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GUIDE TO THE GEOLOGY
OF
MOUNT DIABLO STATE PARK
Part I - THE ROCKS OF MT. DIABLO -
Their type and history |
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PART I: THE ROCKS OF
MT. DIABLO - Their type and history
Rising 3,849 feet, Mt. Diablo forms a prominent feature in the East Bay landscape. Our understanding of the geological history of the rocks and structure of Mt. Diablo has undergone major changes during the past 30 years, and even now geologists are still trying to unravel the complicated history of the mountain. This complex history is not unique to the mountain, but to our region as a whole, since Mt. Diablo has been caught up in the processes that have shaped the Coast Ranges over the past several million years.
There are no rocks in our area older than Jurassic in age, nor is there any
strong evidence that any part of the North American continent extended this far west prior
to the Jurassic. It was a time when the North American continent was slowly drifting
westward over the earth’s surface, over-riding the ocean crust of the Pacific Ocean,
a scenario that would lead to the creation the rocks of a future Mt. Diablo and setting in
motion forces that would shape the mountain itself.
Plate tectonics played a major part in the formation of the Mesozoic
rocks of Mt. Diablo. We now recognize at least 11 separate major plates of oceanic crust
and rigid upper mantle rocks around the globe. These plates "float" on a layer
of semi-molten rock, all moving and jostling against each other, crowding for room, and
creating, among other things, new land forms in the process. Continents ride atop these
ocean plates, being rafted along as the plates move. New ocean crust is being continually
created by the eruption of submarine volcanic material forming along ocean spreading
ridges such as the Mid-Atlantic Ridge. As new ocean crust is created, older oceanic crust
is driven beneath continental crust and recycled. This process is referred to as
“subduction”.
To more easily understand the complex geological setting of Mt.
Diablo, it is useful to divide the mountain’s rocks into three main groups. Each
group has a different history and is characterized by different types of rocks.
Group 1 - Mt. Diablo Ophiolite
(Mesozoic)
Group 2 - Franciscan Assemblage (Mesozoic)
Group 3 - Great Valley Group (Mesozoic) and
Younger Sedimentary rocks (Cenozoic)
This is a very generalized SW to NE cross-section across the
summit of the mountain.
The Ophiolite is not exposed in this section. The section emphasizes the strong folding of
Mesozoic and Cenozoic sedimentary formations against the Franciscan core of the
mountain.
Suggest you also use the
Rock Descriptions and Ages and Geologic Map
pages to help understand the nature of the rocks on the mountain.
Group 1 - Mt. Diablo
Ophiolite (Mesozoic)
The exposure of an old ocean crust on Mt. Diablo has been named the Mt. Diablo Ophiolite. The term ophiolite refers to “ocean crust found on
land.” The Mt. Diablo Ophiolite is exposed on the mountain north of a line drawn from
Long Ridge through Murchio Gap, encompassing the Zion Peak rock quarry, Mitchell Rock and
Eagle Peak.
Some geologists believed that near the close of the Jurassic, a subduction zone existed in the Sierra foothills. At that time, subduction jumped to a zone along the coast of present day California. The ocean crust caught between the new subduction zone and a volcanic arc to the east (present day Sierra Nevada) was preserved and later partially exposed as the Coast Range Ophiolite. The mid-Mesozoic Coast Range Ophiolite (of which the Mt Diablo Ophiolite is a remnant) formed immediately before the initiation of the subduction zone that led to the formation of the Franciscan accretionary complex. It seems likely that collision of either an island arc or the Coast Range Ophiolite itself with the earlier Nevadan active margin in eastern California resulted in the westward step-out of active subduction into what are now the Coast Ranges.
Radiometric and fossil age determinations date the ophiolite as
having been at a oceanic spreading ridge approximately 169 million years ago during the
mid-Jurassic. Based on the age of overlying
continentally derived sediments, one can infer that this part of the ocean crust arrived
at the continental edge in late Jurassic, perhaps from as far west as 5000 miles.
Where ophiolites are exposed, it is rare that complete vertical sequences of oceanic crust rock types are present. In the Mt. Diablo area, only basalt, diabase, harzburgite, pyroxenite and associated serpentinite members are exposed.
