Systematic study of Early Carboniferous palynological assemblages from the Llanos Orientales Basin, Colombia∗

This paper concerns the description of palynomorphs recovered from subsurface Early Carboniferous strata of the SM-4 well located in the Llanos Orientales Basin, Colombia. Thirty-two species of spores are recognized within the palynoflora. A new species is proposed: Spelaeotriletes colombianus Dueñas and Césari sp. nov. The assemblages were referred to the Tournaisian-Viséan by the presence of distinctive spore species with previous records in the Viséan and Tournaisian of Western Europe and Western Gondwana.

The Colombian Llanos Basin is a structural depression located in the eastern part of Colombia (Fig. 1).The sedimentary sequence, which fills this depression, is divisible into three time units palynologically dated as Paleozoic, Cretaceous and Tertiary.Early Carboniferous strata found in interval 2010-2340 ft of the SM-4 well yielded assemblages dominated by terrestrial palynomorphs (Dueñas & Césari, 2003).This is the only reference to Early Carboniferous sediments in the whole Llanos Basin.Stratigraphic data and miospore range distribution were discussed by Dueñas & Césari (in press).The aim of this paper is to describe the Early Carboniferous palynological assemblages from the SM-4 well of the Los Llanos Orientales Basin, Colombia.

MATERIAL AND METHODS
Ten cutting samples from interval 2010-2340ft of the borehole SM-4 yielded miospores and acritarchs.Laboratory preparation followed the standard techniques using fluorhidric and hydrochloric acids.Sample residues were mounted on microscope slides in Canada balsam and all slides are stored at the National Core Library, Colombian Petroleum Institute, Bucaramanga (Colombia).The preservation is variable within the samples, but frequently is poor to moderate, and the palynomorphs are thermally mature and display dark brown to black color.Miospore locations on slides are based on England Finder graticules.Calamospora liquida Kosanke, 1950 (Fig. 2 P) Description.Spores radial, trilete.Amb circular to subcircular.Laesurae simple, straight, extending three-quarters of spore radius.Exine laevigate, 1µm thick, usually with large compression folds.
Comparisons.Calamospora liquida is char- * Contribution to the IGCP Project 471 acterized by its long laesurae (greater than half the spore radius).
Previous records.This species has a widespread occurrence in Carboniferous strata.
Comparisons.The present specimens are smaller than those described by Naumova (1953).
Previous records.This species was originally described from the Upper Devonian by Naumova (1953) and later recognized in the Tournaisian by Higgs et al. (1988).Dimensions.35-56 µm (8 specimens).

Genus
Comparisons.The specimens are treated at the generic level, because they are not clearly assignable to any described species.Punctatisporites irrasus Hacquebard, 1957 (Fig. 2 A) Description.Spores radial, trilete.Amb subcircular.Laesurae distinct, occasionally with raised narrow lips, extending up to three quarters of the spore radius, frequently with dark intertectal areas.Exine laevigate to finely infragranulate, about 1µm thick.Compression folds common, particularly in the equatorial regions.
Retusotriletes crassus Clayton in Clayton, Johnston, Sevastopulo & Smith, 1980 (Fig. 2 G) Description.Spores radial, trilete.Amb subcircular to subtriangular.Laesurae straight, simple, approximately four-fifths of the spore radius in length.The laesurae end in curvaturae perfectae which delimit the contact areas.Large thickened pads are present in the contact areas, separated from each other by radial zones of thin exine along the laesurae.Also thin exine separates the pads from the equatorial margin.
Previous records.This species is recorded from the latest Strunian to the earliest Carboniferous from Europe (Clayton et al., 1980).Sullivan, 1964 (Fig. 2 Q)

