The long-term organization exhibited
by peritidal sections occurs on a scale of 10s to 100s of meters; yet,
the question persists, what specific stratigraphic characteristics are
responsible for these changes? Two end-member hypotheses present themselves.
First, thickness variation within peritidal sections could occur across
all facies types, wherein regions of a stratigraphic section are generally
thicker or thinner bedded than the long-term average. Such cross-lithologic
thickness variation could be driven by gradual changes in long-term rates
of accommodation space creation. Alternatively, a second hypothesis suggests
that there exist thickness-frequency relationships unique to specific lithologies.
Vertical variation in the abundance of these different facies would then
result in variation about the average bed
thickness. Long-term variation
in
bed
-thickness as driven by lithology-specific thickness relationships
could be the result of shifting facies mosaics across the depositional
surface, or to more strictly allogenic changes in facies abundance in an
updip or down-dip direction driven by eustasy or tectonism. Preliminary
analysis of the Ordovician Kindblade and West Spring Creek Formations of
southern Oklahoma supports the hypothesis of lithology-specific thickness
variation. Algal boundstone facies average 0.78 m while mud and grain-rich
particulate carbonate beds while more abundant, average only 0.25 m in
thickness. Stratigraphic distribution of boundstone within the sequence
strongly correlates with the shape of the Fischer plot constructed for
this sequence. Elementary forward models of carbonate deposition tend to
support the notion of lithology-specific thickness variation as being the
dominant factor in controlling large-scale stratal architecture within
peritidal sequences.
AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana