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1

Integrated biostratigraphy and palaeoenvironments of the Upper Cretaceous in the Petrich section (Central Srednogorie Zone, Bulgaria)

Pavlishina Polina, Dochev Docho, Wagreich Michael, Koukal Veronika

Acta Geologica Polonica

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2023

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Vol. 73, no. 1

33--62

EN

The Upper Cretaceous succession (Coniacian to lowermost Maastrichtian, with focus on the Campanian) at Petrich, Central Srednogorie Zone in Bulgaria, is described and calibrated stratigraphically based on nannofossils, dinoflagellate cysts and inoceramids. The following standard nannofossil zones and subzones are identified: UC10-UC11ab (middle to upper Coniacian), UC11c-UC12-UC13 (uppermost Coniacian to Santonian), UC14a (lowermost Campanian), UC14bTP-UC15cTP (lower Campanian to ‘middle’ Campanian), UC15dTP-UC15eTP (upper Campanian), UC16aTP (of Thibault et al. 2016; upper part of the upper Campanian), and UC16b (Campanian-Maastrichtian boundary). The base of the Campanian is defined by the FO of Broinsonia parca parca (Stradner) Bukry, 1969 and Calculites obscurus (Deflandre) Prins and Sissingh in Sissingh, 1977 (a morphotype with a wide central longitudinal suture). The Areoligera coronata dinoflagellate cyst Zone (upper lower Campanian to upper upper Campanian) is identified, corresponding to the UC14bTP-UC16aTP nannofossil subzones. The inoceramid assemblage indicates the ‘Inoceramus’ azerbaydjanensis-‘Inoceramus’ vorhelmensis Zone, correlated within the interval of nannofossil subzones UC15dTP-UC15eTP. The composition of the dinoflagellate cyst assemblages and palynofacies pattern suggest normal marine, oxic conditions and low nutrient availability within a distal shelf to open marine depositional environment during the Campanian.

2

Tidally dominated Miocene deltaic deposits and pipe rocks in the Tebessa Basin, eastern Algeria: sedimentological and ichnological characteristics

Mazrou Samia, Mahboubi Mohamed

Geologos

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2021

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Vol. 27, No. 1

15--34

EN

Analyses of sedimentology, ichnology and depositional environments of Langhian–Tortonian siliciclastic deposits in the southern Tebessa Basin (eastern Algeria) have revealed a tidally dominated deltaic setting with a high concentration of vertical burrows. From south to north, two depositional environments are here distinguished in the Tebessa Basin: a subaerial lower delta plain in the Oum Ali region with the trace fossils Ophiomorpha and Skolithos linearis, and a proximal delta front with numerous Skolithos, situated in the Hjer Essefra area. The occurrence of Skolithos pipe rock in this Miocene subtidal domain is unusual. Pipe rocks have been commonly reported from shallow and well-oxygenated environments, especially from Cambrian strata; their number decreased significantly during the Ordovician, coupled with an increase in biodiversity. Here different deltaic subenvironments and tidal signals that are exceptionally well preserved in outcrops are analysed and interpreted. Different ichnotaxa are briefly described, and the uncommon density and size of Skolithos pipe-rock in these Miocene strata are discussed.

3

Foraminiferal and calcareous nannoplankton biostratigraphy of the upper Badenian-lower Sarmatian strata in the SE Polish Carpathian Foredeep

Peryt Danuta, Garecka Małgorzata, Peryt Tadeusz Marek

Geological Quarterly

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2021

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Vol. 65, No. 1

art. no. 18

EN

The Badenian/Sarmatian boundary in the Central Paratethys has been traditionally identified by the faunal turnover recording an important environmental change possibly controlled by the change from marine to brackish conditions. The strata below the Badenian/Sarmatian boundary in the northern Carpathian Foredeep are included into the Pecten beds, and those above it into the Syndesmya beds. Foraminiferal study of the Babczyn 2 borehole which is one of the crucial sections in the northern Carpathian Foredeep, well-known for the depositional age of rhyolite tuff within the Pecten beds dated by Śliwiński et al. (2012) at 13.06 ±0.11 Ma, indicated that in fact the boundary occurs within the Syndesmya beds. This conclusion is based upon the rapid change from a stenohaline foraminiferal fauna to a euryhaline one, and the appearance of the species Anomalinoides dividens, the taxon regarded as the marker for the Sarmatian. In the Babczyn 2 and Cieszanów 1 (located ~2.5 km basinward of Babczyn 2) boreholes, Anomalinoides dividens appears 3.1-3.8 m above the replacement of stenohaline by euryhaline foraminifers. The calcareous nannoplankton study shows that the upper Badenian and the lower Sarmatian strata in the studied sections represent the NN6, undivided NN6-NN7, and NN7 zones.

