ABSTRACT: 

The aim of this study is to describe density-flow facies variability and interpret their flow evolution in a fresh-water rift-lake system. The data were collected from the 500-m-thick Paleogene lacustrine oil-prone source-rock succession penetrated by the Enreca-3 core-hole at the intersection of the Song Hong and the Beibuwan basins, Vietnam. The sedimentological data collected are supplemented with source-rock screening data to get an insight into the origin of mud in the density flows. A wide range of density-flow facies are recognized and can be assigned to turbulent, transitional, and laminar flow processes. The beds range from centimeter-scale, mudstone-rich beds to meter-scale cohesive debrites and hybrid beds. Similarly to the “classic” hybrid beds, the centimeter-scale beds are interpreted to record flow transformation and concentration. The hybrid bed motifs include essentially similar bed divisions (H1–H5; Haughton et al. 2009) to those described from marine basins. Moreover, transitional-flow facies are particularly common and include transitional current ripples and variously developed cyclic banding and lamination. These facies occupy a fixed position below and/or at plug-flow units in the bed motifs, suggesting that flow dynamics related to plug-flow development governed their development. Source-rock screening data show that most of the mudstone in hybrid beds represents lake-bottom and lake-margin mud, which was likely assimilated into the density flows along their path. Similarly, sedimentological data show common evidence for interaction between flow and the muddy lake floor such as sheared flame structures, deformed mud intrusions, and interbeds in turbidite facies. The assimilation of the lake-bottom mud into the density flows probably played a key role in modulating flow turbulence and explains the common occurrence of transitional-flow facies and indications of late-stage flow concentration in these strata.

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