Cozy Dell Shale
Encyclopedia
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. A relatively soft and easily weathered unit, it forms saddles and passes between the more resistant Coldwater and Matilija formations, which form the prominent scenic peaks and ridges in the range. It outcrops infrequently, weathering instead to clayey soil which supports dense stands of chaparral
.
of the Cozy Dell is in Cozy Dell Canyon, on the south slope of Nordhoff Ridge north of Ojai
in Ventura County. The formation extends continuously from the mountains north of Ojai westward along the south slope of the Santa Ynez Mountains into Santa Barbara County, crossing the crest of the Santa Ynez range under and west of San Marcos Pass
and then back again, forming much of the crest of the range north of Goleta. In the western portion of its range it grades into the Sacate Formation; north of the Santa Ynez crest it disappears underneath other units including the Coldwater Sandstone
. South of the mountains, the unit dips under the coastal plain and then under the Santa Barbara Channel, where it is encountered in drill cores many thousands of feet below the sea floor. While the unit appears in a few small scattered locations north of the Santa Ynez River, there it has mostly been eroded away.
Thickness of the formation varies widely. It is thickest west of San Marcos Pass, in the approximate center of its distribution, where it reaches 4,000 feet; within the Santa Ynez Mountains overall it averages about 1,700 feet thick. In drill cores in the Santa Barbara Channel, it is considerably thinner, measuring 650 feet thick in the Hondo Oil Field and only 350 feet in the Molino Gas Field, west of the Hondo field. The formation dips to the south from the Santa Ynez Mountains, underneath the coastal plain, and then under the Santa Barbara Channel; in the Molino field the top of the formation is 13,000 feet below the sea floor.
The unit was originally considered to be the middle part of the three-unit Tejon Formation, and appears that way in publications beginning in 1928. By the middle 1960s, it was considered to be a formation in its own right, along with the underlying Matilija and overlying Coldwater formations.
, most significantly west of San Marcos Pass. The unit breaks apart quickly when exposed to the elements, weathering to clayey soil supporting dense chaparral
growth. Since it is sandwiched between two highly resistant rock units – the Matilija and Coldwater sandstones – and upended onto its side through most of its extent, it erodes to passes and saddles between the sandstone peaks and ridges of the adjacent units. Where it occurs in the subsurface, such as in the oil fields on the coast and offshore, it is usually highly impermeable – typical of shaly units – and serves as a cap rock, trapping hydrocarbons below. In some cases, such as in the now-abandoned Cuarta Offshore Oil Field, about six miles west-southwest of Gaviota, the individual sandstone beds within the unit are sufficiently permeable to contain oil or gas, which in the case of the Cuarta field are 7,000 feet below the ocean floor.
As a potential hydrocarbon resource, the unit is grouped with above- and below-lying sandstone units as part of the Gaviota-Sacate-Matilija Sandstone Play. While the Cozy Dell itself does not likely contain large pools of oil or gas, it may be a source rock for petroleum found in other units.
The Cozy Dell represents the era when the Eocene sea covering the region reached its maximum depth. Only the finest-grained sediments reached the ocean bottom, and the land providing the outflow had been eroded to low relief. The sediments were principally a micaceous mud, although a fairly high organic content indicates microorganisms flourished on the ocean bottom at that time. Recent magnetostratigraphic
dating has determined that the Cozy Dell unit was deposited between approximately 46 to 42 Ma (million years ago). Additionally, studies of the correlation of the unit with similar units in the California Central Valley, along with paleomagnetic data, show that the tectonic block on which the Cozy Dell and subsequent formations was deposited – including the western part of the Transverse Ranges – has rotated clockwise almost 90 degrees since the time of deposition.
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 principally in 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 north of Santa Barbara, California
Santa Barbara, California
Santa Barbara is the county seat of Santa Barbara County, California, United States. Situated on an east-west trending section of coastline, the longest such section on the West Coast of the United States, the city lies between the steeply-rising Santa Ynez Mountains and the Pacific Ocean...
, from 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...
in the west to 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...
in the east. A relatively soft and easily weathered unit, it forms saddles and passes between the more resistant Coldwater and Matilija formations, which form the prominent scenic peaks and ridges in the range. It outcrops infrequently, weathering instead to clayey soil which supports dense stands 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...
.
