------------MT. DIABLO TERTIARY ROCKS-----------

Green Valley Formation (upper Miocene)
The Green Valley formation is only mapped on the south side of the mountain. The name applies to the uppermost Miocene and lowermost Pliocene beds that overlie the Neroly. The sediments indicate a change in deposition from a marine to non-marine environment during this time. The 9 million year old Blackhawk Ranch quarry vertebrate fossil horizon lies within this formation as well as at least one leaf-bearing marker bed along with several shell beds.

The Neroly formation (upper Miocene) underlies the Green Valley Fm and forms the grass covered rounded hills immediately south of the ridge-forming Briones/Cierbo strata on the south side of the mountain. The sedimentary rocks have been described as a typical blue-gray volcaniclastic sandstone containing abundant plagioclase and andesitic fragments. Intertongues westward with the non-marine siltstones of the Orinda formation. Andesitic Neroly sandstones alter easily and in many places the sand grains are coated with a thin layer of bluish clay (montmorillinite). Beds rich in fossil clams are well exposed in Sycamore Canyon.

The San Pablo Group (upper Miocene) on the south side of the mountain is normally divided into three marine formations - the Briones, Cierbo, and Neroly formations (oldest to youngest). It is often difficult to distinguish the Briones and Cierbo from each other and some geologists have mapped the two as one unit (called either Briones or Cierbo). The joint unit is very fossiliferous and the vertical beds form the prominent "hogback" ridges known as Fossil Ridge and Blackhawk Ridge. The rocks contain Franciscan fragments. and have been described as dark poorly sorted fine to coarse grained sandstones with interbedded shales and pebble beds. Hard, reef-like shell beds weather to bold relief near the base of the unit and contain abundant Astrodapsis brewerianus fossils. Building materials from Fossil Ridge were used to construct the museum building, and numerous clam shells can be seen in the exterior walls of that building.

"Monterey formation" (middle Miocene). Rocks of this age are represented by the "Monterey formation". The oldest Miocene rocks on the south side of the mountain have been mapped as an age equivalent to the widespread Monterey formation, a major siliceous deposit to the west. The sediments were derived from the east with deposits thickening to a bathyal basin near the Hayward Fault. During Monterey time, a thin deposit with coarser pebbles, sands and shales of this age blanketed our area which lay closer to a shoreline to the east. The rocks have been described as massive gray siltstone with some sandstone beds and locally a conglomerate at its base.

During the Eocene, the climate warmed globally, 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. Conditions were subtropical. On-shore vegetation included palms, avocados, and other plants similar to those found in southern Mexico today.

Northside: On the north side of the mountain, the Eocene is not present inside the park boundary. Good exposures are present in the Black Diamond Mines Regional Park farrther north including coal and glass sand deposits.

Southside: Eocene deposits form the ridges of tan colored sandy rock formations that wrap around the south and southwest side of the mountain. They are well exposed at Rock City and Castle Rocks.

Domengine Formation (middle Eocene)
The vertically oriented strata of middle Eocene age rocks are represented by the Domengine formation. These sands on the south side of the mountain are characteristic of deep off-shore slope turbidites, shedding sands off the southwestern flank of the "Mt. Diablo high" toward an open sea to the west. The rocks have many features of deep marine sediment flows, including graded bedding and flame structures. Radiolaria present are consistent with a deep marine environment.

The formation has been described as a tan, arkosic sandstone. The thick bedded sandstones are medium to coarse-grained with scattered pebbles and are well sorted. The sands are the product of a warm wet climate producing a large amount of quartz sands from extensive "Sierran" erosion. The rocks have been deeply weathered yielding interesting erosional features including the so called Wind Caves (see photo)

--------JURASSIC/CRETACEOUSE SEDIMENTARY ROCKS--------

The Great Valley sequence is composed mostly of deepwater marine shales, sandstones 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. Most of the material derived from the ancestral Sierra and Klamath highlands. Near-tropical and tropical temperatures persisted in the Great Valley region throughout this time. The Cretaceous ended with a major period of erosion.

