64 Geodiversity

The CIasts Petrography of Oligocene from The Getic Depression: A graveI facies provenance

Formations and sand

Roban Relu - Dumitru l, Anastasiu Nicolai University ofBucharest, Faculty ofGeology and Geophysics, N Balcescu, No. 1, Bucharest, Romania reludumitru. roban@g.unibuc.ro: nicanastasiu@gmail.com

The comparative petrographical analysis was used in an attempt to reconstruct the Oligocene relationships between tecto-structural units of South Carpathians and their peripheral foreland basin.

Geological settings and the studied formations The foreland area situated between South Carpathians and Moesian Platform contains three sedimentary cycles: upper

Cretacic- lower Burdigalian, upper Burdigalian - lower Sarmatian, middle Sarmatian - Pliocene. The frrst two were strongly deformed during the Burdigalian and Sarmatian tectogenesis (Maţenco, 1997), and are known in romanian classicalliterature as Getic Depression. The interest formations are part ofthe first cycle and are exposed on the north- eastem side ofGetic Depression. These are characterized, according to Roban and Melinte, (2005) by the succession ofthe following lithostratigraphic units: (1) the Călimăneşti Formation (Ypresian), (2) the Olanesti Formation (Lutetian Priabonian), (3) the Cheia Formation (Rupelian-lower Chattian), (4) the Corbi Formation (Rupelian-lower Chattian), and (5) the Brăduleţ Formation (Rupelian lower Burdigalian). The conglomerates of Cheia Formation from west area are coeval with the sandstones of Corbi Formation exposed in the eastem part and both, are bounded by mudstone facies of Olăneşti and Brăduleţ formations. The study focuses only the clasts of Cheia and Corbi coarse grain formations .

.. Crystaline basement

~ Calimaneşti ~ Fonmation

l1IIIIJJ11I1III OIăneşti -.ru Fonmation

f"777}\ Brădule\ rLLLL1 Formation

Hi~ţ;;~~~~~ ~~::tf~~ EfHHH Gura Vaii IIHHB Formation

~Sarata ~Gypsum ~ Maţau ~ Conglomerates

L=:J Quatemary

• TownNillage

-- Fault

......... Sections

5km

Sections investigated

G) Cheia Valley

@ Olaneşti Valley

@ Muiereasca Valley

@ Valsan Valley

® RAul Doamnei Valley

Fig. 1. Geological map ofthe NE Getic Depression (modified after Romanian Geological Survey)

Abstracts 65

The Cheia Formation is well exposed in Cheia, Olăneşti and Muiereasca valleys (fig. 1). Ithas a total thickness of500 m and contains mainly gravelly and sandy facies (Table 1). Depositional facies suggest gravitational processes like debris falls , coesive and cohesionless debris flows, high-density turbidite currents and settling of suspension, within a deep sea coarse grain deltas environment (sensu Postma 1990) with a complex evolution marked by intense processes of resedimetations caused by instability.

The Corbi Formation, exposed between Vâlsan and Doamnei valleys (fig. 1), has a total thickness of250 m and it is mostly composed of sandy and subordinately gravely and muddy facies (Table 1). The facies suggest cohesionles debris flows, grain flows, and high-density turbiditic flows process. Sedimentary environment in the first stage, similar with Cheia Formation is thought to be a deep sea coarse grain fan delta, and in the second stage, a mouth bar delta system (sensu Postma, 1990). The petrological types corresponding to the sandy and gravelly facies.are sub-lithic and quartz sandstones and less lithic and sub­fe1dspathic sandstones and polymictic matrix and grains supported conglomerates.

Grain size Cheia Corbi percent % Formation Formation

GraveI 80 35 Sand 17 50 Mud 3 15

Table 1. Grain size proportion of Cheia and Corbi formations

The both formations are depositional systems with point source.

Petrographic analyses The petrographic examination of silicic1astic formations Cheia and Corbi is based on optical microscopy.

In Cheia Formation (FCR) the main petrological types are polymictic matri x and grains supported conglomerates, sub-lithic, sub-feldspathic and lithic sandstones. (Fig. 2)

Q

A)

• Microconglomerates * Sandstones B)

Fig. 2 A) Modal QFL composition ofmicrogravelly and sandy samples of Cheia Formation. B) Modal composition Lm Lv Ls of rock fragments from microcloglomerates

The roundness (Ro) of the gravelly and sandy elements falls into sub angular to rounded categories. The quartz is dominated by undulatory extinction, oriented solid inc1usions suggesting metamorphic source. The potassic feldspars (microc1ine) and plagioc1ase (albite-oligoc1az) have metamorphic origin, and their different state (8:2, fresh/altered) is suggesting different sources and evolutions, determined by the alteration.

