Sedimentary geology of the middle Carboniferous of the Donbas region (Dniepr-Donets basin, Ukraine)

The Paleozoic Dniepr-Donets Basin in Belarus, Ukraine, and Russia forms a major hydrocarbon province. Although well- and seismic data have established a 20 km thick stratigraphy, field-studies of its sediments are scarce. The inverted Donbas segment (Ukraine) exposes the middle Carboniferous part of the basin's stratigraphy. Here, we provide detailed sedimentological data from 13 sections that cover 1.5 of the total of 5 km of the Bashkirian and Moscovian stages and assess the paleoenvironment and paleo-current directions. Middle Carboniferous deposition occurred in a shelf environment, with coal deposition, subordinate fluvial facies, and abundant lower and middle shoreface facies, comprising an intercalated package of potential source and reservoir rocks. Sedimentary facies indicate a paleodepth range from below storm wave base to near-coastal swamp environments. Sedimentation and subsidence were hence in pace, with subtle facies changes likely representing relative sea-level changes. Paleocurrent directions are remarkably consistently southeastward in time and space in the different sedimentary facies across the Donbas Fold Belt, illustrating a dominant sedimentary infill along the basin axis, with little basin margin influence. This suggests that the middle Carboniferous stratigraphy of the Dniepr-Donets basin to the northwest probably contains significant amounts of fluvial sandstones, important for assessing hydrocarbon reservoir potential.


Section:
Bulavinskoye Location: 37U 0451370 UTM 5344133, central area Situation: Bulavinskoye section is situated just to east of the village Булавинское (Bulavinskoye) in a river outcrop on the road to Олбховатка (Olbchovatka). Where the road goes down the top of the section starts immediately to the south of the road, on the west-flank of a small river.
Age: upper Bashkirian, C 2 3 , below limestone J 1 % Sand: 31 % Thickness: 180 m Sedimentology: The Bulavinskoye section is characterised by fairly identical well-sorted fine sandstone units that contain hummocky cross-stratification or current induced large scale cross stratification (set height ~50 cm) and belong to Groups C and D. Paleocurrent directions in these sandstones are consistently northeast ( Figure XB). A few sandstones in the section show coarser grains of which one bears tree trunk prints, bad sorting, and structure-less massive beds (Group A). This unit also shows some trough-like structures. Limestone beds are present in the middle of shaley intervals, and one close to a sandstone interval. One coal interval has been observed. Below the studied section a long interval of shales (~200 m?) is present and below that a thick sand-rich interval (~200 m?) has been observed in which among others coarse fluvial sandstones are present (Group A).

Environmental interpretation:
Most sandstones in the Bulavinskoye section belong to the Groups D and C, and are interpreted as middle to lower shoreface depositional environments. The presence of a sandstone of Group A indicates a pronounced sea level lowering in the middle of the section, also the occurrence of a coal layer close to the latter sandstone reveals this observation. The long interval of shales below the section is interpreted as a pronounced sea level high-stand.