4

Geochemical analysis for palaeoenvironmental interpretations : a case study of the English Wealden (Lower Cretaceous, south-east England)

Akinlotan O.

Geological Quarterly

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2017

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Vol. 61, No. 1

227--238

EN

The elemental composition of terrigenous sediments is extremely important for interpreting the depositional conditions of sedimentary basins especially when traditional data are not available or insufficient. For the first time in the published literature, the elemental composition of the Lower Cretaceous Weald Basin (south-east England) is presented via X-ray fluorescence (XRF) and Spectral Gamma-ray (SGR) methods. The concentrations of Zr, Cr, Ni, Rb, and Sr in varying quantities suggest that the sediments may have been sourced from a stable craton while the enriched redox-sensitive elements such as Mo and Co confirm the presence of anoxic and reducing conditions at the sites of deposition. The spectral gamma-ray data show that the sandstone facies are more radioactive than the adjacent mudstone and shale and this trend is consistent with a freshwater origin. The higher quantity of thorium in relation to uranium indicates humid and hot palaeoclimatic conditions at the source areas that favoured intense weathering. The results of these analyses reinforce the importance of the geochemistry of sedimentary rocks as useful tools for understanding the depositional conditions of sedimentary basins.

5

Palaeoenvironmental changes in the Transylvanian Basin during the Early Miocene revealed by the foraminifera assemblages

Székely S. F., Beldean C., Bindiu R., Filipescu S., Săsăran E.

Geological Quarterly

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2016

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Vol. 60, No. 1

167--180

EN

The evolution of the Transylvanian Basin during the Early Miocene has been restored from the succession of palaeoenvironments inferred from the sedimentological trend and succession of specific foraminifera assemblages from Lower Miocene Tihău section in northwestern Transylvanian Basin. Planktonic foraminifera suggest a Burdigalian age and recorded sea-level changes, climatic and productivity events. Benthic foraminifera offered valuable data on the palaeoenvironmental evolution, with a large-scale progradational (coarsening up) sedimentary succession influenced by regional tectonics. The succession of depositional events include: i) transgressive coarse grained deposits with typical mediterranean assemblages of bivalves in beach environments; ii) the glauconitic facies which can be associated to the maximum flooding surface of the transgression; iii) the sedimentation continued on a narrow shelf influenced by deltas during the highstand; iv) influence of regional tectonics and subsequent filling with turbidites associated to fan deltas.

6

The Silurian of southwestern margin of the East European Platform (Ukraine, Moldova and Romania) : lithofacies and palaeoenvironments

Radkovets N.

Geological Quarterly

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2015

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Vol. 59, No. 1

105--118

EN

Silurian strata, stretching along the western margin of the East European Platform from the Baltic to the Black Sea, represent a potential target for both conventional and unconventional hydrocarbon exploration. Distribution of the black shale facies, prospective for shale gas, and the reef facies, prospective for oil, has been studied in respect of palaeoenvironments. The Silurian sequence has been investigated in the territory of Ukraine (Volyn-Podillyan Plate, Dobrogean Foredeep) and correlated with the data on Moldova and Romania (Moldovian Platform). The occurrence of Silurian strata, their thickness, and petrographic and lithological characteristics allowed reconstructing the distribution of open-shelf, reef and lagoonal facies. The reef facies migrated during the Wenlock–Middle Pridoli, shifting towards the open sea and back towards the shore, and therefore has been termed a migrating reef facies. Correspondingly, the boundary between the open-shelf and reef facies was shifting. The facies distribution was controlled by the transgressive-regressive cycles, which caused the fluctuations of the shelf water depth in different time intervals of the Silurian. The shelf water depth of about 100 m, where the top of the oxygen-minimum layer impinged on the sea bottom, was the boundary between the open-shelf facies, represented by organic-rich sediments, and the reef buildups.

