Chapter 5 Provenance and Palaeoenvironmental Interpretation of Superficial Deposits, with Particular Reference to Post-Depositional Modification of Heavy Mineral Assemblages

Abstract

A typical complete sedimentary cycle involves five phases (pre-erosional, in transit, post-depositional pre-burial, post-burial, exhumation) and six processes potentially modifying the original composition of the sediment. Some processes add mineral species to an assemblage, others subtract minerals, and yet others can both add and subtract minerals; moreover, some can operate in only one of the five phases, whereas others can affect up to three phases. Authigenic growth and anthropogenic addition are relatively easy to recognise and compensate for, occurring in predictable and limited environmental conditions. Although mechanical weathering can substantially alter heavy mineral assemblages, its effects also are predictable, and prolonged or intense abrasion is required. In contrast, differential sorting of minerals during transport can have a rapid and profound modifying effect on a heavy mineral assemblage that is less readily compensated for but does broadly follow the laws of physics. In an analogous effect, any contrasts between heavy mineral studies in the size fraction selected by each analyst can seriously undermine data comparison. The widely recognised boundary between superficial pedochemical weathering and deeper geochemical weathering is largely artificial. Both phenomena frequently and often profoundly modify heavy mineral assemblages, but the resulting patterns of depletion vary considerably. For example, intrinsic properties of minerals conferring vulnerability to acid pedochemical weathering include well-developed multiple cleavage planes and/or inclusions, high-energy lattices and high Fe 2+ content. However, these properties are insufficient to accurately predict a stability series in any particular heavy mineral assemblage, as both relative and absolute depletion rates are strongly influenced in the post-depositional environment by both abiotic factors (e.g., pH, redox potential) and under-researched biotic factors. Five case-studies of heavy mineral assemblages in British Tertiary and Quaternary deposits are used to illustrate these principles and to demonstrate the benefits of (a) using multivariate methods to elucidate patterns of variation in heavy mineral assemblages, (b) characterising a weathering front using 90% depletion ( D 90 ) values and (c) integrating heavy mineral data with those derived from other geoanalytical and field-based techniques. This approach allows not only more accurate reconstruction of provenance but also of palaeoenvironments; both can then usefully inform correlation.