TIMING RECOVERY AFTER THE CRETACEOUS/PALEOGENE BOUNDARY: EVIDENCE FROM BRAZOS RIVER, TEXAS

The Cretaceous/Paleogene boundary successions in Texas and Alabama provide a sedimentary record of events relatively close to the Chicxulub impact site. Recent work in both areas has shown that there was a single ‘impact’ event that is coincident with the extinctions of planktic foraminifera and calcareous nannofossils, although the dinoflagellate cyst community was little affected. The benthic foraminifera in the Texas successions are, remarkably, little affected, with many taxa being found in both the Corsicana Mudstone Formation (uppermost Maastrichtian) and the Kincaid Mudstone Formation (lowermost Paleocene). In the sediments just above the erosive surface that marks the ‘impact’ event (and the K/Pg boundary) there are large, beautifully-preserved, benthic foraminifera, including nodosariids <1.5 mm in length and lenticulinids <1.5 mm in diameter. As Lenticulina rotulata Lamarck occurs throughout, this taxon has been used for stable isotope analysis (δ¹⁸O and δ¹³C) in a range of different size fractions. The results show both a consistent variation in oxygen and carbon isotope values with size as well as a distinct cyclicity which reflects an astronomical tuning. It is possible, therefore, to use these precession cycles to determine the probable duration of zones P0 (~30,000 yrs) and Pα (~75,000–80,000 yrs), and the timing of biotic recovery following the ‘impact’ event. The large, negative δ¹³C excursion in the lowermost Paleocene that is recorded in distal sites (e.g., Stevns Klint) is not evident, probably because the reworked spherule-rich bed and the sandstones of the ‘Event Bed’ represent a disturbed environment in which the stable isotope signal has been lost. The size of the stable isotope excursions (close to the base of zone P1a) is indicative of the Dan-C2 and the Lower 29n hyperthermal events, allowing direct correlation with the two other locations where these have been described; most notably in the Gubbio succession where there is also a good record of the magnetostratigraphy and comparable biostratigraphy.