Krzysztof Starzec

A regional cross-section based on geophysical, borehole, and structural field data reveals the subsurface structure of the western Polish Outer Carpathians, a relatively understudied segment of the Carpathian thrustand- fold belt. A key feature is an antiformal stack, where the Dukla Unit underlies the central part of the Magura Unit. This structure probably was formed due to a thrust ramp within the Palaeozoic cover of the North European Platform. New apatite fission-track ages, along with a re-evaluation of published thermochronologic age data, indicate two exhumation phases. The first occurred in the Early Miocene (~20 Ma), affecting the Magura and Silesian units. The second, in the Late Miocene (~10 Ma), is confined to the Magura Unit and linked to an antiformal stack formation either through underthrusting of the Magura Unit by the Dukla Unit or large-scale normal faulting that drove arc-parallel extension. Both structures are supported by outcrop and seismic evidence. The present authors propose that the lateral termination of the antiformal stack triggered a normal fault system, forming a hanging-wall drop fault along a lateral culmination wall. The structural data of the present study do not support a Middle–Late Miocene arc-perpendicular extension, previously documented in the eastern Polish Outer Carpathians, as a viable explanation for the Late Miocene exhumation, recorded in the western sector.

Facies were recognized based on the results of well logging data, using these measurements and conducting a comprehensive interpretation. This was done by first using shaliness (volume of shale) as the main parameter for a geological profile division into petrophysical lithofacies before secondly calculating the Flow Zone Index (FZI), utilizing porosity and permeability estimated from well logging interpretation and defining Rock Types on the basis of FZI frequency distributions. FZIs were also determined according to porosity and permeability from laboratory measurements whenever the results were available. Descriptions of cores and cuttings together with the results of geological field investigations were the basis for lithofacies determination. Maps of average porosity, permeability, and FZI values were constructed for the study area. Three types of facies data and maps of reservoir parameters were used to support the recognition of geological structures and their assessment of the potential for fluid movement in the rock formation. The facies analyses were performed on the example of the Istebna Beds of the Silesian Unit in the Carpathians.

Based on the interpretation of 2D seismic profiles integrated with surface geological investigations, a mechanism responsible for the formation of a large scale normal fault zone has been proposed. The fault, here referred to as the Rycerka Fault, has a predominantly normal dip-slip component with the detachment surface located at the base of Carpathian units. The fault developed due to the formation of an anticlinal stack within the Dukla Unit overlain by the Magura Units. Stacking of a relatively narrow duplex led to the growth of a dome-like culmination in the lower unit, i.e., the Dukla Unit, and, as a consequence of differential uplift of the unit above and outside the duplex, the upper unit (the Magura Unit) was subjected to stretching. This process invoked normal faulting along the lateral culmination wall and was facilitated by the regional, syn-thrusting arc–parallel extension. Horizontal movement along the fault plane is a result of tear faulting accommodating a varied rate of advancement of Carpathian units. The time of the fault formation is not well constrained; however, based on superposition criterion, the syn -thrusting origin is anticipated.

An appropriate velocity model from well logs is a key issue in the processing and interpretation of seismic data. In a deep borehole located in the central part of the Polish Outer Carpathians, the sonic measurements were inadequate for seismic purposes due to the poor quality of data and gaps in the logging. Multiple regression (MR) and a modified Faust equation were proposed to model the velocity log. MR estimated the P-wave slowness as a dependent variable on the basis of sets of various logs as independent variables. The solutions were verified by the interval velocity from Check Shots (CS) and by the convergence of synthetic seismograms and the real seismic traces. MR proved to be an effective method when a set of other logs was available. The modified Faust method allowed computation of P-wave velocity based on the shallow resistivity logs, depth, and compaction factor. Faust coefficients were determined according to the lithology and stratigraphy divisions and were calibrated with the use of the velocity previously determined in the MR analysis. The modified Faust equation may be applied in nearby old wells with limited logging data, particularly with no sonic logs, where MR could not be successfully applied.

The Dukla Nappe in the Skrzydlna area exposes two types of mélange reflecting two different phases of basinal and tectonic evolution of the Outer West Carpathian orogen in its Polish sector. The Oligocene-age sedimentary mélange (olistostrome) is related to growth of the accretionary wedge, whereas the Miocene-age diapiric mélange postdates the orogenic thrusting. Textural and structural features of the very coarse-grained sedimentary mélange suggest non-cohesive debris flows and high-density turbidity currents as predominant emplacement mechanisms. Growth strata, associated with progressive unconformities, and facies contrast between the underlying fine-grained unit and the overlying olistostrome reflect a considerable uplift of the source area and rotation of the adjacent part of the basin floor. The olistostrome and the overlying turbidite succession form a retrogressive sequence interpreted as a submarine canyon infill grading to a small submarine fan. The diapiric mélange, injected into the Oligocene-age succession of the Dukla Nappe, contains the Early and Late Cretaceous-age blocks and matrix derived from the underlying Silesian Nappe. The features reflecting diapiric emplacement include matrix proportion increase and block content decrease towards the mélange margins, scaly fabric and shear zones. Both mélanges, interpreted in the past as chaotic bodies, upon detailed examination reveal genesis-related subtle internal organization.

Mélanges and broken formations at the boundary zone of the Magura and Silesian nappes (Gorlice area, Polish Outer Carpathians) - a result of sedimentary and tectonic processes