ABSTRACT

Five Tertiary units ranging in age from Eocene to Late Miocene are described from the Quesnel area of central British Columbia. From oldest to youngest they are: 1, an Eocene assemblage of pyroclastics and lavas, dated by K-Ar as 42-49 Ma and correlative with the Kamloops Group; 2, a tilted and locally folded early Oligocene unit, the Australian Creek Formation, about 500 m thick, that includes lignites, clay, silts, sand and gravels; 3, flat-lying gravels, sands, silts, and clays, with minor lignites, about 150 m thick, here defined as the Fraser Bend Formation, and considered early Middle Miocene in age; 4, a widespread diatomite up to 12 m thick, here named the Crownite Formation, and estimated as late Middle Miocene in age; and 5, one or more thin lava flows forming plateau basalts, and dated by K-Ar at about 8.5 Ma (Late Miocene).

Sediment deposition occurred in a tectonic trench (Oligocene), and a broad valley under fan, floodplain, swamp, and lacustrine conditions (Miocene). Changes in spore/pollen and leaf assemblages indicate a cooling climatic trend, from a paratropical climate in the mid-Eocene (Kamloops Group equivalent), to warm temperate and drier during both the Early Oligocene (Australian Creek) and the Middle Miocene (Fraser Bend). Further cooling, marked by a sharp increase in conifers, occurred between the deposition of the Fraser Bend and that of the Crownite Formation. Climatic changes are correlated with those recognized in other regions on the basis of floral, faunal, and oxygen isotope studies, and are related to such possible causative factors as orogeny, continental separation, changes in the earth's rotational axis, and radiation regime.