Margaret Formation
The Margaret Formation is a geologic formation of the Eureka Sound Group in the Sverdrup Basin in Northwest Territories and Nunavut, Canada. The unit belonging to the Eureka Sound Group which crops out at Ellesmere Island preserves fossils dating back to the Early Eocene period, or Wasatchian in the NALMA classification. The Margaret Formation comprises sandstones, sandy siltstones, clay-rich sandstones, coal seams and clay-rich coal seams and volcanic ash beds. The thickness of the formation, which overlies the Mount Moore Formation, reaches about . Radiometric dating of the formation provided ages of 52.6 ± 1.9 Ma (2010 ) and 53.7 ± 0.6 Ma (2017). The area where the formation was deposited in the Early Eocene experienced a much warmer climate than the High Arctic today, with mean annual temperatures ranging from and warmest month mean temperatures from . The deltaic to swamp environment of the Margaret Formation has provided a diverse fauna of various groups of mammals, birds ("Presbyornis" and "Gastornis"), reptiles (turtles, snakes, lizards and crocodiles) and fish. Description. The Margaret Formation comprises sandstones, sandy siltstones, clay-rich sandstones, coal seams and clay-rich coal seams and volcanic ash beds. The thickness of the formation, which overlies the  Mount Moore Formation, reaches about . The formation was probably deposited in a lush proximal delta front to delta plain environment, with abundant channels and coal swamps. Dating. A volcanic ash layer containing crandallite in the middle of the formation was dated using U-Pb radiometric dating in 2017 to 53.7 ± 0.6 Ma. In 2010, ashes of the formation were dated to 52.6 ± 1.9 Ma. Arctic climate of the Early Eocene. During the Early Eocene, the climate of much of northern North America was warm and wet, with mean annual temperatures (MAT) as high as , mean annual precipitation (MAP ) of , mild frost-free winters (coldest month mean temperature >), and climatic conditions that supported extensive temperate forest ecosystems. Ensemble estimates of mean annual temperatures for the high-latitude fossil localities in Arctic Canada ranged from , with the range of coldest month mean temperature from and warmest month mean temperatures from . Mean annual precipitation estimates for the Margaret Formation ranged between . The mean summer precipitation has been estimated at and mean winter precipitation at . The fossils and sedimentology indicate a lush, rain forest community on a coastal delta plain. Multiple palaeoclimate proxies, ranging from oxygen isotope analysis of vertebrate bones  and teeth to palaeofloral analyses, estimate a mild temperate climate for the Eocene High Arctic, where winters remained at or just above freezing and summer temperatures extended to or higher. These temperatures are a far cry from today's High Arctic, where central Ellesmere Island experiences a mean annual temperature of , a warm month mean temperature of about and a cold month mean temperature of or colder. Despite the mild Eocene Arctic climate, the vertebrate fauna would have experienced months of total darkness and cooler temperatures during the winter. Recent isotopic work suggests that some mammals, including the hippo-like  Coryphodon, were year-round residents in the High Arctic. Given that "Gastornis" was large (approaching ) and flightless, it likely also was a year-round resident of the Arctic. In contrast, the volant "Presbyornis" might have been a seasonal migrant to the Arctic. Fossil content. The formation has provided the following fossils: Correlations. The formation has been correlated with the Allenby Formation and Kamloops Group of British Columbia, the Chickaloon Formation of Alaska, and with the Wishbone, Chuckanut and Iceberg Bay Formations. The upper Margaret Formation also has been correlated with the Buchanan Lake Formation of the Eastern Arctic Archipelago. Margaret