Lithology and geochemistry of the Guadalupe Group base around Tunja, Boyacá, Colombia

The base of the Guadalupe Group, in the Tunja area of Colombia, contains cherts, porcellanites, mudstones, and siltstones with subordinate quartz arenites. The lithostratigraphic description of two stratigraphic sections showed that the dominant facies have fine granular textures and siliceous compositions, which considerably differ from those of the prevailing sandy terrigenous facies described in the type locality in the Eastern Hills of Bogotá, in the Arenisca Dura Formation, the basal unit of the Guadalupe Group in this sector. The units that form the Guadalupe Group (base of the Guadalupe Group, Plaeners, and Arenisca Tierna) markedly differ from each other morphologically, which facilitates their mapping because the base and top units generate a steep morphology, and the intermediate units form surface depressions or valleys, similar to the morphology of the Guadalupe Group in its type locality in the Eastern Hills of Bogotá. The base of the Guadalupe Group consists of cherts and porcellanites toward the NW of the study area (Alto del Gavilán section), with mudstones, siltstones, quartz arenites, and to a lesser extent porcellanites and cherts prevailing toward the SE (Vereda Salitre section). Geochemical analysis of total rock samples by XRD and XRF confirmed the primarily siliceous nature of the base of the Guadalupe Group, with SiO2 ranging from 62 to 98%, CaO less than 3.0%, and P2O5 peaking at 15.0%. Etayo-Serna (2015) conducted paleontological determinations of ammonites found in the stratigraphic section of Alto del Gavilán and assigned the base of the Guadalupe Group mainly to the Lower Campanian.


IntroductIon
From 2012 to 2015, the Servicio Geológico Colombiano (SGC) conducted detailed prospecting of 1600 km 2 in the Tunja area, in the department of Boyacá, in a sector of Plate 191, Tunja (Renzoni et al., 1976); such prospecting (Terraza et al., 2016) made it possible to redraw the geologic map of this region of the Eastern Cordillera of Colombia into a new stratigraphic panorama of the Upper Cretaceous corresponding to the Guadalupe Group. This new geological knowledge is partly reported in this article.
The study area is located in the central region of the Eastern Andes (Eastern Cordillera) of Colombia, in the Tunja area, where two stratigraphic sections were measured that included the base of the Guadalupe Group: one in Alto del Gavilán in the municipality of Motavita, and the other in Vereda Salitre in the municipality of Soracá (see the location in the geological framework), where the lithostratigraphic and geochemical characteristics of this unit were described in detail.

