[X] My Assistant

[tdemko]

(alternative topic title: Opportunity lost its lens cap!)

http://umn.edu/home/tdemko/webpage_photos/...tahberries.html

I've put up this small website showing some pictures of spherule-like concretions in the Lower Jurassic Wingate Sandstone from the San Rafael Swell in central Utah that are at least superficially similar to the Meridiani Planum concretions imaged by the MER Opportunity.

The Wingate Sandstone was deposited by large wind-blown sand dunes in the great western Pangaean desert during the Early Mesozoic. It is a totally continental deposit (deposited in a terrestrial desert setting). These concretions are presumably related to groundwater-related cementation during early diagenesis and have been accentuated in these outcrops by weathering in the modern desert environment.

Enjoy!
--
Tim Demko
http://umn.edu/home/tdemko

[djellison]

Now that IS analagous to meridiani - seen lots of people say "isnt this just like..." - when really - it isnt - but your site actually is - interesting stuff

What sort of time-scale does one need the water to be around for, for concretions such as those to form? A few thousand years, hundreds of thousands? Longer?

Doug

[tdemko]

Doug:

Concretion formation is a funny thing...it seems to happen when it happens! Relative dating is sometimes easy to do (i.e. which came first, second, last, etc.), but not much has been done on their timing of growth in an absolute geologic sense (numbers of years). There is some anecdotal evidence about very fast concretion or nodule formation (siderite concretions forming around metal shrapnel in a tidal flat used during WWII for artillery practice, and around Coke cans in some wetlands in Louisiana, etc.).

These small concretions in the Wingate on my webpage seem to have formed rather early (soon after deposition of the eolian sand during the Early Jurassic). They seem to have been related to some upward movement of the water table and are at least spatially and stratigraphically related to what I have interpreted to be some some damp- or wet-looking interdune deposits (fairly common in the Wingate erg). If I had to guess, I'd say the water was around for a couple of months to a couple of 10's of years when these spherule-like concretions would have formed. The bed they occur in is overlain by huge dune cross-strata which indicate a much lower groundwater table and a return to very dry (arid-hyperarid) conditions.

[tdemko]

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[tdemko]

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