Type locality and extent
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 Cozy Dell is in Cozy Dell Canyon, on the south slope of Nordhoff Ridge north of Ojai
Ojai, California
Ojai is a city in Ventura County, California, USA. It is situated in the Ojai Valley , surrounded by hills and mountains. The population was 7,461 at the 2010 census, down from 7,862 at the 2000 census.-History:Chumash Indians were the early inhabitants of the valley...
in Ventura County. The formation extends continuously from the mountains north of Ojai westward along the south slope of the Santa Ynez Mountains into Santa Barbara County, crossing the crest of the Santa Ynez range under and west of San Marcos Pass
San Marcos Pass
San Marcos Pass is a mountain pass in the Santa Ynez Mountains in California.It is traversed by State Route 154. The pass connects Los Olivos and the Santa Ynez Valley with Santa Barbara, California...
and then back again, forming much of the crest of the range north of Goleta. In the western portion of its range it grades into the Sacate Formation; north of the Santa Ynez crest it disappears underneath other units including the Coldwater Sandstone
Coldwater Sandstone
The Coldwater Sandstone is a sedimentary geologic unit of Eocene age found in Southern California, primarily in and south of the Santa Ynez Mountains of Santa Barbara County, and east into Ventura County. It consists primarily of massive arkosic sandstone with some siltstone and shale...
. South of the mountains, the unit dips under the coastal plain and then under the Santa Barbara Channel, where it is encountered in drill cores many thousands of feet below the sea floor. While the unit appears in a few small scattered locations north of the Santa Ynez River, there it has mostly been eroded away.
Thickness of the formation varies widely. It is thickest west of San Marcos Pass, in the approximate center of its distribution, where it reaches 4,000 feet; within the Santa Ynez Mountains overall it averages about 1,700 feet thick. In drill cores in the Santa Barbara Channel, it is considerably thinner, measuring 650 feet thick in the Hondo Oil Field and only 350 feet in the Molino Gas Field, west of the Hondo field. The formation dips to the south from the Santa Ynez Mountains, underneath the coastal plain, and then under the Santa Barbara Channel; in the Molino field the top of the formation is 13,000 feet below the sea floor.
The unit was originally considered to be the middle part of the three-unit Tejon Formation, and appears that way in publications beginning in 1928. By the middle 1960s, it was considered to be a formation in its own right, along with the underlying Matilija and overlying Coldwater formations.
Characteristics and deposition environment
The Cozy Dell Shale is gray to dark gray, weathering to brown and olive, and is mostly an argillaceous to silty shale unit. It also contains small beds of sandstones and a few areas of conglomerateConglomerate (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...
, most significantly west of San Marcos Pass. The unit breaks apart quickly when exposed to the elements, weathering to clayey soil supporting dense 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...
growth. Since it is sandwiched between two highly resistant rock units – the Matilija and Coldwater sandstones – and upended onto its side through most of its extent, it erodes to passes and saddles between the sandstone peaks and ridges of the adjacent units. Where it occurs in the subsurface, such as in the oil fields on the coast and offshore, it is usually highly impermeable – typical of shaly units – and serves as a cap rock, trapping hydrocarbons below. In some cases, such as in the now-abandoned Cuarta Offshore Oil Field, about six miles west-southwest of Gaviota, the individual sandstone beds within the unit are sufficiently permeable to contain oil or gas, which in the case of the Cuarta field are 7,000 feet below the ocean floor.
As a potential hydrocarbon resource, the unit is grouped with above- and below-lying sandstone units as part of the Gaviota-Sacate-Matilija Sandstone Play. While the Cozy Dell itself does not likely contain large pools of oil or gas, it may be a source rock for petroleum found in other units.
The Cozy Dell represents the era when the Eocene sea covering the region reached its maximum depth. Only the finest-grained sediments reached the ocean bottom, and the land providing the outflow had been eroded to low relief. The sediments were principally a micaceous mud, although a fairly high organic content indicates microorganisms flourished on the ocean bottom at that time. Recent magnetostratigraphic
Magnetostratigraphy
Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their characteristic remanent magnetization , that is, the...
dating has determined that the Cozy Dell unit was deposited between approximately 46 to 42 Ma (million years ago). Additionally, studies of the correlation of the unit with similar units in the California Central Valley, along with paleomagnetic data, show that the tectonic block on which the Cozy Dell and subsequent formations was deposited – including the western part of the Transverse Ranges – has rotated clockwise almost 90 degrees since the time of deposition.