The Allenby formation is a

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondar ...

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, for ...

which was deposited during the

Ypresian stage In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by the Eocene Lutetian Age. The Ypresian ...

of the

Early Eocene In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by the Eocene Lutetian Age. The Ypresian ...

. It consists of conglomerates,

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...

s with interbedded

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

s and

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

. The shales contain an abundance of insect, fish and plant fossils known from 1877 and onward, while the

Princeton Chert The Princeton Chert is a fossil locality in British Columbia, Canada, which comprises an anatomically preserved flora of Eocene Epoch age, with rich species abundance and diversity. It is located in exposures of the Allenby Formation on the east ...

was first indented in the 1950's and is known from anatomically preserved plants. There are several notable fossil producing localities in the Princeton & Tulameen basins. Historical collection sites included Nine Mile Creek, Vermilian Bluffs, and Whipsaw Creek, while modern sites include One Mile Creek, Pleasant Valley, Thomas Ranch, and the

Extent and correlation

The Allenby is estimated to have an overall extent of approximately , though actual outcroppings of the formation make up less than 1% of the formation, while other exploratory contact is via boreholes and mines. The

half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...

which contains the formation is separated into two major depositional basins, the Princeton basin around Princeton, British Columbia and the Tulameen basin centered approximately west. The grabens

extensional fault An extensional fault is a fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere. In most cases such a fault is also a normal fault, but may create ...

s at the eastern side of the basin place the hanging wall Allenby strata in contact with much older

foot wall In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

strata of the

Nicola Formation The Nicola Formation is a geologic formation in British Columbia. It preserves fossils dating back to the Triassic period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, enter ...

which dates to the

Upper Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. ...

. The Allenby Formation is the southern-most of the

Eocene Okanagan Highlands The Eocene Okanagan Highlands or Eocene Okanogan Highlands are a series of Early Eocene geological formations which span a transect of British Columbia, Canada, and Washington state, United States. Known for a highly diverse and detailed plant an ...

lakes in British Columbia, and second most southern site after the

Klondike Mountain Formation The Klondike Mountain Formation is an Early Eocene (Ypresian) geological formation located in the northeast central area of Washington state. The formation, named for the type location designated in 1962, Klondike Mountain north of Republic, Wash ...

Republic, Washington Republic is a city in Ferry County, Washington, United States. The population was 1,073 at the 2010 census, a 12.5% increase over the 2000 census. It is the county seat of Ferry County. It was the largest mining camp in the Republic Mining Dis ...

and northern

Ferry County Ferry County is a county located on the northern border of the U.S. state of Washington. As of the 2020 census, the population was 7,178, making it the fourth-least populous county in Washington. The county seat and largest city is Republic. Th ...

. In British Columbia, the formation is coeval to the Tranquille Formation, known from the

McAbee Fossil Beds The McAbee Fossil Beds is a Heritage Site that protects an Eocene Epoch fossil locality east of Cache Creek, British Columbia, Canada, just north of and visible from Provincial Highway 97 / the Trans-Canada Highway (Highway 1). The McAbee Fo ...

and

Falkland site Falkland may refer to: * Falkland, British Columbia, a community in Canada * Falkland, Nova Scotia, a community in Canada * Falkland Islands, an archipelago in the south Atlantic Ocean * Falkland, Fife, a former burgh in Fife, Scotland ** Falkla ...

, the

Coldwater Beds The Coldwater Beds are a geologic formation of the Okanagan Highlands in British Columbia, Canada. They preserve fossils dating back to the Ypresian stage of the Eocene period, or Wasatchian in the NALMA classification.Quilchena site, and

Driftwood Canyon Provincial Park Driftwood Canyon Provincial Park is a provincial park in British Columbia, Canada. Driftwood Canyon Provincial Park covers 23 ha of the Bulkley River Valley, on the east side of Driftwood Creek, a tributary of the Bulkley River, 10 km northe ...

