COAL FACIES
The use of petrographic data as the only source in facies analysis has been controversial by different authors, such as Crosdale 1993, Dehmer 1995, Wüst et al. 2001, Scott 2002, Moore and Shearer (2003), Diessel 2007; Hower and Ruppert (2011), Sahay 2011, Sen 2016, Dai et al. 2020. The present study deals with the facies and the coal deposit environment, obtained from the petrographic indices and models. We argue that the models apply the fundamentals related to the active processes at the time of peat formation, the appearance, and the nature of the maceral groups. We consider the facies concept as ¨a body of sediment characterized by specific textural, structural, and compositional properties, and definition of the sedimentary environment as characterized by a particular suite of physical, chemical and biologic parameters that operate to produce facies (Boggs 1987). We relate the results of the coal facies with other disciplines such as palynology and stratigraphy, for the deposit environment interpretation, which solve some points. Several models have been developed from coal composition to interpret the mires environment (Rimmer et al. 20000; Mukhopadhyay 1986; Singh and Singh 1996; Singh and Singh 2000; Singh et al. 2013; Smyth 1979, 1984).
The maceral composition allocated to "genetic" groups, based on processes that may have been active in the formation of peat swamp (Preservation, Degradation or Fusinization) of the Piedemonte Llanero coals applying a facies model proposed by Rimmer et al. 2000, which suggests: - Facies A, as characteristic of the San Fernando Formation, reveals low content of inertinite macerals, higher content of degraded organic matter and moderate of preserved macerals, derived from woody tissue composed of cellulose and lignin (Suarez and Crelling 2008), possibly in a low pH environment; - Facies B in the Arcillas del Limbo Formation, the Palmichal Group, and the Chipaque Formation, indicates an intermediate state of preservation and degradation of the organic matter, as a result of alternation of construction and destruction processes in the peat deposit, and variations in the water table; - Facies C has been identified in some seams of the Chipaque Formation that contain a low concentration of liptinite and the highest percentages of collotelinite, as indicators of better conservation of organic matter (Fig. 8).
Based on the maceral composition or microlithotypes, and the criteria used in the classification of peatlands, mainly the influence of the water table, the Mukhopadhyay 1986; Singh et al. 2013 and Singh and Singh (1996) models attempt to understand the redox conditions and the peat-forming ecosystems in the past (Fig. 9-12). In the case of Piedemonte Llanero coals, the models indicate peat formed in a swamp forest to moor-reed plants (low-peat wetlands, like the grass of humid zones) in an open water swamp where the oxic to anoxic conditions prevalence, with good tissue preservation (Mukhopadhyay 1986; Singh and Singh 1996; Singh and Singh 2000, Singh and Singh 2013).
PETROGRAPHIC INDEXES
The parameters formulated by Diessel 1985, 1986, 1992 denominated Gelification Index (GI) and Tissue Preservation Index (TPI), have been determined for the analysis of the level water that covers the peat. According to the maceral composition, four accumulation zones are clearly defined: terrestrial, telmatic, limnic and limnotelmatic. The GI and TPI are calculated as under:
GI = Vitrinite + Macrinite/ Semifusinite + Fusinite + Inertodetrinite
TPI = Telinite + Collotelinite + Semifusinite + Fusinite/ Collodetrinite + Macrinite + Inertodetrinite
The GI also determines the degree of persistence of wet or dry conditions (Diessel 1992). TPI is the measure that corresponds to the ratio between preserved and not preserves macerals, the index quantifies the degree of humification suffered by the precursor macerals of the organic matter and the proportion of woody matter that contributes to the total groundmass of the peat (e.g. Singh and Singh 2000; Singh 2016).
To determine the factors that control the peatlands, Calder et al. 1991 introduced the GWI and VI indices. The Groundwater Index (GWI) deals with the ratio of the strongly gelled tissue and poorly gelled tissue, depending on the water supply and the pH. The Vegetation Index (VI) corresponds to the contrast of macerals of forest affinity related to those of marginal herbaceous and aquatic affinity (Suárez et al. 2012).
