Cracked pebble conglomerates form in situations where, first, there's conglomerate, i.e. sedimentary rocks the main component of which are clasts larger than sand. Second, that rock is exposed to shear stress great enough to deform it, but, critically, not enough confining pressure (or heat) to allow it to deform plastically. In more straightforward terms, it deforms by breaking. These fractures, very small faults, cut across the clasts, giving the appearance of cracks, hence the name "cracked pebble conglomerate." From the hand sample I once had from here, my recollection is that typical offsets were very small, a millimeter or less. While the resolution of my camera is better than my eyes, we're simply too far from the outcrop for me to spot any of the small faults in this photo.
What I
can see, on the other hand, is a very poorly sorted, texturally immature conglomerate. This tells me that it's a high energy environment, and that the sediment has not been transported far from its source. While I was hesitant to accept it at first, I've more or less convinced myself that the prominent lineations dipping about 35-40 degrees to the right are bedding planes. The sorting is just so poor, though, that I'm not certain just how the bedding planes are so clearly defined. However, they don't look tectonic in origin.
Photo unmodified. May 7, 2013.
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4 comments:
nice post .....
I like to learn something new every day and today i did. Didn't know a fault could be a millimeter in size. I always thought it had to have visual shear displacement otherwise it was just a joint or a crack.
Anything down to 0.1 mm is considered "naked-eye visible," so the offsets here were easily seen without a hand lens. And in metamorphic rocks, "microfaults" are not uncommon, shearing across individual or multiple mineral grains.
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