Sedimentological and structural geologic observations at the eastern slope of the Gorba High (“Gyökér Ravine”, Western Gerecse Mts, Hungary)

Abstract

The deposition of the Jurassic – Lower Cretaceous sequence in the Gerecse Mts was defined by differentiated
palaeotopography likewise in other parts of the Transdanubian Range (GALÁCZ & VÖRÖS 1972, GALÁCZ 1988, VÖRÖS &
GALÁCZ 1998). The most characteristic element of the palaeomorphology was the uplifted Gorba High (CSÁSZÁR 1995)
where condensed, discontinuous sedimentation occurred. The studied section is situated in a small valley called Gyökér
Ravine in the eastern flank of the Alsó-Látó Hill (Figure 1) where multiple recurrence of an approximately 5 m thick
Jurassic–Berrriasian condensed succession can be observed (Figure 3). The lowermost Jurassic beds are crinoid-bearing
limestones containing thin calcareous turbidite layers of Liassic age overlain by Saccocoma- and Calpionella-bearing
Upper Jurassic – Berriasian limestones and Lower Cretaceous sandstones (Figure 6). The Middle Jurassic is missing,
apart from a local appearance of condensed variety of the Tölgyhát Limestone Formation (Bositra limestone) which
related to the coeval sediments of the Asszony and Szél Hills. The sedimentary sequence suggests deposition in the upper
part of the palaeoslope of the Gorba High. This marginal position of the profile is also indicated by the presence of
Hierlatz Limestone and a Liassic fault or dyke margin at the western end of the southern part of the Gyökér Ravine.
In contrast with the former olistolithic models (LANTOS 1997, FODOR & LANTOS 1998, BÁRÁNY 2004) low angle, west to
north-west dipping normal faults can explain the repeated occurrence of the Mesozoic series. The tectonic reconstruction
(backtilting) suggests that the normal faults formed due to ~ESE–WNW extension, before the tilt of the sequence, which was
induced by NE–SW compression. The normal faults supposed to be Aptian to early Albian, while the tilt itself could be
Albian.