GWI = Gelinite + corpogelinite + clay mineral + quartz + vitrodetrinite / telinite + collotelinite + collodetrinite
VI = telinite + collotelinite + semifusinite + fusinite + suberinite + resinite / collodetrinite + inertodetrinite + alginite + liptodetrinite + sporinite + cutinite
The macerals gelation degree of the Piedemonte Llanero coals indicates a limnic environment (lacustrine zones) in the Chipaque, Palmichal Group and Arcillas del Limbo formations coals, and telmatic conditions for San Fernando Formation coals. The water contribution in the peat formation with scarce nutrients from precipitation according to low GWI (Calder et al. 1991) and interdigitation to rheotrophic conditions are also observed (Fig. 13-14).
The indices values obtained in the Piedemonte Llanero coals are shown in Table 4, where TPI and VI indices reveal the preservation of vegetation tissues made up of woody plants and trees, wet forest swamps, and herbaceous vegetation (VI <3) similar values of Suárez et al. 2012. The TPI value between 1 and 7 is an indicator of the good preservation tissues. The Low TPI value indicates an important contribution of oxidated material vegetal (Dai et al. 2020).
The inertinite group has a high proportion in the analyzed coals, except in the San Fernando Formation. These concentrations are also observed in the results of the petrographic indices which are of importance in the interpretation of the coal formation conditions.
DEPOSIT ENVIRONMENT
Smyth's (1979, 1984) model based on microlithotypes, defines five areas that signify different sedimentary environments: lake, river, brackish, upper deltaic, and lower deltaic. The model reflects the oxidation degree of the organic matter prior burial and the vitrinite content in order to determine the environment of coal deposition. Dai et al. 2020 indicate the importance of variations in "constructive” and "destructive" processes at the interface of environments as important considerations in the analysis and interpretation. According to Sen et al. 2016, this model does not provide the existence of ombrogenous environments, nor does it attempt to determine the vegetation types. The Piedemonte Llanero coals are vitrinite rich and located between the ombrotrophic to the mesotrophic bog zone. The vegetation type is inferred from our petrographic analysis and previous contributions from other disciplines.
We consider the microlithotypes association at the vertices of the triangular model to be correct regarding its genesis. This allows us to associate the microlithotypes results to the obtained facies in the Piedemonte Llanero coals, and to identify the differences in the deposit setting of the samples for each formation from the Upper Cretaceous to Middle Miocene age. However, we consider that the areas delimited by Smith (1979, 1984) can be modified in the upper delta plain to identify more transitional subenvironments (Fig. 15).
The Durita + Inertite association indicates greater oxidation of the organic matter of the Arcillas del Limbo Formation, where the low water table level is associated to dry peat (Diessel 1992). Vitrita + Clarite show greater subsidence, and the preservation of the tissue in the San Fernando Formation. The so-called intermediate microlithotypes are the indicative to local fires or major transport, and high energy in the coal units.
The Piedemonte Llanero coals reflect variations in the humidity conditions, oxidation, and the influence of the brackish water. The petrography results, together with previous studies of stratigraphy, and palynology, allow us to deduce the evolution from an estuarine to a deltaic system-lacustrine (Fig. 15). The preservation processes of peat, or the estuarine environment that allowed the development of the coals, follow the definition of estuary given by Dalrymple et al. 1992. According to these authors, the term is supported in sedimentological criteria and defined as "the portion towards the sea of a flooded river system that receives sediment from river and marine source areas and contains sedimentary facies influenced by tides, waves and river processes.".
