Paleoecology of Eocene lacustrine varves at Horsefly, British Columbia

Abstract

Middle Eocene lacustrine sediments, cropping out in the valley of the Horsefly River, British Columbia [Canada], contain abundant fossils of fishes, fish scales, fish coprolites, insects, leaves and diatoms. The fish scales, insects and leaves are preserved in at least 3 sequences of alternating light tuff and dark sapropel laminae, separated stratigraphically by coarse-grained structureless sequences. The proportions of the main types of fossils occurring in the light laminae compared with the dark laminae are significantly different, and are consistent with the hypothesis that the laminations are varves, with dark organic winter laminae and light inorganic summer laminae. Occasional graded sandy layers contain carbonized allochthonous plant remains and represent turbidity deposits caused by storms in the drainage basin. The varves were probably deposited in the deeper regions of a stratified, monomictic or meromictic lake in a warm temperate climate. The depositional environment was anaerobic, containing abundant H2S, and was free of turbulence and benthos. Fish were entombed mostly during the winter, insects during the spring and summer coprolites during the summer, and deciduous leaves during the late summer and autumn. The fish died of starvation and/or overturn-induced anoxia.