Volcanics (~10.6 to 8.5 Ma; Wagner and 

others, 2005) and the Sonoma Volcanics (~8.5 to 2.5 Ma; Wagner and others, 2005). A system of 

Pliocene and younger normal faults bound the Santa Rosa plain on the southwest, and in the 

subsurface, the plain is partitioned by west-northwest-trending thrust faults, into the Cotati basin (to 

the south) and Windsor basin (to the north – not figured). The Cotati basin is bounded on the 

northeast by the active northwest-trending Rodgers Creek-Healdsburg fault zone. Detailed 

stratigraphy in these buried basins is important to our understanding of local groundwater supplies 

and the structural configuration of the basins has important earthquake hazards implications

(McPhee and others, 2005). The complexity in stratigraphic and structural relations across faults 

bounding the Santa Rosa plain make it difficult to tie the geology of the surrounding uplands 

directly to the basin stratigraphy. However, previous work on the exposed basin margin

 

1

 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 

4
River Road fossil locality

SANTA ROSA

TRENTON

SEBASTOPOL

122°45'

38°30'

21 2 0 4 6 8 10 12 KILOMETERS

SCALE 1:150 000

2 1 0 2 4 6 8 10 MILES

CALIFORNIA

Sonoma County Water Agency Water Well

Sonoma County Water Agency Water Well which has penetrated basementCity of Rohnert Park Municipal Water WellMonitoring Well

DESCRIPTION OF MAP 

NONMARINE ALLUVIAL DEP (Holocene and P

FLUVIAL AND LACUSTRINE  (Pleistocene an overlies 2.78-2 Volcanics, but l interfingers wit Locally includes  3.4 my. old ash  contains obsidi

SONOMA VOLCANICS (Late  Late Miocene 2. to rhyodacitic fl tuffs. Volcanics  across strike-sl interfinger with  nonmarine to m Wilson Grove an Formations, and  gravels

WILSON GROVE FORMATIO Pliocene?-late M deposits, littora locally interfing Roblar tuff of S

PETALUMA FORMATION (Pl middle Miocene  lacustrine and e interfinger with  Tolay Volcanics with 6.2 MA Ro Sonoma VolcanTOLAY VOLCANICS (late an Miocene ~8-10  dacite

COAST RANGE OPHIOLITE  VALLEY SEQUEN -Middle Jurassic

FRANCISCAN COMPLEX (Eo Jurassic)—Includ belt and Coasta

Major strike slip faults and associated  lineaments, arrows denote relative slipThrust faultsMinor strike slip faults and associated lineaments

^

^

Occidental Road pilot well

Sebastopol Road pilot well

Wilson Grove fossil localitytype Wilson Grove Formation

Figure 1. Geologic map of the of the Cotati Basin part of the Santa Rosa plain area showing the location of surfac

Wilson Grove Formation (upside down triangles) at its type section at Wilson Grove and along River Road at TrentCounty Water Agency Occidental Road and Sebastopol Road pilot wells (triangles), Sonoma County, central Califo

5
stratigraphy (Fox, 1983; Allen, 2003) work on the stratigraphy in oil and gas wells (Wright and 

Smith, 1992; Zieglar, Wright, and Smith, 2005), and the compilation of water well data by 

Cardwell (1958) has provided a modern context (i.e. Valin and McLaughlin, 2005) for detailed 

biostratigraphic and chronostratigraphic analysis of the subsurface of Santa Rosa plain. 

This report expands the above framework by documenting, for the first time, an invertebrate 

fauna in the subsurface of Santa Rosa plain that allows correlation to a paleontologically wellstudied Pliocene section along the western margin of Santa Rosa plain and to other correlative 

Pliocene sections elsewhere in central California. Based on the stratigraphic relations of correlative 

marine strata around the upland margin of Santa Rosa plain and the chronostratigraphic relations of 

correlative strata in the Santa Cruz area, an age younger than 5.3 Ma is indicated for the 

fossiliferous horizons in the pilot wells. Although some of the presently available stratigraphic 

evidence also permits an age as young as late Pliocene for the upper part of the fossiliferous section 

(Powell, Allen and Holland, 2004), other stratigraphic relations around the margins of the Santa 

Rosa Plain outlined here, suggest that the marine mollusk-bearing strata could have been deposited 

over a narrower time interval between 5.3 and ~4.5 Ma. In addition, this correlation, furthermore, 

identifies a stratigraphic interval in the subsurface that will be useful in constraining displacements 

and deformation rates across basin bounding faults and on intra-basin fault partitions. 

Fossil occurrences in pilot wells 

Washed ditch samples from the two pilot wells have produced residues containing 

mollusks, mollusk fragments, and a microfossil. The Sonoma County Water Agency Occidental 

Road pilot well is located at 38.4229°N, 122.8088°W (NAD27), on the north side of Occidental 

Road about 1.7 km northwest of Llano, Sebastopol 7.5’ quadrangle (Figure 1). This well contains 

fossiliferous samples in two shell beds that appear to be continuous, or nearly continuous. One 

shell bed is located between 320-690 ft (97-210 m) and the other is between 800-840 ft (244- 256 

m), total drilled depth. 

The Sonoma County Water Agency Sebastopol Road pilot well is located at 38.4085°N, 

122.7995°W (NAD27), along the Petaluma and Santa Rosa railroad line about midway between 

Llano and the Sebastopol Grange (north of Gravenstein), Sebastopol 7.5’ quadrangle (Figure 1). 

This well contains several shell beds based on continuous, or nearly continuous, fossil occurrences 

between 470-600 ft (143-183 m), 620-640 ft (189-195 m), 780-910 ft (238-277 m), 940-950 ft 

(287-290 m), and 1000-1070 ft (305-326 m). 

Molluscan taxa identifiable at order level or below are present in the Occidental Road pilot 

well between 320-330 ft (98-101 m), 340-360 ft (104-110 m), 370-380 ft (113-116 m), 390-440 ft 

(119-134 m), 460-480 ft (140-146 m), and 500-510 ft (152-155 m) (Appendix 1). Identifiable taxa 

are present in the Sebastopol Road pilot well between 510-520 ft (155-158 m) and 560-570 ft (171-

174 m) (Appendix 1). 

The microfossil taxon is identifiable to the species level and the specimen was recovered 

from the 380–390 ft (116-119 m) interval in the Occidental Road pilot well. 

Mollusks 

In the Occidental Road pilot well, the 320-500 ft (98-152 m) cutting depth interval contains 

a fauna similar to that found in the type Wilson Grove Formation at Wilson Grove, and along River 

Road at Trenton, both localities in northern Sonoma County (Figure 1). The small fauna reported 

here consists of eight identifiable taxa (one bivalve, seven gastropods; see Table 1). This fauna is 

too small for detailed age and environmental assessment. However, the occurrence of specimens 

questionably attributed to Nassarius grammatus (Dall) and Ophiodermella graciosana Arnold 

6
indicate a Pliocene age. Nassarius grammatus (Dall) has been reported in Pliocene formations 

from Humboldt County, northern California, to San Diego and possibly Baja California Norte, 

Mexico (Addicott, 1965). Ophiodermella graciosana (Arnold) occurs in the Careaga Sandstone in 

northern Santa Barbara County (Woodring and Bramlette, 1950), the Merced Formation (Yancey, 

1978) on the San Francisco Peninsula, and the Purisima Formation (references in Powell, 1998) 

between Año Nuevo, San Mateo County and the Sargent oil field area in southern Santa Cruz 

County. All outcrops containing Ophiodermella graciosana Arnold are considered Pliocene. 

Moreover, environmentally, all of the taxa are marine and modern representatives of these genera 

mainly represent shelf water depths (≤100 m). 

The Sebastopol Road pilot well contains only two specifically identified taxa, Nassarius sp., 

cf. N. grammatus (Dall) and Ophiodermella graciosana Arnold(?) both found between 560-570 ft 

(171-174 m). Other collections from this well contained only unidentifiable mollusk, bivalve, and 

gastropod fragments. The two species recovered from between 560-570 ft (171-174 m) are both 

present from the type Wilson Grove Formation at Wilson Grove and along River Road (Trenton), 

both in northern Sonoma County (Figure 1). Other occurrences are discussed above, but both 

species are restricted to the Pliocene. There is again too few taxa in the cuttings from this well for 

detailed age or environmental assessment. But, both taxa are restricted to the Pliocene and are 

marine forms with modern representatives of the genera most often encountered at shelf water 

depths (~100 m or less). 

Mollusca 

 Bivalvia 

Arcoida, indeterminate 

 Gastropoda 

Nassarius sp., cf. N. grammatus (Dall) 

Nassarius sp. 

Naticidae, indeterminate 

Olivella pycna Berry 

Olivella sp. 

Ophiodermella graciosana (Arnold)? 

Ophiodermella? sp. 

Table 1. Faunal list of mollusks identified at Order level or below, in the Sonoma County Water 

Agency Occidental Road pilot well, Sonoma County, central California. 

Samples between 320-690 ft (98-210 m), depth in the Occidental Road pilot well and 

between 560-570 ft (171-174 m), depth (and likely above) in the Sebastopol Road pilot well 

contain the same species. These species are common in the type Wilson Grove and the River Road 

at Trenton fossil localities of the Wilson Grove Formation in northern Sonoma County (Powell, 

Allen and Holland, 2004) where these and associated species indicate a Pliocene age. The Pliocene 

age is supported by a biocorrelation of the River Road (Trenton) locality with the Pliocene part of 

the Purisima Formation section in the sea-cliffs at and south of Capitola in Santa Cruz County, 

central California (Powell, Allen, and Holland, 2004). 

