SNAPSHOTS IN TIME? TESTING THE ASSUMPTION OF SIMULTANEOUS DEPOSITION AND PRESERVATION OF TEPHRA BEDS

Silicic tephras simultaneously blanket entire landscapes and are deposited in a myriad of depositional environments. A great deal of work has been done to characterize and date discrete tephra beds in the western US, which now allows the placement of unknown tephras, and their surrounding sediments, into a firm chronostratigraphic context. One of the fundamental assumptions in tephrochronology is that the deposition and preservation of a particular tephra happens simultaneously, essentially representing a “snapshot in time.” The question remains, however, is this really true?

To address this question, the calibrated radiocarbon ages of proximal tephras erupted from the Mono-Inyo Craters (MIC) in eastern California were compared to the ages associated with distal tephras found in the western Great Basin, to determine if there were significant differences. For the youngest tephra (Tephra 1; ~600 cal BP), the durations over which the tephra was reworked into younger deposits is within the error range of the radiocarbon ages used to date the distal tephras. In contrast, Tephra 2 was erupted ~1300 – 1350 cal BP but was episodically reworked into younger deposits for up to 600 years after original deposition.

In the western Great Basin and surrounding areas, the MIC tephras have been found in stream deposits, deltas, beach and other lake deposits, alluvial fans, eolian deposits, and on hillslopes. At any given point in time, however, most parts of semi-arid landscapes are relatively static with little to no geomorphic activity and deposition, which leads to few opportunities for tephra to be concentrated, buried, and preserved before being dispersed and mixed with other clastic sediments.

How then are coherent and identifiable beds of older tephras incorporated into younger deposits, sometimes after hundreds of years? One explanation might be that the relatively low density and platy shapes of the glass shards lead to preferential transport and concentration into discrete beds. For example, light winds blowing across a sand dune or gentle waves lapping against a shore may provide just enough shear stress to initiate transport of individual glass shards but not enough to move the surrounding, denser grains, thereby effectively winnowing and concentrating the shards into a coherent layer that is then buried and preserved.