The rock fragments (fig. 3) are giving the best information regarding the sedimentary source areas. The present types are: - metamorphic rock fragments : orthoc1ase gneiss, microcline gneiss, gamet gneiss, muscovite and biotite gneiss,

paragneiss, mica-schist, gamet mica-schist, amphibolites, ec10gites presenting different grades of retromorphism, crystalline limestones, mylonites, migmatites;

- magmatic rock fragments: pegmatites, granitoides, graphic granitoides; - sedimentary rock fragments: lithic sandstones with mostly sedimentary rock fragmentsand c1ay. The limestones are

also often present. There have been identified the next structural types: massive limestones and limestone depositional and tectonic brec6ia. After Dunham (1962) nomenc1ature, these lith-c1asts correspond to the categories: grainstone, packstone, mudstone, baffiestone, framestone, floatstone, rudstone. After Folk (1962) nomenc1ature, they correspond to the petrotypes: oosparite, pelsparite, pelmicrite, biomicrite, biolitite, micri te, dismicrite.

66 Geodiversity

Fig. 3 Rockfragments typesfrom Cheia Formation: a - orthoclase gneiss, b - eclogite with different grades of retromorphism; c­limestone pelsparite, d sedimentary rock fragment with a graphic granitoide clast.

Keliphitic coronas ofthe gamets, alteration ofbiotite into chlorite and serpentinization of the mafic minerals indicate retrogressive metamorphic sources. Graphic myrmekites and perthites indicate that sub-solidus magmatic processes affected the source rocks. The presence, frequency and the characteristics of the foliation some in metamorphic clasts indicate poly­metamorphism processes within source area. Heavy minerals from arenites and the matrix of the gravei is embracing a spectra more varied: zircon, tourmaline, gamet, apatite (pyroxenes, amphiboles). Their affiliation to magmatic or metamorphic sources requires supplementary analysis.

The petrological types corresponding to the sandy and gravelly fac ies of Corbi Formation are sub-lithic and quartz sandstones and less lithic and sub-feldspathic sandstones and polymictic matrix and grains supported conglomerates (fig. 4).

Q

Microconglomerates Sandstones

Lm

B)

Fig. 4 A) Modal QFL composition of microgravelly and sandy samples of Corbi Formation. B) Modal composition Lm Lv Ls of rock fragments from microcloglomerates

The petrographic exam ofthe clasts from offered information about their petrography and origin (fig. 5):

- the gravei clasts have high roundness index, falling into sub-rounded and rounded categories; the arenitic clasts are sub­angular to sub-rounded, presenting indication of re-processing during the transport and re-deposition; - the quartz grains belongs to different generations (through recycling) and presents features indicating their affiliation to distinct petro-types; - plagioclase (An,s) is more frequent then the potassic feldspars (ortho.clase and microcline); - the rock fragments are mainly metamorphic petro-types: gamet and staurolite gneiss, biotite and muscovite gneiss, mylonite. - sedimentary rock fragments are: polymictic ortho-conglomerates, lithic sandstones and limestones. The last ones are massive in structure and are classified as: biosparite with a diversified biotic presence, mostly big foraminifera, biosparite with very large numulites (mm), pelsparite, limestones with earthy elements. The siliciclastic sedimentary rock fragments ( clast -supported conglomerates and sandstones) have a low maturity degree, suggested by the presence in large quantities ofthe feldspars and rock fragments, usually with poikilitic calcitic cement. - volcanic rock fragments are rare, being represented by porphyritic micro-granodiorite and dacite; - heavy minerals is represented by zircon, tourmaline, rotile, gamet, staurolite, green homblende a very good one for markers which can allow lithostratigraphic correlations.

Abstracts 67

Fig. 5 Roekfragments typesfrom Corbi Formation: a - sub-lithie sandstone, the main petrotype of Corbi Formation, b- mylonite formed on the gneisse, e­sedimentary litho-clast, limestone (biosparite with numulites), d-voleanie litho-clast, dacite with zoned and weathering feldspar and bipiramidal quartz.

