Paleonet: Biostratigraphic datums - a question

Jean Guex Jean.Guex at
Thu Apr 19 09:33:55 GMT 2007

>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).

The above sentence of Martin's posting provides 
an excellent introduction to his problem:

"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)."

In some rare cases, the three dimensional surface 
he mentions can be "flat" but, usually, it is a 
landscape with peaks and low points, due to the 
usual diachronism of datums. In fact the problem 
of reliability of a given datum (local FO or LO) 
is mainly related to its diachronism: (its age is 
not the same everywhere because 
of  discontinuous  record , implying that the 
local appearances/disappearances are not 
synchronous at large geographical scale). Such 
diachronisms can be demonstrated by constructing 
sequences of concurrent range zones of minimal 
duration (ie the zones or units are defined by 
maximal sets of intersecting taxon ranges). A 
datum is demonstrated to be diachronous when it 
is recorded in one zone at one given locality and 
in a different zone at another locality. Such 
diachronisms can be mathematically quantified 
(see the graph theoretical approach of Guex 1991, 
Biochronological correlations, Springer Verlag).

Another way to study the disachronism of datums 
is by examining classical "age-depth" bivariate 
graphs : such diagrams often show conflicting 
chronological relationships of the following type 
: The worst case is when the "top" (=last 
occurrence "datum" or LAD or TOP or HO etc) of a 
taxon X  is numerically older but 
stratigraphically higher than the "bottom" (=FAD= 
LO) of a taxon Y  (i.e. X and Y co-occur 
stratigraphically whereas they ought to be in 
sequence according to their respective numerical 
age). This kind of situation appears to be very 
common when numerical ages of successive last and 
first occurrences of species are tested against 
good biostratigraphic evidence by means of 
"age-depth" graphs. In such cases it is clearly 
the calibrations of the datums with the numerical 
ages which are false and not the stratigraphic 
observations which are biased. One very common 
contradictory relationship  is when the bottom 
( of a given taxon is numerically older 
but stratigraphically higher than the bottom 
(resp. top) of another taxon. This kind of 
situation is obviously generated by the general 
and common diachroneity of the usual "datums" 
(first and last occurrences of taxa).  A 
spectacular example can be found in the age-depth 
model of DSDP leg 90, site 586B (See Guex in 
Savary & Guex 1999 (p 43-45: Discrete 
biochronological scales; Mem . Geol. Lausanne Vol 34).

Any one interested in such deterministic approaches can contact me.

Prof Jean GUEX
Institut de Paléontologie
UNIL- BFSH-2 - Anthropole
CH 1015 Lausanne

phone 004121 6924346
fax                 6924305

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