ALBERT

Description: The increasing use of detailed detrital-zircon provenance data calls for an understanding of the controls that sedimentary processes exert on changing detrital-zircon provenance signatures in time and space in addition to the well-established influence of tectonics. Our U-Pb detrital-zircon data from seventeen samples taken across a series of modern second-order rivers to deep-marine depositional systems in central California show that marine mixing processes and sediment pathway partitioning exert important controls on detrital-zircon provenance signatures. The second-order rivers that input sediment into this system produce differentiable detrital-zircon provenance signatures as a function of the unique geology of their drainage areas. The mixing of sediment through the marine realm can be traced through the evolution of distinct detrital-zircon signatures as sediment is transported from rivers down system through longshore drift into submarine canyons. Sediment input from second-order streams is mixed in the littoral and upper submarine-canyon environments, yielding provenance signatures that are not thoroughly mixed until the lower reaches of submarine canyons. Sediment partitioning in these dispersal systems results in distinct detrital-zircon provenance signatures for different dispersal systems over short length scales (〈 50 km). These data suggest that sediment mixing in the marine realm is likely an important control on detrital-zircon provenance signatures in any system in which sediment is transported through longshore drift and sediment pathways are distinctly partitioned. Changes in the way sediment pathways are partitioned through time has the potential to create heterogeneous detrital-zircon provenance signatures in stratigraphic successions through time and between closely spaced deposits of the same age. When seen in an ancient system, this heterogeneity may be mistakenly attributed to tectonic controls without the recognition of the influence of sedimentary process.

Description: New high-resolution datasets across La Jolla submarine fan, offshore California, illuminate low-relief, down-dip widening conduits emanating from a deep-sea channel that deposited a combination of laterally extensive sand strata seemingly crisscrossed by distributary patterns. Extensive coverage of this sector of the seafloor shows submarine-fan architecture and morphologies essentially different than distributary channelized patterns characteristic of subaerial systems and previous conceptual models of submarine fans. The main La Jolla channel, connected to La Jolla Canyon, loses confinement by widening, decreasing in relief, and developing scoured margins across kilometers-long down-slope and lateral distances. Two scales of distributary patterns are associated with sand-rich deposits down-system from, and outside of, fully formed channels. A larger-scale distributary pattern is identified in backscatter and bathymetry from trains of preferential erosion associated with laterally continuous repetitive steps that extend for kilometers outside channel confinement and may represent net erosional upper-flow-regime transitional bedforms. Smaller-scale distributary backscatter patterns in unconfined sand-rich deposits originate from the wide, low-relief channel. We suggest that the newly imaged La Jolla seascape displays sedimentary features that may be common on deep-sea fans but missed in previous lower resolution studies of submarine fans. Thus, La Jolla provides the basis for integrating previously enigmatic and (or) incomplete images of submarine fans. High-resolution seafloor, subsurface, and sample datasets highlight the importance of channel widening, headward erosion, and unconfined flows in La Jolla submarine-fan development, and may be relevant to other sandy submarine fan systems.

Description: : Ultra-high-resolution (1 m * 1 m * 0.25 m) bathymetry was acquired with an autonomous underwater vehicle (AUV) over a sector of the Navy Fan offshore Baja California. The survey specifically targeted an area where the former interpretation of the fan showed a channel–lobe transition; however, the lobe and the transition were not recognized. Instead, the newly acquired bathymetry shows that the previously identified channel continues basinward changing its overall morphology and stratigraphic architecture, becoming gradually but significantly wider (650–1000 m) and of lower relief (3–4 m). Cores from the channel thalweg recovered mud-poor (〈 5%) well-sorted sands, interpreted as deposited by fully turbulent flows. The cores also show several mud-rich (9–18%) poorly sorted sands, probably indicating deposition from more cohesive flows. The high-resolution bathymetry shows large sectors of the seafloor sculpted by elaborate bedforms and scours. The overbank area north of the channel exhibits the most numerous and prominent scours, interpreted to have been largely generated by flow stripping at a bend in the channel. Along high-gradient sectors (more than approximately 1¯) of this area, the scours are largest and deepest. Some of these scours show an erosional headwall and a distal upflow-dipping depositional bulge, forming repetitive bedforms interpreted as erosional cyclic steps associated with locked-in-place trains of hydraulic jumps. The scours seem to coalesce to form an incipient channel, which would likely drive the avulsion of the main channel. Further basinward, average gradients decrease (〈 0.6¯ ) and scours become smaller and less deep suggesting a gradient control on erosion. The southern channel margin and adjacent overbank area exhibit a trend of scours that are elongated transverse to flow, that successively repeat themselves basinwards, and that at times merge with sediment waves. Probably these scours are genetically linked to sediment waves, and they may have been formed by cyclic-step-like processes as well. The acquired bathymetry represents a breakthrough in the imaging of the proximal sectors of deep-sea fans, which provides the basis for an accurate morphometric characterization and the understanding of sedimentary processes and morphodynamics associated with the delivery of sediment into the deep sea.