СТРАТИГРАФИЯ. ГЕОЛОГИЧЕСКАЯ КОРРЕЛЯЦИЯ, 2013, том 21, № 3, с. 60-71
PROBLEMATIC MICROSCOPIC TRACE (?) FOSSILS, OLIGOCENE, SLOVAKIA
© 2013 R. Mikulas", D. Boorova*, and K. Holcovac
aInstitute of Geology, Academy of Sciences of the Czech Republic, Rozvojovâ 269, 16502 Prague
e-mail: mikulas@gli.cas.cz bSlovak Geological Survey, Mlynskâ Dolina 1, Bratislava, Slovakia e-mail: daniela.boorova@geology.sk cInstitute of Geology and Paleontology, Charles University, Prague, Albertov 6, 128 43 Czech Republic
e-mail: holcova@natur.cuni.cz Received February 7, 2012
Abstract—Meioscopic to microscopic capsules found in reddish, probably marine or brackish shales (Oligocene, Pannonian Basin; South Slovakia) are interpreted as possible trace fossils. They may represent burrows of meioscopic in-fauna. Ferruginous walls of the capsules appeared very probably as late as during diagenesis; however, the (possibly organic) matrix had to exist before the diagenetic processes. Other discussed explanations (inorganic "ironstones"; coprolites) are not plausible.
Keywords: Pannonian Basin, Oligocene, microfossils, meiofauna, burrows DOI: 10.7868/S0869592X13030058
INTRODUCTION
The term "problematica" involved in the descriptive palaeontology of the 19th and early 20th century body fossils of uncertain systematic affinity, most trace fossils and structures of doubtful organic origin. The paper by Fritsch (1908) named "Probematica Silurica" illustrates well this attitude. During the time, the group of "problematica" dwindled as the knowledge of fossil fauna and flora—and especially of trace fossils—expanded (cf. Abel, 1935; Seilacher, 1953; Hantzschel, 1962, a.o.). Nowadays, the term "problematica" is rather a rarity in palaeontology. The find we describe below pertains to "classical" problematica. Herein, we bring its detailed description and interpretation as possible meioscopic trace fossils, which alone is a "problematic", overlooked size category of ichnologic record. More detailed description of bioturbate structures caused by meiofauna and data on meiobenthic tracemakers were given by Bromley (1996); recently, numerous papers on crypto-bio-turbation and cryptobioturbated rocks appeared (e.g., Pemberton et al., 2008). On the other hand, as far as we know, none structures have so far been interpreted as domichnia of meiofauna, except borings. The structures focused in further text thus may be the first ones that can be understood (through their size, morphology and chemical composition) as meiofaunal domichnia.
The aim of the contribution is also to discuss other possible explanations and aspects. Finally, the paper is aimed to bring empiric knowledge on the surrounding beds. We believe that the study may lead to finds of similar structures, both in the fossils record and in modern settings.
GEOLOGIC SETTINGS
The studied borehole was situated in the geomor-phological unit called the South Slovak Basin (Central Western Carpathians). Basement of the unit is built by Triassic shales and limestones which originated in the continental shelf of the hypothetical Meliata (or Me-liata-Hallstatt) Ocean (a part of the Tethys Ocean; Ivan, 2002). After closure of the Meliata Ocean in the Middle Jurassic, the area was repeatedly inundated by the epicontinental sea. Firstly it happened probably during the Upper Cretaceous (Vass, 2002). Later, from the Oligocene to the Middle Miocene (Rupelian— Langhian), the South Slovak Basin was periodically flooded by the epicontinental seas belonging to the Central Paratethys System (Rogl, 1998; Popov et al., 2004). Two sea-level cycles were recorded during the Oligocene. The first one is correlated with the calcareous nannoplankton Zone NP23 and lower part of the NP24 Zone (the Central Paratethys regional stage Kiscellian, equivalent of the Rupelian); the second one is correlated with the upper part of Zone NP24 and Zones NP25 and NN1 (the Central Paratethys Stage Egerian corresponding to the Chattian; Vass et al., 1989). During these two cycles, the South Slovak Basin represented the northern part of the Buda Basin (Baldi, 1986; Vass, 1996). Litostratigraphicaly, the Kiscellian is built by variegated terrigenous shales of the Skalnik Formation and by marine transgressive coarse clastics of the Blh Formation. Both formations are overlain by open marine dark claystones to silt-stones of the Lenartovce Formation (Vass and EleCko, 1982; Vass et al., 1989; Vass, 2002). During the Kiscel-
i_i_i_i_i_i
0 5 10 15 20 25 km
Fig. 1. Location of the studied borehole (modified from Halasova et al., 1996). 1, pre-Cenozoic rock; 2, Hungarian Paleogene Basin (Oligocene); 3—4, Fil'akovo-Petervasara Formation (Early Miocene, Eggenburgian, marine sandstones to siltstones); 5, lower rhyolite and rhyodacite tuffs (Early Miocene, late Eggenburgian + Ottnangian); 6, Nograd/Novohrad Formation (Early Miocene, Ottnangian and Karpatian, terrigenous siltstone to sandstone and marine "schlier"); 7, middle rhyolite tuffs (Early/Middle Miocene, Upper Karpatian—Lower Badenian); 8, Middle Miocene volcanics; 9, Middle to Upper Miocene sediments (terrigenous); 10, Upper Miocene to Pleistocene Basalts.
lian, the first of endemic Paratethyan assemblages appeared due to the early isolation of the Central Parat-ethys area. The isolation leaded to hypoxic to anoxic environment at the bottom (Baldi, 1986; Rogl, 1998).
