Freshwater Swamps at the Latest Danian Event Hyperthermal, Hooper Formation (Wilcox Group), Bastrop County, Texas

Hooper Formation (Wilcox Group) deposits at McKinney Roughs Nature consist of siltstones, carbonaceous siltstones and very fine-grained sandstones, with an estimated total thickness of 99 m. A 6-m-thick fine-grained sandstone within this succession includes wavy and flaser bedding from a tidally-influenced depositional setting. Prolific and widespread crayfish colonies in the upper 30–40 cm of this sandstone indicate subsequent freshwater flushing during prolonged subaerial exposure prior to deposition of a paleomire.

Overlying claystones, carbonaceous siltstones and lignites up to 32 m thick are poorly exposed except for the lowermost 5 m. Palynomorphs from this lower part include common to abundant pollen and spores with abundant and diverse freshwater algal cysts, with no marine forms. Sparse rhizoliths extend down from a 2-m-thick lignite into a dark grey silty underclay. An ashfall deposit (now a tonstein) 50 cm above the base of this lignite contains intact and fragmentary fossil leaves, bark and wood fragments. Tonstein thickness ranges from 3 to 20 cm thick at five locations, indicating ash transport in shallow moving water in a swamp or freshwater marsh.

Sanidine crystals from this tonstein, 62.08 ± 0.19 Ma old, provide the first radiometric age date for the Gulf Coast onshore Wilcox and the first chronostratigraphic tie-point within the lower Wilcox, with major implications for regional correlations. Freshwater siltstones and lignites enclosing the tonstein represent the first terrestrial record of the Latest Danian Event (62.18 to 61.99 Ma).