A Late Pleistocene-Holocene wetland megafan in the Brazilian Amazonia

Author(s)

Rossetti, D. F. ^[1] ; Zani, H. ^[1] ; Cohen, M. C. L. ^[2] ; Cremon, E. H. ^[1]

Affiliation

^[1] Brazilian Natl Inst Space Res, Remote Sensing Div, BR-1224597 Sao Jose Dos Campos, SP - Brazil

^[2] Fed Univ Para, Grad Program Geol & Geochem, Coastal Dynam Lab, BR-66059 Belem, Para - Brazil

• Document type: Journal article
• Source: SEDIMENTARY GEOLOGY; v. 282, p. 276-293, DEC 30 2012.
• Web of Science Citations: 10

Abstract

Despite the growing interest in megafans, definitions provided for this type of environmental setting have not yet been widely agreed upon. A record of sedimentary fades distribution in both space and time including a larger number of analogs is particularly needed for improving megafan facies models. This work focuses on a large fan-like feature from an Amazonian wetland in northern Brazil. Morphological data based on remote sensing, as well as sedimentary facies and radiocarbon analyses, were integrated to propose that this feature is related to a megafan system active during the Late Pleistocene to Holocene. The megafan displays a divergent drainage network, gently-dipping slope, and concave-up and convex-up longitudinal and transverse profiles, respectively. Near surface deposits correspond to fining and coarsening upward sands related to active channels and overbank sand sheets/terminal fan lobes. Sediments are interbedded with abandoned channel/floodplain and lake/pond muds. Morphostructural analyses and drainage anomalies revealed a geological setting affected by reactivation of pre-existing faults contemporaneous with sediment accumulation. Establishment of a megafan system in this wetland most likely occurred within a slightly tectonically subsiding basin under favorable climatic conditions. During wet seasons, high water discharge would have favored sediment transport from highlands into this depositional site. High summer temperatures and drought under a monsoonal regime kept the water levels low. The described megafan could serve as an analog for contemporary tropical wetland megafans formed under a monsoonal climate regime. (c) 2012 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 11/23317-4 - Quaternary evolution of Amazonian ecosystems
• Grantee: Dilce de Fátima Rossetti
• Support Opportunities: Scholarships abroad - Research
• FAPESP's process: 09/00672-3 - Remote sensing applied to quaternary sedimentary analysis at the interfluve of Negro and Branco rivers (AM)
• Grantee: Hiran Zani
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 10/09484-2 - Quaternary megafans in the state of Roraima, Northern Amazonia: sedimentological, climatic and tectonic implications
• Grantee: Dilce de Fátima Rossetti
• Support Opportunities: Regular Research Grants