Abanicos Cuaternarios

Revista do Instituto Geológico, São Paulo, 23(1), 1-11, 2002.

QUATERNARY FANS AND COLLUVIUM AS INDICATORS OF ENVIRONMENTAL CHANGE AND LANDSCAPE SENSITIVITY Michael F. THOMAS

ABSTRACT Unconsolidated sediments in the humid tropics range from highly altered weathering products involved in minimum, low energy transport, to coarse fans and landslide deposits. But most can be regarded ascolluvium and alluvium showing imprints of episodic sedimentation. These deposits may span the entire Cenozoic, but 14C dating has led to a focus on Late Quaternary sediments. Many studies have indicated concentration of activity between the Last Glacial Maximum (LGM) and the early Holocene ‘pluvial’, but recent use of Optically Stimulated Luminescence (OSL) dating indicates episodic sedimentationthroughout the Last Glacial Cycle in some landscapes. The low resolution of most OSL studies limits attempts to construct true event stratigraphies. But, despite this restriction, these sediments record the detailed history of landscape sensitivity to environmental change, and this history includes the effects of continuing weathering and pedogenesis. Bringing together studies of weathering, landscapesensitivity to environmental change, and source-to-sink models of sedimentation, provides a conceptual framework to understand earth surface systems within a Quaternary time frame. Keywords: Colluvium, fans, landscape sensitivity, Quaternary sediments, sediment sources.

1 INTRODUCTION Unconsolidated sediments blanket large parts of the tropics. They include Tertiary deposits such as the BarreirasFormation of Brazil, the Continental Terminal and Bullom Series sediments in West Africa, and the Kalahari Sands of central and southern Africa. Such formations are particularly associated with Cenozoic denudation and the sedimentary history of tropical cratons, and the sediments are frequently classified as fluviatile, lacustrine, aeolian or marine facies deposited at the margins of the landmass or in intra-cratonic basins. In the humid tropics, the sediments are characteristically derived from ferrallitic (lateritic, bauxitic) weathering profiles often of Mesozoic age, by erosion under conditions of low relief. These source materials, and the mainly low energy conditions of transport, produce sediments ranging typically from sands to ‘lateritic’ clays. Their advanced weathering ensuresa paucity of carbonates and sulphates that form common cementing materials in other environments. Silica cementation is also rare in humid environments, though common across Australia and southern Africa. On the other hand, Fe cementation, forming

varieties of ferricrete and enrichment by Al, due to further weathering is common. Such sediments may be particularly sensitive to erosion followingperturbations in the environment, arising from whatever cause. In fact, one of the problems of interpretation that they pose is a history of repeated mobilisation and redeposition, often involving only minimum distances of transport. Another, is the effect of continued weathering of the sediments following deposition, which can obscure original sedimentary structures. This style of sedimentationcan be considered part of a ‘cratonic regime’ (FAIRBRIDGE & FINKL 1980), embracing prolonged periods of weathering followed by shorter episodes of stripping of the saprolite mantle. ERHART (1955) and MILLOT (1964/1970) anticipated these ideas, and the former’s concepts of biostasie and rhexistasie have passed into the vocabulary of geo-ecology. However, our understanding of these sedimentaryenvironments and their modern analogues is limited. FAIRBRIDGE & FINKL (1980) envisaged very long timescales for these events: 107-108 y for cratonic stability and weathering; 104-105 y for episodes of tectonic or climatic crisis and erosion.

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Revista do Instituto Geológico, São Paulo, 23(1), 1-11, 2002.

But more detailed studies have shown that events on shorter timescales involving...