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Stratigraphic surfaces are really complicated

Professor Harry Wheeler was one of the architects of modern stratigraphy. His ideas seemed abstract and difficult to many of his contemporaries in the 1950s, and there wasn’t a whole lot of data to support them. While his ideas initiated a revolution in sedimentary geology, it was a few more years before the increased availability and quality of seismic reflection profiles offered unprecedented insights into the large-scale architecture of sedimentary rocks.

In the 1970s, using many seismic cross sections from continental margins, Peter Vail and his colleagues at Exxon’s research centre in Houston identified a whole range of large-scale stratigraphic patterns and interpreted them as the effects of sea-level change and subsidence on sedimentation and erosion. By doing so, they created sequence stratigraphy, an entirely new way of looking at sedimentary rocks. Erosional surfaces became important in sequence stratigraphic analysis, as stratigraphers realized that deposits of tectonically quiet continental margins have numerous stratigraphic discontinuities that are more subtle than major angular unconformities, yet can be confidently traced across great distances. These sequence boundaries divide the stratigraphic record into sequences. Deposits below and above the boundary have little to do with each other. This is in contrast with a relatively continuously deposited stack of sediments in which the location of deposition — and therefore the nature of the sediments — can only change gradually.

One of the most important discoveries of sequence stratigraphy was the impact of sea-level changes on the morphology and stratigraphy of continental shelves. As sea level drops below the shelf edge, rivers erode into the shelf and incised valleys form. These valleys were thought to be erosional and accumulate little or no sediment during sea-level fall; they would only be filled during subsequent sea-level rise. If this was true, it would also mean that the erosional surface — or sequence boundary — at the base of the valley was a topographic surface at some point in the past, and all of the ‘missing’ sediment was transported beyond the shelf edge, into deeper water. But studies of incised valleys that formed during glacial periods of the Quaternary, and flume experiments in which both topography and stratigraphy can be carefully tracked have started to suggest a different, more complex story: overall fluvial incision can still be accompanied by deposition, the sequence boundary is a ‘composite’ surface that does not correspond to any topographic surface from the past, and significant parts of the valley fill have formed during incision. As the title of a paper in the Journal of Sedimentary Research suggested, an incised valley is a ‘valley that never was.’

If this wasn’t confusing — or, depending on your point of view, enlightening — enough, experimental stratigraphy has also cast doubts on one of the other tenets of sequence stratigraphy: the idea that all the deposits above the sequence boundary are younger than the deposits below. In a flume tank at Saint Anthony Falls Laboratory in Minneapolis, coastal deposits that were time-equivalent to fluvial sediments above the sequence boundary ended up lying below the erosional surface as the system slowly advanced into the miniature sea. This concept is not easy to visualize, but it turns out that one of the well-known small-scale sedimentary structures, which is mentioned in every sedimentology textbook, shows all the characteristics that seem so surprising in the case of large-scale erosional surfaces. Climbing ripples that have a relatively small angle of climb deposit cross-laminated sands with erosional surfaces that are clearly different from the topography of the ripples at any time; and it is tempting but foolish to think that all the sediment above one of these surfaces is younger than everything below.

climbingripples.png

As geomorphologists and stratigraphers come up with new and more precise methods of age dating, as experimental sedimentologists and numerical modellers refine their techniques for modelling sedimentary systems in flume tanks and on powerful computers, and as field geologists and seismic interpreters adopt a more sophisticated view of stratigraphy, Harry Wheeler’s ideas seem more relevant than ever. He initiated the revolution in our thinking about stratigraphic surfaces, but the revolution is not over yet.


References
Strong, N and C Paola (2008). Valleys That Never Were: Time Surfaces Versus Stratigraphic Surfaces. Journal of Sedimentary Research 78, 579–593, DOI 10.2110/jsr.2008.059.
Wheeler, H (1964). Baselevel, lithosphere surface, and time-stratigraphy. Geological Society of America Bulletin 75, 599–610, DOI 10.1130/0016-7606(1964)75[599:BLSAT]2.0.CO;2.

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Stratigraphic surfaces are complicated

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