Paleonet: Biostratigraphic datums - a question

Wed Apr 18 16:20:14 UTC 2007

It also depends on how common the fossil is in the sample interval--if it is found in only a quarter of the samples in the fossils range the actual top will most likely be farther up than another fossil found in every sample in the fossils range. Strange that no one has mentioned the extensive literature on the Signor-Lipps effect.
Peter Sheehan

-----Original Message-----
From: paleonet-bounces+sheehan=mpm.edu at nhm.ac.uk [mailto:paleonet-bounces+sheehan=mpm.edu at nhm.ac.uk]On Behalf Of Martin J. Head
Sent: Tuesday, April 17, 2007 6:46 PM
To: PaleoNet
Subject: Re: Paleonet: Biostratigraphic datums - a question

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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