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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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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