Basalt: The ophiolite is an igneous rock solidified from sub-marine lava flows. The basalt making up a part of the Mt. Diablo Ophiolite is mainly interbedded pillow basalts and basalt flows. As the basaltic lava erupts under water, the outer surface of the flow "freezes" in contact with the water. More lava breaks through and again the outer surface "freezes". This process leads to the accumulation of "pillow" structures and the resultant rock is referred to as pillow basalt. The basalt has a microscopic crystalline texture with a black to greenish brown color, weathering to a yellowish brown to dark reddish-brown soil. Well developed pillows can be seen on Mitchell Rock
Pillows shown here in the Marin Headlands are similar to structures found in the basalt flows on Mt. Diablo
Diabase: The pillow lavas are fed by a series of vertical fissures, or dikes, that allow the molten rock from below to reach the surface. The molten material in the dikes solidify into a rock called diabase which has the same chemical composition as the basalt, but with a courser texture, although still fine grained. Diabase is exposed in the quarries on Mt. Zion and on the flanks of Eagle Peak. The rock has a mineral composition that averages about 50% plagioclase feldspar, and 35% augite or hornblende.
Serpentinite: Serpentinite is a rock frequently found in association with an ophiolite. Serpentinite is derived from the basal portion of the original ocean crust and upper-most part of the mantle (peridotite, harzburgite or dunite), but has been metamorphosed by hydration from ocean water circulating at depth through fractures in the ocean crust.
Since it is derived from mantle material, the chemistry of serpentinite is unlike that of most rocks in the earth's crust. Serpentinite rock is mostly composed of a mineral called serpentine. This mineral is low in potasium and calcium, both of which are plant nutrients, and also contains high levels of potentially toxic elements such as magnesium, chromium, and nickel. Plants that live on serpentinite are adapted to survive in these unusual chemical conditions. The new minerals formed are commonly the serpentine minerals antigorite, chrysotile and lizardite. Serpentinite, incidentally, is California’s state rock.
On Mt. Diablo, serpentinite occurs in several localities. The largest is the prominent east-west band that runs through Murcho Gap extending west along Long Ridge, separating the ophiolite on the north from the Franciscan rocks exposed in the central core of the mountain to the south. This band is characterized by a noticeable change in vegetation due to the high magnesium content of the serpentinite. Exposures of the serpentinite are typically pale green to greenish-gray, locally black, weathering to grayish orange forming rounded boulder covered slopes.
Bodies of harzburgite and pyroxenite occur in the serpentinite band. On Long Ridge there is a body of coarse-grained pyroxenite a quarter mile in area consisting almost entirely of hypersthene and augite. As you walk over the exposure, the area can sparkle from the sun’s reflected light. In the altered, but unsheared massive serpentinized harzburgite, many of the veins are filled with fibrous asbestos.
Diagram of a subduction zone
showing the position where the
Franciscan Complex formed.
The Mt. Diablo Ophiolite is part of the Coast Range Ophiolite that underlies the Great Valley Sequence. The ophiolite was underthrust and uplifted by Franciscan wedges driven eastward by subduction.
(courtesy National Park Service)
Group 2 - Franciscan
Complex (Mesozoic)
An assemblage of Mesozoic rocks that were a puzzle to California geologists for many years, underlies the central summit area and North Peak. Our relatively new understanding of plate tectonics and subduction has finally provided an important clue to unraveling this mystery. This diverse complex of rock types is common up and down the coastal ranges of California and has been given the name Franciscan Complex or Assemblage. The processes of subduction can account for the mixing of such a wide variety of rock types.
The Franciscan Complex ranges in age from 90 to 190 million years old. They represent volcanic lava flows and sedimentary rocks deposited on the ocean floor millions of years ago. The rock types include greenstone (altered volcanic basalt), chert, graywacke sandstone, and minor amounts of schist and argillite (altered shale). This variety of rocks is due to plate tectonics where an eastward moving oceanic plate carrying theses rocks collided with the westward moving North American plate continuously over millions of years. In the area of collision along western California (subduction zone) the oceanic plate, along with the sedimentary deposits and volcanics, were forced downward beneath the North American plate. Some of the rocks on this plate were scraped off in the collision zone, then later buried and altered by pressure and heat. Uplift and erosion has exposed the rocks today forming the core of Mt. Diablo.
The two main peaks are composed of faulted
blocks of resistant basalt and chert with some graywacke and minor shale, and are
expressed topographically as rugged and jagged rock masses.
