Sequence Stratigraphy of the Volhynian (Late Middle Miocene) Deposits from the North Sector of Eastern Carpathian Foredeep

Abstract

An exposed sedimentary succession, ca 115 m of a total of 1000 m, from the Eastern Carpathian foredeep was, for the first time, analyzed using facies analysis and scale- and time-independent sequence stratigraphy methods to reveal the depositional environment and its cyclic sedimentation. The outcropping deposits, belonging to the Șomuz Formation, dated on the basis of molluscs, foraminifera, and ostracods, are uppermost Volhynian (upper Serravalian). The three recurrent facies associations we have distinguished indicate a storm-dominated shoreface–offshore transition environment. Five-decametre-thick high-frequency sequences (HFS1–5), at most of 4th order, bounded by maximum regressive surfaces, were defined in the studied interval. The maximum thickness of the Volhynian deposits in the area, known both from well sites and outcrops, allowed us to estimate the sedimentation rate at ca 1.5 m/kyr. The fossil content shows that the entire sedimentary succession was deposited in very shallow to shallow water during the whole Volhynian (12.65 - ca 12.01 Ma). The time interval we studied was estimated at ca 75 kyr, so the average time of one HFS is ca 15 kyr. At this scale, considering that both high subsidence and Eastern Paratethys sea-level rise added to accommodation, the sediment supply must have been the main control of cyclic sedimentation, which, in turn, must have been controlled by precession climatic changes in the source area. The estimated time of an HFS is shorter than a precession cycle, but better dating might support or refute this hypothesis. This paper may awaken the interest of the owners of better data, especially from subsurface (seismic, well logs), to complete the data from natural exposures.

1. Introduction

The late Middle Miocene (Volhynian substage of Sarmatian sensu lato stage) sedimentary succession, which is the topic of this paper, was deposited in the northern part of the Romanian Eastern Carpathian foredeep, belonging to the Eastern Paratethys (Figure 1A).

By the end of the Eocene, after the rising of mountain ranges consequent to Alpine orogeny, the former Tethys Ocean was succeeded by the Mediterranean Sea to the south and the Paratethys Sea to the north, the latter covering large areas of Europe and Asia [1,2]. The two successors of the Tethys Ocean had only intermittently communicated. Having different palaeogeographic and palaeobiotic evolution, a different chronostratigraphic scale was developed for each of them; the Mediterranean one is considered standard, while the one for Paratethys is regional. The different areas of the Paratethys Sea had different evolutions both in time and in space. In time, by geodynamic and palaeobiological data, four evolution stages were recognized [3,4]: Protoparatethys (early to middle Oligocene), Eoparatethys (late Oligocene to Early Miocene), Mesoparatethys (late Early Miocene to early Middle Miocene), and Neoparatethys (Middle Miocene to Pleistocene). In space, three main regions were distinguished: Western Paratethys, Central Paratethys, and Eastern Paratethys. The Western Paratethys, mostly covering the North Alpine Foreland, ended its evolution in the Early Miocene [5]. The Central Paratethys evolved up to the beginning of the Late Miocene (end of Sarmatian sensu stricto) in the Alpine–Carpathian foreland Basin and the Pannonian Basin, giving place to Pannonian Lake, which was filled up around the Pliocene. The Eastern Paratethys, extending from the Romanian Carpathian foreland to the east of the Caspian Sea (Figure 1A), continued to evolve while shrinking until the Pleistocene; its remnants are still represented in the Black Sea, Caspian Sea, and Aral Sea [6]. For the Southern and Southeastern Carpathians’ foreland, beginning with late middle Sarmatian, the so-called Dacian Basin was defined as part of the Eastern Paratethys [7,8].

There are only a few papers regarding the Miocene sequence stratigraphic evolution of the Eastern Paratethys covering the Carpathian foreland, most of them discussing the part of the basin in the Southeastern and Southern Carpathian foreland [8,9,10,11,12,13,14], and there are even fewer for its northern area [15,16,17]. The aim of this paper is to take a step forward in the sequence stratigraphy knowledge of the early Sarmatian sedimentary succession from the northern part of the Eastern Carpathian foreland to understand the controlling factors of cyclic sedimentation in the foredeep established after the last major thrust of the Eastern Carpathian.

2. Geological Setting

The study area (Figure 1B) is located within the foredeep zone of the Eastern Carpathian (EC) foreland basin system [8,15,16,18].

The studies in the Eastern Carpathian (EC) foreland on Romania territory began after the mid-nineteen century, most of them focusing on general lithostratigraphy, biostratigraphy, and chronostratigraphy. Among the main results acquired during more than 160 years of work, especially for the northern area of the EC foreland, we consider the following the most important:

(1)

In the sedimentary succession, three megacycles were defined, namely the Vendian (Ediacaran)–Devonian, Cretaceous–middle Eocene, and Middle Miocene–Late Miocene [19]; within these megacycles, different cycles, bounded by unconformities, were defined.

