ABSTRACT

Exposures of the Aguja Formation (84 to 74.5 m.y.a.) at Big Bend National Park contain deltaic sequences deposited on the southwestern margin of the Western Interior Seaway. Nearshore-marine, shelf sandstones interfinger with floodplain mudstones, recording multiple regressive-transgressive cycles. Palynomorphs, previously undescribed, occur in regressive depositional phases and are preserved in brown to black organic-rich, dinosaur-bearing shales or lignite beds within sandstone horizons.

Samples from five stratigraphic sections in the western part of Big Bend National Park contain vitrinite, wood fragments, tree resin, dispersed leaf cuticle, petrolitic filaments, pollen/spores, fungal hyphae, and dinoflagellates. Numerous and varied pollen/spore grains suggest that the region supported a rich and diverse tropical to subtropical flora. Dry seasons are indicated by abundant thin (2 to 5 micrometers thick) dispersed cuticles. Quantitative analysis of 65 samples revealed 43 genera of angiosperms, 11 genera of gymnosperms, 36 genera of ferns, 2 genera of freshwater algae, 12 genera of dinoflagellates, and 8 genera of fungi. Common genera are Aercipites, Liliacidites, and Palmaepollenites (monocotyledons); Momipites, Plicapollis, Vitipites (dicotyledons), Cycadopites, and Inaperturopollenites (gymnosperms); and Cyathidites, Granulatisporites, and Gleicheniidites (ferns). Coastal marsh deposits possess the richest assortment of fungi, ferns, gymnosperms, tricolpate and triporate angiosperms; intertidal nearshore deposits contain dinoflagellates, and fluvial-deltaic deposits contain a major component of dispersed leaf cuticles and a minor component of dicotyledon pollen.