From: Darryl Maddox (dpmaddox@arn.net)
Date: Wed Apr 02 2003 - 23:22:23 EST
Hello Bill,
Sorry I haven't gotten back to you but it is going to be a few more days before I can do so.
On a related topic addressed in a post I just read -
Personally I like the idea of a paraconformity rather than saying that if followed far enough it will show signs of non-deposition and therefore should be classified as a disconformity. Where it shows signs of discontinuous deposition it is a disconformity but to say that the area that shows no such signs should be classified as a disconformity because it will become one if you follow it far enough is like saying a granite is a gabbro because you can follow it far enough to go through all the interveening rock types. It is what it is based on the definition of the term and the properties exibited where you are standing. Ok, I will admit some things are indeterminant at a particular point and you have to look around a bit. Maybe the geologists who hold this view are saying you should follow it a few feet instead of the miles or so that I immediately thought of when I read the comment.
Or maybe it's just late and I am tired.
Question: Can you give a pre-test after the class? Yes, according to the education people.
I am going to give a one day session on historical geology and drawing inferences from data for middle school teachers this Saturday and I was given three questions and told to be sure to emphasise the answers to them because after the class was over they would be given a pre-test and a post test to compare what they knew before the class to what they knew after it. No how the ______ can you give a pre-test after the fact? Well I guess the same way a paraconformity becomes a disconformity if you follow it far enough. But what do you do if the bedding planes develop an angular relationship? Is the paraconformity that isn't really a paraconformity because it becomes a disconformity just become a non-disconformity because it is really an angular unconformity? What exactly is the ranking here? Why is it not just as logical to day some disconformities are really paraconformities because if you follow them far enough they lose their indications of non-deposition? And if we are going to play that game why have unconformities at all? IF you follow most of them far enough in the right direction you wind up with deposition between them and then there is no unconformity. Beats me. Sounds too much like the question: When is a transition fossil not a transition fossil?
I really did enjoy the post that it sounds like I just blasted and I didn't intend to make light of what the person had to say. It was a good bit of information but at some point we geologist have to start sticking to some definitions. I get tired of telling my students we have used a word so many different ways for so long it no longer has a meaning but here is one of the definitions and you need to learn it because it will be on a test. A couple of months ago I was giving my historical geology lab a quick review of rock types and we had some graywackes. Then that night I was looking for photos of various rock types on the internet and found a picture of some kind of rock and at the end of the description it said something to the effect of - this is the same kind of rock we used to call graywacke. Well the textbook I am using and the lab book I am using are still calling it a graywacke and so as far as I am concerned it's a graywacke; paraconformities are not disconformities; and pre-tests are given before the material is presented, not afterward. Guess I just flunked education.
Darryl
----- Original Message -----
From: Bill Payne
To: dpmaddox@arn.net
Cc: asa@calvin.edu
Sent: Tuesday, April 01, 2003 10:09 PM
Subject: Re: Paraconformities (was test questions)
On Mon, 31 Mar 2003 06:52:58 -0600 "Darryl Maddox" <dpmaddox@arn.net> writes:
> I have seen the Texas Gulf
> Coast and I have seen pictures of the desert plains of Kuwait and Iraq and
> am convinced that either will produce surfaces flat enough to be considered
> peneplains when seen from the top or when their surface topography is
> mapped, and/or essentially straight line paraconformities or
> non-conformities - nonconformity in the case of PDC since there is a
> lithological difference across the boundary but in many places it isn't much
> more, if any more different that other boundaries within the Permian and
> within the Triassic which are not considered unconformities.
>
> Confirming or contrary thoughts or facts I should know about?
There is an interesting article at http://www.grisda.org/origins/15075.htm - "Those Gaps in the Sedimentary Record", by A.A. Roth, which discusses (from YEC POV) the PDC you refer to.
Just off the top of my head, I would think that you don't consider the effect of millions of years that either subaerial or submarine exposure would have on topography.Yes we do see some fairly flat areas on the earth today, but they are not as extensive nor as planar, and they have not been eroding as long as many in the geologic record.
Glenn's post and another I received offline, and some other info cited below, suggest that these contacts are not as simple as what I had said - there is some evidence of both erosional and chemical weathering. It'll take me a while to digest what I have, but my initial thoughts are that the erosional features, while present in some areas, are not deep enough to account for millions of missing years.
There are 3 posts in the ASA archives dealing with paraconformities:
http://www.asa3.org/archive/ASA/200101/0131.html
http://www.asa3.org/archive/ASA/200101/0099.html
http://www.asa3.org/archive/ASA/200101/0069.html
From what I understand at this point, the paraconformities do often exhibit some erosion, but the generally planar features are difficult to explain if there were truly millions of years in the intervening gaps. The conclusion to Roth's article (cited above) summarizes this question:
"This flat and parallel, or near-parallel, arrangement at these gaps seems to be different from the eroding surfaces of much of our present earth.
The difficulty with the extended time proposed for these gaps is that one cannot have deposition, nor can one have much erosion. With deposition, there is no gap, because sedimentation continues. With erosion, one would expect abundant channeling and the formation of deep gullies, canyons and valleys; yet, the contacts are usually "nearly planar." Over the long periods of time envisioned for these processes, erosion would erode the underlying layers and much more. One has difficulty envisioning little or nothing at all happening for millions of years on the surface of our planet. The gaps seem to suggest less time.
Our current topography does not represent an extension from the ancient past. Ashley's (1931) provocative study points out how recent our present topography is and argues that 99% of it was formed in an assumed 15 Ma, which would be very recent on an earth assumed to be thousands of millions of years old. Thornbury (1969, p. 25) states that little of earth's topography is older than Tertiary (67 Ma ago), and most of it is no older than Pleistocene (2 Ma ago).
This raises the question of what happened to the topography for the assumed hundreds of millions of years before that. Our present topography is so dramatic in places that it is difficult to think of ancient topography being so poorly represented. Yet, our Everests and Grand Canyons seem conspicuously absent in the record of the past, while that past is still very well-represented in the older sedimentary layers of the earth. Dramatic topography should be especially noticeable at the assumed long time periods (gaps) between the layers, when there would be ample time for uplift and erosion.
It is often difficult to discern what happened in the past; however, the assumed gaps in the sedimentary layers witness to a past that was very different from the present. In many ways, that difference is readily reconciled with catastrophic models such as the Genesis flood that proposes the relatively rapid deposition of these layers."
Bill
This archive was generated by hypermail 2.1.4 : Wed Apr 02 2003 - 23:23:16 EST