Climate and sea‐level changes across a shallow marine Cretaceous–Palaeogene boundary succession in Patagonia, Argentina

Abstract

Upper Maastrichtian to lower Paleocene, coarse‐grained deposits of the Lefipán Formation in Chubut Province, (Patagonia, Argentina) provide an opportunity to study environmental changes across the Cretaceous–Palaeogene (K–Pg) boundary in a shallow marine depositional environment. Marine palynological and organic geochemical analyses were performed on the K–Pg boundary interval of the Lefipán Formation at the San Ramón section. The palynological and organic geochemical records from the San Ramón K–Pg boundary section are characteristic of a highly dynamic, nearshore setting. High abundances of terrestrial palynomorphs, high BIT‐index values and the occasional presence of plant fossils are indicative of a large input of terrestrial organic material. The organic‐walled dinoflagellate cyst (dinocyst) assemblage is generally dominated by Senegalinium and other peridinioid dinocyst taxa, indicative of high‐nutrient conditions and decreased salinities, probably associated with a large fluvial input. The reconstructed sea surface temperatures range from 25°C to 27°C, in accordance with the tropical climate inferred by palynological and megafloral studies. As in the Bajada del Jagüel section, ~500 km north‐north‐east of San Ramón, peaks of Senegalinium spp. were recorded below and above the K–Pg boundary, possibly related to enhanced runoff resulting from more humid climatic conditions. The lithological, palynological and organic geochemical records suggest the occurrence of a sea‐level regression across the K–Pg boundary, resulting in a hiatus directly at the boundary in both sections, followed by a transgression in the Danian.

The fossil flip‐leaves (Retrophyllum, Podocarpaceae) of southern South America

“…The new species demonstrates that the divergence of Retrophyllum from its sister clade comprised of Afrocarpus and Nageia , or, very conservatively, from Podocarpus (see Introduction), must have occurred at least by the K–Pg boundary (66 Ma). This finding is concordant with recent molecular divergence dating of the Retrophyllum – Podocarpus split at ∼85 Ma (Quiroga et al, 2016), while otherwise the same node has been considered Cenozoic (Biffin et al, 2011, 2012; Crisp and Cook, 2011; Leslie et al, 2012, 2017). The Quiroga et al (2016) estimate was calibrated using the then‐undescribed 52.2 Ma occurrence (citing Wilf, 2012) of what is here fully presented as R. spiralifolium , along with several other recently discovered (or recently well‐dated) fossil podocarp occurrences from Patagonia that were not used in the other molecular studies.…”

“…Our handheld GPS trace along the main productive exposure of the LefE bed extended ∼40 m along strike on a 108° bearing, from S 42.68366°, W 69.83430° to S 42.68378°, W 69.83382° (WGS 1984 datum). Biostratigraphic data from dinoflagellates, palynomorphs, and marine invertebrates collectively place the locality within the last 1 Myr of the Maastrichtian, 67–66 Ma (Medina et al, 1990; Barreda et al, 2012a; Scasso et al, 2012; Aberhan and Kiessling, 2014; Vellekoop et al, 2017). The K–Pg boundary clay is not preserved in the area, because of bioturbation or regression (Scasso et al, 2012; Vellekoop et al, 2017).…”

“…The Lefipán Formation is a suite of deltaic deposits from a paleo‐Atlantic embayment in northwestern Chubut that is exposed close to Laguna del Hunco (Scasso et al, 2012; Vellekoop et al, 2017). The single Lefipán Formation fossil considered here (Figs.…”

Araucaria lefipanensis(Araucariaceae), a new species with dimorphic leaves from the Late Cretaceous of Patagonia, Argentina

“…This age was confirmed by terrestrial pollen and spore assemblages (Baldoni, ), and more recently and in more detail by Barreda et al. (), who showed a clear Maastrichtian‐Danian transition associated to dinoflagellates (see also Vellekoop et al., ).…”

Agathistrees of Patagonia's Cretaceous‐Paleogene death landscapes and their evolutionary significance

“…These three localities (LefE, LefW, and LefL of Donovan et al., ; GPS coordinates available upon request to MEF collection staff) are approximately coeval and yield fossil leaves from horizons that are stratigraphically located from 8–24 m below the K–Pg unconformity (Barreda et al., ; Scasso et al., ; Donovan et al., ). Biostratigraphic indices include the presence of the Danian Turritella marker bed as well as pollen and dinoflagellate markers for the uppermost Maastrichtian and lowermost Danian (Barreda et al., ; Scasso et al., ; Vellekoop et al., ). The fossil leaves studied here belong to the latest Maastrichtian portion of the Lefipán, 67–66 Ma.…”