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Showing results for tags 'bif'.
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...also oncolites, thrombolites, microbialites, related things can be included. We can also include BIFs (Banded Iron Formations) and suchlike, being indirectly created by early life, but there will be a preference for biogenic stuff. I don't think this topic has been started already (I would have thought it has), so it falls to me. If one already exists, maybe this can be merged with it and I'll edit accordingly. Some of us have been showing each other our stromatolite finds/acquisitions in other topics and it seemed like a good idea to make a central depot. Links to preexisting posts are welcome, if you don't want to make duplicate posts here. I'll start it off with some of mine. I did a photographing blitz and then finally got done editing them. Some of the slices have not yet been polished, so I had to photo them wet, though some wouldn't hold the water so I gave up and photo'd them dry. First, my Australian examples... Apparently this one is (mid?) Cambrian, from the Barkly Tableland of the Northern Territory, west of Camooweal, Queensland. I like that it has some of the natural eroded surface as well as the sliced (and thankfully polished) surface: I wish this piece were bigger. It must have at one time passed through the hands of that dealer in England who chops everything up into small pieces to maximize profit, but I don't know. Earaheedia kuleliensis, Paleoproterozoic/Statherian (~1.75 b.y.), Kalele (Kulele?) Limestone, Wiluna, Nabberu Basin, Western Australia: I doubt this one is technically a stromatolite or microbialite - just layers (varves?) of BIF sediment, but it did exist at a time when there was single-celled life on Earth, pumping oxygen into the seawater, turning iron suspended therein into iron oxide which precipitated out to form the Banded Iron formations. And anyway it's too cool to not include. The rockhounds refer to this stuff as 'Snakeskin jasper'. 2.45 b.y. (Paleoproterozoic/Siderian), Weeli Wolli Fm (Hamersley Group), Turee Creek Station, SW of Newman, WA. (Pilbara Region) A couple more BIF pieces from Australia, just because it's so interesting and indirectly biogenic... Rockhounds refer to this stuff as Tiger Iron (ie. Tiger-eye in Banded Iron). I understand there are 2 different locations: the Ord Ranges/Port Hedland location (Cleaverville/Nimingarra Iron Fm, Mesoarchean 3.02-3.1by) and the Marra Mamba/Brockman location (Marra Mamba Fm, Neoarchean 2.6by), both in the Pilbara Region of Western Aus, but I'm not sure how to tell which one this might be from. I lean toward the former as the latter is apparently less common. If anyone has more expertise on this I would appreciate a tip. The Tigereye is a pseudomorph (replacement in silica) of selenite growths in the sediment after deposition, if I'm not mistaken? I need to check that to be more confident. ( wet ^ --- dry v )
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In southeast Utah, a gravel road leads from the highway to Mexican Hat, a rock formation in the Permian Halgaito Formation: The 'brim' of the 'sombrero' is 60 feet across. Anyway, the road continues along the San Juan River: As we walked up the slope high above the river, we began to find many rounded pebbles on the bench: There was a diverse variety of rock types represented: One was this beautiful piece of conglomerate: So what are the four generations? The fourth is this alluvial deposit on the bench. It was emplaced at some point in the late Cenozoic before the river had cut down to its present level. The third generation is the conglomerate, which is Proterozoic or possibly early Paleozoic. The drainage basin of the San Juan River is quite large, so the pebble's provenance would be difficult, if not impossible to track down. The second generation is represented by the clasts within the pebble. Most striking is this red fragment of banded ironstone formation (BIF): These BIFs are almost exclusively Precambrian. They formed at a time when excess dissolved iron in sea water was being oxidized and incorporated into sediments. The first generation is represented by whatever deposits were eroded to provide the sediment for the BIF. It's even possible that the various components were involved in additional generations that left no trace.
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