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A Tradition of Looking Behind the Curtain
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 Post subject: roadside geology of mississippi
PostPosted: Mon Jul 03, 2017 9:32 pm 
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Joined: Fri Apr 04, 2008 10:48 am
Posts: 4317
Location: Broomfield, Colorado
Interesting enough, given that the state doesn’t have much in the way of visible geology. Outcrop illustrations are nice color pictures of one or two feet of section, pathetic little rocks peeking out from behind the foliage. And the authors warn that even these are ephemeral, as the Mississippi Highway Department conspires with kudzu to quickly eliminate any roadcuts. The subsurface is interesting, though, and mostly elucidated by Mississippi’s status as an oil-producing state – not a big one, but enough for a liberal pincushioning with well bores. The major subsurface attraction is the Mississippi Valley Graben, which is a failed rift dating to the breakup of the Rodina supercontinent in the late Proterozoic, and which still controls the general channel of the Mississippi River 750 My later. The current Mississippi dates to the end of the Pleistocene – prior to that a lot of the headwaters drainage basin was covered with continental glacier – but there’s probably been some sort of watercourse in that general path for a long time (of course, for a lot of that time it was under an epicontinental sea). I was surprised to find that the Ohio River once flowed into Mississippi, as far south as Natchez. Apparently, the Ohio and the Mississippi paralleled each other for a long distance during the late Pleistocene, before the Mississippi captured the Ohio near their current junction.

There was yet another rift in Mississippi geohistory during the Triassic, during the breakup of Pangea. This one runs diagonally across the state from southeast to northwest and displays all the classic features of rifts – volcanics, rift lakes, sedimentary fill – all now covered by later sediments and only detectable in oil well bores.

In the Cretaceous, most of the state was under seawater as part of an epicontinental sea – the Cretaceous Interior Seaway. However, twin chains of offshore volcanoes developed – not active margin volcanoes but hotspots (at least that the interpretation authors Stan Galicki and Darrel Schmitz give). The Midnight Volcano to the north and the Jackson Dome to the south both have additional volcanoes “downstream” or “downplate” – to the west in Louisiana and Arkansas. They do look a lot like a shorter version of the Hawaii chain, and the authors claim they are the result of North America moving over the “Bermuda Hotspot”. Other traces of the Bermuda Hotspot are not known, the explanation being that they are lost under the Appalachians. We’ve had a couple of threads about hotspot theory controversy and the situation is still unsettled; could be, could be something else. At any rate, the closest the Jackson Volcanoes get to the surface now is around -3500 feet.

Northeastern Mississippi sports the “Kilmichael Dome” or “Kilmichael Crater”, which is most likely an impact crater, probably Eocene. Needless to say, nothing is visible on the surface. An alternative explanation is “flower structure”, which is a complicated pattern formed by multiple converging faults; I’d never heard of “flower structures” before but they are apparently of some interest in petroleum geology because they can act as traps; this, in turn, explains why there are enough boreholes around Kilmichael, Mississippi to figure out what’s going on in the subsurface.

Mississippi is also a nuclear bomb test state; the AEC detonated two bombs in a salt dome back in the 1960s, to test If they could pick up seismic signals from other countries doing tests in similar formations. Once again there’s nothing to see at the surface except a plaque saying “DON’T DRILL HERE”.

All of this is interesting, but alas none is visible from the roadside; the roadside part of the book consists of checking mile markers and having the authors inform you that something interesting is going on underneath. Now and then you can see a few rocks. Still, the paleogeographic maps are interesting and well done – and a reminder on how quick geological understanding changes.

 Post subject: Re: roadside geology of mississippi
PostPosted: Thu Jul 06, 2017 8:50 pm 

Joined: Fri Apr 04, 2008 10:45 am
Posts: 8222
I spent a few months in Mississippi as a young man. Lovely area, but, yeah, not a lot in the way of rocks.

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