When discussing fossil deposits with the public the subject of reworked specimens occasionally comes up. Reworked specimens are often (but not always) easy to spot. Shown above are two teeth from the same bed at Carmel Church, both from the genus Galeocerdo. The one on the left is reworked, while the one on the right is non-reworked or “fresh”. The heavy abrasion on the reworked tooth is obvious. But I frequently get the question “What do you mean by reworked, and how does it happen?”.
Reworking can theoretically happen to any fossil. It simply means that the fossil has been removed from its original sedimentary layer and redeposited in a different, usually younger, layer. There are lots of ways this can happen, many of which are unique to particular depositional environments. In marine deposits we’re usually dealing with two sources of reworking; transgressive erosion and bioturbation.
In geology a transgression is a rise in sea level relative to the land (the ocean transgresses onto the land). During the early phase of transgression, when the water is shallow, there is typically a lot of erosion of the seafloor, while later on in the transgression there is deposition of new sediments. When the transgression ends and regression begins, many of these newly deposited sediments are left behind as a new sedimentary layer; that’s how we get the marine deposits we see on the coastal plain. Those deposits are often full of fossils that were buried as they died, scattered vertically throughout the deposit. Our first diagram below shows our sediment at the end of the regression; the sea has gone out, and the red objects are fossils preserved in the sediment that remained behind:
Sometime later the ocean transgresses again. In doing so, it erodes away the top portion of the old sediment layer and deposits new sediment on top of it. However, some of the components of the old sediment are not easily removed by the transgression. These might include rocks, teeth, and bones that are too large and hard to be ground to bits or completely swept away. Instead they are rolled around on the bottom, beaten up and abraded – they’re reworked. In the diagram below, the reworked fossils are indicated by pink, while the blue fossils are newly deposited:
Notice that the reworked fossils are deposited in the younger, light-green sediment even though they date from the older, dark-green sediment. They are also found in the same bed as the younger, “fresh” fossils (represented by blue). So our light green sediment has fossils from two different time periods. We still have a hope of telling them apart, however, because the younger fossils don’t show the same degree of wear.
In most shallow areas, the seafloor is teeming with life, but most of those organisms live buried in the mud. This leads to our second major source of reworking, bioturbation. Organisms burrowing in the mud churn up the seafloor, and as they dig their burrows they throw sediment up into the water. In some cases they may burrow right across the sediment boundary into the older beds underneath, and they may encounter older fossils on the way:
To the burrowing critter, the fossil is just another sediment grain and it’s tossed out (note that it doesn’t have to be this specific; the general churning up of the sediment can have the same effect):
The bioturbated fossil is shown in dark pink. Consider its history relative to the other reworked fossils. While they’ve been rolled around on the seafloor by waves, perhaps for years, the bioturbated fossil has had a pretty short, easy trip to the younger sediment layer. It may arrive in its new younger bed with little or no abrasion. This type of reworking is tougher to spot, because many of the signs of reworking may be absent. Also note that the burrow itself is eventually filled with the younger sediment. A blue fossil could potentially be preserved in this burrow, and it would be located below the sediment boundary. This could appear to be a fossil occurring in a bed that was deposited before the blue fossil organism evolved! This might be a little easier to spot because the burrow is filled with the younger sediment; however, if the younger and older sediments are the same lithology (that is, if they look the same) it can be tricky.
As the transgression continues, eventually enough new sediment accumulates that erosion of the older beds is cut off, because they’re buried:
Notice how the fossils are distributed vertically through the section. We have a lower layer that contains only older (red) fossils and an upper layer that contains only younger (blue) fossils. But at the boundary between the layers there is a mixed layer that contains mostly reworked older fossils (pink), with some fresh younger fossils. The mixed bed is typically dominated by reworked fossils, because it represents the accumulation of all the fossils from the eroded part of the older layer. In contrast, the fresh fossils represent only the organisms that happened to die in that spot while deposition was occurring.
This mixed layer is called a “lag deposit” or a “transgressive lag”, and it’s a marker for the beginning of a new transgression. Almost every recognized sediment bed on the coastal plain begins with a lag deposit at its base. So each lag represent a time when the ocean regressed and then transgressed, and each one represents a significant loss of sediment by erosion.
Here are two examples of lag deposits:
There are at least four lag deposits in the Carmel Church section, including a portion of the bonebed we’re excavating. In Part 2, we’ll look at what the presence of reworked fossils can tell us about the depositional history at Carmel Church.