Reworked fossils, Part 3

Yesterday we saw that the Carmel Church bonebed, which is only about two feet thick, contains a whole variety of shark teeth from different ages. Reworked fossils are often treated by paleontologists (including me) with a certain amount of mild disdain, because of their relatively uncertain ages and frequent destruction of important anatomical details. But they aren’t completely useless, and we can apply the general principles of reworked fossils to Carmel Church site to see if we can try to understand its history a little better.

First we should revisit yesterday’s stratigraphic column to see the known ranges of the relevant sharks. This time I’ve modified the column, so that the beds preserved at Carmel Church are highlighted in blue:

I’ve simplified things a little in the chart. For example, the entire thicknesses of these formations are not preserved at Carmel Church, but usually only single thin beds. I have indicated this with the Calvert Formation though; the only Calvert bed preserved at Carmel Church is the top one, Bed 15. That means that at Carmel Church Calvert Bed 15 sits directly on top of the Nanjemoy Formation, as you can see in the photo at the top of the page (the Calvert is brown, while the Nanjemoy is gray-green).

To portray the history here, will start with our base diagram from two days ago. We need a key in this case to identify our different teeth. We’re going to divide the teeth into four stages, with Stage 1 as the oldest and Stage 4 as the youngest. We’ll also use different shades to tell if the tooth is fresh, reworked, or reworked through bioturbation.

Stage 1 represents Carmel Church during the regression immediately after deposition of the Nanjemoy Formation, and it includes fresh Otodus obliquus teeth:

Now we need Stage 2 to bring the ocean back (a transgression), to bring Carcharocles sp. to Carmel Church. The beginning Stage 2 might look like this:

The transgression has eroded part of the Stage 1 sediments, so there are fresh Stage 2 teeth (green) mixed with reworked Stage 1 teeth (pink). But the bottom part of the Stage 1 sediments escaped erosion and survived in place. The end of Stage 2, at the next regression, could look like this:

Now we need to introduce Stage 3 teeth, which are reworked C. megalodon. So we need another transgression:

An important point to notice is that ALL of the stage two sediments were eroded by the transgression, so our Stage 3 sediments are lying directly on top of Stage 1. As a result, no fresh Stage 2 teeth (green) have survived; they’ve all been reworked (light green). Now we have reworked Stage 1 and Stage 2 teeth mixed in with fresh Stage 3 teeth (and the Stage 1 teeth have been reworked twice). At the end of Stage 3 we might see this:

This still doesn’t get us where we need to be. The problem is that the Stage 3 teeth (C. megalodon, in purple) are all fresh, but at Carmel Church we have both fresh and reworked C. megalodon. We need another transgression, Stage 4:

Just like last time, notice that ALL the Stage 3 sediments were removed by the transgression, so that Stage 4 sits directly on Stage 1. I’ve also added some bioturbation here (it might have happened in the earlier stages too). The bioturbation has brought almost completely fresh Stage 1 teeth up into the Stage 4 sediments. (Why doesn’t bioturbation bring fresh Stage 2 and Stage 3 teeth up? There are none available – remember that Stage 2 and Stage 3 were completely eroded by the next transgression, so no fresh teeth from these units survived.) Notice also that our bed has gotten very crowded; we have fresh Stage 4 teeth, reworked Stage 3, 2, and 1 teeth, and bioturbated Stage 1 teeth all found together. This is what we see at Carmel Church, and it’s characteristic of lag deposits. The end of Stage 4 would look like this:

There’s one more important step. There are more units above the Carmel Church bonebed. How did it survive? Here’s our next transgression, Stage 5:

Just as happened with Stage 1, Stage 4 wasn’t completely eroded by the Stage 5 transgression, so the bottom of Stage 4 is preserved.

If we go back to our stratigraphic section, what does this all tell us?

