SEDIMENT DEPOSITION AND REWORKING IN LAVA TUBE CAVES

Just as in karstic caves, sediments within lava tube caves may yield informationm on Holocene, Pleistocene, and perhaps some pre-Pleistocene events. Few have been studied in detail.

Except in arid regions (where thick deposits of air-transported spelean sediments may accumulate over long periods of time), sediment deposition in lava tube caves primarily occurs during flood pulses. Piracy of turbid surface streams is a less common factor. Perieruptive lahar and other mudflow deposits may be solitary and massive (e.g., Lava River Cave, OR and Ape Cave, WA), or successive, small volume events may produce and rework a succession of thinly layered deposits (e.g., Gremlin Cave, WA).

Depending on topographic settings, flood events unrelated to eruptions are known to deposit a specrum ranging from large boulders (e.g., caves of the Honokaa area, HI) to fine-grained particulates morphologically resembling the commonly-observed clay fills of karstic caves.

Where flood waters cover the entrance of a lava tube cave (e.g., Turtle Cave, HI), backflooding may leave extensive backwater deposits.

Potentially of special interest, Oregon's Mowich Cave recently was opened to initial geological reconnaissance. The entrance of this cave is at the base of a canyonside 25+ m cliff where the cave was truncated by downcutting of the Clearwater River. This cave contains an extraordinary sequence of thin, complexly bedded sediments. These and other sediments of lava tube caves merlit the same careful study as do those of karstic caves.