138-4 Distribution of Spherules and Foraminifera in the Cretaceous/Paleogene Ejecta Layer in Ocean Drilling Program Hole 1259B

Session: The Cretaceous-Paleogene (K-Pg) Boundary Interval: From Large-Scale Geological Events to Mass Extinction Mechanisms

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Abstract:

The Cretaceous/Paleogene boundary cores recovered during Ocean Drilling Program (ODP) Leg 207 have been interpreted to contain primary ejecta deposits sorted during settling through the water column but otherwise unaffected by reworking. Notable features include a basal layer inferred to be seismogenic, normal grading of the spherules, planktonic foraminifera that may have been direct victims of the impact, and lower and upper ejecta sublayers representing dominantly ballistic and vapor phase ejecta, respectively. To test and refine these inferences, we mapped the distribution of spherules and foraminifera via manual pixel segmentation on a high resolution, 2 cm wide SEM mosaic of the boundary interval from ODP Hole 1259B that spans the ~2 cm thick ejecta layer and 2 cm of both overlying and underlying deposits. We also documented textural differences among different grains and tested for compositional differences using point analyses, line scans, and X-ray elemental maps.

Qualitative examination of foraminifera confirm that the ejecta layer occurs precisely at the biostratigraphic boundary. Elemental and textural differences confirm the presence of homogeneous and mottled spherules suggesting there were initially both glassy spherules and microkrystites. The matrix between spherules has a composition expected if the material was derived from collapsed and/or dispersed spherules. Carbonate grains with porous textures and other previously reported exotic grains are recognized in the upper few mm of the ejecta layer and continue to be present in the overlying Danian interval. The ‘white layer’ between the spherule layer and underlying Cretaceous marl is Ca-rich relative to the overlying ejecta layer and contains common small to very small Cretaceous (where identifiable) foraminifera consistent with fine material mobilized from underlying deposits by seismic shaking. Both spherules and planktonic foraminifera in the ejecta layer are normally graded, but foraminifera are only common in the upper 2/3’s of the layer, a pattern predicted if the foraminifera were killed by the impact and were entrained with the settling ejecta but had a lower maximum settling rate than the largest spherules. Work is continuing to estimate a standing population of latest Cretaceous planktonic foraminifera in the tropical Atlantic if the foraminifera in the ejecta layer were the individuals living at the time of the impact.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9609

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