Can Strata Above An Unconformity Be Older Than Strata Below It

I just could not resist this. A paper on sequence stratigraphy in the August issue of Journal of Sedimentary Research by Nikki Strong and Chris Paola discuss just how such stratal relations can develop during the evolution of a passive margin lowstand systems tract. That is a lot of terminology. Passive margins are continental margins that form when plates rift and separate as result of sea-floor spreading. The east coast of the U.S and the west coast of India are passive margins.

These margins are characterized by quite a broad continental shelf. Large thicknesses of sediment accumulates on these shelves over time. These deposits are not one homogeneous body of sediment. Rather they are arranged in packages or bundles of sediment known as sequences. The controlling factor on the deposition of these sequences is the rise and fall of sea-level. Of interest here are fluctuations of sea-level spanning roughly 1-10 million years. These are known as 3rd order cycles and the sequences deposited within are called third order sequences. 2nd and 1st order cycles span larger time frames and 4th and 5th order span much smaller periods.

3rd order sequences have become something of a "working unit" for sequence stratigraphers. They are readily recognized in outcrop and in the subsurface using seismic profiling of basins. They also span a long enough period to record different phases of global sea-level history and basin development. Hence the focus on these sequences.

Several types of 3rd order sequences are recognized depending on what phase of a sea-level cycle they are deposited in. Each exhibit distinctive facies arrangements and stratal relationships (should I call it architecture? :)). Sequences deposited during a sea-level rise are called transgressive systems tracts. Those deposited when the sea-level rise has peaked are called highstand systems tracts. And those deposited during a sea-level fall are called lowstand systems tracts. The study of how these sequences evolve and how they are distributed in time and space is known as sequence stratigraphy.

Right. So as sea-level drops, it eventually exposes the continental shelf and an erosional surface develops. When sea-level rises again drowning the shelf, sediment will get deposited on this erosional surface. Geologists recognize this type of surface as an erosional unconformity. It is also a sequence boundary since it separates older sediment of the underlying lowstand systems tract from the subsequent transgressive systems tract. Knowing this how then can strata above an unconformity be older than strata below it?

It is something of a trick question since the researchers were not concerned with the subsequent transgressive systems tract but the complex evolution of the erosional unconformity. The researchers used an experimental basin at the Experimental Earthscape facility at the Univ. of Minnesota. The basin has a subsiding basin floor and a sediment supply system and these were manipulated to recreate scaled versions of lowstand systems tracts.

When I was doing graduate research in carbonates, modeling of sequences to simulate stratigraphic evolution of basins had gained recognition as a powerful tool which complimented outcrop and subsurface evaluation. Now you can get experimental data by recreating scale models of sequences.

That's cool!

The somewhat counter-intuitive stratal relationships develop because sea-level drop across the continental shelf is not instantaneous. The entire shelf is not exposed to erosion all at once or very rapidly. Instead as sea-level starts dropping, the shallower parts of the basin (think near the coast) are exposed early and an erosional surface develops on the exposed sea-floor. At this time the deeper parts (think shelf edge) are still below sea-level and may be active centers of delta formation.

Eventually on the exposed part of the shelf, i.e on the erosion surface a river valley may form and a fluvial depositional system may develop. Sediment is now accumulating on the erosion surface, while towards the shelf edge marine conditions prevail and deltaic sedimentation continues.

As sea-level continues to drop the erosion surface will get extended and keep overriding younger and younger deltaic sediments in shelf edge areas. But sea-level fall and the gradual exposure of the shelf has spanned so much time (often 100's of thousands of years) that often the very initial fluvial sediments deposited on top of the erosion surface will be older than the youngest deltaic sediments that have been recently overridden by the migrating erosion surface.

This may not come as something terrible new to geologists working in sequence stratigraphy but I thought the use of experimental data combined with a theoretical understanding of sequence development is a pretty powerful way to analyze basin evolution. Will this have any practical relevance when it comes to say correlating strata based on their position relative to a sequence boundary? I don't work in this field so I can't say how important such a result will be. But a work like this does remind us that those squiggly lines you see in a geological column and dismiss as just an unconformity, a time of no deposition, a hiatus when nothing but erosion took place, often have complex, protracted and interesting geologic histories of their own.