COMPARATIVE ANATOMY OF QUATERNARY SHORELINE CLASTIC AND CARBONATE SEDIMENTARY DEPOSITS USING 3D GPR: EXAMPLES FROM ANCIENT LAKE BONNEVILLE AND BAHAMAS

Carbonate and clastic depositional systems, despite being controlled by drastically different physical and chemical processes, can exhibit strikingly similar internal structure. We use high-resolution 2D and 3D ground-penetrating radar (GPR) from two disparate sedimentary environments, a Pleistocene regressive-phase remnant Provo shoreline of ancient Lake Bonneville in northwestern Utah and a Holocene strandline plain in San Salvador island, Bahamas, to compare and contrast the internal structure of a lacustrine clastic and a marine carbonate depositional system, respectively. The GPR data, aided by strategic stratigraphic sampling (cores and/or outcrops) covered the full range of each of the two environments, about 30 meters wide for the lake deposits and over 1000 meters wide for the strandline deposits. In order to afford exact comparisons, we used the same 400-MHz antenna and GPR profile-line spacing for both areas. The GPR profile amplitudes were re-binned into 3D volumes to allow any desired vertical view or horizontal depth slice. In both cases, sharp variations in porosity across stratal boundaries control reflectivity and the overall expression of the sedimentary structure. Both data sets show prominent sea- or lake-ward sets of dipping reflectors associated with a linear topographic ridge; however, for the Lake Bonneville case, the reflectors describe well-defined sequences of clinoform, sigmoidal strata with a discrete base, whereas for the Bahamas case, the reflectors are imbricated and appear to be more uniformly planar along strike without a clear base. The stratal reflectors in the Lake Bonneville case show a transition from proximal offshore to foreshore deposition, while those in the Bahamas case can be interpreted as linear eolianite dunes and swales that parallel the coastline and which represent seaward-migrating accretion into accommodation space.