Paleoenvironmental interpretation and palynoflora of Devonian–Carboniferous subsurface sections from the eastern part of the Moesian Platform (Romania)

https://doi.org/10.1016/j.revpalbo.2020.104338Get rights and content

Highlights

  • Five exploration wells were analyzed.

  • Devonian–Carboniferous marine and terrestrial palynomorphs are presented.

  • Miospores and palynofacies composition contribute to environmental interpretation.

Abstract

Palynomorph assemblages and palynofacies analyses have been performed on several core samples from the Devonian–Carboniferous deposits identified in five wells located within the eastern region of the Moesian Platform. The investigated sections include, in ascending stratigraphic order, the Ţăndărei, Smirna, Călăraşi and Vlaşin formations. Based on the stratigraphic distribution of key taxa identified (miospores, chitinozoans, acritarchs), seven biozone intervals (four for the Devonian and three for the Carboniferous) have been recognized. The oldest samples were dated as being part of the micrornatusnewportense (MN) – lower part of breconensiszavallatus (BZ) Assemblage Zones (Early Devonian), while the younger ones are assigned to the kosankeivarioreticulatus (KV) – nobilisjunior (NJ) Assemblage Zones (Pennsylvanian). Palynofacies observations suggest a fairly distal depositional environment during the period between the Lochkovian and Pragian times, followed by some proximal/fluvio-deltaic conditions in Emsian–Early Eifelian (top of the Ţăndărei Formation). The upper Tournaisian to Serpukhovian sedimentary rocks of the Călăraşi Formation and lower part of Vlaşin Formation were deposited in inner neritic environments. Mud-dominated dysoxic/anoxic conditions of the Bashkirian, abruptly transitioned to deltaic deposition and oxidizing environments that persisted to the Moscovian.

The Early Devonian terrestrial palynoflora is dominated by trilete spores which belong to lowland vegetation (vascular plants), the latter preferring areas with open connections via rivers to marine sedimentary basins. Carboniferous samples yielded only terrestrial miospores of various types of arborescent and herbaceous lycopsids and ferns, suggesting different habitats such as non-flooded wetlands or swamps within coastal plain and continental interiors. These assemblages of Carboniferous miospores are an indication of neutral-humid climatic conditions which existed at the time of deposition.

Introduction

The Moesian Platform represents a continental block extending 250–300 km north–south and ~ 600 km east–west, located mainly in the present territories of Romania and Bulgaria (Fig. 1).

Starting with the mid-twentieth century, this unit has been intensively explored for conventional hydrocarbons (Paraschiv, 1979). Dispersed plant spores, as well as marine organisms (chitinozoans, acritarchs), are the primary tools used for biostratigraphic age determination and correlation of subsurface Paleozoic deposits across the Moesian Platform. Only a few papers (i.e., Venkatachala and Beju, 1961, Venkatachala and Beju, 1962, Beju, 1967, Beju, 1971, Venkatachala et al., 1969, Vaida and Verniers, 2005, Vaida and Verniers, 2006) are devoted to the palynostratigraphy of the Moesian Platform. Among these, the most detailed palynological study was presented by Beju (1971). Based on the stratigraphic distribution of the palynomorphs (chitinozoans, acritarchs, miospores), the author proposed 6 biozones for the Devonian–Carboniferous interval. During the Early Devonian (Lochkovian–Emsian, D1 Biozone), the chitinozoans are in continuous decline and become extinct near the end of this period. The palynological assemblage of the D1 Biozone consists mainly of miospores such as Anapiculatisporites burtonensis, Apiculiretusispora toriensis, Emphanisporites rotatus, Leiotriletes simplex and Retusotriletes rotundus. The microflora of the mid–Late Devonian (D2–D3 Biozones) includes abundant large miospores (e.g. Calyptosporites div. sp., Grandispora sp., Hymenozonotriletes div. sp.) or miospores with “smooth or ornate distal thickening” (Archaeozonotriletes variabilis, Geminospora svalbardiae, G. maculata, Rhabdosporites parvulus). The Carboniferous sedimentary succession, assigned to C1–C3 Biozones (Beju, 1971), has a diversified and well-preserved palynological assemblage. Among the commonly encountered taxa, various species of the Anulatisporites, Cingulizonates, Densosporites, Granulatisporites, Lycospora genera have been documented (Venkatachala and Beju, 1962, Venkatachala et al., 1969, Beju, 1971).

Other biostratigraphic information was previously obtained based on a diverse macrofossils, the Paleozoic age of the deposits being indicated by the assemblages with graptolites, brachiopods and bivalves (Iordan, 1990, Iordan, 1999, Seghedi et al., 2005).

