Sunday, October 5, 2008

Mapping Upside Down?

Recently I've had the frustrating experience of looking at stratigraphy that has been divided into upside-down units.

Well, that's not exactly the case. This is what I mean. In geology it's fairly common to divide major rock units or formations into sub-units, especially when dealing with large scale maps that show a lot of detail. Sometimes the units or sub-units are formal parts of the stratigraphy of a formation; sometimes they are informal units made up by people working in an area where detailed stratigraphy isn't broadly recognized.

When dividing a section or formation into units or sub-units, it is standard practice for geologists to number the units from the bottom up, that is, with smaller numbers at the bottom or older parts of the sections and larger numbers at the top or younger parts of the section*.

For example, I might start mapping some large section of limestone in Nevada, maybe one that is fairly similar throughout but not entirely, and maybe one that hasn't previously been divided into subunits. Or maybe I'm working in some volcanic terrane where units and even major formations haven't been subdivided at all, but I need some way to distinguish rock formations and type on my map, especially so I might be able, then, to figure out the structure.

I'd start at the bottom of some hill, presuming that the section is right side up with youngest on top and the oldest on the bottom, and I'd call the first unit, Unit 1. (Not very original, but it works.) So the first unit, a light gray, cliff-forming, sandy limestone is Unit 1. The next identifiable rock formation I run across as I walk up the hill is a buff-weathering, slope-forming shaly limestone. I call that Unit 2. Unit 2 is younger than Unit 1. That's the way it is. We number things from the bottom up. Unit 3, above Unit 2, just happens to be a gray, cliff-forming sandy limestone almost identical to Unit 1, but Unit 3 has a few scattered brachiopod fossils in it, and Unit 1 doesn't. And so it goes, all the way up the hill - and down the next hill if need be - and all the way across the area, going from older to younger, with the unit numbers getting larger and larger.

The frustrating thing I've had to deal with recently is a particular rock formation that has two already-described stratigraphic sections. One is a section with numbers for the sub-units; one section has letters for the sub-units. Both the numbering and lettering systems are upside down. I can choose from using a numbered section that starts at the top of the formation with Unit 1 and works it's way down-section into older rocks with Unit 2 on top of Unit 3, on top of Unit 4, on top of Unit 5, and so on into however many subunits this rock formation has. Or I can choose a lettered section that starts with Unit A at the top, older than the Unit B beneath it, which is on top of Unit C, on top of Unit D, and so on to Unit Z. (I kid you not about the Z.)

I have a somewhat difficult time keeping track of numbers that start at the bottom of some formation with Unit 8 and work their way up-section, younger, often uphill (not always) to Unit 1. I find it almost impossible to start at the bottom of the formation with Unit Z and work my way up-section into younger rocks, often uphill, with Unit Y on top of Unit Z, with Unit X on top of Unit Y, and so on.

Perhaps the obvious the thing is to start at the top of the section, in this particular case and work downward into older rocks - and that's fine, until I have to start thinking about the rocks I just looked at. I keep having to say the alphabet over and over again, sometimes near the beginning, sometimes in the middle somewhere, and sometimes near the end, just to keep track of these upside-down letters.

At least they are upside down to me, even though the stratigraphy is right side up.

Maybe these numbered and lettered measured sections were from some area where it turned out that the formation in question was upside down when they described the section. Maybe. Then, maybe, when they started at the bottom of the hill and measured there way up, they started with 1 or A, which turned out to be the youngest unit of the formation because everything was overturned sometime back in the Paleozoic (a long, long time ago - more than 250 million years ago!). Well, maybe.

*Geologic convention is to number or letter units from oldest to youngest because the first layer deposited will then be called Unit 1 (or A). It was the first, it is the oldest, in a right-side up section it will be on the bottom or underneath all later and younger units.

NOTE: The section in question was described using letters A through W, with A at the bottom of what turned out to be an upside-down section (a section overturned by a regional-scale fold). Read a little more here and here.

Updated on 17Jun16.


Anonymous said...

This reminds me of one of our student's exam howlers ...
[Superposition:] a very simple concept ... continued to confuse many geologists of the past as it was firstly believed that the rocks were placed in some form of alphabetical order.

Silver Fox said...

Hmmm... that would really screw things up in the Grand Canyon. I would have to re-learn the section. The Redwall Limestone would end up above the Navajo Sandstone, for one thing. And maybe the Chinle would be first? Or should we do it alphabetically from the top down??!

Anonymous said...

Another possible(?) explanation:

Maybe the section was described by an oilfield geologist! We're used to looking at beds from the surface downward, in the order they are penetrated by a drill bit. Though it is of course stratigraphically backward, it's a common practice in the oilfield, especially when doing informal mapping and naming/numbering of units. It's counterintuitive to say "the 1st unit we penetrated was unit 4, the 2nd was unit 3, the 3rd was unit 2...".

Perhaps your section was mapped by someone who was trying to compare the outcrop section to his/her subsurface work, and numbered the units to coincide with an informal subsurface nomenclature that had been developed.

I can think of a couple of examples of this upside-down terminology coming into common use. Here in the Alberta basin, early workers nicknamed the oil-producing Devonian rocks "D1" (Wabamun Fm/Gp), "D2" (Nisku Fm.) and "D3" (Leduc Fm.). They were numbered in the order of penetration (youngest to oldest). Similarly, a project I'm currently working on, in SE Saskatchewan, has units labelled "M1, M2, M3" from the top down.

--Howard (geologist, Calgary, AB, Canada)

Silver Fox said...

Howard, thanks for the alternate explanation - and that's something I didn't realize about the oil patch. In this particular case, I think the section was first described on the ground, and that there hadn't been any drilling for oil. It's possible the oldtimers doing the section had worked elsewhere in the oilfields, though. I'll ask if I see them sometime in the next year or two.

I've done a fair amount of drilling for minerals in my time, but our sections have usually been described on the surface first, so are in the usual order - and also, the drilled intervals aren't hardly ever deeper than 2000 feet!