Section:
Chegharniki Location: 37U 0426556 UTM 5355167, central area Situation: Chegharniki section is named after the abandoned mercury quarry with the same name situated in the northern part of the city Горловка (Gorlovka). It can be reached by taking the main road southwest around the city to the area called Комарова (Kotarova). North of this area several quarries are situated on a row from east to west. The Chegharniki can be reached by taking a small road to the east (just after a bus stop) where the old white quarry tower is situated just to the west. The tower can be seen from distance.
Age: lower Bashkirian, C 2 2 , just below limestone H 1 % Sand: 39 % Thickness: 118 m Sedimentology: The stratigraphy of the Chegharniki section is characterised by a relatively high sand content with respect to shales. In the middle and largest part of the section these sandstones are fine and well sorted and belong to Group D. The basal sandstone is fine too but shows current induced cross beds and belongs to Group C or D. The topmost very thick sandstone is coarser, show larger scale foresets, and some intervals with very poor sorting, therefore it has most characteristics from Group B and some of Group A. In this sandstone unit CH.D some bundling of finer and coarser individual foreset laminae is observed. In the section plant remains are common and no tree trunk prints have been found. The sedimentary log of this section contains details for the shaley and silty intervals because the section has been studied in a good quarry outcrop. Too few paleocurrent measurements could be measured to derive a statistically correct paleocurrent direction for the complete section ( Figure XC). The measured directions point to an eastward flow of currents, except for one measurement in the lower part of the section. Environmental interpretation: The characteristics of the Chegharniki section point at a lower to middle shoreface depositional environment for the largest part of the section. Only at the top the sandstone unit aims at proximity of the continent and the sediments are interpreted as deltaic to upper shoreface with maybe minor fluvial intercalations. The presence of small scale wave cross bedding may be confusing as these sedimentary characteristics also often occur in very shallow water environments. The facies association indicates however deeper water environments and wavy lamination is known to also occur at greater bathymetry, even between depth of occurrence of hummocky cross-stratification and storm wave base (Duke et al., 1985). Above the upper shoreface sandstone at the top the environment becomes deeper again.

Section:
Fashchivka Location: 37U 0472949 UTM 5339402, central area Situation: The Fashchivka section is named after the village called Фащевка (the one south of M-03), that is situated just northeast of the section. The section can be reached by taking the road to the east from a statue of Lenin at the village's main square and following that road to south and southwest for about 3 km, also outside the village.
Age: upper Moscovian, C 2 7 , around limestone M 7 % Sand: 37 % Thickness: 109 m Sedimentology: The Fashchivka section is characterised by fairly prominent sandstone units intercalated in shales and silts. Half of these units are well-sorted fine to middle sandstones with current induced intermediate scale (~25 cm) cross-beds and some indications for hummocky cross-stratification. These belong to Group C. The basal and top unit consists of poorly sorted coarse sandstones with intervals showing a massive character and tree trunk prints, and intervals with large scale current induced cross-beds (~ 50-100 cm) to variable directions.  Environmental interpretation: The coarse poorly sorted sandstones that belong to Group A are interpreted as fluvial deposits. From all studied sections sandstone unit F.C has the best developed fluvial current induced cross bedding at its top (PHOTO 3329). This interval is seen as a prolonged interval of fluvial environment. The finer better sorted sandstones are interpreted as middle shoreface deposits according to the interpretation of Group C. We interpret the silts and shales as open deeper water deposits.

Section:
Illinka Location: 37U 0457528 UTM 5347233, central area Situation: Illinka sandstone is named after the village Ильцнка (Illinka). The sandstone can be found along a small footpath that starts to the west at the point in the northern end of the village, where the paved road ends.
Age: upper Moscovian, C 3 1 , below limestone O 1 % Sand: n / a Thickness: 8 m Sedimentology: The Illinka sandstone is not directly fitting into one of the defined sandstone groups. This is because the sandstone consists of very coarse and poorly sorted grains, but show very well-developed current induced large scale cross bedding. None of the defined groups shows these characteristics, however they come closest to a mixture of Group A and B. The basal and top parts of the sandstone show slightly better sorting and finer material. Also in the top trough cross beds occur. Paleocurrent analysis shows a fairly consistent flow to the east-northeast with around 90° of variation. Above the sandstone outcrop there is a shale interval and a thick limestone. Downwards a few sandstones have been found also showing huge lateral continuity. The whole interval is not very sandstone rich. Environmental interpretation: The depositional environment of the Illinka sandstone is interpreted to have been relatively close to the coast. This is inferred from the poor sorting, (very) coarse grains, and large scale current induced cross bedding. However no signs for a well-developed fluvial system is found, because there is for example no large scale trough cross bedding. Also in the basal and top parts indications are present for deeper water environments like from Group B and C. Therefore we prefer an interpretation in which the main environment of the Illinka section is situation very close to the river mouth at the sea-side.

