DACIAN BASIN - GeoEcoMar

6.1. Sedimentary characteristics of the fluvial deposits in the Dacian Basin
Coarsening upward grading also occurs in the fluvial environment. This grain
size trend is related to processes of progradation that take place in the crevasse
splay development.
The well logs offer the best way to examine the grain size grading. The Dobretu
well logs point out four main grading types, each one with varieties (Fig. 6.10).
6.1.3. Sedimentary structures
The Dacian Basin fluvial deposits display the whole array of current lamination
structures, in accordance with the general characteristic of the sediment accumulations
generated by alluvial processes.
The current structures evolve within the fluvial graded sequences, following
the fining upward trend and the corresponding level of sedimentary dynamics
(Figs. 6.7; 6.11 and 6.12).
Frequently, the fluvial unit displays structures showing the effect of discharge
pulses. The apparently unitary sequence in the figure 6.11A has three stages of clay
pebbles accumulation, but the grain size of the consecutive pebble levels is fining
upward. Closely successive sedimentary events can generate polygenetic units
(Fig. 6.12 A) or separate units narrowly superposed (Fig. 6.12 C and D).
The characteristic erosional lower limit of the fluvial sequence has been observed
in many cases in the fluvial sequences of the Dacian Basin (Figs 6.3C and
6.9), excepting the sedimentary units with fine-grained detrital material at the base
(Fig. 6.8 C). Channels with basal erosional surface are sometimes present in the
outcrops (Fig. 6.3 B), showing erosional irregularities at the decimetric scale. But the
erosion within the Dacian-Romanian fluvial deposits can locally reach hundreds
of meters, expressed by the absence of Late Dacian or Early Romanian deposits
(Papaianopol, Marinescu, 2003).
The magnitude of the fluvial transport in the Dacian Basin is indicated by large
scale current structures, occurring in coarse-grained deposits (Fig. 6.3 A) but also in
sandy accumulations (Figs 6.4 C and 6.15).
6.1.4. Inclined bedded units
The lateral migration of the fluvial channels is the main process that generates
large scale inclined stratification (Thomas et al., 1987), as the characteristic of the
point bar facies. In the fluvial sediments of the Dacian Basin, a few point bar accumulations
have been identified. One of the studied point bar unit has been
observed in the terminal Dacian-aged deposits from Călugăreni village (Prahova
County) (Fig. 6.13). Another point bar, cropping out in a coal quarry from Oltenia
(west of Romania), is an outstanding feature which will be described in a separate
chapter of this book (see Chapter 7).
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