A summary of the ground water geology of Sonoma County along with a discussion of the basic principles of geology and hydrology were presented in Chapter II of this bulletin. Contained in this appendix is a detailed discussion of the geologic features of the county as they pertain to ground water. Included is information on the geologic history; the physical, water-quality, and water- yielding characteristics of the various nonwater-bearing and water- bearing formations; and the effect of geologic structures on the movement and quality of ground water.
The oldest rocks exposed in Sonoma County occur at Bodega Head, west of the San Andreas Fault. The granitic rocks found here represent the core of an ancient landmass that once stretched southward and included what is now the Point Reyes Peninsula, the Farallon Islands, and the granitic mountains of Santa Cruz and Monterey Counties. These intrusive rocks were formed at great depth when they crystallized during the early part of the Jurassic Period. Subsequent uplift brought them to the surface, and erosion has reduced them to the isolated remnants seen today.
East of the San Andreas Fault are found somewhat contemporaneous marine sediments, which were formed as geosynclinal deposits in an oceanic environment during the Jurassic Period. These deposits, which together comprise the Franciscan Formation and the Great Valley Sequence, are formed of sediments derived from an area located west of the present shoreline. The area which received these sediments was of broad extent and was slowly subsiding.
During the 50 million years that the sediments were being deposited, they were intruded with basic and ultrabasic rocks in the form of dikes, sills, and pillow lava flows, most of which subsequently have been completely serpentinized. During the Creataceous Period, deposition was halted and was followed by a brief period of uplift. During this uplift, coarse conglomeritic detritus was deposited in certain areas, while in others, sand and clay continued to mantle the sea bottom. In a few areas, notably along what is now the present sea coast, basaltic flows poured out from localized volcanic centers.
In the early part of the Tertiary Period, marine deposition again was the rule. Paleocene and Eocene sediments, ranging from sand to clay, were deposited in areas to the west and east of the San Andreas Fault. During Miocene time, the sea invaded the area, and shales and sands of the Neroly and related formations were deposited. Also in Miocene time, there were several periods of transgression and regression of the sea as the land surface oscillated above and below sea level. Toward the close of the Miocene Epoch, crustal compressional forces began forming the Coast Ranges.
Beginning in the early part of the Pliocene Epoch, a volcanic sequence was deposited in the south-central part of the county. After a subsequent period of erosion, deposition began anew, forming the sediments which now constitute the Petaluma Formation. The depositional environment of the Petaluma Formation consisted of shallow to brackish water embayments which had been eroded into the landscape formed from the Jura-Cretaceous and later rocks. Following deposition of the Petaluma beds, a period of orogeny occurred, which tilted, folded, and uplifted these beds while still young. This caused a widespread period of vigorous erosion, during which much of the then-soft Petaluma sediments were stripped off. By that time, much of eastern Sonoma County was above sea level, and here and there were lakes filled with water, which supported large communities of diatoms. At about this same time, movement began along the Tolay Fault, which caused a displacement of several thousand feet of formerly adjacent sediments.
During the latter part of the Pliocene Epoch, volcanic activity broke out in the eastern part of the county. Vents spewed out vast amounts of tuff-breccia, lava, ash, and cinders, all of which now constitute the Sonoma Volcanics. Meanwhile, to the west, in a shallow marine environment, sands and clays were being deposited to form the Merced Formation. This latter formation was deposited in an embayment that covered the central part of the county as far north as Healdsburg. At the close of the Sonoma volcanic episode, reworked volcanic detritus began to be deposited under lagoonal and deltaic conditions; these sediments now are the Glen Ellen Formation. Sediments in the lower part of the Glen Ellen Formation were deposited contemporaneously with those of the Merced Formation as shown by the interfingering of these two formations.
In the northwestern part of the county there was a broad, shallow depression with a northwesterly orientation. Deposition of sandy material in this depression occurred contemporaneously with Merced- Glen Ellen deposition farther south; sediments in this depression now comprise the Ohlson Ranch Formation. Subsequent uplift of this basin brought the Ohlson Ranch Formation to the elevation of the ridgetops where much of it was removed by erosion, leaving only the remnants seen today.
Uplift of the Ohlson Ranch sediments occurred at about the same time as uplift and faulting of the Merced and Glen Ellen sediments. By the close of the Pliocene Epoch, the Kenwood and other synclines had been formed, and the structural feature that now is Sonoma Valley began to take shape. To the west, however, were still rolling hills leading down to the sea, and no evidence of the Santa Rosa Plain as yet could be seen.
During the Pleistocene Epoch, uplift and erosion continued, with much of the Ohlson Ranch, Merced, and Glen Ellen Formations being stripped off. Present drainage courses were established and Older Alluvium was deposited in many areas. Downwarping along the Windsor Syncline in the Santa Rosa Plain area caused the creation of a broad valley which sloped southward from Dry Creek all the way to what is now San Pablo Bay.
In the late part of the Pleistocene Epoch, sea level stood some 300 feet (91 m) lower than it does today. Streams draining the highlands cut deep valleys as they meandered southward toward the Golden Gate. Local uplift in the vicinity of the Washoe Anticline subsequently created a gentle arch in the vicinity of Penngrove. This formed the present drainage patterns and turned the Russian River westward to the sea. Since the beginning of the Holocene Epoch, Mark West Creek has changed its course slightly. This event has been coupled with rapid aggradation along the creek and minor subsidence along Laguna de Santa Rosa to the south, all of which helped to create the swamp and marsh condition found today along Laguna de Santa Rosa from near Sebastopol downstream toward Occidental Road.
Nearly all geologic formations in Sonoma County yield some degree of water to wells. Well yields range from 1,000 8pm (3,780 1/m) in wells completed in coarse-grained Holocene deposits, to less than 1 8pm (3.78 1/m) in wells in the Jura-Cretaceous and Tertiary marine sediments. In general, the Jura-Cretaceous and Tertiary marine sediments, along with the granitic rocks and serpentine, yield less than 5 8pm (19 1/m). Mineral constituents, such as chloride, iron, manganese, and boron may be present in sufficient amounts to make ground water nonusable. In contrast, the Pliocene to Holocene materials are the principal water producers in the county; the water derived from these materials usually is of good to excellent quality, although some water quality problems also may be present.
Each of the various geologic formations occurring in Sonoma County is discussed below. Included in the discussion is a description of its general Ethology, its water-yielding characteristics, and the general character of ground water produced.
An area of intrusive granitic rocks is exposed along the western side of Bodega Head. Named the Bodega diorite by Johnson (1943), the rocks consist of deeply weathered, sheared, and faulted hornblende-biotitie-quartz diorite. Pegmatite, aplite, and lamprophyre dikes occasionally are present; some of the rock mass exhibits gneissose banding. Johnson correlated the Bodega diorite with similar rocks occurring in the Santa Lucia and Gabilan ranges to the south; it is considered to be of preFranciscan age.
A spring-fed pond is located at the abandoned site of the Bodega Nuclear Power Plant. The pond is formed from ground water entering the foundation excavation area. The water level in the pond stands about 20 feet (6 meters) above sea level, and there is a constant outflow from the pond into Bodega Bay of from 10 to 20 8pm (38 to 76 1/m). The water in this pond is an acceptable quality sodium chloride water.
There are no known wells in the granitic rocks of Bodega Head. Wells drilled into this rock mass may be able to produce limited quantities of potable ground water from fractures, shears, and deeply weathered zones.
General Character. Much of the mountainous area in the northwestern and northeastern parts of the county is underlain by an assemblage of marine sediments with a stratigraphic thickness of at least 40,000 feet (12,000 meters) that has been identified as part of the Franciscan Formation and the Great Valley Sequence. Lesser exposures of these rocks also occur in the southwestern and southeastern parts of the county.
