Def. of "Thapto"

Does anyone know exactly what the descriptor ""thapto"" means? As in a
Thapto-Histic Fluvaquent? I'm assuming that it refers to the buried
Histosol or histic epipedon, but I'm trying to find it's exact
definition. An example of a series would be like a Wallkill. . .Thanks to
anyone who knows from the soil scientists at SDI Consultants, Inc. in Oak
Brook, IL!