reef facies

1. Depth transect of an Upper
Triassic (Rhaetian) reef from
Gosau, Austria: Microfacies
and community ecology
Martindale et al., 2013

2. • A depth transect along a lower Rhaetian (Upper
Triassic) barrier reef (Dachstein Limestone
Formation)
• Microbial-sponge-coral barrier reef
• Northern Calcareous Alps- sedimentation on the
NCA passive margin typically arranged in shore
parallel carbonate environment (barrier
reefs/carbonate platforms
Fig. 1. Map of Austria with the locations of major cities (circles) and
theGosausee reef locality (star) marked. Modified from Wurm (1982).

3. Fig. 2. The Gosausee margin of the Dachsteingebirge and sample localities from this study
(transect A–A′–A″ refers to reef cross section in Fig. 3). A) Geological map of the Gosausee
region with sample localities, modified from Krystyn et al. (2009) and Mandl (2001).

4. Fig. 2. The Gosausee margin of the Dachsteingebirge and sample localities from this study
(transect A–A′–A″ refers to reef cross section in Fig. 3). A) Geological map of the Gosausee
region with sample localities, modified from Krystyn et al. (2009) and Mandl (2001).

5. The Gosausee reef
Strong depth zonation
Five classic reef zones:
1. Fore reef
2. Reef Front
3. Reef Crest
4. Back reef
5. Lagoon
Fore reef & reef front:
Gradational transitions between reef zones in these deeper
portions
Reef crest, back reef, and lagoon:
In these shallower portions of the reef, the zones are clearly
resolved based on the fossil assemblage and sedimentology.

6. Fore reef facies
(Samples from sites GS1–GS4)
• An abundance of a mud-rich skeletal
wackestone matrix
• Increases with depth
• Transported reef debris
• Dominance of “Tubiphytes”-like
encrusters (Fig. C)
• Scarcity of thick microbialite fabrics
• Lack of well-developed isopachous
cements

7. Reef front facies
(GS19, GS5–7, GS14, GS11–12, and GS18)
• The patches of coral, sponge, and
microbial framestones mixed with
brecciated and cemented reef debris, and
the compositional transitions as the reef
front shallows
• As the reef shallows up the fore reef and
reef front, the amount of muddy matrix
decreases and is replaced by microbial
fabrics
• As the reef front shallows, more robust
branching corals occur with the highest
diversity of coral species, coral colony
structure and coral growth morphologies
towards the top of the reef front (Fig. D)

8. Reef crest facies
(GS13 and GS8)
Bioconstruction attributed to encrusting
organisms such as encrusting sponges,
microbial crusts and red algae
Key features:
• Thick isopachous cements
• Brecciated intraclasts
• High diversity of sponges and encrusters
• Lack of muddy or fine-grained carbonate
matrix
• Scarcity of corals

9. Back reef facies
(GS9–10, and GS SALM)
Abundance of large, robust phaceloid
(Retiophyllia) coral colonies (More
abundance, less diversity than other
facies)
Distinguishable from the reef crest and
lagoon :
• Lower degree of marine
cementation
• Higher abundance of microbialite
fabrics than the reef crest
Microbialite-coated phaceloid corals
are dominant in the back reef facies

10. Lagoon facies
(GS LAG)
• Dominantly microbial or sponge-
microbial bindstones
• Abundance of microbialite fabrics
in the lagoon
• Lack of breakage and intraclasts
that is characteristic of the high-
energy zone of the reef crest.
• Lacks the large corals that
dominated the back reef facies
• Heavily cemented oncoids-
abundance of cements much greater
than typical for a lagoonal facies
• Possible Triassic environments may
have had thicker cements than their
modern analogues.

11. …then click the placeholders to add your own pictures and captions.
Strong depth zonation in the bioconstructors, reef community paleoecology,
cryptobiont communities, and the inorganic reef fabrics, such as the amount of
sediment, brecciation, and early cementation

12. Gosausee corals exhibit a subtle change in coral ecomorphotype (colony growth
form) with water depth
Shallow water phaceloid colonies (back reef) tend to be thicker and more robust
than the deeper water (fore reef and lower reef front) phaceloid colonies
The phaceloid ecomorphotype is dominant throughout.
Corals are often interpreted to have favored quiet, low-energy waters in the
Triassic
Corals of the upper reef front comprise thamnasterioid, meandroid, and phaceloid
colonies as well as robust branching, and more massive forms
Gosausee corals

13. Conclusion
The facies of the Gosausee reef exhibit strong depth control Five classic reef
facies or zones are identified:
Fore reef: Thin, rare microbial fabrics and a high abundance of mud-rich
wackestones or transported reef debris characterize the deepest fore reef.
Reef Front: As the reef shallows up the reef front, muddy sediments decrease
in abundance and are replaced by microbial fabrics, corals, and cements.
Reef Crest: The shallowest, and highest energy zone is the reef crest, which
features abundant sponges, microbial crusts, and thick, aragonite and high-
Mg calcite cements (intraclast grainstones or heavily cemented bindstones).
Back Reef & Lagoon: Behind the crest, microbially coated phaceloid corals
carpet the narrow back reef facies and grade into the lagoon, which is
differentiated by the lack of large coral colonies, an abundance of oncoids or
microbial-sponge bindstones and thick, early aragonite and high-Mg calcite
cements

14. Conclusion
Gosausee reef exhibits some intriguing departures from canonical
Dachstein reef ecology :
• Microbialites in shallow water
• Few corals on the reef crest
• Abundant sponges in shallow water
Variations suggest the need to revisit some previous assumptions
about ecological zonation in Triassic reefs