Paleonet: Biostratigraphic datums
chrking at globalnet.co.uk
Sun Apr 22 20:38:04 BST 2007
Only just read this discussion, but would make two (not unrelated) points:
1. Coincidentally, I had a discussion with the wellsite geologist about biostratigraphic datums, while working offshore as wellsite biostratigrapher in the North Sea last week. The justification for the (apparently European) convention on datums in cuttings (i.e. to use the base of the sample interval) is this: it clearly indicates that the highest occurrence of the relevant fossil is at (or above) the base of the sampled interval: this is an objective statement. Using the top of the sample interval gives the misleading impression (especially when in logs and reports the datum depth is likely to be represented as a single figure) that the datum is at the top of the interval: this may or may not be correct . As Formation or other lithological unit boundaries, which may be unconformities, often lie within sample intervals, this can result in assigning the datum to an incorrect formation ! I have seen this in practise !
Both geologists this was discussed with agreed on this point.
In my opinion, fossil ranges should never be extrapolated above their highest recorded occurrence.
2. Most of the preceding contributions have treated fossil records largely as though they were simply data-points in a vacuum rather than being (once) living organisms in a complex world. In the real world, depositional environments and depositional rates are generally fluctuating continually, and stratigraphic breaks at a variety of levels are the rule rather than the exception; Jere Lipps clearly pointed out the fallacy of 'joining up the dots' of fossil records, and that 'gaps in the record' are often a reflection of population fluctations rather than lack of data. Unless close attention is paid to lithostratigraphy and depositional environments, the use of graphic correlation and similar 'objective' techniques can yield inaccurate and sometimes seriously misleading results. Holostratigraphy rules !
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