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Republished from Martin, R. and Panuska, B., eds., 2004, Proceedings of the 11th Symposium on the Geology of the Bahamas and Other Carbonate Regions, Gerace Research Center, San Salvador Island, Bahamas, p. 145-155

UIS KHS Commission
Pit cave morphologies in eolianites: variability in primary structure control

The landforms of San Salvador, Bahamas, demonstrate extensive karst development, in particular epikarst features called pit caves. Studies on Hog Cay, an interior dune ridge located north of the San Salvador International Airport runway, indicate that some pit caves have morphologies controlled by bedding. These pit caves, initiating within the vadose zone, have a tendency to follow the foreset beds of the dune for some distance and are analogous to solution chimneys found in continental settings. These solution chimneys are distinguished from vertical shafts, which propagate vertically into the vadose zone of the
subsurface with little, if any, horizontal offset.

Previous field observations have described how eolian deposits can be sorted by grain size into alternating coarse-grained and fine-grained strata. The alternating strata undergo selective cementation, where the coarse-grained strata become poorlycemented and the fine-grained strata become well-cemented because of retention of pore waters. This is observed in weathered outcrops as poorly-cemented micro-recesses and well-cemented micro-ledges. In the subsurface, the coarse-grained, poorlycemented strata are the preferred flow path for vadose water. This water is perched upon and flows laterally along the foreset beds on the well-cemented, fine-grained strata. Pit caves forming under these conditions are described as solution chimneys. Also found on Hog Cay are pit caves that extend from the surface down to near sea level. These vertical shafts are generally found on the crests of dunes, with the deepest shaft being over 15 meters. They commonly display a near-perfect cylindrical shape and extend vertically with no horizontal offset. The walls of vertical shafts exhibit micro-ledge and micro-recess morphology; however, the vertical shafts have no indication of bedding control, which may be due to cementation in the fine-grained layers
being less complete in certain areas, facilitating vertical shaft development.

Preliminary XRD analysis of the pit caves shows that the top and bottom wall rocks of one pit is almost entirely calcite, but the wall rocks in the middle of the pit have a high aragonite content. These observations are consistent with long residence time of meteoric water in the epikarst at the top of the pits, and in the fill material at the base of the pits, such that aragonite was inverted to calcite. However, the rapid transit time of the vadose water along the pit walls allowed dissolution to enlarge the pit, but without inversion of the primary aragonite.