Ichnofabric patterns in shallow-marine carbonate tidal flats of the Middle Cambrian in the Precordillera.

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

San Miguel de Tucumán

Congreso; IV Reunión Argentina de Icnología y II Reunión de Icnología del Mercosur; 2001

Ichnofabric patterns have not been appropriately described from the Argentine Precordillera where an exceptional Early Cambrian-Early Ordovician carbonate succession of Laurentian affinity is exposed. A semiquantitative methodology based on ichnofabric indexes (ii) is used to recognize the amount of biogenic disruption and reworking before cementation of carbonate muddy substrates from the Rivadavia Member in the La Laja Formation, San Juan Province. Environmental analysis is based on field observations of physical sedimentary features and facies analysis. The studied section outcrops throughout the Zonda Range and is composed by a rhythmic interval of about 150 m thick of thin bedded alternations of slightly dolomitic light-gray fine-grained carbonate silts with discernable sedimentary structures and dark gray carbonate muds. The heterolithic couplets yield 1-5 cm in average thickness with a thicker muddy term. When mixed by infaunal activity the two terms are not properly recognized, however, the ichnofabric pattern is diagenetically enhanced by the contrasting colors. It is this contrast (representing variable bioturbation) which usually allows separating different beds. Unfortunately, late diagenetic features and pressure-solution seams bound many beds yielding a stylo-bedding pattern and preclude determination of primary thickness. However, primary mud cracks are apparent and well represented toward the base. Interbedded intraclastic packstone-wackestone lenses represent shallow channel fills due to cut & fill effects related to storm activity. This member has a remarkable lateral continuity along the Sierra Chica de Zonda and is devoid of siliciclastics. Extent and depth of bioturbation was examined throughout. Gradual to pervasive bioturbation variably affects the thin-bedded tidal rhythmites and allow interpreting substrate conditions, rates of bioturbation-sedimentation, and bottom energy and oxygenation. No beds completely homogenized (ii 6) or with no bioturbation (ii 1) were recognized. Most beds have been reworked by poor, moderate, high or intense bioturbation (respectively, ii 2, 3, 4 & 5). Tiering is not apparent in the carbonate softgrounds however, slightly more cohesive and hardened substrates in the transition layer (firmgrounds) and 3D exposures allow recognition of superimposed bioturbation patterns. Although no ichnogeneric name was applied to discrete trace fossils these assemblage postdates early soft-sediment bioturbation. Poorly developed three dimensional tube-like galeries (with circular to subcicrular cross sections) and no apparent spreiten fill can be recognized. Distinct burrows are not strictly horizontal and yield 0.5 cm in average diameter. This study shows a greater extent and depth of bioturbation than previously recognized with patterns more similar to those developed later during the Upper Cambrian and Early Ordovician. Amount of bioturbation in the tidal flat environment is comparable to that defined in inner shelf facies by previous workers. Depth of bioturbation is of at least 5 cm due to good control on historic preservation. Hence, we suggest a better development of the infaunal biological benthic boundary layer and earlier radiation of soft-bodied burrowing organisms colonizing the infaunal ecospace in marine carbonate tidal flats. Considering the environment, this means that by Middle Cambrian incipient tiering had already occurred among soft-bodied infaunal detritivorous organisms, probably related to environmental pressure and opportunistic strategies. Moreover, proper recognition of ichnofabrics allows ruling out thrombolitic fabrics seldom confused with bioturbation patterns in Cambrian-Ordovician carbonates of the Precordillera.