Geology Reference
In-Depth Information
0.5 cm
Fig. 20.6.
Parautochthonous debris from a small coral-
dominated patch reef, with large solitary rugose corals
(
Streptelasma
sp.), and abundant gastropods of varying
size, fragments of crinoids, trilobites, and ostracods
forming a rudstone with packstone matrix. Geopetally
filled cavities inside and under fossils are common and
exhibit graded infilling. After rapid deposition of the
sediment the cavities were filled with suspension which
settled according to grain size. The primary calcitic
organisms (corals, brachiopods, crinoids, ostrocods) are
well preserved whereas the originally aragonitic
gastropod shells have been completely replaced by clear
blocky calcite spar. Shell abrasion-derived material in
the matrix. Abundant tiny quartz grains indicate a nearby
land mass. Late Ordovician: Anticosti, Canada.
BRACH - Brachiopod shell (note the geopetal under-
neath), GA - Gastropod. RUG-C - Rugose corals
Fig. 20.7.
Poorly sorted reef limestone (SMF 5) from a
stromatoporoid-dominated biostrome rich in fragments
of stromatoporoids and crinoid ossicles. In addition,
brachiopods and bryozoans are present. The sediment
is poorly washed, geopetal structures are frequent. The
components are not abradedand delicate particles such
as fenestrate bryozoans are not destroyed. This, in com-
bination to the poor sorting and the high amount of fine-
grained material, points to a very short transport dis-
tance. Silurian (Ludlow): Gotland, Sweden.
BRACH - Brachiopod, BRY - Bryozoans, encrusting,
F .-BRY - Fenestrate bryozoans, CRIN - Crinoids, STR
- Stromatoporoids, STYLO - Stylolithic contact under-
neath the stromatoporoid colony