Microfossils 

Samples were examined for microfossils while logging the Occidental Road and Sebastopol 

Road pilot wells. Elsewhere in this region, in cores from oil and gas wells in the Petaluma basin, 

and in surface exposures surrounding the Santa Rosa plain and Petaluma basin, fresh to brackish 

7
water diatoms and ostracodes and rare benthic foraminifers are preserved in fluvial, lacustrine and 

estuarine lithofacies of the Petaluma Formation (Starratt and others, 2005). Only a single benthic 

foraminifer was encountered during the course of this study. 

A well-preserved benthic foraminifer, Nonionella basispinata (Cushman and Moyer) 

(Figure 2), was recovered from between 380-390 ft (116-119 m) core interval in the Occidental 

Road pilot well. N. basispinata is a long-ranging (Tertiary) taxon typically observed as a dominant 

species in inner shelf (0-50 m) benthic foraminiferal assemblages. However, it has been found in 

deeper, generally nutrient-rich, middle shelf (50-90 m) water depths off of San Francisco (Bandy, 

1953, Quinterno and Gardner, 1987, McGann, personal communication). Off southern California, 

N. basispinata is characteristic of outer shelf (50-200 m) assemblages (Douglas and Heitman , 

1979, K. McDougall, 2006, personal communication). Though an isolated specimen, the presence 

of N. basispinata supports the marine environmental interpretation indicated by the molluscan 

assemblage in this well. 

Figure 2. Nonionella basispinata (Cushman and Moyer). Digital photomicrograph of benthic 

foraminiferal specimen from Sonoma County Water Agency Occidental Road pilot well, Sonoma 

County, California, 380-390 feet, depth. Left: ventral view, center: profile, right: dorsal view. 

Magnification 160X. 

Tephrochronology 

Tephra or volcanic ash layers can be extremely useful for dating sedimentary sequences. 

Tephrochronology includes the use of established geochemical “fingerprinting” (usually by 

Electron Microprobe) and correlation techniques to identify and indirectly date sediment intervals 

containing isotropic glass shards from known volcanic eruptions or source areas (Sarna-Wojcicki, 

1976; 1979). 

The search for tephra-containing sediment layers from the Sonoma County pilot wells 

yielded largely negative results. Although some of the clay-rich horizons probably included finegrained tephra, inspection with binocular and petrographic microscopes failed to identify any 

recoverable unaltered glass shards. All of the examined intervals showed only quartz, feldspar, 

lithic grains, and altered or clayey composite grains that may have been vitreous or glassy prior to 

devitrification. In several stratigraphic intervals, very fine, globule-like grains resembling minute 

obsidian clasts were noted in the lithologic log (Appendix 2). Upon detailed examination, most of 

8
these grains turned out to be very fine sand-sized, rounded, and polished particles of chromite, 

ilmenite, chert or cherty argillite. 

Well stratigraphy and regional stratigraphic relationships 

Detailed lithologic descriptions of the washed cuttings from the Occidental Road and 

Sebastopol Road pilot wells (Appendix 2; summary - Figure 3) indicate the southwestern side of 

Santa Rosa plain southwest of Santa Rosa is underlain by 50-150 ft (15-46 m) of Pleistocene and 

Holocene floodplain and basin deposits. In turn, the deposits are underlain by 250-300 ft (76-90 m) 

of Pliocene and Pleistocene fluvial to deltaic silt, sand, and gravel. Moreover, these sediments are 

underlain by at least 650 feet of dominantly shoreline to shelfal marine strata with minor nonmarine intercalations. These marine strata correlate with parts of the Petaluma and Wilson Grove 

Formations, which overlap in age and interfinger, transitioning from southeast to northwest, from a 

fluvial-lacustrine to a deltaic-estuarine, then to a marine littoral, and finally, to a shelfal 

depositional system (Allen, 2003; Powell, Allen, and Holland, 2004; Staratt and others, 2005). In 

addition, textural and sorting characteristics of the sediments suggest that one or more shoreline 

fluctuations are preserved in the lithologic records of these wells. Here we describe and 

stratigraphically correlate between the Occidental Road and Sebastopol Road pilot wells and to 

outcrops of the Wilson Grove Formation on the southwest side of Santa Rosa plain. These 

correlations are based on sedimentologic interpretation of textural and sorting characteristics of the 

cuttings in the wells, together with the paleontologic data and age constraints implied by 

stratigraphic and sedimentologic relations of the exposed surface deposits. 

Late Pliocene and Quaternary non-marine deposits 

The upper ~50 feet of the Occidental Road drill hole and the upper ~150 feet of the Sebastopol 

Road drill hole consist of Pleistocene and Holocene clayey to pebbly, dominantly lacustrine and 

non-marine basinal or distal floodplain deposits. These sediments are underlain in both wells, 

probably unconformably, by ~265’ (Occidental Road) and ~300’(Sebastopol Road), respectively, 

of silt, sand and gravel, interpreted on the basis of textural and sorting characteristics, as distal 

fluvial, lacustrine, deltaic, and dune deposits. These deposits correspond at the surface to mildly to 

moderately tilted and locally folded fluvial and lacustrine sediments of late Pliocene and 

Pleistocene age, derived from several different local paleodrainage basins. These deposits include 

the Glen Ellen Formation, the Huichica Formation, and several other locally named and unnamed 

gravels. 

Pliocene marine and non-marine deposits 

Unconformably beneath the late Pliocene and Pleistocene non-marine section in the 

Occidental Road and Sebastopol Road wells are strata assigned here to the Petaluma and Wilson 

Grove Formations. The dominantly marine character of these strata and the shelfal affinities of the 

mollusks lead us to assign most of the beds to the Wilson Grove Formation. Some poorly sorted 

pebbly strata in the section, which are devoid of fossil or bioclastic debris, could represent minor 

fluvial or estuarine intertongues of the Petaluma Formation in the dominantly marine shelfal and 

littoral section. Most of the pebbly to gravelly component of this section is characterized by wellrounded and polished clasts and locally the gravels are clast supported and nearly devoid of fines. 

These deposits are interpreted here to be indicative of well-winnowed (washed) shoreline deposits 

(i.e., intertidal depositional setting).

9
poorly sorted sand, pebbly sand 

and pebbly coarse to medium 

sand, general fining up cycles

unsortd granule to fine sand-distal fan delta?

pebbly silt-low energy distal 

fan delta?

well sorted (wind/dune?) sand

well sorted (wind/dune?) sand

silty pea gravel-beach face

to outer littoral?

 FAULT?

 granule pea gravel, winnowed

gritty sand and silty-sandy matrix 

pea-gravel-wave base-inner littoral

clay, mud-basinal/lacustrine

silt, sand , grit, gravel-fluvial to 

distal deltaic? general fining up

tuffaceous? silt, fine sandlacustrine/basinal

FAULT?

 biocl. slty sand-inner-cent. shelf?

unsorted granule-silt with 

bioclastics-beach to inner shelf?

concentrated bioclstics

unsorted granule sand to silt and

pebbly mud w/ bioclasticsestuarine?/central shelf?

poorly sorted bioclastic granule/

grit, sand to fine sand-low energy 

estuarine/shelf?

unsorted, rounded, polished gravel, 

granules, sand and silt

inner shelf or estuary?

Bioclastic silt to fine sandinner shelf or estuary? Low energy

 SF-98003-Occidental Road Pilot Well 

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1000

840 TD

1050

1070 TD

1100

SUMMARY STRATIGRAPHIC COLUMNS, SCWA PILOT WELLS

Holocene and Pleistocene

(not discussed in text)

late Pliocene and Quaternary

nonnamrine deposits

Pliocene marine and 

nonmarine deposits

pea gravel-wave base

SF-98003-Sebastopol Road Pilot Well 

pebble gravel grit and fine sand, 

general fining up-distal fluvial

-deltaic 

UNCONFORMITY?

Bioclastic silt, sand, grit and 

pebbly sand with mollusks

-inner to central 

shelf (or estuarine?)

pebbly silt

clay+rare pebbls-basinal/lacustrine

pebble gravel pebb e g a e

clay, grit, pebly clay-lacustrine

/basinal

UNCONFORMITY?

UNCONFORMITY?

UNCONFORMITY

pebbly sand and silt

distal deltaic to estuarine/river mouth?

pea gravel-wave base

Bioclastic pea gravel-wave

base to inner littoral

unsorted sandy silt, moderately 

sorted silt and silty fine sand, fines 

up-distal fan delta to basinal/

lacustrine 

foraminifer

(380-390')

Cherty porcellanite clast 

(Cotati) lithofacies

Cherty porcellanite clast 

(Cotati) lithofacies

mollusks

(320-680')

mollusks

(800-840')

mollusks

(470-600')

mollusks

(620-640')

mollusks

(690-700')

mollusks

(780-790')

mollusks

(800-920')

mollusks

(940-950')

mollusks

(1000-1070')

Figure 3. Correlations between Sonoma County Water Agency Occidental Road and Sebastopol 

Road pilot wells, Sonoma County, central California, based on mollusks (red) and lithology (blue). 

Also shows stratigraphic occurrence of fossils and the cherty porcellaneous clast (Monterey-like 

pebble lithofacies) deposits. 

