GeologyRocksGeopetal infills.
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They're better than nowt!
Mon, 07/23/2007 - 18:24They're better than nowt! The assumption is that the sediment is deposited horizontal (as with all sediments). Of course lot's of things can violate that, as you suggest. Taking averages over different readings and different types mitigates the error somewhat. Just bear this in mind when a paper uses them to "prove" the angle of deposition...
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I'm investigating the
Mon, 07/23/2007 - 22:33I'm investigating the northern tip of the Derbyshire carbonate platform towards the close of the Dinantian. There seems to have been a complete removal of the upper layers of limestone present over much of the rest of the area, and a karstified erosion surface proves emergence at some point. There are two features of particular interest- the first is a boulder bed developed on top of the erosion surface consisting of limestone blocks up to several metres in diameter with mixed shaley and calcareous matrix. The second is the 'beach beds' which are now thought to have formed at the foot of the fore reef slope at the mouth of a submarine channel rather than on a beach. These are thought by some to be equivalent to the boulder beds, but I have not seen any conclusive proof. I hope to:
-Â use conodonts to assign dates to both of these to see whether they formed side by side.
- Investigate layers in boreholes claimed by some to be distal portions of the beach beds and/or boulder beds and see whether they match petrologically and date-wise.Â
The reason I ask about geopetal infills is that they have been used to assert that the fore reef slope was deposited at about 22 degrees and tectonically steepened by a further 8 degrees soon after. 8 degrees seems rather precise.
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That sounds really
Tue, 07/24/2007 - 11:42That sounds really interesting! Whereabouts in Derbyshire are you looking at (I'm not sure where the northern tip is - I know Castleton and Hope, but that's about it).
Yes, 8 degrees is very precise...Other papers I've seen have used averages of readings (although they haven't tested for significance - naughty!) or prefixed the reading with "approximate". I've got a couple of papers about the Capitan Reef in Texas and the argument over whether the deformation seen in syn- or post-depositional:
Hunt et al. "Syndepositional deformation of the Permian Capitan reef carbonate platform, Guadalupe Mountains, New Mexico, USA" Sedimentary Geology, Volume 154, Issues 3-4, 1 January 2003, Pages 89-126
Saller, 1996. A.H. Saller , Differential compaction and basinward tilting of the prograding Capitan reef complex, Permian, west Texas and southeast New Mexico, USA. Sediment. Geol. 100 (1996), pp. 1–10.
Wood et al., 1994. R.A. Wood, J.A.D. Dickson and B.L. Kirkland-George , Turning the Capitan reef upside down: a new appraisal of the ecology of the Permian Capitan reef, Guadalupe Mountains, Texas and New Mexico. Palaios 9 (1994), pp. 422–427
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Matt, Quote: The reason I
Tue, 07/24/2007 - 20:25Matt,
The reason I ask about geopetal infills is that they have been used to assert that the fore reef slope was deposited at about 22 degrees and tectonically steepened by a further 8 degrees soon after. 8 degrees seems rather precise.
Just a thought - If the current dip is 30° then it is the 22° of fore reef slope that is precise - the 8° is just the difference.
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Geopetal infills.
Submitted by Matt on Mon, 07/23/2007 - 04:28.How good are geopetal infills at recording the exact angle at the time of deposition? If an upturned brachiopod valve lands on a tilted surface, won't the infill/cavity retain the tilt of the surface?
If mud gets in through a hole in the shell, won't it pile up higher towards the opening creating an illusion of tilt? Perhaps the angle of repose is too low for this to be a problem, or perhaps an average angle of a number of infills is used... If anyone can help clear this up I'd appreciate it.