Geochemistry of a Tertiary continental basalt suite, Red Sea coastal plain, Egypt: petrogenesis and characteristics of the mantle source region

Major and trace element data on Tertiary continental basalt flows from the Shalatein area, Red Sea coastal plain of Egypt, have been presented and used to obtain more information about their source region and the processes involved in their generation. The rocks are mainly alkali olivine basalt with MgO and Mg no. in the range of 9.8–5 wt % and 65–46, respectively. They display wide variations in incompatible element concentrations, particularly LREE, Zr, Nb, K, Y, Ba and Sr. There is no evidence of significant crustal contamination or a lithospheric mantle signature in these rocks. Normalized trace element patterns and diagnostic elemental ratios are very similar to those of modern ocean-island basalts (OIB) a feature which suggests that the mantle source region was the asthenosphere. Comparison with the different types of OIB indicates that the basalts may be derived from a high U/Pb (HIMU) source with slightly elevated K and Ba contents. The basalts show general trends of increasing incompatible elements (K2O, Nb, Y, Sr and Yb), and decreasing contents of compatible elements (Cr, Ni, Sc and Ca) with decreasing Mg no. Furthermore, TiO2, P2O5, LREE and Th define maxima at about Mg no.=56, suggesting late fractionation of Fe–Ti oxides and apatite. Although these variations are consistent with fractional crystallization processes, the wide variations in LREE contents and the incompatible trace element ratios Ce/Y (1.2–3.8), Zr/Nb (2.3–7.1) and Nb/Y (0.6–4) in the least fractionated samples (Mg no. > 56) suggest that fractional crystallization involving the observed phenocryst assemblage (olivine and clinopyroxene) cannot fully explain such compositional variations. Modelling of the mafic rocks (Mg no. > 56) using REE suggests varying degrees of partial melting of an enriched mantle source region in the garnet stability field. Partial melting is attributed to plume-related mantle upwelling beneath the Red Sea rift system.

Automated identification of rock boundaries: An application of the Walsh transform to geophysical well‐log analysis

A system of orthogonal functions, known as Walsh functions, which assume only the values +1 and −1, are presented as a tool appropriate for the analysis of well‐log data. Many of the characteristics of these rectangular waveforms, such as discrete transitions in signal level, make them ideal for processing borehole data. Using the generalized Fourier transform, the basic equations for the Walsh transform and Walsh power spectrum are developed showing how the energy in a given signal is distributed among these rectangular wave components. As a first application of the Walsh transform to logging data, a method of computeraided rock boundary identification is developed and demonstrated on a set of well logs from a continental basalt sequence. This technique provides a fast, simple yet accurate means of dividing well‐log data into groups of measurements corresponding to different rock units.