MATERIALS AND METHODS

Our analysis is based on the study of approximately 4,560 m of Upper Cambrian-Ordovician strata. The Lampazar Formation is exposed in the western region of Cordillera Oriental (Figure 3.1), while the Santa Rosita is exposed in the eastern region (Figure 3.2) and the Santa Gertrudis Formation in the southern margin of Cordillera Oriental (Figure 3.3). The Acoite Formation is exposed in both the western and eastern regions of Cordillera Oriental (Figure 3.1-3.2). Sections were measured in various localities of Cordillera Oriental (Table 1).

Fieldwork activities are part of a long-term project dealing with the integration of sedimentologic, ichnologic, and sequence-stratigraphic data in order to characterize the early Paleozoic depositional evolution and macroevolutionary events in the Northwest Argentina Basin. Activities include basic mapping and standard sedimentary facies analysis based on bed-by-bed measuring of stratigraphic sections. Facies analysis was based on careful characterization of lithology, physical sedimentary structures, bed boundaries, and geometry. Ichnologic analysis involves trace-fossil sampling; recognition and identification of the ichnofossils present; measurement of abundance and distribution of individual ichnotaxa; measurement of degree of bioturbation; estimation of ichnodiversity; identification of trophic types and ethologic groups; reconstruction of tiering structure, and relationships among trace fossils, physical sedimentary structures, and bedding types. Photographs were taken to document the most significant sedimentologic attributes and those trace-fossil specimens that were not collected. Laboratory work was aimed to refine ichnologic observations performed in the field (e.g., study of polished slabs). In addition, we have compared our results with trace-fossil information from other areas in Gondwana, Baltica, and Laurentia based on a literature survey and a review of specimens housed at the Geological Institute of the University of Tübingen. Additional observations were made in the field in the Ordovician of the High Atlas of Morocco.

For this specific study, we have focused our analysis on deposits that, on sedimentologic criteria (see below), are identified as formed in the upper offshore. By restricting our study to a single bathymetric zone, we avoid comparing ichnofaunas formed under different environmental conditions and can, therefore, detect evolutionary controls. We follow the environmental zonation of MacEachern et al. (1999), and consider fairweather wave base as the base of the shoreface, and storm wave base as the base of the lower offshore. Within this scheme, the region between the fairweather wave base and the storm wave base is subdivided into offshore transition, upper offshore, and lower offshore, based on sedimentologic attributes (e.g., sandstone/mudstone ratio, tempestite architecture).

To evaluate the degree of infaunalization in lower Paleozoic upper-offshore deposits, we have framed our observations within the ichnofabric approach sensu Bromley and Ekdale (1986). An ichnofabric refers to any aspect of the texture and internal structure of a substrate resulting from bioturbation and bioerosion at any scale (Bromley and Ekdale 1986). As biogenic structures are almost always in situ records of behavior, ichnofabric analysis may provide important information on the ecology of endobenthic communities (e.g., Wetzel 2008). As part of analysis of the ichnofabric, the degree of bioturbation has been estimated, using the scheme of Taylor and Goldring (1993). This scheme comprises seven grades of bioturbation, ranging from 0 for unbioturbated sediments to 6 for those that have suffered complete bioturbation.