Wrapping around the two peaks in a rough "figure 8" shape are the
more gentle treeless slopes of “melange”. The Franciscan “melange” is
essentially a chaotic mixture of an intensely sheared sandstone and shale
"paste" in which are embedded blocks of basalt, chert and graywacke along with
rare exotic rocks. It is often difficult to distinguish between the melange topography and
local landslides.
Franciscan basalt (Greenstone): The basalt blocks in the Franciscan are believed to be fragments scraped off of the upper part of subducting basaltic ocean crust. The blocks of basalt exposed in the Franciscan on Mt. Diablo are altered oceanic pillow basalt. During burial following subduction, pressure and hot fluids changed the basalt into a rock called “greenstone”. Eventually the greenstone was uplifted and exposed along with other Franciscan rocks during formation of the mountain. Some of the basalt blocks are up to 190 million years old.
On the surface the rock weathers to a dark yellowish-brown to dark reddish-brown while fresh exposures are grayish-green to light-olive drab. The green color comes mostly from chlorite, a green alteration mineral.
Franciscan chert: The chert bodies in the Franciscan form prominent dark red exposures and talus slopes. Made up of silica, they are resistant to erosion and form such features as Devil’s Pulpit and Turtle Rock. Typically red in color (green and white less common), the chert layers are typically interbedded with reddish colored shale. The banded rocks are often referred to as "ribbon chert". The red color is derived from iron oxides.
The chert in the Franciscan was formed far out at sea millions of years ago. Silica skeletons of minute ocean animals called radiolaria settled to the ocean floor forming a silica ooze that ultimately solidified into chert. The chert continued to slowly accumulate on top of the ocean floor as the ocean crust drifted away from the spreading center on its long journey toward subduction. The chert ranges in age from 190 myo to 90 myo, representing 100 million years of accumulation
Franciscan cherts are formed from the tiny (0.5 to 1.5 mm) silica shells of radiolaria. Many of these radiolaria are tropical species indicating that the sediments were deposited near the equator and were later transported northeastward by plate movements.
Franciscan graywacke: Graywacke is less common on Mt. Diablo than the greenstone and chert. It is typically fine to medium grained and massive (no stratification or bedding visible). It breaks along distinct jointing planes (see photo to right) that helps distinguish it in outcrop from the more "shatter fracturing" of the greenstone. It consists mainly of angular quartz, plagioclase feldspar, chert fragments, and dark volcanic rock fragments. Calcite and quartz occur commonly in the criss-crossed white veins.
The graywacke is younger in age than the greenstone (basalt) or chert, ranging from 90 to 100 million years in age. These rocks are thought to have formed in a subduction trench environment off the coast of North America (some researchers suggest Mexico and subsequently moving north) as intermittent submarine landslides cascading westward down the continental slope and coming to rest on top of the ocean crust or overlying chert. Geologists think that several eroding land area contributed grains to the diverse greywacke sands. The degree of metamorphism of the graywacke indicates burial to at least a depth of 15 miles before uplift and exposure.
Franciscan shale: Approximately 10% of the Franciscan on Mt. Diablo is made up of shale. Most of this clay sized material was probably deposited in less turbulent current conditions in association with the graywacke deposition.
Franciscan exotic rocks: Exposures of the so-called “exotic rocks” of the Franciscan are not uncommon on Mt. Diablo. These rocks have undergone a different pressure-temperature history than the rest of the Franciscan, an intriguing story yet to be fully explained.
The most common exotic rock present on Mt. Diablo is a glaucophane schist, or "blue schist", named for the noticeable blue color of the glaucophane. Blue schist is largely altered basalt and reflect a history of hi-pressure/low-temperature metamorphism, a condition found in subduction environments and rarely any other place. One boulder can be found just north of the Junction Office in a gully on the east side of Northgate Road. On the Summit Road toward the summit just past the Rocky Point Picnic area, you will notice a dark blue-black boulder of blue schist about 5 feet across protruding from the bank on the left side of the road.
Much less common are bodies of green schist (rich in actinolite and talc minerals) and eclogite (garnet-pyroxene).
