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Identity HelpDistinguishing sedimentary from volcanic rocks

10th Jan 2017 20:33 UTCIsaac Carman

Hey guys,

First off not sure if this belongs in this forum or in General, but thought I would put it here as it seemed most appropriate.

Just wondering if you guys had some tips for distinguishing sedimentary from volcanic rocks in the field? Especially for fine grained sedimentary rocks such as siltstones.

I know you are supposed to be able to tell by looking at the grains and if they form interlocking crystals then it is volcanic whereas if they look cemented together then it is sedimentary. But I'm not convinced anyone can do this to any reasonable extent, especially in such fine grained rocks. I know this is easy to do for intrusive igneous rocks where the crystals are large, but not so much for basalts, rhyolites, etc. For example telling a black shale apart from a fine grained basalt (with no visible olivine) is often very difficult to do.

You could also probably get an idea from the minerals each rock is composed of, but again in really fine grained rocks it can be hard to determine mineralogy without lab-based techniques (petrography, etc).

This has been one thing I have struggled with since starting geology and I would much appreciate some help.

Regards

10th Jan 2017 21:49 UTCIsaac Carman

-- moved topic --

10th Jan 2017 21:58 UTCRalph S Bottrill 🌟 Manager

Hi Isaac

Good question, it can be difficult with finer grained rocks.

Coarse grained rocks usually show obvious intergrowth textures.

With fine grained rocks you need to look for vesicles and phenocrysts; lacking those prepare a thin section to study the textures under magnification. XRD can be very useful; if its quartz rich its probably a sediment.

With tuffs it becomes a grey area, where you may need to look at the sequence in outcrop, and look for sedimentary features.

10th Jan 2017 22:10 UTCRichard Gibson 🌟

If you are in the field, look at the context. As Ralph suggested, the sequence, sedimentary features which could be confused sometimes with flow features, or even with metamorphic foliation. When I helped teach field geology we'd take students to a metamorphic area, all kinds of cool stuff, then without elaboration to an outcrop of what appears to be a foliated quartzose rock - but which is just tightly cemented sandstone, which they had seen elsewhere. I'd say 80% call it metamorphic. Point is, it is not always simple or straightforward, and you certainly can't always tell from a hand sample. Interlocking angular grains ("crystals") could be a completely sedimentary arkose. Loose, friable, rounded grains, not especially interlocking, could be a poorly welded volcanic tuff, or metamorphic garnetite. You're right about black shale vs basalt, often enough - but the context should tell the tale. But it might take a fair bit of exploration and mapping (miles/km) to define the context enough to be sure.

11th Jan 2017 00:14 UTCIsaac Carman

Hey guys,

Thanks for the help, some good info there.

Ralph I never thought about vesicles, that is a good point that I will have to remember. Also most of the fine grained rocks I was remembering/thinking about did not have phenocrysts, but their presenece would make it a little easier. Though a 'phenocryst' may also turn out to just be a larger grain incorporated into a poorly sorted sedimentary rock.

Richard I was always taught that interlocking crystals = igneous and that cementation = sedimentary. I never thought that interlocking crystals could actually just be a well compacted arkose or similar sedimentary rock. Also I'm not sure whether one can actually spot a cement without a large grain size? What are your thoughts on this?

I also forgot to mention in my first post that sedimentary structures (cross-bedding, etc) are a bit of a give away normally, even though you can get some sedimentary structures in lava flows.

11th Jan 2017 12:34 UTCDale Foster Manager

Isaac Carman Wrote:

-------------------------------------------------------

For example telling a black shale apart from a fine grained basalt (with no visible olivine) is often very difficult to do.

Just taking this example, shale is normally partly defined by an ability to be relatively easily cleaved into thin plates, whereas basalt doesn't tend to break in this manner.

11th Jan 2017 13:30 UTCReiner Mielke Expert

Of course the whole subject gets exponentially more complicated when dealing with volcaniclastic rocks and even more so if they are metamorphosed and or hydrothermally altered. The only way to deal with these is thin sections and sometimes geochemistry as well.

11th Jan 2017 16:11 UTCRichard Gibson 🌟

Isaac Carman wrote:

Richard I was always taught that interlocking crystals = igneous and that cementation = sedimentary. I never thought that interlocking crystals could actually just be a well compacted arkose or similar sedimentary rock. Also I'm not sure whether one can actually spot a cement without a large grain size? What are your thoughts on this?

=============

only thought is that it can be complicated! Few things are "always" - and yes, recognizing cement in hand sample in fine-grained rocks can be virtually impossible. As others have said, sometimes the only way to really figure it out is with thin sections and other types of analysis; and even then, some things will be subject to interpretation - even such basic things as sedimentary vs igneous vs metamorphic. When you get beyond the standard things, "boundaries" can be pretty fuzzy.

13th Jan 2017 06:26 UTCGregg Little 🌟

Although not as cheap as a hand-lense, a binocular microscope can go a long way in ID. They are used extensively in wellsite work and I have used them in hard rock as well. It is true that there are a few chameleons out there like separating volcanoclastics from lithic sedimentary rocks and low grade metamorphism in a sedimentary terrain but with practice and reviewing the regional geology much can be sorted out. You may have to look at many different samples, hopefully with a mentor, but that is really the only way to hone ID skills.

For example, using up to 40X magnification, it is easy to determine (once you know what you are looking for) siliceous overgrowth cement on a quartz sand grain from the interlocking crystals of igneous and metamorphic rocks. Lithic grains in sedimentary rocks have a transported look (rounding, sorting, surface argillaceous material, etc) to them and they don't have that inter-grown textural appearance.

Back in the 90's (1990's, not that old) I managed a volcanogenic hosted gold deposit of Archean age and the core consisted of thousands of metres of almost featureless fine grained rock which could easily have been clastic or volcanic. The only way to decide what to call it was when we saw some faint sedimentary textures we hung our hats on the volcanoclastic label.

Work with known samples and branch into familiar territory (described outcrops) to develop your own personal database. Geology is a tactile science.

13th Jan 2017 17:18 UTCD. Peck

Years ago, I read that a good, well labelled and documented rock collection is scientifically more valuable than a similar sized mineral collection. I have no idea as to the validity of that claim, but it started me on a rock collection. My intent was trim a rock specimen to 2.5" x 4" x 1" and slice a chip of the bottom, one side, and one end. I polished the cut sides, and ground oriented thin sections from the chips. Well, it was I think a great idea but somehow it never got done. I finished about two of them.

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