Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Roadrunner on 19/10/2014 23:53:10
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Hello There. I've been in here now and then, but this is my first post here. I spend a lot of time at "thefossilforum.com." Years ago I started looking for good rock samples for my class, and started finding a lot of fossils.
The last "unusual boulder" that I found was a 180 pound highly silicified stromatoporoid. I was fortunate enough to get our local Natural History Museum scientist out to identify it, as pictures and samples weren't enough.
This however, seems more a geologic process that I can't make out. It is unusual - at least to me. I've taken a couple of geology courses at our local University but obviously that is very little. My degrees are in Education.
We have a LOT of limestone in the area, are in the foothills of the Sandia Mountains north of Albuquerque, New Mexico. The area is a cross-section of many pale ontological periods. It has plenty of detritus that in times of flood - have washed down from the mountains, and a lot of deep gullies.
This high desert area has crinoids, gastropods, rugose corals, and other shallow sea type(s) of fossils. I also find petrified wood. About 5 miles from here there have been mastodon finds.
Hiking this area I normally see limestone boulders covered with what appear to be additional sedimentary and also volcanic-type of rock (the first and last pictures below show this).
Yesterday however, I came across a limestone boulder that appeared to have this same reddish/black material protruding out of the boulder in what appear to be knobs and other shapes. Looking closely, you can see the knobs have in some areas cracked the limestone as they appeared to push out.
What could account for this?
Thank you for any/all help!
Most sedimentary rocks with what I believe to be volcanic rock covering it look something like this;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi633.photobucket.com%2Falbums%2Fuu60%2Fgabbriell-photos%2FRocks-Fossils%2FDSCN1085_zps6367de22.jpg&hash=ab0ba4a2c0259a0b7dd937f684994cb9) (http://s633.photobucket.com/user/gabbriell-photos/media/Rocks-Fossils/DSCN1085_zps6367de22.jpg.html)
This unusual boulder with the knob-like protuberances looks like this;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi633.photobucket.com%2Falbums%2Fuu60%2Fgabbriell-photos%2FRocks-Fossils%2FDSCN1076_zpsb515e45b.jpg&hash=b52a03552d3cb08b32612f05fabfb58f) (http://s633.photobucket.com/user/gabbriell-photos/media/Rocks-Fossils/DSCN1076_zpsb515e45b.jpg.html)
...and here;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi633.photobucket.com%2Falbums%2Fuu60%2Fgabbriell-photos%2FRocks-Fossils%2FDSCN1078_zps1957b051.jpg&hash=a47110963759736df87b3dfbcca22ef4) (http://s633.photobucket.com/user/gabbriell-photos/media/Rocks-Fossils/DSCN1078_zps1957b051.jpg.html)
There was also a nice limestone crinoid trace fossil (25 pound rock) close by a "regular" coated rock. Sorry for the red dog leash at the top of the picture;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi633.photobucket.com%2Falbums%2Fuu60%2Fgabbriell-photos%2FRocks-Fossils%2FDSCN1082_zps76b45081.jpg&hash=0a547f56c3cfa5c9636a5ed74ea18bf4) (http://s633.photobucket.com/user/gabbriell-photos/media/Rocks-Fossils/DSCN1082_zps76b45081.jpg.html)
I'm looking forward to hearing your thoughts. Thanks again for any assistance!
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... I came across a limestone boulder that appeared to have this same reddish/black material protruding out of the boulder in what appear to be knobs and other shapes. Looking closely, you can see the knobs have in some areas cracked the limestone as they appeared to push out.
It looks like the pale rock has weathered (http://www.geolsoc.org.uk/ks3/gsl/education/resources/rockcycle/page3461.html) back/out , exposing the more resilient darker rock, (rather than the darker rock pushing through).
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Thank you for your reply.
Were my pictures good enough to see that the limestone has cracked around the knobs, and appear to have been pushed upwards?
(BTW, I find most teacher websites a bit too rudimentary. Do you have any relevant resources that are "30" or lower on the Flesch reading scale)?
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I went back twice (the first time I didn't locate it), as I decided to break off some knobs to get a better look. I got 2 of them off and of course, you were right. They were limestone underneath although the texture of the limestone knobs themselves were finer grained.
What is still a mystery to me at least, is that the knobs were so hard that when I broke them off, they came off mostly whole and left holes in the boulder.
Can anyone explain to me how those nodules might have been formed? I know this area was once shallow seas but don't understand the process that would create the nodules.
