The Pannonian deposits of the Buda Mountains

Abstract

Deposits of the Late Miocene Pannonian stage occur at surprisingly high elevations — up to almost 500 m above sea
level — in the southern part of the Buda Mountains (on the top and the slopes of the Sváb-hegy in the western part of
Budapest, Figure 1). The sedimentary succession usually starts with gravel and sand/sandstone, overlain by silty deposits.
The latter are in turn capped by silts, clays, and freshwater limestone (Figure 2). Fossils of Late Miocene vertebrates and
molluscs have long been known from these deposits. The uppermost siliciclastic units of the sequence, from right below the
limestone, yielded partly freshwater and terrestrial snails (such as Planorbarius, Lymnaea, and Succinea), and partly species
of Melanopsis and Theodoxus, the latter that were endemic to the nearshore, strongly freshwater-influenced regions of the
brackish Lake Pannon. The freshwater limestone contained poorly preserved moulds of freshwater and terrestrial snails and
thus its age remained uncertain until Late Miocene (Pannonian) vertebrates were recovered from its layers. However,
unambiguous indications that the giant brackish Lake Pannon ever flooded the Buda Mountains is missing.
This paper introduces the first brackish endemic Lake Pannon bivalve fauna from the Buda Mountains. The fossils
were collected in two locations. Construction works in Fodor street, at 305–310 m above sea level (Figure 2) —
temporarily exposed a 7-metre-thick section of Pannonian sediments (Figure 3). The sequence consisted of a finegrained quartz sandstone with thin pebbly intercalations and silt layers. The grey sandstone was unevenly tinted
reddish by goethite and also contained some kaolinite. The fossils occurred as ornamented moulds within the
sandstone. The following species were identified: Paradacna cf. wurmbi (LŐRENTHEY), Congeria cf. simulans turgida
ANDRUSOV and Dreissenomya sp. (Figure 4).
The argillaceous marls intercalating the freshwater limestone (that outcrops at the eastern edge of the hilltop plateau
in two sections at Normafa [Figures 2, 3]) contained endemic Pannonian molluscs characteristic of lagoonal
environments. The following taxa were identified: ?Anodonta sp., Unio sp., Dreissena sp., Lymnocardium decorum
(FUCHS), Viviparus sp., Hydrobiidae sp., Micromelania cf. laevis (FUCHS), Theodoxus radmanesti (FUCHS), Theodoxus
sp., Melanopsis cf. sturii FUCHS, Melanopsis sp., Lymnaeidae sp., Planorbidae sp., and terrestrial species (Figure 4).
These faunas lived in the brackish Lake Pannon. The Normafa mollusc assemblage is similar to the classic Tihany
fauna, and belongs to the Lymnocardium decorum zone (8.0–8.7 Ma). The Fodor street fauna is either of the same age or
slightly older (Lymnocardium ponticum zone, 8.7–9.6 Ma; Figure 5).
The Pannonian layers in the Buda Mountains comprise one depositional cycle. The lower part of the sequence
indicates high-energy lacustrine conditions. The gravel was derived from the underlying Palaeogene or Lower Miocene
sediments. The sandstones may have originated from more distant sources and suffered fluvial transport. The overlying
silty and shaly layers were deposited in low-energy environments, such as lagoons. The closing member of the sequence,
the Nagyvázsony Limestone probably formed in places where the ancestors of the present-day karstic thermal springs
charged into very shallow lagoons and floodplain lakes, causing the deposition of calcareous mud. Any minor rise in the
level of the brackish Lake Pannon, however, could switch off limestone deposition and facilitate the dispersal of shallowwater endemic mollusc species. The alternation of limestone layers, containing freshwater fossils and silty-clayey layers
with endemic Lake Pannon fossils may be attributed to such environmental changes.
Flooding of the Buda Mountains by Lake Pannon was probably a consequence of a lake level rise. The Pannonian
deposits are in conspicuously higher positions than the earlier Neogene formations in the vicinity. Although this pattern
can be a result of tectonic subsidence right before the flooding by Lake Pannon and subsequent differential uplift, we
suggest that the high level of Lake Pannon — significantly higher than the coeval sea level — also played an important
role in the present-day high elevations of the lacustrine sequence