Sedimentary organic matter as a proficient tool for the palaeoenvironmental and palaeodepositional settings on Gondwana coal deposits

Palynofacies is based on the different types of the dispersed/sedimentary organic matter (DOM/SOM) and has been used as a proficient proxy for the palaeoclimatic reconstructions in sedimentary deposits of various time spans. It has also been acknowledged as an effective tool in the different domains like sequence biostratigraphy, palyno-biostratigraphy, palaeodepositional history, identification for depositional processes, oxic–anoxic environment, and variations in the water depth. It has been emerged as an analytical tool in palaeoclimatic reconstruction, which could complement geophysical and geochemical datasets. Since long palynofacies analysis has been exclusively applied in the marine sediments, it has recently dragged the attention of many researchers as a significant parameter for palaeoclimatic interpretation in continental deposits. In the last few decades, more consideration was focused on palynofacies that have become an essential proxy in the biostratigraphic and other non-biostratigraphic fields due to its requirement in the petroleum industries. The present study provides a basic idea of dispersed organic matter characterization, methodology, interpretations, and its application with special emphasis on the Gondwana deposits. The study also includes the summary of the worldwide distribution of the Gondwana sediments, especially for palaeodepositional settings through palynofacies along with other parameters.


Introduction
coined the term palynofacies and was described by Powell et al. (1990). Since long, palynofacies analysis has been extensively applied in the various sediments of different periods by various workers (Singh et al. 1992;Traverse 1994;Graz et al. 2010;Garcia et al. 2011;Aggarwal et al. 2019a, b, c) for the source rock potential by using the transmitted light microscopy, as illustrated by Tyson (1995). Previously, the palynofacies analysis was only confined to the marine settings solely for the petroleum sectors. It has not only played a substantial role in the palaeoenvironmental settings in the marine succession (Götz et al. 2003;Oboh-lkuenobe et al. 2005;Hermann et al. 2012;Carvalho et al. 2013;El Atfy et al. 2014) but also has been applied on the continental successions of different sedimentary horizons, including coal/lignite (Singh et al. 1992;Closas et al. 2005;Cazzulo-Klepzig et al. 2009;Götz 2016, 2017;Cornamusini et al. 2017;Götz et al. 2017;Aggarwal et al. 2019a, b, c). The palynofacies study has now been expanded in various types of lacustrine (Aggarwal et al. 2019b) and fluvial systems delineating its importance to coal deposits (Aggarwal et al. 2019a, b, c), understanding the processes and factors responsible for the sediment deposition.
In palynofacies investigation, the total organic matter in the sediments is scrutinized, which decreases the taphonomical preconceptions to a great level and permits interpreting source rock provenance for applied scientific aspects. It has recently been incorporated in the basin analysis, producing geological models to represent the direct quantification and qualifications of the different types of organic matter in a specific sediment/rock. This is found to be helpful in designing the geological models, especially in the sedimentary carbonate systems, due to various chemical, geological, physical, and biological parameters that govern deposition 1 3 in these environments. This proxy is also helpful in providing information about the provenance of the organic matter sedimentary and palaeobiological dynamics and gives an idea about the gas and oil generation potential.
Palynofacies analysis has also been acknowledged as an efficient tool in domains like sequence biostratigraphy, palyno-biostratigraphy, proximal-distal trends, identification for depositional processes, oxic-anoxic environment, and variations in the water depth (Zobaa et al. 2011;Mueller et al. 2014;Tyson and Follows 2000). It is being used as an analytical proxy in palaeoclimatic reconstruction, which could complement the combination of geophysical and geochemical datasets (Mueller et al. 2014;Zhang et al. 2015). Palynofloral record, along with palynofacies, has become a reliable tool for enhancing the accuracy and consistency of palaeoclimatic reconstructions based on palaeophytogeography, palaeoecology, and palaeoclimatology (Modie 2007;Césari and Colombi 2016;Lindström et al. 2016).

Palynofacies analysis
The accumulation of various forms of the organic matter components (pollen grains, spores, plant tissues, tracheids, resins, filaments, fungi, algae, acritarchs, foraminiferal test linings, etc.) depends upon the different processes like sedimentary, physical, and biochemical (Aggarwal et al. 2019a, b, c). After decay, demise, and burial, diverse plant communities represent different types of taphonomical and preservational characters demonstrating the setting of their transportation and primary procedures (autochthonous and allochthonous) in the dispersed organic matter.

