Trace element concentrations in the Mexico‐Belize ejecta layer: A link between the Chicxulub impact and the global Cretaceous‐Paleogene boundary

Abstract— Four exposures of Chicxulub impact ejecta along the Mexico‐Belize border have been sampled and analyzed for major and trace element abundances. The ejecta deposits consist of a lower spheroid bed, containing clay and dolomite spheroids, and an upper diamictite bed with boulders and clasts of limestone and dolomite. The matrix of both beds is composed of clay and micritic dolomite. The rare earth element (REE) compositions in the matrix of both units show strong similarities in concentrations and pattern. Furthermore, the Zr/TiO₂ scatter plot shows a linear correlation indicating one source. These results indicate that the basal spheroid bed has the same source and was generated during the same event as the overlying diamictite bed, which lends support to a single‐impact scenario for the Albion Formation ejecta deposits. The elevated concentrations of non‐meteoritic elements such as Sb, As, U, and Zn in the matrix of the lower spheroid bed are regarded to have been derived from the sedimentary target rocks at the Chicxulub impact site. The positive Eu and Ce anomalies in clay concretion and in the matrix of the lower part of the spheroid bed in Albion Island quarry is probably related to processes involved in the impact, such as high temperature and oxidizing conditions. Analogous trace element anomalies have been reported from the distal Cretaceous‐Paleogene (K/T) boundary clay layer at different sites. Thus, the trace element signals, reported herein, are regarded to support a genetic link between the Chicxulub impact, the ejecta deposits along the Mexico‐Belize border, and the global K/T boundary layer.     

Potential Tsunami Hazard Associated with the Kerepehi Fault,Firth of Thames,New Zealand

Geophysical data have identified four submarine segments of the Kerepehi Fault, roughly bisecting a back-arc rift (Hauraki Rift). These segments have been traced through the shallow waters of the Firth of Thames, which lies at the southern end of the Hauraki Gulf, New Zealand. No historical or paleotsunami data are available to assess the tsunami hazard of these fault segments.Analysis of the fault geometry, combined with paleoseismic data for three further terrestrial segments of the Fault, suggest Most Credible Earthquake (MCE) moment magnitudes of 6.5–7.1. Due to the presence of thick deposits of soft sediment, and thesemi-confined nature of the Firth, the MCE events are considered capable of generating tsunami or tsunami-like waves. Two numerical models (finite element and finite difference), and an empirical method proposed by Abe (1995), were used to predict maximum tsunami wave heights. The numerical models also modelled the tsunami propagation.The MCE events were found not to represent a major threat to the large metropolitan centre of Auckland City (New Zealand's largest population centre). However, the waves were a threat to small coastal communities around the Firth, including the township of Thames, and 35,000 ha of low-lying land along the southern shores of the Firth of Thames.The Abe method was found to provide a quick and useful method of assessing the regional tsunami height. However, for sources in water depths < 25 m the Abe method predicted heights 2–4 times larger than the numerical models. Since the numerical models were not intended for simulating tsunami generation in such shallow water, the Abe results are probably a good guide to the maximum wave heights.     

地震综合效应场函数及其在地震预报中的应用（四）计算流程的实现及程序使用说明

本文介绍了实现地震综合效应场计算的程序流程和数据流程，程序中数据的形成、所需参数的选取方法，程序运行的软、硬件环境。为便于地震综合效应场方法的推广应用，本文还给出了程序使用的简单说明。     

Stellungnahme zu den „Anmerkungen“

Ohne Zusammenfassung     

Glaukonitkörner in rezenten Sedimenten des Persischen Golfs

The field-name “glauconite” is applicable to a great variety of green grains, particularly mud clasts, which are found in the fraction >63μ in surface sediments from the Persian Gulf. These grains occur in areas of low sedimentation rates in all water depths to 110 m, the shelf break in the Gulf of Oman (Fig. 1). Glauconite grains coarser than 250,μ were magnetically separated from 15 samples and then handpicked. Their carbonate fractions (75->90%, predominantly aragonite) are basically different from those of the corresponding total samples (50–65%, predominantly calcite). The clay minerals in the glauconite grains fall into two groups. The samples away from the shore contain abundant montmorillonite, compared to the small amount in the 2μ (fractions of the total samples; the near shore samples are predominantly amorphous material and illite. Because of these and other differences from the remaining parts of the samples (Tab. 1) the glauconite grains are considered to be parautochthonous relicts from the underlying late Pleistocene. They were apparently formed under the special conditions of a transgressing shallow marine environment.     

Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania

 Alkali basalts and nephelinites from the volcanic province of northern Tanzania contain pyroxene and nepheline that show evidence for chemical and/or isotopic disequilibria with their host magmas. Olivine, pyroxene, nepheline and plagioclase all appear to be partially xenocrystic in origin. Five whole rock/mineral separate pairs have been analyzed for Sr, Nd, and Pb isotopic compositions. The ²⁰⁶Pb/²⁰⁴Pb ratios are distinct by as much as 20.94 (whole rock) vs. 19.10 (clinopyroxene separate). The Sr and Nd isotopic disequilibria vary from insignificant in the case of nepheline, to Δ ⁸⁷Sr/⁸⁶Sr of 0.0002 and Δɛ_Nd of 0.7 in the case of clinopyroxene. The mineral chemistry of 25 samples indicates the ubiquitous presence of minerals that did not crystallize from a liquid represented by the host rock. The northern Tanzanian magmas are peralkaline and exhibit none of the xenocrystic phases expected from crustal assimilation. The disequilibria cannot be the result of mantle source variations. Rather the xenocrystic phases present appear to have been derived from earlier alkali basaltic rocks or magmas that were contaminated by the crust. Material from this earlier magma was then mixed with batches of magma that subsequently erupted on the surface. Disequilibrium in volcanic rocks has potentially serious consequences for the use of whole rock data to identify source reservoirs. However, mass balance calculations reveal that the ²⁰⁶Pb/²⁰⁴Pb isotopic compositions of the erupted lavas were changed by less than 0.25% as a result of this indirect crustal contamination. Received: 15 February 1995 / Accepted: 4 May 1996