10
A distinctive lithofacies of these marine deposits, noted in the Sebastopol Road pilot well, is 

characterized by banded porcellanite pebbles. This lithofacies resembles a cherty porcellaneous 

lithofacies common in middle Miocene part of the Monterey Formation and the equivalent 

Claremont Shale of the Monterey Group of the eastern San Francisco Bay region (Allen, 2003; 

Allen and Repenning, 2005). In addition, this lithofacies is a common component of the gravels in 

the sand and gravel of Cotati (Fox, 1983), exposed at the surface on the southwest side of the Santa 

Rosa plain near Cotati, as a lithofacies of the Petaluma Formation. In the Sebastopol Road pilot 

well Monterey-like porcellanite pebbles, some of which appear to contain foraminifers, occur 

within two depth intervals, between 720-810 ft and 940-990 ft. No pebbles of this lithology were 

noted in the Occidental Road well, which was drilled to a total depth of only 840 ft. On the basis of 

the mollusks and lithologic correlations, it appears that the Occidental Road pilot well did not reach 

the depth of the sand and gravel of Cotati of the Petaluma Formation, providing another important 

lithologic constraint to our correlation to the Wilson Grove Formation type section. 

Fossil occurrences and stratigraphic relations at the River Road at Trenton and Wilson 

Grove fossil localities of the Wilson Grove Formation permit an age range for these strata of 

between 5.3 Ma to ≥ 2.8 Ma, based on the extinct (Pliocene) mollusks discussed above and age 

constraints of an unconformity between the Wilson Grove Formation and an overlying unit of 

fluvial gravel containing obsidian from a Napa Valley source (McLaughlin and others, 2005). 

Other regional stratigraphic relations could indicate that sections of Wilson Grove Formation along 

River Road at Trenton and at Wilson Grove could be significantly older than 2.8 Ma, and are 

confined to a relatively shorter depositional window between 5.3 to ~4.5 Ma. 

The Wilson Grove Formation is overlain unconformably by late Pliocene and(or) 

Pleistocene fluvial sediments that have been assigned by others to the Glen Ellen Formation (Fox, 

1983; Fox and others, 1985) east of Wilson Grove. The Glen Ellen Formation in this area locally 

includes gravels with minor but conspicuous obsidian pebbles that have been geochemically 

fingerprinted and correlated to a radiometrically dated source material from the Sonoma Volcanics 

in the Napa Valley (Napa-Glass Mountain obsidian source area) (McLaughlin and others, 2005). 

The Napa Valley obsidian source has an 40Ar/39Ar age of 2.8 Ma (McLaughlin and others, 2005). 

This implies that the Glen Ellen Formation overlying the Wilson Grove Formation is younger than 

2.8 Ma. Although we assume they are coeval with obsidian-bearing gravels northeast of the 

Trenton and Wilson Grove localities, non-marine fluvial and lacustrine sediments that overlie 

Wilson Grove equivalent strata in the Occidental Road and Sebastopol Road Pilot wells lack 

obsidian pebbles. The lack of obsidian detritus in Pliocene and Pleistocene strata south of Trenton 

may be related to structural partitioning of the subsurface of Santa Rosa plain into two separate 

basins by the Trenton thrust fault, but the lack of obsidian cannot be resolved here. 

Other fluvial gravels that crop out east of the River Road at Trenton and Wilson Grove 

fossil localities along the east side of Santa Rosa plain near Santa Rosa, unconformably overlie 

Petaluma Formation strata containing a Monterey-like pebble lithofacies. The Monterey-like 

pebble lithofacies is similar to one seen in the lower part of the Sebastopol Road pilot well section. 

The gravels overlying the Petaluma Formation in this area include obsidian pebbles. However, the 

source of the obsidian pebbles, based on their geochemical signature (McLaughlin and others, 

2005), is directly east of Santa Rosa at Annadel State Park. The obsidian source at Annadel State 

Park is dated by 40Ar/39Ar techniques at ~4.5 Ma (McLaughlin and others, 2005). The stratigraphic 

position of these gravels unconformably above the Monterey-like pebble lithofacies of the 

Petaluma Formation, indicates that the overlying gravel is ≤ 4.5 Ma but possibly could be much 

younger than the age of the obsidian clast source. The Annadel–sourced gravel appears to be 

underlain by andesitic volcanic rocks and the 4.8 Ma Lawlor Tuff that are intercalated with or 

overlie the Petaluma Formation in this area. Although exposures are complicated by faulting, this 

11
relation suggests that the Monterey-like pebble lithofacies of the Petaluma Formation could be ≥

4.8 Ma in age. 

The notable lack of any obsidian clasts in the Wilson Grove Formation on the west side of 

Santa Rosa plain and in the pilot well drill holes, could be interpreted to indicate that the Wilson 

Grove strata at River Road at Trenton, at Wilson Grove, and in the Occidental Road and Sebastopol 

Road pilot wells are: 1) entirely older than the 2.8 Ma and 4.5 Ma obsidian clast sources for the late 

Pliocene and Pleistocene gravels overlying the Petaluma and Wilson Grove Formations; 2) that 

neither of the obsidian clast sources were exposed to erosion until after deposition of the Wilson 

Grove Formation late in the Pliocene and early Pleistocene; or 3) that depositional systems carrying 

sediment sourced in the Napa-Franz Valley and Annadel areas never reached the southwestern 

Santa Rosa plain. Alternative 1) implies that the Wilson Grove Formation is entirely older than 4.5 

Ma and a significant unconformity between ~4.5 and ~ 2.8 Ma. Alternative 2) permits relatively 

uninterrupted Wilson Grove Formation deposition well into the late Pliocene, with major uplift and 

unroofing of depositional sources in the Sonoma Volcanics after ~2.8 Ma. Alternative 3 explains 

the lack of obsidian clasts in the Wilson Grove Formation merely by a limit in the westward 

reaches of the fluvial systems carrying obsidian detritus from their source regions. It is noted, 

however, that Napa-Franz Valley obsidian pebbles are present in Pliocene and Pleistocene gravels 

that overlie marine Wilson Grove strata immediately east of Wilson Grove in the Windsor basin, 

which argues against alternative 3. None of these alternative interpretations uniquely explain the 

stratigraphic relations presented herein, although alternative 1) is favored as a better fit to the 

available data and field relations.

The lower part of the River Road section of the Wilson Grove Formation at Trenton 

includes the 6.3 Ma Roblar tuff of Sarna-Wojcicki (1992) (Sarna-Wojcicki, 1992; McLaughlin and 

others, 2005). The Roblar tuff of Sarna-Wojcicki (1992) is also present along the east side of the 

Santa Rosa plain, stratigraphically beneath local Monterey-like pebble lithofacies gravels. The 

fossiliferous sections described from River Road at Trenton and Wilson Grove are stratigraphically 

above the Roblar tuff (Allen, 2005; Powell, Allen, and Holland, 2004), consistent with the age 

based on fossils. The Roblar tuff of Sarna-Wojcicki (1992) is absent in the Occidental Road and 

Sebastopol Road pilot wells. The well stratigraphy suggests that the tuff is probably present below 

the drilled depths of the wells. 

Acknowledgments 

We thank Chris Farrar, Scott Starratt, and Don Sweetkind (all USGS) for their careful 

reviews of the manuscript and Mary McGann (USGS) for providing the foraminifer photographs. 

We also thank the Sonoma County water agency for access to the samples. 

References Cited 

Addicott, W.O., 1965, Some western American Cenozoic gastropods of the genus Nassarius: U.S. 

Geological Survey Professional Paper 503B, p. B1-B24. 

Allen, J.R., 2003, Stratigraphy and tectonics of Neogene strata, northern San Francisco Bay area: 

San Jose, Calif. San Jose State University, M.S. thesis, p. 1-188. 

Allen, J.R., and Repenning, C.A., 2005, Lithology and vertebrate fossils of the Petaluma 

Formation, Sonoma County, California: Geological Society of America Abstracts with Programs, 

v. 37, no.4, p. 70. 

12
Bandy, O.L., 1953, Ecology and paleoecology of some California foraminifera, Part I. The 

frequency distribution of recent foraminifera of California: Journal of Paleontology, v. 27, p. 

161-203. 

Cardwell, G.T., 1958, Geology and ground water in the Santa Rosa and Petaluma Valley areas, 

Sonoma County, California: U.S. Geological Survey Water-supply Paper 1427, p. 1-273. 

Douglas, R.G. and Heitman, H.L., 1979, Slope and basin benthic foraminifera of the California 

Borderland: Society of Economic Paleontologists and Mineralogists Special Publication 27, p. 

231-256. 

Fox, K.F., Jr., 1983, Tectonic setting of late Miocene, Pliocene, and Pleistocene rocks in part of the 

Coast Ranges north of San Francisco, California: U.S. Geological Survey Professional Paper 

1239, p. 1-33. 

Fox, K.F., Jr., Fleck, R.J., Curtis, G.H., Garniss, H., and Meyers, C.E., 1985, Implications of the 

northwestwardly younger age of the volcanic rocks of west-central California: Geological 

Society of America Bulletin, v. 96, no. 5, p. 647-654. 

Langenheim, V.E., Roberts, C.W., McCabe, C.A., McPhee, D.K., Tilden, J.E., and Jachens, R.C., 

2006, Preliminary Isostatic Gravity Map of the Sonoma Volcanic Field and Vicinity, Sonoma and 

Napa Counties, California: U.S. Geological Survey Open-File Report 2006-1056, 1 sheet, scale 

1:100,000. http://pubs.usgs.gov/of/2006/1056/ . 

McLaughlin, R.J., Wagner, D.L., Sweetkind, D.S., Sarna-Wojcicki, A.M., Rytuba, J.J., 

Langenheim, V.E., Fleck, R.J., Jachens, R.C., and Deino, A., 2005, Late Neogene transition from 

transform to subduction margin east of the San Andreas fault in the wine country of the northern 

San Francisco Bay Area, California: Fieldtrip 10, Guidebook and volume for the joint meeting of 

Cordilleran Section GSA and Pacific Section AAPG, Book 98, 112p. 