Palynomorphs (percentage of total kerogen)
Palynomorphs include sporomorphs, phytoplankton, and zoomorphs. Their specific organization defines the appearance of palynomorphs in the sedimentary organic matter at the cellular level. A high proportion of continental palynoflora (Cordiates, Glossopterids, conifers, lycopsids, sphenopsids, filicopsids, etc.) indicates proximity to terrestrial sources (Tyson 1995), while the presence of the marine palynomorphs (dinoflagellate cysts, foraminiferal test linings, scolecodonts, prasinophytes, etc.) indicates the proximity of the source towards the marine regime.

Structured organic matter (percentage of total kerogen)
The predominant occurrence of phytoclasts is mainly related to the oxic conditions (Mendonça Filho et al. 2010). The structured phytoclast specifically includes brown translucent woody fragments (poorly/well preserved) with tracheids (uni-or biserial arrangement of rows of pits), vessels, and elongated elements (Batten 1996), tubes and filaments, plant tissues. One more category that lies under this class is planar organic remnants (cellular or non-cellular). Cellular structures are mainly associated with the yellowish brown, thin, unevenly folded, transparent or translucent cuticles (Batten 1996). Recently a new category has also been included in it called gelified phytoclasts (Zhang et al. 2015). The principal prevailing reason of phytoclasts allocation is the distance travelled over which particles have been transported (Carvalho et al. 2013). The large-sized phytoclast particles are deposited near the sources, which are considered as the proximal. In contrast, the small-sized phytoclasts are deposited in the distal part like in deep waters by turbidity currents (Habib 1982).

Opaque black woody tissues (percentage of total kerogen)
The opaque particles/charcoal (OP) is formed of wood particles in highly oxidizing conditions that range from black to dark brown under transmitted light. Black opaque phytoclasts are produced due to the oxidation of the translucent organic matter transported over a prolonged period at a normal or elevated temperature (Ercegovac and Kostić 2006).

Degraded organic matter (percentage of total kerogen)
The biodegraded organic matter is produced due to fungal and bacterial activities on the structured organic matter. A fungus is one of the most viable degrading agents which convert plant fragments into diverse organic matter types with the help of biological processes. The influx of fungi on the organic matter stimulates bacterial degradation in the later stages. Amorphous organic matter is derived from phytoplankton or by the action of fungi/bacteria on partially degraded organic matter that converts them into a structureless amorphous mass. Such amorphous matter looks to be porous and spongy (ranging from partial to complete degradation).

Methodology
Palynofacies involve the classification of diverse types of the dispersed organic matter remains (like brown/opaque phytoclasts, palynomorphs and light/dark AOM matters), size, and state of preservation. Identification of the palynofacies is chiefly established by Batten (1996), Tyson (1989Tyson ( , 1993Tyson ( , 1995, and later various other authors (Tschudy 1969;Minshall et al. 1985;Bustin 1988;Van Bergen et al. 1990;Williams 1992;Wood and Gorin 1998;Aggarwal et al. 2017Aggarwal et al. , 2019a have enhanced the criteria of the identification. For the processing of the samples of palynofacies analysis, a standard non-oxidative procedure is usually followed (Faegri and Iversen 1989;Tyson 1995;Batten 1999;Prasad et al. 2013). About 10-20 gm of the material from each sample was taken and crushed into small-sized pieces (not powder) with the help of a pestle and mortar. First, treated with hydrochloric acid (HCl) and followed by hydrofluoric acid (HF) to remove the carbonate and silicates in the sediments, respectively. The material is thoroughly is washed and sieved 37 µm (400 mesh)/25 µm (500 mesh) size, and later on, the material was spread and dried on a coverslip with the help of the polyvinyl alcohol (PVA). Finally, the dried coverslip is mounted using Canada balsam, and permanent slides are prepared. Three to four permanent slides 1 3 from each sample are observed with the help of a microscope with a transmitted light mode. As a minimum, 500 organic particles are counted from each sample at variable magnifications to attain statistically significant organic matter content and diversity (quantitative and qualitative data) for the study. The palynofacies parameters were computed to calculate relative abundances. For identifying numerous palynofacies zones/assemblages in the studied succession, any statistical software that may be used like CONISS can be performed with the help of TILIA ver 1.7 (Grimm 1987(Grimm , 1990Hammer et al. 2001).

