Espada Formation
Encyclopedia
The Espada Formation is a sedimentary
rock formation widespread in Santa Barbara County, California. Of late Jurassic
to Cretaceous
age, the unit consists primarily of shale
with some interbedded thin layers of sandstone
, conglomerate
, and limestone
.
of the Espada Formation is in Hondo Canyon, near Point Arguello
, as first described by Thomas Dibblee
in his 1950 book on the geology of southwestern Santa Barbara County. While the unit is 4,000 feet thick at the type locality, it is much thicker elsewhere; along the southern slope of the San Rafael Mountains
it is exposed from its base to its top, where it is probably in conformable contact with the Jalama Formation
, and the total thickness is around 16,000 feet – three miles of uninterrupted sedimentary deposition that took place over 50 million years.
The formation throughout its geographic range consists of layer upon layer of well-bedded argillaceous silty to sandy shales, with smaller interbeds of arkosic sandstone. The shales are generally brown with a greenish tinge in fresh, unweathered exposures; upon weathering they develop a variety of colors. A few beds of pebble conglomerates and limestone, dark gray and not pure, are also found, but the shales are predominant throughout, accounting for around 90 percent of the entire unit. In general, the sandstone layers become more common towards the top of the stratigraphic column, and the largest conglomerate bed is in the highest part of the Espada, exposed north of the Santa Ynez River near its junction with Mono Canyon. The sandstones are composed of grains which are similar mineralogically to the underlying Franciscan Formation.
, even though the two units may have been deposited at great distances from each other, and brought together by tectonic forces along the boundaries of the Pacific
and North American Plate
s.
The crustal block on which the Espada was deposited has been shifted northwestward along the plate boundaries and rotated approximately 90 degrees since the time of its deposition. Most of the movement took place recently, compared to the time of deposition – in the Miocene and Pliocene eras. As the block shifted northward, the formation was also subject to folding
, resulting in creation of structures such as the Mono syncline, which includes Camuesa Peak north of Gibraltar Reservoir, and the parallel Agua Caliente anticline.
s, baculite
s, and pelecypods.
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
rock formation widespread in Santa Barbara County, California. Of late Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
to Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
age, the unit consists primarily of shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
with some interbedded thin layers of sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
, and limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
.
Type locality and description
The type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
of the Espada Formation is in Hondo Canyon, near Point Arguello
Point Arguello
Point Arguello is a headland used as a launch site by the United States Navy. Point Arguello was first used in 1959 for the launch of military and sounding rockets. It was transferred to the United States Air Force in 1964, at which time it became part of Vandenberg Air Force Base.There were 6...
, as first described by Thomas Dibblee
Thomas Dibblee
Thomas Wilson Dibblee, Jr. was an American geologist best known for his extensive geological mapping...
in his 1950 book on the geology of southwestern Santa Barbara County. While the unit is 4,000 feet thick at the type locality, it is much thicker elsewhere; along the southern slope of the San Rafael Mountains
San Rafael Mountains
The San Rafael Mountains are a mountain range in central Santa Barbara County, California, U.S.. They are part of the Transverse Ranges system of Southern California which in turn are part of the Pacific Coast Ranges system of western North America....
it is exposed from its base to its top, where it is probably in conformable contact with the Jalama Formation
Jalama Formation
The Jalama Formation is a sedimentary rock formation widespread in southern Santa Barbara County, California. Of late Cretaceous age, the unit consists predominantly of clay shale with some beds of sandstone...
, and the total thickness is around 16,000 feet – three miles of uninterrupted sedimentary deposition that took place over 50 million years.
The formation throughout its geographic range consists of layer upon layer of well-bedded argillaceous silty to sandy shales, with smaller interbeds of arkosic sandstone. The shales are generally brown with a greenish tinge in fresh, unweathered exposures; upon weathering they develop a variety of colors. A few beds of pebble conglomerates and limestone, dark gray and not pure, are also found, but the shales are predominant throughout, accounting for around 90 percent of the entire unit. In general, the sandstone layers become more common towards the top of the stratigraphic column, and the largest conglomerate bed is in the highest part of the Espada, exposed north of the Santa Ynez River near its junction with Mono Canyon. The sandstones are composed of grains which are similar mineralogically to the underlying Franciscan Formation.
Deposition environment and geologic history
The Espada Formation represents a long period of geologic history – tens of millions of years – in which sediments were deposited in warm, quiet water in a basin that was gradually subsiding. The water was usually shallow, as indicated by the frequent sandstone beds (coarse sediment such as sand is deposited nearer shore unless carried far offshore by submarine landslides and other subsea mass movements). The Espada resembles some strata of the underlying Franciscan FormationFranciscan Assemblage
The Franciscan Assemblage is a geological term for an accreted terrane of heterogeneous rocks found on and near the San Francisco Peninsula. It was named by geologist Andrew Lawson who also named the San Andreas Fault which bounds the Franciscan Assemblage....
, even though the two units may have been deposited at great distances from each other, and brought together by tectonic forces along the boundaries of the Pacific
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
and North American Plate
North American Plate
The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...
s.
The crustal block on which the Espada was deposited has been shifted northwestward along the plate boundaries and rotated approximately 90 degrees since the time of its deposition. Most of the movement took place recently, compared to the time of deposition – in the Miocene and Pliocene eras. As the block shifted northward, the formation was also subject to folding
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
, resulting in creation of structures such as the Mono syncline, which includes Camuesa Peak north of Gibraltar Reservoir, and the parallel Agua Caliente anticline.
Paleontology
Relatively few fossils are found in the Espada, considering its extent and immense depth, but those that are found mostly indicate a Cretaceous age. Buchia piochii, found in the lowest part of the Espada, has been dated to the late Jurassic, which gives the low extreme for age of the unit; Coralliochana orcutti is found near the top, indicating late Cretaceous age and warm water, suggestive of tropical conditions. Other fossils found include mollusks, ammoniteAmmonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s, baculite
Baculite
Baculites is an extinct cephalopod genus with a nearly straight shell that is included in the heteromorph ammonites...
s, and pelecypods.