Settling and Deposition of AD 181 Taupo Pumice in Lacustrine and Associated Environments

Pumice is an unusual geological material. It is of low density, its density can vary, reversibly, through time, and it is capable of floating in water. These properties result directly from the abundance and interconnectedness of vesicles, which in sedimentary environments contain air and water in varying proportions. The variable density and sometimes positive buoyancy of pumice in water lead to unusual transport properties that complicate attempts to interpret the energy of depositional environments in which it is deposited. Experimental settling of AD 181 Taupo pumice confirms the general observation that larger clasts are the last to settle, indicating that progressive saturation and sinking of clasts from a pumice raft can produce a reverse-graded bed (saturation grading). Saturation of pumice clasts with water is mediated by inhomo-geneities in the vesicle population, and in particular by more rapid transport of water through larger vesicles into the interiors of the clasts. Experiments designed to evaluate the behaviour of pumice clasts after they have become saturated show that although larger clasts retain slightly lower bulk density than smaller ones, fall velocities are nevertheless proportional to grain size. Sorting of saturated pumice by fall velocity therefore produces normally graded pumice beds (redeposition grading). Subaqueous deposition of pumice from currents results in a range of conventional styles of cross-bedding, but also produces distinctive steeply imbricated clast fabrics developed by the progressive growth of bedload cluster bedforms.