DACIAN BASIN - GeoEcoMar

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Chapter 6. Upper Neogene Fluvial Sedimentation in the Dacian Basin
6.2.2. Fluvial environments in the Dacian Basin
One of the best known classifications of the fluvial sedimentary facies considers
the following genetic categories (Galloway and Hobday, 1983):
■ fluvial channel facies and
■ out-of-channel facies.
The out-of-channel environment includes:
» sediments accumulated near the fluvial channel (levees and crevasse splays);
» sediments accumulated away from the channel (fluvial floodplain).
The Dacian Basin fining upward units described in this chapter constitute the
facies of the sediment accumulated in the fluvial channel. Certain sedimentation processes
(disturbance of the fluvial channels, erosion immediately after sedimentation,
advanced sorting of sand sediments etc.) restrict or modify the fining upward trend
of the channel-fill deposits, which leads to the apparition of distinct types of grain
size grading in the deposits of the fluvial channel.
Well log analysis shows that Middle Dacian – Romanian channel fill deposits are
thick (10 - 20 m) and numerous. Many channels are discrete, separate bodies (Fig.
6.16 A) with fining upward trend. The non-graded channel fills (cylindrical pattern
on the well log) are infrequent (Fig. 6.16 B). Some channel fill units are superimposed
(Fig. 6.16 C), but this is not a dominant feature. Clayey, flood plain deposits
separate the channel fill, sandy deposits. The well logs reveal the existence of coarsening
upward alluvial cycles (Fig. 6.17 E) in the Middle Dacian. However, this trend
is not common in the fluvial Pliocene sequence.
Images based on log data from a detailed hydrogeological wells network, supplied
information on the geometry of fluvial channels in the Pliocene to Quaternary time
(Jipa et al., 1999) (Fig. 6.17). The reconstructed channels are large features, up to several
kilometers wide (between 300 m and over 4 km, especially between 1.5 and 2 km). The
smallest channel sections are several hundreds of meters in width. The density of the
channel system is remarkable. The individual fluvial channels have 5 to 25 m thick. No
constant lateral switching trend emerges from the channel network reconstruction.
Due to the spectacular exposures created by the mining activities in the coal
quarries from Oltenia, the facies of the sediments accumulated on the edge of the
channel (levees) was occasionally recognized. We attributed to this category the
fine and very fine sands that are bounding deposits interpreted as channel-fill sediments.
The channel levee deposits interpretation is supported by the position on
the edge of the channel, and by the presence of fossil wood (tree trunks in growing
position) found in these deposits (Fig. 6.18).
The crevasse splay environment was identified in the fluvial deposits investigated
from the Dacian Basin. The sediments of this fluvial environment occur as
coarsening upward, fine-grained (silt to fine sand) sequences, with reduced thickness
(decimeter to less than one meter).