Geology Reference
In-Depth Information
Fig. 7.9.
Radiaxial cement formed on the slope of a distally steepened ramp.
Radiaxial calcite cements are common in
depositional or diagenetic cavities of ancient reefs and slope settings. They consist of cloudy or turbid, fibrous to bladed
crystals several millimeters long, growing perpendicularly to the substrate and exhibiting undulose extinction. The fabric is
defined by a combination of curved twins, convergent optic axes and divergent subcrystals within single crystals.
A
: The pore space was filled with a first generation of marine inclusion-rich crystals, then covered by thinner (arrows) and
thicker (double arrows) dark zones of reddish silt.
B
: A second radiaxial generation characterized by shorter and expanded crystals (arrows) can be seen under crossed nicols. It
was formed subsequent to the cover of reddish silt on top of the crystals. The deposition of silt indicates influx from adjacent,
subaerially exposed, and karstified platforms (Flügel and Koch 1995). The remaining pore space is occluded with coarse
blocky cement. Polarized light. Late Triassic: Steinplatte, Tyrol, Austria. Scale is 1 mm.
developed in Late Paleozoic phylloid algal reefs) may
be related to now-extinct algae (James et al. 1988).
A specific type of botryoidal cement known from
Late Paleozoic platform carbonates is
array calcite
(Davies and Nassichuk 1990) characterized by coarse
spherulitic and botryoidal arrays of calcite replacing
radial-concentric (botryoidal to spherulitic) pore-fill-
ing aragonite cements with a convex outer margin and
internal concentric growth lines.
RFC as a first generation of cement, uncompacted shel-
ter-pore fabrics, growth of RFC in marine and skeletal
substrates, growth as isopachous rims around pores,
interlayering of RFC and marine internal sediment, and
analogues to modern submarine Mg-calcite cements.
The present consensus favors a precursor Mg-calcite
mineralogy. Evidence for a High-Mg calcite precursor
is the textural similarity to modern Mg-calcite cements
(Aissaoui 1988), microdolomite crystals within RFC,
and relatively high amounts of Mg.
Radiaxial fibrous cement (RFC):
These calcite ce-
ments (Fig. 7.9) are most pervasive and volumetrically
the most important cements in many Paleozoic and Me-
sozoic reef and fore-reef limestones (Pl. 28/2), but ra-
diaxial fibrous cement is also known from ancient
beachrocks and sheet voids (Gray and Adams 1995).
They have been thought to be cements of syndepos-
itional marine, shallow-burial marine (Halley and
Scholle 1985), or subaerial meteoric origin. Current
interpretations have focused on a neomorphic replace-
ment by calcite of radiating bundles of acicular Mg-
calcite that precipitated as early submarine cement
(Davies 1977; Davies and Nassichuk 1990). Evidence
of a marine-phreatic origin of RFC is the occurrence of
Radiaxial fibrous cements were described in detail
by Kendall and Tucker (1973) and revised by Kendall
(1985), who differentiated three categories distin-
guished by the extinction patterns of subcrystals within
each larger crystal (Pl. 35/2):
• Radiaxial fibrous calcite exhibiting a pattern of
subcrystals within each subcrystal that diverge away
from the substrate in an opposing pattern of distally-
convergent axes, yielding a corresponding curvature
to twin lamellae and cleavage (Bathurst 1959, 1977,
1982; Kendall and Tucker 1973; Kendall 1985).
• Fascicular-optic calcite (Kendall 1977; Chafetz
1979; Marshall 1981) characterized by the presence of
