Tuesday, December 9, 2014

Unconformity? Disconformity?

Here's a purely geologic question by someone who has already taken at least one course in geology. The question opens up and highlignts the three-dimensional aspect of geology - and why mathematics (especially geometry) is such a fundamental prerequisite for studying geology. Some people persist in saying that a geologist is just someone who didn't do well in physics or math. The hard reality is that physics, math, chemistry, and English composition are the building blocks - the basic tools - of a modern geologist. Some of the most sophisticated geology being carried out these days is done with computers. Drill-hole information is fundamentally three-dimensional, and the ability to construct three dimensional landscapes from surface mapping and drill-hole intercepts is just so very cool. To rotate this 3D landscape on one or several computer screens, showing how individual components evolved in time in a single giant cubic space... is absolutely essential to numerically assessing any resources the land under the geologic map may host.

Q: I have a question regarding identifying unconformity on geological map. I have attached a map as an example. How do we identify unconformity on such 2D geological maps if each colour represents a different rock? Please advice.
Thank you and hope to hear from you soon. Regards
- Hazel A

A: I have not downloaded your map and looked at it in detail, but just looked at it via the attached thumbnail. We are discouraged pretty strongly from downloading and opening any files from unknown individuals that might potentially be vectors for malware. For the purposes of this Q/A, a map is not really necessary, however. 

I'd like, instead, to address your question on a somewhat broader level: The inherent problem with a geological map is that it represents the surface of the land. It's a view looking downwards from space, which is not always the same as looking downwards in time. Sometimes, with tectonic and erosional events, older in time doesn't necessarily mean deeper in the Earth.

An unconformity is a gap in sedimentary deposition for one of several fairly specific reasons: non-deposition, subsequent erosion, etc. It is not easily represented in a geologic map, which only shows just one sub-horizontal surface - the part exposed to the sky. An unconformity means that there has been a time break in the geologic record. This is quite different from a juxtaposition of different geologic units due to, say, a thrust fault (though they could both be involved at the same time). 

In practicality, this means that the geologist who produces the map must somehow indicate or convey any unconformity (or disconformity, or nonconformity, or paraconformity, etc.: see http://en.wikipedia.org/wiki/Unconformity ) in her/his *Correlation of Map Units* columns on the side of the geologic map.

For most people not intimately familiar with a particular local or regional geology, it would be very difficult if not impossible to determine if some break between units is an unconformity or a fault juxtaposition just from looking at a geologic map alone. A change in rock-type could mean any of several much more common things: a change in sedimentary regime (like an ocean transgression), an intrusive event (like a big granite body punching up from the Mantle), a volcanic eruption, any of several different kinds of fault, etc., exposed at the earth's surface. 

It comes down to the fundamental difference between a map view (looking down at the ground from space),  and a cross-section view (looking at the ground side-ways, as if a giant trench had been cut in the landscape). However, even in an exposed cross-section, considerable sleuthing is required to determine if a break is an unconformity or not.

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