STRATIGRAPHIC DISPLACEMENT OF A TEPHRA BED IN ORGANIC LAKE SEDIMENTS

Volcanic tephras are often used in paleolimnology and other stratigraphic applications as a chronostratigraphic marker, particularly where radiocarbon or other dating methods are either unavailable or make precise comparison between sites difficult due to inherent imprecisions in dating. Used in this context, tephras are considered to provide an absolute stratigraphic reference that can be used to assess the relative ages of events between multiple sites. Lacustrine applications of tephrochronology assume that a tephra is deposited at what was the top of the sediment column at the time of deposition, and that the contact between the tephra and underlying lake sediments is an isochronous horizon wherever it occurs. If this assumption is violated, tephras cannot be accurately used as absolute stratigraphic markers or indicators of the absolute age of surrounding sediments. Inspection of an exposure of organic lake sediments from the recently drained Doal Lake, near Faro, Yukon Territory indicates that the 1200-year-old White River tephra moved more then 8000 years down-sequence and created conformably-bedded tephra units at a stratigraphic level corresponding to the early Holocene. These tephra beds, if seen in core, would appear to represent primary deposition of a tephra at a time thousands of years prior to the actual event. This type of stratigraphic displacement has obvious consequences for the use of tephrochronology in lakes and underscores the importance of properly identifying tephras and critically assessing their stratigraphic context within a lake core.