section:
Illyria Location: 37U 0499835 UTM 5361662, northern area Situation: Illyria section is named after the village Иллирия (Illyria). To reach the section by car from the east, the centre is passed by a turn to the right on a T-crossing, then another turn to the right after passing a bridge. Keep right on the unpaved roads afterwards, but do not cross the water anymore.
Age: lower Bashkirian, C 2 2 , sandstone H 1 in top of section % Sand: 27 % Thickness: 167 m Sedimentology: The Illyria section is characterised by a relative high amount of nonoutcropping shales and silts. The sandstone outcropping grade is very different for the three units present. The lowermost reveals characteristics of Group A to B in the middle and some of Group C especially at the top, but is also disturbed by diagenesis that makes the observation of sedimentary characteristics difficult. The middle unit shows features of Group C all over with one thin level with Group A in the middle. The uppermost unit show features that are not described in one of the defined groups. It is characterised by fine to middle well sorted sand with large scale swaley cross stratification. This unit is relatively thick and very continuous in outcrop.

Environmental interpretation:
The Illyria sediments show all characteristics for shelf sedimentation (the shales) with intercalations of lower to upper shoreface environments. The two fluvial impulses are not prominent. Swaley cross stratification is usually found in an identical environment as hummocky cross stratification, although more energy is involved. We therefore place the unit that is full of this stratification (IL.C1) in a little higher energetic environment as of Group C, the middle to upper shoreface regime, also because this unit is coarser grained than the normally hummocky cross stratified sandstones. Figure XIK1. Rose diagram of all measured paleocurrent directions in the Illyria section. Petals in groups of 10°, largest petal has 2 measurements and represents 28% of the total.
Age: lower Moscovian, C 2 6 , just below limestone M 1 % Sand: 12 % Thickness: 233 m Sedimentology: The Malo-Orlovka section is characterised by a very high amount of shales and silt versus sandstones. It is also the longest studied section and the outcrop even permits to study more stratigraphy in future. Only three sandstone units are present that have a grainsize larger than very fine sand. These all exhibit characteristics of Group C. Other present sandstone are very fine, intercalate with silts and belong to Group D.

Environmental interpretation:
The sediments of the Malo-Orlovka section all show characteristics of middle to lower shoreface and shelf environments (Group C and D and shales). The section clearly shows the intercalation of lower shoreface sandstones (Group D) in between huge intervals of shales and silts, and therefore strengthens the relatively deep water interpretation of these fine sediments. The large intervals without clear outcrops are not at all expected to contain considerable sandstones because these would have been weathered out. Although these intervals might contain some extra sandstones of Group D or more well possible some extra silts.

Section:
Orlovo-Ivanivka Location: 37U 0459526 UTM 5334117, central area Situation: Orlovo-Ivanivka section is named after the nearby called Орлово-Ивановка (Orlovo-Ivanovka). The section is situated on the west flank of the biggest lake just south of the village and can be reached by the unpaved road that departs just before the first building of the village from the paved road into the village from the west. The section is situated along the lake shoreline.
Age: upper Bashkirian, C 2 4 and C 2 5 , limestone K 1 is situated in the section.
% Sand: 28 % Thickness: 190 m Sedimentology: The upper part of the stratigraphy of the Orlovo-Ivanivka section is characterised differently than the larger lower part. The lower part is characterised by long shale and silt intervals with well sorted very fine to fine sandstone of Group D and some features of Group C. In the upper part sandstone OI.C1 and OI.C2 show characteristics of Group B and C, while topmost unit (OI.D) has features from Group A. In the village Orlovo-Ivanovka sandstone have been found that belong older stratigraphy than the studied section, while in between not much outcrop has been found.

Environmental interpretation:
The Orlovo-Ivanivka section shows for the larger lower part mainly shelf environments and some lower to middle shoreface environments during sandstone deposition. A gradual shoaling is revealed by the sandstones in the upper part of the section. The section uppermost sandstone shows fluvial characteristics of Group A. The long probably shaley interval below the section until the sandstones in the village itself might indicate deep environments for a long time, such that no big sandstone were deposited, which is seen as a 3 rd order sea level cycle.