Stratigraphic nomenclature of the Guadalupe Group
The term "Guadalupe" was introduced by Hettner (1892, pp. 1-351), referencing Upper Cretaceous rocks, present in the Eastern Andes of Colombia, or Bogotá mountain range, which he designated the Guadalupe Stage (Table 1).
To the east of Cundinamarca and the Bogotá savanna, Hubach (1931, pp. 126-150) divided the Guadalupe Stage into two sets: "an upper set of sandstone and a lower set of schist". The upper set, which is currently known as the Guadalupe Group, was subdivided into three horizons at the time: an upper horizon of soft sandstones, a middle horizon of Plaeners, and a lower horizon of hard sandstones. Hubach (1951, pp. 10-13) assigned the formation rank to the Guadalupe Stage in the Bogotá savanna, dividing it into two sets: an upper set, consisting of sandstones and Plaeners, which is currently known as Guadalupe Group; and a lower schistose-clayey set with intercalating quartzitic sandstone horizons and strata, which is currently known as Chipaque Formation. He divided the upper set into three horizons: an upper horizon, which he termed Arenisca Tierna (Soft sandstone); a middle horizon, which he termed Plaeners and which included Arenisca Labor; and a lower horizon, designated Arenisca Dura (hard sandstone). In addition, based on the level of Exogyra mermeti and on the fauna of crushed ammonites next to bivalves found in the lower set of the Guaduas Formation, he stratigraphically ranked the Guadalupe Formation between the Coniacian and the Maastrichtian. Hubach (1957, pp. 39-46) promoted the Guadalupe Formation to the group rank, subdividing it into a lower clayey formation, or Lower Guadalupe, and an upper sandy formation, or Upper Guadalupe. The Lower Guadalupe Formation, consisting of clayey and quartzitic sandy facies, currently belongs ticosta Gabb, Ostrea falcata Morton, Siphogenerinoides clarki Cushman & Campbell, and Sphenodiscus, among others. In the Tunja area, Renzoni (1981) and Renzoni et al. (1976) divided the Guadalupe Group into the Plaeners Formation (kg2) and the Arenisca Labor-Tierna Formation (kg1) and indicated that the Arenisca Dura Formation (kg3) of the Bogotá savanna might correspond to some sandstones found in the top of the Conejo Formation of this locality (Table 1). They described the following fauna: Baculites sp. in the Plaeners Formation, Ostrea abrupta in the Labor-Tierna Formation, and Lenticeras baltai in the top of the Conejo Formation, the last of which is from the Santonian, according to Etayo-Serna (1968b, 1969. Pérez and Salazar (1978) described in detail the formations that compose the Guadalupe Group, located east of Bogotá (Guadalupe hill and Rajadero páramo), thereby formally defining its units, which from the base to the top are the Arenisca Dura Formation, the Plaeners Formation, the Arenisca Labor Formation, and the Arenisca Tierna Formation (Table 1). Arenisca Dura would range from the Coniacian or Santonian to the Campanian, considering the age of the overlying Plaeners Formation, which these authors assigned to the Lower Maastrichtian based on the observed Ostrea tecticosta Gabb, Orthocarstenia cretacea (Cushman), Orthocarstenia clarki (Cushman and Campbell), and Globigerinacea. Föllmi et al. (1992), in a section in Tausa (Cundinamarca), collected fossil fauna from the Lower Plaeners Member, overlying the Raizal Member (=A. Dura) of the Guadalupe Formation. According to the authors, these fossils included Nostoceras (Nostoceras) liratum, Exiteloceras jenneyi, Libycoceras sp., and remains of the dinoflagellate Andalusiella polymorpha, dated between the Upper Campanian and the Lower Maastrichtian (Table 1).
In the Llanero foothills (municipality of San Luis de Gaceno), in a section measured in the San Antonio ravine, Guerrero and Sarmiento (1996) divided the Guadalupe Group, from the base to the top, as follows: Lower Guadalupe (Arenitas de San Antonio Formation), Middle Guadalupe (Lodolitas de Agua Caliente Formation), and Upper Guadalupe (Arenitas de San Luis de Gaceno Formation). Based on palynological analyses, they assigned the following stratigraphic ranks to these units: Lower Campanian to Lower Guadalupe, Upper Campanian to Middle Guadalupe and Lower Maastrichtian to Upper Guadalupe.
More recently, in geologic mapping studies of the SGC, Montoya and Reyes (2003 a, b;2005 a, b;2007), in the Bogotá savanna and Plate 209, Zipaquirá, Colombia, identified the Upper Lidita Formation (nomenclature of the Upper Magdalena Valley (Valle Superior del Magdalena)) at the base of the Guadalupe Group, a unit considered contemporary to and heteropic with the Arenisca Dura Formation of the Bogotá savanna (Table 1). Montoya and Reyes (2003 a, b) assigned the Upper Lidita Formation to the Lower Campanian, based on the fossil fauna found in the top of the Conejo Formation (municipality of Ubaté), corresponding to "Texanites (Texanites) cf. quinquenodosus (Redtenbacher) cf. Kennedy et al. (1981); Glyptoxoceras sp. cf. crispatum (Moberg), cf. Kennedy et al. (1995); Eulophoceras sp. indet., cf. Kennedy et al. (1995), and Baculites sp. Indet".
In this article, the informal nomenclature proposed by Martínez (2018) for the area located between Ventaquemada and Toca ("base of the Guadalupe Group") ( Table 1) will be used while the SGC decides whether to accept the nomenclature of Montoya and Reyes (2003 a, b;2005 a, b;2007).