In the north coastal area, west of the San Andreas Fault, the rocks of this group have been identified by Huffman (1972) as being of Cretaceous age and have been named the Stewarts Point Strata and the Anchor Bay Strata. These rocks are massive sandstone, conglomerate, and mudstone, all of marine origin. Underlying the Stewarts Point Strata near Black Point is a complexly faulted mass of spilitic basalt.
The rocks east of the San Andreas Fault have been divided into two main groups, the Franciscan Formation and the Great Valley Sequence. Rocks of the Franciscan Formation predominate throughout most of the area. These rocks are of Jura-Cretaceous age and consist of three main rock types. The sedimentary sequence
consists of interbedded graywacke and shale with minor amounts of greenstone, conglomerate, chert, and limestone. Much of the rock is highly shattered and commonly is veined with zeiolite minerals. The metamorphic sequence contains metagraywacke with lesser amounts of weakly metamorphosed greenstone and chert; some glaucophane schist also is present.
The sheared sequence includes sheared sandstone and shale with discrete masses of serpentine and other rock types such as ultramafic, silicacarbonate rock, chert, greenstone, pillow lava, metabasalt, glaucophane schist, eclogite, and amphibolite. All of these rock types are intensely folded and faulted; zones of shearing and crushing are common. Rocks of the Great Valley Sequence are composed of well-bedded sandstone, shale, siltstone, and conglomerate. Included near the base of the Great Valley Sequence are masses of pillow lava, basalt breccia, diabase, gabbro, quartz diorite, and ultramafic rocks.
Water Quality. Only meager data are available on the quality of ground water contained in the fracture and shear zones of the Franciscan Formation and Great Valley Sequence. Excellent quality water is found at a number of cold springs which issue from these rocks. Thermal areas, such as The Geysers and Skaggs Springs, yield hot to boiling water with compositions ranging from highly mineralized sodium bicarbonate water to unpotable magnesium sulfate and ammonium sulfate water.
Water quality data are available for only one well tapping the rocks of the Franciscan Formation. This well Number 4N/7W-8R80, is located south of Petaluma and is 854 feet (260 meters) deep. The well produces an excellent quality sodium bicarbonate water that has a total hardness of only 5 mg/1. The water is used for both domestic purposes and dairy operations.
Well Yield. Ground water is present in the Franciscan and Great Valley Sequence rocks as indicated by the great number of springs in the areas of outcrop (see Figure 18). Ground water is not present in primary openings, as with the water-bearing materials, but rather in secondary openings such as joints, fractures, and shear zones. Wells drilled in these rocks frequently are completed as "hard rock" wells; that is, they usually are uncaged. Well yields generally are low and range from less than 1 to at most 3 8pm (<4 to 12 1/m). These meager yields, however, may be sufficient for domestic purposes provided that water storage facilities of at least 1,000 gallons (3.78 (m3) are available.
Well log data are available from 27 wells drilled into the JuraCretaceous rocks. These wells range in depth from 20 to 257 feet (6 to 78 meters); the range of yield of water is from 0.2 to 688pm (0.7 to 257 1/m), with the average being 18.48pm (70 1/m). Static water levels ranged from 2 feet to 160 feet (0.6 to 48.7 meters); one well was reported as flowing. An indication of the ability of a "hard rock" well to yield water is its discharge per unit of saturated rock. For wells in the Jura-Cretaceous rocks, this value ranged from 0.01 to 1.5 8pm per foot (O.1 to 18 1/m per meter); the average was 0. 22 8pm per foot ( 2.7 1/m per meter).
Elongate masses of serpentine and related ultramafic rocks occur within the outcrop area of the Franciscan Formation. Major areas of this rock type have been identified on Plate 1. The rock areas so identified consist of blocks of greenish-black serpentinized periodotite enclosed in a bluish-green matrix of sheared serpentine. The weathered surface of these masses commonly is reddish-brown due to concentrations of iron oxides. Serpentine is not usually considered a reliable source of potable ground water. Mineral analyses are available from two springs in an area of serpentine (see Table 15) . One spring yields a highly alkaline unpotable calcium hydroxide water; the other yields a magnesium bicarbonate water of acceptable potability.
Although usually mapped as part of the Great Valley Sequence, the Dry Creek Conglomerate is considered a separate unit for ground water studies because of its ability to transmit and yield appreciable quantities of good-quality ground water to wells. The conglomerate is situated in the fold of the Geyserville Syncline which trends northwesterly from east of Fitch Mountain through Lytton to Pritchett Peaks, a distance of about 18 miles ( 29 kilometers). The surface exposure of the conglomerate ranges from one to two miles (1.6 to 3.2 kilometers) in width. The conglomerate consists of beds up to 100 feet (30 meters) thick of well-rounded cobbles and boulders of granodiorite, porphyry, chert, quartz, and greenstone; the matrix is an arkosic sandstone. The conglomerate is extremely massive; Gealey (1950 estimated its thickness at 5,ooo feet (1,500 meters). Exposures of a similar conglomerate have been reported to the southeast in Napa County.
Water Quality. Ground water in the Dry Creek conglomerate is an excellent quality calcium bicarbonate water. Table 2 presents a summary of the quality characteristics of ground water in the
Dry Creek conglomerate. No data have been gathered concerning
the presence of any deleterious mineral constituents in ground water in this conglomerate.
Well Yield. Wells in the Dry Creek Conglomerate yield sufficient water for most domestic purposes. Data are available for 11 wells tapping this conglomerate. The wells, situated mostly near the intersection of Canyon Road and Walling Road, range in depth from 50 to 341 feet (15 to 104 meters). After completion, depths to standing water ranged from 22 feet (6.7 meters) in a 50-foot (15- meter) well to 145 feet (44 meters) in the 341-foot (104-meter) well. All wells were tested by the well driller with a bailer; reported yields ranged from 20 to 60 8pm (76 to 227 1/m) with drawdowns ranging from 15 to 105 feet (4.5 to 32 meters).
General Character. Two areas of Tertiary marine sediments occur in Sonoma County. West of the San Andreas Fault, near Fort Ross and south of the Gualala River, marine sediments of early Tertiary age are exposed. These sediments overlie Cretaceous sediments and have been identified by Blake, et al, (1971) as two distinct units. Paleocene to middle Eocene strata near Fort Ross have been called the Strata of German Rancho; these rocks consist of thin to thick interbeds of sandstone and mudstone. Near the Gualala River, Miocene marine sediments, consisting of mudstone, siltstone, and glauconitic sandstone, have been tentatively classed by Huffman (1972) as being part of the Gallaway Formation. Associated with these latter beds are two small areas of basalt.
In the upper part of Nuns Canyon, near the eastern border of Sonoma County, Weaver (1949) described an exposure of Tertiary marine sediments and assigned them to the Neroly Formation. The beds are of Miocene age and are composed principally of northwesterly dipping medium "rained tan sandstone containing casts of marine pelecypods.
Water Quality. Ground water in the Tertiary marine sediments along the coast is a moderately hard calcium bicarbonate water suitable for most domestic purposes. There are no water quality data available from the Tertiary marine sediments of the Nuns Canyon area.
Well Yield. Yield data are available from eight wells drilled into the Tertiary marine sediments north of Fort Bragg; no wells have been drilled in the Tertiary marine sediments of the Nuns Canyon area.
