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Stratigraphy Part 6 - Post depositional processes

MIKE SMITH: Now I want to move on to look at post-depositional processes. Whatever stratigraphic sequence we’ve got can be overprinted by post-depositional processes. Soil formation is the classic one, but there are others.

The point I want to make here is that, when you sit there and look at, say, the stratigraphic sequence of a North Australian rock shelter with organic staining at the top and paler sands at the bottom, by and large you’re not looking at actual stratigraphy; you’re looking at overprinting of the stratigraphy by younger processes.

The classic example, as I said, is soil development. In a soil horizon you’ve got a zone of organic enrichments; you’ve got a zone where the fine mobile material is leached out, particularly the clays; and you get a zone of enrichment, the sea horizon, where you get enrichment in carbonates and clays and things like that. Now these same processes will operate on a site, an archaeological deposit, if it’s subject to a lot of weathering and if it’s stable enough and has enough time for this to start to operate.

Characteristically, what you might get is compaction of deposit and loss of organics over time. You might get bioturbation by invertebrates and a loss of the fine detail in the deposit as interfaces and features are disturbed by burrowing insects and things. You might get re-organisation of the matrix as the clays are washed down through the system or the iron is mobilised. You might get the formation of new minerals. Carbonates may precipitate out – sesqui-oxides. And you might get pedogenesis which is generally just the formation of soil horizons, clay aggregates, grain cutans. The voids in the sediments might get filled and lined by clay and so forth.

The real message here is that, when you look at a stratigraphic sequence, some of the things that may be most obvious to you in terms of colouring may be due to the secondary alteration of the sequence. So don’t get misled by simple things like colour and organics. Try and look beneath them to look at the structure of the sedimentary sequence. It’s very common in north Australia that the actual stratigraphic sequence is more cryptic, and the most obvious thing is the post-depositional processes that move fine grain organics around.