Paleonet: Biostratigraphic datums - a question

David Kopaska-Merkel davidkm at
Wed Apr 18 13:28:57 GMT 2007

I have not used the technique that Dr. Head proposes, but I have encountered
analogous problems many times.  The desire is to include the very useful
information that the taxon in question is absent from the lowest sample
above its highest observed occurrence.  If the samples are closely spaced
then you know with considerable precision where the highest occurrence would
be if you sampled the entire section.  If the samples are far apart your
precision is much less.  You hate to throw away the information you get when
your observation is precise.
I am not sure it is really correct to name the midpoint of the non-sample
interval as the estimated highest occurrence.  What if you used a
single-sided error bar?  All of your error is above that highest occurrence
that you have observed.  So I suggest you put a dot or what have you at the
highest observed occurrence and an error bar reaching up to the lowest
observed absence.  

David C. Kopaska-Merkel
Geological Survey of Alabama
P.O. Box 869999
Tuscaloosa AL 35486-6999
(205) 247-3695 (direct line/voice mail)
(205) 349-2852 (switchboard)
fax 349-2861

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From: at
[ at] On Behalf Of
Tony D'Agostino
Sent: Tuesday, April 17, 2007 6:05 PM
To: PaleoNet
Subject: Re: Paleonet: Biostratigraphic datums - a question

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
"The limits of a tyrant are determined by the endurance of those that oppose
him" Frederick Douglass

-----Original Message-----
From: at
[ at]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 J. Head


Department of Earth Sciences


500 Glenridge Avenue

St. Catharines, Ontario L2S 3A1


Tel 905-688-5550 ext. 5216

Fax 905-682-9020

Email mjhead at

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