Chipaque Formation (Late Cenomanian- Santonian)
Coal petrology indicates low oxidation conditions as shown by the low percentages of durite and inertite microlithotypes, and significant contents of vitrite and clarite microlithotypes that indicate humid environments. Guerrero and Sarmiento (1996) indicate an estuarine environment and micro-tidal dominated by waves according to the stratigraphy and palynology study of a mixture of Dinoflagellates and pollen in the lower segment of the formation (Fig. 16). The coals developed in a partially closed lagoon, which it is corroborated by the increase in the content of detrovitrinite and oxidized vitrinite that indicates a pH increase due to the incursion of marine waters and the degradation in the tissues. The Inertinite percentage reflects the alternation of wet and dry periods with water oxygenated under the tidal influence (Diessel 1992). The low oxidation and good preservation of the macerals are corroborated by deposition that occurs in extensional basins with half-graben geometry (post-rift). The accommodation space generated the subsequent deposition of marine facies and the beginning of a transgressive event (Guerrero and Sarmiento 1996; Horton et al. 2020).
The Palmichal Group (Early Campanian- Maastrichtian).
The coals of this unit show a medium degree of oxidation with a moderate concentration of the vitrinitic, inertinitic, and liptinitic micro-lithotypes. Botryococcus colonies are indicative of swamp environments, periods of the shallow watershed, and the climate variations (Diessel 1992), and the high concentrations of inertinite indicate periods of greater oxidation. The coals were deposited in shallow bays, swamps, and coastal lagoons of an estuarine system (the Guaduas Formation in Guerrero and Sarmiento 1996; Ulloa 1976a Fig. 16), along an open lagoon with tidal and wave processes active, possibly. In the regional geological framework, the oxidation periods are related to the initial pulse of the Colombian Andes uplift (Central Cordillera), with the first great crust shortening and the foreland basin formation as the evidence (Horton et al. 2020). For this reason, tectonics controlled the sedimentation, the accommodation space decreased, and shallow coastal marine environments developed (Villamil 1999).
Arcillas del Limbo Formation- Late Paleocene.
The coals show the highest-level oxidation in the studied sequence. According to Guerrero and Sarmiento 1996, the coals were deposited in a lagoon of estuarine coastal plain environment (from pollen and spores studies Fig. 16). The content of intermediate micro-lithotypes is low, as to indicate a freshwater deposit and low salty water influence. The monomaceral and bimaceral micro-lithotypes proportion is considerable, mainly composed of durite and inertite due to the important inertinite concentration. The inertodetrinite indicates high oxidation and desiccation of the peat, caused by an intermittent drop in the local water table, the cutinite and sporinite have thick-walled protect against dehydration, indicating dry environments (Diessel 1992). The dry period with greater oxidation in coals is a consequence of accommodation space, which decreases due to the crust uplift as result of the initial phase of tectonic inversion from prior normal faults, and shallow-coastal environments are developed (Villamil 1999).
San Fernando Formation (Late Eocene- Early Miocene).
The coals reflect a higher concentration of vitrite and clarite as a product of wooded peat bogs and low oxidation under telmatic conditions. The spores and cutinite are very well preserved with a thin wall, which indicates the humid environments of terrestrial plants. The low oxygen concentration in the environment favored the preservation of plant tissues in a lacustrine setting. Some brackish incursions are suggested in previous studies (Dueñas and van der Hammen 2007; Parra et al. 2008; Caballero et al. 2020 Fig. 16), related to the intermediate microlithotypes content and sulfides in framboidal shape. The low oxidation of the organic matter in this period is related to the accommodation space increases, as a result of the subsidence of the basin and a transgressive event that allows the entry of marginal marine systems (Villamil 1999).
PALEOVEGETATION
Previous palynological studies reveal the dominance of angiosperms in the Upper Cretaceous (Guerrero and Sarmiento, 1996) up to Lower Eocene (Pardo and Jaramillo 2014). For the Chipaque Formation, Plamichal Group, and the Arcillas del Limbo Formation, the petrology analysis reveals a transition from forest swamps to reed swamps, characterized by short trees, shrubs, and herbaceous vegetation, since they develop mainly in bogs.