Palynomorphs
Palynomorphs in the sedimentary organic matter ( Fig. 2a-b) are recognized by their specific cellular structure, shape, and morphology. This group is divided into three subgroups which include (a) sporomorphs, (b) phytoplankton, and (c) zoomorphs.
Sporomorphs include two categories spores and pollen grains. Continental spores include triletes, monoletes alete spores, and fungal spores. These spores ( (Fig. 3b), and Scolecodonts (illustration: plate 8, Fig. 3; Mendonça Filho et al. 2011). Acritarchs are defined as small microfossils of unknown and probably varied biological affinities. Foraminiferal test linings are organic linings of benthic foraminifera. On the other hand, Scolecodonts are defined as the mouth and body parts of some polychaete worms (primarily marine). Chitinozoa (illustration: plate 8, Fig. 4; Mendonça Filho et al. 2011) is the group characterized by the appearance of flask-shaped hollow bottles (30-2000 µm). It is an extinct group of microfossils found in Palaeozoic marine sediments. Their occurrence has been recorded from the Early Ordovician to Late Devonian.
(a) Tubes and filaments (T/F) are recognized by the variable size of structured, unstructured, filamentous, unbranched/branched tubes ( Fig. 3c-d), either of algal/ fungi (septate/aseptate) or vascular origin has been considered under this subgroup. This subgroup is continental, mainly in origin. (b) Poorly preserved woods (PW; Fig. 3e) are the bright brown-orange and elongated structureless remains. Woods (W) are the organic matter with a lath-shaped/ blocky outline, changeable from pale yellow to brown/ dark brown (Fig. 3f), with cellular structure, partially/ clearly visible. (c) Transparent lignocellulosic fragments (TLF) are characterized by the secondary xylem (cellular structures of wood) and the grey, light brown, or black colour of the lignin (Fig. 3g-h). The lack of fluorescence, low 1 3   After experimental examination, it has been concluded that these are the parts of the fossilized leaf that probably result from the higher biodegradation and more substantial weathering of small GP debris than the large leaf fossils preserved well (Zhang et al. 2015). As a result of which GP is ubiquitous in the palynofacies assemblages.

Opaque phytoclasts
Opaque phytoclasts have no visible structure and appear to be homogeneous. These include black-coloured organic matter that showed blade/equidimensional/angular-shaped palynodebris with irregular shapes under the light microscope. Opaque phytoclasts (Fig. 3k) are included in the inertinitic maceral group (mainly fusinite and inertinite). The opaque black phytoclasts comprise carbonized brownish black/oxidized to black-coloured woody tissues, including tracheids. These are further divided into two subgroups: a) structured charcoal (SC) and b) non-structured charcoal (NSC). SC is defined by the opaque and black particles with the noticeable structure are incorporated (mainly trachedial structures). NSC (Fig. 3k) is described as opaque and black particles without noticeable structures.

Degraded organic matter (DOM)
This group is divided into three subgroups: (a) degraded terrestrial (DT), (b) amorphous organic matter (AOM), and (c) resins. (a) DT is formed as an effect of fungi and bacteria. Fungi are well-known visible degrading agent, which transforms vegetal parts into the different organic types through biochemical mechanisms. Multiple states of preservation partial to completely degraded have been observed (Fig. 3l). It is purely terrestrial in origin. (b) AOM (Fig. 3m) is derived from phytoplankton or by the action of fungi/bacteria on partially biodegraded organic matter, which converts them into brown to structureless, porous/spongy, dark or slightly pale amorphous masses/structureless amorphous mass. Such formless matter looks to be porous and spongy (ranging from partial to complete degradation). AOM is further divided into two categories based on their origin (Combaz 1980;Batten 1983;Courtinat et al. 2003;Ercegovac and Kostić 2006;Pacton et al. 2011;Suárez-Ruiz et al. 2012), i.e. marine and continental/ terrestrial. Its gelified appearance defines terrestrial AOM, while the granular AOM is produced by the degradation of the phytoplankton with a granular/ fluffy appearance (Ţabără et al. 2015). Granular AOM with patchy fluorescence (depending on its oxidation state) from marine sources derived from microbial reworking (illustration: Ţabără et al. 2015). It colour varies like in coastal areas its yellow to green in oxic sedimentary conditions, while it is brown in dysoxic/ anoxic conditions in pelagic zones (Marson and Pocock 1981;Valdés et al. 2004;Ercegovac and Kostić 2006;). It is mainly yellow to brown under natural light and exhibits irregular aggregated shapes formed by fibrous and ultramicroscopic organic particles (< 3 μm). Gelified AOM (illustration: Ţabără et al. 2015) is orange to brown and sometimes contains internal structures (Pacton et al. 2011) derived from terrestrial sources degraded by bacteria, being a secondary microbial product. It appeared to form clumpy masses with sharp angular shapes. (c) Resins are derived from higher plants of tropical and subtropical forests. Resins are recognized by structureless particle, hyaline, homogeneous, fluorescent, rounded, sharp to diffuse outlines derived from higher plants, so this category is purely continental in origin.