(2)

Only the newest deposits (upper Middle Miocene) of the last megacycle are exposed in the area; the exposed deposits were initially divided based on their lithology and fossil content in three units [20], ascribed to the regional stage Sarmatian (sensu lato), which latter were given names [21]: Volhynian, Bessarabian, Khersonian. It is worth mentioning that the Sarmatian s.l. has a larger time range than the Sarmatian s.s., the latter corresponding only with the Volhynian and early Bessarabian substages of the former [22,23].

(3)

A lateral lithology and fossil content change was remarked in the exposed deposits, three or four “facies” and/or biofacies being distinguished (e.g., [7,19,24,25]), namely littoral, neritic-arenitic, neritic-pelitic, and reef facies (Figure 1B).

(4)

The deposits of the last megacycle thicken toward the orogen as revealed by well data [26,27,28].

(5)

The last megacycle, which lasted longer toward the south, was sedimented in the foreland basin system developed as a consequence the last EC Moldavian tectogenesis (regional subage Volhynian) [15,16]; the four depozones of a foreland basin system defined by [29] were recognized by [15,16] (Figure 1C); the upper Volhynian deposits overlay the lower Volhynian folded deposits after an angular unconformity [17,26] in the wedge-top depozone.

Figure 1. (A) Palaeogeographic map of the Paratethys (after [1]) with the studied area marked with the red star; (B) The lower and lower middle Sarmatian (Volhynian–lower Bessarabian) lithofacies map (after [7,19,30]) with the studied area marked by the green square (the depth contours of the Badenian–Sarmatian boundary are shown with orange lines); (C) XY cross section through the Sarmatian foreland basin system of the Eastern Carpathians (after [15,16]) with the four depozones corresponding more or less with the lithofacies lateral variation. Key: 1—the outer fold and thrust nappes of the Eastern Carpathians; 2—gravels, sands, mudstones, and coal beds; 3—sands and sandstones, mudstones, sandy ooliths; 5—mudstones and sandy mudstones; 6—limestones (reefs) with Serpula and bryozoans; 7—limestones and mudstones.

Figure 1. (A) Palaeogeographic map of the Paratethys (after [1]) with the studied area marked with the red star; (B) The lower and lower middle Sarmatian (Volhynian–lower Bessarabian) lithofacies map (after [7,19,30]) with the studied area marked by the green square (the depth contours of the Badenian–Sarmatian boundary are shown with orange lines); (C) XY cross section through the Sarmatian foreland basin system of the Eastern Carpathians (after [15,16]) with the four depozones corresponding more or less with the lithofacies lateral variation. Key: 1—the outer fold and thrust nappes of the Eastern Carpathians; 2—gravels, sands, mudstones, and coal beds; 3—sands and sandstones, mudstones, sandy ooliths; 5—mudstones and sandy mudstones; 6—limestones (reefs) with Serpula and bryozoans; 7—limestones and mudstones.

The Volhynian sedimentary succession analyzed in this work belongs to the Middle Miocene (regional stage Badenian)–Pleistocene megacycle, that lasted not later than Maeotian (latest Tortonian–early Messinian) in the northern part of the EC foreland, within which two unconformities were defined [31,32]: in between Badenian and Sarmatian; in between Bessarabian and Khersonian.

Although the cyclic nature of the sedimentary cover at different scales is acquainted by decades, a proper sedimentological study using the standard facies analysis as well as a sequence stratigraphic approach lack. Here we present the results of such analyses made at the outcrop scale, where the highest-frequency sequences were recognized in the Șomuz Formation (uppermost Volhynian) sedimentary succession. For this study, we chose the best exposure in the area (ca 115 m stratigraphic thickness) that was biostratigraphically dated in the stratotype of the named formation [28].

3. Materials and Methods

The methods used in this study are:

(1)

The field analyses, including detailed logs and sampling of outcrops, were made in the Șomuz Formation stratotype area. The sedimentological investigations consisted of standard bed-by-bed logging after Nemec [33]’s methodology and photo shootings along Livijoara creek. A rough grouping of facies in facies associations was performed in the field; the facies associations were laterally traced on the photos where inaccessible. The descriptive sedimentological terminology is after [34,35,36,37] while for abbreviation we followed the method of Miall [38] who uses capitals for lithology and smalls to abbreviate the sedimentary structures (e.g., Shcs means sands with hummocky cross stratification). The facies analysis and sedimentary process interpretation leaded us to the presented palaeodepositional environment interpretation based on the existing facies models [39,40,41,42,43].

