that thick anhydrite sections can not form in tidal flat settings
because of hydrologic constraints rests on the assumption that most
of the evaporites there are displacive in origin. But,
what if their formation was not purely displacive and developed
through the consumption of precursor carbonate ("pacman anhydrite"
name proposed by K. Leyrer 2001) instead?
research suggests there is considerable evidence for "pacman
anhydrite" (see figures) and that when the consumption of anhydrite
reaches completion, only massive anhydrite devoid of internal structure
remains. Typically, such anhydrite and even massive 'chickenwire
anhydrite' are interpreted to represent strongly coalesced nodules
whose origin resides in subaqueous gypsum growth even in the absence
of evidence for some vertically elongated nodules. Observed gypsum
to anhydrite transformation models show that primary gypsum crystal
fabrics have at least some influence on anhydrite nodule shape and
orientation. We contend that some inclined and vertical elongated
nodules should be preserved in 'chickenwire anhydrite' if it has
a subaqueous origin. In the absence of such evidence, we contend
that thick beds of 'chickenwire anhydrite' may be purely diagenetic
structures attributable to "pacman anhydrite" processes.
Such an interpretation would be consistent with not only with the
generally equant anhydrite structures present, absence of displacive
fabrics, and dearth of carbonate.
gypsum to anhydrite transformation model adopted from Loucks and
is this important? Sequence stratigraphic models very much depend
on the correct interpretation of a depositional model. For example,
one stratigrapher considers the evaporites as salina deposits formed
during a lowstand, whereas a second worker may assign a tidal flat
origin and include them in the highstand systems tract. In the first
interpretation, the sequence boundary occurs at the base of the
evaporites and the deposits delineate an intrashelf basin. The sequence
boundary occurs at the top of the evaporites in the second model
and the evaporites define a progradational wedge. Correlation frameworks
will differ fundamentally and this has profound impact on reservoir
characterization or play concept development.
1 — Felted "pacman" anhydrite exhibit carbonate
rim (stained for calcite) partially replaced by anhydrite laths.
1a and 1b show details of attacked carbonate rim in plane-polarized
(figure 1a) and plane cross-polarized (Figure 1b) transmitted light.
2 — Mudstone with coarsely crystalline carbonate rim developed
around felted "pacman" anhydrite. Note white areas (anhydrite)
within carbonate rim and corroded carbonate crystal margins adjacent
to anhydrite core. Plane-polarized transmitted light.