Abstract
The Cretaceous-Tertiary boundary is exposed in shallow marine coastal plain sediments near Braggs, Alabama. Magnetic stratigraphy across the boundary indicates that Chron 29R is represented by a stratigraphic thickness of 2.91 (±0.48)m. The average sedimentation rate in this chron is therefore 5.1 m/m.y. The apparent macrofaunal extinction at C29.15R is significantly below the nannofossil extinction event at C29.45R. In the pelagic realm, the microfaunal extinction event is consistently found between C29.7R and C29.8R. The discordance between the microfaunal and macrofaunal extinction events at Braggs, and between the microfaunal extinctions in pelgic sections and at Braggs, may be due to local changes in sedimentary environment in the Braggs section, which culminate in an erosional uncomformity at C29.15R. The minimum estimate for the apparent time lag between the two extinction events at Braggs is about 80,000 yr. Rock magnetic experiments and the analysis of magnetic separates indicate the presence of magnetite, hematite and pyrrhotite as potential magnetic remanence carries. Two phases of magnetite one detrital titanomagnetite and one euhedral low-titanium magnetite, have been identified by electron microscopy, X-ray diffraction and electron diffraction techniques. The euhedral low-titanium magnetite is fine grained (< 0.1 μm) and may be biogenic.
Published Version
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