(2)

During the sedimentary succession logging, several samples were collected (ca 1.5 m apart) for micro- and macrofauna analyses and photos were taken. The samples for the foraminifera and ostracods analyses were prepared using standard micropalaeontological methods. An amount of 200 g of previously dried sediment of each sample was washed by decantation method. In order to facilitate the microfossil handpicking, the remaining materials was separated into four fractions through a set of sieves of 0.466, 0.236, and 0.122 mm. The microfossils were handpicked using a micropalaeontological needle from a picking tray (manufactured by Dr. F. Frantz Rheinisches Mineralien-Kontor GmbH & Co., Bonn, Germany) under a Carl Zeiss Stemi 508 stereomicroscope (manufactured by Carl Zeiss Microscopy GmbH, Jena, Germany). The micropalaeontological content was deposited in Franke cells. The most representative taxa were photographed using a SEM microscope Hitachi S-3400N (manufactured by Hitachi High-Technologies Corporation, Tokyo, Japan) from the RAMTECH Laboratory of our University. The fossil content, as well as the one known from previous papers (e.g., [28]), was used for the biostratigraphic age determination. The zonations proposed by [28,44,45,46,47], on the basis of molluscs, foraminifera and ostracods, were used. Some palaeoecological inferences were made taking in consideration the habitats of the contemporary relatives of some taxa (molluscs and foraminifera).

(3)

The sedimentary succession was also studied from a sequence stratigraphic point of view. For this purpose, we followed the terminology and model- and scale-independent work methodology especially useful at the outcrop scale [48,49,50,51], where the stratigraphic sequences are defined on the basis of their stratal stacking patterns and are bounded by recurrent sequence stratigraphic surfaces, irrespective of their allogenic or autogenic origin. Such a methodology can be applied at any temporal and space scale (form outcrop to seismic scale), enabling avoiding the confusions that can occur when the classic methods from the dawn of rather low-resolution seismic stratigraphy, proposed in 1970s–1980s, e.g., [52,53,54,55,56], are used. The data gathered from natural exposures have the disadvantage as being difficult to correlate, considering their sparsely distribution, as well as the possibility of estimating the extension significance of the stratigraphic surfaces, being they local or regional. Accordingly, the surfaces bounding the sequences may be either unconformities or conformities; the former are relevant sedimentologic hiatuses [51], even if they do not cover temporal gaps long enough to be biostratigraphically proven, while the later mark conformable changes in the stacking patterns (e.g., from progradational to retrogradational stacking). The stacking pattern change is often associated with changes in sedimentary trends, which facilitate the recognition of the bounding stratigraphic surfaces at the outcrop scale [49].

4. Results

4.1. Biostratigraphic Data

The biostratigraphic dating was made on the basis of the samples whose fossil content (Figure 2) consists of molluscs (bivalves and gastropods), foraminifera, and ostracods. Molluscs with biostratigraphic value are (Figure 3): Podolimactra eichwaldi (=Mactra eichwaldi), Plicatiformes plicatus (=Plicatiforma plicata), P. latisulcus (=Plicatiforma latisulca), Potamides disjunctus, and Tiaracerithium pictum (=Potamides mitralis). We have used the accepted names, proposed by [57,58,59], for the taxa which are found in the older literature with the bracketed names.

The Podolimactra eichwaldi Range Zone is superposed on the Volhynian substage, while the P. eichwaldi–Plicatiformes praeplicatus and P. eichwaldi–Plicatiformes plicatus Concurrent-range Zone correspond with the lower Volhynian and upper Volhynian, respectively, according to Kojumdgieva et al. [46]. The occurrence of Sarmatimactra pallasi (=Mactra vitaliana pallasi) would coincide with the base of Bessarabian, while the presence of Plicatiformes latisulcus would indicate the uppermost Volhynian. However, the biozonations proposed by [28,44,45] (Figure 2) do not recognize any taxon range zone, but an Assemblage Zone with Podolimactra eichwaldi and Plicatiformes plicatus. Tiaracerithium pictum is the revised and accepted name of the former Potamides mitralis, P. nimpha, and P. bicostatus, previously defined as different species (e.g., [44]), as shows Harzhauser et al. [58]. Its range zone extends the whole Volhynian and the lower part of Bessarabian (Figure 2). All mollusc biozones mentioned before indicate that the studied deposits are uppermost Volhynian.

Among the foraminifera with biostratigraphic value we mention (Figure 4): Elphidium hauerinum; E. rugosum; E. macellum; Porosononion subgranosus; Pseudotriloculina (=Quinqueloculina) consobrina. They belong to the ACME Zones of Elphidium rugosum and Pseudotriloculina consobrina [44,45,60]. Additionally, the ostracod fauna is characteristic to the lower Sarmatian assemblages of the Vienna Basin [61], the Polish Carpathian Foredeep [62], and the Eastern Carpathian Foredeep [60,63]. In the studied outcrop, the ostracod assemblage consists of Cyprideis pannonica, Loxoconcha minima, Aurila mehesi, and Callistocythere postvallata. Another biostratigraphically important ostracod species, Aurila notata, was identified in the neighbourhood outcrops by Loghin [64]. The latter species would place the studied deposits in the NO12 Neocyprideis (N.) kollmani–Aurila notata Zone of Jiříček and Říha [47], indicating the lower Sarmatian in the Vienna Basin.

