The rims of the Carolina Bays have inverted stratigraphy, which is characteristic of impact cratering. This presentation discusses the structure of the raised rims of the Carolina Bays and the deductions that we can make about the age of the bays taking into consideration the mechanism of rim formation.
Many geologists propose that the Carolina Bays evolved as a result of processes active episodically over thousands of years. The paper by Brooks, et al. says: "Based on 45 OSL dates, active shorelines and associated eolian deposition occurred during marine isotope stage (MIS) 2 to late MIS 3 (12 to 50 thousand years before the present), MIS 4 to very late MIS 5 (60 to 80 thousand years before the present), and late MIS 6 (120 to 140 thousand years before the present). These age ranges also correspond with the ages of other eolian landforms in the Coastal Plain, including sand sheets and dunefields, and suggest a climatic threshold was crossed during the transition toward stadials, initiating both bay and dune activity." The image of Big Bay shows dates ranging from about 2000 to 74,000 years ago.
Proponents of the impact origin of the Carolina Bays argue that since the bays have a mathematically elliptical geometry they must have originated as inclined conical cavities and that viscous relaxation reduced the depth of the cavities to produce shallow elliptical basins. In this scenario, all the bays formed contemporaneously within a few minutes rather than over thousands of years.
Some scientists favor uniformitarian explanations while others prefer catastrophic explanations for the Carolina Bays. To overcome the bias from these mindsets, it is useful to draw parallels between everyday experiences and deep geological concepts. I am going to tell you the story of the inverted stratigraphy cheese sandwich.
The parable of the inverted stratigraphy cheese sandwich uses ordinary language that everyone can understand, but it makes a few substitutions. Sponge cake is used to represent stratigraphic layers. Bread crumbs are sprinkled to simulate eolian deposits. Fauna are represented by paper cut-outs. I was not able to buy animal crackers because the grocery store was closed when I made this video. I was going to stand the animal crackers to represent live animals and I was going to knock them over to represent fossils. Instead, the fossils are represented by pine nuts. Slices of cheese represent the layer of ground at the time of an impact.
Once upon a time, a geologist was making a cheese sandwich, when he got the idea that he could use it as a model for the impact origin of the Carolina Bays.
He sliced a sponge cake, and laid part of it to represent an ancient stratigraphic layer where diverse fauna multiplied and prospered under typical uniformitarian geological processes. Erosion from wind and water modified the landscape through the seasons in a predictable way.
When the fauna died their skeletons returned to the earth and sometimes fossilized. The landscape continued to change with its usual consistency and dust carried by the wind gradually buried the landscape.
Over the centuries, new stratigraphic layers were created by ordinary eolian and sedimentary processes. The uniformitarian geological changes allowed life to thrive with its usual rhythm. Animals lived, propagated, died and evolved in gradual steps as they adapted to the predictable changes in environmental conditions.
The wind continued to deposit dust over the landscape creating new stratigraphic layers. The geologist covered the landscape with a layer of cheese and said: "This is the base layer that marks the time when uniformitarian processes are interrupted by a catastrophic impact."
The geologist grabbed a stratigraphic layer and overturned it over the base layer of cheese to form a raised rim adjacent to the newly formed cavity made by an imaginary projectile.
The gap left by the overturned layer was replaced with a new layer to level out the terrain.
The world returned to its uniformitarian regimen and eolian deposition continued like before. Everything was covered with dust and new stratigraphic layers continued to form. Dust, dust and more dust.
The geologist was pleased with what he saw and said: "That is a good model for the rim of a Carolina Bay. I am going to eat it now."
The moral of this parable is that when you dig through the older material in the inverted layer, you will find the base layer with cheese, which was the surface of the terrain at the time of the impact. Inverted stratigraphy can be detected by examining at least three sections of a core sample in the rim. Proceeding from the top down, the surface layer contains the youngest material that accretes by ordinary eolian and sedimentary processes. Immediately below the youngest layer there is a layer of older material that was excavated and flipped over during the formation of the elevated rim. Below the layer of older material is a layer of base material that was the original surface of the terrain before the impact cavity formed. Deeper layers contain progressively older material.
Impact cratering displaces material laterally by horizontal compressive forces and ejects debris ballistically to produce stratigraphically uplifted rims around the cavity. The book by Prof. Jay Melosh illustrates the inverted stratigraphy of a crater rim. If we obtain a core sample from the rim of an impact crater, we would see the youngest material in the top layer, followed by older material excavated by the projectile from a deeper layer, and going deeper we would find the base material that was the surface of the terrain at the time of the impact.
A paper published in 2012 by Moore et al. reported the dates of a core taken from the rim of a Carolina Bay called Flamingo Bay in South Carolina. The sequence of dates shows inverted stratigraphy with a layer of older sediments lying stratigraphically higher than younger sediments. This is expected in the rim of an impact crater.
The rim of an impact crater will usually have inverted stratigraphy, and this is what we see in the rim of Flamingo Bay. The base layer at the time of impact is 13.1 thousand years old. The base layer is overlaid by a layer of older material with a date of 15.5 thousand years. This older material presumably originated from an overturned flap when the rim of Flamingo Bay was formed. The old material is covered with new material that accumulated after the impact that made the bay with dates from 11.5 to 5.0 thousand years ago.
A similar case of inverted stratigraphy was reported by Bunch, et al. in 2012 for the rim of a Carolina Bay near the town of Blackville, South Carolina. The base layer with a date of 12.96 thousand years ago is overlaid with material dated at 18.54 thousand years ago. This older material was excavated by the impact to form the overturned flap. The old material was overlaid with a younger layer dated at 11.5 thousand years ago, which consists of material that accumulated after the impact.
This is the secret of the rims of the Carolina Bays. The base layer under the inverted stratigraphy of the rim likely corresponds to the date when the bay formed. The dates of the base layers obtained by Bunch and by Moore are in the range of the date determined for the Younger Dryas boundary by Kennett, et al. in 2015. Is this a coincidence? In any case, this approach for dating the time of formation of the Carolina Bays is worth pursuing.
Dating cores taken from the rims of the Carolina Bays is the most logical way of determining the age of the bays because the procedure takes into consideration the mechanism of crater formation, which includes the creation of the overturned flaps on top of the base terrain at the time of the impact. One suggestion for future dating of the rims is to take additional samples below the base layer to confirm an increase of age with depth. It is also important to take into consideration that many bays were emplaced on top of each other, so the most accurate dates will be obtained from bays that were emplaced first and not the ones that formed on top of previous bays. The law of superposition should be used as a guide.
