Juncal Formation
Encyclopedia
The Juncal Formation is a prominent sedimentary geologic unit of Eocene
age found in and north of the Santa Ynez Mountain
range in southern and central Santa Barbara County
and central Ventura County
, California. An enormously thick series of sediments deposited over millions of years in environments ranging from nearshore to deep water, it makes up much of the crest of the Santa Ynez range north of Montecito
, as well as portions of the San Rafael Mountains
in the interior of the county. Its softer shales weather to saddles and swales, supporting a dense growth of brush, and its sandstones form prominent outcrops.
of the Juncal Formation is found north of the Santa Ynez River and east of Agua Caliente Canyon, near Juncal Campground in the Los Padres National Forest
, where the unit is well-exposed. At the type locality the Juncal Formation is 3,360 feet thick, which is about average; elsewhere it is both thicker and thinner, reaching 5,400 feet thick near the Romero Saddle north of Carpinteria
, and thinning to 1,500 feet north of the Santa Ynez Fault in Oso and Redrock Canyons.
The unit consists of shales and sandstones, with the shales being generally more abundant, particularly near the top of the stratigraphic column. Overall the unit shows a persistent alternation of shale and sandstone layers, of widely varying thickness, indicating frequent and erratic changes in water depth at the point of deposition. The lithologic characteristics of the unit are different north and south of the Santa Ynez River. South of the river, on the north slope and on the crest of the Santa Ynez Mountains, the shales are darker and a thick interior sandstone member can be mapped as a separate unit. Page, Marks, and Walker, in their 1951 report on the region mapped it as the "Camino Cielo Sandstone member." North of the river, the unit contains fossil oyster beds in a sandstone member at its base, and an interior sandstone member is massive and cliff-forming, unlike anything in the Juncal of the Santa Ynez Mountains.
Shales of the unit are micaceous and dark, usually gray or brown, weathering to various colors. Sandstones are arkosic, hard, and range from white to tan in color. The shale weathers readily, and supports a dense growth of chaparral
, similar to the other shale units in the Santa Ynez Mountains, such as the Cozy Dell
.
The unit lies conformably on the Sierra Blanca Limestone, a relatively thin, separately-mapped limestone unit, in some areas. Where the Sierra Blanca is not present the unit lies unconformably
on older units – including the Jalama Formation
and Espada Formations, both of Cretaceous
age; and in some places the Juncal lies unconformably on the Franciscan Formation
, which is probably of Jurassic
age.
.
The first unit deposited in the Eocene was the limestone Sierra Blanca Formation, although only north of the Santa Ynez Fault, and then the ancient sea deposited the Juncal Formation on top of the Sierra Blanca. At this time, the crustal block on which these units were deposited was nearer to the present-day position of San Diego; it has twisted approximately 90 degrees clockwise during the last 15 million years, as part of a process of regional, extensional deformation associated with motion along the boundary of the Pacific
and North American Plate
s.
At first the sea was shallow, and the first part of the Juncal Formation to be deposited was in a shallow-water environment with reefs. This portion of the unit includes calcareous sandstones and shales and sandstones with lenses of limestone, as well as come conglomerates. Gradually the sea deepened, resulting in the deposition of finer-grained sediments, although mixed in with the shales that resulted are coarser-grained sediments such as sandstones, indicating an alternating regime of deep and shallow water over the millions of years in which the Juncal Formation was laid down. Another peculiarity of the unit is that fossil foraminifers from very different depth environments are often mixed together; this has been interpreted as the result of turbidity currents and other submarine mass movements. Rip-up clasts and conglomerates which appear in the Juncal also support this interpretation. Some of the foraminifers are characteristic of lower bathyal depths, indicating that some of the Juncal formation was deposited at a depth of up to 2,000 meters.
, larger fossils are uncommon. However, some of the stratigraphically lowest beds – such as the portion which is in contact with the Sierra Blanca limestone formation – contain larger fossils characteristic of nearshore and reef environments. These include oysters, especially in limestone-rich beds and limestone lenses. Other fossils from the Juncal include algae, worms, brachiopods and molluscs.
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
age found in and north of the Santa Ynez Mountain
Santa Ynez Mountains
The Santa Ynez Mountains are a portion of the Transverse Ranges, part of the Pacific Coast Ranges of the west coast of North America, and are one of the northernmost mountain ranges in Southern California.-Geography:...
range in southern and central Santa Barbara County
Santa Barbara County, California
Santa Barbara County is a county located in the southern portion of the U.S. state of California, on the Pacific coast. As of 2010 the county had a population of 423,895. The county seat is Santa Barbara and the largest city is Santa Maria.-History:...
and central Ventura County
Ventura County, California
Ventura County is a county in the southern part of the U.S. state of California. It is located on California's Pacific coast. It is often referred to as the Gold Coast, and has a reputation of being one of the safest populated places and one of the most affluent places in the country...
, California. An enormously thick series of sediments deposited over millions of years in environments ranging from nearshore to deep water, it makes up much of the crest of the Santa Ynez range north of Montecito
Montecito, California
Montecito is an unincorporated community in Santa Barbara County, California. As a census-designated place, it had a population of 8,965 in 2010. This does not include areas such as Coast Village Road, that, while usually considered part of Montecito, are actually within the city limits of Santa...
