Classic Clastics Trivia Quiz

Answer: True

Sandstones rich in K-feldspar generally come from granites or gneisses. Sands containing only well-rounded grains of quartz tend to come from older sandstones. And, if the sand contains rock fragments, the rock fragments are dead ringers for the original source rock. Many a happy hour I have spent figuring this out, in hand-sample or in thin section...Ah, the joys of petrography...

Answer: All of these

Compaction, the grains are smushed closer together; Dissolution, parts of grains can dissolve, sometimes even entire grains; Cementation, mineral growths, usually quartz or calcite, bind the grains together. Compaction increases at greater depth, and cementation usually stops compaction, so with a combination of those two, you can roughly tell the depth at which the rock was cemented. Why do we care? Porous sandstone forms good reservoir rocks for hydrocarbons. Obviously, less compaction and cementation and more dissolution are GOOD.

Answer: Lean back and float out

In normal wet sand, the water fills the pores between the grains, and the sand grains support their own weight. In quicksand, the water supports the sand grains, making the quicksand behave like a fluid. The best thing to do is to lean back. The sand grains will settle out, and you, being the biggest grain around, will end up floating at the surface. Anything which stirs up the quicksand keeps the grains in suspension.

Not good.

Answer: Fine, wind-blown dust

The other answer choices refer to more of my favorite words. A hill shaped by the wind would be a yardang. An intermittent stream channel in a desert area is a wadi (plural: wadii). Seasonal light and dark layers are varves.

Answer: Icebergs

As glaciers travel downhill, they pick up loads of sediment from fine grains to huge boulders. When the glaciers reach the ocean, icebergs can calve (break off), and as these floating ice rafts melt, they drop their clastic cargo. Of course, this is much more common in polar areas than, say, the equator.

Answer: Alluvial fan

Alluvial fans generally form in tectonically active areas at junctures between uplands and much flatter, lower areas, say, at the base of a mountain. The stream carries very coarse material, and avulses periodically when it chokes up its bed, forming a cone-shaped deposit of sediment. There can also be debris flows on the upper part of the fan, since the gradient is so high.

Answer: Desert

While dunes are the most distinctive and memorable features of deserts (think Lawrence of Arabia), deserts can also be characterized by several other types of geologic features. Deflaitionary pavements occur where the wind has blown away all the sand, leaving behind a pavement of small pebbles too heavy to transport. Playas form as temporary lakes, with the water quickly evaporating and leaving behind evaporite minerals, such as gypsum and halite. Ventifacts are pebbles that have been faceted by the wind.

Answer: Point bar

In the curve of a meandering stream, the erosion happens on the outer curve, the cut bank, where the speed of the current is faster. Deposition occurs on the inside of the curve, the point bar, so you will have more land at no additional cost.

Answer: Herringbone cross-strata (reversing current directions)

On a tidal flat, the current direction reverses daily. Hence, the tide creates two layers of ripples, in opposite directions to each other. Alternately, the current can be much stronger one direction than the other, so one set of ripples is constantly eroded and redeposited.

In that case, you look for erosional features on the ripples. For erosional scour features, the current generally isn't that strong on the flat itself (though it may be in a tidal channel.) For large-scale dunes-- it's a *flat*, isn't it? When was the last time you saw a flat dune? As for the last choice, well, unless beach umbrellas have been in the geologic record longer than I thought...

Answer: All of these

Porous sandstone can form really nice reservoirs for oil, and impermeable shale makes for good hydrocarbon traps. Just what you need to know before you drill. Knowing the behavior of streams on floodplains is useful, especially how they deposit sediment. Ask the ancient Egyptians-- their land was so fertile because the flooding of the Nile deposited a fresh layer of nutrient-rich mud every year.

As for beaches, well, what can I say?