Abstract The origin and deposition of spherule-bearing, dominantly sandy beds in a sandstone complex (also called “event deposit”) below the biostratigraphic Cretaceous–Tertiary (KT) boundary plays a key role in models linking the KT mass extinction to the Chicxulub impact. This study, which focuses on the chemostratigraphy of this complex exposed in a ca. 60-cm-thick succession along the Brazos River, Falls County, Texas, U.S.A., aims to constrain the source of the material as well as the depositional conditions and postdepositional history of this highly controversial stratigraphic unit. Major and trace elements, as well as the isotopic composition of the Ca carbonate, contrast sharply with the underlying Corsicana Formation, indicating a dramatic change in the source of material and depositional conditions. Evaluation of geochemical data by principal-component analysis permits identification of (1) siliciclastic components, (2) ejecta material, consisting of altered impact-glass spherules, and (3) Ca carbonate. The ejecta material, originally represented chiefly by glass spherules with carbonate infill, is strongly altered to clay minerals with dominantly smectitic composition and is characterized by the element association Al 2 O 3 , TiO 2 , Fe 2 O 3 , P 2 O 5 , and SO 2 and the trace elements (TE) V, Cr, Ni, Cu, Zn, Ga, and Mo. The occurrence of two moderately high Ir peaks (0.2 and 1.1 μg/kg) suggests the presence of tiny amounts of extraterrestrial material within the sandstone complex. Based on the contrasting abundance of Ni and Cu in chondritic meteorites and middle crust, the Ni/Cu ratio was used to trace the portion of extraterrestrial material in the sequence. The distribution of this ratio reflects changes in the amount of siliciclastic components added during deposition of the sandstone complex rather than variations in the amount of meteoritic material. The disagreement between evidence suggesting a prevalence of reducing conditions during the alteration of the ejecta material (pyrite inclusions in spherules; accommodation of Mn 2+ by secondary calcite) and sedimentologic features which indicate that the sandstone complex was deposited in a dominantly oxic, high-energy environment strongly supports the case that the ejecta material in these deposits was subjected to reworking.

Abstract Variations in the major-element and trace-element composition of impact spherules were investigated in the sandstone complex subjacent to the KT boundary in a section exposed along the Brazos River, Falls County, Texas, in order to identify whether any regular pattern can be discerned in the vertical distribution of the compositions of the spherules. Stable isotopes of carbon and oxygen strongly suggest that most of the carbonate in the cores of the spherules is not a secondary alteration product of glass, as currently thought, but a primary intrinsic component of spherules segregated from a melt supersaturated in carbonate. The investigation of 90 spherules (1,209 points) by μ-XRF, complemented with ICP-MS analyses on individual grains, shows that the silicate glass of the shell that encased the carbonate core has been altered to Mg-rich smectite, while mafic components have been altered to a mineral which is considered to be a low-Fe chlorite. Spherules of the hummocky cross-bedded sandstone unit (referred to hereafter as HCS) can be distinguished from those of the spherulitic conglomerate bed (referred to hereafter as SCB) not only by their much smaller size, but also by their lack of a carbonate core and by the presence of a mixed-layer, more K-rich clay mineral in their composition. Spherules obtained from different layers of the SCB do not show any distinguishing characteristics with respect to bulk chemical and mineralogical compositions, per se. However, statistically significant vertical gradients can be observed in the abundance of the elements Mg, P, S, and Cr in the carbonate core of these spherules. Further high-resolution chemostratigraphic studies in other KT sections are needed in order to verify the validity and significance of such compositional gradients in the sandstone complex.