Graded Bedding

Most sedimentary layers show a basically consistent grain size throughout their thickness or show essentially random variations in grain size. However, some beds have their largest particles at the base and gradually move toward the smallest ones at the top. Such beds are said to be normally graded. More rarely, others show the opposite pattern (small at base, coarse at top) and are said to be reversely graded. Graded beds form when a steep pile of sediment on the sea floor (or lake floor) suddenly slumps into a canyon or off a steep edge. As the sediment falls, water mixes in with it, creating a slurry of sediment and water that flows quickly down a sloping bottom. When the bottom levels out, the flow begins to slow. The coarsest sediment is deposited first and progressively finer and finer sediment is deposited until finally the area sees only normal sedimentation again. Such mixed sediment-water flows are termed turbidity currents, because the flow makes the water cloudy (turbid).

These are tilted layers of lake bed sediments exposed in Death Valley, CA. The graded bed has a sharp base that contains large cobbles. It fines up through sands and into silts and clays. Then comes a conglomerate layer.
This is an excellent example of reverse grading (behind the knife). The lowest layer represents normal lake sedimentation. Then there was a large volcanic eruption that put a lot of ash and pumice into the air. The pumice initially floated on the lake, but with time it became waterlogged and sank. The small particles waterlogged first, then the larger ones.
This is at the same outcrop as before. After the reversely graded bed comes sediment rich in ash that was washed into the lake from the surrounding landscape some 760,000 years ago. Then came a small turbidity current that deposited the normally graded bed. Click here to get a super-zoom image. Bishop, CA
The man is standing on titled marble layers some 600 million years old. Behind his legs are fine gravels. These coarsen upward and reach cobble and boulder size. Given the angularity of these clasts and their size-sorting, this deposit is a reversely graded breccia. It probably represents a debris flow from the nearby mountains that is only a few million years old. Mosaic Canyon, Death Valley, CA
I hope to get for you some classic normally graded turbidite photos, but I don't have any yet in my possion.

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