Meeting of Young Geoscientists, 5th April 2013. The results of sedimentological and gamma-ray logging of Upper Miocene lacustrine turbidites of the Transylvanian Basin Lilla Tőkés Department of Physical and Applied Geology, Eötvös University, Budapest István Róbert Bartha Institute of Geology, Babes-Bolyai University, Cluj Napoca, Romania

Aims Krézsek et al (2010) have studied the sedimentology of the Upper Miocene sediments of the Transylvanian Basin, but an in-depth study is lacking from the Southwestern part of the basin Study the relationship between natural gamma-ray radiation and facies Paleoenvironmental reconstruction

9,5 Ma Magyar et al 1999 Krézsek 2011 Upper Miocene

Krézsek et al 2010 Micesasa Tau Seica Mare Daia Ocna Sibiului Gusterita

Methods Detailed facies analysis Gamma-ray logging Portable gamma-ray spectrometer Scintillation Bismuth Germanate Oxide (BGO) detector of 20 cm3 volume

Test measurements Ocna Sibiului (Vízakna) Height Dose rate

Test measurements 30 sec measuring time unsteady, too scattered Ocna Sibiului (Vízakna) 30 sec measuring time unsteady, too scattered 60 sec measuring time 180 sec mesuring time correlates less with grain size Hereafter: measuring distance 3 or 10 points/meter Height Dose rate

Gusterița (Szenterzsébet) 50 m clay, silty clay Rarely very fine, fine grained sand beds are intercalated

Gusterița (Szenterzsébet) Height Gamma dose rate does not correlate with clay and silty clay variability – at macro scale 10 cm thick sand bed is evident on gamma graph Interpretation: Background sedimentation with occasional low-density turbidity currents Overbank deposit or outer lobe Dose rate

Daia (Dolmány) 4 5 1 2 4 3 3 2 1 6 5

Daia (Dolmány)

Interpretation: Gravelly and sandy high-density turbidity currents Dolmány (Daia) Interpretation: Gravelly and sandy high-density turbidity currents Channel erosion and fill – the finer part of the turbidity current was deposited on channel flanks Complex channel: channel fills are eroded repeatedly 1,5 km away from Gusterita, about 50 m higher – what is their relationship?

Micasasa (Mikeszásza) SW NE NE

Micasasa 4 + TG 3B 3A 2 1

Micasasa (Mikeszásza) Interpretation: I. thin-bedded clay, sand alternation – levee deposit II. erosion, 3 upward thinning cycles – channel III. erosion, upward thinning cycle – channel Channel-levee system 1,5x vertical exaggeration SW NE

Tau (Székástóhát) NW SE 2x vertical exaggeration

Tau (Székástóhát) NW SE

Channel-levee system: Tau (Székástóhát) 2x vertical exaggeration NW SE Interpretation Clay and channel fill sand sequences alternating horizontally and vertically Channel-levee system: Some 10 meters away from a channel filled with sand, thin- bedded clay and sand is deposited After filling, the channel moves away, clay is deposited Later another channel arrives: thick sand, numerous erosion surfaces

Ocna Sibiului (Vízakna)

Ocna Sibiului (Vízakna)

Ocna Sibiului (Vízakna)

Ocna Sibiului (Vízakna)

Ocna Sibiului (Vízakna)

High-density sandy turbidity currents or sandy debris flows Ocna Sibiului (Vízakna) Interpretation: High-density sandy turbidity currents or sandy debris flows Thinning and fining upward cycles, lack of erosion Mid-fan, lobe – the system gets filled, the centre of deposition migrates

Şeica Mare (Nagysejk)

High-density sandy turbidity currents or sandy debris flows Şeica Mare (Nagysejk) Interpretation: High-density sandy turbidity currents or sandy debris flows Lobe or channel on mid-fan

Conclusions Gamma-ray and facies logs correlate Small scale heterogeneity (on the order of 10 cm) can be roughly mapped with 10 cm measuring distance In a homogene succession a different lithology with only 10 cm thickness alters the graph for 1 m Rip-up clay clasts should not be overlooked No significant difference in gamma-ray response for different composition of sand

Conclusions Paleoenvironment ? Complex channel fill – Daia (Dolmány), Tau (Székéstóhát) Channel-levee systems – Micasasa (Mikeszásza), Tau (Székástóhát) ? Lobe – Ocna Sibiului (Vízakna), Seica Mare (Nagysejk) ? Overbank or outer fan – Gusterita (Szenterzsébet) Richards et al 1998

Thank you for your kind attention! We are grateful to Dr. Orsolya Sztanó, Dr. Csaba Krézsek and Dr. Lóránd Silye for the supervision; to Dr. László Lenkey for his help in interpreting gamma-ray graphs to Szabolcs Kövecsi for gamma-ray measuring to Papp Simon Foundation and Koch Antal Geological Society for funding our work. Thank you for your kind attention!