Palaeoenvironmental interpretations based on the different forms of the dispersed organic matter
Besides providing biostratigraphic ages, various fossil groups and taxa play a significant role in providing palaeoenvironmental information. The palynofacies studies are based on the qualitative (presence/absence of taxa) or quantitative data (ratios between benthonic and planktonic foraminifera; dinoflagellates and spore/pollen, etc.). Some particular taxa are attributed to specific environmental conditions such as salinity (hypo, normal or hyper), anoxia/ dysoxia and bathymetry. All organic-walled fossils can be observed in a palynological slide. These slides include several fossil groups (spore, pollen grains, algae, dinoflagellates, foraminiferal test linings, fungal hyphae, acritarchs, etc.) derived from different environments resultantly; it usually allows palaeoenvironmental elucidation. The palynofacies symbolize the primary composition of the organic matter in the sedimentary rock after the long process of organic diagenesis and sedimentation. The composition of organic matter is directly controlled by the palaeoenvironments. Palynofacies uses the relative proportions of several fossil groups for the interpretation of the sedimentation settings, such as land-derived Vs. marine-derived and spores Vs. Acritarchs. The quantity of the organic matter exhibits an exceptional ability to trace palaeoenvironmental evolution under different climatic and sedimentary settings (Carvalho et al. 2006;Sebag et al. 2006;Zhang et al. 2015).
Resultantly, palynofacies investigation has been applied as one of the fundamental parameters for palaeoenvironmental interpretation (Buchardt and Nielsen 1991). As the relative proportions of the various organic matter particles reflect oxygenation conditions, this proxy has also been widely applied to estimate the lake-level changes and distances from the lakeshore (Sebag et al. 2006).
Botryococcus (Carboniferous to recent) is a diversified green alga commonly occurring in different water bodies like lakes, ponds, wet mud, ditches, and bogs (Round 1965;Guy-Ohlson 1992). Botryococcus belongs to the family Botryococcaceae. It is reported to be distributed in broad geographical regions, found in fresh to brackish water conditions (Chisti 1980;Tennant et al. 2019). Botryococcus contains unsaturated fatty hydrocarbons and can generate high hydrocarbons (Maxwell et al. 1968;Taylor et al. 2012;Hirose et al. 2013).
The occurrence of prasinophytes algae has been documented in the sediments of the Precambrian to the recent age. These are produced by tiny quadriflagellate phytoplankton. Recent forms occur in freshwater to the hypersaline settings, but fossilized forms are mainly marine (Mendonça Filho et al. 2011). But on the other hand, prasinophytes algae (Cymatiosphaera-like cysts) from the Palaeozoic to early Mesozoic (Clausing 1993;Dotzler et al. 2007;Kustatscher et al. 2014) sediments have been reported from brackish to freshwater sediments (Zippi 1998;Mudie et al. 2011;Mays et al. 2021).
Dinoflagellate cysts or dinocysts (division Dinoflagellata) are produced by single-celled red algae classified as Pyrrhophyta. These are defined as one-celled aquatic organisms bearing two dissimilar flagella and having characteristics of both plants and animals. Mostly the organisms are marine and form an essential part of the marine microphytoplankton, 1 3 though some live in freshwater habitats (Masure et al. 2017). These are very common in Upper Triassic to Holocene sediments and act as an excellent biostratigraphic indicator due to their geographical distribution and evolution. Different marine environments can be easily identified based on their morphology and assemblages (Tyson 1995;Vincent 1995).
Acritarchs are organic-walled microfossils that are interpreted as unicellular marine algae (Riegel 2008). Mainly the acritarchs, genera of the Gondwana time, are represented by genera of Veryhachium, Micrhystridium, Dictyotidium, and Leiosphaeridia. However, some workers considered Leiosphaeridia of freshwater green algae (Mays et al. 2021). These acritarchs probably represent stress-tolerant phytoplankton (Luo et al. 2013;Rampino and Eshet 2018). These are the excellent biostratigraphic indicators in the Palaeozoic (most diverse during the Ordovician-Silurian) but less important in Mesozoic and Cenozoic sediments. The organisms which produced these palynomorphs are either extinct or unknown, and these have been classified exclusively based on morphological characteristics.
Foraminifers are defined as single-celled organisms, members of the class of amoeboid protists. These are tectinous linings derived from certain marine benthic foraminifera. The external shell is called a test which is made up of diverse forms and materials. A test can have either one or multiple chambers. Most of the foraminifera are marine. The majority of them are benthic, while very few are planktonic. Fossilized linings are mainly produced by benthic foraminifers and predominated by the planispiral foraminifers (Tyson 1989(Tyson , 1995. They are known as excellent markers of marine or brackish marine shelf conditions (Tschudy 1969;Tyson 1993Tyson , 1995. Scolecodonts are represented as mouth and body parts of some polychaete worms, which are primarily marine in nature (Tyson 1995). The most abundant occurrence has been recorded during the Ordovician to Devonian while the geologically ranges from the early Ordovician to recent sediments.
Chitinozoans are flask-shaped extinct marine organicwalled microfossils that occur in rocks of the Palaeozoic (Ordovician to Devonian) age. However, they are of uncertain affinity but may be considered as the eggs of marine metazoans. Due to their restricted geological occurrence, these organisms are considered excellent biostratigraphic indexes, useful palaeoenvironmental markers, and thermal maturity indices. Due to the thermal alteration, the test changes from translucent and amber colour to brown and finally black/opaque (Tyson 1995).
Another group known as zooclast group comprises animal-derived organic particles (graptolites, crustacean eggs, tintinnids, insect cuticle fragments, arthropod cuticle fragments, etc.). These are materials of definite animal origin having specific morphological characteristics. The most common varieties of zooclasts include organic linings from some bivalve shells and ostracod carapaces, arthropod exoskeletal debris, and graptolite fragments (Tyson 1989(Tyson , 1995.

