Renalcys in the Laurentian derived Middle Cambrian rocks of the Argentine Precordillera: morphologies, distribution and stratigraphic position.

GOMEZ, F.J., FOGLIA, R., Y ASTINI, R.,A.

Belem (Brasil)

Congreso; III Latin American Congress of Sedimentology; 2003

The La Laja Formation is the lowermost unit of the Cambro-Ordovician passive-margin carbonate sequence of the Argentine Precordillera, considered to be an exotic terrane accreted to the western Gondwana margin in Middle Ordovician time. It outcrops along the Eastern Precordillera morphostructural province, between 31º-32ºS latitude in the foothills of the Andes, San Juan Province. Faunal arguments and strong stratigraphic similarity argue in favor of Laurentia being the parent continental mass from where the Precordillera broke apart. Recent surveys in the region and detailed logging through the La Laja Formation in both the Sierra Chica de Zonda to the south of San Juan city and the Sierra de Villicum to its north, have provided new material and data which allow further comparisons with the sedimentary successions that characterize the low latitude passive marign carbonate banks of Laurentia. In this abstract we report the first finding of Renalcis, presently regarded as a morphological generic group of "calcareous algae" or cyanobacteria (coccoid cyanobacteria) of paleontological and sedimentological importance. These rock-formers are also of importance from a paleogeographic veiwpoint due to their close association with reefs (bioherms) of Cambrian age commonly developed surrounding Laurentia and Siberia. This continental masses were by that time located at low latitudes. Modern classifications of these extinct groups are based on comparative morphology and also on evaluations of their affinity based on similarities to Recent algae. The small size of these forms makes them susceptible to diagenetic alterations, being best known from cratonic areas with less overburden and relatively undisturbed.   The La Laja Formation (560 m) can be divided into five members recognized from their contrasted stratofabrics and slightly different composition, reflecting overall contrasted depositional environments within a carbonate attached platform succession, separated from the craton by an internal detrital belt. Renalcis herein described appears in the uppermost section, recently included in the Las Torres Member (~60 m). This dark gray member is composed of variously bioturbated thin-bedded stylo-nodular mudstones and wackestones. Thin- to medium-bedded tabular layers of intraclastic, oncolitic and bioclastic packstones, with sharp bases and gutter-cast development, frequently punctuate the lower section, whereas brecciated horizons (exposure breccias) and thick packages of cross-bedded oolitic grainstones are common toward the top, in the transition with the overlying Zonda Formation. The latter shows an increasing amount of thick-bedded yellowish dolomites with abundant domal stromatolites. Within the middle section of the Las Torres Member (within the bioturbated mudstones and wackestones) two meter-scale horizons with abundant renalciform aggragates were detected. This interval of maximum 5 m thick can be traced laterally for more than 20 kms along the west slope of the Sierra Chica de Zonda. When observed at detail in the type section they form two relatively massive matrix-rich individual beds of 2.0 and 0.5 m thick that exhibit crude internal lamination. Few more distinct muddier intervals (0.01-0.05 m thick), with sparse non-oriented trilobite shells are bounded by stylolites, but could coincide with internal hardground surfaces. A mosaic of mostly shallow subtidal environments are represented in the upper Las Torres Member, ranging from well-defined storm-influenced platform facies to oolitic shoals and with some subaerial exposure as well that contrast with peritidal cycles in the above Zonda Formation.   Renalcis are regarded as clusters of thick-walled botryoidal chambers of various particular morfphologies embracing end terms from saccate to very irregular masses, sometimes ill defined and hence, described as clotted. In our examples thally (used here for convention and convenience to refer to the cluster without implying that it represents a single plant) range from less than a mm to few millimeters (exceptionally reach 1cm) and usually do not represent chambers, but more irregular reniform sacs. Individual lobes range from ~150-300 mm. Few real ramifying elongate sacs comprise an uncommon morphotype. Concave-curved V-shaped invaginations pierce their thick recristallized walls giving the appearance of botryoidal aggregates. Bare-eye or with little augment aggregates are frequently subrounded and resemble oncoids. However, they do not classify as coated grains. Renalcis is usually found in growth position in Cambrian age bioherms ranging from patch reefs in shelf interiors to buildups along shelf margins. Contrary to usual our findings do not form evident buildups, however they may be regarded as essentially in place calcareous deposits created by sessile organisms and may have an important environmental meaning. Considering this more broad definition although largely biostromal in appearance they would classify as low-relief mud-supported spaced cluster reefs (unit distance³1). Alternatively, they could represent important destructional events.