Climbing-ripple successions in turbidite systems: depositional environments, sedimentation rates and accumulation times

摘要

Climbing-ripple cross-lamination is most commonly deposited by turbidity currents when suspended load fallout and bedload transport occur contemporaneously. The angle of ripple climb reflects the ratio of suspended load fallout and bedload sedimentation rates, allowing for the calculation of the flow properties and durations of turbidity currents. Three areas exhibiting thick (>50m) sections of deep-water climbing-ripple cross-lamination deposits are the focus of this study: (i) the Miocene upper Mount Messenger Formation in the Taranaki Basin, New Zealand; (ii) the Permian Skoorsteenberg Formation in the Tanqua depocentre of the Karoo Basin, South Africa; and (iii) the lower Pleistocene Magnolia Field in the Titan Basin, Gulf of Mexico. Facies distributions and local contextual information indicate that climbing-ripple cross-lamination in each area was deposited in an 'off-axis' setting where flows were expanding due to loss of confinement or a decrease in slope gradient. The resultant reduction in flow thickness, Reynolds number, shear stress and capacity promoted suspension fallout and thus climbing-ripple crosslamination formation. Climbing-ripple cross-lamination in the New Zealand study area was deposited both outside of and within channels at an inferred break in slope, where flows were decelerating and expanding. In the South Africa study area, climbing-ripple cross-lamination was deposited due to a loss of flow confinement. In the Magnolia study area, an abrupt decrease in gradient near a basin sill caused flow deceleration and climbing-ripple cross-lamination deposition in off-axis settings. Sedimentation rate and accumulation time were calculated for 44 climbing-ripple cross-lamination sedimentation units from the three areas using TDURE, a mathematical model developed by Baas et al. (2000). For T_c divisions and T_(bc) beds averaging 26 cm and 37 cm thick, respectively, average climbing-ripple cross-lamination and whole bed sedimentation rates were 0·15 mm sec~(-1) and 0·26 mm sec~(-1) and average accumulation times were 27 min and 35 min, respectively. In some instances, distinct stratigraphic trends of sedimentation rate give insight into the evolution of the depositional environment. Climbing-ripple cross-lamination in the three study areas is developed in very fine-grained to fine-grained sand, suggesting a grain size dependence on turbidite climbing-ripple cross-lamination formation. Indeed, the calculated sedimentation rates correlate well with the rate of sedimentation due to hindered settling of very fine-grained and fine-grained sand–water suspensions at concentrations of up to 20% and 2?5%, respectively. For coarser grains, hindered settling rates at all concentrations are much too high to form climbing-ripple cross-lamination, resulting in the formation of massive/structureless S_3 or T_a divisions.