Structured organic matter
Terrestrially derived structured organic matter, mainly including the woody material, cuticles, biostructured organic matter, are mainly related to the deltaic fronts and proximal settings (Marson and Pocock 1981;Tyson 1995;Ercegovac et al. 1997;Peters et al. 2003). This group of organic matter represents the oxidizing conditions. Cuticles are derived from the leaves, which are considered to be the most buoyant type of structured organic matter cuticles deposited from the suspended load when the energy conditions are shallow. These are found in high percentages in deltaic distributary and prodeltaic facies (Parry et al. 1981;Tyson 1993;Vincent 1995). The abundant occurrence of the lath-shaped woody particles represents ( Fig. 4a-b) the short transportation of the sediments (Carvalho et al. 2013;Aggarwal et al. 2019a, b, c). Structured organic matter particles commonly occur in delta-fronts, prodelta, lagoons, and prodelta settings (Marson and Pocock 1981;Ercegovac et al. 1997). On the other hand, they are rare to common in lake and intertidal sedimentary environments and are absent to rare in beach mudflats (Ercegovac and Kostić 2006). Small-sized phytoclast particles represent the distal deposition and vice versa (Tyson and Follows 2000). On the other hand, the abundance of the equidimensional phytoclasts (Fig. 4c) in the sediments represents the variable energy conditions for the deposition of the sediments (Aggarwal et al. 2019a;. In general, the percentage of phytoclasts decreases in the distal direction and is strongly affected by the proximity to the source, granulometric composition of the sediments, and fragmentation through sample preparation (Tyson 1993;Vincent 1995;Mendonça Filho 1999).