Stage 1 is the Nanjemoy Formation, while Stage 4 is Calvert Bed 15. Carcharocles megalodon only starts showing up around Bed 12, so the Stage 3 teeth (reworked C. megalodon) were likely coming from an older Calvert Bed (Bed 12-13) that was completely eroded by the Bed 15 transgression. Carcharocles sp. ranges in various forms from the top of the Nanjemoy Formation into the upper part of the Calvert, probably as high as Bed 12. The size and shape of our Carcharocles sp. suggest that they are older rather than younger, however. It’s likely that they came from either the Piney Point or Old Church Formations, or from an earlier Calvert transgression. Moreover, these aren’t mutually exclusive – all of these units may have been in the area and contributed teeth to the bonebed, as long as they were completely removed by the next transgression.

So, from these beaten-up, reworked teeth, we can infer the existence of at least two ocean transgressions over the Carmel Church area between about 40 and 14 million years ago, for which no sediment has survived!

There’s one other important point to make. You may ask, given all this evidence for reworking, and the fact that bones and teeth are concentrated in lag deposits, why can’t you explain the entire existence of the Carmel Church bonebed as being just another lag deposit? What’s the big mystery? Here are my responses:

You can’t easily rework articulated skeletons of whales, and we have multiple examples of them. There are also tons of other fossils at Carmel Church that aren’t easily reworked, like articulated bones from the lower jaws of fish, and almost paper-thin fish skull and vertebral elements. There is a lag deposit at Carmel Church, but there’s something else going on there that’s superimposed on the lag. That’s what makes Carmel Church so difficult to interpret, and what makes it such an exciting place to work.

This entry was posted in Carmel Church Chondrichthyans, Carmel Church Geology, Carmel Church mysticetes, Carmel Church Quarry, Chesapeake Group, General Geology. Bookmark the permalink.

4 Responses to Reworked fossils, Part 3

  1. Tony Edger says:

    Excellent series of posts. It’s a fascinating set of puzzles created by the repeated regressions and transgressions. What would you recommend for reading about the geological history of Carmel Church?

  2. boesse says:

    Very interesting! I’ve read before taphonomists hypothesizing that there could (or should) be bonebeds like this, where there are ‘ghost strata’ that are entirely eroded and a few fossils are the only remaining material record of them.

    WRT the associated/articulated specimens – if they’re preserved slightly above the bonebed, the most parsimonious hypothesis I can think of is applying Kidwell’s (1985) model, where the sedimentation rate increases from ~0 to positive net sedimentation, allowing preservation of more complete fossils. Sort of a condensed section superimposed on a bonebed (Pyenson et al 2009 argued for a similar mode of formation for the STH bonebed, and for my Masters thesis, I’m making similar interpretations).

    That’s really interesting! I think you need to do a taphonomic analysis of Carmel Church…

  3. Alton Dooley says:

    Thanks, Tony!

    The best place for Carmel Church info is this blog. All the Carmel Church pages are linked on the Carmel Church archive page.

    There are 4 peer-reviewed papers on Carmel Church. Marr and Ward published a geologic overview in Virginia Minerals in 1986, but that was before the bone bed had been discovered.

    Next was the description of Eobalaenoptera in JVP (Dooley, Fraser, and Luo, 2004), which included an updated strat column. I published on the land mammals in 2007, and Brian Beatty and I published on Diorocetus in 2009. Both of those were in Jeffersoniana and are available online; there’s a link on the blog.

    Check back, though. The next paper on Carmel Church should be out next week, possibly as early as Monday!

  4. Alton Dooley says:

    Bobby, it’s possible that a change in sedimentation rate is going on at Carmel Church, perhaps even likely. I even have some ideas about the source of the sediment, but I’m not ready to reveal them in print!

    A few things complicate the picture, though. One is that the reworked fossils are distributed throughout the bonebed, but this may be a case of bioturbation and local topographic variation. There are also some indications that the fresh bones accumulated relatively rapidly and were buried fairly quickly. Also, the fresh bones and teeth make up a major percentage of the bonebed; for example, at least 20% of the cetacean vertebrae are part of articulated or associated specimens.

    As for ghost strata, I may have additional evidence independent of the fossil components, but I’m still evaluating that. But I think the case is pretty good based on the teeth alone. I’ve been hoping to find a reworked Squalodon tooth. Squalodon almost perfectly overlaps with C. angustidens and C. chubutensis, but it doesn’t occur in Bed 15. No luck so far, though!

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