CONICET | Buscador de Institutos y Recursos Humanos

The Precambrian-Cambrian sedimentary sequences of northwest Argentina are represented by the Puncoviscana Formation and equivalents, which are widely distributed in Cordillera Oriental and Northern Pampean Ranges, Argentina. This unit represents a passive margin basin formed from the breakup of Rodinia that changed into an active continental margin basin consequence of collision of the Arequipa-Antofalla terrane during the Early Cambrian. The sedimentary sequence provides an important record of trace fossils and synsedimentary volcanism. Palaeoenvironment of the Puncoviscana Formation has traditionally been interpreted as deep turbidites contemporaneous with shallow carbonate platform sequences to the east. New paleoenvironmental interpretations, presented herein, arise from multidirectional paleocurrents, trace fossil bathymetry and diverse subfacies containing a diagnostic suite of shallow water sedimentary structures. Caution should be exercised for deciphering paleoenvironments, particularly for Proterozoic rocks, where one has to depend only on physical sedimentary structures. Comparison with modern analogs helps understanding processes and flow characteristics and these resemble intertidal and subtidal storm-influenced environments. The most intensively studied modern tidal environments are tidal flats, tidal inlets, deltas and subtidal shelf banks, with attention focused on siliciclastic tidal flats of North Sea, Gulf of California, Bay of Fundy and Hudson Bay in Canada; Gironde estuary, Mont Saint-Michel Bay in France; Bohai Bay and West Yellow sea in China; Pacific coast of Japan and Bay of Bengal in India. Sedimentary structures in Puncoviscana Formation include ripples of diverse forms (linguoid, linear crested with symmetric and asymmetric surfaces, double crested ripples with mud drapes, interference ripples). Subfacies association suggests that these bedforms developed in intertidal settings by ebbing tides. Rhomboidal ripples associated with 3-D ripple migration structures are common when there is a strong effect of swash/backwash system of waves. Asymmetrical ripples in two successive levels, with different orientations to demonstrate multidirectional flows affected by tides and coastal currents possible driven by wind. Up-sequence, tidal rhythmites with mud/silts couplets, isolated ripple trains (subordinate), fining upward layers and wide variety of trace fossils indicate a tidal flat system with high suspended load and quite environmental conditions for organism development. Parallel lamination with uniform and non-uniform lamina in sandy layers and hummocky cross stratification are common in modern wave-dominated flats affected by storm episodes. Presence of tidal rhythmites, herringbone cross stratification, wrinkle and rill marks at the bottom of the profile are found in tidal flats with sub-areal exposures in the intertidal zone. Those paleoenvironmental changes are thought to be related to seasonal variation (summer/winter and storms) when processes and hydrodynamical change. Facies analysis carried out in Rancagua and Palermo Oestelocalities allows recognition of shoreface to open shelf environments, with transition from wave- to tide-dominated systems possible related to seasonal cycles occurred during basin deposition. Harmonic analysis on tidal rhythmites allow recognition of four major cycles due to its spectral power: 2.03, 26.64, 50.33 and 119 events (tidal days)/cycle. These can be interpreted as representative of ancient tidal periods including semidiurnal tides, the tropical month, seasonal episodes (possible storm episodes) and a semiannual cycle, respectively. This last could represent variation in sedimentation between winter and summer seasonality, which was manifested in the sedimentary profiles.