Title

Dispersed Organic Matter in Samples from the Western Continental Shelf of Southern Africa: Palynofacies Assemblages and Depositional Environments of Late Cretaceous and Younger Sediments

Abstract

The composition and distribution of dispersed microscopic organic matter particles (phytoclasts) and palynomorphs in sediments and sedimentary rocks reflect factors that influenced their production in their original environments, transport to depositional sites, burial, and post-depositional changes. Thus, they can be used as proxies for reconstructing provenance, temporal and spatial paleoenvironmental conditions, and diagenetic changes. In this study, phytoclasts and palynomorphs in 55 spot samples from Upper Cretaceous, Eocene, Miocene and uppermost Pleistocene sediments on the western continental shelf of Namibia and South Africa were examined. They were used to test lateral depositional facies inferred from unpublished micropaleontological analysis, and to decipher any stratigraphic trends in the distribution of the organic particles in the sediments. Thirteen types of phytoclasts and palynomorphs were identified and analyzed in two ways: visually with basic quantitative techniques and statistically with two multivariate techniques. Three components, namely amorphous organic matter, wood, and black debris, were visually and statistically significant. Average linkage cluster analysis of these three components and discrete changes in the amounts of marine palynomorphs and other terrigenously derived organic material, such as spores and pollen, brown-black fragments, and plant tissues, delineated seven palynofacies assemblages. by contrast, principal components analysis revealed two groups of samples based on the amounts of amorphous organic matter and wood. Five groups of samples (referred to as associations) were identified visually from the pattern of organic matter distribution and compared with the statistical results. A palynofacies depositional model is proposed to re-evaluate the fluvial to inner shelf settings previously inferred for the sediments. Some clear stratigraphic trends were obvious in both sets of results, such as the dominance of amorphous organic matter in Turonian, Coniacian, and uppermost Pleistocene sediments, and higher percentages of black debris in the older sediments. These trends appear to be controlled more by provenance and depositional conditions than by age. The dominance of amorphous organic matter in several uppermost Pleistocene sediments (deposited during a global fall in sea level) were attributed to in-situ physical and biological degradation of mostly marine algal material in an inner shelf setting.

Department(s)

Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Namibia; South Africa; Palynofacies; Palynomorphs; Phytoclasts

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2003 Elsevier, All rights reserved.

Share

 
COinS