McPhee, D.K., Langenheim, V.E., Jachens, R.C., McLaughlin, R.J., and Roberts, C.W., 2005, 

Basin structure beneath the Santa Rosa Plain, northern California, and its possible influence on 

damage patterns from the 1906 and 1969 earthquakes [abs.]: Geological Society of America 

Abstracts with Programs, v. 37, no. 4, p. 84.

Powell, C.L., II, 1998, The Purisima Formation and related rocks (upper Miocene-Pliocene), 

greater San Francisco Bay area, central California: review of literature and USGS collections 

(now housed at the Museum of Paleontology, University of California, Berkeley): U.S. 

Geological Survey Open-File Report 98-594, p. 1-102 http://pubs.usgs.gov/of/1998/of98-594/ . 

Powell, C.L., II, Allen, J.R., and Holland, P.J., 2004, Invertebrate paleontology of the Wilson 

Grove Formation (late Miocene to late Pliocene), Sonoma and Marin Counties, California, with 

some observations on its stratigraphy, thickness, and structure: U.S. Geological Survey OpenFile Report 2004-1017, p. 1-106 http://pubs.usgs.gov/of/2004/1017/ . 

Quinterno, P.J., and Gardner, J.V., 1987, Benthic foraminifers on the continental shelf and upper 

slope, Russian River area, northern California: Journal of Foraminiferal Research, v. 17, no. 2, p. 

132-152. 

Sarna-Wojcicki, A.M., 1976, Correlation of late Cenozoic tuffs in the central Coast Ranges of 

California by means of trace-and minor-element chemistry. U.S. Geological Survey Professional 

Paper 972, 30 p. 

Sarna-Wojcicki, A.M., 1979, Chemical correlation of some late Cenozoic tuffs of northern and 

central California by neutron activation analysis of glass and comparison with X-ray 

fluorescence. U.S. Geological Survey Professional Paper 1147, 15 p.

Sarna-Wojcicki, A.M., 1992, Long-term displacement rates of the San Andreas fault system in 

northern California from the 6-Ma Roblar tuff [abs.], in Borchardt, G., and others, eds., 

Proceedings of the second conference on earthquake hazards in the eastern San Francisco Bay 

13
area: California Department of Conservation, Division of Mines and Geology Special Publication 

113, p. 29-30. 

Starratt, S.W., Allen, J.R., Peterson, D., Powell, C.L. II, Ruck, E., and Sarna-Wojcicki, A.M., 2005, 

New paleontological evidence supporting the Neogene transition from marine to non-marine 

conditions in Marin and Sonoma counties, California, USA [abs.]. Geological Society of 

America, Abstracts with Programs, v. 37, no. 4, p. 69. 

Valin, Z.C., and McLaughlin, R.J., 2005, Locations and data for water wells of the Santa Rosa 

Valley, Sonoma County, California: U.S. Geological Survey Open-File Report 2005-1318, p. 1-

16, http://pubs.usgs.gov/of/2005/1318/ . 

Wagner, D.L., Fleck, R.J., McLaughlin, R.J., Sarna-Wojcicki, A.M., Clahan, K.B., and Bezore, S., 

2005, New constraints on the age and distribution of Cenozoic volcanics north of San Pablo Bay, 

California: implications for displacement along faults inboard of the San Andreas fault [abs.]. 

Geological Society of America Abstracts with Programs, v. 37, no. 4, p. 83. 

Woodring, W.P., and Bramlette, M.N., 1950, Geology and paleontology of the Santa Maria 

District, California. U.S. Geological Survey Professional Paper 222, 185 p. 

Wright, T.L., and Smith, N., 1992, Right step from the Hayward Fault to the Rodgers Creek fault 

beneath San Pablo Bay, in Borchardt, G., and others, eds., Proceedings of the second conference 

on earthquake hazards in the eastern San Francisco Bay area: California Department of 

Conservation, Division of Mines and Geology Special Publication 113, p. 407-417. 

Yancey, T.E., 1978, Stratigraphy of the Plio-Pleistocene strata in the Twelvemile Creek area, San 

Francisco Peninsula, California: Proceedings of the California Academy of Sciences, Fourth 

Series, v. 41, no. 5, p. 357-370. 

Zieglar, D.L., Wright, T.L., and Smith, N., 2005, Subsurface geology in the northern San Francisco 

Bay region, California [abs.]: Geological Society of America Abstracts with Programs, v. 37, no. 

4, p. 69.

14
Appendix 1: Macrofossil samples 

Forty-one washed ditch samples from Sonoma County Water Agency-Occidental Road Pilot 

Well were found to contain mollusk shells or fragments. The well is located at 38.4229°N, 

122.8088°W (NAD27), on the north side of Occidental Road about 1.7 km northwest of Llano, 

Sebastopol 7.5’ Quadrangle, Sonoma County, California. 

Field No.: 320’-330’ (98-101 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius sp., cf. N. grammatus (Dall) 

Olivella pycna Berry 

Indeterminate fragments 

Field No.: 330’-340’ (101-104 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 340’-350’ (104-107 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius sp. 

Indeterminate fragments 

Field No.: 350’-360’ (107-110 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Naticidae?, indeterminate 

Indeterminate fragments 

Field No.: 360’-370’ (110-113 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

15
Gastropoda 

Indeterminate fragments 

Field No.: 370’-380’ (113-116 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius sp. 

Indeterminate fragments 

Field No.: 380’-390’ (116-119 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 390’-400’ (119-122 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Olivella pycna Berry 

Indeterminate fragments 

Field No.: 400’-410’ (122-125 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Olivella pycna Berry 

Indeterminate fragments 

Field No.: 410’-420’ (125-128 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius? sp. 

Olivella pycna Berry 

Indeterminate fragments 

Field No.: 420’-430’ (128-131 m). 

Mollusca 

16
Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius sp., cf. N. grammatus (Dall) 

Olivella pycna Berry 

Indeterminate fragments 

Field No.: 430’-440’ (131-134 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius sp., cf. N. grammatus (Dall) 

Olivella pycna Berry 

Ophiodermella graciosana Arnold(?) 

Indeterminate fragments 

Field No.: 440’-450’ (134-137 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 450’-460’ (137-140 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius? sp. 

Olivella sp. 

Indeterminate fragments 

Field No.: 460’-470’ (140-143 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius? sp. 

Olivella sp. 

Indeterminate fragments 

Field No.: 470’-480’ (143-146 m). 

Mollusca 

Bivalvia 

17
Indeterminate fragments 

Gastropoda 

Nassarius? sp. 

Olivella sp. 

Ophiodermella? sp. 

Indeterminate fragments 

Field No.: 480’-490’ (146-149 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 490’-500’ (149-152 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 500’-510’ (152-155 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius sp., cf. N. grammatus (Dall) 

Indeterminate fragments 

Field No.: 510’-520’ (155-158 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 520’-530’ (158-161 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 530’-540’ (161-165 m). 

Mollusca 

18
Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 540’-550’ (165-168 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 550’-560’ (168-171 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 560’-570’ (171-174 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 570’-580’ (174-177 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 580’-590’ (177-180 m). 

Mollusca 

Bivalvia 

Arcoida, indeterminate (taxodont hinge fragment) 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 590’-600’ (180-183 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

19
Gastropoda 

Indeterminate fragments 

Field No.: 600’-610’ (183-186 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 610’-620’ (186-189 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 620’-630’ (189-192 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 630’-640’ (192-195 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 640’-650’ (195-198 m). 

Mollusca 

Indeterminate fragments 

Field No.: 650’-660’ (198-201 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 660’-670’ (201-204 m). 

Mollusca 

Indeterminate fragments 

20
Field No.: 670’-680’ (204-207 m). 

Mollusca 

Indeterminate fragments 

Field No.: 680’-690’ (207-210 m). 

Mollusca 

Indeterminate fragments 

Field No.: 800’-810’ (244-247 m). 

Mollusca 

Indeterminate fragments 

Field No.: 810’-820’ (247-250 m). 

Mollusca 

Indeterminate fragments 

Field No.: 820’-830’ (250-253 m). 

Mollusca 

Indeterminate fragments 

Field No.: 830’-840’ (253-256 m). 

Mollusca 

Indeterminate fragments

Thirty-five washed ditch samples from Sonoma County Water Agency- Sebastopol Road 

pilot well were found to contain mollusk shells or fragments. The well is located at 

38.4085°N, 122.7995°W (NAD27), along the Petaluma and Santa Rosa railroad line about 

midway between Llano and the Sebastopol Grange (just north of Gravenstein), Sebastopol 

7.5’ Quadrangle, Sonoma County, California. 

Field No.: 470’-480’ (143-146 m). 

Mollusca 

Indeterminate fragments 

Field No.: 480’-490’ (146-149 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 490’-500’ (149-152 m). 

Mollusca 

Bivalvia 

21
Indeterminate fragments 

Field No.: 500’-510’ (152-155 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 510’-520’ (155-158 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Astyris? sp. 

Indeterminate fragments 

Field No.: 520’-530’ (158-162 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 530’-540’ (162-165 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 540’-550’ (165-168 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 550’-560’ (168-171 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments

22
Field No.: 560’-570’ (171-174 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Nassarius sp., cf. N. grammatus (Dall) 

Ophiodermella graciosana Arnold? 

Indeterminate fragments 

Field No.: 570’-580’ (174-177 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 580’-590’ (177-180 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 590’-600’ (180-183 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Gastropoda 

Indeterminate fragments 

Field No.: 620’-630’ (189-192 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 630’-640’ (192-195 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 690’-700’ (210-213 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

23
Field No.: 780’-790’ (238-241 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 800’-810’ (244-247 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Vertebrata 

Bone fragment(?) 

