Depth transect of an Upper Triassic (Rhaetian) reef from Gosau, Austria: Microfacies

and community ecology

Martindale et al., 2013

• 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).

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 Gosauseeregion with sample localities, modified from Krystyn et al. (2009) and Mandl (2001).

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 Gosauseeregion with sample localities, modified from Krystyn et al. (2009) and Mandl (2001).

The Gosausee reef Strong depth zonationFive classic reef zones:

1. Fore reef2. Reef Front3. Reef Crest4. Back reef5. 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.

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

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)

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

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

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.

…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

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

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

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