Subsidence-sinkhole development in light of mud infiltrate structures within interstratal karst of the Coastal Plain,southeast United States

Mud infiltrate structures (MIS) are a class of clastic sedimentary structures that have formed by high-angle gravitational sedimentation in vadose cavernous limestones beneath the Miocene Hawthorn Group. Observations of MIS in caves beneath the Pelham Escarpment, southwest Georgia, suggest three main types occurring either beneath, within, or contiguous to subvertical feeder pipes in the ceiling: (1) rubbly, conical masses; (2) polished, striated, cylindrical masses; and, (3) mud-flow, -drape, and-splash forms. MIS abundance suggests gravitative deposits are a major component to the total volume of cave mud. The muds apparently do not include soil-mantle materials; rather, their sources are poorly cemented clay-rich layers within the overlying Hawthorn Group. In cases where an individual MIS volume is large in proportion to the clastic-cover thickness, but no surface depression is found, the MIS location hypothetically predicts a directly overlying site of potential cover-collapse sinkhole development. Smaller-volume and/or shallower MIS predict the location of potential cover-subsidence sinkholes. The MIS forms suggest plastic and fluid behavior of the sediments occurs during displacements. The capacity for subsurface voids to accept mobile sediments depends on several factors that influence sinkhole development: (1) void size and interconnectedness; (2) spacing and size of subvertical solution pipes which can act as MIS feeders; (3) depth beneath the sediment source of any main horizontal cave development; (4) thickness of the cover sediments; and (5) the presence and depth of any breakout domes. The Hawthorn is present over more than 50 percent of the area underlain by the Floridan Aquifer system and is the system's major upper confining unit. MIS-forming processes are likely to be widely distributed within this stratigraphic setting, promoting ground subsidence and subsurface porosity obliteration. Given the existence of feeder pipes, MIS also are expected to form within the phreatic zone.