Group 3 - Great Valley (Mesozoic) and Younger Sedimentary Rocks (Cenozoic)
The third major rock sequence on and around Mt. Diablo are the thick sections of sedimentary rock formed from material derived, not from subduction to the west, but from material eroded from ancient highlands to the east, an ancient “Sierra” if you like, and proto-Klamath ranges in the north. Beginning about 10 million years ago, river, lake and stream sediments replaced marine deposits as the sea retreated from the Great Valley for the final time.
These rocks are exposed today wrapping around the mountain with
bedding dipping steeply away from the Franciscan and ophiolite core. As you drive up South
Gate Road to the summit, you are driving over rocks continually increasing in age - a 190
million year journey back through geologic time. In the Rock City and Castle Rock areas
along the south and west side of the mountain, the original flat-lying beds are now
upturned and stand almost vertical. Differential weathering of alternating resistant
sandstone and soft shale layers are responsible for the ridge-valley topography seen
around the mountain’s west, south and east sides.
Great Valley Group (Upper Jurassic through Cretaceous):
The name Great Valley Group refers to the thick sedimentary deposits of Upper Jurassic through Cretaceous age that were deposited in the marine basin west of the present day Sierra. The Great Valley sequence is composed mostly of deepwater marine shale, sandstone and some conglomerates accumulating to a thickness of 60,000 feet near the western margin of the present day Sacramento Valley and then thinning toward Mt. Diablo. The mode of deposition was as overlapping lobes of sedimentary material derived from ancient river deltas along the front of the highlands to the east. The oldest beds (140 to 145 myo "Knoxville" in this area.) were deposited on top of a remnant of oceanic crust (ophiolite). Great Valley deposits on-lap the Mt. Diablo area and thinner deposits intermittently covered it during this time. One interpretation suggests the Mt Diablo area probably was an area of non-deposition during early and middle Cretaceous. The entire area was located on a westward dipping gentle shelf, far from sediment source.
One study describes a gradual change that takes place in the rock character from the lowest beds to the top of the Great Valley Group - from dark-gray to dark-green, thin bedded sandstone and olive-drab mudstone to the clean, light-brown, massive sandstone and interbedded mudstone at the top of the section. Large cannonball concretions 2 to 5 feet in diameter are common is the upper parts of the sequence.
Although uplifted and folded to vertical in places, The Great Valley rocks were never involved in subduction and are significantly less altered than the Franciscan rocks of the same age. The Cretaceous ended with a period of exposure and widespread erosion.
Cenozoic Tertiary and Quaternary Rocks
To summarize the Cenozoic in this area, it is perhaps easiest to think of the Central Valley of California as a low elongate basin, flooded intermittently by an encroaching shallow sea, and slowly being filled by sedimentary material from the surrounding exposed land masses, primarily the "Sierra" until mid-Miocene when highlands developed to the west and south. During the last part of the Tertiary, these new highlands became the primary source area for the material found in the upper Tertiary rocks.
The Mt. Diablo area (along with the Kirby Hills to the north) seemed to represent a persistent “high”, frequently underwater, but less deep than surrounding areas and periodically exposed to erosion. Many of the formations seem to shoal out on the flanks of this area and when submerged, the strata thin over the "high". However the area was not a "mountain" in the sense we see today, but rather a land of low relief. The rising of Mt. Diablo would have to wait until the Pleistocene, still far in the future.
Paleocene Rocks (58-64 mya):
There are few Paleocene deposits present in our area indicating that the region was probably above sea-level and undergoing erosion following the close of the Cretaceous. The only nearby rocks of this age are restricted to the north side of the mountain outside of the park.
Eocene Rocks (36.5-58 mya):
During the Eocene, the climate warmed, resulting in heavy "ancestral Sierra" weathering that yielded large quantities of sands that washed into and across the Central Valley providing material to the Eocene deposits of Mt. Diablo. A shallow marine basin, a sandy shoreline, a swampy backwater area - all existed in this area at different times, or at the same time in different places.
On the north side of the mountain, the Eocene is present in the Black Diamond Mines Regional Park. This strata contains coal beds and glass sands and have been described as a near-shore lagoonal swamp or tidal flat esturarian environment. On Mt. Diablo, Eocene deposits form the ridges of tan colored sandy rock formations that wrap around the south and west side of the mountain. They are well exposed at Castle Rocks, Rock City, Knobcone Point and Cave Point. Sands on the south side of the mountain are characteristic of deep off-shore slope deposits, shedding sands off the southwestern flank of the "Mt. Diablo high" toward an open sea to the west. Shallow near-shore deposits contain beds rich in Turritella fossils (marine snails).