Opaque phytoclasts
The opaque black phytoclasts are mainly derived from the oxidation of the structured phytoclasts (chiefly woody particles) during prolonged transport or post-depositional distinctions through usual or higher temperatures (Closas et al. 2005;Cincotta et al. 2015). The abundant occurrence of opaque black phytoclasts specifies highly oxidizing environments and degradation of terrestrial organic matter or proximity to terrestrial sources (Tyson 1989;Carvalho et al. 2013). The massive amount of opaque black phytoclasts accompanied by the low quantity of the other organic matter types has been recognized in oxic swamps, 1 3 delta-fronts, prodelta, etc. (Marson and Pocock 1981;Closas et al. 2005;Pieñkowski and Waksmundzka 2009) and primarily associated to the proximal side of the deposition (Carvalho et al. 2013) of the organic matter (Tyson 1989;Cincotta et al. 2015). The presence of many structured opaque phytoclasts may represent palaeofire evidence (Scott and Glasspool 2007;Kumar et al. 2011;Brown et al. 2012;Glasspool and Scott 2013). Opaque phytoclasts are frequently predominant in high-energy environmental settings, such as delta-fronts, prodelta, and near-shore habitats (Pieñkowski and Waksmundzka 2009), due to its inertness nature. The presence of opaque phytoclasts accompanied by a little amount of the palynomorphs and other types of organic matter, possibly replicate flooding events in the oxic environment. Consequently, the occurrence of the opaque phytoclasts is usually found towards the proximity to the inland source or its redeposition of the organic matter from the fluvio-deltaic fronts (Tyson 1989;Cincotta et al. 2015;Aggarwal et al. 2019a, b, c). Additionally, it may be concluded that flooding may have been occurred due to the higher terrestrial input because of the high impounding of the water due to fluvial activity. The presence of the equidimensional opaque phytoclasts (Fig. 4d-e) reflects the deposition of the sediment under the relatively high-to moderate-energy-level conditions Götz 2016, 2017). These particles are prevalent in coarse-grained, high-energy environments (distributary channel sands and point bars) (Denison and Fowler 1980;Parry et al. 1981;Batten 1982aBatten , 1982bBustin 1988;Tyson 1989Tyson , 1993Tyson , 1995.

Degraded organic matter
Degraded organic matter particles are the resultant of the oxidized structured/translucent phytoclasts (mainly woods, woody material, cuticles, and plant tissues). The degraded organic matter has been recorded in low-energy settings like waterlogged/standing water column/flooded palaeomires/ peat swamps/lakeshores (Tyson 1993;Zhang et al. 2010;Peters et al. 2013;Aggarwal et al. 2019a, b, c). It is purely continental.
The organic component, which appears structureless, refers to the AOM under light microscopy in palynological slides, including bacterially and phytoplankton-derived organic matter, higher plant resins, and amorphous products macrophyte tissue diagenesis (Tyson 1993(Tyson , 1995. The high percentage of AOM is also assumed as a result of the preservation of benthic microbes or autochthonous planktonic organic matter or eradication of the distal palaeodepositional setting of the dynamic sources of diluent terrestrial organic matter (Tyson 1993(Tyson , 1995Mendonça Filho et al. 2010;Pacton et al. 2011) in the sluggish and oxygen-depleted water conditions (Bryant et al. 1988). The predominance of AOM can be inferred as dysoxic/anoxic (reducing environment) conditions in low-energy settings. Oxygen content is one of the most critical parameters for the preservation of degraded OM and the AOM (Pacton et al. 2011). He suggested that anoxia stimulates an advanced darkening due to polymerization, which can be inferred that AOM has been deposited under anoxic conditions. Granular AOM has been considered marine in origin, while gelified AOM is purely continental.

Marine deposits
The APP (AOM-Phytoclasts-Palynomorphs) ternary plot (Fig. 5) after Tyson (1993Tyson ( , 1995 has been widely applied to understand the palaeodepositional environment and the transport pathways of the organic matter in shallow and deep-sea depositional settings. It correlates the percentage of the three main groups of organic matter identified under white transmitted light, and based on it, palynofacies assemblages are formed (Mendonça Filho et al. 2011). In the ternary diagram, the most proximal components (phytoclasts) are used to put on the top of the ternary diagram. The components that belong to the most reducing conditions (AOM) are placed on the bottom left corner of the diagram. The plot can trace the relative proximity to the source of the terrestrial organic matter particles, transportation pathways, and reducing conditions of the depositional sub-environments, which denotes AOM preservation. The top to the base of the diagram represents the proximal-distal trend, while left to right represents the change from the reducing to the oxidizing (redox) settings. As in the marine ecosystems, more than 60% of the occurrence of the palynomorphs are sporadic and dominated by AOM, so in most cases, the lower right corner of the diagram is usually empty. Based on the palynofacies components and their assemblages, nine distinct environments (I-IX) have been identified in marine settings using the three main organic matter types. These nine environments are range from a highly proximal shelf or basin environment to a distal shelf or basin environment. The top of the diagram shows the shallowest sediments and are generally both proximal (high phytoclast supply) and oxic. If the basin is oxic, then most of the area of the ternary diagram would be covered by the phytoclasts due to the abundance of the opaque phytoclasts and the structured organic matter particles, resultantly the left basal portion of the ternary would be not be filled due to the least presence of AOM. If AOM is predominant in any succession, then it does not mean that the particular rock would be oil-prone, for that fluorescence of AOM has to be checked (Mendonça Filho et al. 2011). Table 2 shows a key to marine palynofacies fields recognized on the APP diagram according to Tyson (1993Tyson ( , 1995. Beside APP diagram some other ternary diagram has also been defined to better resolve the results for the palaeodepositional settings. Some examples of other ternary diagrams are SPP (spore-microplankton-pollen grain), DSA (dinocyst-sporomorph-acritarch), PAD (prasinophytes-acritarch-dinocyst), GPG (gonyalacoid-peridinioid-other gonyalacoid), APB (AOM-Pediastrum-Botryococcus), etc. (Mendonça Filho et al. 2011).