Yields from the wells along the coast range from a low of 0.2 8pm (0.4 1/m) to a high of 37 8pm (140 1/m); the median yield is 5.5 8pm (21 1/m). Static water levels range in depth from 7 to 33 feet (2 to 13 meters) and average 22 feet (6.7 meters). Using the criteria of well yield per foot of saturated thickness, the average value for the Tertiary rocks is 0.104 8pm per foot (1.3 1/m per meter) of saturated thickness; the range is from 0.001 to 0.33 8pm per foot (0.01 to 4 1/m per meter).
General Character. The Petaluma Formation is exposed at various localities in Sonoma County, from Sears Point northward nearly to Santa Rosa. The formation consists of folded continental and brackish water deposits of clay, shale, sandstone, with lesser amounts of conglomerate and nodular limestone; occasional thick beds of diatomite are present. Work done by Dickerson (1922) indicates that the formation is of upper Miocene age. According to Morse and Bailey (1935), the Petaluma Formation is more than 4,000 feet (1,220 meters) in stratigraphic thickness. Weaver (1949) measured a section of the formation near Lakeville and found a total thickness of 1,059 feet (323 meters). The section contained 70 percent clay shale, 25 percent sandstone, and 5 percent pebbly conglomerate. A 94-foot (29-meter) thick section was measured by Cardwell (1958) near Waugh School on Corona Road. Medium-grained, thinly cross-bedded, friable sandstone accounted for 48 percent of the total. The remainder was silty clay with large nodules of limestone.
The area of exposure of the Petaluma Formation from the vicinity of Lakeville southeast to Sears Point has long been stratigraphically indefinite. Morse and Bailey (1935) originally mapped these beds as the "Petaluma Beds" and assigned them to the Pliocene, on the basis of ostracod fossils and leaf impressions. Subsequent work by Weaver (1949) and Cardwell (1958), placed these beds in the younger Merced Formation. Fox, et al (1973), Sims, et al (1973), and Blake, et al (1974) remapped much of this area and placed these sediments again in the Petaluma Formation. In the current investigation, the Petaluma Formation has been defined as being contemporaneous in part with the Merced Formation.
The upper part of the Petaluma Formation apparently interfingers with sediments of the Merced Formation, thus causing some of the confusion in identifying areas of the Petaluma Formation. Logs of water wells analyzed during this current investigation indicated that a number of wells east of the Denman Flat area reported "blue sandstone with shells" in areas previously identified as Petaluma Formation. Because
fossiliferous sandstone is diagnostic of the Merced Formation, these areas have been assigned to that latter formation.
Water Quality. Ground water in the Petaluma Formation ranges from sodium bicarbonate to sodium chloride and calcium chloride in composition. Water analyses indicate that wells in the area of outcrop will yield water with a range of electrical conductivity from 840 to 1,400 micromhos; chloride ion ranges from 78 to 177 mg/1. Deep wells underlying Petaluma Valley tap beds of the Petaluma Formation. Here, wells yield a calcium-chloride water; electrical conductivities are 800 to 900 micromhos and chloride contents are 175 to 200 mg/1. In all cases, boron concentrations are 0.2 mg/1 or less.
Well Yield. The Petaluma Formation is noted for its low well yield. Bailer test data are available from 42 wells tapping the formation. These wells yielded from as low as 5 8pm (19 1/m) to as much as 300 8pm (1,134 1/m). Drawdowns were reported to be as much as 200 feet (61 meters) at one well which yielded only 10 8pm (38 1/m). One 739-foot (225-meter) well in the Bennett Valley area penetrates the Petaluma Formation; the results of a 72-hour pump test are available for this well. Using the method developed by Brown (1963) and utilized in the study of the ground water resources of Livermore Valley, Ford and Hills (1974), a transmissivity of 3,754 gpd/ft (46.54 m3/day) was derived for the Petaluma Formation.
The Sonoma Volcanics were named by Weaver (1949) for a thick sequence of volcanic ejecta and related volcanic sediments that are exposed in the Sonoma Mountains. Weaver identified related volcanic materials, also assigned to the Sonoma Volcanics,, occurring in the Mayacmas Mountains and the mountains separating Sonoma Valley from Napa Valley. Cardwell (1958) extended the volcanic sequence to include isolated volcanic exposures to the west of the Santa Rosa Plain.
Included with the Sonoma Volcanics,, but differentiated on Plate 1, is a sequence of volcanic sediments which have been differentiated by Fox, et al (1973). Also differentiated on Plate 1 are exposures of the St. Helena Rhyolite,, which first was described by Osmont (1905) and further identified by Weaver (1949). In the area of Healdsburg and Alexander Valley much of the area now shown as Sonoma Volcanics originally was considered as the Sonoma Group by Gealey (1951). Later work by Blake, et al (1971) placed these materials within the Sonoma
Volcanics. Exposures of the Sonoma Volcanics in the northeastern corner of the county have been identified by McLaughlin (1974) as the Caldwell Pines Basalt and the Cobb Mountain Rhyolites.
The Sonoma Volcanics comprise a great thickness of mixed volcanic materials consisting of flows, dikes, plugs, and beds of andesite, rhyolite, basalt, tuff breccia, agglomerate, tuff, and related intermediate to acidic flow rocks. Banded flows of welded tuff, perlite, and obsidian occur locally. Some obsidian zones are up to 10 feet (3 meters) in thickness and range from glassy to prophyritic. Volcanic ejecta comprise some 60 percent of the total mass, with the remainder being composed of a variety of volcanic- related sediments such as black volcanic sandstone, ashy clay, tuffaceous sandstone, and diatomite. It is this latter, the diatomite, which allowed for the dating of a part of the Sonoma Volcanics. Axelrod (1944) studied samples of the Sonoma diatomite and identified it as being middle to late Pliocene in age, on the basis of plant fossils contained therein.
The Sonoma Volcanics accumulated in a basin that was some 30 miles (48 kilometers) wide in an east-west direction and 40 miles (64 kilometers) long from north to south. With a maximum thickness of well over 1,000 feet (300 meters), the volcanics cover an area of about 350 square miles (91 square kilometers). The volcanics usually overlie the older JuraCretaceous sediments with a pronounced unconformity. Certain parts of the volcanics interfinger with partly contemporaneous beds of the Petaluma, Merced, and Glen Ellen Formations. In some areas the volcanics unconformably overlie or are in fault contact with the Petaluma Formation.
Lower portions of the Sonoma Volcanics are strongly deformed as a result of intense folding and faulting. This condition and the extreme lateral variability of the flows make it nearly impossible to trace flows and beds over any great lateral distance. According to Huffman (1971), upper portions of the Sonoma Volcanics are but little deformed and occur as gently sloping flows of basalt and andesite.
In the Sonoma Valley area, Kunkel and Upson (1960) reported a great number of thick flows of tuff breccia containing blocks of andesite up to 4 feet (1.2 meters) across contained in a matrix of fine-grained ash. Also noted were locally abundant beds of red scoria having high permeability. In contrast to the andesitic nature of the Sonoma Volcanics found elsewhere, the volcanics in Alexander Valley are composed of basaltic flows and related material. Many of the basalt flows are up to 100 feet (30 meters) in thickness; pillow structure is common.
Water Quality. Ground water in the Sonoma Volcanics usually is a satisfactory quality sodium bicarbonate water. Boron concentrations of up to 1.0 mg/1 have been reported. Because of a higher than usual geothermal gradient, some ground water from deep wells in the volcanics is warmer than that found at equal depth in other formations. The unusual gradient illustrated by the water from Well 7N/7W-32G1, which is 403 feet (023 meters) deep and produces water with a temperature of 74°F (23 C), a temperature somewhat warmer than that of usual ground water.
