TS
Tectonics and Structural Geology
Samuele Papeschi

Samuele Papeschi

Samuele Papeschi received his PhD in Earth Sciences at the University of Pisa in 2019. His work combines classic structural geology with several laboratory techniques - like EBSD - to understand how rocks deform deep in the Earth's crust. His natural habitat is the field where he maps structures and geological formations - taking snapshots of impressive geological features. He is a member of EGU, AGU and GSA.

Features from the field: Ripple Marks

Features from the field: Ripple Marks

Earlier this year, Ian Kane, geologist at the University of Manchester, captured the iconic snapshot shown above. The picture reveals ripples, developed due to waves and currents in the sand of White Strand (near Killard, county Clare, Ireland) right next to Carboniferous sandstone that contains ‘petrified’ ripple marks!

The image is powerful, because it shows the basic principle of geological actualism, which can be summarized in the famous quote by Charles Lyell:

‘the present is the key to past, the past the key to the future’

The physical processes that are active in the world today occurred in the past and will continue to occur in the future. And ripples can tell us a lot about the past of our planet!

Sedimentologists study and analyze bedforms, like ripples, in present-day shorelines, river systems, deserts, and in deep marine environments like submarine fans to understand past environments. It is pretty obvious, when walking on a strand, to tell ripples were shaped by waves along the coast or by blowing wind on sand dunes, but would you be able to tell how they developed in rocks, without actually seeing the ambient where they formed?

 

Ripple marks in the Moenkopi Fm., Capitol Reef National Park (Utah). Photo credits © Daniel Mayer/Wikimedia.commons

 

The first and most obvious information we can get from the presence of ripples in sedimentary rocks is that a current must have been present- either a water current or a blowing wind. Their crests are always oriented perpendicular to the current that formed them, telling us what the direction of currents in past environments was.

Their shape, size and symmetry depend on the type of sedimentary process that is associated with their formation. There are two types of ripples: asymmetric and symmetric.

 

Asymmetric ripples exposed in the intertidal zone near Lawrencetown (Nova Scotia). They were formed by a current (likely the tide) that was flowing from left to right. Photo credits © Michael C. Rygel/Wikimedia.commons

 

Asymmetric ripples show a gently-dipping side (stoss side) and a short inclined side (lee side). The sediment is dragged and eroded from the stoss side until it reaches the crest and deposits on the lee side, which is downstream with respect to the current. The continuous removal of sediment from the stoss side and the re-deposition on the lee side causes the ripple crest to migrate in the same direction of the current. Recognizing asymmetric ripples tells us immediately where the flow was directed. We can, for example, reconstruct the direction of a river, or a marine current, or the dominant wind in sandstone that deposited millions of years ago.

 

Symmetric ripple marks formed by waves in Permian rocks from Nomgon, Mongolia. Photo credits © Matt Affolter/Wikimedia.commons

 

Not all bedforms are the result of a single and dominant current. Symmetric ripples are formed by bidirectional currents: currents that move in one direction and then in the opposite one. Does it ring a bell? Waves!

Waves cause ripples to be symmetric because both sides of the ripple become alternatively sites of erosion and deposition while water moves back and forth. Recognizing wave ripples can tell us whether an ancient sandstone deposited on a shoreline rather than on a river bank or a dune field.

Finally, ripples are very useful in structural geology because, as they mark the surface of deposition, they are useful indicators of the stratigraphic top in a sedimentary sequence, for example when we have to deal with overturned beds.

Features from the field: Foliation

Features from the field: Foliation

Have you ever walked on a mountain trail, passing past outcrops of rocks and noticed that many rocks appear to be split along a well-defined orientation? If you have, you might have seen one of the most important structures in metamorphic rocks – called foliation.

The term ‘foliation’ derives from the Latin folium, meaning ‘leaf’. A rock with a foliation looks like a pile of ‘leaf-sheets’ that appear piled up one upon the other. Any set of planes that is pervasively repeated in a rock volume defines a foliation, irrespective of its origin, thickness or composition. In the example shown on top of the page, the rock easily splits along foliation surfaces that dip to the left. In this case, the foliation is defined by the preferred orientation of tiny platy minerals – called phyllosilicates – that cause the rock to break apart easily along a specific direction.

Triassic metabreccia from Punta Bianca (La Spezia, Italy). Note the foliation defined by deformed clasts of white marble. The image is 90 cm in width. Photo © Samuele Papeschi

In the example on the left, a deformed breccia, the foliation is defined by clasts that were flattened by tectonic forces and that are now all oriented parallel to each other.

There are many processes that can lead to the development of a foliation in rocks (I will save them for another post). What is important is that a foliation is a pure and simple geometric term, which means that you can use it to describe any pervasive set of planes in a rock, even if you don’t know their origin or composition.

Given this definition, foliations are not restricted just to metamorphic rocks. Bedding planes in sedimentary rocks define a foliation, and so do flow structures in volcanic rocks or compositional bands (schlieren) in intrusive rocks. This kind of foliation is called primary foliation – formed during deposition or igneous crystallization of rocks – to distinguish them from secondary (or tectonic) foliation that develops when rocks are deformed.

 

There is a foliation inside this granitic dyke from the Calamita Peninsula (Elba Island, Italy). In this case it is defined by black bands rich in tourmaline and changes in grain size. Photo © Samuele Papeschi

 

Tectonic foliations are widespread in metamorphic rocks. They form as a result of different processes, which require two basic ingredients:

  • First, you need deformation processes. Luckily, we are on a trembling planet and at depth rocks are squeezed by tectonic forces.
  • Second, you need heat and pressure. You can squeeze rocks as much as you want, but if temperature is not high enough, they will break rather than develop a foliation. Some rocks can deform at relatively low temperature (claystones for instance) while others -for example a granite, require several hundreds of degrees to start deforming.

If all the conditions above are satisfied, congratulations! Your rock will develop a foliation. During deformation, old grains rotate and parallelize with each other. Eventually, new minerals will grow already oriented parallel to the foliation. This is because the foliation plane lies perpendicular to the direction of maximum tectonic force (called stress in geology) and mineral grains find less resistance to their growth along the foliation plane. Difficult? Here is a sketched example, modified after Raymond (1995).

 

The progressive development of a foliation overprints primary structures leading to the development of a new, metamorphic structure. Redrawn after Raymond (1995).

 

The more deformation goes on, the more primary structures are obliterated. A foliation progressively overprints primary structures, becoming more and more penetrative as deformation continues. If rocks face only limited deformation or are deformed at relative low temperatures, they can preserve original structures, but if the intensity of deformation or temperature (or both) are high, primary structures will likely be destroyed.

We have barely started to scratch the surface of this interesting topic. In the next chapters of the ‘Features from the Field’ series I will look in greater detail at the complicated world of foliation. Stay tuned!

 

References and further reading

Dieterich, J.H., 1969. Origin of cleavage in folded rocks. American Journal of Science 267, 155-165.
Fossen, H., 2016. Structural Geology. Cambridge University Press.
Ramsay, J.G., and Huber, M.I., 1983. The techniques of Modern Structural Geology. Vol. 1: Strain Analysis. Academic Press, London.
Raymond, L. A., 1995. Petrology: the study of igneous, sedimentary, metamorphic rocks. Dubuque, IA : Wm. C. Brown (editors)