Attached is a picture before I knocked off 2 nodules, and then after. It got dark before I got home to take pictures of the nodules themselves.
With nodules still intact;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi633.photobucket.com%2Falbums%2Fuu60%2Fgabbriell-photos%2FRocks-Fossils%2FDSCN1202_zps82799b8b.jpg&hash=f40760285b6c3b759f41165e637079e4) (http://s633.photobucket.com/user/gabbriell-photos/media/Rocks-Fossils/DSCN1202_zps82799b8b.jpg.html)
After breaking them off;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi633.photobucket.com%2Falbums%2Fuu60%2Fgabbriell-photos%2FRocks-Fossils%2FDSCN1203_zpsb03efadd.jpg&hash=f93df719471127110f56c5e735ef0389) (http://s633.photobucket.com/user/gabbriell-photos/media/Rocks-Fossils/DSCN1203_zpsb03efadd.jpg.html)
And one more;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi633.photobucket.com%2Falbums%2Fuu60%2Fgabbriell-photos%2FRocks-Fossils%2FDSCN1205_zps335129be.jpg&hash=f90a7a7fd62aba5185aba139b19370f2) (http://s633.photobucket.com/user/gabbriell-photos/media/Rocks-Fossils/DSCN1205_zps335129be.jpg.html)
The hole left from the nodule toward the top of the rock is harder to see than the larger one toward the bottom.
The nodules have a think dark crust on top of them and as mentioned earlier, the limestone underneath the thin crust is a finer grain.
I'm sorry if my questions seem stupid.
My husband and I are both scuba divers (and my husband studied marine geology as a graduate), and we are thinking that we may have seen something "nodule-like" while diving in different parts of the world, but can't quite place how these nodules were formed.
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The area where I mostly hike has the following written about it;
GEOLOGY
About 40-50 percent of the rocks exposed in the Sandia Mountains are of Precambrian age and include the Sandia Granite (1,445±40 m.y.), which intrudes older biotite schist and gneiss of Kelley and Northrop (1975) and the Cibola Gneiss, and a still older northeast-striking greenstone belt along Tijeras Canyon. The Tijeras Greenstone of Kelley and Northrop (1975), of probable Proterozoic X (Early Proterozoic) age, consists of predominantly dark-greenish-black, blastoporphyritic metabasalt flows, and minor thin lenses of metarhyolite. The absence of pillow structures in the greenstone suggests predominantly subaerial extrusion. The slightly younger Cibola Gneiss is a granitic paragneiss containing intercalated and isoclinally folded quartzite beds. The gneiss and schist of Rincon are highly foliated, biotitic-sillimanitic, metapelitic rocks that have been intricately intruded by the post-orogenic Sandia Granite. Numerous pegmatite and aplite dike swarms that are largely fracture controlled intrude the metapelitic rocks.A relatively thick (2,500 ft) section of Mississippian, Pennsylvanian, and Permian strata rests unconformably on the Precambrian rocks and forms an extensive east-dipping dip slope on the tilted fault block. The Mississippian strata are thin (50 ft), discontinuous, erosional remnants of cherty limestone. The Pennsylvanian section is about 800 ft thick and is represented by carbonaceous shale and sandstone of the Sandia Formation and an overlying thick (650 ft) sequence of shallow marine limestone beds of the Madera Group (Myers, 1973). No attempt was made in this study to differentiate the various units in this group. A complex Permian and Triassic red-bed section of sandstone, siltstone, mudstone, and thin limestone beds crops out farther down the dip slope. From oldest to youngest these are the Permian Abo, Yeso, and San Andres Formations, and the Triassic Santa Rosa Sandstone and Chinle Formation.The remaining Mesozoic strata are folded into synclinal basins, such as the Tijeras and Hagan synclines and also fill ramplike depressions near Placitas (Kelley and Northrop, 1975, p. 83).
The Jurassic and Cretaceous sedimentary rocks include the Jurassic Todilto, Entrada, and Morrison Formations and the Upper Cretaceous Mancos and Mesaverde Formations.
Black carbonaceous shale and thin coal beds are commonly interbedded with thick sandstone beds in the Mesaverde Formation. Middle Tertiary(?) lamprophyric dikes cut the Sandia Granite along the steep west-facing slope of the Sandia Mountains, and a basaltic dike cuts the Mesaverde Formation about 1 mi northwest of Placitas.