Well Yield. The productivity of water wells drilled into the Sonoma Volcanics is highly variable and unpredictable. In some areas a driller might complete a well producing adequate quantities of water for domestic use, while only a short distance away a nonproducer, or dry hole, had previously been drilled. In general, successful wells drilled into the volcanics should yield from 10 to 50 8pm (38 to 189 1/m) and drawdowns should be on the order of from 10 to 120 feet (3 to 37 meters). Because of the large expected drawdowns and the fact that standing water may be as deep as 200 to 300 feet (60 to 90 meters), domestic wells ranging in depth to 500 feet (150 meters) are not uncommon.
Three examples illustrate the nature of the Sonoma Volcanics as a water-producing unit. The first is a well on Mountain Home Ranch Road, near the Napa County line. The well originally was drilled to a depth of 200 feet (60 meters) in "yellow sandstone, blue sandstone, blue clay, shattered rock, and hard rock". When tested with a bailer, the well produced 20 8pm (76 1/m) with a drawdown from standing level of 25 feet (7.6 meters); standing water was at a depth of 40 feet (12 meters). These data indicate that the specific capacity of the well was o.8 8pm per foot (9.9 1/m per meter) of drawdown. Six years after the well was completed, it was deepened to 400 feet (120 meters), passing through 200 feet (60 meters) of "black volcanic rock". The water level in the deepened well was 330 feet (100 meters) and when tested with a bailer, it produced 20 8pm (76 1/m) with a drawdown of 20 feet (6 meters); the specific capacity of the deepened well was 1.0 8pm per foot (12.4 1/m per meter) of drawdown.
A pair of wells also along Mountain Home Ranch Road illustrates the unpredictive nature of the Sonoma Volcanics. One well was drilled to a depth of 241 feet (73 meters) and was perforated from 220 to 240 feet (67 to 73 meters) in "black volcanic rock". When tested, this well yielded 50 8pm (189 1/m) with a drawdown of 5 feet (1.5 meters). The specific capacity of the well was 10.0 8pm per foot (124 1/m per meter) of drawdown, and the standing water level in this well was 180 feet (55 meters). A short distance away, another well was drilled to a depth of
256 feet (78 meters). The perforated interval in this latter well was 156 to 256 feet (48 to 78 meters) in "volcanic conglomerate". On test, the well produced 40 8pm (151 1/m) with a drawdown of 35 feet (11 meters); the specific capacity was 1.1 8pm per foot (13.6 1/m per meter) of drawdown. The depth to standing water in this well was reported to be 165 feet (50 meters).
Finding water in the Sonoma Volcanics is not always possible. This is illustrated by the records from four test wells drilled near St. Helena Road. The first test well was drilled onequarter mile (0.4 km) from an existing well and went to a total depth of 841 feet (256 meters). The log indicated a succession of "hard blue rock, cemented gravel with volcanics, multicolored rock, white volcanic ash with pebbles, and hard rock". When tested with a bailer, the well yielded but 2 8pm (7.6 1/m) with a reported drawdown of 650 feet (198 meters); the specific capacity was 0.004 8pm per foot (0.05 1/m per meter) of drawdown. Three other test wells then were drilled on the same property to depths of 270, 225, and 190 feet (82, 69, and 58 meters), all in sequences of "sandstone, broken rock, and blue clay". Being dry holes, all were abandoned.
The water-yielding characteristics of the Sonoma Volcanics in the area of the Sonoma Mountains is similar to that in the mountainous area to the east. A well was drilled at Jack London State Historical Monument to a depth of 136 feet (41 meters). The well log indicated "red rock, fractured gray basalt, and red and black clay". Tested for 24 hours, the well yielded 315 8pm (1,190 1/m) with a drawdown of 12 feet (3.6 meters); the depth to standing water was 28 feet (8.5 meters). The specific capacity of this well was 26.2 8pm per foot (324.9 1/m per meter) of drawdown. In contrast, a deep municipal well was drilled several miles to the south. This 1,005-foot (306-meter) well intercepted "black rock, multi-colored rock, black and red rock, green rock, and conglomerate". When tested, it yielded 60 8pm (227 1/m) with a drawdown of 720 feet (219 meters). The specific capacity was 0.075 8pm per foot (0.9 1/m per meter) of drawdown. Standing water in this well was at a depth of 80 feet (24 meters).
General Character. The Ohlson Ranch Formation has been described by Higgins (1960) as consisting of marine sandstone, siltstone, and conglomerate up to 160 feet (49 meters) thick similar in appearance to the Merced Formation. The Ohlson Ranch Formation occurs only in the northwestern part of Sonoma County in the vicinity of the town of Annapolis. Here, resting atop ridges composed of dipping marine strata of the Franciscan Formation,
flat-lying beds of Pliocene age fluvial sediments occur. Waveterraces and sea stacks buried beneath the formation indicate that it was formed in a shallow-water embayment. Since deposition, the shallow basin has been uplifted and dissected by erosion, leaving the isolated areas as they are today. It is not known if the sediments of the Ohlson Ranch Formation are a northwest extension of the similar sediments of the Merced Formation to the south, but it is assumed that the two formations are contemporaneous.
Water Quality. Water quality data are available from two wells in the Annapolis area. These wells yield an excellent quality sodium bicarbonate water.
Well Yield. Well yield data are available from five well tapping the Ohlson Ranch Formation in the vicinity of Annapolis. Yields from these wells range from 2 to 36 8pm (7.6 to 136 1/m), with drawdowns ranging from 30 to 125 feet (9 to 38 meters).
General Character. The Merced Formation is one of the principal water-producing formations in Sonoma County. The formation consists of massive beds of fine to very fine-grained sandstone which is exposed over a broad area extending from Petaluma, on the south, to the Russian River, and from the west edge of the Santa Rosa Plain westward to beyond Occidental. Exposures of pebble conglomerate and siltstone in the area east of Cloverdale have also been included in the Merced Formation, although the exact stratigraphic relationship of this latter unit is not clear. In the subsurface, the Merced Formation has been identified at depth beneath the Santa Rosa Plain as well as beneath a cover of younger alluvium in Petaluma Valley.
The color of the Merced sandstone ranges from red, to orange, to white in exposed sections and from blue to gray in the subsurface where the beds have been under reducing conditions since deposition. Many well drillers report "clam shells" and "oysters" when drilling in the Merced Formation, indicating the wells have penetrated one of the numerous fossiliferous zones known to exist throughout the formation. Paleontological studies reported by Cardwell (1959) show that most shells belong to five reported species of pelecypods and four of gastropods. So abundant are many of the shell beds that they resemble coquina.
Much of the sandstone is loose and poorly cemented, although some beds, principally the more fossiliferous ones, are cemented to some degree with calcium carbonate and iron
oxide. Near the base of the formation there is a bed of white tuffaceous material about 10 feet (3 meters) thick. This bed is exposed near the western edge of the outcrop area where it can be seen as white patches on the hillsides. Interbedded with the beds of the Merced Formation are several beds of tuff breccia, one of which attains a thickness of 10 feet (3 meters). Whether these tuff breccia flows represent distal ends of flows from the Sonoma Volcanics or whether they are from some local source is not known. Johnson (1934) found a volcanic neck northeast of Bodega and suggested that as a possible source. Travis (1952), however, stated that there is no evidence to support this view.
The Merced Formation is of late Pliocene age and was deposited in a subsiding embayment that was open to the ocean. Cardwell (1959) has postulated that the Merced sediments were derived from older Franciscan rocks to the north and were brought southward by a major trunk stream to be deposited in a lagoonal environment that was protected from the ocean by an off-shore bar. The sediments were deposited on a surface of high relief carved into the underlying Franciscan sediments. Occasional outliers of Franciscan rocks seen today surrounded by Merced sediments represent former islands that were partially buried during Merced sedimentation.
