Calciturbidites and calcidebrites: Sea-level variations or tectonic processes?

Abstract

The compositional variation of Pleistocene carbonate gravity deposits from the Exuma Sound Basin, Bahamas, and of Turonian–Conacian deposits from the Southern Provence Basin (France) was determined. The Bahamian carbonate platform is located in a tectonically stable passive margin setting and the gravity deposits were deposited in an environment controlled by eustatic sea-level fluctuations. In contrast, the Southern Provence Basin sediments were deposited in a tectonic active regime.Two types of gravity deposits were present in the cores of ODP Leg 101, Site 632A, i.e. calciturbidites and calcidebrites. In analogy with earlier studies (Haak and Schlager, 1989), the compositional variations in the calciturbidites could be linked to different sources on the carbonate margin, i.e. platform interior, platform edge, platform slope. Interglacial (sea-level highstands) calciturbidites show a dominance of non-skeletal grains derived from the platform interior, while glacial (sea-level lowstands) calciturbidites show a dominance of skeletal platform edge to platform slope derived grains. Hence, the calciturbidite composition can be used to reconstruct the position in relative sea level. In contrast, the composition of the calcidebrites remained unaltered through time and showed a clear dominance of platform edge derived sediments. In addition, it was shown that the calcidebrites were deposited during regressions.In the Southern Provence Basin (SPB), the gravity deposits were deposited during episodes of sea-level rise coinciding with a series of earthquakes associated with the opening of the SPB. The SPB-megabreccias show a mix of sediment grains derived from the platform interior, the platform edge and the upper slope. Thus, in contrast to the Exuma Sound calcidebrites with their platform margin domain specific composition, the SPB-megabreccias have a composition that could not be assigned to a single domain on the platform margin and comprise a total mix of platform margin domains.In conclusion, this study shows that the composition of calciturbidites, calcidebrites and megabreccias can be used to determine which processes, eustatic variations or tectonic events, triggered the redeposition of these gravity deposits.