Neogene carbonate-platform slopes (NEOCARPS)
Final Report Abstract
Slope angles in temperate carbonates vary between 20° in the steepest areas and 8° in the most distal settings. In subtropical carbonate platforms, the inclination of the slopes is comprised between 24° in the proximal slope and 10° in the distal slope. Tropical carbonate platform slopes shows a higher variability of angles along the slope from a maximum of 80° in the talus slope to 45-30° in the proximal slope and 10° in the distal slope. In temperate carbonates, facies are arranged in clinobeds and locally lenses. Within this clinobeds there is no significant facies variations along the slope, as typically occurs in tropical settings, where facies show a certain depth dependence. These makes the temperate systems more comparable to siliciclastic depositional systems. In the analysed sub-tropical slopes, facies are dominated by rhodoliths. The occurrence of rhodoliths along the slope is related to offplatform shedding and in-situ growth along the slope. During episodes of relative quiet waters, in-situ rhodoliths growth and red algal binding took place in the platform and along the slope generating composite clinoform bodies filled back from the toe of the slope. The final facies distribution results from the interplay of complex downslope gravitational processes, in-situ carbonate production and binding, intercalated with periods of low sedimentation and intense bioturbation. Tropical carbonate platform slopes show the highest slope facies variability. Facies composition and distribution are the result of the interaction of several factors such as coral growth, in-situ slope carbonate production, rockfalls and sediment gravity flows, hemipelagic rain, and reworking of reef-slope facies. The relative impact of the factors controlling the facies distribution is determined by changes in accommodation related to sea-level fluctuations. The patchy distribution of reef-framework debris, Halimeda accumulations, serpulids bioherms and alluvial fans along a tropical carbonate slope indicates that a facies mosaic characterizes the slope rather than facies belts. In the frame of this project, a major finding is that Holocene and Miocene tropical carbonate platform slopes contain facies with distinctive microfabrics indicative of microbial activity, which is proposed as an early-stage slope stabilizing factor, as is the case in Palaeozoic and Triassic carbonate platforms. It is therefore proposed that microbial binding is a previously underestimated factor of slope stabilization in Neogene and Holocene carbonate platform slopes. The outcrop, microscopic and geochemical analysis of cement paragenesis merged with terrestrial LIDAR data provide a precise approach to trace past sea-level positions in a fossil tropical carbonate platform. This allows reconciling outcrop and pelagic sea level proxies, which to our knowledge has been demonstrated for the first time with this exactitude and accuracy. Data show that between 5.89 and 5.87 Ma sea level changes related to precessional cycle reach 23 +/- 1 m for the sea-level fall and 31 +/- 1 m for the sea-level rise.
Publications
- 2014. Reef Slope Geometries and Facies Distribution: Controlling Factors (Messinian, SE Spain): Facies, v. 60, 737-753
Reolid, J., Betzler, C., Braga, J.C., Martín, J.M., Lindhorst, S., and Reijmer, J.G.
(See online at https://doi.org/10.1007/s10347-014-0406-4)
