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.

The formation comprises mudstones, shales and tuffs deposited in a lacustrine environment and has provided many insect fossils, as well as indeterminate birds and fossil flora.

Description

The Coldwater Beds were defined by Dawson (1895) based on a section along the Coldwater River in the Okanagan Highlands. The formation reaches a thickness of 230 metres (750 ft), and comprises mudstones, shales and tuff deposited in a lacustrine environment. U-Pb dating of thick tephra, combined with Ar-Ar dates of sanidine from same bed provided an Early Eocene age. The tephra was deposited within insect-bearing shales.

Climate

During the Early Eocene, the climate of much of northern North America was warm and wet, with mean annual temperatures (MAT) as high as 20 °C (68 °F), mean annual precipitation (MAP) of 100 to 150 centimetres (39 to 59 in), mild frost-free winters (coldest month mean temperature >5 °C (41 °F)), and climatic conditions that supported extensive temperate forest ecosystems.

The Quilchena fossil locality is dated to 51.5 ± 0.4 Ma corresponding to the Early Eocene Climatic Optimum (EECO), and is reconstructed as the warmest and wettest of the Early Eocene upland sites from the Okanagan Highlands of British Columbia and northern Washington State. Mean annual temperature (MAT) is estimated from leaf margin analysis as 16.2 ± 2.1 °C (61.2 ± 3.8 °F) and 14.6 ± 4.8 °C (58.3 ± 8.6 °F). Using bioclimatic analysis of 45 nearest living relatives, a moist mesothermal climate is indicated (MAT 12.7 to 16.6 °C (54.9 to 61.9 °F); cold month mean temperature (CMMT) 3.5 to 7.9 °C (38.3 to 46.2 °F) and mean annual precipitation (MAP) of 103 to 157 centimetres (41 to 62 in)/yr. Leaf size analysis estimates a mean annual precipitation of 121 ± 39 centimetres (48 ± 15 in).

Fossils

A wide variety of fossils occur in the formation, including abundant fish remains, insects, and plants, and rare occurrences of molluscs, ostracods, and birds:

Flora

Fossil plants were first reported from the Coldwater Beds at the Quilchena site and nearby by Penhallow (1908) with an expanded taxonomic list given by Mathewes et al (2016).

Pollen taxa

Molluscs

Mark Wilson (1987) noted, without taxonomic identification, that unidentified small bivalves are a component of the Quilchena invertebrate paleofauna.

Insects

The insect fossils studied by Wilson (1987) showed Bibionidae dominating the paleoentemofauna, at 28% of all specimens examined at that time. An additional 13% of the fossils were other dipterans while up to 41% of all insects still had attached wings. The invertebrates trace fossils included two undescribed species of Trichoptera larval cases and burrowing or tracks in the sediment.

Fish

• Amia sp. scales
• Amyzon brevipinne

Birds

• Aves indet. feathers

Mammals

• Insectivorous mammalian indet. tooth

Correlations

The formation has been correlated with the Eocene Okanagan Highlands floras including the Allenby Formation, Kamloops Group, Horsefly shales, and Driftwood Canyon site of British Columbia, along with the Klondike Mountain Formation of Washington State. Additionally its correlated with the Margaret Formation of Ellesmere Island, Nunavut, the Chickaloon Formation of Alaska, Wishbone, Chuckanut and Iceberg Bay Formations, all of similar age. The flora of the Coldwater Beds has been correlated to the Chu Chua Formation of southeastern British Columbia. The formation also correlates with the Springbrook, Kettle River and O'Brien Creek Formations in Washington, United States.

See also

• List of fossiliferous stratigraphic units in British Columbia