Section:
Pervomaysk Location: 37U 0459526 UTM 5334117, central area Situation: The Pervomaysk sandstone is named after the nearby city Первомайск (Pervomaysk). To reach the sandstone a small paved road that starts to the south on the west side of the river from the bridge between the cities Первомайск (Pervomaysk) and Теплогорск (Teplogorsk) has to be followed for a while. At a split to the southwest and south a very small bushy footpath starts between two houses to the west. The sandstone is a few hundreds of metres to the west along this path.
Age: upper Moscovian, C 2 7 . % Sand: n/a Thickness: 10 m Sedimentology: The Pervomaysk sandstone is characterised by features of coarsening upwards to the middle and fining upwards to the top of the unit. The lower fine to middle sand part is characterised by current induced cross bedding and belongs to Group B or C. The middle part is much coarser grained and less sorted. Big trough structures and tree trunk prints occur, and this part must belong to Group A. The top part is finer again and full of mega and intermediate scale current induced cross bedding. This part is grouped within Group B. The three measured paleocurrent directions aim at an average southeast direction with around 100° variability. The unit fits within a short outcropping stratigraphy below it. This consists of mainly shales with a limestone (see photo in log) and just below the Pervomaysk sandstone a transition interval of sandy silts occurs.

Environmental interpretation:
The Pervomaysk sandstone shows a change from middle to upper shoreface to clearly fluvial environments and back to deltaic to upper shoreface environments in the top. It can therefore be seen as a 4 th order sea-level lowstand deposit. Figure XIK1. Rose diagram of the three measured paleocurrent directions in the Orlovo-Ivanovskaia section. Petals in groups of 10°.

Section:
Stepano-Krynka Location: 37T 0450178 UTM 5306144, southern area Situation: The Stepano-Krynka section is named after the village called Стелано-Крьінка, that is situated around 3 kilometre north-east of the section. The section can be reached from the road between Стелано-Крьінка and Кутейниково and is situated just north of a small lake a few hundreds of meters at the west side of the road.
Age: lower Bashkirian, C 1 n e 1 or e 2 (Note that on the bigger geological map this section is positioned in two suites younger, however the more detailed map clearly showed the in this report used age) % Sand: 77 % Thickness: 65 m Sedimentology: The Stepano-Krynka section is characterised by a very high amount of sandstone versus clays and silts, that is not seen in any of the other studied sections. Most sandstones in the section are characterised as well-sorted middle sand that are full of intermediate scale current induced cross bedding. These sandstones belong to Group B or sometimes C. Short intervals exist of coarser and less sorted sandstones with tree trunk prints. These sandstones belong to Group A. One relatively thin sandstone (SK.2A) is present that is much finer grained and contains hummocky cross stratification. This unit belongs to Group D. There is not a lot of variation in paleocurrent directions measured that are mainly pointing at an east-southeast direction (see Figure XSK). One coal layer has been found that however could not be sampled due to its very weathered character.

Environmental interpretation:
The sandstone characteristics of the Stepano-Krynka section aim at a deltaic to upper shoreface depositional environment with intercalations of fluvial regimes. Between sandstone unit SK.2B and SK.1 there was a short period of deeper water environment, interpreted to be a lower shoreface environment. The low variability in paleocurrents might indicate the open water environments in which the sediments where deposited, instead of more continental (fluvial) systems. This section is interpreted as a 3 rd order but even also 2 nd order sea level low-stand.

Section:
Stepnoye Location: 37U 0458711 UTM 5333739, central area Situation: The Stepnoye section is named after the settlement Смепное (Stepnoye) that is close to the village Михайловка (Michailovka). The section can be reached by taking the right split of the road (in bad shape) to the north from the latter village and going sharp right after passing small lakes. The section then starts in between the trees near the river and continuous along the road.
Age: upper Bashkirian, C 2 5 % Sand: 19 % Thickness: 157 m Sedimentology: The Stepnoye section has a low amount of sand with respect to shales, which might be biased by the bad outcrop scale in between big sandstone units due to which some thinner sandstone might be missed. The sandstones present in the section consist mainly of moderately and well sorted middle to fine sand that have small to large scale current induced cross bedding. Paleocurrent measurements aim at a consistent southeast direction with around 120° of variation. These sandstones are grouped within sandstone Group B and C. At the base of the section one thin sandstone that is coarser and less sorted and that belongs to Group A occurs.

