Geoscience Reference
In-Depth Information
Fig. 9.28 Extended RASC ranges for Cenozoic Foraminifera in Gradstein-Thomas database.
Letters for taxon 59 on the right represent (A) estimated RASC distance, (B) mean deviation
from spline-curve, and (C) highest occurrence of species (i.e., maximum deviation from spline-
curve). B is shown only if it differs from A. Good markers such as highest occurrence of taxon
50 (
Subbotina patagonica
) have approximately coinciding positions for A, B And C. Note that as a
first approximation it could be assumed that the highest occurrences (C) have RASC distances
which are about 1.16 units less that the average position. Such systematic difference in RASC
distance is equivalent to approximately 10 million years (Source: Agterberg
1990
, Fig. 8.9)
variance of deviations between observed and estimated mean event positions. It was
concluded that taxon 50 is an exceptionally good marker. Maximum deviations
between observed and estimated positions tend to be the same (
1.16) for most
other events. This suggests that, on average, the truly last appearance datum (LAD,
cf
.Sect.
9.1.1
) occurred at least 10 million year later than average observed last
occurrence (LO) for most Cenozoic Foraminiferal taxa.
9.4.2 Central Texas Cambrian Riley Formation Example
As explained in Sect.
9.1.1
, the rationale underlying Shaw's method of graphic
correlation differs from the one underlying RASC and CASC, which is based on
simultaneously averaging biostratigraphic events in all stratigraphic sections for a
Search WWH ::
Custom Search