Which are the biggest methodological differences between the archaeological and geological approaches to stratigraphy?

Question

Stratigraphy, or study of rock or soil layers (strata), was originally introduced as a branch of geology. However, it is often applied in other disciplines, especially in archaeology and paleonthology. As an archaeologist (though not a field archaeologist), all I learnt about stratigraphy was taught from archaeological perspective. So I'm curious how the approach of other disciplines to stratigraphy differs from the archaeological. I'm especially interrested in geology - because it is the main and original discipline for stratigraphy, and because the geological time is very different from the archaeological time.

I know the basic principles such as law of superposition and principle of original horizontality are common. On the other hand, the time (hundreds or thousands of years x millions of years before present), materials on which stratigraphy is studied (sediments often created by human culture x rocks) and dating methods differ.

But how do these differences affect the "theoretical" methodology? That it is usually not possible to dig 100 million old strata with a spade is natural, but how about interpretation?

Is geological stratigraphy visualized with Harris matrix or something very similar too? Are there any special principles that usually apply only on very old strata? Or is geological stratigraphy simpler, so some archaeological stratigraphy rules don't apply? Is there any distinctions between various stratigraphic units (layers and cuts, or other distinctions)? If yes, which categories are usually distinguished?

Answer 1

As I see it there are no clear differences. There are, however, processes that are not compatible with a superposition principle. In geology, it is possible for layers to be folded and result in inverse age relationships. Packs of sediments can also be pushed over other layers during, for example mountain building, so that sequences are out of order or even repeated, almost like shuffling a deck of cards. It is also possible for, for example, magma to penetrate into rocks and generate much younger layers within an older rock mass. A depth, mass is added to the crust by melting and solidification while cooling such that rock units of different age can be found parallel (in a stratigraphic context) to each other. Hence the superposition principle only applies in sedimentary rocks and unconsolidated sediments that has not undergone and significant tectonic activity. But, I can also imagine that archaeological material can (although not extensively) be subject to disruptions in certain environments where active tectonics are at play or for example where glaciers act to push material forward and generate repeat sequences.

To expand on the second part of the post. Geological stratigraphy is established from physical exposures and by measuring structural shapes such as the strike and dip of strata (a branch of geology called Structural geology. Very little is gained from created exposures such as open day mines, mine shafts of other dug pits. It is also possible (as in archaeology) to use geophysical methods to infer layer by ground penetrating radar, seismics etc. It should be mentioned here that methods and possibilities vary between bedrock and unconsolidated sediments, where I would venture to say that archaeology and studies of the unconsolidated sediments are very similar using test pits and coring with the addition of understanding transport mechanisms to understand how and why certain sediment layers exist where they are.

Visualisation pof geological data is usually done in sections or logs of boireholes with attempts to connect between observed sections. In bedrock where there is not only vertical variability but also significant horizontal, such as in igneous rocks, obtaining a precise image is very difficult

Answer 2

The main difference between archaeology and geological stratigraphy is the archaeological deposits are unbound sediments that are dug and, in geology, some samples or fossils are taken to describe the outcrops, but the main tasks are description and correlation, while in archaeology the main tasks are only digging and description.

The other huge difference as you mention is the geological ages are based in chronostratigraphic chart, which is based in geological events, and the archaeological ages are based in the kind of tools the human used in the prehistory and the history.

Dating methods to estimate the ages are done by the same principles, but a geological age covers millions of years and the epsilon is not so important, being more easy to grant an age to a level in the chronostratigraphic chart.

There are differences in how strata are correlated in both sciences. While in geology spatial correlation can be as large as the whole world (for example for the Cretaceous-Paleogene mass extinction, where a level enriched in iridium is seen in different places all over the Earth), archaeology correlation is based on the spatial distribution of human populations with similar culture, less extensive in the spatial dimension.

Is geological stratigraphy visualized with Harris matrix or something very similar too? Is geological stratigraphy simpler, so some archaeological stratigraphy rules don't apply?

The Harris Matrix and other techniques and principles used in archaeology are sophisticated and not used in geological stratigraphy. In archaeological stratigraphy the level of detail of the description of the succession is higher, becoming essential to correctly establish a detailed register succession.