Coastal dune fields at the S?o Francisco River strandplain, northeastern Brazil: morphology and environmental controls

摘要

Extensive coastal dune fields occur on the Quaternary strandplain associated with the Sao Francisco River mouth. Two different generations of dunes are identified. One is inactive, already fixed by vegetation, comprising parabolic dunes. The other generation is active, bordering the present-day shoreline and transgressing over the inactive dune field. Three morphological provinces in the active coastal dune fields are recognized. On the updrift side of the Sao Francisco River mouth, they are: (a) sand-sheet with shrub coppice and shadow dunes; (b) isolated dunes of the barchan-transversal type up to 5 m high, and interdune areas; and (c) a 23 m high compound dune, with superimposed small dunes. The same provinces are recognized on the downdrift side of the river mouth, with two important exceptions: the barchan-transversal and compound dunes are replaced, respectively, by (i) zibar-type dunes up to 5 m high, and (ii) a 19 m high precipitation dune, which is associated with numerous blowouts. The prevailing eastern winds from August to January favour the development of the aeolian bedforms and the migration of dunes. The shoreline orientation almost transversal to the winds and the great supply of fine-grained sediments contribute to the formation of barchan-transversal types and compound dunes in the updrift side. On the other hand, in the downdrift side the shoreline orientation is almost parallel to the prevailing winds. This fact, in association with a coarser grain size in the beachface, favours the formation of zibar-type and precipitation dunes with numerous blowouts. The rate of migration of individual dunes is about 20 to 24 m per year. This study suggests that the aeolian sedimentation is a relatively recent phenomenon at the Quaternary strandplain of the Sao Francisco River. The first generation of dune fields initiated some time after 3000 years BP and the second generation originated some centuries ago.