Based on Paleozoic palynomorph assemblages (including chitinozoans, acritarchs and miospores; Beju, 1972, Iordan et al., 1985, Vaida et al., 2005), as well as macrofaunas (Iordan, 1972, Iordan et al., 1985), a northern Gondwanan origin for Moesian Platform can be suggested.

In the past, the palynological data (see the works quoted above) have been applied for petroleum exploration to determine a stratigraphic framework of subsurface Paleozoic deposits within the Moesian Platform. In the same context, this palynological study aims to contribute to a better understanding of the Devonian–Carboniferous stratigraphic succession, based on the analysis of 14 core samples from 5 exploration wells located in the eastern part of the Moesian Platform. The palynomorph assemblages were evaluated in terms of their chronostratigraphic significance and their bearing on the understanding of paleoclimatic and paleofloristic evolution across the Devonian–Carboniferous. The biostratigraphic data obtained allowed the correlation of several lithological levels assigned to Ţăndărei, Călăraşi and Vlaşin formations, penetrated by the five exploration wells. This study also aims to interpret the depositional environment during the Devonian–Carboniferous periods based on palynofacies data and palynomorph taxa.

Section snippets

Geological setting

The Moesian Platform is an E–W elongated structural unit, bordered to the north and west by the South Carpathians, to the south by the Balkanides, and extends eastwards to the Black Sea (Fig. 1). In Romania, the Moesian platform is separated from the Carpathian belt by the Pericarpathian Fault, oriented E–W and dipping north, with its eastern part being curved northward. Most authors divide the Moesian Platform in two sectors based on crustal thickness, simply named as East and West Moesia (

Stratigraphical framework

The Paleozoic deposits of discussion are part of the Cambrian–Carboniferous sedimentary cycle and, within the Meosian Platform, are evaluated based on a limited number of deep wells penetrations. According to Paraschiv, 1974, Paraschiv, 1982, Ionesi (1994), Seghedi et al. (2005), the main Devonian–Carboniferous lithostratigraphic units of the northern Moesian Platform are represented by the Ţăndărei, Smirna, Călăraşi and Vlaşin formations.

Material and methods

The study focuses on the palynostratigraphy of five petroleum exploration wells located in the eastern part of the Moesian Platform (Fig. 1). From these wells, 14 core samples were processed using standard palynological techniques described by Batten (1999). Approximately 50 g of rock from each sample were treated with HCl (37%) to remove carbonates and HF (48%) to remove the silicate minerals. Denser particles were separated from the organic residue using ZnCl2 with a density of 2.0 g/cm3. The

Palynological content

The palynological assemblages are well-preserved, show a low to moderate diversity and include both terrestrial (miospores, phytodebris) and marine elements (chitinozoans, acritarchs and very rarely scolecodonts). A total of 102 taxa (Appendix A), mainly consisting of miospores (83 taxa), acritarchs and chlorophytes (9 taxa) and chitinozoans (8 taxa), were observed. Rare occurrences of scolecodonts and phytodebris are noted.

The following palynomorph assemblages were identified in the studied

Biostratigraphy and correlation with other known biozonations

In this paper, the standard Western European biozonation of the Devonian (Richardson and McGregor, 1986, Streel et al., 1987) and the Carboniferous (Clayton et al., 1977, Owens et al., 2004) is used to establish a biostratigraphic framework for the studied wells. Biostratigraphic age determination is based on the first occurrence (FO) and the last occurrence (LO) of significant marker taxa (mainly miospores). The ranges of the key miospore taxa recorded from the Ţăndărei, Călăraşi and Vlaşin

Conclusions

Palynological analysis was carried out for 14 core samples collected from the Devonian–Carboniferous sections of five petroleum exploration wells drilled within the eastern part of the Meosian Platform. Abundant, well-preserved palynomorph assemblages were identified, consisting mainly of miospores and phytoplankton taxa. Based on the assemblages identified and established palynofacies data, interpretations of age, paleoclimate, paleoenvironments and paleobiogeography are made. The main

Author Agreement Statement

We the undersigned declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere.

We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

We understand that the Corresponding Author is the sole contact

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study would not have been possible without the help of Hunt Oil Company of Romania who provided the core samples, lithological data and other valuable well information. The constructive reviews and thoughtful comments of the two anonymous reviewers and M. Stephenson, the Editor-in-Chief, are greatly appreciated. Also, we are grateful to Emery Hauser (Hunt Oil Company of Romania) for his assistance in the revision of the English version of this manuscript.

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