4.2. Palaeoecologic Inferences

The macro- and microfossil assemblages allowed us to infer some paleoecologic conditions, among the most important for our study being the water depth. Such inferences are made on the basis of contemporary fauna habitats. For instance, Actaeocina, which occurs in the studied sedimentary succession (Figure 4), is a small predatory gastropod living in shallow waters (on muddy or sandy bottoms) of different salinities [65].

Donax trunculus, a possible contemporary relative of Donax dentiger, uses to populate populate beaches at depths between 0 and 2 m on the Mediterranean coasts and between 0 and 6 m on the Atlantic coasts, respectively, being an exclusive species of the Superficial Fine Sand (SFS) biocenosis, strictly dependent by a certain sediment grain size (fine to medium sand) [66]. Signorelli [59] shows that the mactrids prefer muddy subtidal sediment to bury in. Polititapes aureus, a contemporary relative of Polititapes gregarius tricuspis, can be found in waters of 10 to 35 m depth [67] (https://obis.org/taxon/246150). Potamides disjunctus was a highly frequent species in shallow littoral to sublittoral settings, preferring sandy bottoms [58]. In the Mediterranean, Gibbula populates shallow waters from the lowest level of spring tides down to 73 m [68].

The microfossil assemblages yielded by samples not only from the outcrops in the study area and its neighbourhoods, but also from several well sites around the study area (e.g., down to −1392 m in Suceava area; Figure 1B) consist of foraminifera (Elphidium, Porosononion, Quinqueloculina, and Ammonia genera [28,44,60,69]) and ostracods (Cyprideis, Loxoconcha, Aurila, and Callistocythere [60,63,64]). Both foraminifera and ostracods indicate shallow water environments. For instance, Elphidium and Quinqueloculina are epifaunal organisms living in oxic, inner shelf high-energy environments [70,71,72,73], while Ammonia beccarii prefers water depths between 0 and 50 m.

4.3. Sedimentary Environment

Three recurrent facies associations (FA1–3) were defined in the studied sedimentary succession along Livijoara Creek. Their characteristics, compared with the existing facies models [40,41,42,43], indicate a storm-dominated shoreface–offshore-transition sedimentary environment (Figure 5 and Figure 6). Its facies model enhances the vertical facies successions in relation with the depth of fairweather and storm-wave bases. The water shallowness is also proved by both macro- and microfossil content (Section 4.2).

Facies association 1: Greyish–blue sandy mudstones and muddy sands with ripple cross lamination (RCL) of offshore–transition subenvironment.

Description: The peculiarity of this facies association is given by the greyish–blue sandy mudstones with lenticular cm–dm-thick bioclastic accumulations and muddy sands (Figure 5A,B). The most common sedimentary facies of this association are: dm-thick greyish–blue sandy mudstone (M), shell beds (SB) with Tiaracerithium, Potamides, Acteocina, Obsoletiformes, Polititapes, and Donax, among others, muddy sands with wave ripple cross lamination (Swrcl; Figure 5C), but also some rare thin beds (<15 cm) of sands with hummocky cross stratification (HCS), resting on shell beds (Figure 5A). Thin interlayers (one shell thick) of bioclastics may also occur unassociated with sands. The mudstone thicknesses decrease upward from 50–60 cm to <10 cm.

Interpretation: The accumulation of sandy mudstone facies indicates an environment with calm waters, somewhat deeper than in the shoreface area, characterized by conditions of suspension sedimentation, episodically affected by storms during which the fines were mixed with sands. It should be highlighted that most of the microfossil content of this sedimentary succession, consisting mainly of benthic foraminifera (e.g., Ammonia, Porosononion, Elphidium, and Quinqueloculina), comes from the mudstones of this FA and indicate shallow water (infra- to upper circalittoral environments). Moreover, we have not observed bioturbations neither in sandy mudstones, nor in muddy sands. The role of storms in shell bed formation was tackled by Kreisa and Bambach [74], among many others. They are seen as reworked and winnowed rather than in place accumulations, resting on sharp erosional surface. Usually, the shells are oriented parallel to bedding or aligned if they have a suitable morphology (such is of Tiaracerithium).

The Swrcl and Shcs (Figure 5A), the latter especially when following the shell beds, indicate periods of storm waves and currents’ events followed by periods of calmer sedimentation, which allowed the micro-epifauna and shallow micro-infauna to thrive. A more detailed discussion on the Shcs will be given in the FA2 interpretation. The important participation of the Swrcl and Scrcl facies in this association suggests the frequent lowering of wave base, probably during storms, when previously sedimented fines were reworked and mixed with sands. According to the models used [40,41,42,43] for FA interpretation, we consider this association as a record of an offshore-transition sedimentary subenvironment. Although the FA1 represents the deepest sedimentation sub-environment in the studied succession, the lack of bioturbation, the high frequency of Swrcl and Scrcl, and also Shcs with shelly storm lags, as well as the sandier nature of mudstones, indicate a storm-dominated offshore–transition.

Facies association 2: Sands with hummocky cross stratification (HCS) of lower shoreface subenvironment.

Description: This association contains mostly very fine to fine sands, greyish muddy sands, and mm–cm-thick mudstone interlayers. The most frequent facies are: sands with wave and current ripple cross laminations (Swrcl, Scrcl), with HCS (Shcs), with low angle cross stratification (Slacs), and with plane-parallel stratification (Spp). Shcs frequently occur as amalgamated beds (Figure 5D) or may grade lateral in Swrcl, the later filling up the swales between hummocks, or may be truncated by Swrcl, which can reach sub-metric thicknesses (Figure 5D). Ripples are well preserved in many situations under thin (mm to cm thick) continuous or discontinuous mud drapes (Figure 5E). Sands with cross stratification (Stcs) may also occur as isolated sets. The Shcs are fine or very fine and may occur either as simple sets of 20–30 cm thicknesses or as thicker (sub-metric) amalgamated units. Where it could be measured, the wave length between two swales was 1–2 m. Some of these units are truncated by undulating surfaces, bounding intervals characterized by Swrcl.

Interpretation: The Shcs are considered by most authors a diagnostic for storm waves in shallow waters. Other opinions exist, e.g., [75,76], but here we have adopted the mentioned interpretation, considering the macro- and microfossil content (e.g., Tiaracerithium, Gibbula, Duplicata, Plicatiformes, Polititapes, and Quinqueloculina, Ammonia, Elphidium, and Porosononion, respectively) in resedimented beds or in mudstone interlayers of FA1 supports it. The fossil content in FA1 belongs to the deepest sub-environment of entire sedimentary succession, being a benthic mixture of infralittoral and upper circalittoral taxa. The formation of hummocky cross stratification (HCS) is one of the important topics in process sedimentology since it was defined by Harms et al. [34]. Strong oscillatory currents [35,39,77,78,79,80] with a unidirectional component (e.g., [81,82]), which may be a geostrophic current [83], were proposed as formation mechanism and reproduced in laboratory experiments [84,85,86]. The HCS represents the internal structure of the large (metre to several metres diameter) circular to ellipsoidal, dm-high, mounds (hummocks) separated by similar plan form depressions (swales), so that it consists both of large convex and concave laminasets. Both the 3D swales and the 3D hummocks with their internal structures (swaley and hummocky cross stratifications, SCS and HCS) are primary sedimentary structures highly recognized as diagnostic for oscillatory currents generated by storm waves in shallow marine environments. Such topography is built during storms, so that it often occurs above the average storm wave base, but below fair-weather wave base [78], considering the mounded relief may be readily eroded by normal waves. The Slacs may represent distal portions of the large scale Shcs. The intervals consisting of Swrcl and Scrcl indicate returning to normal conditions, yet characterized by high energy as proven by the frequent Scrcl sets, while the thin draping or lenticular thin mudstone layers indicate short periods of quiet sedimentation under the wave base.

Based on the existing models where Shcs are well represented [40,41,42,43] and on the scarce presence of mudstones, we consider that these deposits were sedimented on the lower shoreface of a storm-dominated coast.

Facies association 3: Sands with trough cross stratification (TCS) and swaley cross stratification (SCS) of upper shoreface.

Description: The facies belonging to this association consist mainly of coarse to medium, well-sorted, yellowish sands without matrix. Rare interlayers of thin mudstones and bioclastic materials, may also occur. The most frequent facies are: sands with trough cross stratification (Stcs), containing shell hash, sands with swaley cross stratification (Sscs), sands with plane-parallel stratification (Spp), sands with current ripple cross lamination (Scrcl). Convolute lamination was also observed in several cases. The Stcs and Sscs give the characteristic note to this facies association. The Stcs (Figure 5F) occur in sets of 30–50 cm-thick bounded by erosive surfaces, while Sscs as singular sets of 20–30 cm thick and sub-metric width. The Scrcl occur as cosets of sets of 10–15 cm-thick bounded by slightly undulated surfaces.

Interpretation: This mostly well-sorted sandy facies association indicates sedimentary processes characterized by high energy. The swaley cross stratification (SCS) is the internal structure of circular to ellipsoidal depressions (swales) formed among mounds with similar plan forms (hummocks). The occurrence of only swaley cross stratification is a result of the smaller preservation potential of convex structures (hummocks) under conditions of energetic processes, likely due to the mounded relief readily eroded by normal waves [86,87,88,89]. Also, the bioclasts (predominantly gastropods of Tiaracerithium and Potamides or Plicatiformes and Polititapes bivalves) are concentrated sometimes onto the erosive bases of SCS, indicating storm lags.

The Stcs sets are the result of sedimentation in complicated bar-trough systems created under the action of longshore, onshore and/or “rip” currents hosting medium scale 2D and 3D megaripples in the upper shoreface area [40,41,42,43,90]. These sets, accumulated during fairweather conditions, are truncated by erosive surfaces bounding the Sscs and Spp. The high-rate transport and accumulation during storms over deposits not yet consolidated create instability at the interfaces between the sands accumulated in different stages, the consequence being the formation of convoluted structures [91] observed in the studied outcrops.