. The highlands, including the Allenby Formation, have been described as one of the "Great Canadian '' Lagerstätten''" based on the diversity, quality and unique nature of the biotas that are preserved. The highlands temperate biome preserved across a large transect of lakes recorded many of the earliest appearances of modern genera, while also documenting the last stands of ancient lines. The warm temperate uplands floras of the Allenby Formation and the highlands, associated with downfaulted

lacustrine basin A lacustrine plain or lake plain is a plain formed due to the past existence of a lake and its accompanying sediment accumulation. Lacustrine plains can be formed through one of three major mechanisms: glacial drainage, differential uplift, and inla ...

s and active volcanism are noted to have no exact modern equivalents, due to the more seasonally equitable conditions of the Early Eocene. However, the formation has been compared to the upland ecological islands in the Virunga Mountains within the

Albertine Rift The Albertine Rift is the western branch of the East African Rift, covering parts of Uganda, the Democratic Republic of the Congo (DRC), Rwanda, Burundi and Tanzania. It extends from the northern end of Lake Albert to the southern end of Lake Tan ...

of the

African rift valley The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of ...

. The earliest work in the region was on exploratory expeditions in 1877 and 1878, with fossils collected in the areas of Nine-Mile Creek, Vermilian Bluffs on the

Similkameen River The Similkameen River runs through southern British Columbia, Canada, eventually discharging into the Okanagan River near Oroville, Washington, in the United States. Through the Okanagan River, it drains to the Columbia River. The river is said ...

, and

Whipsaw Creek Whipsaw Creek is a creek in the Similkameen Country, Similkameen region of British Columbia. The creek flows into the Similkameen River from the west and is approximately upriver from Princeton, British Columbia. The Crowsnest Highway, Highway 3, ...

. While reporting on additional plant fossils collected from British Columbia, Penhallow (1906) noted the likely coeval status of the Princeton basins with many of the sites now considered the Okanagan Highlands. Modern collecting has centered on the areas around One Mile Creek, Pleasant Valley, and Thomas Ranch.

Age

The age estimates for the Allenby Formation have varied a number of times since the first explorations happened in the 1870s. Shaw (1952) dated the formation as

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

, an age followed by Arnold (1955). Half a decade later, the older age of was first suggested, with a younger age being suggested at in 2000 and an older date of obtained from

uranium–lead dating Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4.5 billion years ago with routi ...

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

s from Vermilion Bluffs shale in 2005.

Lithology

The Allenby is composed of cyclical sedimentation events that were deposited along the course of a river-system in conjunction with depositional areas from nearby lakes and wetlands. Coeval volcanic eruptive events are recorded as interbeds of

tephra Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they r ...

s and lavas, while the riverine course is marked with depositional areas of conglomerates and sandstones. The quieter environments are noted for finer layers of shales and coalified layers. The coal seams throughout the formation are typically sub-bituminous. Notable in conjunction with the coal seams are sections of

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

which formed during silica rich periods. The rapid cyclical changes from coal to chert and back are not noted in any other fossil locality in the world. An estimated 49 coal-chert cycles are known, though the exact conditions for this process are not well understood. Silica rich volcanic episodes in the region during deposition would have been needed for formation of the cherts, while slowly moving waters and gently subsiding terrains would be needed for the peats and

fen A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetlands along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as mires ...

s to accumulate. Rates of organic deposition in swamps have been estimated at approximately in modern temperate climates, this suggests the time needed for each chert layer would be at least 100 years or more, with the full sequence of cycles taking place over no more then 15,000 years.

Palynoflora

Palynological analysis of samples from the Thomas ranch site by Dillhoff ''et al'' (2013) resulted in the identification of 32 pollen and spore types that were assignable to family or genus level, with a total number of distinct pollen and spore types, including unassignable morphotypes, number over 70. The predominant pollens of the site are

conifer Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All ext ...

s, which make up between 85%–97% of the total pollens, while the angiosperm pollens are dominated by members of

Betulaceae Betulaceae, the birch family, includes six genera of deciduous nut-bearing trees and shrubs, including the birches, alders, hazels, hornbeams, hazel-hornbeam, and hop-hornbeams numbering a total of 167 species. They are mostly natives of the ...

. Several

pteridophyte A pteridophyte is a vascular plant (with xylem and phloem) that disperses spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as " cryptogams", meaning that their means of reproduction is hidden. Fer ...

families and genera are represented as spore fossils alone, without corresponding megafossil records, including

Lycopodiaceae The Lycopodiaceae (class Lycopodiopsida, order Lycopodiales) are an old family of vascular plants, including all of the core clubmosses and firmosses, comprising 16 accepted genera and about 400 known species. This family originated about 380 m ...

