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Friday Field Foto #24: Conglomerate injectites

July 27, 2007

I don’t have a lot of time right now…still traveling…but wanted to get this Friday Field Foto posted while I wait for my flight (hopefully a better experience than the trip here). I’m not traveling at all for the next 5 weeks, so I should be blogging more frequently (and finishing my dissertation).

A paper I’m a co-author on just came out, which I will post about in more detail next week. The photo above (that’s me for scale) is a great view of some conglomerate injectites in Cretaceous sedimentary rocks down in Patagonia. I showed a photo of some smaller injectites previously on this blog.

Note the flat lying shale and thin-bedded sandstone that I am standing on juxstaposed against a more resistant conglomeratic body that is nearly vertical.

That must be a fault, right? Wrong. I know this may sound crazy, but this feature represents the upward injection of coarse-grained material through the overlying deposits. Yeah….right. Give me a few days and I’ll put together a better explanation of all this. It’s pretty wild stuff.

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2 Comments leave one →
  1. John permalink
    June 16, 2010 9:01 pm

    Hello,
    I’m not a geologist, but used to map soils. I first read about injectites (sand injectites) a couple years ago. I saw some sandy shale injectites or sandy shale dikes in Montana doing soil sampling for an oil company not long after reading an article about sand injectites in Utah. Some of the spire type formations were caused by injection.

    Conglomerate injectites is something new, but it makes sense. I wonder if there have not also been conglomerate flows. In southwest Missouri we have some ‘channel sandstone’ stringers in western Greene, Christian, and Polk counties. But no idea where the material came from, though the coarse parts of the conglomerate are of chert from the Burlington and Elsey limestones the conglomerate rests upon. I’ve not ever heard of any conglomerate filled joints underneath conglomerate areas, but maybe it came from below. It is known to have filled old sinks and karst depressions.

    There reportedly was a conglomerate in eastern Missouri that was later re-classified as a peridotite. Some peridotites are calculated to have erupted at freezing temperatures due to expansion of CO2 gas content on the way up.

    Farmers and other landowners in the conglomerate areas of southwest Missouri often commment that the rock which we know as conglomerate appears to have been fused, and some think it resulted from volcanic activity. I always assumed it was due to iron and manganese coatings, but maybe some of it was not, considering that peridotite report. I have also seen sandstone rims around sinkholes northwest of Walnut Grove, Missouri, in Polk County, and now wonder about the odds of something like injectite activity having been involved. Galena, zinc, and iron have been associated with such sinkholes.

    Injectites could have been a way that caves started or formed, faster than has been thought in the past. A material might have been squirted up from below, and along the intersection of a bedding plane and a lineament or joint, creating an initial tube. This material might have been of something more susceptible to chemical erosion, and eroded out to create an open space, and current processes of chemical erosion expanded it afterward. Or, processes with more acidic rainwater or soil-leached rainwater could have accelerated chemical erosion.

    There is a feature of some caves in southwest Missouri called an upside down well. Crystal Cave (a commerical cave) north of Springfield has one. The current explanation is complicated. It’d be alot easier to visualize one of these forming from an injectite that stopped before breaching to the surface or turning.

    Sand volcanos have been reported in the past along the Mississippi River, behind Corp of Engineer dams, as the water level rose and put pressure on unconsolidated sand strata, squirting them upwards through more coherent material.

    John

  2. June 17, 2010 9:43 pm

    John, thanks for the comment … yeah, the more we map the sea floor at high resolution the more we find mud/sand “volcanoes”. I still haven’t heard of a cobble volcano being documented on the modern sea floor though.

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