Borehole FGRK-1 is situated in the eastern part of the South Slovak Basin (in the subunit called the Rimava Basin), NW from the spa CiZ and SE from
the village Ivanice (Y(JTSK) = -339820.82 m, X(JTSK) = -1278265.13 m, at the altitude of 200 m). Depth of the borehole was 1050 m (Fig. 1). The borehole was preliminary described by Marcin et al. (2009).
The studied variegated violet, red and brown shales and siliceous sandstones containing problematic microscopic trace fossils were recorded at the base of the boreTOM 21 № 3 2013
Ma 21
22 23 ■ 2425 ■ 26
27 ■
28 ■
29 ■
30 ■
31
32
Paragloborotalia nana (Bolli)
Coccolithus pelagicus (Wallich) Schiller 50 M-m Helicosphaera compacta
'.eticulofenestra Bramlette & Wilcoxon
100 p,m
Globigerina anguliofficinalis Blow
1050
Fig. 2. Stratigraphical correlation of the studied interval of the FGRK-1 borehole. Stratigraphical range of microfossils according to Aubry (1990), Young (1999), Berggren et al. (1995), Rogl (1986), Cicha et al. (1998). 1, sandstones; 2, siltstones; 3, claystones; 4, coarse sandstones to conglomerates with clasts of overlain rocks; 5, calcareous sandstones with fragments of echinoderms; 6, trace fossils; 7, benthic foraminifera; 8, planktonic foraminifera; 9, calcareous nannoplankton; 10, no fossil content.
hole in depths interval 900—1050 m. Lithologically, the shales are similar to the lower Triassic Bodvaszilas Formation of the Silica Unit as defined by Kovacs et al. (1989). However, new finding of calcareous nannoplankton and foraminifera enables us to correlate this interval with the Oligocene marine transgression (see the next chapter).
Boorova in Marcin et al. (2009) determined the boundary of the local Central Paratethys stages Kiscel-lian/Egerian at the depth of 628.90 m. Zlinska (2009) also correlated the upper part of the borehole (15— 625 m) with the Egerian regional stage.
BIOSTRATIGRAPHY AND PALEOECOLOGY
Analysed succession (900—1050 m) is very poor in fossils. Macrofossils are represented only by undetermin-
able fragments ofechinoderms accumulated in small len-ticles of the calcareous sandstones (Boorova in Marcin et al., 2009). Trace fossils were recorded only in the lower part of the interval (1005-1050 m; Fig. 2).
To precise biostratigraphy and paleoecology of the studied interval, we tried to isolate foraminifera, calcareous nannoplankton and palynomorphs. Unfortunately, palynomorphs have not been found (KovaCova, 2011, personal communication). Other microfossils are rare; however, their assemblages enable biostrati-graphical correlation and also limited paleoecological interpretation. Calcareous nannoplankton was recorded in two horizons (Fig. 2). Only Thoracosphaera spp. was recorded in the sample 997 m. It represents vegetative calcareous cysts of several marine peridin-
iphycidean dinoflagellate species. The genus is considered generally as indicator of stress environment. However, the reasons of primary accumulations of Thoracosphaera spp. have not been satisfactory explained yet: Holl et al. (1998, 1999) observed enhanced abundances of the calcareous dinoflagellates in periods with reduced productivity, while Wendler et al. (2002) reported higher abundance during the increased productivity. The cysts are relatively resistant to chemical and physical degradation (Vink et al., 2002; Vink, 2004). More diversified and abundant calcareous nannoplankton assemblage was recorded in sample 900 m. The assemblage is probably incomplete, influenced by diagenetic dissolution. It can be interpreted from poorly preserved nannoliths as well as from higher abundance of species very resistant to dissolution: Coccolithuspelagicus and Helicosphaera compacta (Roth and Berger, 1975; Roth, 1994; Aubry, 1990; Flores et al., 2003). The assemblage is dominated by Reticulofenestra minuta which indicates environmental stress in usually near-shore environments characterized by quick changes within that environment including oscillation of salinity (Wade and Brown, 2006) and oscillation of nutrient content (Gartner et al., 1983; Hallock, 1987; Beaufort and Aubry, 1992; Wells and Okada, 1997; Flores et al., 1997; Bollmann et al., 1998; Kameo, 2002). Also Pontosphaera multipora indicates a near-shore environment, though it reflects stable marine conditions and only slight salinity fluctuations (Melinte, 2005). Co-occurrence of Helicosphaera compacta (NP16-NP23; Aubry, 1990; Young, 1999) and Cyclicargolithus abisectus (31.1-23.5 Ma = NP23-NP24; Berggren et al., 1995; Young, 1999; Lourens et al., 2004) is ofbio-stratigraphical significance and enab
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