Ballast Brook and Beaufort Formations (late Tertiary) on Northern Banks Island, Arctic Canada

Abstract

Excellent exposures of late Tertiary terrestrial sediments occur in the Ballast Brook region of northwestern Banks Island (Canadian Arctic Archipelago). These sediments contain abundant concentrations of organic detritus as well as autochthonous peats which can be traced for some kilometres. All of these deposits were previously assigned to the Beaufort Formation. In this paper, the deposits formerly grouped as the Beaufort Formation are subdivided into two formations: the newly named Ballast Brook Formation and the Beaufort Formation. The Ballast Brook Formation is characterized by alluvial sand, silt-clay and peat with compressed, altered wood. Paleobotanical evidence indicates an early to mid-Miocene age, with mid-Miocene being the most probable. This formation is believed to overlie the Eureka Sound Group and is in erosional contact with the overlying Beaufort Formation. Plant macrofossils from the Ballast Brook Formation include a number of taxa now found in northern hardwood and southern boreal forests, while the prominent peat which characterizes the formation contains a number of taxa that now grow or could be expected to grow in a cypress swamp type of environment. For example the conifer Glyptostrobus is well represented in the peat. The Beaufort Formation at Ballast Brook consists of alternating sands and gravels, and silt in the lower part. It is characterized by an abundance of unaltered wood and other plant materials. Paleobotanical evidence indicates an early Pliocene to early late Pliocene age (i.e. approx. 5-3 Ma). Like the Ballast Brook Formation, the Beaufort Formation contains an abundance of fossils of conifers, but in contrast with the Ballast Brook Formation nearly all of them belong to Pinaceae, a group well represented in the present boreal forest. The Beaufort Formation also contains insect fossils, which are very rare in the Ballast Brook Formation. Together, the insect and plant macrofossils suggest an environment similar to, but richer in species (some extinet; others now found only in Asia) than present boreal forest and taiga. The Ballast Brook Formation flora resembles the flora from Mary Sachs gravel (formerly Beaufort Formation) on southwestern Banks Island and the flora from the West River beds on the Northwest Territories mainland. It also is similar to a 40 Ar 39 Ar dated mid-Miocene flora from east-central Alaska and to the Mamontova Gora (mid-Miocene) flora from Russia.