7

Middle Turonian trace fossils from the Bystrzyca and Długopole sandstones in the Nysa Kłodzka Graben (Sudetes, SW Poland)

Chrząstek A.

Geological Quarterly

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2013

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Vol. 57, No. 3

443–-466

EN

The Middle Turonian sediments of the Nysa Kłodzka Graben (Bystrzyca Sandstone in the Stara Bystrzyca outcrop and the Długopole Sandstone in Długopole Górne Quarry) contain trace fossils, which include Curvolithus simplex, ?Macaronichnus isp., Ophiomorpha nodosa, Ophiomorpha isp., Palaeophycus tubularis, Thalassinoides cf. paradoxicus, T. suevicus and Thalassinoides isp. The assemblage of trace fossils points to the proximal Cruziana ichnofacies, that characterizes the distal lower shoreface and the archetypal Cruziana ichnofacies, typical of upper offshore settings. The trace fossils evidence implies that sedimentation took place in a shallow basin with periods of a sudden sediment input, good oxygenation and normal salinity. The Bystrzyca and Długopole sandstones are deposits of the shallow epicontinental sea that were deposited between the fair-weather and storm-wave base, in the distal lower shoreface–upper offshore setting. The Bystrzyca Sandstone is recognized as storm-originated deposits, whereas the Długopole Sandstone is probably the part of prograding “accumulation terrace”. The source of material for the sandstone was the East Sudetic Island and probably also the Orlica–Bystrzyca Uplift. The studied sandstones are related to a regression that started in the early/middle Middle Turonian and caused a relative uplift of the surrounding land.

8

Neptunian dykes in the Middle Miocene reefs of western Ukraine : preliminary results

Jasionowski M., Peryt D., Peryt T. M.

Geological Quarterly

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2012

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Vol. 56, No. 4

881–-894

EN

Neptunian dykes were recently recorded within the Middle Miocene (Upper Badenian and Lower Sarmatian) Medobory reef complex of the Carpathian Foreland in western Ukraine. The Upper Badenian reefs are cut by a regular, semi-perpendicular network of intersecting fissures that penetrate the Badenian reef limestone down for more than 10 m. The dykes are filled by several generations of Sarmatian microbialites coating the fracture walls and by bedded bioclastic sediment (including foraminifers) which is more important volumetrically. The fissure fillings containing both the Sarmatian material as well as rare clasts of Badenian rocks indicate that the fissures were open during the onset of Sarmatian deposition. They originated following the emergence and fracturing of the Badenian limestones, either due to fault tectonics at basin margins induced by basin subsidence, around the Badenian-Sarmatian boundary, or to gravitational instability of large lithified Badenian reef bodies. Only one phase of fracture opening occurred. The filling of fractures was episodic, with pulses of cementation and microbial growth and sediment injection. Four different foraminiferal assemblages recorded in the neptunian dykes indicate that the process of fracture filling was long-lasting.

9

Foraminifers and stable isotope record of the Dubivtsi chalk (upper Turonian, Western Ukraine): palaeoenvironmental implications

Dubicka Z., Peryt D.

Geological Quarterly

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2012

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Vol. 56, No 1

199-214

EN

A combined micropalaeontological and stable isotope study of the Dubivtsi chalk in the Western Ukraine indicates its middle late Turonian age. One long-term and clearly distinguishable positive excursion of the isotope curve (d1318

10

Sarmatian foraminiferal assemblages of cavern fillings in the Badenian reefs of Medobory (Polupanivka, Western Ukraine)

Peryt D., Jasionowski M.

Biuletyn Państwowego Instytutu Geologicznego

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2012

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nr 449

175--184

EN

The Middle Miocene (Upper Badenian) coralline algal reefs of Western Ukraine contain caverns (up to 1 m across) and fissures that are filled by Sarmatian deposits: marly clays, clayey carbonate sand and bioclastic, bryozoan-rich sand. These deposits often contain abundant and very well preserved calcareous benthic foraminifera; agglutinated forms have not been recorded. Foraminiferal tests quite commonly show morphological abnormalities, e.g. twin forms or tests with irregularities in size or shape in last chambers. Each of the three analysed samples is characterized by a different foraminiferal assemblage: Elphidium aculeatum assemblage, Hauerinidae assemblage and Lobatula lobatula assemblage. These assemblages are characteristic for a shallow marine environment. Low taxonomic diversity and high dominance or monospecific foraminiferal assemblages indicate generally a restricted marine environment. It seems that the increasing salinity and very high-energy environment were the most probable factors controlling the composition of the foraminiferal assemblages.

