If you have seen the geological spectacle called the Grand Canyon of the Colorado River, your first impression might have been that the stratigraphic record is not that different from a layer cake. Most bedding surfaces visible in the canyon walls are roughly parallel to each other and many sedimentary layers seem to go on forever. However, this picture breaks down if you look more carefully. At a large scale, layers that seem to have a constant thickness turn out to be consistently thinning in one direction and/or they are truncated by large erosional surfaces; at a small scale, as you look closer at some of the rock faces, interesting patterns with inclined bedding and smaller-scale erosional boundaries emerge. The geometry and origin of stratigraphic surfaces and their relationship to ancient land- and seascapes may seem to be simple geometric problems. In fact they are far from simple.
To better understand why, it is helpful to view stratigraphy as the result of cumulative changes in the surface of the earth. The change can occur either through addition (deposition) or subtraction (erosion). While it may be easier to think of the surface — or the landscape — as undergoing either deposition or erosion everywhere at any given time, there is no reason why parts of it can’t be eroding while deposition is taking place not too far away. Yet conventional stratigraphic thinking is strongly influenced by the idea of a clear separation of erosion and deposition in space and time.
There are certainly cases when and where this view is valid; classic angular unconformities come to mind. When James Hutton saw the unconformity at Siccar Point, where only slightly tilted 345 million year old layers of the Old Red Sandstone are sitting on top of near-vertical beds of 425 million year old Silurian greywackes, he realized that such structures could not have formed in only six thousand years, widely believed to be the age of the earth in the late eighteenth century. He reasoned that:
• The sediment in the older formation was deposited in horizontal layers.
• It got buried, compacted, and became hard rock.
• It was tilted to an almost vertical position and lifted above sea level.
• It was then eroded by subaerial erosion.
• It was buried again by much younger sediment that was itself later cemented and tilted by tectonic forces.
Although the eroded material that corresponds to the erosional surface must have been deposited somewhere, we know that this place was far away from the location of the erosion.
For two centuries aft er Hutton’s discovery of unconformities, stratigraphy essentially remained the study of quasi-horizontally deposited sedimentary layers; the study of deposits — and the fossils they contained — rather than surfaces, with little interest in the nature of erosional boundaries. However, in the 1950s Harry Wheeler, professor of geology at the University of Washington, started to explore ideas about where the eroded material went and what happened to stratigraphic discontinuities if one was able to follow them laterally over long distances. Wheeler realized that:
1. The time gap represented by an unconformity must include both the duration of deposition of the eroded sediments (he called this the ‘degradation vacuity’) and a time of erosion and non-deposition (hiatus); and
2. This time gap must decrease if we follow it toward the area where the eroded material is deposited.
These concepts are best illustrated on a cross-section-like diagram that has time on its vertical axis, instead of elevation or depth. Nobody before Wheeler had looked at stratigraphy using such an ‘area–time section’, as he called it, and the Wheeler (or chronostratigraphic) diagram ultimately became the spark that ignited a revolution in sedimentary geology.
This essay continues in Stratigraphic surfaces are really complicated.