Sequences of Strata

Decoding the handful of past processes encoded into a single cobble is powerful, but more power still comes from looking at sedimentary rock in outcrop, and comparing the conditions of one layer with those below it and those above it. The lingua franca of historical geology is sedimentary rock, and vast epic tales have been inscribed in the sequence of accumulated strata, or rock layers.

If a shale means former mud, and that mud’s deposition implies calm, offshore conditions, and sandstone means former sand, and that sand’s deposition implies more energetic nearshore conditions, then what does this photo show?

A sequence of sedimentary layers can tell a story of changing conditions over time.
Photograph by Zoltán Sylvester; reproduced with permission.

A sedimentary sequence of mud-rich strata overlain by sand-rich strata is the record of a shift in depositional conditions at a site over time. In the image above, from Chilean Patagonia, the vertical sequence of layers shows that as time went by, there was less and less mud being deposited at this location and more and more sand. The progressive stack of layers is a record of time, with older, earlier chapters at the bottom and more recent chapters at the top.

So we’ve made an observation (the shift from muddy to sandy), and that observation begs the question: Why was there less mud and more sand? (In other words, what’s the story?)

Maybe it shows a shift from deeper water to shallower water conditions. Maybe it shows the tectonic drift of a landmass into a more energetic, storm-dominated location. Maybe it shows the advance of a submarine fan system spreading onto the abyssal plains of the deep ocean. Maybe it shows the development of a nearby mountain belt, shedding the source sediment. Each of these “maybes” is a hypothesis, a potential explanation for the observation of the sandier layers overlying the muddier layers. The goal of historical geologists is to probe each of these possible explanations and test the ideas against other local details of the geology. Storms rip up mud chips, for instance. Are there mud chips in these sands? Submarine fans often show graded beds. Are there any? A change in the fossil fauna often marks shallower conditions. Tectonic drift can be marked by changes in the paleomagnetic inclination. There is a profusion of additional data we can collect to match up with the different potential interpretations. Generating and considering multiple working hypotheses allows historical geologists a “toolkit of the imagination” when they head into the field.

Outcrops in Inuyama, Japan, show a Triassic sequence of sedimentary strata that record changes in deep ocean oxygen levels after the end-Permian mass extinction event. The strata have been tectonically rotated due to accretion with the Japanese mainland.
Photo by Yukio Isozaki; reproduced with permission.

Here is another example from Inuyama, Japan:

Here, the strata are not in their original horizontal orientation. They have been rotated during tectonic accretion of these deep ocean strata to the Japanese mainland. The layers get younger to the right (rather than “up”). Don’t let that throw you off – You can twist your head to the right if you need to.

The important thing is to note that a distinctive color change occurs across the outcrop: the layers shift from black to red. This is a signature of changes in the amount of oxygen in the deep ocean: there was almost no oxygen (black), and then there was plenty (red). This is interesting because a major mass extinction occurs just before this sequence: the end-Permian mass extinction (also called “The Great Dying”). These strata are 247 to 242 Ma, early middle Triassic, and show the recovery after the mass extinction.

Evidence such as this sequence of strata suggests that ocean anoxia (low oxygen levels) were associated with this major die-off in animals, either as a cause or as a consequence.  And most everything on the planet died: 95% of species. It was the worst thing that ever happened to life on our planet, and this sequence of jet black layers transitioning to red layers is trying to tell us why it happened. Can we speak Rock well enough to hear the story? Our own species’ future might depend on our ability to translate. This work is important.

Again, no single layer tells the whole story – we must look at the succession of layers over time.