What thrusting does to porous sandstones (surprisingly little)

The situation: porous sandstone overthrust by Sevier-age thrust sheet. How much did the sandstone suffer? Surprisingly little.

The situation: porous sandstone overthrust by Sevier-age thrust sheet. How much did the sandstone suffer? Surprisingly little.

The place

The place

To many of us, thrust nappes and thrusting are things related to metamorphic rocks or shale-limestone sequences in places such as the Alps, the Caledonides or the Canadian Rockies, while rifts are more commonly associated with porous sandstones and shales. When shale-limestone sequences are shortened, thrusts follow shale beds and ramp up through the more competent limestone units, forming fault-bend folds and fault-propagation folds. This is the style of foreland thrust tectonics that we know from many orogens. But what happens to porous sandstones involved in foreland thrusting?

Porous Aztec Sandstone. The thrust to the overlying Lower Paleozoic Bonanza Group can be seen in the cliff

Porous Aztec Sandstone. The thrust to the overlying Lower Paleozoic Bonanza Group can be seen in the upper part of the cliffs in the background, just above the person’s head. The variations in color in the sandstone are caused by meteoric water dissolving and precipitating iron oxide, a process which is controlled by bedding and shear-enhanced deformation bands alike.

Recent work in Nevada, USA (and also in Provence, France) suggests that the sandstone can survive a lot of overthrusting without much internal strain. It develops deformation bands in high-porosity parts. Specifically it develops compaction bands where porosity is very high, and shear-enhanced compaction bands. Compaction bands are relatively rare and mainly involve band-perpendicular compaction, and are oriented perpendicular to the shortening direction and sigma1. Shear-enhanced deformation bands form two (conjugate) sets as illustrated in the figure (although both sets may not be present in the same outcrop). Hence we develop sets of small-scale structures that constrain the shortening direction very well.

Arrangement of SECB (shear-enhanced compaction bands) and PCB (pure ompaction bands)

Arrangement of SECB (shear-enhanced compaction bands) and PCB (pure compaction bands)

Later during thrusting, as the sandstone is buried and somewhat more lithified (but still porous), cataclastic deformation bands form where grains are more strongly crushed.

Perhaps the most surprising thing that we observe is that these structures are fairly sparsely distributed in the sandstone, and only in a meter-thick zone at the thrust do we see intense deformation of the sandstone. It is amazing (and surprising) to have a several kilometer-thick thrust nappe moving tens of kilometers over a porous sand(stone) without doing more damage (grain crushing, porosity reduction): The sandstone is still highly porous and of good reservoir quality (0.1-1 darcy in large parts of the area). Hence overthrust reservoirs may be of good quality.

Shear-enhanced compaction bands (wide vertical bands) cut by low-angle cataclastic deformation bands.

Shear-enhanced compaction bands (wide vertical bands) cut by low-angle cataclastic deformation bands.

Why is this surprising? Because there is no weak shale or salt layer between the sandstone and the thrust nappe that can take up the shear strain. Just the meter-thick cataclastic zone. It is also difficult to create overpressure in this zone since the sandstone, porous and very thick and probably connected to the surface at the time of thrusting.

Papers:
Fossen, H., Zuluaga, L.F., Ballas, G., Soliva, R. & Rotevatn, A. 2015: Contractional deformation of porous sandstone: insights from the Aztec Sandstone, SE Nevada, USA. Journal of Structural Geology 74, 172-184.

Gregory Ballas, G., Soliva, R., Sizunb, J-P., Fossen, H., Benedicto, A., & Skurtveit, E. 2013: Shear-enhanced compaction bands formed at shallow burial conditions; implications for fluid flow (Provence, France). Journal of Structural Geology 47, 3-15.

Brock, W.G., Engelder, J.T., 1977. Deformation associated with the movement of the Muddy Mountain overthrust in the Buffington window, southeastern Nevada. Geol. Soc. Am. Bull. 88, 1667-1677.

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The Caledonian bedrock layer cake in S Norway

Cake model

Cake model

In the South Norway Caledonides – the Himalaya-style mountain chain that formed ca. 420 million years ago when Greenland and Scandinavia smashed into each other – there are three principally different rock units that are stacked on top of each other (top to bottom):

Crystalline thrust nappes (Strong, partly mylonitized)

Phyllite (décollement zone) Lower Paleozoic, very weak and micaceous

Proterozoic basement (Granitoid, Proterozoic structures, very strong)

Basement

Basement

These are theologically very different units: The phyllite-dominated middle layer is very weak compared to the others and acted as a weak décollement on which the overlying Jotun Nappe and other nappes moved. Most of the nappes are thought to have moved several hundred kilometers!

Because the phyllites localized deformation very effectively, the basement was barely deformed during the Caledonian continent-continent collision in many places, at least at the outcrop scale. Proterozoic cross-cutting relations are well preserved!

Phyllite

Phyllite. Shear bands dipping to the right indicate top-to-the-NW (left) sense of shear.

