Introduction

Deltas are broadly defined as coastal accumulations of sediment extending both above and below sea level, formed where a river enters an ocean or other large body of water (Fig. 3.1). The key element in this definition is the presence of a fluvially derived point source of sediment (Boyd, Dalrymple & Zaitlin, 1992). This chapter concentrates on the Late Quaternary evolution of marine deltas.

The term ‘delta’ was first applied about 450 BC by the Greek historian Herodotus, who noted the similarity of the Greek letter to the shape of the accumulation of sediment around the mouth of the Nile River. Despite the fact that many of the world's deltas do not show this particular morphology, the term has remained.

Deltas are extremely important depositional systems, both ecologically and economically. They often contain extensive wetlands, whose high biological productivity makes them vital nursery grounds for fisheries. Just as significant are the agricultural activities supported by the fertile soils of the world's deltas. Many of the world's largest ports are located on distributaries of major deltas. Huge amounts of coal, oil, and natural gas, still the principal fuels of the world economy, are derived from subsurface deltaic deposits.

The purpose of this chapter is to examine the geomorphology and sedimentology of modern deltas and provide a synthesis of the evolutionary elements. This will be done in several steps: 1. Examination of the processes responsible for deltaic deposition; 2. discussion of basic deltaic environments, 3. development of idealized, end-member models of deltaic sedimentation; 4. discussion of the evolutionary implication of these concepts.