Latest Bajocian Bio-Events of Ammonite Immigration and Colonization in the Tarapaca Basin (Northern Chile): Palaeoenvironmental Implications for Sequence Stratigraphy

Abstract

The uppermost strata of the Torcazas Formation from Quebrada San Pedro and Caracoles outcrops, Comuna Sierra Gorda, Region de Antofagasta (Chilean Precordillera), over a distance of 20 km, have yielded latest Bajocian ammonite fossil assemblages characterized by the dominance of juvenile individuals belonging to endemic or pandemic taxa. Also found are very scarce preadult individuals of dimorphic taxa previously unknown in circum-Pacific basins but relatively common in West Tethyan areas, such as the morphoceratids Dimorphinites [Macroconch]–Vigoriceras [microconch] and the strigoceratids Strigoceras [M]–Cadomoceras [m]. This report presents the main taphonomic, palaeoecological, and palaeobiogeographical observations related to these recent palaeontological findings and their implications for sequence stratigraphy. The maximum deepening, relative sea-level rise, and oceanic accessibility of a Bajocian–Bathonian, second-order, transgressive/regressive facies cycle in the marine, back-arc Tarapaca Basin were reached during the latest Bajocian Parkinsoni Biochron.