In the middle Eocene period, Pardo and Jaramillo (2014) registered the spore domain of the Pteridophytes. This time interval includes the lower segment of the San Fernando Formation. Petrographically, it is evidenced with a high percentage of thin-walled sporinite and sporangium, low content of inerts, and mono-bimaceral microlithotypes (Vitrite, Liptite, Clarite). In the intermediate segment of the San Fernando Formation, Pardo and Jaramillo (2014) report mangrove vegetation, which may be related to M304 and M305, where sporangium is no longer evident and sporinite-microsporinite is abundant (Fig. 16).
Our results procured from petrographic, facies and paleoenvironment identification are like to those obtained in other countries (e.g. German, Australian, Polish, Canadian, North America, Spain, India, China, Brasil). Singh and Singh (2000) studied Eocene coals of platform basins (Meghalaya, India) and indicate lagoon and estuarine environments subject to recurrent transgressions and regressions; Singh et al. 2013, researched Oligocene coals from the foreland basin of northeast India, where they identified tertiary flora and the occurrence of inertinite macerals related to tectonic upheavals; Singh 2016, studied the Karharbari (Lower Permian) coals, India, concluding that the accumulation of peat in forest mud is affected by oxic to anoxic fluctuating conditions with good tissue preservation; and Feng et al. 2019 developed the study in the district of Hanshuiquan, Santanghu Coalfield, Xinjiang, NW China, in the Middle Jurassic coals, indicating wet forest swamp facies with a high value of TPI and VI, low GI and GWI, and high inertinite content in the maceral composition, as occurs in the Piedemonte Llanero coals.
From these issues, we consider that the basic concepts of peat formation can be applied in any place and age of the geological context, considering the evolution of each particular basin.
COAL PETROLOGY APPLICATION
The coal petrology is an indicator of the organic matter type, range, distribution, lateral continuity, thickness, and organic and mineral composition, which can support research in several applied topics. Although our research does not have such scope, we mention here some potential approaches for future studies.
BASIN ANALYSIS
The coal petrology in basin analysis contributes to the organic facies understanding. The coal facies are important to deduce the accumulation and subsidence rates of the basin, to define the depositional systems, and to infer the sedimentary processes in the analysis of stratigraphic sequences, and the thermal history evaluation (e.g Diessel 1992, 2007; Qiu et al. 2011; Suarez et al. 2012; Singh and Singh 2013).
CBM.
The coal sedimentary environment is important in areas with scarce or without exploration studies, understanding of the depositional environment allow to infer the enrichment of CBM (Hou et al. 2019). As an unconventional reservoir, the coal acts as source and reservoir rock. As source rock, it is of vital importance to define the type and degree of maturation of the organic matter, which is related to the range and content of organic components that generate hydrocarbons. The coal range and the burial depth play a significant role in the adsorption capacity of the methane gas and in the maceral composition (e.g. Laxminarayana and Crosdale 1999; Bustin and Bustin 2016; Li et al. 2016; Hou et al. 2016; Zhao et al. 2019; Busch et al. 2019), as well as defining the quality of the coal deposit (Farhaduzzaman et al. 2012; Miao 2016 in Hou et al. 2019). Also, according to the deposit environment, it allows the evaluation of the seal conditions of the adjacent rocks (Li et al. 2014).
Rare Earth Exploration.
In recent decades, the rare earth elements occurrence (REE and REY) in coal and coal ash has been studied according to the economic interest, and Colombia is not an exception. Rare earth elements in coal can be found in organic or mineral associations or dissolved in water in pores in low-ranking coals (Dai 2020). The coal environment allows a link to the identification of REY, according to the occurrence mode, abundance, and origin, as well as in the definition of the associated mechanisms and the enrichment mode when determining anomalies (Dai 2020; Finkelman 2019; Hower 2016; Seredin 2012).