The mineral composition of the Pannonian s.l. Formations in the Great Hungarian Plain (II). - Tendencies of the changes of the mineral composition of the Pannonian s.l. sands and sandstones and their geological significance

Abstract

Sedimentological as well as seismic and sequence stratigrapic studies show that during the Late
Miocene - Pliocene (Pannonian s.l.) the main sediment input into Lake Pannon in the area of today' s
Hungarian Plain came from the NW and NE and subordinately from SE directions. Sediments were
carried along by large fluvial and deltaic systems into the lake. The characteristics of the mineralogical
composition and origin of sediments transported from different directions were studied on the basis of
the heavy mineral data of 868 sand and sandstone samples from 53 boreholes, also using statistical
methods. During the interpretation the studied samples were identified in terms of depositional facies,
depositional cycles, and lithostratigraphic units. The detailed results of the work can be seen in THAMÓBozsó et al. 2006. As a conclusion of this earlier detailed study, in this paper it is possible to highlight
major tendencies in the mineralogical composition of the sediments.
Different kinds of tendencies have been recognized and these have different backgrounds. There are
the characteristic differencies between the NW and NE sediment input of the basin. Sands which came
from the NW and reached the middle and southern parts of the Hungarian Plain, have significantly
more minerals deriving from metamorphic rocks, while in sands which originated from the NE and
which reached the Békés Basin, are characterised by the dominance of volcanic rocks. There is a minor
sediment source from the SE from the Apuseni Mountains. The average amount of chlorite gradually
increases from fluvial sands, through delta and basin-slope sediments to the deep-basinal turbidite
sandstones. This is in contrast to garnet, volcanic pyroxenes and amphiboles, the respective frequencies
of which gradually decrease with water depth as well as distance from the source area. These tendencies
were caused by the selective sorting of minerals during transport and sedimentation and the changing
of source rocks. The maturity of the studied sands and sandstones primarily depends on the distance
from their source areas. There are characteristic vertical changes in the mineralogical composition of the
sedimentary succession. These changes are connected to depositional cycles rather than vertical facies
changes. The closer the minerals are to the source area, the stronger the changes are. The most
considerable changes are connected to tectonically controlled 3rd order sequence boundaries, especially
in those cases when they indicate a considerable hiatus. In the NE area, close to the sediment source,
even the 4th order cycles clearly show changes in the mineralogical composition.