Field No.: 810’-820’ (247-250 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 820’-830’ (250-253 m).

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 830’-840’ (253-256 m) . 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 840’-850’ (256-259 m) 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 850’-860’ (259-262 m). 

Mollusca 

Indeterminate fragments 

Field No.: 860’-870’ (262-265 m). 

Mollusca 

Indeterminate fragments 

Field No.: 870’-880’ (265-268 m). 

Mollusca 

Indeterminate fragments 

24
Field No.: 880’-890’ (268-271 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 890’-910’ (271-277 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 910’-920’ (277-280 m). 

Mollusca 

Indeterminate fragments 

Field No.: 940’-950’ (287-290 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 1000’-1010’ (305-308 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 1020’-1030’ (311-314 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 1030’-1040’ (314-317 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 1040’-1050’ (317-320 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

Field No.: 1050’-1060’ (320-323 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

25
Field No.: 1060’-1070’ (323-326 m). 

Mollusca 

Bivalvia 

Indeterminate fragments 

26
Appendix 2: Lithologic logs of ditch samples 

Lithologic logs of washed ditch samples from Sonoma County Water Agency-Occidental 

Road Pilot Well located at 38.4229°N, 122.8088°W (NAD27), on the north side of Occidental 

Road about 1.7 km northwest of Llano, Sebastopol 7.5’ Quadrangle, Sonoma County, 

California. 

Depth(ft) Lithology Notes 

20-30 (6-9 m) Mudstone; clayey grey. May be tuffaceous. 

30-40 (9-12 m) Mudstone; clayey dark grey. May be tuffaceous. May have ostracodes or diatoms. 

40-50 (12-15 m) Mudstone; clayey dark grey, with 

angular white fragments to 5 mm. 

May be tuffaceous. May have ostracodes or diatoms. 

50-60 (15-18 m) Sandstone; silty-muddy, with white 

tephra-like fragments, much fine 

quartzo-feldspathic matrix, and Tertiary 

volcanics, abundant 

Franciscan/Mesozoic lithics. 

Probably tuffaceous silt. 

60-70 (18-21 m) Sand; quartzo-feldspathic with hard, 

white, rhyolitic lithics and lithified 

sandstone fragments. 

Minor white tuff(?) fragments. 

70-80 (21-24 m) Sand; quartzo-feldspathic, lithic, gritty. Includes rounded to subrounded, black chert and 

argillite (chalcedonic) pebbles + mostly Mesozoicderived lithics. 

80-90 (24-27 m) Grit; quartzo-feldspathic. Clasts include white rounded vein quartz, dark chert, 

scattered ashy rhyolite, subrounded, hard metasandstone. 

90-100 (27-30 

m) 

Same as 80-90'. Includes a 1-cm diameter, subrounded, fine-grained, tuff 

pebble. 

100-110 (30-34 

m) 

Sand; fine to medium grained, lithic 

quartzo-feldspathic. 

Includes probable minor tephra component and fine to 

medium-grained, rounded, black chert clasts. 

110-120 (34-37 

m) 

Sand; pebbly lithic, quartzo-feldspathic. 

Like 90-100', but no tuff. 

Includes prominent 0.5-2.0 cm graywacke clasts. 

120-150 (37-46 

m) 

Gravel; rounded pebble. Clasts include red chert, andesite-rhyodacite, quartzite, 

black chert/argillite. 

150-160 (46-49 

m) 

Sand; medium to fine grained. Includes black rounded "globules" of obsidian glass; 

doubly terminated, clear, euhedral quartz, what appear 

to be fine rounded white water-transported tephra. 

Other detritus include serpentinite + other quartz, 

argillite, chert (Franciscan Complex and Great Valley 

sequence-derived) detritus and Tertiary volcanic 

(andesitic-rhyodacitic) detritus. 

160-170 (49-52 

m) 

Sand; orange-brown stained, silt and 

silty. 

Includes some minor gray pumice grains, clear euhedral 

to rounded quartz and feldspar, also cloudy clasticderived rounded quartz + argillite + serpentinite grains, 

and some minor very fine grained black glass globules. 

170-180 (52-55 

m) 

Sand; locally gritty, orange-brown 

stained, silty to fine grained. 

Includes clear euhedral quartz and feldspar, some very 

fine black glassy globules + very fine white clumps of 

possible tephra, and other Tertiary volcanics, Franciscan 

Complex /Great Valley sequence detritus, as above. 

180-190 (55-58 

m) 

Same as 170-180, orange-brown stained. Includes some rounded fine sand (0.3-0.5 mm), white, 

euhedral tephra grains. Clear feldspar and some clear 

double-terminated quartz, glass globules if present not 

obvious. Some red/black grains of possible welded(?) 

27
tuff. 

190-200 (58-61 

m) 

Sand; gray, quartz-rich, angular silty. Some clear feldspar + quartz; possibly some minor 

black glass (obsidian?). Significant Franciscan 

Complex/Great Valley sequence component. No 

obvious tephra. 

200-210 (61-64 

m) 

Sand; gray silty sand, same 190-200’. No obvious tephra; includes clear + some minute 

rounded brown glass(?) "globules." 

210-220 (64-67 

m) 

Silt; gray. Appears to have very fine tuffaceous(?) matrix and 

minor very fine glassy globules + abundant clear to 

milky, subhedral to subrounded feldspar and quartz, 

mixed with Mesozoic-derived clastics including schist, 

and serpentinite. 

220-230 (67-70 

m) 

Silty sand. Mostly quartz-feldspar, angular to subangular grains, 

with mafic rock fragments (gabbro-diabase). 

230-240 (70-73 

m) 

Sand; gritty, medium to coarse grained; 

like 220-230'. 

Mostly quartz + feldspar, angular to subangular, with 

rounded, grit to small pebble-sized clasts of bull quartz, 

veined hard argillite, minor gabro-diabase, granitic 

quartz-feldspar, and fine-grained rhyolite-dacite 

(Tertiary volcanic?) and possible chromite/picotite. 

240-250 (73-76 

m) 

Same as 230-240'. 

250-260 (76-79 

m) 

Pebbly grit (< 0.75cm). Rounded, with clasts largely of bull quartz, veined dark 

grey black argillite, red chalcedonic, felsite volcanic, 

and red Franciscan-type radiolarian chert. 

260-270 m (79-

82 m) 

Same as 250-260'. Rounded clasts up to 1 cm dominate; prominent red 

chert pebble. 

270-280 (82-85 

m) 

Same as 260-270'. 

280-290 (85-88 

m) 

Pebbly to gritty gravel (< 0.5 cm). Well rounded, polished clasts, largely of hard, quartz 

veined argillite, bull quartz; mostly of Franciscan 

melange derivation. 

290-300 (88-91 

m) 

Pebble gravel (< 1 cm). Well rounded as in 280-290', largely Mesozoic-derived 

clasts, spilite/metabasilt, but also including greywacke, 

porphy, diabase, quartz, quartzite, quartz-veined 

argillite, and red radiolarian chert. 

300-310 (91-94 

m) 

Pebble gravel (< 1 cm); same as 290-

300'. 

Same well rounded Mesozoic-derived clasts and some 

aggregate clasts of coarse rounded sand/grit cemented 

by possible altered fine-grained tuff. 

310-320 (94-98 

m) 

Pebble gravel (< 1 cm); same as 300-

310'. 

Rounded clasts include hard silica cemented chert, 

felsite-clast-gritty sandstone, red radiolarian chert, and 

composite lumps of quartzose sand with altered tuff 

matrix (observed one definite pumice shard in 

matrix/interclast area of one sandstone lump). 

320-330 (98-

101 m) 

Shell fragments (> 80%+) with sporadic 

pebbles (< 0.5 cm). 

Includes sporadic pebbles of bull quartz + a few lumps 

of lithic quartz-rich sand. 

330-340 (101-

104 m) 

Shell fragments > 85% +. One or two broken pebble fragments. 

340-350 (104-

107 m) 

Mixed thin-shell hash (~70% +) with 

sand + grit. 

Secondary pyritic coatings locally on shell material. 

350-360 (107-

110 m) 

Shell hash (~60%), + sand grit. In sand/grit matrix. 

360-370 (110-

113 m) 

Shell hash, in silty sandy grit; ~90%. Silty, sandy matrix. 

370-380 (113-

116 m) 

Shell hash in gritty, rounded sandstoneclast matrix; ~75% Gritty, rounded, sandstone matrix. 

380-390 (116- Silty-sand with bivalve/gastropod hash. Foraminifer picked (slide slot #19). 

28
119 m) 

390-400 (119-

122 m) 

Pebbly (< 1.3 cm) silt with admixed shell 

hash. 

Matrix is silty with pebbles. 

400-410 (122-

125 m) 

Same as 390-400' One possible green glauconite pellet(?) observed. 

410-420 (125-

128 m) 

Same as 400-410' Includes some lumps of what look like altered grey ash. 

420-430 (128-

131 m) 

Silty-sand; fine-grained with ~75% fossil 

shell material. 

Silty fine-grained-sand matrix. 

430-440 (131-

134 m) 

Sand; pebbly (< 0.66 cm), fine-grained, 

with shell fragments and minor 

fragments of carbonate-cemented, 

quartzo-feldspathic clasts. 

440-450 (134-

137 m) 

Shell fragments (~50%), in silty matrix 

(~30%) and pebbles (~20%). 

Few round cherty veined argillite pebbles (< 1 cm) 

make up ~20%. 

450-460 (137-

140 m) 

Shell hash with silt matrix; ~same as 

440-450', cements. 