The Galisteo Formation of early Tertiary age crops out as an incomplete section north of the wilderness boundary west of Placitas.The dominant structural element is the east- tilted Sandia fault block, which is controlled on the west by the Sandia and Rincon-Ranchos range-front faults. These faults have as much as 20,000-28,000 ft of throw. The eastward tilt of 15°-20°, formed in conjunction with displacement along these faults, steepens where the Paleozoic section underlies the folded Mesozoic sedimentary rocks of Tijeras and Hagan basin synclines.The plunging northern terminus of the fault block is near Placitas, but, to the south, the Sandia fault block is contiguous with the Manzanita and Manzano fault blocks.
Numerous north-trending faults along the dip slope are considered coeval with the Pliocene and Miocene Sandia-Rincon- Ranchos faults and commonly displace older faults of post-Permian (principally Laramide) age. Most of the north-trending dip-slope faults are downthrown to the east, but the large Ellis fault is an exception and locally reverses the regional east dip so that near Capulin Peak the strata dip gently to the west. Probably the most important effect of these faults has been to bring Precambrian rocks to the surface along the back-slope between Tejano and Tecolote Canyons.
The Placitas-San Francisco, Tijeras and Gutierrez faults strike northeast to east-northeast along the north and south boundaries of the Sandia Mountains. These faults, principally of Laramide age, locally displace the Upper Cretaceous Mesaverde Formation and are commonly branching or splayed fault systems having combined throws as much as 1,000-3,000 ft. The Tijeras and Gutierrez faults bound the Monte Largo horst and the Tijeras graben and syncline; a scissorslike displacement with some strike-slip displacement is characteristic of these faults. Most of the northeast and east-northeast faults have had numerous periods of movement. For example, the Tijeras fault was probably active in the Precambrian with some left-slip displacement (Connolly, 1982), and again during late Paleozoic, Laramide, middle Tertiary,and Holocene times. The Placitas-San Francisco fault system probably had a similar history with the principal displacement taking place during
Laramide time.
http://pubs.usgs.gov...631a/report.pdf
http://pubs.usgs.gov/mf/1984/1631a/report.pdf
I hike in the "Placitas" area.
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I found a rock sort of like that once. In a bed of yellow clay there was a dark rock about a foot in diameter made of dark sand and it had nodules of harder rock embedded in it. the nodules were hollow. It was alone in the clay bank. Makes no sense to me how it formed.
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Firstly, i have to agree with RD- the black "knobs" are more resistant to erosion, so they stick out.
The "knobs" are probably very large pyrite nodules. The dark red-brown to black color means they are made of iron and manganese oxide, which I would expect in weathered specimens.
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Firstly, i have to agree with RD- the black "knobs" are more resistant to erosion, so they stick out.
The "knobs" are probably very large pyrite nodules. The dark red-brown to black color means they are made of iron and manganese oxide, which I would expect in weathered specimens.
Well now, old boy, let a sedimentologist have at it. This ain't your grandma's iron ore.
(Clears throat)
In areas where there is a large supply of available iron in the rocks, concretions of iron minerals are often formed when water leaches the iron bearing layers. If this water then become concentrated by heat and other environmental factors iron bearing nodules will precipitate out (over a long period of time.) These nodules then get buried in mud. After eons this becomes stone (limestone of slate, dependent on mud composition) and ...
Voi la! black looking concretions in a grey matrix. (Isn't this fun, kids?)
The white material is the result of subsequent hydrothermal activity. I cannot fit the formation in to a time bracket because of lack of info.
Bass, you need me
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Bass is correct.
BUT .........
"probably very large pyrite nodules." & "... iron and manganese oxide.' " but which is the correct answer?
from https://en.wikipedia.org/wiki/Manganese_oxide
Manganese may also form mixed oxides with other metals such as Fe, Nb, Ta, ... :
Bixbyite, a manganese iron oxide mineral
Jacobsite, a manganese iron oxide mineral
Columbite, also called niobite, niobite-tantalite and columbate
Tantalite, a mineral group close to columbite
Coltan, a mixture of columbite and tantalite
Galaxite, a spinel mineral
Todorokite, a rare complex hydrous manganese oxide mineral
The largest possibility is an iron-manganese concretion.
No, Neil, it is not dinosaur poo
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JB
Great to see you posting again. We need a curmudgeon on the geology forum!
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Who asked you, sonny?