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In my text book I came across this multiple-choice question

A sedimentary rock consists of only minerals and lacks fossils, it could be classified as:

i) a clastic sedimentary rock

ii) a chemical sedimentary rock

iii) a biochemical sedimentary rock

Of course I would exclude "a biochemical", but I am confused between clastic and chemical. I did some research on google but didn't find a precise answer. However, I found that both of them could contain only minerals. So, I hope someone could help me here!

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    $\begingroup$ Everything is made of minerals. I assume what it means is that it lacks lithic fragments or bioclasts. I can still think of examples of all three types that fit these criteria though so not a it's not really a very good question. $\endgroup$ – bon May 2 '17 at 21:30
  • $\begingroup$ @bon, OK, but if you had this question in an exam which of the choices you would choose? $\endgroup$ – Asmaa May 2 '17 at 21:53
  • $\begingroup$ Why do I only have to pick one? I would give examples of all three. $\endgroup$ – bon May 2 '17 at 21:56
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    $\begingroup$ Second that, all three would be the only correct answer if I had to correct that exam. $\endgroup$ – Tactopoda May 2 '17 at 22:54
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This is a really bad question, because all three can lack fossils, or all three can have fossils. The definition of "minerals" is also a bit unclear, because fossils are also made out of minerals. I'll assume that by minerals they mean non-biogenic minerals. Let's go through each one of them:

a clastic sedimentary rock

This rock will have minerals, because this is what is being transported. Usually quartz, feldspars, clays, heavy minerals, etc. But, it can also have fossils. Fossils can be transported as well, and it is not uncommon to find fossils in clastic sedimentary rocks. They are usually not intact, but still fossils nonetheless.

a chemical sedimentary rock

This will have minerals, obviously. This might also have fossils: what if there were organisms around while the chemical rock formed? Could be fossils falling from above into a carbonate precipitate, or insects in an evaporite basin. However, this is probably the least likely rock to contain fossils. If you have to choose only one, this would be it (unfortunately).

iii) a biochemical sedimentary rock

This sounds like an obvious one to contain fossils, but it may not be. What about limestones where all of the fossils were broken down to unrecognizable fragments of calcite? Also known as a micrite? This would be biogenic, but it wouldn't be clear what fossils are there.

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  • $\begingroup$ Sir, your explanation is just awesome. I think I agree with you now that the question was a bad one. I really benefited a lot from you! $\endgroup$ – Asmaa May 2 '17 at 23:40

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