Estimation of duration of subaerial exposure in shallow-marine limestones - An isotopic approach

Subaerial exposure surfaces in shallow-marine limestones may represent a significant period of nondeposition and/or erosion. The minimum duration of subaerial exposure is estimated through modeling the maximum whole-rock C-13 depletion toward exposure surfaces in vadose meteorically altered limestones. The model simulates the water-rock interaction that caused the depletion, and calculates the volume of meteoric water needed to achieve the observed delta C-13 value. Then, the duration of vadose meteoric diagenesis, which approximates exposure duration, is the volume of water divided by the infiltration rate of vadose water. Sensitivity tests indicate that estimated duration is moderately sensitive to many input parameters, particularly the isotope composition and concentration of total dissolved carbon in vadose water, sediment porosity, and infiltration rate of vadose water. The method is mostly effective in estimating exposure durations of less than 400 ky. The estimated durations of nine subaerial exposures in Pleistocene reef tracts of Barbados, West Indies agree well with available radiometric dates. Application of the method to subaerial exposures in Mississippian, Pennsylvanian, and Cretaceous shallow-marine limestones confines their durations to 21-54 ky and distinguishes exposures of different durations. The proposed method offers a potential tool and direction in estimating minimum duration of stratal hiatuses in meter-scale stratigraphic studies.