The source area presumed: Getic and Supragetic Units of Southern Carpathians

The supposed source areas are the metamorphic complexes from the Getic and Supragetic Units, together with their sedimentary covers, Jurassic - Cretaceous and, older deposits from Getic Depression unit. Considering stability grade (Anastasiu, 1986) ofthe mineral constituents from the preexistent rocks in the exarnined elasts, petro-types with high conservation potential and high certainty grade for the reconstruction of the source areas were found. Applying these criterions, we could define diagnosis mineral associations and we realized the comparative analysis.

The results confmn major similarities between the Getic and Supragetic types ofmetamorphic rocks and the Cheia Formation petro-types. Therefore:

• There is a high certainty grade to state likeness between: i) metamorphic rock fragments (symplectite eelogites) and similar petro-types from the Getic metamorphic belt; ii) sedimentary rock fragments (limestone breccia and biosparite, biolitite, Jurassic and Cretaceous in age Vanturarita Mountains) and the limestones from the sedimentary cover ofthe Getic Unit similar in age;

• Medium certainty grade for: i) metamorphic rock fragments (microeline gneiss, orthoelase gneiss, gamet gneiss, paragneiss, mica-schists, mylonites, amphibolites) and for ii) magmatic rock fragments (pegmatites, graphic granitoides). Forthe Corbi Formation,

• High certainty grade for the sedimentary rock fragments such limestones with numulites, specific for Eocene (Albeşti limestone?! from northern part ofGetic Depression);

• Medium certainty grade for gamet and staurolite gneisses, biotite and muscovite gneisses, which are similar with the Supragetic units. We also can tell that the mylonites formed on the gneisses are mentioned in shear zones from the Supragetic metamorphic belt.

Discussion and conclusions The premises for the reconstruction of source areas for FCR and FCO took into consideration the comparative

examination ofthe used criterions, enunciated before: 1. The pattern ofthe petro-types distribution from the interest areas FCR and FOC is difIerent and this fact is suggesting that in Oligocene were two point sources with difIerent behavior and a slightly distinct evolution; 2. The grains size spectra ofFCR have a gravelly tendency against the FCO, which is sandy. Probably FCR had a proximal position in respect to source area. 3. The comparative examination between rock fragments and supposed petrotypes from source area suggest:

i) the elastic material for deep sea fan delta of Cheia Formation is original from Getic Unit and their sedimentary covers; ii) the source area for deep sea fan /mouth bar delta of Corbi Formation was Supragetic Units, their sedimentary cover and older rock from Getic Depression .

4. The presence ofthe volcanic rock fragments in Corbi Formation raises many questions about there origin, making possible the elaboration of a few hypotheses:

i) an explosive event during the Oligocene; ii) the erosion of some volcanic facies which are not anymore present in Supragetic units specific for Făgăraş Mountains; iii) the origin from the Getic unit specific to Sebeş Mountains.

There are no proofs for the existence of an Oligocene volcanic arc, but the other hypotheses are plausible. 5. The paleocurrents directions registered into silicielastic _~equences from Cheia and Corbi formations (Jipa, 1994) sustains the transport from the NW and N sources referred to the present position. Considering the continuous evolution of the Southern Carpathians, marked by translations, rotations and intense decollements (Miocene) (Schmid et al., 1998) we suggest that position of entire ensemble source area /foredeep bas in it must rotatescounterelockwise.

References ANASTASIU N., (1986), Conceptul de petrologie comparată şi reconstituiri1e de arii sursă în formaţiunile de fliş (Comparative Petrology Concept and Flysch Formations), St.cerc.geologie. Academia Română, 31, pp.89-100. Jipa, D., (1994), Studiul fenomenului de sedimentare laterală. PhD Thesis, University ofBucharest, 230 pp. MAŢENCO, L. (1997). Tectonic evolution of the Romanian Outer Carpathians: Constraints from kinematic analysis and flexural modeling. PhD thesis, Vrije University -Amsterdam, 160 pp.

Postma G., (1990), Depositiona1 architecture and facies of river and fan deltas: a synthesis, in Coarse- Grained Deltas, Ed. by Colella A. and Prior D. B. , Spec. Publs int. Ass. Sediment, 10, p . 13-27 ROBAN, R . D ., MELINTE, M. C., (2005), Paleogene Litho- and Biostratigraphy of the NE Getic Depression (Romania), Acta Palaeontologica Romaniae, v. 5, p. 335-353 SCHMID, S. M., BERZA, T., DIACONESCU, v., FROITZHEIM, N., FUGENSCHUH, B. , (1998), Orogen-parallel extension in the Southern Carpathians, Tectonophysics, 297, p. 209228.