Reconstructing late Early–Middle Jurassic environments, ecosystems and climate based on palynological and sedimentological data of the Los Molles Formation, Neuquén Basin, northwestern Patagonia, Argentina

Abstract

The Los Molles Formation (Early–Middle Jurassic) in the Picún Leufú Sub-basin, Neuquén Basin, Argentina, is a clastic unit accumulated in a marine-delta system. Its deposits are rich in organic matter, and provide a detailed record for palaeoenvironmental reconstruction. The present contribution integrates sedimentological and palynological data from the Painemilla section (0–100 m interval) to infer palaeoenvironmental, palaeoecological, and palaeoclimatic conditions. The palaeoenvironment evolved from a hyperpycnal littoral delta during wetter periods to a marine littoral delta system during drier ones. The five identified palynofacies-types show a high terrestrial input due to the predominance of phytoclasts. PT-1, 2 and 5 are associated with light hyperpycnites, while PT-3 and 4 characterise offshore/prodelta deposits. Sporomorph assemblages dominated by Hirmeriellaceae and Araucariaceae are consistent with a seasonally dry (winterwet) biome. The highest values of Araucariaceae show a good correlation with the parasequences, representing the maximum water input into the basin. The Callialasporites complex record suggests an age no older than late Toarcian. A Sporomorph Ecogroup (SEG) analysis is applied for the first time in the Los Molles Formation. Principal Components Analysis points to variations in palaeoecology and the taphonomic component in the record (water/wind-transport taxa). This analysis reveals an active participation of the Coastal (Hirmeriellaceae), Lowland (Araucariaceae and Umkomasiaceae) and Riverside (Caytoniaceae) SEGs throughout the entire section, showing an increase in the Upland SEG supply towards the upper part of the section. This study highlights the role of light hyperpycnal flows in the sedimentary and palynological records, providing new insights into Jurassic palaeoenvironmental and ecological dynamics in the Neuquén Basin.