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.
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:
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:
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.