Chock full of data, the chapter is really in two parts:
1. Review of hydrodynamics (which you usually get in Sed/Strat).
2. A group-by-group review of the literature
Let's start with
Part I: hydrodynamics:
Figure 2.3 is a classic diagram that you'll find in every sedimentology textbook (Silent j, the name is pronounced HUL-strom in an American accent). It basically relates the size of sedimentary particles to the velocities required to move them. So, for example, a 1 mm diameter particle will be picked up (eroded) at velocities above 20 cm/s, and transported as long as the velocity remains above about 7 cm/sec. It will settle out (deposition) at velocities less than that. HOWEVER, we learn later in the chapter, especially in the section about microfossils, that the density of the particle (ie, shell) and shape will affect its hydrodynamic properties.
Unlike the previous chapter, the diagrams in the hydrodynamic section are for the most part self-explanatory and clear, probably because they describe idealized behavior of an idealized sphere. However, once we start talking about the behavior of fossils, things get messier---
I like fig. 2.7, but let's skip Fig. 2.8.
What conclusions can we draw from Fig. 2.9?
Part II: the group-by-group taphonomy summary
Well. As you start reading this section, trying not to get bogged down in specifics, a pattern starts to emerge.....that for every conclusion there is an equal and opposite conclusion by another researcher. By the end of the section I was left with the impression that it's a miracle taphonomy has emerged as an area of viable research!
Your thoughts and questions?
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I would agree with you about diagram 1.7; any one of those lines alone might be okay, but all together it is too much. Figure 2.9 seems to indicate that each shape of particle has a different settling velocity pattern which is dependant on grain size. In other words, settling velocity is determined by the interplay of shape and size of the particle.
ReplyDeleteI'm still working my way through the second part; it seems to have a lot of long lists of what happens to various types and kinds of organisms. Also, a lot of it does seem contradictory. In general, though, decay, type of transport (i.e. flow type), size and strength of the pieces seem to be determining factors for the taphonomy of all the groups.
I meant 2.7, in line 1 above.
ReplyDeleteI really do like figure 2.7... It not only explains itself, but it does prove the cause of weight distribution and size difference on settling paths of grains. And although shells are not grains, could this be applicable in some way to shells of similar shape? Like the third example on the rods? Anyway, although we are skipping 2.8, it seems to me that none of the laws really work all too well unless you get down to 1 or less mm in diameter. Would this be because of something affecting the particles?
ReplyDeleteFor figure 2.9 it seems to me to show that grain size does has an effect on settling velocity, but so does size (which was somewhat shown in fig. 2.7) I mean all shapes do follow a similar trend with small grain diameter, but as they get larger size seems to play a part. But I think this makes a lot of sense. Perhaps the weight might be increasing, perhaps the resistance increases greatly do to size related to diameter. Although I know this seems out of the topic however, what is technically the diameter of a block? I'm afraid that phrase is a little confusing to me...
The rest of chapter two is exactly like that. Back-forth-back-forth. Although I have to ask a question: why are these results so different? In some cases, could it just have been a different flume with different dimensions? I know that these experiments were probably done in a more extensive lab with maybe similar flumes, but the dimensions do matter. I found this out when I met with Dr. Hampton about using the department's flume which is very long and only about 6 in wide. Thus the larger the shell used the less z plane effects you get out of it. Could this explain a difference? (Even though it is probably more complex then that...)
But I do agree with you about the issue of all the contradictory results released within the chapter. After a while I just started to except that right after a sentence of results that "However" was going to pop up somewhere and state something totally opposite. Although if we meet to discuss the rest of chapter two I would really like to talk about all the issues and experiments dealing with this. Besides, there must have been a few things they agreed on at least. Otherwise I don't think there would be this book right under my right elbow on my desk right now....
Good comments. Our challenge is to look for the "rules" that are most helpful/useful for our purposes--the ones that permit taphonomy to function as a subdiscipline, and to be aware of the WHOLE literature on whatever group we might concentrate on. Not good to ignore a study that doesn't fit our preconceived notions; better to try to understand why the contradictions exist. Which brings me to--
ReplyDeleteA's comment about flume dimension is correct--size does matter because of "edge effects" of the sides, and in Dr. Hampton's small flume that would be a big factor.
Diameter of a block? Let's ask our mathematician--T, any comment?