Geology Reference
In-Depth Information
1980 for discussion). Reverse grading in larger reef
cavities may also result from seawater pumping through
voids (Scoffin 1993).
Internal geopetals
: The most commonly used geo-
petals are fossils, half-moon ooids (Fig. 4.23), borings
(Pl. 80/7) and cavities within the sediment, character-
ized by fine-grained internal sediment fillings at the
base and sparry calcite at the top. These fabrics repre-
sent frozen spirit levels (
Wasserwaagen
) which record
the water level in the cavity at the time of deposition of
the internal sediment. Partial infilling occurs in many
fossils, including among others calcareous algae (Pl.
122/2), brachiopods, bivalves, gastropods (Pl. 28/1, Pl.
89/7, 8), serpulids (Pl. 92/1) and cephalopods (Richter
1968).
Particularly favorable conditions for the formation
of intraskeletal geopetal fillings are low sedimentation
rates (e.g. in stratigraphically condensed horizons) and
rapid burial of fossils (e.g. by storm events).
Important internal geopetals in micritic reefal lime-
stones are related to cavities with a flat bottom and ser-
rated roofs, known as 'stromatactis' (Pl. 17/3, Sect.
5.1.5.3). The fine-grained micritic internal sediment in
geopetal cavities consists of micrite, micrite with pe-
loids, or carbonate silt (Aissaoui and Purser 1983). Mi-
crite and peloidal micrite may represent infilled sedi-
ment, but some of these internal 'sediments' are in fact
fine-grained magnesian calcite cements, particularly in
carbonate slope deposits (Wilber and Neumann 1993).
Other micritic infillings represent microbial sediments.
Gray silt-sized internal sediment may correspond to 'va-
dose silt' (Sect. 7.4.2.1). Internal fillings and fine-
grained matrix sediment often differ distinctly in tex-
ture and crystal size (Pl. 28/1).
Plate 17 Geopetal Fabrics of Limestones: TopandBottom Criteria in Thin Sections
Both sedimentary and diagenetic features are used to determine the top and bottom of carbonate rocks. Paleon-
tological criteria are also important (in-situ growth structures, light-dependant algal crusts). Geopetal fabrics aid
in determinating of the stratigraphic order of successions, reconstructing the dips of paleoslopes and reconstruct-
ing of complex reworking and depositional patterns (-> 6). Widely distributed geopetals are particles and mud
trapped in bottom reliefs or in intra- or interskeletal cavities (-> 4, 5), grains deposited on flat surfaces (-> 1, 2)
and normal grain grading with coarse particles at the bottom and fine grains at the top (-> 1). Truncated micro-
cross-bedding and valves deposited with the curvature facing upwards (convex up) are further top-and-bottom
criteria. Abundant geopetals are fossils filled with sediment at the bottom and calcite at the top (-> 3).
1
Grain grading.
Calcareous turbidite. The figure shows the lower and the middle section of a turbidite bed characterized
by significant changes in packing, sorting and grain size. Geopetal fabrics are represented by peloids deposited on the
surfaces of the bivalve shells (lower part of the picture) and vertical grading of lithoclasts and peloids. Note 'normal'
(lower part) and 'reverse' grading of peloids and lithoclasts (upper part), the latter caused by infilling of small grains in
interstices of larger grains. The sample is from a section composed of thin-bedded micritic limestones and calciturbidites.
The turbidite beds consist of peloids, lithoclasts and benthic biota derived from a nearby coral reef and deposited in a
shallow basin. Late Jurassic (Tithonian): Kapfelberg near Kelheim, southern Germany.
2
Deposition of grains on free surfaces.
Oncoid rudstone. Geopetals are represented by the uniform deposition of grains on
flat surfaces of aligned oncoids (arrows). The nuclei of the oncoids are phylloid algae (Pl. 58). Early Permian: Carnic
Alps, Austria.
3
Internal geopetals.
Fenestral mudstone. Geopetals: Basal infilling of silt-sized sediment (arrows) in synsedimentary cavi-
ties (stromatactis type). Peritidal environment. Late Triassic (Hauptdolomit, Norian): Northern Calcareous Alps, Austria.
4
Geopetal infilling.
Storm wave layer consisting of a coquina with brachiopod (B) and trilobite shells (T). Geopetals:
Infilling of fine lime mud within and between the fossils after deposition of the shells. Note the divergent orientation of
brachiopod valves indicating event deposition, and the multiple cementation of large shelter pores (SP). The sample is an
example of a 'storm wave shell concentration' (Fürsich and Oschmann 1993), characterized by good preservation of
skeletal elements, bimodal sorting (complete shells and shell debris), horizontal lateral orientation and absence of signs of
abrasion, bioerosion or encrustation. The limestones originated in subtidal environments between reef mounds (see Pl.
143). Early Devonian (Emsian): Anti-Atlas, southern Morocco.
5
Geopetal deposition
of peloids, fine quartz grains and microfossils in a gastropod shell. Note the compositional coinci-
dence of infill and surrounding sediment. Early Permian: Carnic Alps, Austria.
6
Reoriented geopetals.
Gravity-flow breccia. The geopetal fabric in intergranular voids (black arrows) is distinctly differ-
ent from the original sedimentary geopetal fabric (white arrows). This indicates resedimentation of lithified sediment
subsequent to submarine cementation as part of a megabreccia (Flügel et al. 1993). The pore-filling sequence consists of
a white calcitic crystal seam followed by light gray internal sediments, overlain by micrite and (dark) dolomitized crystal
silt. The remaining pore space was occluded with granular calcite (white). Middle Permian (Murgabian): Sosio, Sicily,
Italy.
