The Cozy Dell Shale is a
geologic formation
A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...
of middle
Eocene
The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
age
that crops out in the
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. It is the westernmost range in the Transverse Ranges.
The range is a large fault block of Cenozoic age create ...
and
Topatopa Mountains
The Topatopa Mountains are a mountain range in Ventura County, California, north of Ojai, Santa Paula, and Fillmore. They are part of the Transverse Ranges of Southern California.
Etymology
A name for the mountains was first inscribed within the ...
of California, extending from north of
Fillmore in
Ventura County
Ventura County () is a County (United States), county in Southern California, the southern part of the U.S. state of California. As of the 2020 United States Census, 2020 census, the population was 843,843. The largest city is Oxnard, California ...
westward to near
Point Arguello
Point Arguello ( Spanish: ''Punta Argüello'') is a headland on the Gaviota Coast, in Santa Barbara County, California, near the city of Lompoc. The area was first used by the United States Navy in 1959 for the launch of military and soundin ...
, north of
Santa Barbara. Because the Cozy Dell easily weathers to a clay-rich soil, it crops out infrequently and generally forms dense stands of
chaparral in saddles between peaks and ridges of the more resistant
Matilija and
Coldwater formations.
Type locality and extent
![CozyDell2](https://upload.wikimedia.org/wikipedia/commons/4/4b/CozyDell2.jpg)
The Cozy Dell was first described in 1928 as the middle member of the Tejon Formation. However, Dibblee (1966) reclassified it as a formation, along with the underlying
Matilija Sandstone
The Matilija Sandstone () is a sedimentary geologic unit of Eocene epoch in the Paleogene Period, found in Santa Barbara and Ventura Counties in Southern California.
It consists of thick layers of sandstone, made up of grains of feldspar and qu ...
and overlying
Coldwater Sandstone
The Coldwater Sandstone (or Coldwater Formation) 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 pri ...
. He also named the formation after its
type locality at
Cozy Dell Canyon, which is located on the south side of Nordhoff Ridge, just north of
Ojai
Ojai ( ; Chumash: ''’Awhaỳ'') is a city in Ventura County, California. Located in the Ojai Valley, it is northwest of Los Angeles and east of Santa Barbara. The valley is part of the east–west trending Western Transverse Ranges and is ...
in Ventura County.
The Cozy Dell crops out continuously in the mountains to the north and east of Ojai, where it is exposed along much of the south slope of the Topatopa Mountains in Ventura County, and continues westward along the south slope of Santa Ynez Mountains into Santa Barbara County. Here, it is overlain by younger rocks at
San Marcos Pass
San Marcos Pass (Chumash: ''Mistaxiwax'') is a mountain pass in the Santa Ynez Mountains in southern California.
It is traversed by State Route 154. The pass crosses the Santa Ynez through a southwestern portion of Los Padres National Forest, ...
, on the crest of the Santa Ynez Range, cropping out west of the pass and forming much of the crest north of Goleta. It disappears north of the Santa Ynez Mountains beneath younger formations, and is removed, for the most part, by erosion north of the Santa Ynez River, where it appears at only a few localities. It grades into and is overlain by the
Sacate Formation in the western part of its outcrop belt, and dips south beneath the modern California coastal plain in the southern part.
Continuing south into the offshore region beneath the
Santa Barbara Channel
The Santa Barbara Channel is a portion of the Southern California Bight and separates the mainland of California from the northern Channel Islands. It is generally south of the city of Santa Barbara, and west of the Oxnard Plain in Ventura Cou ...
, the Cozy Dell is found in oil wells at
Molino field some feet below the sea floor.
The thickness of the formation increases, more or less, from east to west and averages about in the Santa Ynez Mountains. However, thicknesses vary widely, ranging from a maximum of west of San Marcos Pass,
to thick in the subsurface of the offshore
Hondo Oil Field,
and only thick in the offshore Molino Gas Field, which is located west of Hondo.
Description
The Cozy Dell is a gray to dark gray
argillaceous
Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.
Clay minerals ...
to silty shale that weathers to brown and olive. It contains minor sandstone beds at some localities, and some
conglomerate at others, most significantly west of San Marcos Pass.
When exposed to surface conditions, it breaks down quickly to form a clay-rich soil that supports dense
chaparral growth. Because the Cozy Dell is layered between the highly resistant Matilija and Coldwater sandstones, and turned up on end through most of its extent, it erodes to create passes and saddles between peaks and ridges of the adjacent units.