MT. DIABLO GREAT VALLEY STRATIGRAPHY
Upper Cretaceous Moreno
Upper Cretaceous Panoche
Lower Cretaceous Undifferentiated
Upper Jurassic Knoxville

Colburn (1961) describes a gradual change that takes place in the rock character from the lowest beds ("Knoxville") to the top of the Panoche. He states that "it is easy to distinguish the clean, light-brown, massive sandstone and interbedded mudstones at the top of the section (Moreno) from the dark-gray to dark-green, thin bedded sandstones and olive-drab mudstones of the lowest part ("Knoxville"), but it is impossible to pick a boundary where the change takes place."

He describes the Moreno as characterized by limestone beds, large cannonball concretions 2 to 5 feet in diameter, chocolate-brown mudstones, and thick beds of light-tan sandstones.

The Great Valley rocks were never involved in subduction and are significantly less altered than the Franciscan rocks of the same age. The presence of potassium feldspar minerals in the Great Valley rocks helps distinguish them from the Franciscan of similar age and appearance.

------------FRANCISCAN COMPLEX---------

GREENSTONE
The oldest and most common rocks exposed in the core of the mountain is greenstone. Greenstone is an altered oceanic pillow basalt (lava) believed to have been formed during the Jurassic and Cretaceous far out in the Pacific along a spreading oceanic ridge. The summit of the main peak exposed inside the Visitors Center is greenstone.

CHERT
Second in volume to greenstone in the summit core is the rich reddish-brown finely folded chert. This sedimentary rock is typically red in color (although there are exposures of green and white chert), often thin-bedded, interbedded with reddish colored shale. The hard chert is made up of extremely small silicious skeletons of an oceanic animial called radiolaria. These skeletons were deposited on the oceanic basalt (greenstone) in deep ocean, subsequently turning into chert..

GRAYWACKE
Graywacke is less common than the greenstone and chert. This sedimentary rock is typically fine to medium grained and massive (no stratification or bedding visible). It breaks along distinct jointing planes that helps distinguish it in outcrop from the more "shatter fracturing" of the greenstone. Graywacke is a sandstone consisting mainly of angular quartz, plagioclase feldspar, chert fragments, and dark volcanic rock fragments, and was derived from continental slope and terrestrial sources.

SHALE
Approximately 10% of the Franciscan rocks on Mt. Diablo are made up of shale.

EXOTIC ROCKS
On Mt. Diablo, the most common so-called exotic rock is a sodic amphibole glaucophane schist, or "blue schist", named for the noticeable blue color of the glaucophane. Blue schists are largely altered basalt and reflect a history of hi-pressure/low-temperature metamorphism, a condition found typically in subduction environments.

------------COAST RANGE OPHIOLITE-------------

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Description of the Mount Diablo Ophiolite Rocks

BASALT
Mainly interbedded pillow basalts and basalt flows. Along Olofson Ridge one intact sequence reaches a thickness of about one mile. Most of the sequences are not intact, separated by several major faults. The rock has a microscopic crystalline texture with a black to greenish brown color, weathering to a yellowish brown to dark reddish-brown soil.

DIABASE
Diabase is exposed in the Kaiser and Lone Star quarries and on the flanks of Eagle Peak. The rock is an altered diabase with fine to medium grained crystals. Its composition averages about 50% plagioclase feldspar, and 35% augite or hornblende.

SERPENTINITE
On Mt. Diablo, serpentine 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 from the Franciscan rocks exposed in the central core of the mountain (see Geology Map). This band is characterized by a noticeable change in vegetation due to the high magnesium content of the serpentinite. Chemically, serpentinite is a hydrated magnesium-iron silicate. David Jones maps the serpentinite band as two units - unsheared harzburgite and pyroxenite overlain by a sheared serpentinite melange.

Exposures are typically pale green to greenish-gray, locally black, weathering to grayish orange forming rounded boulder covered slopes. A characteristic alteration product of serpentinite on Mt. Diablo, is the "silica-carbonate" rock associated with the mercury mines on the northside of the mountain.

HARZBURGITE and PYROXENITE
On Long Ridge within the serpentine belt, there is a body of coarse-grained pyroxenite a quarter mile in area. According to Pampeyan, it consists almost entirely of hypersthene and augite. In the unsheared massive serpentinized harzburgite, some of the veins are filled with fibrous asbestos. Chrysotile is the serpentine mineral commonly forming the fibrous vein fillings.