Osmundaceae Osmundaceae (royal fern family) is a family of ferns containing four to six extant genera and 18–25 known species. It is the only living family of the order Osmundales in the class Polypodiopsida (ferns) or in some classifications the only ord ...

, and

Schizaeaceae Schizaeaceae is a family of ferns in the order Schizaeales. In the Pteridophyte Phylogeny Group classification of 2016 (PPG I), it includes only two genera. Alternatively, two families kept separate in PPG I, Lygodiaceae and Anemiaceae, may be ...

. Similarly, at least three additional conifer genera are only present as pollen fossils and up to 12 angiosperms are present in the pollen record. Sometimes considered a Biostratgraphic

index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...

, the angiosperm palynospecies '' Pistillipollenites macgregorii'' has been recovered from several sites in the Allenby Formation, while the palynospecies '' Erdtmanipollis pachysandroides'' is rare, having only been reported from the formation twice.

Compression paleobiota

A group of six mosses were described from the Allenby Formation by Kuc (1972, 1974) representing the genera '' Ditrichites'', '' Hypnites'' and '' Plagiopodopsis'', with two species placed in the morphogenus '' Muscites''. Dillhoff ''et al'' (2013) identified twelve distinct

gymnosperm The gymnosperms ( lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes, forming the clade Gymnospermae. The term ''gymnosperm'' comes from the composite word in el, γυμν ...

taxa spanning the families

Cupressaceae Cupressaceae is a conifer family, the cypress family, with worldwide distribution. The family includes 27–30 genera (17 monotypic), which include the junipers and redwoods, with about 130–140 species in total. They are monoecious, subdioeci ...

Ginkgoaceae The Ginkgoaceae is a family of gymnosperms which appeared during the Mesozoic Era, of which the only extant representative is ''Ginkgo biloba'', which is for this reason sometimes regarded as a living fossil A living fossil is an extant tax ...

, and

Pinaceae The Pinaceae, or pine family, are conifer trees or shrubs, including many of the well-known conifers of commercial importance such as cedars, firs, hemlocks, larches, pines and spruces. The family is included in the order Pinales, formerly kn ...

. While being the minority component of the Thomas Ranch flora by total fossil numbers,

angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...

s have a higher diversity, with 45 distinct morphotypes represented as foliage, reproductive structures, or both. Seventeen of the morphotypes are identifiable to genus or species, with members of the family

being most prominent. At least common one leaf type is suggested to possibly represent an extinct plant order, but has not been described. Only two

species have been described from the compression flora, ''

Azolla primaeva ''Azolla primaeva'' is an extinct species of "water fern" in the family Salviniaceae known from Eocene fossils from the Ypresian stage (), found in southern British Columbia. The species was first described from poorly preserved material co ...

'' by Penhallow (1890) and ''

Equisetum similkamense ''Equisetum similkamense'' is an extinct horsetail species in the family Equisetaceae described from a group of whole plant fossils including rhizomes, stems, and leaves. The species is known from Ypresian sediments exposed in British Columbia, ...

'' by Dawson (1878). The following fossil conifers, pteridophytes, ginkgophytes and bryophytes have been described from the Allenby Formation:

Bryophytes

Pteridophytes

Gingkophytes

Pinophytes

Angiosperms

Insects

Coleopterans

Dipterans

Hemipterans

Hymenopterans

Mecopterans

Neuropterans

Odonata

Raphidiopterans

Vertebrates

Princeton Chert biota

The

Princeton chert The Princeton Chert is a fossil locality in British Columbia, Canada, which comprises an anatomically preserved flora of Eocene Epoch age, with rich species abundance and diversity. It is located in exposures of the Allenby Formation on the east ...

biota is unique in the Allenby formation due to the silicification of the chert, which has resulted in cellular and anatomical preservation of the organisms. As of 2016 over 30 different plant taxa had been described from chert fossils along with a number of fungal species.

Fungi

Ferns

Conifers

Angiosperms

References

{{commons category, Allenby Formation Sandstone formations of Canada Shale formations Coal formations Coal in Canada Lacustrine deposits