The Sandstone Complex in the Brazos Riverbed Section: Geochemical Constraints on Genesis and Depositional Conditions

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 lg/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 2þ 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.

Trace fossil evidence suggests widespread dwarfism in response to the end-Cretaceous mass extinction: Braggs, Alabama and Brazos River, Texas

“…Scale bars = 1 cm. Liu, 1993;Gale, 2006;Schulte et al, 2006;Keller et al, 2007;Abramovich et al, 2011;Adatte et al, 2011;Munsel et al, 2011). At each previously described section, all bioturbation structures were examined for surficial and architectural morphology, as well as fill characteristics.…”

“…These units comprise the upper portion of the Corsicana Formation and were deposited in a low-oxygen, shallow marine paleoenvironment (Keller et al, 2007). Overlying Unit A3, across an erosional unconformity, is a hummocky, cross-bedded sandstone (Unit HCS) with multiple fining upward sequences and abundant impact-derived spherules (Schulte et al, 2006;Keller et al, 2007;Abramovich et al, 2011;Adatte et al, 2011;Munsel et al, 2011). This unit is commonly referred to as the 'event deposit' by many workers and is interpreted as a single catastrophic tsunami deposit or a series of tempestites generated by the bolide impact (Bourgeois et al, 1988;Smit et al, 1996;Gale, 2006;Schulte et al, 2006;Hart et al, 2012).…”

“…Approximately 2 km to the northeast, HCS and its bounding units are also exposed in bedding planes and cross-sections along the Brazos River (Munsel et al, 2011;31°7′ 41″ N, 96°49′ 19″ W). In these sections, the K-Pg boundary coincides with the erosional surface between Units A3 and HSC, based on the International Commission on Stratigraphy (ICS) definition of the boundary as the base of the ejecta layer (Cowie et al, 1989;Schulte et al, 2006).…”