Zane Richards Jobe, Donald R Lowe, William R Morris

Climbing-ripple cross-lamination (CRCL) 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 (> 50 m) sections of deep- water CRCL deposits are the focus of this study: 1) the Miocene upper Mount Messenger Formation in the Taranaki basin, New Zealand, 2) the Permian Skoorsteenberg Formation in the Tanqua depocenter of the Karoo basin, South Africa, and 3) the lower Pleistocene Magnolia Field in the Titan basin, Gulf of Mexico. Facies distributions and local contextual information indicate that CRCL 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 CRCL formation. CRCL 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, CRCL 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 CRCL deposition in off-axis settings. Sedimentation rate and accumulation time were calculated for 44 CRCL sedimentation units from the three areas using TDURE, a mathematical model developed by Baas et al. (2000). For Tc divisions and Tbc beds averaging 26 and 37 cm thick, respectively, average CRCL and whole bed sedimentation rates were 0.15 and 0.26 mm/s and average accumulation times were 27 and 35 minutes, respectively. In some instances, distinct stratigraphic trends of sedimentation rate give insight into the evolution of the depositional environment.
CRCL in the three study areas is developed in very fine- to fine-grained sand, suggesting a grain size dependence on turbidite CRCL formation. Indeed, the calculated sedimentation rates correlate well with the rate of sedimentation due to hindered settling of very fine- 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 CRCL, resulting in the formation of massive/structureless S3 or Ta divisions.