the San Andreas fault. Because of the level of movement and activity along this section of earth,
the geology and soils are a variety of different types mixed together and scattered throughout the
state. The primary soils around Lake Berryessa come from sedimentary rock, which means the soil
textures are most commonly sandy loams and sandy clay loams. Soils that are sandy generally have a
low water-holding capacity, meaning they drain water through quite rapidly, leaving little time for
the vegetation to make use of it. This causes droughty conditions, and thus requires species that
can survive on low water levels within the soil profile, or have the capability to extend deep roots,
reaching far down into the soil profile. Gaining an understanding of the geology and soils can be
useful in understanding the complex functions of the ecosystems in and around Lake Berryessa.
This information can also be useful in making important management decisions regarding the
conservation and preservation of the Lake and the habitat that surrounds it.
Background
Soils
Testing
Quality Projects
Concerns
Opportunities
long-term subduction of an oceanic plate under the Western margin of the North American
craton. The Franciscan complex is composed of three distinguishable belts: the eastern belt (Yolla
Bolly and Pickett Peak terranes), the central belt, and the coastal belt. Age and metamorphic
grade of the belts decreases to the west (Blake and Jones, 1981). Formation of the accretionary
complex began during the late Jurassic in the eastern belt and has continued into the Miocene
along the western coastal belt. The complex trends NNW and is bounded by the San Andreas
Fault to the east and by the coastal range fault to the west. The coast range fault separates the
Franciscan complex with the partly coeval Great Valley sequence. Debate exists over the tectonic
evolution of the Franciscan, centered around the geographic origin of the Franciscan rock units.
Characterization of the Three Belts
The coastal belt of the Franciscan Complex is composed of the youngest and least deformed
units and makes up the western quarter of all Franciscan rocks. The rocks of the coastal belt are
composed of arkosic sandstones, andesitic graywackes, and quartzofeldspathic graywackes
interbedded with radiolarian chert (turbidite deposits) (Blake and Jones, 1981). These
sedimentary rocks suggest a depositional environment of deep-sea fan systems with both oceanic
and continental provenance. Parts of the belt show evidence of later metamorphism, principally
due to subduction. Low-grade blueschist mineral facies are indicated by the presence of minerals
such as laumonite and prehnite-pumpellyite (Blake and Jones, 1981). All rock units show evidence
of thrust (imbricate) faulting due to the compressional forces of subduction. Ages of the coastal
belt run from as little as 40 Ma (Eocene) to as old as 100 Ma (middle Cretaceous).
The central belt of the Franciscan Complex represents older and more metamorphosed units of
rock best characterized as a melange. Blocks of graywacke, greenstone, chert, limestone, and
blueschists are sheared and thrust upon one another in a chaotic mix (Isozaki and Blake, 1994).
In contrast to the coastal belt, metamorphism is higher in grade here and dominated by
pumpellyite which formed within the matrix of graywacke (Hagstrum and Murchey, 1993). The
mixing of these units makes a stratigraphic subdivision difficult but analysis of the graywacke slabs
indicates that the depositional environment was also deep sea, near to the continent. Turbidity
currents in this environment deposited much of the sediment in both the coastal and central
belts. Structurally, the central belt is dominated by gently to moderately east-dipping faults that
again indicate a compression of the belt. Ages for the belt range from late Jurassic to Paleocene
(150 - 60 Ma) (Hagstrum and Murchey, 1993).
The eastern belt is commonly subdivided into the Pickett Peak and Jolla Bolly terranes which are
juxtaposed along an east dipping, low angle fault. Both contain metamorphosed clastic rocks
(quartz-lawsonite-mica schist), metachert (quartz-riebeckite) and metagreenstone along with
smaller amounts of septentinite (Isozaki and Blake, 1994). The Pickett Peak terrane is structurally
higher and of higher metamorphic grade than the Jolla Bolly terrane. It is the easternmost unit of
the Franciscan. The Pickett Peak and Jolla Bolly terranes collectively represent the highest grade
of blueschist (high P, low T) metamorphism seen in the Franciscan complex as well as the oldest
rocks. Structurally, the belt is dominated by imbricate faulting with large imbricate thrust sheets
(nappes) extending from the central belt to the coast ranges fault to the east. Like the other
belts, the eastern belt rocks represent the off-scrapings of an oceanic plate as it collided with the
North American craton.