A primary unconformity is locally preserved between the phyllite unit (décollement) and the basement. A conglomerate and weathering arkose can be found in places, showing that the phyllite layer was deposited on the basement during the Cambrian transgression (rise in sea level). Hence the phyllite was mud deposited in a shallow continental ocean.

Conglomerate with phyllitic matrix

Conglomerate with phyllitic matrix

The nappes are also crystalline basement rocks, such as migmatites, granites and gabbros/anorthosite, probably ripped off the basement somewhere to the northwest of the present coastline. But the base of the nappes (base of the orogenic wedge) is strongly mylonitized (sheared), with a strong banding or schistosity. The mylonite zone is typically something like 200 m thick.

Unconformity with basal conglomerate on top of the Proterozoic basement. Phyllite in upper part of the picture

Another view of the unconformity with basal conglomerate on top of the Proterozoic basement. Phyllite in upper part of the picture.

What is perhaps surprising is that the kinematic indicators (asymmetric structures) in both the mylonite zone and the décollement phyllites consistently show top-to-the-NW sense of shear, contrary to the expected collision-related nappe translations. The explanation for this is that the orogenic wedge (nappes) moved back toward the hinterland shortly after the collision finished, suggesting a change from convergent to divergent plate motions shortly before 400 Ma.

Mylonites at the base of the nappe layer (Jotun Nappe)

Mylonites at the base of the nappe layer (Jotun Nappe). Top-to-the-NE (right) sense of shear.

You can read more about the  transgression in “The making of a land  Geology of Norway” (http://www.geologi.no/the-making-of-a-land) and about tectonic aspects in my papers, for example in publications 47 and 86 in my publication list: http://folk.uib.no/nglhe/Publications.html)

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Jointing and mechanical layering

Two sets of joints in Permian eolian sandstone, Canyonlands National Park

Two sets of joints in Permian eolian sandstone, Canyonlands National Park. Two joint sets at high angles are quite common in competent beds.

Jointed Cretaceous sandstone layers, Book Cliffs, Utah

Jointed Cretaceous sandstone layers, Book Cliffs, Utah. Note how the distance between joints is larger in the thick layers.

Joints occur pretty much everywhere in rocks exposed at the surface of the Earth. And they can be pretty important, both during construction activities, intrusion of magma and flow of fluids (groundwater, hydrocarbons, CO2, magma). Joints represent conduits of fluids, particularly in the vertical direction, and are made artificially during fracking (hydraulic fracturing) operations.

Jointed silica-rich layers in green shale, Green River Formation, Utah

Jointed silica-rich layers in green shale, Green River Formation, Utah. Note higher joint spacing in thick layer at the top.

Joints form naturally in a variety of settings, but particularly during exhumation due to pressure release and cooling. Thermal contraction is very important, and it is the stiff layers that fracture most easily. Hence we typically see fractured sandstone or limestone layers between non-fractured or less fractured siltstones or shale layers. Layers that develop different joint patterns define what is called mechanical stratigraphy.

Relationship between layer thickness and joint spacing

Relationship between layer thickness and joint spacing, based on field observations.

Typically, stiff layers develop fractures with a characteristic spacing that depends on layer thickness: The thicker the layer, the farther apart the fractures, as shown in some of these pictures.

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Great stereonet applications

Screendump of Stereonet 3D

Screendump of Stereonet 3D

Stereonet plotting programs are extremely valuable, particularly to those of us that are collecting field data. I have enjoyed using my iPhone app (GeoID) to collect geo-tagged measurements over the last year or two, and being able to easily import data into Stereonet3D is nice. Stereonet3D is written by Nestor Cardozo and Rick Allmendinger, and the last version of this application does most of what I want a stereonet program to do:

3D view of Stereonet3D

3D view of Stereonet3D

•importing data with no hassle
•showing the data in a map/satellite window
•exporting data to kml files for display in GoogleEarth

While Stereonet3D is MacOS software, Rick Allmendinger’s Steronet 9, which is a similar application with many of the same features, comes in Mac, Windows, and Linux versions. The advantage of this is that the same binary file is compatible across all different versions. 

These two programs are closely related, but with certain differences.

Screendump of Stereonet9 (from Rick's website)

Screendump of Stereonet9 (from Rick’s website)

Which one will serve your purpose best is something you have to figure out through a little testing. If you are using it for educational purposes, for example, Stereonet3D’s interactive 3D view may be particularly useful. Try it out!

Stereonet 9 is downloadable from Rick’s programs website, while Stereonet3D is now available from Apples App Store for a nominal fee. Many thanks to these two structural geologists for putting a lot of work into something that we all benefit from!

Link to Stereonet9:
http://www.geo.cornell.edu/geology/faculty/RWA/programs/stereonet.html

Link to Stereonet3D:
http://www.ux.uis.no/~nestor/work/programs.html

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Structural geology and tectonics

This video point at some basic aspects of structural geology, emphasizing the importance of being able to read information out of structures, and to put this information into a larger-scale tectonic picture.

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San Francisco geology: a diversion from AGU

Serpentinite block at Marshall Beach (SW side of the Golden Gate Bridge). The road with folded chert can be seen on the far (north) side of the bridge.

