Turbidite depositional patterns and flow characteristics, Navy Submarine Fan, California Borderland

Abstract

The late Pleistocene and Holocene stratigraphy of Navy Fan is mapped in detail from more than 100 cores. Thirteen 14C dates of plant detritus and of organic‐rich mud beds show that a marked change in sediment supply from sandy to muddy turbidites occurred between 9000 and 12,000 years ago. They also confirm the correlation of several individual depositional units. The sediment dispersal pattern is primarily controlled by basin configuration and fan morphology, particularly the geometry of distributary channels, which show abrupt 60° bends related to the Pleistocene history of lobe progradation. The Holocene turbidity currents are depositing on, and modifying only slightly, a relict Pleistocene morphology. The uppermost turbidite is a thin sand to mud bed on the upper‐fan valley levées and on parts of the mid‐fan. Most of its sediment volume is in a mud bed on the lower fan and basin plain downslope from a sharp bend in the mid‐fan distributary system. Little sediment occurs farther downstream within this distributary system. It appears that most of the turbidity current overtopped the levée at the channel bend, a process referred to as flow stripping. The muddy upper part of the flow continued straight down to the basin plain. The residual more sandy base of the flow in the distributary channel was not thick enough to maintain itself as gradient decreased and the channel opened out on to the mid‐fan lobe. Flow stripping may occur in any turbidity current that is thick relative to channel depth and that flows in a channel with sharp bends. Where thick sandy currents are stripped, levée and mid‐fan erosion may occur, but the residual current in the channel will lose much of its power and deposit rapidly. In thick muddy currents, progressive overflow of mud will cause less declaration of the residual channelised current. Thus both size and sand‐to‐mud ratio of turbidity currents feeding a fan are important factors controlling morphologic features and depositional areas on fans. The size‐frequency variation for different types of turbidity currents is estimated from the literature and related to the evolution of fan morphology.

Grain-size characterization of Amazon Fan deposits and comparison to seismic facies units

“…Other factors must be contributing to this fining-upward trend. Piper and Normark (1983) demonstrated that when a turbidity current is thick relative to the channel depth it can overtop the levee, stripping off the upper part of the flow and allowing the residual coarser portion of the flow to continue down channel. This process, termed "flow stripping," occurs when thick turbidity flows travel in channels having sharp bends.…”

“…Flow density, and consequently velocity within the channel on the other hand, may increase if sediment is eroded from the channel floor and added to the turbidity current load. Denser, faster turbidity currents will also tend to become thinner, decreasing the potential to export sediment overbank (Piper and Normark, 1983). Turbidity currents traveling several hundreds of kilometers downfan must then continuously maintain a balance between flow thickening and dilution by entrainment, and increase in flow concentration by flow stripping and erosion.…”

Mangarara Formation: Exhumed remnants of a middle Miocene, temperate carbonate, submarine channel‐fan system on the eastern margin of Taranaki Basin, New Zealand

“…Similar stages of channel drift have been recognised in other submarine sinuous channel systems (Peakall et al 2000;Lien et al 2003). In the final stage, spillover deposits overtop the channel levees (Piper & Normark 1983). In the Mangarara case, no evidence for levees has been observed.…”

Physiography and recent sediment distribution of the Celtic Deep-Sea Fan, Bay of Biscay

“…The right-hand levees are higher and broader than the left-hand levees in the north hemisphere, this being controlled by the Coriolis force deflecting the turbidity currents to the right. The centrifugal force associated with flow stripping processes is responsible for the preferential building of sediment waves at the outer corners of bends of the levees (Piper and Normark, 1983). The upper-middle fan boundary is the result of the disappearance of the channel-levee systems due to the progressive downpath decrease of the fine-grained fraction in sediment transported by the channelised flows.…”

“…The difficulty of coring and the very low penetration of the 3.5 kHz seismic waves does not make the study of such environment easy. The transition from a muddy channel-levee system on the upper fan to flat-lying sand lobes on the middle and lower fan, represents a facies shift related to particle sorting and segregation due to channel-levee overflow as shown in the Amazon Fan (Flood and Piper, 1997), Mississippi Fan (Bouma, 1985), Navy Fan, (Piper and Normark, 1983).…”