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:

In cross section, this specimen is remarkably boring; just a mottled grey limestone. This specimen is actually not technically a stromatolite; it’s a thrombolite, an algal mound that lacks the internal laminations that define stromatolites. Compare it to this stromatolite, from Minnesota, in which the curved laminations are visible:

Even though the Boxley specimen is a thrombolite, it does have a stromatolitic outer layer up to about 5 mm thick (note the curved bands near the 5 cm mark):

Thrombolites first became common in the Cambrian period. Walter and Heys (1985) suggested that the appearance of large numbers of burrowing animals in the Cambrian may have led to the appearance of thrombolites, with the internal laminations being destroyed by the burrowing.

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):

In other parts of the Conococheague, fossils of the chiton Matthevia have been found associated with thrombolites. Chitons, like the modern example below, are mollusks with segmented shells that live in intertidal zones:

Runnegar et al. (1975) suggested that chitons may have been grazing on the algal mounds. While Matthevia has not been found at the Blue Ridge Quarry, the grooves on the thrombolite are the correct width to have been made by Matthevia.

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):

On top of the coarse-grained rock is one of the most prevalent rock types: “ribbon rock”, which consists of alternating thin beds of dolestone and limestone:

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:

Often the stromatolitic beds are accompanied by desiccation cracks, which form when wet mud dries out:

Demicco (1982, 1983) interpreted these units as representing algal mats that formed above the normal high tide line, kind of like the modern sabkha in the Persian Gulf region.

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):

Also note the huge piece of ribbon rock at Tim’s knees.

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.

References:

Demicco, R. V., 1982. Upper Cambrian Conococheague Limestone, in P. T. Lyttle (ed.), Central Appalachian Geology. Geological Society of America Northeast-Southeast Joint Section Field Trip Guidebook, American Geological Institute, pp. 217-254.

Demicco, R. V., 1983. Wavy and lenticular-bedded carbonate ribbon rocks of the Upper Cambrian Conococheague Limestone, central Appalachians. Journal of Sedimentary Petrology 53:1121-1132.

Runnegar, B., J. Pojeta, Jr., M. E. Taylor, and D. Collins, 1975. New species of the Cambrian and Ordovician chitons Matthevia and Chelodes from Wisconsin and Queensland: evidence for the early history of polyplacophoran mollusks. Journal of Paleontology 53:1374-1394.

Walter, M. R. and G. R. Heys, 1985. Links between the rise of the Metazoa and the decline of stromatolites. Precambrian Research 29:149-174.