The sedimentary processes interpreted on the basis of the described sedimentary facies are representative for the upper shoreface characterized by the permanent action of fairweather waves, as well as by the periodic incidence of storm surges. Idealized models of this sub-environment have been proposed [40,41,42,43].

4.4. High-Frequency TR Sequences

The three FAs presented above may occur gradationally on top of each other in coarsening and shallowing upward trends (Figure 6). Such a gradational succession suggests a continuous sedimentation associated with a progradation of the shoreface—offshore-transition sedimentary environment, accompanied by a regressive shoreline. The opposite situation also occurs, where a change from shallow to deeper water sedimentation, associated with a finning upward trend, indicates the retrogradation of sedimentary environment, accompanied by a transgressive shoreline.

Both shallowing and deepening upward trends (ShU and DU) are results of the relationships between accommodation and sedimentation, a highly debated topic recently reviewed by Catuneanu [92]. The former situation occurs when sedimentation outpaces accommodation, while the latter occurs when accommodation outpaces sedimentation. In the studied succession, the shallowing upward intervals may be one order thicker than the deepening upward ones. The ShU and DU intervals are bounded rather by nonerosive surfaces, although some dm-thick shell beds might rest onto erosive surfaces. Because of the limited exposures, we could not trace them laterally more than 1–2 m, so this hypothesis remains to be proven. As the interpreted sedimentary environment shows, the most proximal sub-environments (foreshore, backshore) lack from succession, so that the position of the examined deposits should have been at some distance from the shore, where the development of subaerial unconformities was rather unlikely. Accordingly, here, we have chosen to use transgressive–regressive sequence model [93] bounded by maximum regressive surfaces.

Five high-frequency sequences were defined in the ca 115 m-thick exposure, abbreviated as HFS1–HFS5. These sedimentary cycles are decametre-thick regularly repeated successions of facies associations. Four of the HFSs are dominated by thicker regressive deposits (Figure 7). In the HFS4, the transgressive and regressive deposits have almost equal thicknesses. In the same sequence, the upper shoreface lacks. It seems that there is a thinning upward trend from HFS1 to HFS3, followed by a thickening upward trend from the HFS3 to HFS5. Minor intervals showing similar trends (ShU and DU) may occur mainly in the FA1 and the lower part of the FA2, but they were not taken into consideration, being rather results of normal sedimentation in offshore-transition and lower shoreface subenvironments. They are bedsets bounded by erosional or non-depositional discontinuities, belonging to the corresponding facies associations. For instance, the bedset in Figure 5A, with two amalgamated storm beds, characterized by erosive lower bounding surfaces and finning upward trends, is rather related to local energy level, namely lowering the wave base during successive storms. Returning to normal sandy mudstone sedimentation after the second storm is recorded as a finning (/deepening) upward trend. Such bedsets represent sedimentological cycles [94]. In defining the HFSs, we looked for a consistent progradational and retrogradational stacking pattern associated with the increasing presence of those sedimentary facies that describe the best each sedimentary subenvironment. Each HFS, especially the FA1 and FA2 intervals, contains numerous sedimentological cycles (Figure 7).

5. Discussion

The cyclic nature of the deposits studied in this paper is obvious. The high-frequency sequences are part of the Volhynian–Bessarabian cycle, belonging to the last sedimentary “megacycle” defined for the Carpathian foreland. This “megacycle” includes several unconformities proven on different criteria [19,32], as we have mentioned in Section 2.

The unconformity between Badenian and Sarmatian was established based on a sudden change in micro- and macrofauna from marine to brackish assemblages [95], and occurs in the entire Eastern Carpathian foreland. In between the middle Sarmatian (Bessarabian) and upper Sarmatian (Khersonian), there is a disconformity which was observed in the field, especially toward the Carpathians, where the upper Bessarabian lacks [31]. Consequently, the HFSs belong to the Volhynian–Bessarabian cycle.

If we were to rank the entire sedimentary succession of the foreland, then the three megacycles would be 2nd order sequences, and the Volhynian–Bessarabian sedimentary cycle would be a 3rd order sequence. A similar ranking was proposed for the western Central Paratethys [96], where the Sarmatian s.s. (= the Volhynian–lower Bessarabian of the Sarnatian s.l.) sedimentary succession was equivalated with the global cycle TB2.6 (3rd order). For the Transylvanian Basin (belonging to the Central Paratethys), 3rd order sequences (MLM5 and MLM6) were defined for the early Sarmatian, the authors [97] mentioning also the existence of lower rank sequences. The MML5 and MLM6 sequences would belong to global cycle TB2.5 and TB2.6, respectively. Consequently, HFS1–5 presented in this paper may be 4th order at most. One thing worth mentioning here is that the entire Volhynian sedimentary succession known from wells and outcrops is ca 1600 m in the proximal foredeep (Figure 1B), but the deposits are always of shallow water, as it is indicated by the fossil content mentioned in Section 4.2. Older high-frequency sequences (but still Volhynian) of shoreface deposits are also preserved in the wedge-top depozone northward of Solca [17] (Figure 1B). Equivalent age deposits are one order thinner away from the Eastern Carpathians (Figure 1B,C), in forebulge and backbulge depozones [7,19,24].