PL

Górnobadeńskie rafy koralowe zachodniej Ukrainy posiadają kawerny o przekroju do 1 m oraz szczeliny, które obecnie są wypełnione osadami sarmackimi – iłami marglistymi i zasilonym piaskiem węglanowym i bioklastycznym, często mszywiołowym. Osady te często zawierają liczne i bardzo dobrze zachowane węglanowe otwornice bentosowe; nie stwierdzono natomiast otwornic zlepieńcowatych. Dość częste są skorupki z morfologicznymi nieprawidłowościami, np. formy bliźniacze albo skorupki z nieregularnością rozmiaru lub kształtu ostatnich komór. Każda z trzech próbek zawiera inny zespół otwornicowy: w pierwszej występuje zespół z Elphidium aculeatum, druga zawiera zespół z Hauerinidae, a trzecia – zespół z Lobatula lobatula. Wszystkie te zespoły są charakterystyczne dla środowiska płytkomorskiego. Niskie zróżnicowanie taksonomiczne i wysoka dominacja lub jednogatunkowe zespoły otwornicowe wskazują na ogólnie ograniczone środowisko morskie; wydaje się, że w takich warunkach najbardziej prawdopodobnym czynnikiem warunkującym skład zespołów otwornicowych było podwyższone zasolenie i środowisko bardzo wysokoenergetyczne.

11

Palynology and palaeoenvironments of the Silurian Coosglass Slate and Ferriter’s Cove formations in the Dunquin Inlier (Dingle Peninsula, Ireland)

Higgs K. T., Williams B. P. J.

Geological Quarterly

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2011

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Vol. 55, No. 2

95-108

EN

This study documents the first palynological data from the Silurian of the Dunquin Inlier of the Dingle Peninsula and focuses on the lower part of the succession. The previously undated Coosglass Slate Formation contains a low diversity Llandovery to early Wenlock acritarch assemblage, confirming the previously enigmatic Coosglass Slate Formation as the oldest formation in the Silurian succession. The lithofacies and acritarch biofacies suggest the mudstones of the Coosglass Slate Formation represent a low energy, offshore, muddy shelf palaeoenvironment. The Ferriter' s Cove Formation, a succession of five offshore to tidal-flat regressive marine cycles yielded more diverse palynological assemblages. Acritarchs and cryptospores from the Ferriter's Cove Formation confirm a Wenlock (Homerian) age for this formation. The integration of palynological and sedimentological data from four of the offshore to tidal-flat regressive cycles reveals three consistent palynomorph distribution trends: (1) offshore and shoreface depositional facies contain the most diverse palynological assemblages in which acritarchs with long and ramified processes dominate; (2) back-barrier tidal-flat facies are characterized by terrestrially derived palynomorphs, particularly cryptospores, together with a restricted acritarch assemblage of mainly micrhystrid and veryhachid forms with small simple processes; (3) back-barrier lagoonal facies with patch reefs are dominated by the sphaeromorph acritarch Leiosphaeridia.

12

Reefal environments and sedimentary processes of the Anisian Karchowice Beds in Upper Silesia, southern Poland

Matysik M.

Annales Societatis Geologorum Poloniae

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2010

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Vol. 80, No 2

123-145

EN

The Anisian shallow-marine Karchowice Beds of the Upper Silesia represent reefal habitats and circum-reefal environments, where biological-mechanical interactions determine sedimentary processes and facies pattern. The purpose of this study was recognition of the interaction between biological and mechanical controls of carbonate deposition. Such interdependence resulted in considerable lateral variability of thickness and lithological features, observed at a distance of 25 km. The western part of the basin is dominated by proximal facies (reefal facies), whereas the eastern one represents distal facies (fore-reef). Sedimentary succession in the western area is twice as thick than the eastern one. It resulted from different rate of subsidence owing to block tectonics, controlled by reactivated ancestral Silesian-Moravian Fault. Small-scale synsedimentary faults confirm syndepositional tectonic activity in the region. Palaeogeographical position caused that the Upper Silesia was strongly affected by monsoon climate, generating storms. These storms contributed to episodic deposition, prevailing during the sedimentation of Karchowice Beds. However, most of time was represented by prolonged non-deposition periods, recorded as interstratal hiatuses, but also by forming of firmgrounds, micritization and coating of bioclasts or substrate recolonisation by organisms.