Environmental interpretation:
The sandstones of the Stepnoye section are interpreted to be deposited in a upper to lower shoreface setting (Group B and C) and the shales in a deeper environment. The sandstone interpretation is strengthened by the low amount of variation in paleocurrent directions measured (see Figure XST). One short interval of fluvial environment or close proximity of the continent is documented in the base of the section. Figure XST. Rose diagram of all measured paleocurrent directions in the Stepnoye section. Petals given in groups of 10°, largest petal represents 3 data points and 16% of the total data.

Section:
Toshkovka Location: 37U 0468392 UTM 5403293, northern area Situation: The Toshkovka section is named after the village Тошковка (Toshkovka). The section can be reached from the cemetery by walking downwards to the west. The cemetery is situated a few hundreds of metres south of the main road at the eastern edge of the village towards the village called Нижнее (Nizhnee).
Age: upper Bashkirian -lower Moscovian, C 2 5 % Sand: 25 % Thickness: 119 m Sedimentology: The lower part of the Toshkovka section is characterised by coarse poorly sorted sometimes massive and tree trunk bearing sandstone units. Some large scale trough cross stratification is present and well graded intervals exist. These units are grouped within sandstone Group A. Above this interval a huge amount of shale exist with a thick limestone in between. Then a few well sorted very fine to fine sandstones occur that are grouped within group D and C. Paleocurrent directions aim at flow to the southeast, although two measurements to other sides considerably bias the rose diagram ( Figure XT), as not too many measurements could be taken. One of these two is a paleocurrent measurement to the northwest that has been measured on 3D very small-scale ripples, which might be caused by wave induced currents. The section ends with a thick light limestone. Two coal layers, of which one has been samples in this project, are present within a shale interval not far below the latter limestone.

Environmental interpretation:
The sedimentary characteristics of the Toshkovka section show a deepening from fluvial (Group A) to shallow marine environments (Group C and D). The uppermost thick limestone has to be deposited in clear water environments without clay suspension, while the close co-occurrence with coal layers possibly requires fast short term sea level variability. The fact that around these layers no shallow sandstones have been deposited is not fully understood.

Environmental interpretation:
The Yur'ivka section reveals sandstones of middle to lower shoreface depositional environments. No fluvial impulses have been found which strengthens this interpretation. The shale-rich interval below the section might be seen as a 3 rd order sea level high-stand and the section itself a 3 rd order sea level low-stand. This section shows that low-stands of the 3 rd order sea level cycles do not involve fluvial impulses to be deposited, and thus that they are relatively high with respect to low-stands at the base and top of the middle Carboniferous.

Section:
Zolotoye sandstone Location: 017D, northern area Situation: The Zolotoye sandstone is named after villages Эолотое (Zolotoye), of which there are several around the section. The sandstone is situated in a small non-official quarry at the northside of the Камыщеваха river at the junction in the road between two Эолотое-villages and Голубовское (Golybovskoie).
Age: lower Moscovian, C 2 5 % Sand: n/a Thickness: 12 m Sedimentology: The Zolotoye sandstone can be divided into two halves that show distinct sedimentary characteristics. The lower part shows poorly sorted bedded massive coarse sandstone, and can be grouped into Group A. The upper half consists of better sorted coarse sand with large-scale current induced cross bedding. Without success there has been searched for stratigraphy around the Zolotoye sandstone.

Environmental interpretation:
The lower part of the Zolotoye sandstone is interpreted to be deposited in a fluvial environment. The upper part has characteristics of deltaic and upper shoreface environments (Group B).