Explanation of an ‘extraordinary rock deformation’ — shear-related structures in the Ördöggát Quarry of the Transdanubian Range, Hungary

Abstract

The aim of the present paper is to describe and analyse the structural elements in the Valanginian Bersek Marl of the
Ördöggát Quarry in the Gerecse Mts, Hungary. Fiftyfour years ago, Bálint BALKAY (1955) published a short communication
on his observations on an ‘extraordinary rock deformation structure’. The first and main goal of this paper is to identify the
origin of the deformation; the existence of both sedimentary and structural mechanisms is supposed. The presence of cogenetical structural deformations and the total absence of sedimentary features suggest pure structural origin.
The formation of an oblique bedded domain in the whole quarry can be related to a bedding-parallel shear. The
internal block controlled by two parallel shear planes was cut by subhorizontal normal fault planes; this resulted in the
rotation of the blocks synthetic to the shearing direction. This mechanism differs distinctly from the well studied classic
block rotation (back tilt) model. Related shear plane related schichtosity, minor thrusts, bedding parallel striaes, the
geometry of the oblique bedded block and bedding controlled the s-c schistosity, this suggests the same origin and same
direction of shear. The direction of shear was top-to-east. Taking the palinspastic reconstructions and the structural
evolution of the region into consideration, the deformation age is possibly early Albian.
Evidences of an older deformation can be also identified in the Ördöggát Quarry. Steep E-NE–W-SW as well as N-S
trending fissures can be observed and these form a frequent network in the marl. Their geometry in the normal and oblique
bedded rock is different. The retilting of the fissures proves the primacy of fission followed by tilting of shear-controlled
blocks. The direction of the compression resulting in the fissures was NE–SW; taking previous results into consideration,
their supposed age can be assumed to be Aptian.
The direction of the observed deformation is opposite to the general shear and thrust direction related to the regional
structurtal trend, but the precise explanation of this contradictory looking deformation style will be the aim of a following
publication.