13

Patterns of occurrence and distribution of crustacean ichnofossils in the Lower Jurassic-Upper Cretaceous of Atlantic occidental margin basins, Portugal

Carvalho C., Baucon A., Rodrigues N., Santos V., Viegas P.

Acta Geologica Polonica

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2010

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Vol. 60, no. 1

19-28

EN

Crustacean ichnofossils are most abundant in the stratigraphic record of Portugal. In this paper is presented a study on crustacean ichnoassemblages from the Sinemurian to middle Cenomanian, during the opening and subsequent filling of Atlantic occidental margin basins (Lusitanian Basin and western part of Algarve Basin). Thalassinoides dominates the lagoonal and inner shelf facies from the late Sinemurian at least to the Turonian, generally defining all of the carbonate sequence with its dense, "nodular" ichnofabric since the Late Jurassic. Asequence is described in the lower Barremian of Cabo Espichel in which burrow mazes of Thalassinoides suevicus occur, containing hundreds of Mecochirus rapax as an obrution lagerstatte. The Cretaceous of the Lusitanian Basin is rich in marly limestones mottled with Thalassinoides but almost devoid of Rhizocorallium. Fairly common in dark marls and biomicrites from the Jurassic, Rhizocorallium irregulare locally occurs in dense monospecific fabrics. Slipper-shaped and oblique forms (Rhizocorallium jenense) as well as spiral and lobate forms are rhizocoralliid foraging modifications usually developed in deeper tiers than Thalassinoides and sometimes evidencing bioimprints in Glossifungites preservation. Siliciclastic facies related to rifting subsidence along major fault scarps and fan deltas/braided river depositional systems show pervasive bioturbation with Psilonichnus tubiformis.Another crab-style behaviour ascribed to Macanopsis plataniformis is described for the first time in Portugal, associated with a coarse-grained sandstone episode in a tidal flat setting from the Kimmeridgian, where monospecific brachyuran burrows were developed in a firmground.

14

Biofacies and palaeoenvironments of the Jurassic Shaqra Group of Saudi Arabia

Hughes G.

Volumina Jurassica

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2008

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Vol. 6, no. 1

33-45

EN

The Jurassic succession in Saudi Arabia consists of seven formations, forming the Shaqra Group, of which most are carbonate and some are partly evaporitic, and is of economic importance because it hosts twelve hydrocarbon reservoirs, including the Arab-D Reservoir within the world’s largest oilfield at Ghawar. The Minjur-Marrat formational boundary marks the Triassic-Jurassic boundary, of which the Marrat is dated as Toarcian. A significant unconformity separates this unit from the overlying Dhruma Formation, of Bajocian to Bathonian ages. The Tuwaiq Mountain Formation, of Callovian age, overlies the Dhruma Formation, with reduced hiatus. The Hanifa Formation, of Oxfordian age, is separated from the Tuwaiq Mountain Formation by a minor hiatus, as are the successive Jubaila, Arab and Hith formations, of Kimmeridgian to Tithonian ages. The Jurassic-Cretaceous boundary is currently placed within the lower part of the overlying Sulaiy Formation. A combination of semi-quantitative micropalaeontological and macropalaeontological analysis of closely-spaced thin sections from these carbonates displays a tiered relay of clearly defined microbiofacies cycles. These cycles reveal palaeoenvironmental trends that subdivide the succession into parasequences, transgressive and highstand systems tracts that are not always evident from the sedimentology alone. The biofacies approach to elucidating palaeoenvironmental variations of the Shaqra Group provides significant insights to the Jurassic history of the Arabian Plate, as well as serving to explain the origin and stratigraphic location of hydrocarbon reservoirs, seals and potential source rocks. The Shaqra Group spans at least 36 Ma, and qualifies as a second order depositional sequence, within which the formations can be considered as third order sequences. Better chronostratigraphic constraint of the depositional sequences reveals elevated subsidence rates for the Dhruma, Tuwaiq Mountain and Hanifa formations that can be related to established episodes of global plate readjustment. Tectonoeustatic and possibly glacioeustatic controls on depositional cyclicity probably exerted an influence on the major unconformities within the Shaqra Group.