Paleontology, age and depositional environments
Microfauna
Microfauna (Ancient Greek ''mikros'' "small" + New Latin ''fauna'' "animal") refers to microscopic animals and organisms that exhibit animal-like qualities. Microfauna are represented in the animal kingdom (e.g., nematodes, small arthropods) and t ...
in the Cozy Dell, primarily
planktonic (i.e., floating)
foraminifera
Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...
, place it in the upper part of the
Lutetian
The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the ...
stage
Stage or stages may refer to:
Acting
* Stage (theatre), a space for the performance of theatrical productions
* Theatre, a branch of the performing arts, often referred to as "the stage"
* ''The Stage'', a weekly British theatre newspaper
* Sta ...
(middle Eocene) of the
geologic timescale
The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochr ...
- the Lutetian correlating, more or less, to the middle part of a local Californian stage named the Narizian that is based largely on
benthic (i.e., bottom-dwelling) foraminifera. Although the formation is rich in microfossils, especially ones indicating deep-marine deposition, no significant larger fossils have been found in southwestern Santa Barbara County.
However, some larger fossils have been found in a few localities in the Topatopa Mountains to the east in Ventura County, particularly in the upper
Sespe Creek area near
Ojai
Ojai ( ; Chumash: ''’Awhaỳ'') is a city in Ventura County, California. Located in the Ojai Valley, it is northwest of Los Angeles and east of Santa Barbara. The valley is part of the east–west trending Western Transverse Ranges and is ...
, which contains an unusual shallow-marine assemblage of molluscs, starfish and brittle stars, as well as plant imprints that indicate close proximity to an ancient shoreline.
The Cozy Dell represents a time when the Eocene sea covering the California border land region reached its widest extent and maximum water depths. Thus, only the finest-grained sediments, principally a
micaceous
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
mud, reached the ocean bottom. A fairly high organic content of up to 3% or more of Total Organic Carbon (TOC) in these sediments indicate that microorganisms flourished and that there were anoxic conditions on the ocean floor conducive to the preservation of organic matter. These observations, together with the fossil evidence, indicate that the Cozy Dell was deposited in an ocean basin with a shoreline on what today would be the east and north side of the basin and gradually deepening to the west to water depths of possibly a thousand feet or more.
Magnetostratigraphy
Recent
magnetostratigraphic dating demonstrates that the Cozy Dell at its type locality was deposited during the
chron 20
geomagnetic reversal
A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's field has alternated ...
through the
chron 19 reversal - approximately 46 to 42 Ma (million years ago), which is consistent with deposition during
Lutetian
The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the ...
time. The paleomagnetic data also show that the tectonic block on which the Cozy Dell was deposited with other Eocene formations has rotated clockwise about 100 degrees since the onset of deposition. Thus, modern-day east-west trends indicated within the formation would originally have been oriented north-south.
Economic importance
The Cozy Dell is grouped with underlying and overlying sandstones as a potential hydrocarbon resource. Where this shale is found in the subsurface of oil and gas fields on the California coast, and offshore in the Santa Barbara Channel, it tends to be impermeable to fluid flow, as with most shales, and represents a cap rock that can trap hydrocarbons in underlying sandstone reservoirs.
However, a few offshore oil fields contain sections of Cozy Dell with sufficient porosity to act as petroleum reservoirs, and sufficient permeability to produce hydrocarbons at economic rates. One example is the now-abandoned Cuarta field, located about six miles offshore from
Gaviota, where the Cozy Dell is mainly shale but contains minor sandstone interbeds that previously produced oil from feet below the ocean floor.
[ PDF file available on CD fro]
www.consrv.ca.gov
Although there is no onshore oil or gas production from the Cozy Dell, its organic content is high enough that some geologists believe it may be a potential source rock for petroleum found in other units.
References and notes
See also
*
Rip-up clasts: often found at the base of sandy turbidites, such as the Cozy Dell
*
Lowe sequence
The Lowe sequence describes a set of sedimentary structures in turbidite sandstone beds that are deposited by high-density turbidity currents. It is intended to complement, not replace, the better known Bouma sequence, which applies primarily to t ...
: describes a set of sedimentary structures found in some
turbidite
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Sequencing
Turbidites wer ...
sandstone beds
{{Paleogene Footer
Geologic formations of California
Shale formations of the United States
Geology of Santa Barbara County, California
Geology of Ventura County, California
Eocene United States
Paleogene California
Santa Ynez Mountains
Topatopa Mountains
Eocene Series of North America