Amazon Fan sands: implications for provenance

The Amazon Fan is the third largest modern 'mud-rich' submarine fan system in the world, located on the equatorial Atlantic margin and abyssal plain. The Fan is believed to have originated as a result of the Andean orogeny in the early Miocene and has continued to build through to the present day. S edimentation rates across the Fan vary in time and space, and can be as hig h as 10-25 m/k.y., especially during periods around and during low sea-leve l stands. These large-scale fan systems offer a unique opportunity to study earth history, in Particular, the comparison between ancient and modern sy stems in terms of reservoir potential, sedimentation processes, fan models and provenance studies. It is the latter that forms the basis of this contr ibution. In 1994, Leg 155 of the Ocean Drilling Program recovered over 4 km of core material from 17 sites on the Amazon Fan. We report on the provena nce (heavy mineral, quartz grain morphology, geochemistry and grain size) o f sands recovered from the Fan. Initial findings indicate that sand composi tions vary through a glacial cycle. The older units in the Upper Levee Comp lex (U.L.C.) have a pyroxene-dominated heavy mineral suite, and a relativel y low proportion of glacially derived quartz grains, whilst the younger uni ts in the U.L.C. have an amphibole-epidote-domina ted heavy mineral suite, and a larger proportion of glacially-derived quartz grains. These results s uggest either a change in provenance or climate, or a combination of both o ver a glacial cycle. Core samples studied appear to be unaffected by diagen esis, whilst abrasion during transport has had little effect on individual grains. (C) 2000 Elsevier Science Ltd. All rights reserved.