Wind Caves in Rock CityThese massive sandstone beds weather easily forming features such as wind caves and open tunnels. Unusual "cannonball concretions" can also be found in these sandstone beds. Rock City is a good place to view these unusual features easily accessible on South Gate Road.
Oligocene Rocks (23.7-36.5 mya):
The only Oligocene in the area is the Kirker Tuff on north side of the mountain outside the park boundary.
Miocene Rocks (23.7-5.3 mya):
On the south and west sides of the mountain, the depositional contact between the Eocene and the Miocene rocks can be recognized by the abrupt change from clean, thick-bedded, light-tan sandstone in the Domengine formation (Eocene) to poorly sorted, dark gray, pebbly sandstone of the marine Miocene rocks.
During middle Miocene time, the general drainage was directed from the east into an open ocean to the west, a pattern similar to the deposition of the earlier Eocene. The shoreline lay not far to the east and clastic “Monterey age” rocks were laid down on older Eocene deposits. By about 10 mya, subduction had ended in central California and there was a major change in the pattern of deposition. A highland developed to the west where the San Francisco Bay currently exists, and the Diablo Range south of Livermore began to rise. The Mt. Diablo area began to accumulate marine and later non-marine deposits from these sources.
Derived from the south and west, the shallow-water marine sediments deposited during this time are referred to as the Briones Formation in most publications. The rocks contain Franciscan fragments probably derived from the rising Diablo Range to the south. They have been described as dark poorly-sorted fine to coarse-grained sandstone with interbedded shales and pebble beds. Hard, reef-like shell beds weather to bold relief near the base of the unit.
DEVILS SLIDE
Hogback ridges plunging into Sycamore Canyon
as viewed from South Gate Road. The ridges are in the Briones Formation (Miocene).Now steeply tilted upward from an original horizontal orientation, the vertical beds form the prominent "hogbacks" on Fossil Ridge and Blackhawk Ridge. Building material quarried from Fossil Ridge was used to construct the summit museum building and numerous clam shells can be seen in the exterior walls of that building.
Following Briones deposition, the direction of sediment transport shifted again, bringing sands derived from the east, rich in volcanic material washed from the Sierran highlands. These volcanic sands have been named the Neroly formation. They form the grass covered rounded hills immediately south of the underlying ridge-forming Briones strata on the south and can be found on the west and north sides of the mountain as well.
Andesitic Neroly sandstone alters easily and in many places the sand grains are coated with a thin layer of bluish clay that is clearly exposed on Shell Ridge in Walnut Creek. Beds rich in fossil marine shells are well exposed on Shell Ridge and in Sycamore Canyon. By around 9 million years ago, during the late Miocene, the sea again receded from the Mt. Diablo area marking a permanent change from marine deposition to non-marine stream and lake deposition.
One of the seven million year old stream deposits on the south side of the mountain has captured and preserved an abundant and diverse collection of animal fossils. The Blackhawk Ranch Quarry has yielded numerous vertebrate fossils of horses, rhinos, camels, and smaller animals.
A large mastodon skull, a Gomphotherium, has been removed from this site. This gives evidence that late Miocene mammals abounded in the newly created forests and flood plains stretching way to low hills to the west and south. There are several volcanic tuff deposits in the late Miocene and Pliocene derived from the volcanic fields of Sonoma County. There was no Mt. Diablo at that time.
Plio-Pleistocene to Recent Rocks (5.3 mya to present):
Non-marine deposits continued to collect in the area during Pliocene time (1.67-5.3 mya). It was during Pleistocene time, beginning about 2 my ago and continuing to the present, that Mt. Diablo was formed as a topographic feature. From that time on, Mt. Diablo has been feeding erosional materials into surrounding valleys. Pliocene sources were predominantly Great Valley rocks. Pleistocene sources were predominantly Franciscan, indicating unroofing and erosion of deeper Franciscan terranes. The 4.83 million year old Lawlor Tuff (a nonmarine, pumiceous, andesitic ash-flow tuff) is a widespread marker bed around the mountain.
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PART II: FORMING THE MOUNTAIN - Mt.
