Upper Cretaceous–Paleocene conifer woods from Big Bend National Park, Texas

Abstract

Conifer wood types from the Upper Cretaceous Aguja and Javelina Formations and the Paleocene Black Peaks Formation of Big Bend National Park, Texas, are briefly described. The Big Bend conifer woods represent the largest assemblage of late Cretaceous conifer wood thus far described from the western interior of North America, and include samples with characteristics of the Araucariaceae and Cheirolepidiaceae, and Cupressaceae and Podocarpaceae. Cupressaceae/Podocarpaceae types of the Maastrichtian Javelina Formation have, on average, narrower rings than those of the Campanian Aguja Formation, consistent with the drier climate already proposed for the Javelina Formation. Angiosperm wood assemblages differ between the lower shale and upper shale members of the Aguja Formation, the Javelina Formation, and the Black Peaks, so do the conifer assemblages. The Big Bend conifer woods differ from those described from other western interior localities and document more variation in growth ring types than previously recognized for the whole of the northern hemisphere Late Cretaceous. The sizes of some logs and width of their growth rings suggest that some trees might have reached diameters of 1 m in approximately 80 years. Woods of the Cupressaceae/Podocarpaceae type show that the strategy of having juvenile wood with narrower tracheids and less distinct growth rings than in mature wood occurred in the Late Cretaceous. The incidence of compression wood in mature trunk wood is relatively high and may reflect either unstable substrates or frequent storms with high winds.