Paleonet: Biostratigraphic datums - a question

[Martin J. Head]

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=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

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