The Merced Formation has been estimated by Cardwell ( 1959) as being not over 2,000 feet (600 meters) thick; however, Travis (1952) estimated the total thickness of the Merced as being only 500 feet (150 meters). Well log data developed during the present study suggest that the Merced is at least 1,000 feet (300 meters) thick.
Water Quality. Ground water in the Merced Formation is of excellent quality and varies from calcium bicarbonate and magnesium bicarbonate to sodium bicarbonate in composition. Typical conductivities range from 140 to 420 micromhos. Wells tapping unoxidized (blue) sandstone may yield water containing excessive amounts of iron and manganese.
Well Yield. The Merced Formation produces large quantities of ground water. The specific yield of the formation ranges from 10 to 20 percent, an unusually high value. This high specific yield is due to the preponderance of even-grained sand found in wells to depths of over 400 feet (120 meters). Yields of wells tapping this formation frequently produce from 20 to 1,000 8pm (76 to 3,780 1/m); drawdowns are minimal, usually from 10 to 150 feet (3 to 45 meters). Domestic wells perforated for only a short distance produce adequate yields for household use, even if wells are located on adjacent lots and lot size is minimal. Deep wells, usually irrigation or municipal, typically are gravel-packed.
Specific capacities of wells, based on bailer tests, indicate that the Merced sands yield about 0.1 to 5.0 8pm per foot (1.2 to 62 1/m per meter) of drawdown. For example, one well drilled along Liberty Road west of Petaluma had a total depth of 163 feet (49 meters). Of this depth, 160 feet (48 meters) was logged as "yellow sand, blue sand, sandstone ledges, and streaks of shells". Tested with a bailer, the well yielded 30 8pm (113 l/m) with a drawdown of 110 feet (34 meters); the standing water level was at a depth of 40 feet (12 meters). These data indicate a specific capacity of 0.27 8pm per foot (3.3 1/m per meter) of drawdown. Farther north, a 385-foot (117-meter) domestic well was drilled on Baker Lane, near Sebastopol. The first 3 feet (0.9 meters) was reported to be "topsoil"; the remaining depth of the well was reported as "sand, yellow sandstone, and blue sandstone". Blank casing was installed in the well to a depth of 270 feet (82 meters). Tested with a bailer, the well produced 16 8pm (60 l/m), with a 30-foot (9- meter) drawdown. The depth to standing water was reported to be 50 feet (15 meters). These data indicate that the specific capacity of the well was 0.53 8pm per foot (6.6 1/m per meter) of drawdown.
Reported standing water levels ranged from 35 to 60 feet (ll to 18 meters). Statements from well owners in the area indicate that water levels decline markedly during the summer months and many wells go dry by early fall. On the basis of an approximate areal extent of 8,ooo acres (3,200 hectares) and an average saturated thickness of 50 feet (15 meters), the Ohlson Ranch Formation has an estimated maximum storage capacity of about 25,000 acre-feet (30 hm3). This total probably is significantly less when water levels have declined to their lowest levels.
General Character. The Glen Ellen Formation is of Plio-Pleistocene age and was first described by Weaver (1949) from outcroppings of poorly sorted clays, sands, gravels, and cobbles occurring near Glen Ellen in the upper part of Sonoma Valley. Not always recognized as a separate formation, the Glen Ellen Formation also has been identified as the "Fresh-Water Merced" by Johnson (1934), the upper part of the "Sonoma Group" by Gealey (1951), and as "Older Alluvium" by Travis (1952). Later work by Cardwell (1958), Kunkel and Upson (1960), and Cardwell (1965) fully defined the formation and its mapped area to its present limit. Exposures of the Glen Ellen Formation, as now mapped, extend from near Sonoma, on the south, through the central part of the Santa Rosa Plain, to Alexander Valley and Dry Creek Valley on the north.
The Glen Ellen Formation is composed of an extremely heterogeneous mixture of pale buff clay, silt, sand, and gravel; some
lignite has been noted. Many beds grade laterally from coarse gravels into clay. The coarse materials are usually of andesitic composition, although some obsidian is present. Particle size ranges up to 6 inches (15 centimeters) in diameter. Near the town of Glen Ellen, a section of the Glen Ellen Formation was measured by Cardwell (1958). The section had a total thickness of 68 feet (21 meters); 18 feet (5 meters) of section was composed of fine to coarse-grained cross-bedded sandstone and conglomerate, the remainder being siltstone with lenses of coarse sand and pebbles. Beds of coarse pebble conglomerate occur in the Rincon Valley area. These beds dip nearly virtically and are believed by Cardwell (1958) to cause artesian conditions in wells located in Township 7 North, Range 7 East, Sections 8 and 9.
The Glen Ellen Formation is up to 3,000 feet (900 meters) thick. It has been deposited in several parallel troughs as a deposit of coalescing piedmont and valley alluvial fans; some clayey portions were deposited in a lagoonal environment. Much of the Glen Ellen overlies the Sonoma Volcanics with some degree of unconformity. At a few localities it is intercalated with volcanic materials belonging to the Sonoma Volcanics.. Likewise, much of the Glen Ellen is known to unconformably overlie sediments of the Merced Formation. Some beds of the continental Glen Ellen, however, are interfingered with beds of the marine Merced Formation. In a few areas, beds of the Glen Ellen directly overlie nonwater-bearing rocks of the Franciscan Group. In the lower Sonoma Valley area, the sediments of the Glen Ellen Formation are believed by Kunkel and Upson (1960) to grade laterally into beds of the contemporaneous Huichica Formation.
Water Quality. Ground water in the Glen Ellen Formation has a greater range of character than any other formation in Sonoma County. Some of the best and some of the poorest quality water is obtained from this formation. Wells generally 100 feet (30 meters) deep yield a magnesium-bicarbonate water of moderately good quality; unusually high content of nitrate ion may be present. Wells up to 800 feet (243 meters) in depth yield a moderately good quality sodium bicarbonate water. Very deep wells, such as those greater than 1,000 feet (300 meters), yield a poorer quality sodium bicarbonate water.
At scattered localities throughout the formation, boron concentrations of up to 1.0 mg/l have been reported, as has water containing over 90 percent sodium.
Well Yield. The Glen Ellen Formation is highly variable in its water-yielding capability. In the Santa Rosa Plain area, wells tapping this formation generally yield adequate supplies
for domestic use, stock watering, or limited irrigation. Yields usually range from 15 to 30 8pm (57 to 113 1/m), with drawdowns of about 10 to 50 feet (3 to 15 meters). Specific capacities based on bailer tests range from 0.5 to 20.0 8pm per foot (6 to 248 1/m per meter) of drawdown..
The highly variable nature of the formation is indicated by yield data from two wells in Section 12, Township 7 North, Range 7 West. One well near Piner Road produced 40 8pm (151 1/m) with a 2-foot (0.6-meter) drawdown. The standing water level in this 102-foot (31-meter) well was reported to be 10 feet (3 meters). The well log indicated a total of 17 feet (5 meters) of "large gravel and sand", with the remainder being "sandy clay, blue clay, and gray clay".
A short distance west on Willowside Road, a well of 128-foot (39- meter) depth was drilled. On test, this well produced 20 8pm (76 1/m) with a 71-foot (22-meter) drawdown. The standing water level was 13 feet (3.9 meters). The log of the well indicated 3 feet (1 meter) of "brown sand with small gravel" with the remainder being "brown clay, blue clay, and blue sandy clay with gravel".