Serpentinite block at Marshall Beach (SW side of the Golden Gate Bridge). The road with folded chert can be seen on the far (north) side of the bridge.

This year I attended the American Geophysical Union fall meeting in San Francisco. This meeting does not offer organized field trips the way that GSA and other conferences with a strong focus on geology do. However, nice outcrops can easily be found near the city, and even though I am not particularly familiar with the geology of the area, renting a bike one day before the meeting was a good decision that I recommend to any geologist who is up for some local hard rock geology.

The closest localities are found on the north and south sides of the Golden Gate Bridge. I started at the north side and found the light to be good during the first part of the day there. The southwest side is great in the evening light.

Iron and manganese-rich chert and related sediments at Conzelman Road.

Iron and manganese-rich chert and related sediments at Conzelman Road.

Crossing the Golden Gate Bridge takes you to the Golden Gate National Recreation Area. I  Just after crossing the bridge (west side), go left onto Conzelman Road. As you wind your way up the road there are nice exposures of folded chert layers several places, both before “Battery Spencer” and farther up the road.

Folded chert.

Folded chert, Conzelman Road. Some quite interesting patterns have formed locally due to layer-parallel shear/slip. Flexural slip is an important mechanism during folding of such layered rocks where the mechanical contrast between the chert layers and thinner shale layers is very high.

The cherts are oceanic (pelagic) sediments of the Franciscan Complex, a rather chaotic assemblage of Jurassic-Cretaceous rocks that were crumbled and sheared during accretion onto the N American continental margin in the pre-San Andreas Fault days when this margin was more of a “standard” subduction zone.

Parts of the Franciscan Complex appear as chaotic in the sense that blocks and fragments of different rock types occur together in what is referred to as mélange. There may be both depositional and deformational components to a mélange, but clearly deformation played an important role during the formation of these rocks. Typically blocks of serpentinite, amphibolite, chert and sandstone occur intermixed. Several of these rocks can be seen on the San Francisco (south-west) side of the Golden Gate Bridge along the bluffs of Marshall Beach (Battery to Bluffs Trail). This area is also affected by landslides, which also give the rocks a chaotic (“melangy”) appearance.

Melange

Melange, Marshall Beach. Blocks of serpentines are most prominent in the picture.

Ultramafics and serpentinites commonly show nice vein systems, and also nice slickensides. You will find them here as well, as shown in the picture. Once serpentine forms in fractures, the rock easily deforms, and this typically happens repeatedly throughout its lifespan. 

Slickensides formed on slip surface in serpentinite.

Slickensides formed on slip surface in serpentinite.

I am sure there are other nice hard-rock localities around, and feel free to comment on that below. With a car the selection of nice outcrops will of course be significantly larger. Here are a couple of resources if you want more information about the geology of the area: http://pubs.usgs.gov/bul/2195/b2195.pdf

http://www.sanandreasfault.org/Geology%20of%20the%20Golden%20Gate%20Headlands%20Field%20Guide.pdf

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Do you sketch in the field?

Normal fault (Bartlett Wash, Utah)

Normal fault (Bartlett Wash, Utah)

Geology is, by nature, a field-based branch of science, and making on-site sketches is the best way to approach an outcrop. Why? Because field sketching sharpens your senses and turns you into a much better observer. It forces you to focus, to make important decisions regarding crucial structures and features, such as cross-cutting relations, sequence boundaries, fault geometry, layer continuity and grain size variations. It makes you discover important details that otherwise might go unnoticed. And it makes the locality stick to your mind.

Student field sketch from Canyonlands, S. Utah.

Student field sketch from Canyonlands, S. Utah.

Students realize this once they are exposed to field sketching during field trips and guided fieldwork. In fact, it is quite amazing to see the change that many, students and professional petroleum geologists alike, go through during a field trip that involves what I find to be a very useful one; to start each new location by making an overview sketch. The first day it seems like hard work. Then the next day – not so bad. By the third day, everyone is enjoying it because they are familiar with the concept and they realize that it works. Sketching is starting to be a fun thing.

Students sketching

Students sketching

Students sketching

Students sketching

Everyone seems to agree on its usefulness, but few if any universities offer a dedicated field-sketching course. Students in Bergen recently decided to do something about this. Through their local geo-student association, they asked for help, and once we had set a date for the course, more than 100 students quickly expressed interest.

Now it is not so easy to go out in the field in November in Bergen, so we put pictures on the screen to practice our skills. The advantage of making sketches in the field is of course that you can move around, check out details, use your hammer, dig, measure things, use your hand lens and so on. But just sketching from a picture is also a good exercise, one that may give you some rewarding surprises every now and then.

Example of a quick sketch

Example of a quick students sketch

If you are a student and want to improve your sketching skills; make at least one quick sketch every day for a few weeks, and you will be amazed by the progress you make. Efficient sketching requires some practice, and you can get much of that at home by sketching from photos.

The picture from which the sketch was made (SW Portugal)

The picture from which the sketch was made (SW Portugal)

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