GeoloGIcal framework
Cretaceous sedimentary rocks of the Conejo Formation and Guadalupe Group crop out in the study area. They are overlaid by the Guaduas, Cacho, and Bogotá formations of the Paleogene. In some sectors, these units are discordantly covered by Neogene and Quaternary deposits. Structurally, the area is influenced by the Combita Syncline, Tunja Syncline, and Puente Hamaca Anticline as well as the Chivatá and Puente Hamaca thrust faults, which place the Upper Cretaceous rocks in contact with the Paleogene rocks ( Figure 1). Morphologically, in the Tunja area, as in the Bogotá savanna, the base of the Guadalupe Group and the Arenisca Tierna Formation generate an escarpment, whereas the Plaeners Formation originates a valley.
Conejo Formation. The Conejo Formation crops out toward the northwest of Motavita and the southeast of Soracá, forming belts with a SW-NE direction (Figure 1). The upper part of the unit forms a valley and is clearly distinguishable from the base of the Guadalupe Group, which generates a ridge. Lithologically, the lower section contains thick bundles of mudstones with dark claystones; the middle section shows quartz arenites interspersed with claystones and siliceous siltstones; the upper section reveals claystones and mudstones with frequent intercalations of quartz arenites and sporadic layers of bivalve wackestone, with layers of siliceous siltstones.
Base of the Guadalupe Group. The Base of the Guadalupe Group forms belts oriented in the SW-NE direction, which are part of the flanks of the Tunja Syncline, Cómbita Syncline, and other, minor folds ( Figure 1). It is characterized by a succession of porcellanites, fossiliferous cherts with wackestone textures of foraminifera and bioclasts, mudstones, quartz arenites, siltstones, and some silicified phosphatic layers that have benthic foraminifera and bioclasts. It lies concordantly  Plaeners Formation. Similarly, to the base of the Guadalupe Group, this unit crops out, forming belts oriented in the SW-NE direction, which are part of the flanks of the Tunja Syncline, San Francisco Anticline, Cómbita Syncline, and other, minor folds ( Figure 1). It is identified by its clayey character, with mudstones, some porcellanites, siltstones, quartz arenites, and some phosphatic layers. It lies concordantly over the base of the Guadalupe Group and underlies the Arenisca Tierna Formation.

method
Initially, the two stratigraphic sections in the study area were located and georeferenced using a GPS. Later, a measurement was performed at 1:100 scale using a Jacob's staff and a Brunton compass. We recorded the primary data in the format adapted by the SGC in 2013.
The following proposals were used to describe the stratigraphic sections: layer and lamina thickness according to Campbell (1967); geometric description of layers and laminae according to Reineck and Singh (1980), degree of progressive destruction of lamination by bioturbation according to Moore and Scrutton (1957); rock color according to the Munsell color system of the Geological Society of America (1991); degree of rock weathering according to the terminology of the Geological Society of London (1990); grain-shape and particle-size comparison diagram according to Krumbein and Sloss (1969); particle selection type and descriptive terminology according to Pettijohn, Potter, and Siever (1973); types of contacts between grains according to Taylor (1950); state of textural maturity of siliciclastic rocks according to Folk (1954); percentages of fossils and other allochems according to the charts of Baccelle and Bosellini (1965); textural classification of siliciclastic sedimentary rocks according to Folk (1954); compositional classification according to Folk (1974); and textural classification of calcareous rocks according to Dunham (1962) and Folk (1962).
Siliceous rocks were classified, both texturally and compositionally, according to Williams et al. (1954) and to Lazar et al. (2015), considering the percentage of microcrystalline quartz of the rock, as follows: The rock was classified as chert when the values ranged from 80% to 100%; as porcellanite, from 50% to 80%; and as mudstone or siliceous claystone, from 25% to 50%.
Both stratigraphic sections were systematically sampled for petrographic and geochemical analysis. The samples were sent to the Chemical Laboratory of the SGC, where they were analyzed by X-ray diffraction (XRD) and X-ray fluorescence (XRF) according to the protocols and methods established by the SGC.
The paleontological material collected in the two stratigraphic sections was analyzed by Dr. Etayo-Serna.

results
The results reported in the present article correspond to the detailed lithostratigraphic description of the base of the Guadalupe Group and to XRD and XRF data of the samples collected in the two stratigraphic sections measured in this study.

Lithostratigraphic Description
This description was performed based on two stratigraphic sections measured by Martínez (2018). The first section is located on the road that connects the municipalities of Soracá and Boyacá (Boyacá), in Vereda Salitre (Figures 1 and 2 In general, in this stratigraphic section, the base of the Guadalupe Group is characterized by the presence of porcellanites and cherts with a wackestone texture toward the base and a portion of the top. In contrast, mudstones, siltstones, claystones, and sporadic layers of very fine-grained quartz arenites prevail in the middle and higher parts.

