A Glimpse Into the Cenomanian: Palynology of the Arlington Archosaur Site, Late Cretaceous Western Interior Seaway, Texas, USA

Abstract

The Arlington Archosaur Site (AAS) between Dallas and Fort Worth, Texas, is known as a rich fossiliferous section. The age of these rocks is generally considered to be mid Cenomanian, but conflicting evidence suggests the age may be as young as the late Cenomanian early Turonian. To address the issue, a palynological study was designed and conducted based on the close sampling of the lithofacies. Palynological samples were processed according to the standard acid preparation. The study was quantitative and focused on associations to determine the paleoenvironment, paleoclimate, biostratigraphy, and age of exposure. The rich palynological assemblages comprise spores from seedless plants, gymnosperms, angiosperms, fungi, algae, and dinoflagellate cysts. Bryophytes were abundant mainly in Facies A and B, with Zlivisporis cenomanianus taking over the bryophytes' habitat in Facies D. Lycophytes abundant in the alluvial and coastal plains are considered to have been transported. Conifers were the predominant group of gymnosperms, also mainly transported into the section. Freshwater algal remains include Schizophacus laevigatus/Ovoidites parvus, Schizosporis reticulatus, Botryococcus sp., and Pediastrum sp. Acanthomorph acritarchs present in low abundance and diversity appear following shallow marine dinoflagellates' spikes and before freshwater colonial algal spikes. The vegetation signal at Noto's Facies A and B indicates tropical to subtropical shallow marine to coastal plains, while Noto's Facies D indicates tidally influenced areas. Also, picks of the diversity and abundance of dinoflagellate cysts are interpreted as an increased marine influence and proposed as possible flooding surfaces. The results support the alternation of marine incursions within deltaic and floodplain sequences, related to regional climate oscillation that affected the vegetation on the upland drainage area. Key palynological markers point to an early Late Cenomanian age, and the presence of the Cyclonephelium compactum - C. membraniphorum (Ccm morphological plexus) signals that the incursion of boreal waters during the Plenus Cold Event of the Ocean Anoxic Event 2 may have reached as far south as the AAS area. This coincides with vegetation trends that suggest a cooler and less humid climate at the start of Facies A, where Ccm is more abundant.