Faults

In the previous post, I discussed folds, which precede faults. That is to say, if the forces that create a fold becomes too strong, the rock layers (beds) eventually fracture, rather than just bend. The dictum is: FOLDING PRECEDES FAULTING. 

Horizontal layers prior to being deformed. The geologically youngest layer is at the top.

Children's clay makes useful fault models, as shown here. The blue layer (bed) is the oldest, and the green layer is the youngest.

In the following images, I shall discuss the different kinds of faults. Afterward, I shall show examples of some of the faults common to southern California.

DIFFERENT KINDS OF FAULTS:

A reverse fault forms when compression causes localized bending and subsequent breakage. The compression causes the "hanging-wall" block on the right side to move up relative to the "foot-wall" block on the left side. The compression causes the layers (beds) to overlap. The fault in this scenario is a high-angle reverse fault. The terms "foot-wall" and "hanging wall" stem from the early days when a miner would dig a mine shaft down along a fault plane (minerals commonly form in this zone of breakage because that is where fluids would easily flow). The miner's feet would be on the "footwall," and the "hanging wall" would be above his head.

If, however, the angle of the reverse fault is low, then the fault is referred to as a thrust fault. The relative motions of the hanging and footwalls are the same as for a high-angle reverse fault.

A normal fault forms when extension (= the opposite of compression) caused the "hanging-wall" block on the right side to move down relative to the "foot-wall" block on the right side. The layers do not overlap themselves. The fault in this scenario is a high-angle normal fault. These kind of faults are uncommon in southern California because this area has mostly undergone (and still is undergoing) compression, rather than extension.

A strike-slip fault has mostly horizontal displacement (shown by the arrows in the diagram above). The sense of displacement is that one side moves in one direction, and the other moves in the opposite direction or is stationary. The San Andreas Fault is an excellent example of a strike-slip fault.

In southern California, all faults, except normal faults, are  common because the area has been and is still undergoing compression. 

EXAMPLES OF FAULTS:

In this reverse fault (the red line), the "OLDER FORMATION (A)," consisting of layers (beds) has been displaced, and is now situated above the "YOUNGER FORMATION (B)." This is not the way things were originally, because, in unfaulted situations, older beds are below younger beds (as is the case on the far left side of the image).

In this thrust fault (red line), older brown sandstone and gray mudstone rocks, in the "shadowy" upper right-hand side of the image, have been moved sideways at a low angle (nearly horizontal) and now overlie younger conglomerate (reddish sandstone and small boulder) beds. Prior to the faulting, the brownish and gray beds had a gradational contact with the underlying reddish beds. Now, they have a fault contact with the underlying reddish beds. The two rock units belong to different formations of considerably different geologic age and much different ancient environments of deposition. Low-angle thrust faults like this one can be very difficult to detect unless the geologist knows full well the lithologic (rock types) and their vertical changes within a formation or a sequence of formations.

The vertical cliff in the image shown above is at least 50 feet high. This fault (indicated by the red arrow) is along the sharp line of color difference between the white rock, which is an igneous rock that formed several hundred million years ago, and the dark gray rock, which formed about 2 billion years ago. This fault is a strike-slip fault and originally involved sideways motion of one rock mass sliding pass the other. 

This is a closeup along the fault shown in the previous image. One can truly put a finger on the fault.

This is a vertical strike-slip fault.  The grayish rocks left of the red-and-white measuring stick (1.5 m in length) are geologically old "basement" metamorphic rocks, and the red rocks on the right of the staff are much younger sedimentary rocks consisting of sandstone and siltstone.

Folds

Folds occur when sedimentary beds (layers) of rock are compressed by tectonic forces. When compressed roughly equally on both sides, the beds can form a syncline (U or V shaped) and/or an anticline (arch shaped). All of the following images are cross-sectional views of beds.

This is a wide, gently deformed syncline.

These are narrow, moderately compressed anticline and associated syncline folds. At the top of this cliff, the folds become less symmetrical and "migrate"sideways. In addition, other folds can develop: notice the addition of another anticline on the upper left side of the image.

This is an overturned fold, which means that the compression was asymmetric (stronger on one side versus the other), which resulted in an asymmetric (partly "flopped over") anticline. The red dashed lines in the image above indicate where beds were present before removal by means of erosion.


These very compressed beds occur in core area of the previously mentioned overturned fold. Notice that beds in the center of this image are initially vertical, become horizontal, and go back to being vertical again. This is a rare sight for field geologists. 



This is a very small (about 10 feet in height) "kink fold," in the immediate area of the where the previous image was taken. These small "kink folds" have the same overall pattern (except in miniature) as the large overturned fold in the area.

This image was taken about 1.5 miles northward of where the previous images of the overturned fold were taken. The thin beds in the core show strong convolutions.



Farther to the north, "spaghetti" bedding occurs in the core of the overturned fold. This type of intensely deformed bedding occurs when the relatively "soft" mudstone beds "flow" like tooth-paste because of intense pressure.

This image shows a cross-section of a smaller overturned fold. There is also a small reverse fault (this kind of fault will be discussed in my next post), indicated by the red arrow. The angulation (= an angular unconformity) between the white bed and the underlying cavernous brownish bed, both in the lower left-hand corner of the image, was caused by channelized erosion of the brownish bed before the white bed was deposited.

Amber and entombed insects

Amber is fossilized resin that blocks gaps in tree bark, especially conifers. It is not the same as sap, which transports nutrients through the heartwood of trees. Amber is very sticky and insects can be trapped in it when they try to burrow or eat the bark. When a tree limb is injured, amber can exude as blobs or drippings and flow down the side of the trunk. Insects can easily be engulfed or trapped in the resin, which commonly falls onto the ground and becomes incorporated into the soil. Other small animals, like lizards, frogs, birds, and even bats have been found in amber. Only a few tree resins (e.g., Kauri pine in New Zealand) can form fossilizable amber. Hardened resin is called copal, and it is easily transportable in streams and rivers, where is becomes part of the non-marine sedimentary record. In some cases, it can be transported into the nearshore-marine environment.

A polished specimen of copal (6 cm width);
 where collected unknown.

The transparency and color of the specimen shown above are typical for most amber. Bubbles, which are also commonly present, make amber lightweight and, in some cases, even floatable in water.

 

Left image: leaf (1 cm long) in specimen of copal shown above.

Right image: bug (7 mm, maximum length) in specimen of copal shown above; with wings and legs intact. 

Left image: Another polished chunk of copal (8 cm length, notice thumbnail--for scale--on left side of image); where collected unknown. Enlarged image shows a termite? (4 mm length) and below that, another insect, both found in the chunk.

These two pebbles, both polished, shown the typical range of color (hue) of most copal. The smallest piece is 1.5 cm in height. Both are from the Baltic Sea area, Denmark. No insects are present in these two specimens, which shows that not all copal has to have insects.

Copal is used in making jewelry and also in rosary beads. Copal is relatively soft, however, and can be scratched by a hard surface. When buying copal, the more insects (inclusions) present means that the price goes up. A word of caution: so-called "copal" can be made out of look-alike plastic.

The oldest known copal  is Late Paleozoic (Late Carboniferous, Pennsylvanian-age coal beds). The oldest copal with insects is Early Cretaceous (when the first flowering plants appeared).