Geology Reference
In-Depth Information
Fig. 5.1.
Common geopetal fabrics in thin sections of limestones.
Geopetal infilling of fine-grained sediment
in lami-
nated microbial and algal carbonates and bioclastic
grainstones is a common feature, see Deelman and
Decoo (1976) for an experimental analysis of floored
interstices in sand-sized carbonate sediments. Intra-
and interskeletal deposition of fine-grained sediment
may be caused by storms (Pl. 17/4).
Growth patterns of sessile organisms
: Sessile co-
lonial organisms (e.g. calcareous algae, sponges, cor-
als) tend to grow upward guided by light and the avail-
ability of nutrients. The growth patterns of these or-
ganisms and other fossils preserved in life position are
excellent top-and-bottom criteria (Pl. 41/1).
Biogenic encrustations
on clasts or shell fragments
can be used as geopetals if the fragments are not re-
worked. Abundant encrustations on lithified surfaces
are valuable geopetals.
Oriented shells:
Convex-upward oriented valves are
commonly considered as the common depositional pat-
tern occurring in low-energy environments (Futterer
1978; Allen 1990). However, bottom currents and bio-
turbation as well as differences between large and
small-sized valves can considerably change this pat-
tern (Salazar-Jiménez and Frey 1982).
Diagenetic criteria:
Gravitative and pending ce-
ments (Pl. 34/6, Sect. 7.4.2.1) are important geopetal
indicators in transitional shallow-marine, marginal-
lacustrine and pedogenic carbonates. Geopetal in-
fillings of dolomite crystal molds with vadose silt
during dedolomitization are further top-and-bottom
criteria (Pl. 39/8).
Reoriented geopetals
Synsedimentary sliding and slumping of weakly
lithified carbonates can lead to the reorientation of geo-
petals, particularly of buried shells filled with internal
sediment. Burrowing may cause the redeposition of
shells (e.g. bivalves, cephalopods, brachiopods) sub-
sequent to the lithification of internal sediment. Inter-
nal sediment in intraskeletal voids may lithify more
rapidly than the surrounding fine-grained sediment and
quickly fill the remaining void space with cement, thus
resulting in reworking and displacement of geopetally
filled fossils (Wieczorek 1979).
Storms may rework geopetally filled shells (Pl. 102/
1). The solution of sulfates may also produce over-
turned geopetal fabrics (Folk et al. 1993)
Significance of geopetal fabrics in limestones
Effect of compaction on reef slopes:
Measurement
of geopetal fabrics in reef cavities allows the effect of
postdepositional processes such as differential com-
paction and basinward tilting of reef and foreslope sedi-
ments to be determined (Saller 1996).