Continental deposits
For the continental set-up, various workers have used the modified ternary models like the ternary diagram proposed by Hacquebard and Donaldson (1969) and modified by Marchionni (1980) and Singh and Singh (1996); later, it was modified by Aggarwal et al. 2019a, b, c. The ternary plot (Fig. 6) has been modified as SP-ST + DT-AOM + CH, where SP stands for spore-pollen or continental palynomorphs including fungal spores, ST stands for structured organic matter, DT stands for degraded organic matter, AOM stands for amorphous organic matter, and CH stands for the opaque phytoclasts. Based on the predominance of the various forms of organic matter, three distinctive environments have been described as forest swamps, lake shores/ flooded palaeomire, and oxidized swamps. Similarly, palaeoenvironmental interpretation of the Yanchang Formation for the continental deposits has been proposed by Zhang et al. 2015. The ternary plot was represented by AOM-TLF + OP-GP + cuticles + palynomorphs (AOM-TO-GCP). AOM is defined as amorphous organic matter, TLF has been described as transparent lignocellulosic fragments, OP is opaque phytoclasts and GP is as gelified particles. Based on these various forms, three different environments have been interpreted as distal dysoxic-anoxic deep basin, shelf-tobasin transition, and proximal sub-oxic shelf.
Besides it, some of the previous workers (Smyth 1979;Goodarzi 1985;Diessel 1986;Mukhopadhyay 1986;Mishra and Cook 1992) have also studied the palaeoenvironmental depositional settings based on the analysis of the coal petrology.

Discussion
Palynofacies is the tool that can be easily applied in the various fields of geology like to recognize the proximal-distal relationships with respect to source, depositional polarity (i.e. onshore-offshore), regressive-transgressive trends in stratigraphic sequences, hydrocarbon source rock potential, in the identification of the various environments (oxic open marine, dysoxic-anoxic marine, and brackish freshwater, flooded palaeomire, swamps, peats, etc.).

Table 2
Various depositional set-ups in the marine/seashore environments proposed by Tyson (1995) S.N Environment

Palynofacies characterization for the interpretation of palaeodepositional settings along with the proximal and distal trend of the sedimentary rock
Palynofacies used in the field of the palaeodepositional settings is one of the well-known and widely accepted tools for the reconstruction of the palaeoclimate (Oboh-Ikuenobe and Villier 2003; Schiøler et al. 2010;Mendonça Filho et al. 2011;Carvalho et al. 2013;Aggarwal et al. 2019a, b, c) not only in the older deposits but also in the recent sediments. It is very easy to identify the proximal and distal trend of any succession with the help of the various morphographical features (size sorting, degradation, oxidation, etc.) of the different types of organic matter particles. Generally, the coarser sediment used to be deposited towards the proximal side in high-energy conditions (higher silt and sand content). The proximal and distal trend in any sequence can be more precisely explained with the help of the Phytoclast Preservation Index (PPI), which directly explains from the proximal to the distal by the increasing oxidation and degradation of the phytoclasts and decreasing trend of the preservation (Tyson 1993(Tyson , 1995Vincent 1995;Mendonça Filho 1999;Mendonça Filho et al. 2011). It denotes that the opaque phytoclasts are dominant at the distal end while the structured organic matter is dominant at the proximal end. Besides it, the proximal-distal concept and the parameters calculated for the groups and subgroups of the different organic matter components have been described by many authors (Tyson 1993(Tyson , 1995Vincent 1995;Mendonça Filho 1999) in detail (see table 12 Mendonça Filho et al. 2011).

