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5.7: Turbidity Currents and Development of Submarine Canyons and Fans

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    Turbidity Currents and Development of Submarine Canyons and Fans

    A turbidity flows is a turbid, dense current of sediments in suspension moving along downslope and along the bottom of a ocean or lake. In the ocean, turbidity currents can be massive episodic events. They typically form and flow down through a submarine canyon (carved by previous turbidity flows) and accumulate near the base of the continental slope on deep-sea fans. Turbidity flows produces deposits showing graded bedding (Figure 5.8). Slowing turbid currents drop their coarser fractions first (gravel and sand) and the finer silt and clay fractions settle out last.

    Turbidity current and deposits Gazos Creek
    Figure 5.8. Turbidity flows are essentially underwater landslides or density-driven currents. Sediments laden with sediment are heavier than clear seawater. Figure 5.9. Seas stacks composed of submarine channel deposits (mostly conglomerate) exposed at Gazos Creek State Beach, California

    A deep-sea fan is a fan- or delta-shaped sedimentary deposit found along the base of the continental slopes, commonly at the mouth of submarine canyons. Deep sea fans form from sediments carried by turbidity flows (density currents) that pour into the deep ocean basin from the continental shelf and slope regions and then gradually settle to form graded beds of sediment on the sea floor. Deep-sea fans can extend for many tens to hundreds of miles away from the base of the continental slope and an coalesce into a broad, gently sloping region called a continental rise.

    Graywacke is a fine-to-coarse-grained sedimentary rock consisting of a mix of angular fragments of quartz, feldspar, and mafic minerals set in a muddy base (commonly called a "dirty sandstone or mudstone" because of its mixed size fractions). Graywacke is the general term applied to sediments deposited by turbidity flows, and they commonly show graded bedding. Graywacke is common in the Coast Ranges of California and other active continental margin regions. It is exposed on land where tectonic forces push up rocks that originally formed in the deep ocean (examples in Figures 5-10 to 5-11). "Turbidites" (deposits associated with turbidity flows) commonly appear as interbedded layers of sandstone and shale. Conglomerate typically occurs in thicker beds and were originally deposited as gravel and mud on ancient submarine fans closer to the mouths of submarine canyons or in channels carved into the seabed.

    Turbidites exposed on Loma Prieta Turbidites at Bean Hollow State Beach
    Figure 5.10. Cretaceous age turbidites exposed on Loma Prieta Peak, Santa Cruz Mountain, California Figure 5.11. Cretaceous age turbidites (turbidity current deposits) at Bean Hollow State Beach, California

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