460-470 (140-

143 m) 

~similar to 450-460' 

470-480 (143-

146 m) 

~similar to 460-470'; matrix slighly 

coarser (i.e., silty fine sand). 

480-490 (146-

149 m) 

Sand; bioclastic, fine to coarse, with 

rounded polished grit-sized clasts. 

Milky quartz prominent grit clast component. 

490-500 (149-

152 m) 

Sand; bioclastic (~50%), same as 480-

490', with rounded polished grit to small 

pebble sized clasts (< 0.5 cm). 

Pebble and grit clasts include quartz and bull quartz. 

500-510 (152-

155 m) 

Sand; bioclastic (~40%), pebbly to gritty. 

Similar to 490-500'. 

A few rounded green pellet-like (glauconite?) grains. 

510-520 (155-

158 m) 

Sand; bioclastic, pebbly to gritty; same 

as 500-510'. 

520-530 (158-

162 m) 

Silty fine sand; quartzo-feldspathic with 

~ 20% shell fragments. 

530-540 (162-

165 m) 

Silt to fine sand; ~ same as 520-530'. Silt to fine sand with a few scattered polished rounded 

grit to < 0.5 cm pebbles, including red chert. 

540-550 (165-

168 m) 

Silty sand; fine to medium grained, 

bioclastic,; ~same as 520-530'. 

550-560 (168-

171 m) 

~same as 540-550'. 

560-570 (171-

174 m) 

~same as 550-560'. 

570-580 (174-

177 m) 

~same as 560-570'. A few clear, euhedral, feldspar grains + altered 

yellowish brown composite tuff grains; any glass gone 

to clay. 

580-590 (177-

180 m) 

Silty sand; bioclastic; ~same as 560-570'. 

590-600 (180-

183 m) 

Silty sand; cemented bioclastic. 

600-610 (183-

186 m) 

Silty bioclastic bed. Agglutinated foraminifers? 

610-620 (186-

189 m) 

Bioclastic bed with minor silt matrix and 

a few, < 0.3 cm, hard, rounded, milkybull quartz, and cherty argillite clasts. 

620-630 (189-

192 m) 

Silt; bioclastic with angular quartzofeldspathic matrix. Matrix silt locally with clumps of CO3 cemented 

sandstone and shell. 

630-640 (192-

195 m) 

Silt; with ~ 20% shell fragments. Silty sediment, angular, quartzo-feldspathic, with sandsized rounded polished, hard black grains, and milky 

29
bull quartz. 

640-650 (195-

198 m) 

Silt; with minor (~15%) shell fragments. Silt, possibly has ash component? 

650-660 (198-

201 m) 

Silt; with slight oxidation to orange-grey. Possible ashy component to silt(?) with a few black (<

0.5%) scoria-like grains. 

660-670 (201-

204 m) 

Pebbly silt; like 650-660'. Possible ashy, pebbly, silt, not as oxidized as above. 

Includes rounded quartz + cherty veined argillite 

pebbles < 1 cm diameter. 

670-680 (204-

207 m) 

Grit; well sorted (clast supported?) 

rounded/polished. 

Includes clasts of spilitic metabasalt, hard recrystallized 

quartose-meta-sandstone, bull quartz, and cherty veined 

argillite. 

680-690 (207-

210 m) 

"Pea gravel;" well sorted with pebbles <

~0.6 cm; clast supported. 

Clasts rounded and polished, including red metachert, 

black cherty argillite; same suite as 670-680'. 

690-700 (210-

213 nm 

~same as 680-690'. 

700-710 (213-

216 m) 

~same as 690-700'. Clasts include metaquartzite and vein quartz. 

710-720 (216-

219 m) 

Pebbly-gravel; ~same as 700-710'; 

includes fine silty sand matrix. 

720-730 (219-

223 m) 

Pea gravel; pebbly to gritty, rounded, and 

polished clasts ~0.25 to 0.75 cm; ~ same 

suite as 680-690'. 

730-740 (223-

226 m) 

Pea gravel; coarse, rounded, 0.6-1.0 cm, 

with minor fine silt as interclast matrix; 

otherwise same as 720-730'. 

Silt locally with pyrite clusters; pebble grit clasts locally

with common CO3 (calcite?) cement. 

740-750 (226-

229 m) 

Pea gravel; ~ < 1 cm rounded clasts. No silt observed, possibly washed out(?). 

750-760 (229-

232 m) 

Pea gravel; coarse, rounded, ~ 0.3 - 1.2 

cm clasts; otherwise ~ same as 740-750'.

760-770 (232-

235 m) 

Silty gravel; clasts ~ 0.3 to 1.0 cm, in 

silty matrix (matrix supported?). 

Pebble clasts include chert + quartzite and ~ same clast 

suite as 750-760’. 

770-780 (235-

238 m) 

~ same as at 760-770. 

780-790 (238-

241 m) 

Pea gravel; silty. ~ same as 770-780. 

790-800 (241-

244 m) 

~ same as 780-790'. 

800-810 (244-

247 m) 

Sand; silt to fine grained with minor 

sporadic rounded pebbles (0.3-1.0 cm). 

Possibly drifted down-hole(?). 

810-820 (247-

250 m) 

Sand; bioclastic, silt to fine sand, with 

sporadic broken pebbles. 

Includes questionable tuff component in fine grained 

silt. 

820-830 (250-

253 m) 

Sand; silty, fine sand, with abundant to 

common bioclastics; ~same as 810-820', 

but no pebbles and fewer shell 

fragments. 

830-840TD 

(253-256 m) 

Silt; fine sandy silt; ~same as 820-830', 

but with slightly greater silt content and 

fewer shell fragments. 

30
Lithologic logs of washed ditch samples from Sonoma County Water Agency- Sebastopol 

Road pilot well located at 38.4085°N, 122.7995°W (NAD27), along the Petaluma and Santa 

Rosa railroad line about midway between Llano and the Sebastopol Grange (just north of 

Gravenstein), Sebastopol 7.5’ Quadrangle, Sonoma County, California. 

Depth (ft) Lithology Notes 

20-30 (6-9 m) Clay; with rare subrounded 0.5 cm 

pebbles, compact, gray. 

Non-calcareous; possibly derived from ash/bentonite; 

appears to include areas of white chunks of fine grained, 

undevitrified, tephra, some with feldspar 

microphenocrysts. Includes rare subrounded 0.5 cm 

pebbles of meta-sandstone and altered mafic rock (silica 

carbonate?). 

30-40 (9-12 m) Pebble-gravel; rounded, 0.5-2.0 cm. Multi-lithologic, with abundant clasts of rounded, banded 

to non-banded, porphyritic, white rhyolite-dacite, rare red 

(baked?) siliceous volcanic clasts (derived from Miocene 

andPliocene volcanics rocks), lithic metasandstone 

(Franciscan Complex/Great Valley sequence), + minor 

chunks of clayey matrix like at 20-30'. 

40-50 (12-15 m) Pebble gravel; rounded to subrounded, 

like at 30-40', but coarser (< 2.5 cm), and 

with more clayey matrix. 

Large subrounded clasts up to 2.5 cm of hard porphyritic, 

gray to white rhyolite/rhyodacite (probably from Tertiary 

volcanic section). Also includes hard welded rhyodacite, 

(also from Tertiary volcanic section - affinity to 

Zammaroni-Warrington Road section?); hard, rounded, 

medium grained, lithic arkosic wacke; fine grained cherty, 

dark grey, argillite; one tan rounded clast of possible 

porcellanite (no foraminifers), or fine-grained banded 

rhyolite; includes chunks of possibly tuffaceous matrix 

clay. Like 20-30' (down hole sluff or in place(?). 

50-60 (15-18 m) Clay; brownish gray, compact; similar to 

clay at 20-30' but more brown. 

May be derived from devitrified ash?? 

60-70 (18-21 m) Grit; rounded to subrounded. Composed of mixed Mesozoic- igneous rocks, argillite, 

and rounded white rhyolite/dacite. 

70-80 (21-34 m) Grit; rounded ~same as 60-70'. Includes rounded white rhyodacite clasts. 

80-90 (24-27 m) Grit; rounded to subangular, otherwise 

~same as 70-80'. 

Includes much subrounded white rhyodacite clasts + 

flaggy rhyolitc (or diatomitic?) shale. 

90-100 (27-30 

m) 

Pebbly clay; brownish grey. Pebbles (~1.0 cm) of banded chalcedony, veined 

rhyodacite, green spilite and milky quartz grit; clay may be 

very fine grained (devitrified?) ash. 

100-110 (30-34 

m) 

Pebbly clay; ~same as 90-100'. Pebbles < 1.3cm; clay may be fine grained devitrified ash. 

Pebbles of rounded, white rhyodacite, hard lithic metasandstone, vein quartz, meta-graywacke, and argillite. 

110-120 (34-37 

m) 

Pebbly clay; ~same as 100-110’. Subrounded to subangular pebbles up to ~1.2 cm include 

greenstone, meta-gabbro, quartzite, and argillite; clay 

somewhat sandy to gritty, may be ashy, like above(?). 

Fine, euhedral, clear feldspar and clear, angular, quartz in 

matrix clay. 

120-130 (37-40 

m) 

Pebbly clay; ~same as 110-120', but with 

only 1 large pebble ~2.0 cm. 

Pebble subangular, hard, grey, recrystalized felsite 

porphyry (probably old, reworked); possibly sluff downhole. Clay with locally prominent(?) white, ash "chunk." 

Clay matrix with fine, angular to subrounded, aggregates 

to single grains of clear, fresh quartz or feldspar? Clay is 

probably devitrified fine grained ash, though can't see 

shards? 