At the scale of the outcrops, although it is possible to recognize the sequences, it is difficult, if not impossible, to interpret the controlling factors, because the extension of the bounding sequence stratigraphic surfaces cannot be estimated, as they rather reflect the importance of the local controls on the accommodation and sedimentation [50]. At the same time, at any scale, the global controls on accommodation and sedimentation may combine in indefinite ways with the local ones, the resulting sequences reflecting both categories. Moreover, for the lower rank sequences, such are the ones we discuss here, the controls may be both allogenic and autogenic [50,51]. Autogenic sequences are more common in the case of deltaic systems, but the system defined by us is shoreface—offshore-transition, so we may ignore this hypothesis. The shoreface–offshore system is very sensitive to allogenic changes in the accommodation space, controlled by sea-level changes and tectonics [54,55]. Sedimentation reflects the supply of sediment to a certain depositional area, but also the energy flux of the environment [92]. The former depends on climate, erosional processes, size of source areas, the existence of intermediary traps along the sediment transport routes in the source areas (e.g., [98]), the source area itself being a result of tectonic uplifting. Hence, the discrimination of the controlling factors must be taken with caution, each of them having its own timing, although tectonics had the lead in the Volhynian foredeep of the Eastern Carpathians.

5.1. Paratethys Sea-Level Control on Accommodation

The placement of the sequence stack defined in the studied outcrops in a regional and global context of sea-level oscillations (Figure 8) is very much an attempt to estimate their control on cyclic sedimentation, on one hand, and to discriminate the role of the tectonic factor on same cyclicity, on the other, given that the sedimentary basin was the Eastern Carpathian foredeep created after the intra-Volhynian Moldavian tectogenesis.

The reconstruction of the Eastern Paratethys sea-level behaviour [99] shows that during the Volhynian, it was rising and reached a highstand to the end of this subage. In the same time span, at global scale [100], sea level was falling from a highstand to a lowstand (Figure 8).

5.2. Tectonic Control on Accommodation

The sedimentation took place during the Moldavian major deformation of the Eastern Carpathians [102,103], responsible for the main nappe emplacement, when the new foreland basin system was organized [15,16]. The nappe load creates a depositional space due to the flexure of continental lithosphere undergoing subduction, forming an accretionary wedge and a foreland basin system (e.g., [29]). During the convergence, the sediment deposited in the foredeep may be incorporated in the thrust belt, uplifted and eroded as was the case with the early Volhynian deposits [17,26].

In the study area, the last thrusting event was intra-Volhynian, considering the angular unconformity between the lower Volhynian folded deposits and the undeformed upper Volhynian deposits in the wedge-top depozone located northward of Solca [17,26].

It is very likely that accommodation space was mostly due to tectonics, an echo of the accelerated subsidence caused by the Moldavian major thrusting event. One more complexity is added by the along Carpathian foredeep migration of the depocenters with an average rate of 380 km/Myr in the Volhynian–Bessarabian [18]. Such a temporary depocenter must have been in the proximal foredeep, south-westward of the study area (Figure 1B). The depocenter migration was driven by the diachronic thrusting event along the Carpathians, which in the Polish Carpathians, for instance, was Badenian, while in the Romanian Carpathians was intra-Sarmatian, still younger from the north to the south [102,104].

5.3. Controls on Sediment Supply

During late Badenian— Sarmatian, the Carpathians rose above the sea level while being shaped by weathering and erosion [105]. The latter authors showed that ca 4 km of exhumed deposits were eroded in the 15–5 Ma time span in the north sector of Eastern Carpathians, the 15–11 Ma interval being characterized by acceleration of erosion rates, so that they maintained a low-altitude [106]. The tectonic landscape of the Carpathian source area underwent the effects of opposite forces, tectonic uplifting and denudation, the latter highly dependent of climate. However, the interaction between the two controls is highly variable, the sediment efflux response to tectonic uplifting being delayed a very long time (10⁶–10⁷ years) in large orogenic systems or rather short time (10⁵ years) for trains of folds in fold-thrust belts (data in [107]). At the same time, if the climate (precipitation) is favourable, any tectonic uplifting may be adjusted by aggressive erosion in reactive tectonic landscapes [107]. One of the controls on climate is generated by periodic oscillations of Earth’s orbital parameters, known as Milankovitch cyclicity, which may have long- to short-periods [108], but we lack such information for our study area.