Palynofacies characterization for hydrocarbon source rock
Sediments which are having the ability to generate petroleum are called 'source rocks'. Organic-rich shales, very fine-grained carbonates, mudstones, and oxygen-deficient waters are considered excellent source sediments. Palynofacies is considered very significant while evaluating source rock potential. Palynofacies is classified into various categories depending on the degree of alteration (Marson and Pocock 1981). Detailed classification is summarized in Table 1; Thakur and Dogra 2011. Structured organic matter contributes to gaseous hydrocarbons, and degraded organic matter (plant-derived organic matter that has undergone biodegradation but still shows visual traces of its cellular structure) is considered to possess a better hydrocarbon source potential than structured organic matter. Amorphous organic matter is considered an excellent source for liquid hydrocarbons; spore-pollens are rich in lipids known for their contribution to the liquid hydrocarbon. Opaque phytoclasts have negligible hydrocarbon source rock potential except for dry gas. Resultantly, the dominance and the sub-dominance of the various forms of the organic matter types also indicate the source rock potential of any rock. Concurrence of TAI (Thermal Alteration Index) with palynofacies is an excellent parameter for the source rock evaluation and has been used widely by different researchers (Misra and Pundeer 1994;Thakur and Dogra 2011;El Atfy et al. 2014;Peters et al. 2015;Christoph et al. 2019).  Hacquebard and Donaldson (1969) and modified by Marchionni 1980;Singh and Singh 1996;Aggarwal et al 2019a, b, c 1 3 The implication of palynofacies in high-resolution sequence biostratigraphy: This field has emerged as an efficient tool for the identification of the sequence boundaries (lowstand system tracts: LST, transgressive system tracts: TST, high-stand system tracts: HST, MFS: maximum flooding surface, SB: sequence boundaries) with the concurrence of the palynomorphs and lithofacies (Batten and Stead 2005). Conceptual models for palynofacies in sequence stratigraphy based on the distribution of the kerogen were proposed by many researchers (Steffen and Gorin 1993;Tyson 1995;Hart et al. 1994;Carvalho et al. 2006). The spatial and stratigraphic variations in the distribution of sedimentary organic matter reflect changes in the depositional system related to relative sea-level fluctuations. LSTs are generally defined by the predominance occurrence of the large phytoclasts, poorly preserved numerous pollen grains, and spores (partially oxidized). Opaque phytoclasts are the common components of such palynofacies. On the other hand, the palynofacies associated with TSTs are predominated by the occurrence of the dinoflagellate cysts and other marine components. At the same time, miospores and phytoclasts are smaller and less varied. HSTs are symbolized as the assemblage represented as AOM as a significant content along with marine components (dinoflagellate cysts as most numerous palynomorphs). Resultantly, palynofacies analysis along with some integrated proxies (lithofacies and geochemical aspects) may be used as a powerful tool for high-resolution sequence biostratigraphy. The different workers have used this in various successions (Jaramillo and Oboh-Ikuenobe 1999;Holz et al. 2002;Vallejo et al. 2002;Gotz et al. 2003;Dybkjaer 2004;Oboh-Ikuenobe et al. 2005;Ruf et al. 2005;Carvalho et al. 2006;Prasad et al. 2013;Davtalab et al. 2017).

Conclusion
(1) The study has its limitations. Accurate identification and observation of the different forms of the organic matter types in the palynological material are required for the successful implication of palynofacies studies in various aspects.
(2) The observation of the different forms of the organic matter types following the change in the lithological patterns through the stratigraphic succession and tagging of the lithofacies as well as the palynofacies is highly recommended for the more acceptable resolution and the better understanding of the palaeodepositional set-up, source rock potential for petroleum and complete information of the stratigraphy.
(3) Recognition of different forms of the organic matter types in conjunction with the biostratigraphy can be employed as an efficient tool in identifying the various sequence boundaries in succession with the absence of the apparent sedimentological evidence. (4) The study is also helpful in guiding the well trajectory during drilling for the discrimination between non-pay zones above, below, and within reservoirs.
Acknowledgements The author expresses her sincere thanks to the director of BSIP, Dr. Vandana Prasad, for permission to publish the paper and for her continuous encouragement. The author is also thankful to the anonymous reviewers for their valuable suggestions to improve the quality of this MS.
Funding The author received no external funding for this research work.

Conflict of interest
The author declares that she has no conflict of interest.
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