Tectonics of the Franciscan Terrane
The tectonics of the Franciscan have been a subject of debate for at least the last 35 years (see
Baily, 1964 or Taliferro, 1943) and theories have evolved over time as advances in plate tectonics,
paleomagnetism, radiometric dating, etc have spaned new hypotheses and required modification
of older interpretation. All recent models recognize plate tectonics as the driving force in the
creation and accretion of the Franciscan to the North American craton, but models disagree on
geometric interpretations and details of the mechanisms involved.
Traditional tectonic models involve collision of an ocean plate (such as the Farralon plate) with the
North American (continental) plate. This collision caused eastward subduction of the Farallon plate
under North America. In this model, the Franciscan represents the accretionary wedge, formed in
place while the Great Valley sequence represents the forearc (arc-trench gap) basin. No
movement other than eastward movement normal to the North American plate is implied in the
traditional model (Blake and Jones, 1981). Objections to this model vary in form and content but
include some of the following: differences in sandstone petrology and subsea fan faces between
the Great valley and some of the coeval coherent Franciscan terranes, variations in
sedimentological characteristics of some of the older rocks of the Great valley and correlative
strata to the east, and differences in the paleomagnetic signature between the Franciscan and
Great Valley rocks (Blake and Jones, 1981) (Hagstrum and Murchey, 1993).
Blake cites differences in sandstone mineralogy, age, and textures as primary reasons to believe
that the traditional model is faulty. Blake and others (such as Seiders and Blome, 1987) have
analyzed modal compositions of the Franciscan and Great Valley rocks, and noticed differences
between the Franciscan and the Great Valley rocks. For example chert, sandstone, and mudstone
are more abundant in the Great Valley sequence while felsic volcanic rocks are more abundant in
the Franciscan rocks. In some cases differences are slight (ex. chert 65% in G.V., 60% in Fran.)
but in other cases marked differences exist (ex. felsic volcanics 1.9% in G.V., 9-15% in Fran.)
Blake and others argue that these differences in composition suggest the rocks have different
provenance (geographic origin) (Blake and Jones, 1981). The model they suggest involves
deposition of the Franciscan rocks away from their present-day location, and later NNE movement
of these rocks to their location today.
This model of deposition far removed from the site of actual accretion is supported by several
paleomagnetic studies such Hagstrum and Murchey (1993). Using radiolarian cherts found in the
central and eastern belts of the Franciscan, Hagstrum and Murchey have taken paleomagnetic
measurements to determine paleolatitude. Their findings suggest large-scale movement of the
Franciscan rocks from the time of deposition to the time of accretion onto the continent.
Summary and Conclusion
The Franciscan Complex is composed of a lithologically heterogenous assemblage of rocks
representing 100 million years of subduction. Broken into three belts, the Franciscan occupies
much of western California and can be seen in outcrop throughout the region. The Franciscan
complex was deposited mainly in deep sea fan systems with the continent to the east. Later,
subduction-related, low P-T metamorphism took place in varying degrees throughout the
complex. Debate still exists over the original location of depositon, some say it is near to the
present-day location, others claim it was to the south or southwest. Further study may or may
not clear up the controversy, however it is clear that workers do not currently agree.
Entire article taken from -
http://geologyindy.byu.edu/faculty/rah/tectonics/Student%20Presentations/2000%20Fall/Peter%
20Fahringer/paper.htm
much of the sedimentary rock of the Great Valley sequence
was deposited. A complete ophiolite sequence consists of
serpentinized harzburgite tectonite at the base, overlain by
cumulate ultramafic and gabbroic rocks, passing upward into
noncumulate gabbroic and related plutonic rocks, then into
diabase dikes, and finally into pillow lavas. The Coast Range
ophiolite, however, generally is highly sheared, dismembered,
thinned, and locally missing, presumably as a result of faulting,
at many places along the fault contact between Franciscan
and Great Valley rocks. Only in a few places is a nearly
complete lithologic sequence of Coast Range ophiolite
preserved, and there the total stratigraphic thickness of the
ophiolite is about 3 to 5 km (Hopson and others, 1981).
Isotopic ages ranging from about 165 to 153 Ma (Hopson and
others, 1981) indicate that the Coast Range ophiolite is Middle
and Late Jurassic in age. Paleontologic and paleomagnetic
evidence suggests that the Coast Range ophiolite formed in
an equatorial setting and was transported great distances
northward before being accreted to North America and
overlain by the Great Valley sequence (Pessagno and others,
1984; Hopson and others, 1986; McLaughlin and others,
1988).
More information on the Coast Range Ophiolite -
http://bioregion.ucdavis.edu/book/09_McLaughlin_Mine/09_03_moores_geo
_ophio.html
