Friday Fault Photo: Where's the Fault?

It's great to see faults with nice outcrops, but not all faults expose themselves well.

This fault, partly hidden by trees (and now totally under snow), shows a nice linear outcrop pattern but nowhere does the actual fault surface crop out, though I suspect the fault plane lies less than 5 feet to the left of the above exposure of brecciated limestone, possibly within a foot or so. Likewise, the fault contact between the two (or more) faulted rock formations is not exposed, at least not in this area. Instead, one can follow a roughly linear outcrop of limestone — sometimes brecciated, sometimes veined or marbleized — and one can find a bit of fault breccia float and a trace of slickenside float here and there where the fault goes under cover. So far, there is no indication as to which way the fault actually moved, though one can assess relative motion and deduce a couple possibilities.

Some faults with large displacements show no surface outcrop at all, at least across broad areas, but instead can be found in relative low spots such as valleys. I found this to be particularly true back east where exposure is not as good as it is in the west, but it can happen anywhere. In fact, some geologists swear by an axiom stating that the faults that actually crop out on the surface are small faults (apparent offset I'd guess of less than a hundred feet or certainly less than 50), and that large faults rarely crop out. I can't say that I buy in to this way of thinking. Some very large faults with huge offsets and some large faults with large offsets have great exposures, some don't. Likewise with small faults. Many of the faults I find in the field are, indeed, small faults with offsets of 5 to 50 feet or so, and that's because there are a lot of faults in the Basin and Range, and they range in size from tiny to very large (as measured by amount of offset).

Many times, small faults are not mapped because they are considered insignificant. If they are part of a larger fault zone, however, they may be cogent to what you are trying to figure out. For example, several en echelon or sub-parallel faults with small displacements can, across a wide fault zone, offset stratigraphy, ore deposits, and faults and other structures in a way that will leave you wondering what is going on, possibly causing you to bend contacts where they don't actually bend. Of course, the scale of mapping has something to do with how many small faults you can show on your map, and you may have to lump several small faults as one fault trace.