Abstract

CARRERA, Marcelo G  and  CANAS, Fernando L. Los biohermos de la Formación San Juan (Ordovícico temprano, Precordillera Argentina): paleoecología y comparaciones. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.2, pp.85-104. ISSN 1853-6360.

The San Juan Formation is a thick sequence of limestones in the Ordovician of the Argentine Precordillera. Two distinctive biohermal horizons occur. The lower horizon (Late Tremadoc) is characterized by a four meter thick complex of reef-mounds built by lithistid sponges, the receptaculitid Calathium and microbial benthic communities. The reef-mounds exhibit two main rock types: (a) massive skeletal and lithoclastic wackestone/packstone, mostly of sponges and Calathium with Girvanella tubules encrusting and binding sponges and bioclasts; lithoclastic wackestones to grainstones were deposited on the irregular mound surface. (b) microbial boundstones, including stromatolitic structures, non-laminated peloidal micritic boundstone and Girvanella crusts. The reef-mounds are dissected by conspicuous channels filled with coarse crinoidal grainstone and lithoclastic rudstone. The upper horizon (Late Arenig) exhibits three types of reef structures: patch-reefs with an organic framework of stromatoporoids, biostromal accumulations of stromatoporoids, algae and sponges, and stromatoporoid-algal mounds. The patch reefs form structures up to ten meters in diameter and 5 meters height with a framework of often more than 90 % globular stromatoporoids in the nucleus and a combination of laminar and globular types in the flank facies. Laterally, the reefs interfinger with high energy deposits (pelmatozoan grainstone/packstones). The biostromes and the stromatoporoid-algal mounds are combined structures with varying amounts of frame-building organisms. Stromatoporoids, lithistid sponges, Calathium and microbial structures occur. Stromatoporoids are the major components of the biostromal accumulations. In these structures the laminar and globular stromatoporoids dominate over the algae and the lithistid sponges. Grainstone and packstones with cross-bedding and channeled structures are the most common lithologies. The stromatoporoid-algal mounds are mainly composed by microbial structures, stromatoporoids and Calathium but also show isolated and scarce lithistid sponges. The most common lithologies in the mounds are wackestone/packestones baffled and bound by the stromatoporoids, microbial comunities, Calathium and lithistids. These structures are developed in and above coarse bio/lithoclastic grainstones. According to the sedimentological and paleontological features, an inner ramp setting has been suggested for all the reef structures. The change in the community composition, especially the constructor guild, was analyzed taking into account local sedimentological variations or biotic interactions among the principal reef builders. The reef-mounds at the base of the formation are dominated by lithistid sponges, while the patch-reefs in the upper horizon were constructed entirely by the stromatoporoids. The paleoecological requirements of both groups may answer the question of whether the biotic, physical or both factors influenced the distribution of these reef structures. The lithistid sponges are solitary organisms, which obtain theirs nutrition from suspended organic matter and bacteria. This heterotrophic condition suggests a preference for nutrient-rich waters. Several authors have pointed out the generally low turbulence and soft substrate settings of these sponges. However the sedimentological features of the sponge-algal mounds from the Precordillera basin suggest a hard substrate (tidal rock terrace) and high energy conditions. The stromatoporoids are clonal organisms with no clear feeding strategy, a mixotroph condition has been suggested by many authors via a symbiotic relationship with cyanobacteria, as well as most of the modern reef building sponges in oligotrophic waters. A trophic analysis of the stromatoporoid patch-reef community shows a very low proportion of primary producers. Taking into account that the benthic macroalgae are almost absent and the phytoplankton and the benthic microbial communities supply a maximum of 40 % of the total primary production, extra photosymbiotic nutrition is suggested in order to maintain the high rate of skeletal production. A hard substrate and very shallow conditions are inferred for these organisms. Although there are local variations, an inner ramp setting is inferred for all the reef structures present in the San Juan Formation. A minor gradient is inferred from the shallower stromatoporiod patch-reefs, the sponge-algal reef-mounds to the combined structures (stromatoporoid-algal mounds or biostromes). It is difficult to explain the important change in the entire reef community considering only local variations of water motion. According to the trofic strategies of the reef-building groups, nutrient fluctuations are suggested as the main factor that controls the distribution of and the change in reef communities. The development of the lower reef horizon follows a Late Tremadoc transgression, representing the first appearance of an open marine fauna. The restricted platform developed in the underling La Silla Formation changes to an open carbonate ramp after the Late Tremadoc transgresive event. These important changes in the basin may have contributed to the runoff of nutrient-bearing currents from nearby land masses and favored the upwelling of nutrient rich waters from the deep basin. The widespread ramp system present at the time of the stromatoporoid-reefs developed and the comparatively minor rise in sea level, may have helped to diminish the amount of nutrients, at least in the inner ramp environment. The stromatoporoid-algal mounds may have developed in a slightly deeper or more protected environment with relatively high amounts of nutrients.

Keywords : Bioherms; San Juan Formation; Ordovician; Facies; Paleoecology.

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