DACIAN BASIN - GeoEcoMar

5.3 Deltaic environment in the Dacian Basin
5.3 DELTAIC ENvIRONMENT IN THE DACIAN BASIN
An important part of the deltaic sedimentation is taking place in the shoreface area.
The two environments are sometimes difficult to be clearly separated in old rocks.
Treating the subject of littoral environment in the Dacian Basin, the shoreline and
deltaic paleoenvironments have been delimitated on the following pragmatic basis:
■ the sediment accumulation displaying features determined by wave and
storm action have been attributed to the shoreline environment;
■ deposits with progradation characteristics but without sedimentary structures
produced by basinal agents (waves and storms), have been considered
as fluvial-dominated delta-front sediment accumulation;
■ a third environmental type is represented by sedimentary sequences with
both fresh-water and brackish-marine features. This kind of sediment accumulation
has been interpreted as representing the deltaic plain environment.
5.3.1. Delta front sedimentation
When it is associated with the absence of wave or storm sedimentary structure, the
coarsening upward grain size trend is the main feature of the Dacian Basin sediment
bodies interpreted as deltaic units. This is the major argument on which the
assignment of these units to the delta front sedimentation environment is based.
Deltaic coarsening upward units. Both surface, outcrop observations and subsurface,
well logs examination produce data concerning the deltaic coarsening upward.
The direct, outcrop observations point out the variable grain size range of the
coarsening upward deltaic bodies. Some deltaic units are fine-grained and evolve
from silty sand in the base, to fine or medium-grained sand at the terminal part
(Fig. 5.22). Other Dacian Basin deltaic sequences show a large grain size variation,
from silt or very fine sand at the base, to fine-grained gravel on top (Fig. 5.23). Even
if the top of the sequence is not much coarser-grained as compared to the grain
size at the base, the coarsening upward trend is sometimes emphasized by small
pebbles (clay but also quartzite) and shell fragments occurring at the terminal upper
part of the unit (Fig. 5.22 B).
The grain size variation of the Dacian Basin deltaic units can also be expressed by
the thickening upward of the sand beds (Fig. 5.24). In this way, the sand/clay ratio is
changing and the sedimentary unit is getting coarser-grained toward the top.
Coarsening upward sequences are confidently observed on the well logs
(Fig. 5.25). The geometry of the deltaic unit is sometimes more completely and
suggestively presented by the borehole geophysical logs. The Dacian Basin deltaic
units are usually multiple, consisting of a succession of coarsening upward units.
Each unit display a coarsening upward pattern (Fig. 5.25 B).
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