Near the town of Glen Ellen, the formation yields somewhat less water than in the Santa Rosa Plain. Here again, however, yields are unpredictable. For example, along Henno Road, a well was drilled to 380 feet (116 meters) in alternating zones of "cemented gravel, yellow clay, brown shale, and clay gravel". Tested with a bailer, the well produced 20 8pm (76 1/m), with a drawdown of 16 feet (4.8 meters); standing water was recorded at a depth of 49 feet (15 meters). Also along Henno Road, three more holes were drilled to 23, 97, and 150 feet (7, 30, and 46 meters); all were dry. On the same property, three more holes were drilled to 82, 200, and 322 feet (25, 61, and 98 meters). Each was tested and found to produce only 1 8pm (3.8 1/m); each was subsequently abandoned.
To the north, in Kenwood and Rincon Valleys, yields from the Glen Ellen Formation are not much better. One 280-foot (85meter) well drilled on Fairway Court yielded only 30 8pm (113 1/m), with a drawdown of 210 feet (64 meters); standing water was at a depth of 30 feet (9 meters). The well log indicated that a successive sequence of clay and cemented gravels was intercepted. Most successful wells drilled in the Glen Ellen Formation in the Rincon Valley, Kenwood Valley, and Valley of the Moon areas tap the underlying, more prolific water-bearing materials of the Sonoma Volcanics.
A great thickness of Glen Ellen materials occurs in the Windsor area. Here, well yields range from 15 to 40 8pm (57 to 151 1/m) for most wells. Drawdowns are less than 50 feet (15 meters)
with depths to water usually being less than 100 feet (30 meters). The highly variable nature of the water-yielding characteristics of the materials is illustrated by the productivity of two nearby gravel-packed wells in the vicinity of Piner Road and Gerhard Drive. One well is 122 feet (37 meters) deep and when tested with a bailer, yielded 30 8pm (113 1/m), with a 65-foot (20-meter) drawdown. Its specific capacity was 0.47 8pm per foot (5.8 1/m per meter) of drawdown. The well log indicated that the well penetrated a succession of blue clay, brown clay, and blue sandy clay, with streaks of gravel in the perforated interval. The other well is 102 feet (31 meters) deep and is perforated from 42 feet (12.8 meters) to bottom. The log indicates that in the perforated interval the well penetrated 49 feet (15 meters) of brown sandy clay, gray clay, and blue clay, and also 10 feet (3 meters) of large gravel and blue sand. Tested with a bailer, the well yielded 40 8pm (151 1/m, with a drawdown of 2 feet (o.6 meter). The specific capacity of this well is 20.0 8pm per foot (248 1/m per meter) of drawdown. Standing water in this well was reported as being 10 feet (3 meters).
Deeper wells in the Windsor area are capable of yielding upwards of 500 8pm (1,890 1/m), as shown by the log of a 400-foot (122- meter) well drilled near Hembree Lane. The well intercepted 78 feet (24 meters) of blue sand and gravel. A 72-hour pump test was made on the well, and it produced 500 8pm (1,890 1/m), with a drawdown of 72 feet (21.9 meters). The specific capacity was found to be 3.7 8pm per foot (45.8 1/m per meter) of drawdown.
The hills east of Windsor contain a number of areas where sediments of the Glen Ellen Formation are exposed. Wells here, such as along Calistoga Road and Chalk Hill Road, produce from 10 to over 100 8pm (38 to over 378 1/m), with drawdowns ranging from 10 to 200 feet (3 to 60 meters). Specific capacities range from 0.2 to 6.2 8pm per foot (2.5 to 76.9 1/m per meter) of drawdown. One well was drilled near Reible Road to a depth of 135 feet (41 meters). The well penetrated sandstone from a depth of 71 feet (22 meters) to bottom. It was reported that the well had an artesian flow of approximately 80 8pm (302 1/m) at the time of construction.
Well yields from the Glen Ellen Formation in the Alexander Valley area tend to be about the same as at wells tapping this formation in other areas. Yields range from 3 8pm (11 1/m) to over 30 8pm (113 1/m), with drawdowns from 20 to 85 feet (6 to 26 meters); specific capacities range from 0.05 to 1.5 8pm per foot (0.6 to 18.6 1/m per meter) of drawdown.
Along Lytton Springs Road, well yields from the Glen Ellen Formation are extremely variable. The lowest yield was from a 198- foot (60-meter) well, which produced 20 8pm (76 1/m), with a drawdown of 35 feet (11 meters); standing water was at
115 feet (35 meters), and the specific capacity was 0. 57 8pm per foot (7.1 1/m per meter) of drawdown. In contrast, another well drilled along Lytton Springs Road went to only 76 feet (23 meters), of which the interval from 33 to 76 feet (10 to 23 meters) was reported to be "coarse cemented gravels, blue sandy clay, and cemented blue gravels with loose streaks". Tested with a bailer, the well produced 50 8pm (189 1/m) with a drawdown of only 1 foot (0.3 meter); the depth to standing water was reported as 27 feet (8.2 meters).
General Character. The Huichica Formation is of Pleistocene age and was named by Weaver ( 1949) . The formation occurs only in a small area in the far southeastern portion of Sonoma County. The formation is of somewhat greater areal extent in adjacent areas of Napa County, and its type locality as described by Weaver (1949), is along Huichica Creek a short distance east of the Sonoma- Napa County line.
The Huichica Formation consists of deformed continental beds, with a maximum thickness estimated by Kunkel and Upson (1960) to be at least 900 feet ( 275 meters). The beds were deposited as alluvial fans that drained a highland to the north composed of Sonoma Volcanics. Hence, the formation consists of reworked tuff, weathered volcanic clay, silt, and similar materials; particles of pumice are common throughout the basal 200 feet (61 meters) of the formation. The beds are all poorly sorted and cross- bedding is locally abundant.
The Huichica Formation is of Pleistocene age and overlies the truncated beds of the Sonoma Volcanics. To the northwest it interfingers with the beds of the contemporaneous Glen Ellen Formation.
Water Quality. Only one analysis is available from a well tapping the Huichica Formation. This well produces a sodium bicarbonate ground water that contains 230 mg/1 sodium ion. With an electrical conductivity of 1,100 micromhos, the water is classed as having a high salinity and high sodium hazard.
Well Yield. The Huichica Formation is considered to be a poor producer of ground water. According to Kunkel and Upson (1960), some wells tapping the Huichica Formation do not yield sufficient water even for domestic use. Well production data are available from five wells tapping this formation. Yields range from 10 to 100 8pm ( 38 to 378 1/m) with drawdowns ranging from 50 to 250 feet (15 to 76 meters). The specific capacities of these wells
range from 0.04 to 1.1 8pm per foot (0.5 to 13.6 1/m per meter) of drawdown. One well, located near the intersection of Napa Road and State Sign Route 121, was drilled to a depth of 750 feet (229 meters). The log of the well indicates "blue sticky clay, yellow sticky clay, sand and gravel, gray clay, and tule mud" to a depth of 423 feet (129 meters). Below this are "blue shattered rock, red clay, and cemented sand and gravel" to a depth of 680 feet (207 meters), below which is basalt to the bottom of the well. A notation on the bottom of the well log indicates, "Flow began at 423 feet (129 meters), big flow is off the bottom".
Another well was drilled in the Huichica Formation along Ramal Road. This well went to a depth of 500 feet (152 meters) in a zone of brown clay, sand and gravel. Prior to testing, the well was reported to be flowing. Tested for 12 hours, the well produced 50 8pm (189 l/m), with a drawdown of 200 feet (61 meters). A notation on the well log indicates that the ground water in this well contained boron, and the well was capped.
