Comparison Between the Early to Middle Cambrian Stratigraphy of the Argentine Precordillera and the Appalachian Margin

The La Laja Formation (Early to Middle Cambrian) is one of the oldest units exposed at the base of the early Paleozoic carbonate platform of the Argentina Precordillera and the key for understanding the early history of the Precordillera terrane. This is important regarding the hypothesis where the Precordillera is conceived as a Laurentia-derived orphan currently located in the southern Central Andes. The La Laja Formation contrasts with the rest of the overlying units of the Cambrian-Ordovician carbonate platform by being partly mixed carbonates–siliciclastics. This dominantly shallow-marine subtidal unit, internally arranged into Grand Cycles, indicates a comparable complex environmental mosaic, with local development of deeper depozones related to variable subsidence as proposed for the Appalachian Cambrian margin.

The detailed stratigraphic and chemostratigraphic analysis in the La Laja Formation allows suggesting a strong similarity with the synchronous interval recorded in the southern Appalachians known as the Conasauga Group (from Tennessee to Alabama). This, points to a comparable geotectonic and depositional setting within this unique time interval. The stratigraphic pattern indicates a progressive stabilization of the margin after the rifting stage that produced the opening of the Iapetus ocean and the final detachment of the Precordillera. During the Early to Middle Cambrian, accommodation was controlled not only by continuous thermal subsidence and eustatism but also by some tectonic component (jerky subsidence). An abrupt change in the depositional pattern is recorded during the Middle to Upper Cambrian boundary, where peritidal dolomites of the Zonda Formation abruptly overlay the La Laja Formation. This is taken to represent the broadening of the carbonate sedimentation and development of a true epeiric sea atop the passive-margin in the drifting Precordillera terrane. A stratigraphic parallelism is observed within the southern Appalachians where the Knox Group dolomites cover the cyclic Conasauga Group.