Log Jams and Juveniles: Unusual Deposits at the Cretaceous-Paleogene (K-Pg) Boundary in the Maastrichtian (66 Ma) Frenchman Formation of Saskatchewan, Canada
BAMFORTH, Emily L.; Royal Saskatchewan Museum, Eastend, SK, Canada.
The latest Mesozoic and earliest Cenozoic rocks of southwest Saskatchewan contain exceptional exposures of the Cretaceous-Paleogene (K-Pg) Boundary. The Frenchman Formation represents a northern extension of the latest Maastrichtian (66 Ma) Hell Creek Formation into Canada. This formation, and the overlying Paleocene Ravenscrag Formation, preserve a continuous sequence of time immediately before, during and after the end-Cretaceous mass extinction. One of the most notable K-Pg sites in the province is a locality known as ‘Hwy 37’. Here, the distinct K-Pg tonstein (‘boundary clay’) is 1-2 cm thick, and is bounded above and below by a coal seam measuring 1.5 m in total thickness. The coal above the boundary is characterized by blocky anthracite and is overlain by 35-45 cm of finely laminated carbonaceous shale. (This shale is a localized feature; it is much thinner or absent at other K-Pg sites in the province). The coal immediately below the K-Pg tonstein is also anthractic, but contains recognizable tree fossils. These fossils are represented by compressed segments of coalified logs, ranging in length from 15 – 40 cm, some with details of the bark and growth rings preserved. Interestingly, a similar deposit reoccurs 3.5 m lower in the Frenchman Formation. This ‘log jam’ deposit, which extends laterally for at least 400 m, is characterized by compressed segments of coalified trees, some with lengths exceeding five meters. Most specimens are so well preserved that bark structure, burls, and blisters of amber can be discerned. These logs often overlap one another, do not appear current-aligned, and are preserved in a fine-grained unconsolidated sandstone. The fine-grained sediment and lack of current ripples suggests this matrix was deposited in a lower energy environment after the original deposition event. Abundant sulfur-rich nodules may reflect low oxygen conditions that developed in pools of water around the jam. There are rare vertebrate fossils associated with this deposit, most of which seem to represent juvenile animals. These include a hatching troodontid metatarsal, a small dentary from a very young ceratopsian, and a juvenile hadrosaur scapula and parietal. These two deposits at and just below the K-Pg Boundary could be reflective of one or more event deposits, similar to that recently described from the Tanis Site in North Dakota. Ongoing research will help to better understand the nature of these deposits, providing more information about the events that occurred around the time of the Chicxulub impact.