The unconsolidated materials are present in a great diversity of grain size, from coarse-grained stream channel deposits to the finest-grained materials occurring in basin areas. Included with these materials are such diverse deposits as alluvial fans, landslides, and both stream and marine terrace deposits. Each of the unconsolidated materials is briefly described below.
Older Alluvium. Older alluvium occurs in Sonoma Valley, Petaluma Valley, and in various parts of the Santa Rosa Plain. The older alluvium consists of lenticular beds of reddish brown, compacted silty clay, silt, sand, and gravel; hardpan and claypan frequently are present. The older alluvium is of Pleistocene to Holocene age and overlies the Glen Ellen and Huichica Formations. According to Cardwell (1958), the older alluvium may be correlative to a part of the Glen Ellen Formation. Cardwell (1958) reported the older alluvium as being up to 200 feet (61 meters) in thickness in the Santa Rosa-Petaluma area; Kunkel and Upson (1960) believed the older alluvium attained a maximum thickness of 500 feet (152 meters) in Sonoma Valley.
Ground water in the older alluvium is a moderately hard sodiummagnesium-bicarbonate water.
The older alluvium is not a prolific producer of ground water, and wells located on this unit usually produce from underlying materials. For example, a 50-foot (15-meter) well drilled near the intersection of Seventh Street East and East MacArthur
Street, southeast of Sonoma, went through a sequence of gray clay, yellow clay, and cemented gravel. Tested with a bailer the well produced only 0.33 8pm (1.2 1/m). A nearby well, drilled near East Fifth Street and Denmark Street, went to a depth of 310 feet (94 meters). After passing through a sequence of brown, yellow, and blue clay from a depth of 58 to 300 feet (18 to 91 meters), the well entered a 10-foot (3-meter) zone of medium sand. On test, the well produced 30 8pm (113 1/m) with a drawdown of 100 feet (30 meters).
Marine Terraces. Deposits of poorly consolidated silt, sand, and gravel occur as discontinuous terraces along the Sonoma coast from Bodega Bay north to the Gualala River. The deposits are from 25 to 50 feet (7 to 14 meters) in thickness and are of Pleistocene to Holocene age. Marine terraces have been formed at a time that sea level stood higher than at present. In some places, notably in the vicinity of Salt Point and Ocean Cove, several distinct-terrace levels may be seen. Median elevations for the various terraces in this area are 100, 300, 500, 700, and 900 feet (30, 91, 152, 213, and 274 meters). The presence of the uppermost terraces at elevations of 700 and 900 feet (213 and 274 meters) suggests vertical movement since deposition in connection with an emerging coastline.
Near Salt Point, a number of wells have been drilled in the area of the terraces. All of these pass through the terraces and bottom some distance into the underlying Tertiary marine sediments. Water levels in these wells usually are at or near the contact with the underlying rock, suggesting that the terraces are nearly all drained. Farther south, near Bodega Bay, shallow wells produce ground water from the marine terraces. Thronson (1963) estimated that the marine terraces contained only 2,700 acre-feet (3.3 hm3) of ground water in storage. There are no data available concerning the quality of ground water contained in the marine terraces. Ground water contained therein should be potable unless affected by mineralized water derived from underlying sources.
Holocene age The deposits cross-bedded feet (61 meters) thick. others are underlain by The terrace deposits were grade level that at present.
Terrace Deposits. River terrace deposits of Pleistocene to occur adjacent to the Russian River and Dry Creek. are discontinuous and consist of unconsolidated sand and gravel that may be up to 200 Some terraces overlie Franciscan rocks; Sonoma Volcanics and Glen Ellen sediments. were formed when streams were at a higher present. They were formed as alluvial fan or stream channel deposits and have been left isolated as the grade level in streams dropped.
Most terrace deposits may yield adequate quantities of ground water for domestic purposes, except where perched on top of knolls and thus drained. Along Dry Creek, several domestic wells have been completed in the terrace deposits. The wells range from 45 to 70 feet (14 to 21 meters) in depth. Yields range from 10 to 60 8pm (38 to 227 1/m), with reported drawdowns being from 0 to 10 feet ( 3 meters). The standing water level in these wells ranges from 10 to 20 feet (3 to 6 meters) below ground. In this same area, a well was drilled through the terrace deposits and completed in the underlying Glen Ellen Formation. This well yielded 175 8pm (663 1/m), with a drawdown of 40 feet (12 meters); the reported standing water level was 180 feet (54.8 meters).
The quality of ground water in the terrace deposits generally is excellent. The water is a sodium to magnesium-bicarbonate water.
Alluvial Fans. Alluvial fan deposits occur along the eastern side of the Santa Rosa Plain; minor fan areas also occur in Sonoma Valley and Alexander Valley. The alluvial fans are composed of a heterogeneous mixture of unconsolidated, poorly sorted gravel, sand, silt, and clay deposited by active streams draining the adjacent mountainous areas. The fan deposits are of Holocene age and range in thickness from 50 to over 200 feet (14 to over 60 meters).
Yields of wells drilled into the alluvial fan deposits are adequate for most domestic needs. One well drilled along Petaluma Hill Road went to a depth of 150 feet (46 meters) in yellow clay, boulders, and rocks. On test, the well produced 10 8pm (38 1/m), with a drawdown of 60 feet (18 meters). The standing water level was 50 feet (15 meters). Other wells in the vicinity produce somewhat less water due to intercepting less coarse material. At Sonoma State College, wells have been drilled to depths of 400 to 450 feet (122 to 137 meters) in the alluvial fan deposits and the underlying materials. These wells produce from 550 to 600 8pm (2,079 to 2,268 1/m), with drawdowns on the order of 85 feet ( 26 meters).
The water produced from the alluvial fans is hard and is of magnesium bicarbonate composition.
Younger Alluvium. Unconsolidated alluvium, consisting of beds and stringers of gravel, sand, silt, and clay, occurs in all valley areas of Sonoma County. The deposits are of Holocene age and were formed as floodplain deposits from such streams as Sonoma Creek, Santa Rosa Creek, and the Russian River. The alluvium usually ranges from 30 to 150 feet (9 to 45 meters) thick, and is highly variable in composition. According to Cardwell (1958), the alluvium may be up to 300 feet (91 meters) thick along Petaluma Creek.
Well yields are highly variable in the alluvium. In the Petaluma area, yields range from 10 to 40 8pm (38 to 151 l/m). A well drilled in the alluvium along Casa Grande Road went to 83 feet (25 meters) in mixed clay, gravel, and cemented gravel. The well produced 40 8pm (151 l/m), with a drawdown of 50 feet (15 meters). The standing water level was 27 feet (8.2 meters). In contrast, wells in the alluvium in areas along the Russian River, such as in Alexander and Cloverdale Valleys, produce large amounts of water. An example of this is a well drilled in cemented and loose gravels along River Road near Cloverdale. The well is 40 feet (12 meters) deep and produces 140 8pm (529 1/m), with no measurable drawdown. Standing water is at a depth of 14 feet (4.2 meters).
Ground water in the alluvium is generally of excellent quality. In the Petaluma area, Well No. 5N/7W-28A2 produces from the alluvium. The well is 99 feet (30 meters) deep and produces an excellent quality calcium-bicarbonate water. Near Cloverdale, Well No. llN/lQW-17Cl, which is 15 feet (4.5 meters) deep, also produces an excellent quality calcium-bicarbonate water.
Landslides. Many landslides occur in the Franciscan terrain in the northwestern and northeastern part of Sonoma County. The deposits occur on steep slopes and are composed of a heterogeneous mixture of broken rock, clay, and some pebbles, cobbles and boulders. Large blocks of Franciscan rock up to 30 feet (9 meters) across are not uncommon. Only the mayor landslides are shown on Plate 1; smaller slides are not shown. All slides are in some degree of failure, thus they are unstable and serve as poor foundations for roads and structures. Many slides have small ponds occupying the depressions at their heads; springs and seeps commonly occur along the toe areas.
