Abstract

At the field scale (m's to 100's m) it is often difficult to define the true three-dimensional form of depositional units or facies within sediments or sedimentary rocks, due to inherent spatial bias in traditional data sources such as outcrops and cores. Full-resolution 3D ground-penetrating radar (GPR) surveying has the potential to overcome this problem. Southwest of Miami, paleotopography on the upper Pleistocene Miami Limestone Formation indicates the former presence of a large oolitic barrier bar, backed by extensive ooid shoals. Data from three closely spaced cores and two large outcrops on the barrier bar allow initial subdivision of the Miami Limestone in to a lower mottled oolitic facies and an upper bedded oolitic facies. However, detailed correlation between the cores and outcrops is problematic due to spatial aliasing. A 250 MHz full-resolution 3D GPR data set was collected over an area of 45 m × 24 m that encompassed the location of the three cores. Depth of radar wave penetration was around 6 m, and both horizontal and vertical resolution was approximately 0.1 m for the 6500 m³ rock volume, such that the upper bedded oolitic facies was imaged without spatial aliasing. Radar surfaces and intervening radar facies define two main radar package types: low-angle sheet and sigmoidal. The low-angle sheet radar packages are interpreted as oolitic shoal deposits consisting of tidal bars covered by ebb-orientated, straight-crested sand waves. The overlying sigmoidal radar packages record barrier-bar formation through alongshore progradation. Comparable sedimentary successions from both Pleistocene and modern deposits in the Bahamas suggest that the three evolutionary stages recorded for the Miami Limestone are typical of many transgressive ooid shoal complexes.