Retusotriletes incohatus
Description.Spores radial, trilete.Amb circular to subcircular.Laesurae distinct, usually extending for three quarters of spore radius, with fine lips.Contact areas depressed, delimited by perfect or imperfect curvaturae.Exine laevigate and darker outside the contact areas.Due to corrosion the specimens seem micropunctate.
Previous records.This species originally described for the Tournaisian has been widely reported from late Famennian -early Viséan sequences (Clayton et al., 1977).
Comments.The specimens are in the lower extreme of the size range of the species.
Previous records.This is a characteristic species of the Viséan in Australia and England (Playford, 1991).Anapiculatisporites concinnus Playford, 1962 (Fig. 2 D, E) Description.Spores radial, trilete.Amb subtriangular with rounded apices and convex to straight sides.Laesurae simple, length three-quarters of spore radius.Proximal surface laevigate.Distal surface sculptured with small coni, 1µm high, about 2-3 µm apart.They are characteristically absent at and around equatorial margin, particularly of interradial areas.Exine about 1 µm thick.
Remarks.Ravn (1991) emended the genus Spinositriletes Dybová and Jachowicz and included the species concinnus, but we consider more appropriate to maintain this species in the genus Anapiculatisporites.
Comparisons.The present specimens differ from the original in having smaller diameter and the sculpture projecting at the equatorial margin.Anaplanisporites delicatus Neves and Ioannides, 1974 differs in having curvaturae imperfectae and grana among the sculptural elements.
Comparisons.Apiculiretusispora fructicosa Higgs 1975, differs in being much larger in size and possessing more varied sculpture including coni, pila and short bacula.Apiculiretusispora microseta Ravn 1991, has smaller diameter and sculpture of fine coni less than 1 µm wide at their bases.
Previous records.This species was originally described by Luber (in Luber & Waltz, 1938)  Dimensions.Equatorial diameter: 35-37 µm Comparisons.The specimens are very dark not allowing a precise description of some morphological characters.Nevertheless, they seem co-specific with R. clavata by the varied sculpture.
Previous records.This species, originally described by Hacquebard (1957) from the Horton Group, has a widely distribution in Tournaisian and Viséan assemblages (Clayton et al. 1977 Verrucosisporites nitidus Playford, 1964 (Fig. 2 M) Description.Spore trilete, amb circular to subcircular.Laesurae often indistinct, extending at least one-half of the spore radius.Exine sculptured by uniform and comprehensively distributed verrucae with rounded to rounded polygonal basal outlines and obtusely rounded lateral profiles.Verrucae 3 µm in basal diameter and 2µm high.
Infraturma PATINATI Butterworth & Williams 1958 Genus Cymbosporites Allen Comparisons.The cavate nature of this specific taxon was suggested by Higgs et al. (2000) who considered the generic assignment to Grandispora made by Byvsheva in 1980 was far more appropriate than her later assignment (Byvsheva, 1985) to the acamerate genus Cymbosporites.Nevertheless, Melo & Loboziak (2003) maintained the proposal of Byvscheva (1985) and illustrated specimens similar to that here illustrated that seem acavate.
Previous records.This species has been recorded from the latest Famennian to Tournaisian in the northern hemisphere and the latest Famennian of Brazil (Melo & Loboziak, 2003).
Comparisons.The specimens are very similar to the original ones and to those illustrated by Melo & Loboziak (2003) from the Amazon Basin.
Previous records.This species, originally described from the Lower Carboniferous of England, has its first Late Viséan record in the Amazon Basin (Melo & Loboziak, 2003).
Subturma ZONOLAMINATITRILETES Smith & Butterworth, 1967Infraturma CINGULICAVATI Smith & Butterworth, 1967 Genus Bascaudaspora Owens, 1983 Type species.Bascaudaspora canipa Owens, 1983.Bascaudaspora submarginata (Playford) Higgs et al., 1988 (Fig. 3 G) Description.Spores radial, trilete, cavate.Amb subtriangular with convex sides and rounded apices.Laesurae indistinct, straight, extending almost to equator with narrow lips.Distal surface reticulate to rugulate with low, smooth, sinous muri which anastomose or terminate freely.Lumina are usually irregular in shape and size.Muri commonly beaded in appearance due to the presence of small rounded nodes.A distinct and continuous cingulum is present in equatorial region which appears darker and thicker.Proximal surface laevigate.Intexine thin, smooth, barely discernible, three quarters or more of the spore diameter.
Comparisons.Owens (1983) defined Bascaudaspora as an acamerate spore, but its type species, B. canipa was described with separation of the exine layers.In accord to Higgs et al. (1988) is here considered that the genus accommodates variably camerate/acamerate spores.B. submarginata is characterized by a reticulate to rugulate distal surface, formed by narrow smooth sinuous muri and by a dark equatorial cingulum.The present specimens are slightly smaller than the original described by Playford (1964)  Densosporites rarispinosus Playford, 1963 (Fig. 3 F) Description.Spores radial, trilete.Amb subtriangular, with convex sides.Laesurae occasionally indistinct, straight, extending on to the cingulum, simple or with thin lips.Cingulum 5-10 µm wide, darker in colour than body.Distal surface sculptured with sparsely distributed spines about 2 µm high and subordinate small verrucae.Apart of this sculpture, exine infrapunctate or laevigate.
