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 and animal paleobiota the paleolake beds as a whole are considered one of the great Canadian ''Lagerstätten''. The paleobiota represented are of an upland subtropical to temperate ecosystem series immediately after the Paleocene–Eocene thermal maximum, and before the increased cooling of the middle and late Eocene to Oligocene. The fossiliferous deposits of the region were noted as early as 1873, with small amounts of systematic work happening in the 1870–1920s on British Columbian sites, and 1920–1930s for Washington sites. Focus and more detailed descriptive work on the Okanagan Highland sites started in the late 1960s.

Extent

The majority of the paleolake deposits are compression fossils in lake bed sediments spanning a transect, which have been grouped informally into "Northern", "Central", and "Southern" sites. The Northern sites are composed of unnamed Ootsa Group formations which outcrop as the "Driftwood shales" near Smithers, British Columbia, and the "Horsefly shales", of an unnamed formation and unnamed group which outcrop around Horsefly, British Columbia. The Central sites represent Kamloops Group formations with the McAbee Fossil Beds, Tranquille River site and Falkland site, all in the Tranquille Formation, the Quichena site and Stump Lake site in the Coldwater Beds near Barriere, British Columbia. The Southern sites include the Princeton Group Allenby Formation sites surrounding Princeton, British Columbia, such as "Nine Mile Creek", "One Mile Creek", "Pleasant Valley", "Thomas Ranch", "Vermilian Bluffs", and "Whipsaw Creek". The most southerly of the Okanagan Highlands lakes, the Klondike Mountain Formation in Northern Ferry County, Washington, include the "Boot Hill site", "Corner Lot site", "Gold Mountain site", "Knob Hill site", and "Mount Elizabeth site". Closely correlated with the Klondike Mountain Formation are the Penticton groups Kettle River, Marama and Marron Formations in the Boundary District along the Canada–United States border.

There is debate as to the affiliation of the, now lost, Quesnel sites with the Greater Okanagan Highlands. Archibald ''et al'' (2018) in the monograph overview of the Highlands Hymenoptera included them as part of the series. However the certainty for the placement was later questioned by Archibald and Cannings (2022) who opted to tentatively exclude Quesnel from the highlands while discussing the history of field collecting in the region.

History

The earliest geological work in south and central British Columbian sites was during exploratory expeditions under the leadership of George Mercer Dawson in the mid-1870s to document the coal and mineral resources of British Columbia. During the 1875 field work, fossil collections were made in the Quesnel region. Two years later, in 1877, the expedition explored the Okanagan, Nicola, and North Thompson valleys with field collecting along areas of the Similkameen River, Whipsaw Creek and Nine-Mile Creek, north at around Quilchena, and at several areas near Kamloops. These expeditions resulted in a series of papers on the plants, authored by John William Dawson, later David Pearce Penhallow and Edward Wilber Berry. The insects were first detailed by Samuel Hubbard Scudder, with follow-up papers by Anton Handlirsch. 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. The first brief work on fish from the Highlands occurred with the 1893 report by Edward Drinker Cope on several fish sent to him by George Dawson from the Tranquille and Princeton areas. While Cope deemed the Tranquille specimens too incomplete to identify, he did provide the description for '' "Amyzon" brevipinne'' from the Similkameen River fossils.

Republic, Washington, area fossils were first reported by Joseph Umpleby (1910), based on fish he collected near the Tom Thumb mine, and given a tentative late Miocene age. Two of the fish were figured and briefly mentioned 7 years later by Charles R. Eastman, who assigned them tentatively to ''"Amyzon" brevipinne'', making one of the early connections between Republic and the British Columbian sites. Report of the plant fossils of the Republic area were first published by Berry (1929) who included the Republic fossils as part of the Latah Formation. The inclusion within an expanded Latah Formation was questioned by Roland W. Brown in 1936, who noted the similarities between Republic and other older fossil sites, combined with the Republic lake bed's overlying basalts then thought to be of similar age to the Columbia River Basalts. This similarity was again noted by Chester A. Arnold during his review of the conifer flora associated with the Princeton basin. Arnold noted the Allenby sites shared over half of the taxa that had been previously reported from Republic.

Starting in the early 1960s and extending through the 1980s were a series of papers on the British Columbian sites combining palynology and the newly devised process of potassium–argon dating to better understand the geochronology of the sites. The first report of K–Ar dating at a group of sites was by Glenn E. Rouse and William H. Mathews (1961) who tested biotite samples from four locations. They sampled volcanic ashes from Princeton and Rock Creek; a Trachyte flow from Savona Mountain and a diabase flow or sill from Tranquille, returning results of , then classified as Early Middle Eocene. They noted the very distinct similarities with the Republic flora, as Arnold had, and posited that the Republic fossils were of the same age, and not Oligocene as then considered. Based on the estimated age, the sites were noted to be coeval