Even though the Conococheague Formation is highly deformed at the Blue Ridge Quarry, there is still a lot of information about the original environment in which these sediments were deposited. The first piece of evidence is the stromatolites.
The large stromatolite shown above is the one that first interested me in this quarry. There is a break in the side of this specimen where we can see the internal structure:
It is thought that the appearance of lots of grazing animals in the Cambrian may have led to the reduction in stromatolite numbers after the Cambrian, and in fact the Boxley specimen shows evidence of grazing, in the form of traces cut across the surface (there’s a long one running below and parallel to the scale bar):
Chitons were likely not the only critters living in the Blue Ridge deposits. There are some beds within the quarry that are covered with burrows of various sizes:
There are other interesting rocks in the quarry that give clues about the depositional environment. The thrombolites are usually overlain by a coarse-grained carbonate rock called a grainstone, that seems to be made largely of ooids and possibly coprolites (ooids are carbonate grains that precipitate from seawater):
The Conococheague ribbon rock was examined in detail by Demicco (1983), who concluded that it represents sediments that were deposited on a tidal flat, an area covered by water at high tide but exposed at low tide.
There are two additional rock types found here, usually in close proximity to one another. Occasionally there are stromatolitic beds with the typical wavy laminations, but that are not arranged into discrete mounds:
It also turns out that the algal mound we collected is a bit unusual. Usually the Conococheaque thrombolites are merged into a continuous thick bed, rather than being discrete domes like we have at the museum. These beds can reach impressive thicknesses and gigantic sizes, like the fragment shown below that fell out of the quarry wall (the block Tim is resting his hand on is entirely thrombolitic):
One final point is that these different rocks are not distributed at random. They form repeating cycles as shown below:
Demicco (1983) interpreted these as regressive cycles; that is, as you move up the rock section (starting at the thrombolitic bed), the rocks represent shallower and shallower water (the sea is regressing). The thrombolites are thought to have formed in shallow subtidal water (under water at low tide). The grainstones were somewhat shallower but still subtidal. (In many Conococheague localities the grainstones are cross-bedded, but I haven’t observed that at the Blue Ridge Quarry). The ribbon rock is from the intertidal zone, exposed at low tide but covered by water at high tide. The algal mats at the top were above the normal high tide line and may have only been covered with water during high spring tides, or during storms. The tops of these beds are erosional unconformities when sea level was low, and the next thrombolitic bed represents the next high sea level stand.
Another noteworthy point is that the Boxley specimens we collected are quite different in shape from the typical thrombolitic beds at the Blue Ridge Quarry. I think they formed under rather different conditions from the other thrombolites, which is one of the things I’m planning to address in the paper I’m currently writing.