Evolution of alluvial fans documented in the stratigraphic record of the Tiyuyacu Formation

Abstract

The sedimentary record of ternary sequences in the Oriente Basin exhibits well-defined characteristics within sequential stratigraphic models reported, defined and analyzed by different authors in previous studies within the Napo Uplift, in the center-north of the basin. Nevertheless, in Southeastern, in topographic front Andean (FCA), few studies reported extensive and continuous exposures of these sequences. In this way, lateral changes of sedimentary sequences have not been reported and part of the history of the evolution of the Andes, has been unknown, this hindered the understanding of the regional geodynamic framework. In order to define the sedimentation environment of the Tiyuyacu Formation; this research included: lithostratigraphic and petrographic characterization and provenance analysis of heavy minerals, in mega outcrop located on Santiago de Tiwintza to Puerto Morona, in the “Mirador la Virgen” sector, in SW of the FCA. Two members were defined in the Tiyuyacu formation: In the Lower Member (MTy-I), channels (CH), gravel bars (GB), bar lateral accretion deposits (LA) and sandy floodplains (FS) macroforms were identified; these macroforms are genetically associated with alluvial plains with braided channels. Meanwhile, in the Upper Member (MTy-S) channels (CH), gravel bars (GB), vertical accretion (UA) and silty floodplains (FF) were defined; these macroforms are genetically linked with alluvial plains with meandering channels. Modal proportions of mineral facies and heavy mineral assemblies indicate that MTy-I was dominated by a cratonic with metamorphic source influence; nevertheless, MTy-S a primary cratonic source and minority volcanic source was identified.