Title

Quaternary sea level changes and the depositional architecture of beach-ridge strandplains along the east coast of Brazil

Date of Award

1987

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine Geology and Geophysics

First Committee Member

Harold R. Wanless, Committee Chair

Abstract

This research uses the information of sealevel history and coastal evolution to document the sedimentary accumulations occurring on the beach-ridge strandplains associated with the Jequitinhonha and the Doce river mouths. This research has focused on: (1) lagoonal fill sequences deposited during two periods of sealevel rise and (2) littoral zone sequences deposited during the last 5,000 years, when sea level dropped 4 to 5 m.The record of the lagoonal fill sequences deposited during the 7.0-5.1 ka sealevel rise was controlled by availability of sediment. At the Jequitinhonha strandplain sediment supply was sufficient to keep most of the lagoon floor at the intertidal zone. Thus, sediments deposited in a mangrove swamp environment dominate the lagoon fill. At the Doce strandplain less sediment was available to the paleolagoon. Thus, sediments deposited in subtidal environments dominate the lagoonal fill.Lagoonal sediments deposited during the 3.8-3.5 ka sealevel rise were investigated only at the Doce strandplain. In this area lagoonal sediments fill elongate depressions occurring on the Holocene beach-ridge terraces. These depressions formed when sand spits prograding coastwise isolated portions of the shoreface, thus, giving origin to elongate lagoons, which later evolved to freshwater lakes. When sea level rose some of these low-lying areas were invaded by the sea and became interconnected bays. Lagoonal fill sequences recovered from those low-lying areas reflect this evolutionary history, being composed of well-laminated muds resting on top of lower shoreface sands.A model for the evolution of littoral zone sequences in response to changes in relative sea level has been developed and tested on the littoral zone sequences of the Jequitinhonha and Doce strandplains. This model predicts that sequences resulting from a rise in sea level, followed by a drop, will be composite sequences in which lower shoreface sediments were deposited during the period of sealevel rise and foreshore/upper shoreface sediments were deposited during the subsequent drop in sea level.Evidences for the existence of such composite sequences were found in only one vibracore. Possible reasons for this are: (1) vibracores did not penetrated the entire Holocene sequence, and (2) most of the vibracores were acquired downdrift of the river mouth. (Abstract shortened with permission of author.)

Keywords

Geology

Link to Full Text

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