Paleonet: Biostratigraphic datums - a question

michael.styzen at shell.com michael.styzen at shell.com
Thu Apr 19 12:44:57 GMT 2007


Actually for cuttings samples I prefer to think of both the top and bottom of the composite interval.  All you can say about a specimen identified in such a sample is that it probably came from somewhere within that interval.  Generally if one is working with cuttings samples it is best to catalog them with both the top depth and bottom depth.  I avoid identifying cuttings samples with a single depth identifier whenever I can. When I write reports I'll express whatever marker I'm picking as being from within a sample interval (25,570'-25,600' - Discoaster petaliformis). UsuallyI work with 30 foot intervals.  So like it or not, any datum I pick is 30' thick. Unfortunately, when we graphically represent this a line that thick looks rather messy, so we have to pick the top, bottom or middle of the interval.  

-----Original Message-----
From: paleonet-bounces+michael.styzen=shell.com at nhm.ac.uk [mailto:paleonet-bounces+michael.styzen=shell.com at nhm.ac.uk]On Behalf Of Paul Britton
Sent: Wednesday, April 18, 2007 4:16 AM
To: PaleoNet
Subject: Re: Paleonet: Biostratigraphic datums - a question



In our experience the datum depth of a cuttings sample depends on which side of the pond you're on. The convention in the US, as Tony points out, is to use the top depth of the depth range of a cuttings sample, but in the UK and elsewhere in Europe the base depth is used. The top depth of the range would be to state that the datum "could be as high as", whereas taking the base depth would state "must at least be as high as". For databasing purposes we try to make it clear which model is in use, and it's important users of data are aware of potential differences.

Paul.

At 00:45 18/04/2007, you wrote:



Dear Tony,

Many thanks for your comments, which I certainly appreciate.  As a matter of clarification, I should just say that this is not a methodology I have yet practiced, but we are considering it as a means of expressing uncertainty in observed HOs (and LOs).  The datum I have in mind would express the age as a mid-point between the two bracketing samples, and provide an error (+/-) based on the ages of those two samples.  This seems a legitimate way of expressing uncertainty, but perhaps it is extending the data too far.  I should also point out that the methodology would be applied to a DSDP hole for which there is a very precise age model, such that sample spacing becomes the main source of error (which is why we are trying to express it somehow).

I can't say I've ever seen this methodology applied to biostratigraphic HOs/LOs etc., but I think it is common to construct a zonal boundary at the midpoint between two samples, and average the age of the samples to calculate the age of the boundary.  In a sense, this is an analogous situation.

Again, thoughts on this approach would be greatly appreciated.

Martin


On 17 Apr 2007, at 19:04, Tony D'Agostino wrote:



In 27 years of industry biostratigraphy I have never seen or heard of usage described by Martin Head (below). In the oil business that would be what we refer to as "made up data". A HO datum (please no Don Imus comments) is routinely adjusted to the top depth of a sample interval when working with cuttings material or any kind of sample that is a composite over a stratigraphic interval. Occassionally I've seen workers assign a datum to a midpoint of such a sample interval, but I've never encountered, let alone used the method described by Dr. Head.  Why would one interpolate a depth for a HO datum at some point that is not sampled or examined, providing a basis for observations? Best practices dictate that you assign the datum to some depth within or at the end points of the sample interval. At least you know with certainty, if not precision, where that the fossil was observed to occur. Leave the "interpolation" to the seismic interpreters.
 
Tony D'Agostino
20746 Prince Creek Drive
Katy, Tx. 77450
281-646-1660 adagostino at houston.rr.com 
 
"The limits of a tyrant are determined by the endurance of those that oppose him" Frederick Douglass


-----Original Message-----


From: paleonet-bounces+adagostino=houston.rr.com at nhm.ac.uk [ mailto:paleonet-bounces +adagostino=  <mailto:houston.rr.com at nhm.ac.uk> houston.rr.com at nhm.ac.uk]On Behalf Of Martin J. Head


Sent: Tuesday, April 17, 2007 5:27 PM


To: PaleoNet


Subject: Paleonet: Biostratigraphic datums - a question



Dear Paleonetters,



I have a question concerning the nomenclature of a particular biostratigraphic datum.



Although highest occurrences often refer to the highest sample in which a taxon is found in a particular section, the real highest occurrence will likely be at some interpolated point (normally placed at the midpoint) between the highest observed occurrence and the next sample up. The difference in position between the observed HO and the inferred HO will depend on the sample spacing.



Is there a term for this inferred/interpolated HO?



I'd wondered if HOD (highest occurrence datum) might be the appropriate term, but I believe this refers to a three-dimensional surface of HOs whereas my "inferred HO" could refer to a single stratigraphic section (i.e a point).



I hope the answer is not too obvious! Thanks for any help or ideas.



Martin



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


Martin J. Head


Professor


Department of Earth Sciences


BROCK UNIVERSITY


500 Glenridge Avenue


St. Catharines, Ontario L2S 3A1


CANADA


Tel 905-688-5550 ext. 5216


Fax 905-682-9020


Email mjhead at brocku.ca


www.brocku.ca/earthsciences/people/mhead.php



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

Martin J. Head

Professor

Department of Earth Sciences

BROCK UNIVERSITY

500 Glenridge Avenue

St. Catharines, Ontario L2S 3A1

CANADA

Tel  905-688-5550 ext. 5216

Fax  905-682-9020

Email  mjhead at brocku.ca
www.brocku.ca/earthsciences/people/mhead.php

_______________________________________________
Paleonet mailing list
Paleonet at nhm.ac.uk
http://mailman.nhm.ac.uk/mailman/listinfo/paleonet

++++++++++++++++++++++++++++++++++++++
Paul D. Britton, StrataData Ltd.
Tel./Fax.: +44 (0)1233 642911
paul at stratadata.co.uk
http://www.stratadata.co.uk
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