15

Growth forms and distribution patterns of stromatoporoids exposed on Devonian palaeobottom surfaces; Holy Cross Mountains, central Poland

Łuczyński P.

Acta Geologica Polonica

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2008

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Vol. 58, no. 3

303-320

EN

Well preserved palaeobottom surfaces with stromatoporoids are exposed in two Devonian localities in the Holy Cross Mountains in central Poland: Skały and Bolechowice-Panek quarries. The stromatoporoids were subjected to morphometric analysis and distribution studies. Stromatoporoids were studied hitherto only in vertical cross sections; the study of three-dimensional stromatoporoid domes has created a need to introduce a new set of parameters describing their shapes, which includes: horizontal length and width, vertical height, elongation ratio and elongation azimuth. In order to make the measurements made by various methods comparable, and thus to allow comparable interpretations, recalculation formulas have been presented. Comparison of the results obtained by the two methods indicates that the measurements made in vertical cross sections substantially bias the dimensions and shapes of the stromatoporoids, which may influence stromatoporoid-based palaeoenvironmental reconstructions. The results of the measurements were interpreted in terms of palaeonvironmental conditions. In Bolechowice-Panek the stromatoporoids lived in a quiet shallow water setting with a low and stable sedimentation rate. In Skały the stromatoporoids grew in a shallow subtidal setting and located themselves on parallel ripples, most probably to escape being buried by deposits accumulating in inter-ripple depressions.

16

Palynofacies of the epicontinental Lower Jurassic deposits of Poland: their distribution in different sedimentary environments

Pieńkowski G., Waksmundzka M. M.

Volumina Jurassica

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2006

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Vol. 4, no. 1

102-103

EN

Diversified continental, marginal-marine and marine epicontinental deposits in Poland of Early Jurassic age yielded unusually rich palynofacies. 210 samples from 6 selected boreholes have been studied in order to determine distribution of palynomorphs and other kerogen particles in various depositional systems, prior determined by detailed sedimentological studies. Several palynofacies parameters, like content of terrestrial phytoclasts, their type and degree of oxygenation, spore/bisaccate pollen ratio, presence of tetrads and sporangia, presence of amorphous organic matter - AOM (both of terrestrial - AOMT and aquatic - AOMA origin), presence of acritarchs and dinoflagellate cysts were collectively found as indicative of certain palynofacies. Generally, six main palynofacies linked to certain depositional systems have been distinguished: 1. alluvial plain - very abundant palynological matter, varied frequency of sporomorphs, varied bisaccate pollens/spores ratio (depending on a local vegetation), very abundant dark-brown to opaque phytoclasts or AOMT (amorphous organic matter of terrestrial derivation); 2. delta plain - abundant dark to opaque phytoclasts (or AOMT) and cuticle, abundant sporomorphs (spores dominate over bisaccate pollens), sporadic presence of dinoflagellate cysts and acritarchs; 3. delta front-lagoon – abundant sporomorphs, varied bisaccate pollens/spores ratio (usually spores and pollens are in approximate ballance, but sometimes huge number of spores accompanied by tetrads and sporangia occurs in “hydrodynamic traps”), moderate amount of phytoclasts - mostly cuticle, relatively rare dark phytoclasts, rare dinoflagellate cysts and acritarchs;4. shoreface-foreshore - dispersed and rounded fragments of black, opaque phytoclasts or AOMT, rare to moderate sporomorphs (bisaccate pollens and spores in ballance or slight dominance of spores), sporomorphs often mechanically destroyed, rare dinoflagellate cysts and acritarchs; 5. offshore brackish marine - rare translucent AOMA (amorphous organic matter of aquatic derivation), very rare small and rounded fragments of opaque phytoclasts or AOMT, rare sporomorphs (bisaccate pollens dominate over spores), occurrence of dinoflagellate cysts and acritarchs; 6. offshore marine - translucent AOMA (amorphous organic matter of aquatic derivation), rare bisaccate pollens, spores absent or very rare, occurrence of dinoflagellate cysts and acritarchs. Transitional palynofacies between these main types can be also distinguished, but it must be coupled with sedimentological data. Some abnormal composition of palynofacies have been observed and they are defined as palynofacies inversions. The first type (type I) of inversion represents coexistence in one sample of palynomorphs of the same geological age but of drastically different colours, which is explained by darkening of some palynomorphs by intraformational oxidation processes. The second type (II) of inversion occurs when characteristic alluvial/deltaic palynofacies elements (like sporangia, tetrads, large phytoclasts) are found together with obviously offshore elements (light AOMA, dinocysts, accompanied by ammonites and open shelf organisms). This inversion is explain by a storm resuspension processes. Abnormally high number of spores observed in few samples of an interdistributary lagoon origin is explained by hydrodynamic entrapment of spores. Some regular fluctuations of palynofacies characteristic within lagoonal/lacustrine laminites are described as a result of seasonal changes. Collectively, palynofacies can be regarded as useful tool for recognition of palaeoenvironments. However, palynofacies studies should be coupled with sedimentological investigations.

