Saturday, February 23, 2008
Dish structures are sedimentary structures found in thick sand (or sandstone) that have concave-up, bowl-like shapes. They form when water is trying to escape from rapidly deposited sand but encounters horizontal barriers of somewhat lower permeability (usually zones with smaller grain size and/or dispersed mud). These force the water to flow laterally until it finds a place where it can go upward again. In the meantime, the subtle permeability differences get enhanced as muddy particles are washed away from the cleaner parts of the sand and concentrated in zones of lower permeability. The sides of these lower perm zones bend upward as the water finds its way up. Eventually pillar structures, vertical zones of cleaner sands can form on the sides of the dishes.
Initially dish structures were thought to be related to the (still somewhat fuzzy) mechanics of sediment transport and deposition in high-concentration gravity flows. However, clear examples that showed primary sedimentary structures (like cross lamination) being cross cut by dish structures proved that the latter are secondary structures, formed soon after deposition.
Probably because rapid deposition of sand is a requirement for the formation of dishes, these sedimentary structures are largely restricted to deep-water sands. Here are some examples that I think are blogworthy:
This one is from the northern California coast. Note the pillar structures between the dishes. [Apologies for the lack of scale - I think this bed is about 4 feet thick].
This is a zoom-in of dish structures in the Cerro Toro Formation of Southern Chile. Lighter-colored areas probably contain less mud than the darker zones.
No scale on this one either (there was no way I could climb up there), but trust me, these are probably among the largest dish structures in the known universe. They were photographed in northern Peru, near the town of Talara.
And to prove that they are really big, here is a photo that gives an idea of their scale: