Characterization and Origin of 1:1 Phyllosilicates within Peloids of the Recent, Holocene and Miocene Deposits of the Congo Basin

Abstract

Abstract--The grey-green peloids from the Miocene period to Recent fine-grained deposits on the continental shelf close to Congo-Zalre River mouth were studied by X-ray transmission diffractometry (XRD), SEM and by EDAX. The peloids have multiphase heterogenous mineral composition. Their most important constituents are detrital minerals like kaolinite, quartz, goethite, 7 A phases with d(001) ~ 7.3 ,~, and in more maturated grains--nontronite. The d(060) values were used to estimate the general composition of phyllosilicate phases to compare with the composition determined by EDAX. It has been found that d(060) equal to 1.504 ~, is common for Fe3+-bearing kaolinite, which is quite abundant for the Recent peloids. The d(060) equal to 1.535 ,~ and 1.55 A is characteristic for the di-trioctahedral and trioctahedral l:l phases, which are abundant within the more evolved Miocene peloids. Nontronite is characterized by d(060) equal to 1.524 A within concordance with its highly ferrous composition, and partly by its potassic interlayer. It shows cabbage-like nannostructures proving neoformational origin of this mineral in the marine environment.It has been shown that areas of the low sedimentation rate within the Congo Basin were favorable for the mineral changes and neofolrnation. For the Holocene vertical profile, we observed levels of slower sedimentation rates. The evolution is expressed by the disappearance of kaolinite at the expense of other 7 A phases and nontronite. Although more advanced stages of maturation of the studied phases were observed in older peloids (10 4 to l07 y), One cannot detect a linear relationship of these processes with burial.

Crystal Chemistry of Layer Silicates of the Miocene Green Grain (Congo Basin) from Transmission Electron Microscopy (TEM) and Analytical Electron microscopy (AEM) Observations

“…Enlargement of the flakes in Figure 6 revealed an interstratified structure where 7-9 ~ layers dominate over 10-,~ layers (Figure 7). These direct observations of 7-A layers and mixed-layer grains of 7-10 A confirm the earlier X-ray identification of a discrete trioctahedral 7-A phase and of a randomly interstratified phase composed of trioctahedral 1:1 and dioctahedral 2:1 layers in an 85:15 proportion, as shown by simulation (Wiewi6ra et al, 1996). In rare cases, spacings of 12 and 13 A were identified.…”

“…However, the Miocene deposits are inferred to represent a variable sedimentation rate because of their thickness, to 700 m in the studied bore-hole. We have selected graygreen peloids without fissures and with the same morphology as those previously described as representative of a mid-evolutionary state process (Wiewi6ra et al, 1996).…”

“…XRD using a position-sensitive detector (PSD) was used simultaneously with energy-dispersive spectrometry (EDS), for structural and chemical characterization. In addition, scanning electron microscopy (SEM) was used to characterize the chemical components of fecal pellets from the Miocene period to Recent on the shelf close to the mouth of the Congo River (Wiewi6ra et al, 1996). XRD studies of single grains, using a PSD, demonstrated the polymineralic character of the grains.…”

“…Kaolinite is abundant in the Recent peloids, whereas the dioctahedral Fe3 § 1:1 phases (Fe-kaolinite) are more abundant within the more mature Miocene peloids (Wiewi6ra et al, 1996). The latter are also characterized by abundant K-bearing nontronite formed via neoformation rather than alteration of preexisting phases.…”

“…In the present work, considering the polymineralic nature of the grains, TEM was used to supplement previous results (Wiewi6ra et al, 1996). Earlier results suffered from the use of analytical methods that involved microscopic studies, coupled with the very small size of all phases present, especially those recrystallized or neoformed.…”