Relative frequency of Neogene volcanic events as recorded in coal partings from the Kenai Lowland, Alaska; a comparison with deep-sea core data

Tephra layers occur as partings in Neogene coal beds from the Kenai Peninsula lowland, Alaska. The coal is time-equivalent with Deep Sea Drilling Project (DSDP) cores from the Gulf of Alaska and along the Aleutian Islands and appears to preserve a more detailed but less complete record of volcanism. Coal from the lower Beluga Formation has abundant thin (<10 cm) and dispersed partings recording an eruption once every 125-500 yr of peat accumulation, probably coinciding with a volcanic pulse more than 10.5 m.y. ago. This pulse is not well recorded in nearby DSDP cores, possibly because of bioturbation and distance from source vents. Coal beds in the upper Beluga Formation were deposited during a period of reduced volcanic activity 10.5-7.5 m.y. ago and record volcanic events that occurred approximately every 9,000 yr. This is also manifested in a near absence of tephra layers in DSDP cores of equivalent age.A volcanic pulse occurred about 7.5 m.y. ago, concurrent with the deposition of the lower Sterling Formation; however, intervals between volcanic events average 11,000 yr or longer. An absence of tephra layers in the Gulf of Alaska DSDP core for this time interval indicates that volcanic centers were distant. A dramatic change in frequency and magnitude of volcanism occurred about 5 m.y. ago. Tephra layers recur at intervals of 1,700-2,400 yr, and thicknesses of some layers exceed 2 m. This increase in volcanism is also recorded in DSDP cores.