November 17, 2011

When Africa bumped North America: cracks underground

I thought about calling this piece "Impacts of the Alleghanian Orogeny", but that just sounded too strange, even for an article based on writings of the patron geologist of the Marcellus Shale pointing out structures that help make drilling here dangerous.

Geology of New York: A Simplified Account is an extremely dense textbook, potent bedtime reading almost guaranteed to cure insomnia. It also, though, tells some critical stories explaining what lies beneath our surface. (It also comes with a great big map, which would look great on my wall except that I need both sides of it!)

So what happened?

the Alleghanian Orogeny lasted from about 330 to 250 million years ago.... proto-Africa was attached to eastern proto-North America. The orogeny produced the Applachian Mountains...

we now think that proto-Africa probably slid southward past proto-North America along a transform margin... As proto-Africa slid southward, it rotated clockwise, pushing westward into the southern part of proto-North America. This westward push produced large faults.... Only portions of New York State were deformed. (19)

Later, Africa moves away, the Atlantic Ocean opens, and this area becomes much quieter as far as earthquakes. However, even though we didn't end up with the Great Smoky Mountains here, that impact had major, if often invisible, effects.

Most of the chapter on the Erie Lowlands and Alleghany Plateau is pretty quiet, focusing on the formation of the rock below us and the waterflows that brought the material. However, at the end, as a supplement, is a section called 'Deformation of "Undeformed" Rocks: Structures in the Allegheny Plateau', which is "Adapted from text furnished by Dr. Terry Engelder, Pennsylvania State University".

If that name sounds familiar, it should. Engelder was one of the first geologists to promote the Marcellus Shale as a source of natural gas, earning himself a profile on This American Life that Penn State apparently didn't like very much. If you want an authoritative geologist who's a fan of the Marcellus Shale drilling, he's the one.

His stories challenge the stable-sounding deposition stories of the previous chapter:

The structure of rocks in the Alleghany Plateau looks deceptively simple — nothing but nearly horizontal sedimentary rock layers: "layer-cake geology". Folds like those commonly seen in the convoluted rocks of the Adirondacks, the Taconic Mountains, and southeastern New York are absent, and faults are rarely seen. However, despite this simple layer-cake appearance of the rocks of the region, subtle effects of the Alleghanian Orogeny are present in most of the rock exposures in central and western New York south of a line between Syracuse and Buffalo. (132)

How much compression was there? "Field Studies show that the Alleghanian Orogeny shortened the crust in the Alleghany Plateau by 10 percent (133)." Even over millions of years, shrinking a rock formation by 10% is pretty impressive. Even more impressive, that shrinking happened in place, without major breaks and folds.

Engelder presents evidence for such compression. That evidence is a set of deformations and breaks that are certainly useful for geological history, but also a warning that the bedrock beneath us is not as solid as we'd like to think:

What evidence for layer-parallel shortening can we see in individual rock exposures? We find the evidence in several types of structures, to be explained below: deformed fossils, pencil cleavage, spaced cleavage, blind thrusting, and drape folds. The first three are associated with flowing in the weak rock layers; the other two are connected with brittle breaks in the strong rock layers. (133)

All of these except the first, deformed fossils, create breaks in the rocks. Here are some of his explanations of cleavage in softer sedimentary rocks:

Rock cleavage refers to very closely-space parallel fractures. Cleavage develops in rocks that are being compressed. Sedimentary rock contains water in the microscopic openings (pore spaces) between its grains. When the rock is compressed, the water pressure is raised, and the water is forced upwards along microscopic passageways. As the water rises it dissolves silica... in the rock. This process is called pressure solution. It results in leaving behind parallel seams of insoluble clay minerals. The removal of rock material by solution along the cleavage planes causes the rock to shorten at right angles to the cleavage... If the rock has thin bedding planes as well as cleavage, the rock breaks along both, to form long narrow pieces called pencil. This kind of cleavage is called pencil cleavage...

Another kind of rock cleavage, called spaced cleavage, is a structure found in the Tully and Onondaga Limestones of western New York. [The Onondaga is below the Marcellus, the Tully above.] Like pencil cleavage, it forms in rocks under pressure, when pore water dissolves part of the rock and leaves and insoluble residue of clay. (134)

There are larger breaks, too:

The thin but strong layers (Tully and Onondaga Limestones and Oriskany Sandstone)... deformed not only by solution along cleavage seams, but also by faulting. Faulted segments were stacked up like roofing shingles... Because the thrust faulting is below the surface and is only rarely seen, it is called blind thrusting...

The faulting and stacking of the thin, strong limestone and sandstone layers created very low mounds beneath the surface. This arrangement caused the overlying shales to drape over these mounds in long, low, wave-like folds, called drape folds... We find such subtle folds scatted throughout the Allegheny Plateau.

Another kind of fracture is often visible on the surface in the Ithaca area, "characteristic of many of the outcrops in the Finger Lakes District of New York." (137)

The most common structures in rocks of the Allegheny Plateau are planar cracks, called joints.... The high water pressure that developed in the rocks during the Alleghanian Orogeny became great enough to drive vertical cracks through the rock. The rock literally split when the internal water pressure exceeded the strength of the rock.

Outcrops in the Finger Lakes district all show abundant vertical joints that were formed in this way. They may exceed 300m in length in cliff-faces. (135)

Those joints run roughly north-south, but there are also east-west release joints formed later, when higher rock was eroded:

rock that was once deeply buried and therefore under great pressure was unloaded and brought closer to the surface. With a lessening of pressure, the rock expanded. This expansion stretched the crust and led to the formation of joints. (137)

There's even one more round of joints, formed later, "as the rock cooled... roughly east-northeast... parallel to stresses found in the crust today." (137)

We have complicated terrain and soil maps here thanks to the glaciers and other erosion patterns. That's just the surface, though. Many more complications, largely unseen and unknown, lurk in the seemingly solid rock below us.

Posted by simon at November 17, 2011 5:41 PM in
Note on photos


Jessica Boyd said:

I swear I'm not stalking you, Simon, but I saw the post on FB about your cat and THEN saw this geology post, and.. well... I'm a rock nerd, too. So there.

Interesting post about East Coast geology. California is kind of a mess, but also a geologist's puzzle in many ways. Have you read any McPhee? He was my dad's favorite. (Dad was another amateur geologist and my grandfather studied it when plate tectonics was a funny theory.)

Anyhoo.. thanks for posting!