Diablo’s Tectonic History
Although Mt. Diablo is old in terms of its rock history, it is very young as a topographic feature. The rising of Mt. Diablo to its present height is the result of a complex interplay of tectonic forces operating during the last few million years. Mt Diablo started growing in Pliocene time, but did not start major growth until late Pleistocene time, about 500,000 years ago. These geologic processes have created a complex uplifted compressional fold. The mountain is continually undergoing the sculpturing effects of erosion and is still rising today at the rate of 1 to 3 mm/year.
There have been a number of explanations for the uplift
of Mt. Diablo. In early publications, Mt.
Diablo was called a diapir. An analogy can be
drawn to a low density cork rising upward through water. In the case of Mt. Diablo, a
relatively low density plastic Franciscan melange and serpentinite, pushed upward forming
an anticlinal fold. The Franciscan core was then exposed by the removal of overlying rocks
by erosion.
Recently, two ideas have been suggested to account for the uplift and
folding of Mt. Diablo. Both involve the recognition that the Mt. Diablo region has been
undergoing compressional stress over the past few million years. However, this compression
and resultant folding may be caused by two different stress mechanisms.
The first scenario (see drawing below)
proposes that the development of the Mt. Diablo tectonic block was the result of
compressional forces acting on a NE-SW direction forming a compressional fold. A northeast dipping thrust fault broke the fold
and plunged under the mountain. This thrust fault is also pulling the San Ramon Valley and
Livermore Valley blocks under the mountain. The under-thrusting
block, moving in a northeast direction, is lifting and folding the mountain above it. The
Mt. Diablo block itself is referred to as a back-thrust block riding up the underlying
thrust plane and being folded in the process.
The source of these compressional forces comes from the fact that
during the past two million years or so, there has been a small compressional component to
the large scale San Andreas strike-slip fault system. The Calaveras, Concord and
Greenville faults are recognized as the eastern most strands of the San Andreas fault
system. Conventional theory suggests that the north-south slip on the Calaveras Fault dies
out near Danville and jumps eastward to the Concord Fault. The earthquake swarm near
Danville in 1990 is sited as an example of a series of ruptures in the transition zone
between the two faults.
The second scenario (see drawing below) requires compression
as well, but the compressional forces acting on Mt. Diablo are generated by horizontal
movement along roughly north-south trending faults (in this case the Greenville-Morgan
Territory Fault on the east side of the mountain and the Concord fault on the northwest
side). Rock caught between two parallel moving faults will undergo compression.
In this case, the folding of Mt. Diablo is the result of the
contraction that is driven by a transfer of slip from the Greenville Fault to the Concord
fault. Most of the slip transfer occurs across the Mt. Diablo anticline. When the
northward development of the Greenville - Morgan Territory Fault is blocked by impinging
on the stable Central Valley block, relief of the stress must jump west to the Concord
Fault. Across Mt. Diablo, the stress is relieved by the development of a blind thrust
fault under the mountain. The westward propagation of the tip of the blind thrust uplifts
and folds the strata. The trace of the fault plane remains underground and does not reach
the surface. It is estimated the fault plane may be as deep as 10 miles beneath the
mountain.
 |
Above are two possible interpretations (greatly simplified) of the stress pattern that resulted in the folding of Mt. Diablo. The cartoon drawings are southwest-to-northeast cross-sections through the summit. Both interpretations assume southwest-northeast regional compression across the Mt. Diablo anticline.
The left image suggests thrust faulting where the downward moving lower block has lifted and folded the mountain rocks in the overriding back-thrust block
The right image suggests a blind thrust (a fault that does not reach the suface) with a westward propagating tip lifting and folding the overhead strata.
The realization that blind thrust faults have been responsible for some of the strongest earthquakes in California, including the Coalinqua quake in 1983 and the Northridge quake in 1994 (that killed 61 people, injured thousands and caused $20 billion in damage) has spurred the USGS to study the populous East Bay more carefully. The recognition of the possible existence of a blind thrust fault under Mt. Diablo has resulted in the issuance of an advisory by the USGS in September of 1999 predicting a four percent probability of a 6.7 or larger earthquake occurring on the blind thrust fault underlying the mountain.
