Jun 28, 2021
Follow a geologist with a Rock ID (II)
Step by step investigation of a rock and how it formed
I participate in a group of rock enthusiasts in Michigan and was asked about this very strange rock. In particular, the asker wanted to know what it was and especially how did it end up looking so bizarre, with these odd patterns all over it. My answer follows…
This one might be the most bizarre I’ve ever encountered. Keep in mind that I haven’t handled it in person, I’m working from all the photos you sent me so this is highly speculative. Now, what you have is a sedimentary rock. In all of your photos, there was not a single crystal large enough for me to identify, so it’s not metamorphic and it’s definitely not igneous. Sedimentary.
I was hoping that the wet photos and close-up photos you provided might show me more, but actually, the original dry photos were most useful. The dark material that makes up the rock: it is very fine-grained and very hard. It is very likely an argillaceous siltstone. Silt is very fine grains of sediment, smaller than sand. Argillaceous means full of clay. An argillaceous siltstone is one with silt grains held together by clay. This forms in the moderately deep ocean, on the slopes of continental shelves offshore from rivers.
The material inside it that makes this so freakin’ weird, though… what is it? Well, from your originally posted pics, I can make out individual grains of sediment which tells me that it is coarser than the surrounding material. What does this tell me? That it doesn’t belong, that it came from higher up the continental shelf, nearer the river source. Not necessarily from very far away, though, since the change in grain size isn’t all that dramatic.
Next, we look at the shape of the clasts. You may notice that many of them appear like puzzle pieces, separated from one another by the darker finer material- that they’d fit together if moved against one another. This is not a coincidence! While there are a number of randomly jumbled clasts in this rock, many are next to each other and seem to fit together nicely. That’s important.
Next, we look to those clasts, themselves. Each one has a ring of lighter material around a darker center. However, it’s the shape of these outer rings that fit together like puzzle pieces. If you ignore the rings and look at the darker centers, they don’t fit together nearly as well. That’s important- the pieces fitting together is not a coincidence so this tells us that the pieces that fit were originally part of larger blocks of material that broke up. It doesn’t make sense for rings of lighter material to be running through those blocks like that, so those rings of lighter material formed later on the clasts, themselves.
Finally, we see evidence of soft-sediment deformation. Several of the clasts look like they’ve been bent. That’s the final piece of evidence. So, when all of this is combined together, what geological event could have created this weird-ass rock?
An underwater landslide along the slope of the continental shelf. Soft, half-lithified (halfway through becoming a rock) materials slid downslope onto softer sediments. Blocks of compacted silt-rich sand slid and settled onto the silty clay below and sank in. Some were small random pieces, but there were larger blocks- all firm enough to remain intact, but just barely. They were buried by more sediments and that burial appears to have been very rapid. Perhaps there were more landslides above or perhaps the river’s discharge was very high, like the Mississippi and there was just a ton of sediment being kicked out. Either way, it was buried quickly and before everything could begin to really harden up and lithify properly, the material began to settle under the weight- like the way soft sand compacts under your foot at the beach.
This settling creates stresses in sediments. The softer, more fluid clay-silt mix was able to flow easily, but the blocks of harder sand-silt couldn’t cope so fluidly and broke apart slightly. The soft fluid clay-silt flowed into these cracks in those blocks and further separated the “puzzle” piece clasts. The stresses also led to the deformation we see in some of the sand-silt clasts, proving they were still pliable at the time- probably with a consistency like half-hardened play-doh.
The silty sand in those clasts is black. That means there is a lot of organic material trapped in there, which is what you’d expect from a major river. You know what really likes to bind with organic carbon, though? Clay. Like the clay in that argillaceous siltstone that surrounds the clasts, which leached the organic carbon from those pieces very slowly, leading to rings of pale rings of carbon depletion surrounding each clast.
So what do you have? It’s incredibly complex. If described among geologists, it would be called: an argillaceous siltstone from a turbidite deposit with clasts of carbonaceous fine-grained silty sandstone exhibiting carbon leaching zonation and soft-sediment deformation.
