This week's Friday Field Photo is from a quick jaunt I took yesterday to scout out some outcrops for the class I'm teaching this term. If you know me or anything about this blog you know that I don't spend a lot of time discussing carbonate sedimentary rocks. This is not because they aren't interesting, it's just my specialty is in clastic sedimentary rocks ... get it, clastic detritus ... yeap. Anyway, now that I'm teaching more general sedimentary geology classes, I need to cover it all. Carbonate sedimentology and stratigraphy is spectacular and utterly fascinating, so look for more photos in the coming months and years.
The image above is from the Cambrian (~500 million years old) Conococheague Formation exposed along the Little River near Radford, Virginia. The bottom part, below the pencil, is a limestone full of pieces, or clasts, of other limestone. If you look closely, you'll see a lot of flat chunks a few centimeters long embedded in the seemingly homogenous texture.
Above that unit, starting a little bit below the pencil, is a distinctly different unit of alternating thin beds of tan and gray rock. The tan rock is a fine-grained deposit composed of silt to sand sized particles called ooids. (See this fantastic post from Evelyn Mervine over at Georneys for more about ooids.) The gray layers are lime mud. It's not the best photo to see it (these beds are slightly overturned and my body was precariously contorted to get this shot), but the tan layers exhibit wavy and ripple lamination.
These deposits are interpreted to be from storms that ripped up material from the carbonate platform, which was deposited as the intraclast layer (the chunky stuff) followed by the finer-grained structured layer as the current energy waned. What's cool is that this two-deposit pattern is repeated three times (you might be able to see the next intraclast layer a bit out of focus at the top of the photo).
Happy Friday!