A few wells have been drilled into the larger slide areas northeast of Cloverdale. These wells passed entirely through the slide area and presumably entered stable rock at depths of about 175 to 200 feet (53 to 61 meters). The intervening area was reported as a mixture of fractured rock, brown sandy clay, red clay, and "serpentine clay". One well has a yield of 40 8pm (151 1/m), with no drawdown; the standing water level was at 40 feet (12 meters). Another well yielded 50 8pm (189 1/m) and was reported to be flowing. Although no water quality data are available for wells in landslides, the ground water in the landslides is believed to be an excellent quality sodium- to magnesium-bicarbonate water.
Stream Channel Deposits .Deposits of coarse sand and gravel occur along the Russian River, Dry Creek, and other active streams in Sonoma County. The deposits are highly permeable and range in depth to 100 feet (30 meters). South of Healdsburg,
a well completed in the stream channel deposits produces 350 8pm (1,323 1/m) with a drawdown of 3 feet (1 meter); the specific capacity of this well is 117 8pm per foot (1,323 1/m per meter) . drawdown. According to Cardwell (1965), test wells drilled for the Sonoma County Water Agency near Wohler Bridge ( 8N/9W-29L) showed 53 to 85 feet (16 to 26 meters) of clean river sand and gravel. Tests at these wells indicated that the stream channel deposits had an average transmissivity of 850, ooo gpd per foot (10,540 square meters).
Water quality data for ground water in the stream channel deposits are lacking. However, the water is presumed to be of the same quality as that in the adjacent stream.
Sand Dunes. Deposits of coastal dunes occur west of Bodega Harbor and at scattered localities along the coast north to the Gualala River. The -dunes are composed of two basic types. Younger dunes are formed of loose, eolian sand that is deposited by the ocean and formed into dunes by the winds. Although highly permeable, the dunes contain little ground water. Some salt crystals may be present dispersed throughout the sand. Older dunes -- those with a cover of salt grass and other coastal plants -- occur some distance inland. The dunes have been arrested in their migration; they contain some lenses of silty clay. Ground water contained within these older dunes may be fresh and also may be perched over saline water.
No well yield data are available for wells drilled in the dune areas. Most dune areas apparently contain little usable ground water. Where ground water is present, wells probably produce adequate ground water for domestic use. Water levels in these wells may fluctuate markedly with the seasons, because long, dry periods will drastically lower the potentiometric surface of water in the dunes. The quality of ground water in the sand dune area is affected by adjacent bodies of salt water.
Bay Mud Deposits Bay mud deposits occur in tidal areas of the lower Sonoma Valley and Petaluma Valley; a small area of bay mud also occurs on the west side of Bodega Bay. The bay mud deposits are composed of organic clay, silt, and fine sand; peat and decomposed tules may be present. The deposits have been formed from flocculation of turbid water flowing down the streams draining the adjacent highlands and also by natural deposition on the floor of San Pablo and Bodega Bay. The deposits are up to several hundred feet thick and are underlain by more consolidated deposits of older alluvium, Huichica Formation, Petaluma Formation, and Franciscan rocks. The deposits of bay mud, where exposed on land, range from fairly stiff to soft; after working, they are subject to heaving. On the floor of San Pablo Bay, the bay mud deposits range downward from ooze to soft fetid clay.
The bay mud is not considered a reliable source of potable ground water. Two wells were drilled in the bay mud deposits north of Midshipman Point on San Pablo Bay. One well went to a depth of 30 feet (9 meters) in gumbo and brown sandy gravel. It produced 1 8pm (3.8 1/m) with a drawdown of 19 feet (5.8 meters); the standing water level was 8 feet (2.4 meters). A nearby well was drilled to a depth of 239 feet (73 meters) and was completed in underlying materials. This well produced 40 8pm (151 1/m), with a drawdown of 25 feet (7.6 meters); the standing water was 30 feet (9 meters). The U. S. Navy Skaggs Island facility has several wells situated on bay mud deposits. All are completed in the underlying Huichica and related materials. These wells produce excellent quality sodium-bicarbonate water in contrast to shallow wells in the bay mud, which produce poor quality sodium chloride water.
Structural features in the nonwater-bearing rocks of Sonoma County generally have little effect on movement or quality of ground water. Conversely, features in the water-bearing materials may act as barriers to ground water movement, as controlling features for the direction of ground water movement, and as source areas for certain mineral constituents found in ground water. Each of the two basic types of geologic structures, folds and faults, are briefly discussed below.
Several anticlines in the water-bearing materials are identified on Plate 1. The Washoe Anticline was named by Weaver (1949) for a structure in Merced sediments and Sonoma volcanics along Washoe Creek, west of Cotati. The anticline extends from the Denman Flat area northwesterly toward Gossage Creek, a distance of about 6 miles (10 kilometers). Meacham Hill lies parallel to and immediately west of the axis of the anticline. The limbs of the anticline are undulatory, and it branches into several parallel folds to the north. The anticline forms part of the southwestern boundary of the Santa Rosa ground water basin.
To the southeast is the Adobe Creek Anticline, also named by Weaver (1949). This feature is entirely within the Petaluma Formation and has a northwest-southeast strike parallel to the grain of the regional structure. The axis of this anticline presumably forms part of the northeastern boundary of the Petaluma ground water basin.
There are three synclines in Sonoma County which have some effect on the movement of ground water. The Windsor Syncline, named by Gealey (1950), is responsible for the formation of the
topographic feature called the Santa Rosa Plain. The axis of this syncline runs from the vicinity of Rohnert Park northwesterly through Windsor toward Healdsburg. At the north end the syncline is an asymmetrical downwarp, with gentle dips on the west side and steeper dips coupled with some faulting on the east side. South of Mark West Creek the syncline becomes very complicated on the eastern side, as secondary folding and faulting has taken place. Another downfold in the water-bearing materials is the Kenwood Syncline. This structural feature runs from the vicinity of Glen Ellen northwesterly through Rincon Valley. Sediments of the Glen Ellen Formation are exposed on the flanks of the syncline, with the axial area being composed of Holocene alluvial materials.
In addition to the structural features in the Tertiary and Quaternary materials, there also is a syncline in the older Dry Creek conglomerate which affects ground water movement. This feature is the Geyserville Syncline, named by Gealey (1950). Folding has deformed the Cretaceous beds into a northwest-trending syncline which extends from near Geyserville northwesterly toward Dutcher Creek.
In all cases, anticlines and synclines affect the movement of ground water by inducing ground water movement downslope along the flanks of the features.
There are a great number of faults in Sonoma County, not all of which are shown on Plate 1. A great number of faults occur in the nonwater-bearing rock and because they have little effect on ground water, most are not shown on Plate 1. A few mayor faults are shown because they could be sources of ground water moving laterally along fracture zones. The faults of Sonoma County are of three basic types: lateral, vertical, and thrust. Lateral faults are those which displace beds in a horizontal direction with little or no vertical component of movement. The San Andreas Fault exhibits this type of movement. Vertical faults displace beds in a vertical direction, and thrust faults move older formations over younger ones.
Faults can form barriers to ground water movement if impermeable materials are brought into position opposite permeable, waterbearing rocks. Furthermore, in relatively nonwater-bearing rocks, faults can create shattered zones which could act as conduits for ground water movement. Ground water along some faults may contain slightly higher amounts of certain mineral constituents, such as fluoride and boron.
Return to the Sonoma State University Website