5.4. Cyclicity Modulator in the High Accommodation and Supply Area

Both the subsidence (proximal-distal and along-Carpathians) and the Eastern Paratethys sea-level rise added to accommodation, the result being a ca 1600 m preserved deposits supplied by the uplifting orogen in a rather short time span. As the fossil content from the wells and outcrops and the interpreted depositional system reveal, there was not a pre-existing bathymetric depression that could have accommodated such a thick package of sediment. Hence, the thicker accumulations of sediment near the thrust line (Figure 1B) mark the position of actively subsiding depocenters and not pre-existing bathymetric troughs.

There are no numerical ages of the Sarmatian substages in the Carpathian foreland. We do not know the numeric age of the Volhynian–Bessarabian boundary, but we may take as guidance the results of magnetostratigraphy and radiometric dating acquired by Vasiliev et al. [109] in the Transylvanian Basin (part of the Central Paratethys) for the Sarmatian s.s.–Pannonian, corresponding with early Bessarabian–late Bessarabian in the Eastern Paratethys, as 11.3 Ma. In the Central Paratethys, however, this boundary is placed at 11.6 Ma and considered approximately equivalent with the Serravalian–Tortonian one, that is at 11.63 Ma (e.g., [101,110]). Harzhauser and Piller [96] put the boundary between Volhynian and Bessarabian at the top of the chron C5An.1n, which was calibrated by Ohneiser et al. [111] at 12.014 Ma. A provisory boundary between the Volhynian and Bessarabian is placed by Raffi et al. [101] at 12 Ma, so we may assume that the Volhynian subage lasted at most 0.65 myr. Data from wells (Suceava, Rӑdӑuți, Fӑlticeni, and Lespezi areas; Figure 1B) and outcrops show that the maximum thickness of Volhynian deposits in the northern EC foredeep is at least 1600 m, but in the studied area would be ca 1000 m [7,27,28,44]. Considering a constant sedimentation rate (1.5 m/kyr), then the studied interval would represent approximately 75 kyr, while an average time span of each HFS would be ca 15 kyr. This figure may be improved by a better age constraint of the Volhynian substage.

At this temporal scale, to explain the development of HFS1–5 in an accommodation created by the cumulative effect of tectonic subsidence and Paratethys sea-level rise, we have to look for a short-period modulator of sediment supply to a rather high energy shoreface—offshore-transition environment. Such a modulator could have been climate changes controlled by precession cycles, which are the closest as time range. As no such studies were made for the Eastern Carpathian foreland, this possible explanation is advanced as a likely hypothesis that needs further support. However, the interplay of all the above-mentioned controls, acting at different cycle periods, produced the sequences described in this paper; further studies are necessary to discriminate the role of each controlling factor. Subsurface data (seismic profiles, well logs) would be necessary to complete this study, at least for the whole Volhynian, to achieve a better ranking of the events that took place during this subage.

6. Conclusions

A sedimentary succession in the northern part of the Eastern Carpathian foreland was for the first time analyzed using sedimentology and sequence stratigraphy methods.

(1)

The sedimentary succession represents the uppermost (ca 115 m) interval of a very thick (ca 1600 m) infill of the north part of the Eastern Carpathian foredeep accumulated during their last major tectonic deformation (Moldavian tectogenesis).

(2)

Based on the molluscs, foraminifera, and ostracods, the sedimentary succession was biostratigraphically dated as uppermost Volhynian (the lower substage of the regional stage Sarmatian s.l., defined for the Eastern Paratethys).

(3)

The fossil content, both from the studied interval and several well sites (down to −1392 m) in neighbouring areas, suggests shallow water, inner shelf (littoral to sublittoral) environments.

(4)

Three facies associations were defined, greyish–blue sandy mudstones and muddy sands with ripple cross lamination of the offshore-transition, sands with hummocky cross stratification of the lower shoreface, and sands with trough cross stratification and swaley cross stratification of the upper shoreface, and interpreted as a storm-dominated shoreface—offshore-transition sedimentary paleoenvironment.

(5)

The vertical recurrence of the three facies associations allowed us to define five decametre-thick high-frequency sequences (HFS1–5) bounded by maximum regressive surfaces, four of them mostly regressive and one of transgressive–regressive type. The HFSs are at most of the 4th order and belong to the Volhynian–early Bessarabian, 3rd order sequence, itself part of the Miocene 2nd order sequence (“megacycle”) of the Eastern Carpathian foreland sedimentary succession.

(6)

The cyclic sedimentation occurred in a high-accommodation setting, where both foredeep high subsidence, a consequence of the major deformation of the Eastern Carpathians in Volhynian, and the Paratehys sea-level rise contributed. The high accommodation was balanced by a high sedimentation rate, so that the depositional environment remained shallow water.

(7)

The time span of the studied interval was estimated to be ca 75 kyr, of the ca 0.65 myr of the whole Volhynian; therefore, each HFS lasted ca 15 kyr.

(8)

To explain the high-frequency cyclicity on a high-accommodation background at the space and time scale of HFSs, we hypothesize as modulator the sediment supply, likely controlled by precession climatic change in the Carpathian source area.

(9)

Further studies (especially on subsurface data) are necessary to confirm the proposed control on cyclic sedimentation or to identify a better one.