Comparisons.Original specimens described by Playford (1963) differ only in having spines up to 6 µm.
Previous records.This species was originally described by Playford (1963) from the Lower Carboniferous of Spitsbergen.
Genus Vallatisporites Hacquebard, 1957 Type species.Vallatisporites vallatus Hacquebard, 1957 Vallatisporites splendens Staplin & Jansonius, 1964 (Figs. 3 C, D) Description.Spores radial, trilete, amb convexly triangular.Laesurae indistinct with narrow lips reaching the equatorial margin.Exine twolayered, cavate.Intexine laevigate, its outline in polar view conformable with the amb.Zona onefifth of total spore radius in width, with little or no equatorial thinning.Inner part of the zone, with uniserial row of internal vacuoles, delimiting equator of intexinal body as a light area.Sometimes, a slight thickening of the inner half of the remainder zona gives a bizonate appearance.Exoexine proximally laevigate, distally bearing galeae and verrucae 2 µm wide and 2 µm high, irregularly arranged and usually fused in irregular ridges and pads bearing scattered minute coni.Equatorial margin with sparse spines.
Comparisons.Our specimens resemble V. splendens in all respects except for their smaller diameter.According to its original diagnosis V. verrucosus includes specimens with sculptural elements predominantly discrete.
Previous records.This species was reported from the Strunian of Canada, uppermost Devonian and/or lower Tournaisian of western Russian Federation and lower and upper Viséan of Iran (see Playford & Mc Gregor, 1993).
Comparisons.The specimens are slightly smaller than the usually described for the species.Lophotriletes coniferus Hughes & Playford 1961 also resembles our specimens, but as suggested by Sullivan & Marshall (1966), this species could be synonymous with C. maculosa.
Comparisons.The specimens studied here differ from the known species of the genus in the very fine and dense distal sculpture.
Remarks.The described specimens have granulate exoexine as well as the original specimens described by Hoffmeister et al. (1955).A. macra Sullivan 1968 is very similar but has a more subtriangular amb with a ratio of diameter of inner body to total spore diameter about three quarters (Van der Zwan, 1980).
Previous records.This species is characteristic of Tournaisian-Westphalian B of the northern hemisphere assemblages.It also occurs in late Tournaisian and late Viséan palynofloras of northern Brazilian basins (Melo & Loboziak, 2003).
Previous records.This species has been recorded from Lower Carboniferous sequences of the northern hemisphere and Northern and Western Gondwana (see Playford et al., 2001, Melo & Loboziak, 2003).Diagnosis.Spores radial, trilete, cavate, with convexly subtriangular amb.Laesurae almost straight, usually distinct, with narrow lips or exinal folds that extend to equator.Exoexine slightly thickened at the equatorial margin, laevigate on the contact faces and sculptured on distal and equatorial areas.Sculpture of densely distributed small galeae, coni and grana 1-2 µm broad at base, 1-2 µm high; elements usually discrete or connected basally to form short, irregular narrow ridges.Intexine laevigate, forming a distinct mesospore usually 50% of the total diameter, with outline in polar view normally conformable with the amb.
Comparisons.According to the detailed revision of Playford et al. (2001) of some different species of Spelaeotriletes our specimens are distinguishable from the Spelaeotriletes triangulus/ Spelaeotriletes arenaceus complex by having diminute, mostly apiculate sculpture of galeae, grana and coni usually discrete, and a slightly thicker exine at the equatorial margin forming a characteristically narrow dark area.According to Playford et al. (2001) and Neves & Owens (1966), S. triangulus Neves & Owens displays distal verrucae, coni and galeae, up to 4 µm broad and 3.5 µm high, regularly distributed and closed spaced that may constitute a reticulum imperfectum.S. arenaceus Neves & Owens is characterized by irregularly distributed verrucae, coni, bacula and pila, up to 2.5 µm broad and 2 µm high.The new species is considered segregated of both, S. triangulus and S. arenaceus type materials, although may be regarded as identical to some Early Carboniferous Amazonian specimens referred to the complex (Melo, pers. comm.).Spelaeotriletes ybertii (Marques Toigo) Playford & Powis emend.Playford et al. 1991, is a distinct form having mostly apiculate sculptural elements like bacula and galeae, usually longer than wide and coalescent in short ridges.

CONCLUSIONS
The palynological assemblages of the SM-4 well, located in the perigondwanic region, provided the only evidence for Early Carboniferous sedimentation in the Colombian Llanos Orientales Basin.The palynofloras contain stratigraphically significant species, including those with Euramerican affinity and those with Gondwanan previous records.As it was pointed out by Dueñas & Césari (in press), the true distribution of the species throughout the sequence is obscured by caving and reworking, but characteristic Viséan species such as Indotriradites morphon, Anapiculatisporites concinnus and Prolycospora rugulosa were identified only above 2250ft.The stratigraphic interval was referred to the Tournaisian-Viséan by Dueñas & Césari (in press).Certainly, future studies in other sequences of the Llanos Orientales Basin will improve the knowledge on the biostratigraphic range of the Colombian assemblages.