A new macroflora from the Turonian–Coniacian Moreno Hill Formation, New Mexico, USA

Abstract

The Moreno Hill Formation of the Salt Lake Coal Field (New Mexico, USA) preserves terrigenous sediments deposited from ∼90.9 until after 88.6 Ma, a critical window of climatic recovery following the ∼94.5 Ma peak of the Cretaceous Thermal Maximum. Recent fieldwork uncovered new macrofloral assemblages: a lower member flora derived from a paralic delta plain, with carbonaceous, pedogenetically altered floodplain sediments indicating often waterlogged conditions, and an upper member flora uncovered from slightly drier, more distal floodplain sediments. We report 18 distinct foliar morphotypes from the study of 116 leaf specimens. Dicot angiosperms are the most diverse in both assemblages, including platanoids, Saliciphyllum, and Cinnamomoides. While only two dicot morphotypes are shared between the lower and upper assemblages, two conifers (Brachyphyllum and taxodiaceous Cupressaceae indet.) are found in both. The fossils at both sites were collected from overbank deposits and lack evidence for significant transport; thus, we infer that the floodplains adjacent to channels during this interval were occupied by a mixed angiosperm-conifer woodland ecosystem. Although the two assemblages are separated by ∼1.5 to 2 million years, they reflect similar floodplain woodlands on a changing landscape in which the Western Interior Seaway was regressing and the local conditions drying. Sedimentological, leaf physiognomic, and wood growth-ring data collectively support persistent warm, yet gradually more seasonal conditions after the thermal maximum. This first report of Moreno Hill Formation leaf macrofossils establishes a regional Turonian–Coniacian palaeobotanical baseline for future studies of vegetation and ecosystem change in the southern Western Interior.