Morphology and structure of Amazon upper continental margin

Integration of extensive geophysical data (seismic refraction, reflection, gravity, and magnetic) with geologic information derived from exploratory drilling and land-based studies allows the delineation of four major phases of development of the Amazon continental margin. Early rifting during the Cretaceous separated a Precambrian craton in the Amazon region from its counterpart in Liberia; Eburnian or Trans-Amazonian (1,800 to 2,000 m.y.) magnetic anomalies, however, are still evident on both margins. During rifting, or perhaps soon after, the Amazon graben was formed, possibly augmented by intersection of the St. Paul's fracture zone onto the Amazon margin. During the Cretaceous and Paleogene, terrigenous sediments were derived primarily from the erosion of nearby shield rocks. Depositional environments ranged from continental to shallow marine, the depocenter being the Marajo basin. The presence of a thick sequence of shelf-edge carbonates suggests that terrigenous influx was low, and/or that the shelf-edge platform restricted terrigenous sedimentation to the inner and middle shelf. Uplift of the Andes Mountains during the Miocene initiated eastward drainage of the Amazon River. Erosion of the mountains provided a 60-fold increase in terrigenous influx to the Amazon margin, which overwhelmed the shelf-edge barriers and began formation of the Amazon Cone. Accumulation since that time has accounted for as much as 12 km of sediment. Quaternary fluctuations in sea level have caused two distinct phases of margin sedimentation. During glacially lowered sea level, Amazon sediment emptied directly into the adjacent outer margin, resulting in high accumulation rates. During high sea-level stands, such as at present, sediment is transported northwest along the inner shelf, with little material escaping to the outer shelf or beyond.