130-140 (40-43 

m) 

Pebbly clay; ~same as at 120-130 . Clay appears ashy as above, with clear euhedral -subhedral 

quartz-feldspar grains. 

31
140-150 (43-46 

m) 

Clay; muddy, grey; same unit as 130-

140', but no pebbles and very fine silt 

component. 

Some clay pieces look graded from mud to clay; chunks of 

questionable fine-white ash present. 

150-160 (46-49 

m) 

Mud; clayey to silty, ~same as at 140-

150'; higher component of silt and fine 

sand than above. 

Approximate base of clay unit; includes definite higher 

component of silt to fine sand than above; observed one 

0.5 cm round pebble of milky bull quartz in clayey (ashy?) 

mud. 

160-170 (49-52 

m) 

Sand; fine-grained, well-sorted, 

dominantly round-subrounded quartz-rich 

sand (dune?). 

Sand dominantly well sorted, rounded to subrounded; 

quartz rich; with bull quartz, black argillite, chert lithics. 

170-180 (52-55 

m) 

~same as 160-170'. 

180-190 (55-58 

m) 

~same as 170-180'. 

190-200 (58-61 

m) 

~same as 180-190. Base of well sorted unit. 

200-210 (61-64 

m) 

Silt; ~same composition as 190-200', but 

less sorted and with mud component. 

Distinctly different hydrodynamic unit from above unit. 

210-220 (64-67 

m) 

Silt; ~same as 200-210'. Matrix with clayey component, possibly tuffaceous, but if 

so ash is completely altered to clay. 

220-230 (67-70 

m) 

Sand; medium to fine grained, 

moderately sorted, rounded to 

subrounded, quartz-rich, multimodal. 

Multimodal sand, with lithics from Franciscan 

Complex/Great Valley sequence/ophiolite sources (black 

chert/argillite,serpentinite, greenstone, mafic igneous, bull 

quartz, etc.). Euhedral quartz + feldspar relatively clear, 

probably from Tertiary volcanics. 

230-240 (70-73 

m) 

~same as at 220-230'. 

240-250 (73-76 

m) 

Sand; Poorly sorted, silty to mediumgrained; same unit as at 230-240', but 

poorer sorting. 

Picotite (or ilmenite?) grains noted in fine-grained 

population. 

250-260 (76-79 

m) 

Sandy-silt; greenish orange, probable top 

of new sedimentation unit; unsorted, with 

weathered, altered clayey matrix 

component. 

Coloration change due to groundwater/fluids? 

260-270 (79-82 

m) 

Silt; quartz-rich; subangular-subround, 

moderately sorted; ~same as at 250-260'. 

270-280 (82-85 

m) 

~same as at 260-270'. 

280-290 (85-88 

m) 

Sand; silty, fine grained, moderately 

sorted, subangular-subround; ~same as 

280-290'. 

290-320 (88-98 

m) 

~ 280-290'. 

320-330 (98-

101 m) 

Sand; silty fine grained; ~same as 290-

320'. 

330-340 (101-

104 m) 

Sand; fine grained, moderately well 

sorted, subangular-subrounded, quartzrich, lithic; ~ 320-330'. 

(dune or beach?). 

340-350 (104-

107 m) 

Sand; fine to medium grain, well sorted; 

same as 330-340'. 

Sporadic subhedral grains of clear feldspar. 

350-360 (107-

110 m) 

Sand; fine-medium grain, well sorted; 

~same as 340-350'. 

No fines (dune sand?). 

360-370 (110-

113 m) 

Sand; medium grain, lithic, quartzofeldspathic, poorly sorted, subangular. New sedimentation unit? 

370-380 (113-

1`16 m) 

Sand; poorly sorted, lithic, subangular, 

medium grained. 

32
380-390 (116-

119 m) 

Sand with grit; lithic, bimodal, with 

medium grained, subrounded to 

subangular, quartzofeldspathic sand 

matrix with rounded grit, 0.25-0.3 cm. 

Grit clasts include bull quartz and dark veined argillite 

390-400 (119-

122 m) 

Sand with grit; ~same as 380-390', with 

small pebbles, < 0.5 cm. 

400-410 (122-

125 m) 

Sand with pebbles and grit; fine to coarse 

grained, poorly sorted, angular to 

subrounded, sand, with sporadic, 

rounded, pebbles and grit. 

Pebbles and grit Franciscan-derived, with cherty-veined 

argillite, bull quartz. 

410-420 (125-

128 m) 

Sand with pebbles and grit; ~same as at 

400-410. 

420-430 (128-

131 m) 

Sand with grit to pebbles; medium 

grained, moderately sorted, rounded to 

angular; ~same as 410-420', but slightly 

coarser. 

430-440 (131-

134 m) 

Sand with grit and pebbles; rounded to 

subangular, coarse to medium, sand, with 

subrounded grit to fine pebble (< .5 cm) 

clasts; ~same as 420-430'. 

440-450 (134-

137 m) 

Pea-gravel; rounded, sandy, clasts < 0.6 

cm); ~same as at 430-440', but 

significantly more pebbles (~50%). 

Beach? 

450-460 (137-

140 m) 

Pea-gravel; ~same as 440-450', but 

pebbles < 0.3 cm. 

Probable beach. 

460-470 (140-

143 m) 

Sandy gravel; ~ same as 440-450'. 

470-480 (143-

146 m) 

Pea gravel; rounded to subrounded, ~0.5 

to 1 cm, with minor fine sand matrix and 

minor abraded angular shell fragments. 

Clasts include veined argillite, bull quartz, red chert, other 

Franciscan-derived clasts. 

480-490 (146-

149 m) 

Shell fragments; ~80% with ~ 20% pea to 

grit-sized rounded clasts of same clast 

suite as seen in 470-480'. 

Top of new bioclastic unit 

490-500 (149-

152 m) 

Pea gravel; Bioclastic, ~50% bivalves and 

pebbles, < 1 cm; ~50% rounded ~0.3-1.0 

cm. 

500-510 (152-

155 m) 

Gravel; sandy, bioclastic, ~50% angular 

shell fragments, ~30% fine-medium sand; 

~20% rounded pebbles; as in 490-500'. 

510-520 (155-

158 m) 

Gravel; ~same as 500-510'; somewhat 

less sand. 

520-530 (158-

162 m) 

Sand; bioclastic, fine grained, subrounded 

to subangular, shell fragments up to ~0.5 

cm, with < 1% rounded grit to ~0.3 cm. 

Rounded grit to 0.3 cm largely includes black argillite/vein 

quartz, (Franciscan Complex/Great Valley sequencederived); note pyritic /sulfide coatings on many shell 

fragments. 

530-540 (162-

165 m) 

Sand; bioclastic, same as 520-530', but 

somewhat more sand matrix. 

Some black spherule-shaped < 1 mm-size grains? 

540-550 (165-

168 m) 

Sand; bioclastic; ~ same as 530-540'. Includeds well rounded, 1 cm, chert (green-gray), also 1 

grain of rare black (~.25-.3mm) glass (obsidian?). 

550-560 (158-

161 m) 

Sand; bioclastic, fine grained, same as at 

540-550', but sand is coarser and includes 

clasts to ~1 cm of subrounded, fine 

grained, banded chert (with probable 

diatoms/foraminifers), resembles 

Monterey Formation chert. 

Sand matrix is fine grained (silty), coarser than above, 

moderately well sorted, quartz-rich, subangular to 

subrounded. 

560-570 (171- Sand; abundant granule-sized rounded 

33
174 m) and polished clasts; sandy matrix < 50%; 

included are uncommon but distinct 

banded porcellaneous chert (Monterey 

Formation-like) clasts. Base of bioclastic 

zone? 

570-580 (174-

177 m) 

“Pea gravel”; granule-sized, polished, 

well rounded, well sorted, with matrix 

largely lacking (winnowed). 

Polished, rounded granule clasts include quartzite, bull 

quartz, black argillite/chert. 

580-590 (177-

180 m) 

“Pea gravel”; some fine sand matrix (< 

10%); largely clast supported. Base of 

same unit as 570-580'. 

590-600 (180-

183 m) 

Granule to grit gravel; without matrix 

(winnowed); granules rounded, well 

sorted finer grained (3-5mm). 

Grains largely dark chert/argillite, bull quartz, quartzite, 

derived from Mesozoic source? Good aquifer? 

600-610 (183-

186 m) 

Granule to grit gravel; but average clast 

size somewhat coarser (~4-6 mm), 

winnowed, matrix lacking, well rounded 

and polished. ~ same unit as at 590-600',.

Same Mesozoic source for clasts as above; probably good 

aquifer? 

610-620 (186-

189 m) 

Granule to grit gravel; ~ same unit as 

600-610'. 

Well rounded and polished granule gravel. Aquifer? 

620-630 (189-

192 m) 

Granule to grit gravel; same as 610-620'. 

Base of unit? 

Granule size range ~3-6 mm; one angular clast of banded 

travertine; either shell fragment or vein material? Aquifer 

base? 

630-640 (192-

195 m) 

Granule to grit; less well sorted than 

above, no fines (winnowed), well 

rounded and polished grains. Separate 

unit? 

A few composite grains of cemented coarse sand grit. 

Matrix of these composite grains is fine grained and white, 

possibly ash? Unit is otherwise like 620-630' 

640-650 (195-

198 m) 

Pea gravel; with clasts ~4-10mm; 

lithologies ~same as above. Top of unit. 

Includes 1 clast (rounded) of fine-grained soft, grey 

probable ash (picked, in vial) 

650-670 (198-

207 m) 

Pea gravel; same unit as at 640-650'. 