, as well as portions 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....
in the interior of the county. Its softer shales weather to saddles and swales, supporting a dense growth of brush, and its sandstones form prominent outcrops.
Type locality, description, and distribution
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 Juncal Formation is found north of the Santa Ynez River and east of Agua Caliente Canyon, near Juncal Campground in the Los Padres National Forest
Los Padres National Forest
Los Padres National Forest is a forest located in southern and central California, which includes most of the mountainous land along the California coast from Ventura to Monterey, extending inland...
, where the unit is well-exposed. At the type locality the Juncal Formation is 3,360 feet thick, which is about average; elsewhere it is both thicker and thinner, reaching 5,400 feet thick near the Romero Saddle north of Carpinteria
Carpinteria, California
Carpinteria is a small oceanside city located in southeastern Santa Barbara County, California, east of Santa Barbara and northwest of Ventura. The population was 13,040 at the 2010 census, down from 14,194 at the 2000 census....
, and thinning to 1,500 feet north of the Santa Ynez Fault in Oso and Redrock Canyons.
The unit consists of shales and sandstones, with the shales being generally more abundant, particularly near the top of the stratigraphic column. Overall the unit shows a persistent alternation of shale and sandstone layers, of widely varying thickness, indicating frequent and erratic changes in water depth at the point of deposition. The lithologic characteristics of the unit are different north and south of the Santa Ynez River. South of the river, on the north slope and on the crest of the Santa Ynez Mountains, the shales are darker and a thick interior sandstone member can be mapped as a separate unit. Page, Marks, and Walker, in their 1951 report on the region mapped it as the "Camino Cielo Sandstone member." North of the river, the unit contains fossil oyster beds in a sandstone member at its base, and an interior sandstone member is massive and cliff-forming, unlike anything in the Juncal of the Santa Ynez Mountains.
Shales of the unit are micaceous and dark, usually gray or brown, weathering to various colors. Sandstones are arkosic, hard, and range from white to tan in color. The shale weathers readily, and supports a dense growth of chaparral
Chaparral
Chaparral is a shrubland or heathland plant community found primarily in the U.S. state of California and in the northern portion of the Baja California peninsula, Mexico...
, similar to the other shale units in the Santa Ynez Mountains, such as the Cozy Dell
Cozy Dell Shale
The Cozy Dell Shale is a sedimentary geologic unit of late Eocene age found principally in the Santa Ynez Mountain range north of Santa Barbara, California, from near Point Arguello in the west to central Ventura County in the east...
.
The unit lies conformably on the Sierra Blanca Limestone, a relatively thin, separately-mapped limestone unit, in some areas. Where the Sierra Blanca is not present the unit lies unconformably
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
on older units – including 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 Espada Formations, both of 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; and in some places the Juncal lies unconformably on the Franciscan Formation
Franciscan 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....
, which is probably of 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...
age.
Deposition environment and tectonic history
Sometime during the late Cretaceous period, the sea withdrew from the area that was to become the California coast in the region of Santa Barbara and Ventura, exposing the sediments that had accumulated during tens of millions of years underwater – the Espada and Jalama Formations – and they began to erode. In the early Eocene, the sea returned, resuming sediment deposition on the eroded surface, making an unconformityUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
.
The first unit deposited in the Eocene was the limestone Sierra Blanca Formation, although only north of the Santa Ynez Fault, and then the ancient sea deposited the Juncal Formation on top of the Sierra Blanca. At this time, the crustal block on which these units were deposited was nearer to the present-day position of San Diego; it has twisted approximately 90 degrees clockwise during the last 15 million years, as part of a process of regional, extensional deformation associated with motion along the boundary 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.
At first the sea was shallow, and the first part of the Juncal Formation to be deposited was in a shallow-water environment with reefs. This portion of the unit includes calcareous sandstones and shales and sandstones with lenses of limestone, as well as come conglomerates. Gradually the sea deepened, resulting in the deposition of finer-grained sediments, although mixed in with the shales that resulted are coarser-grained sediments such as sandstones, indicating an alternating regime of deep and shallow water over the millions of years in which the Juncal Formation was laid down. Another peculiarity of the unit is that fossil foraminifers from very different depth environments are often mixed together; this has been interpreted as the result of turbidity currents and other submarine mass movements. Rip-up clasts and conglomerates which appear in the Juncal also support this interpretation. Some of the foraminifers are characteristic of lower bathyal depths, indicating that some of the Juncal formation was deposited at a depth of up to 2,000 meters.
Paleontology
While the formation is rich in microfossils, particularly foraminifersForaminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...
, larger fossils are uncommon. However, some of the stratigraphically lowest beds – such as the portion which is in contact with the Sierra Blanca limestone formation – contain larger fossils characteristic of nearshore and reef environments. These include oysters, especially in limestone-rich beds and limestone lenses. Other fossils from the Juncal include algae, worms, brachiopods and molluscs.