Tuesday, July 3, 2007

Catching a Whopper

As mentioned earlier in Seismic Fishing, we found the grand-daddy of all data fishing holes, in the tape vaults of Texas. Besides nailing down the deep crustal structure of Ontario, we found the biggest fish of all: a giant Precambrian clastic basin right under Lake Erie!

Lake Erie was always a weird lake for us. It's very shallow, and during a cold winter (which we don't get anymore!), it can freeze right over. Once, we wanted to put in a giant underwater power cable to the States. We eventually found out that the winter ice smashes together like plate-tectonic subduction and forms giant pressure ridges, which can gouge huge trenches in the lake mud. That would mean the cable would have to be deeply buried (too expensive!), even though all those natural gas well-heads also keep getting ripped out (but nobody talks about that!). As well, the cables would have to be DC, and we couldn't find conductive earth for a grounding mat in case of a cable break. Without good grounding, a DC cable fault would turn every cow in the region into instant hamburger!

Enough old-man digression! It is generally assumed by the Paleozoic people (including oil and gas types), that the Precambrian granites (metamorphics), are flat-lying, featureless, and solid (like a rock!). The basement rock forms a solid plate for the pancake sedimentary layers, and has nothing to do with the location of oil, gas, and earthquakes. Our discovery shocked them, and they rushed to drill at the new targets.

The paper for today is "Seismic images of a tectonic subdivision of the Grenville Orogen beneath Lakes Ontario and Erie". Available behind the paywall.

This is the index map of the paper. We are only considering the little piece in the box, although there is a ton more data around the rest of the lakes. Note the tentative location of the other two sisters. The Grenville front is waaaay out in Lake Huron which will make the DGT panel happy. Sister #1 (Elzevir) conveniently stops at the limestones, but this paper shows that Sister #2 has a merry old time crashing right through!

Here's the seismic section.

The section shows what a mess it makes of the Paleozoic. The box marked II is the basin, which is quite large! At least the oil companies knew it was a drilling target, they don't make the 'pancake' assumption. The paper goes on to recontruct how this could happen, mainly by having a giant mountain ranges, which then collapses under its own weight.

And that's all she wrote...

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