670-680 (204-

207 m) 

Pea gravel; same unit as 650-670’, with < 

2% fine sand matrix (largely clast 

supported), and same subrounded quartzveined, red radiolarian chert. 

680-690 (207-

210 m) 

Pea gravel; same unit as 670-680'. 

690-700 (210-

213 m) 

Sand/silt with granule to pebble-size 

clasts, same lithology as 680-690'; with 

~20-30% unsorted fine grained sand/silt 

matrix. Rounded, polished ~3-12 mm 

granules to pebbles. New unit? 

Clasts derived from Franciscan Complex/Great Valley 

sequence/Mesozoic sources. 

700-710 (213-

216 m) 

Silt/sand with granule to pebble size 

clasts; same as 690-700' 

710-720 (216-

219 m) 

Pebbly gravel; polished clasts to ~3-11 

mm, with less silty sand than in 700-710'. 

Base of unit? 

720-730 (219-

223 m) 

Gravel; silty (matrix supported), rounded; 

includes subangular to subrounded, hard, 

banded fine-grained welded tuff and (or) 

porcellaneous (diatomaceous?) 

mudstones (Monterey Formation?) + 

some fine grained tephra-like aggregates 

with fresh, clear feldspars and quartz. 

Fine grained tephra like aggregates with clear fresh 

euhedral feldspar and quartz. 

730-740 (223-

226 m) 

Pebble gravel; rounded, polished, silty 

pebble from 0.8-1.0 cm, with matrix of 

Pebbles of black chert, argillite, and bull quartz. 

34
angular to subangular, fine silt and sand, 

and clear equant, euhedral feldspar and 

quartz grains. ~Same unit as at 720-730'.

740-750 (226-

229 m) 

Pebble gravel; same as 730-740'. includes angular fine, euhedral, clear quartz, feldspar sand 

with possible fine tephra ? Component 

750-760 (229-

232 m) 

Silty gravel; same as at 740-750', but 

probably more silty (~20% silt matrix of 

subrounded to angular, quartzofeldspathic, fine sand). 

Pebble and granule clasts include rounded bull quartz, 

hard aphanitic rhyolite, porcellanite (Monterey Formation 

or freshwater diatomite?). 

760-770 (232-

235 m) 

Silty granule-pebble gravel; more silty 

than at 750-760'; definite bimodality. 

Pebbles/granules ~0.5-1.0 cm; well 

rounded, polished. Base of unit. 

Pebbles and granules include prominent black argillite, 

chert. 

770-780 (235-

238 m) 

Pebbly silt; clast suite same as above unit.Clast suite includes milky vein quartz, banded 

porcellanite/chert (Monterey Formation-like); silt matrix 

with much fine, clear, quartz and feldspar, commonly 

subhedral to euhedral and equant. 

780-790 (238-

241 m) 

Pebbly silt and silty mudstone; same as at 

770-780', but more silt and mud and 

clasts to 1cm as above. 

Clasts include some angular fragments (disrupted thin 

fissile layer?) of thin-bedded fissile diatomite. 

790-800 (241-

244 m) 

Pebbly silt; same as at 780-790'. Base of 

unit. 

Silt includes clear, equant subangular euhedral feldspar. 

Pebbles include prominent banded porcellanite/chert 

(Monterey Formation-like). 

800-810 (244-

247 m) 

Granule sand; poorly sorted to unsorted 

bioclastic, with fine silty to muddy 

matrix. 

Granules include Monterey Formation-like buff banded 

porcellanite, Franciscan-derived fine-grained schist, bull 

quartz, serpentinite. 

810-820 (247-

250 m) 

Granule sand; poorly sorted, silty, same 

as 800-810'. 

Same clast suite as above 

820-830 (250-

253 m) 

Gritty silt and fine sand, with ~1-2% shell 

fragments; grit sized sand clasts are 

rounded to subrounded, and polished. 

Silt and fine sand is angular to 

subrounded, with common clear quartz 

and feldspar. 

Unit probably is nearshore marine. 

830-840 (253-

256 m) 

Gritty silt and fine sand, with < 1% shell 

fragments; granule/grit sized clasts. Same 

as 820-830' 

Granule/grit sized sediment clasts largely Mesozoic 

derived + gray andesite/dacite porphyry (Tertiary), that are 

generally rounded but not highly polished. 

840-850 (256-

259 m) 

Gritty silt and fine sand, with < 1% shell 

fragments; fine silt matrix with 

questionable ash component. Same as 

830-840'. 

850-860 (259-

262 m) 

Gritty silt and fine sand, with < 1% shell 

fragments. Same as at 840-850'. 

Rounded andesitic/dacitic granule clasts common. 

860-870 (262-

265 m) 

Gritty silt and fine sand, with < 1% shell 

fragments; with some chips of muddy 

siltstone up to ~ 1 cm. Granules, largely 

of rounded hard dark argillite; black 

chert, are up to ~ 4 mm diameter. Same 

as 850-860'. 

Unit probably marine shelf or estuarine, based on shell 

fragments and texture. 

870-880 (265-

268 m) 

Gritty silt and fine sand, with < 1% shell 

fragments; with an ~ 1 cm diameter 

subrounded clast of banded dolomitic 

sandstone. Same as at 860-870'. 

880-890 (268-

271 m) 

Gritty silt and fine sand; but with 5-10% 

bioclastic angular, abraded and 

concentrated shell fragments; top of new 

Well rounded bull quartz, black chert/argillite clasts < 5 

mm; and fine-medium-grained angular to subangular 

quartz-feldspar sand matrix; estuarine or shelf? 

35
unit. Same as at 870-880'. 

890-900 (271-

274 m) 

Gritty silt and fine sand; base of 

concentrated bioclastics. Same as at 880-

890'. 

900-910 (274-

277 m) 

Granule-grit-rich silty fine sand; 

unsorted, with rare (< 0.5%) abraded 

shell material. 

Local clots of fine silt and sand with questionable ashy? 

cement. 

910-920 (277-

280 m) 

Granule-grit-rich silty fine sand; with 

clots/aggregates of fine silty sand; base of 

unit. Same as at 900-910'. 

Aggregates of fine silty sand with white ashy cementing or 

intergranular material? 

920-930 (280-

283 m) 

Sand; fine grained, moderately sorted 

quartzo-feldspathic subangular to 

subrounded. 

Probably marine? 

930-940 (283-

287 m) 

Sand; but with rare, grit-sized, rounded 

black chert/argillite. Same as at 920-930'.

940-950 (287-

290 m) 

Sand; with rare, rounded to subrounded, 

white pordcellanite grains (Monterey 

Formation-like). Same as at 930-940'. 

950-960 (290-

293 m) 

Sand; same as 940-950'. 

960-970 (293-

296 m) 

Sand; with sporadic (< 2%) granule and 

grit-sized subrounded to round grains. 

Also includes sporadic "chunks" (< 0.7 

cm) of mud with probable tephra 

component. Same as 950-960'. 

970-980 (296-

299 m) 

Sand; same as at 960-970'. 

980-990 (299-

302 m) 

Sand; but includes 1.5 cm long hard 

Franciscan-like meta-graywacke clast 

fragment; common fine "clumps" of 

ashy(?) cemented fine sand/silt. Includes 

rare angular fragments of light brown 

porcellanite with probable planktic 

foraminifers. Same as at 970-980'. 

990-1000 (302-

305 m) 

Sand; except includes large pebble 

fragments (> 1.2-2.0 cm) of rounded, 

dark gray, dacite porphyry, and the same 

clumps of ashy, grey, cemented sand/silt; 

also includes thin fragments of silicified 

wood with cellular texture (i.e., palmlike?) or possibly same as foraminiferal 

porcellanite fragments as at 980-990'(?). 

Same as 980-990'. 

1000-1010 

(305-308 m) 

Sand; with large pebble fragments, large 

angular fragments of very fine-grained 

ashy silty sand, and fragments of 

silicified petrified wood 2-3 cm in length. 

Same as at 990-1000'. 

Angular pebble fragments include porpyritic, white to 

grey, rhyolite (Tertiary source?), and angular fragments of 

very fine-grained, ashy(?), silty sand 

1010-1020 

(308-311 m) 

Sand; with prominent angular chunks of 

vitric, dacite porphyry (probably 

Tertiary), and a rounded piece of a bull 

quartz pebble (~8 mm); sililcified wood 

fragments largely absent. ~same unit as 

1000-1010’. 

1020-1030 

(311-314 m) 

Granule-silt; unsorted, with subangular to 

angular quartzo-feldspathic-rich matrix. 

Quartz-feldspar includes much relatively "fresh"-looking 

grains (i.e., first-cycle, probably from siliceous volcanic 

36
source); granule-size (< 5 mm) clasts of round to 

subrounded bull quartz, dark chert and argillite derived 

from Franciscan Complex/Great Valley sequence/ophiolite 

sources; and fragments of white rhyodacitic rock in silt to 

fine sand range. 

1030-1040 

(314-317 m) 

Pebbly granule-silt; with rounded -

subrounded pebble/granule clasts up to 

~0.7 cm. Approximate base of unit. 

Same as 1020-1030’. 

1040-1050 

(317-320 m) 

Pebbly to granule-bearing mud; similar to 

above unit, but dominantly mud matrix. 

Mud may be ashy or bentonitic(?). Includes rounded 

pebble of quartzite and some white aggregates/clumps of 

coarser, probable siliceous ash/tuff. 

1050-1060 

(320-323 m) 

Mud; with rounded granules of same 

lithologies as above. 

1060-1070 TD 

(323-326 m) 

Bentonitic?/ashy? mud; same as above. Estuarine? 

37