There is one prominent southeast-directed thrust fault on the
mountain separating the Franciscan from the Great Valley sedimentary rocks. This fault, commonly mapped as the Mt. Diablo
Thrust Fault or the Mt. Diablo Fault or Piercement Fault (not to be confused with the
blind “Mt. Diablo Thrust Fault”) has uplifted the mountain’s Franciscan
core relative to the adjacent Great Valley Cretaceous rocks on the south..
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PART III: MT. DIABLO MINING
HISTORY - Boom to bust
The most important minerals and rocks that have been mined or
excavated on and around Mt. Diablo include mercury, diabase, graywacke, white sands, coal,
blue schists, travertine, copper, and farther north and east, gas and oil.
Mercury
Mercury has been mined on the northeast flank of Mt. Diablo off and on since its discovery in 1863. Prior to that, Indians used the colored mineral for ceremonial purposes. The mercury (also referred to as quicksilver) occurs in the form of cinnabar (red mercury sulfide) and metacinnabar (a black mercury sulfide). The host rock for ore is silica-carbonate rock, itself formed from the hydrothermal alteration of serpentine, lying in the boundary fault zone that separates Franciscan from Great Valley rocks. The silica-carbonate rock is made up of varying quantities of silica (chalcedony and opal) together with magnesian carbonates and stained rusty red by alteration of iron sulfide minerals. The rock is commonly spongy in appearance. Topographically, silica-carbonate rock forms resistant outcrops. It is believed that the mercury minerals were deposited from hydrothermal solutions which formed mostly in fractures in the silica-carbonate rock. Ryne Mine produced most of the cinnabar while metacinnabar was produced at the Mt. Diablo Mine.
A man named Welch discovered cinnabar at what is known as the Ryne Mine in 1863. It operated for about 10 years before becoming uneconomic. In 1933 it was discovered that black metacinnabar in the area known as the Mt. Diablo Mine also contained mercury and was more abundant than cinnabar. The demand for mercury during the second world war resulted in an expansion of operation that was to continue until 1958 when mining operations again ceased. It is estimated that about $1,500,000 worth of mercury was extracted from the mines. Unfortunately a continuing legacy of the mining is the acid mine water emptying into and contaminating Marsh Creek.
Building Stones and Rip-rap
The diabase quarries on the northside of the mountain
(Zion Peak) are currently being excavated for crushed rock and rip rap material. There were several excavations in graywacke on the northside of the mountain for
the same purposes, but they are now
abandoned. Blue
schist from the Franciscan rocks on Mt. Diablo yielded good dimension stone and was
popular for building construction due to its color.
Copper and Precious Metals
About 40,000 pounds of copper was produced from the mines in the diabase in the 1860's, but there is no activity now. Minor amounts of gold and silver associated with the copper were also produced. It was rumored that the best area to discover gold or silver was in the Back Canyon area (unfortunately inside the park boundary).
Travertine
Travertine, a finely crystalline massive calcium carbonate deposit frequently associated with hot springs, was quarried along the northside of Mt. Diablo (Lime Ridge) for many years by the Cowell Cement Company.
Coal and White Sand
North of Mt. Diablo and outside the park in the Black Diamond Mines area, lignite coal beds in the Domengine Formation were the largest known and most extensively mined coal deposits in California. From the 1860's to the beginning of this century, the Mount Diablo coal field supplied coal to the rapidly expanding urban and industrial centers of the San Francisco Bay area. Finally closed as newer and cheaper energy sources were discovered, during its lifetime the mines produced approximately 4,000,000 tons of coal valued between $15 and $20 million.
At the base of the Domengine Formation exposed in the Black Diamond
Mines Regional Park, there is a thin section of white sands called the "Ione" sands, a description carried
across the Central Valley from major white sand deposits in the Ione Formation on the east
side of the Valley. The sands appear to be
continuous across the valley subsurface and of equivalent age. The white sands, that were used for making glass, were mined from two deposits in the area
from 1920 until 1949 when they ceased operation. The caverns are a fascinating place to
visit on guided tours.
Gas and Oil
The Domengine Formation also acts as a reservoir for natural gas and the Martinez Formation produces oil in the subsurface northeast of Mt. Diablo.
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PART IV: SUGGESTED READING LIST
Return to Geology Table of Contents
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MOUNT DIABLO INTERPRETIVE
ASSOCIATION
P.O. Box 346 - Walnut Creek, CA 94597-0346
(925) 927-7222 / FAX: (877) 349-5016