The Collaborative International Dictionary of English v.0.48:

Subcarboniferous \Sub*car`bon*if"er*ous\, a. (Geol.)
   Of or pertaining to the lowest division of the Carboniferous
   formations underlying the proper coal measures. It was a
   marine formation characterized in general by beds of
   limestone. -- n. The Subcarboniferous period or formation.
   [1913 Webster]