17

Biofacies and palaeoenvironments of the Jurassic Shaqra Group of Saudi Arabia

Hughes G.

Volumina Jurassica

|

2006

|

Vol. 4, no. 1

89-90

EN

The Jurassic succession in Saudi Arabia consist of eight formations, of which most are carbonate and some are partly evaporitic, and is of economic importance because it hosts twelve hydrocarbon reservoirs, including the Arab-D reservoir within the world’s largest oilfield at Ghawar. The Minjur-Marrat formational boundary marks the Triassic/Jurassic boundary, of which the Marrat is dated as Toarcian. A significant unconformity separates the overlying Dhruma Formation, of Bajocian to Bathonian age. The Tuwaiq Mountain Formation, of Callovian age, overlies the Dhruma Formation, with reduced unconformity duration. The Hanifa Formation, of Oxfordian age, is separated from the Tuwaiq Mountain Formation by a minor unconformity, as are the successive Jubaila, Arab and Hith formations, of Kimmeridgian to Tithonian age. The Jurassic/Cretaceous boundary is currently placed at the Hith-Sulaiy formational contact. Intensive analyses of the carbonates reveals variably rich micropalaeontological biofacies that contain foraminiferal species of potential palaeoenvironmental significance, especially when applied to cored mudstone to grainstone repeated successions of which the hierarchy is often difficult to elucidate. A combination of semi-quantitative micropalaeontological and macropalaeontological analysis of closely-spaced thin sections from these carbonates reveals clearly defined microbiofacies cycles. Their stacking order provides clearly defined palaeoenvironmental trends that subdivide the succession into potential parasequences, transgressive and highstand systems tracts. The Shaqra Group spans at least 38 Ma, and qualifies as a second order depositional sequence, within which the lithostratigraphic units of formation identity fall into third order sequences. The extensive duration of unconformities spanning the Hettangian-Sinemurian and the Aalenian-Bajocian need explaining, especially when compared to the relatively minimal interformational unconformities that characterize the Bajocian-Tithonian succession. Eustatic sea-level data indicates that the Hettangian to Early Pliensbachian is characterized by a relatively insignificant sea level variation when compared with the gradual rise from the Late Bajocian to the Kimmeridgian. The mid Bajocian event approximates with the global increase in percent dolomite in abiotic muds, and may be associated with a transitional icehouse-greenhouse phase, for which post Bajocian eustatic falls would be expected to display increasing greenhouse affinity. Preservation of calcitic ooids within the Middle and Late Jurassic carbonate reservoirs within Saudi Arabia, together with excellent intergranular porosity testify to the calcitic oceanic conditions associated with greenhouse times. The biofacies approach to elucidating palaeoenvironmental variations of the Shaqra Group provides significant insights to the Jurassic history of the Arabian Plate, as well as serving to explain the origin and stratigraphic location of hydrocarbon reservoirs.