Conococheague Formation, Part 2

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.

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4 Responses to Conococheague Formation, Part 2

  1. Maggie says:

    Hello ,I am a master student in Andong National University,South Korea.I have already read the paper (wavy and lenticular-bedded carbonate ribbon rocks of the upper cambrian conococheague limestone,central appalachians,1983, Demicco) .While I am wondering how the Conococheague cycles changes in that area,the paper told that it is 2~10 cms thick,but the whole Conococheague limestone is about 750 meters in thickness.So, is such kind of cycles repeated again and again in the group? If so ,how can you explain that?The depositional environment can change so rapidly ?If this kind of cycles repeated,there should be more than 70 cycles?I really doubt that.In each cycle ,it is shallowing upward.In that case,there should be 70 times of regressive change in sea level.
    Okay,I really appreciate it if you can reply to me ,that problem really confuse me .Thank you !

  2. altondooley says:

    Maggie, there are two points to keep in mind with the regressional cycles in the Conococheague (and I should note that I don’t have my references in front of me, so I’m not sure what Demicco says on this):

    First, you asked about the depositional environment changing rapidly, but we don’t know how rapidly this deposit was forming. Modern stromatolites seem to grow fairly slowly, so each regressive cycle may represent a relatively long period of time (decades? centuries?). Moreover, there is an unconformity at the top of each cycle (the transgressive part of the cycle), representing some period of time not preserved in the rocks.

    Second, when we talk about these shallowing upward cycles, these are only changes in local relative sea level, not global changes. Local sea level changes don’t necessarily require wholesale changes in climate. And we’re not talking about big changes here; in the deepest preserved part of each cycle (at the base of the thrombolitic beds) the water depth was probably only 2 or 3 meters; it doesn’t take much (on a geologic scale) to move sea level up and down that much.

    And, actually a third point: at least some parts of the Conococheague may represent different setting than an offshore regressive cycle. I suspect the big thrombolite at the top of the post grew in a protected environment, like a back-barrier lagoon. So while the Conococheague may be dominated by the regressive cycles, they aren’t necessarily the only thing in the entire unit.

  3. Maggie says:

    Hello~Sir! I really appreciate it your answered my question so quickly .I think I have already gotten your point.
    1.It always take a longer time than we think to form one of this cycle.
    2.Meanwhile,these changes in the sea level is quite limitted.It only happens in a regional or restricted area.That is to say,this won’t need a regular and repeated change in the golobal sea level,which make it easier to understand.
    3.Although we call it as a cycle,it is just a general idea.There could be some small discordant rocks. The regional sea level changes cann’t effect on everywhere in the sea,just most of it.
    So,is that true the cycles repeated nearly 70times in Conococheague limestone?In the geological history, this area has experienced almost 70 times of regional regressive cycles,which in turn makes all those limestone cycles here?Maybe this is just caused by some consistant in-and-out of water comes throuh some channels in a long geogical time-scale?I just want to know the truth of this strata in the Great Valley of western Maryland?
    Sorry to bother you again.Thank you!

  4. Maggie says:

    no reply?

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