Base of the Guadalupe Group
Segment A (0.0-23.5 m). Segment A was measured in the quarry located by the road from Soracá to the municipality of Boyacá ( Figure 5). The layers show continuous, plane-parallel bedding and very thin, discontinuous, plane-parallel lamination. The low part shows 18.5 m of very thin to thin layers of chert with foraminifera and bioclast wackestone texture ( Figure 6) and light-gray and orange-grayish-pink porcellanites, with sporadic thin layers of yellowish-gray mature quartz siltstones, without internal lamination. The high part of segment A shows 5 m of thin, medium, and thick layers of bluish-white siltstones and some layers of chert. This interval contains benthic foraminifera, fish remains, and peloids, in addition to some very slightly phosphatic layers. Rocks are moderately weathered with low bioturbation.
Segment B (23.5-53.0 m). Segment B was partly measured in the quarry and partly in the road from Soracá to the municipality of Boyacá ( Figures 5 and 9). The layers present continuous plane-parallel bedding and mostly very thin, discontinuous, plane-parallel lamination. From the base to the top, 9.7 m of thick to very thick layers of bluish-white claystones are observed (Figure 7), followed by 4.6 m of medium to thick layers of pale-greenish-yellow mudstones with low bioturbation and sporadic medium, laminated layers of light-brownish-gray claystones. Ascending stratigraphically, a thin, laminated layer of phosphatic porcellanite is found, followed by thin layers of light-brownish-gray chert, with nonvisible lamination and prismatic partitioning. They are followed by 11.6 m of thin to very thin layers of light-brownish-to-brownish gray mudstones, with sporadic layers of siltstones. Segment B ends in a section with 3.2 m of thin to medium, tabular layers of brownish-gray mudstones, with very thin, discontinuous, plane-parallel, lenticular heterolithic lamination and low bioturbation ( Figure 8) and medium to thick, tabular layers of yellowish-gray siltstones. This interval contains benthic foraminifera laminae, fish and crab remains, some bivalves, and ammonites, including Campanian "Didymoceras stevensoni? (Whitfield, 1877), Hoploscaphites sp. Inc.? and Sphenodiscus sp.?" (Etayo-Serna, 2015), as well as peloids, siliceous nodules, and muscovite as an accessory mineral.  Segment C (54.7-99.8 m). Segment C was measured in the road from Soracá to the municipality of Boyacá ( Figure 9). Most layers show continuous plane-parallel bedding and very thin to thin, discontinuous, plane-parallel lamination. The lower, middle, and upper parts of the segment are described below. The lower part begins with a 0.3 m layer of phosphatic porcellanite with peloids, followed by 8.7 m of a very thick set of layers, consisting of thin, medium, and thick layers of porcellanites and siltstones without internal lamination ( Figure 10); bioturbation ranging from low to high; and sporadic medium layers of phosphatic porcellanites. Ascending stratigraphically, 3.5 m of thin layers of mudstones are found with low bioturbation, followed by 2.9 m of very thin to thin layers of siltstones, with low bioturbation, and then by 4.4 m of thin to medium layers of very fine-grained, somewhat phosphatic, mature quartz arenites, with moderate bioturbation, interspersed with thin to medium, tabular layers of siltstones and some mudstones. This interval is characterized by the presence of benthic foraminifera, fish remains, crab claws, gastropods, peloids, and muscovite as an accessory mineral; the typical colors are very light gray, light medium gray, light brownish gray, and pale greenish yellow.
The middle part of the segment ( Figure 11) starts with 4.1 m of very thin, thin, and medium layers of cherts, with low bioturbation, interspersed with tabular, thin, and medium layers of porcellanites, followed by a tabular, (0.3 m) thin layer of phosphatic porcellanite (Figure 12), with peloids and bioclasts, high bioturbation, and horizontal burrows toward the base of the layer. Then we find a 10.6-m-thick section starting with very thin to thin layers of siltstones, low bioturbation, and sporadic layers of chert. This is followed by a tabular, medium layer of very fine-grained, mature quartz arenites, with bioturbation ranging from high to moderate. This layer continues to thin layers of porcellanites with nonvisible lamination, followed by thin to medium layers of siltstones with nonvisible lamination and high bioturbation and, finally, sporadic, thin to medium layers of claystones, porcellanites, and phosphatic mudstones. The typical colors are very light gray, medium gray, greenish yellow, and very pale orange.
The upper part of the segment shows 5.5 m of medium layers of mudstones, with high bioturbation and an intersper-sed, tabular, medium layer of claystones, followed by thin to medium layers of siltstones with high bioturbation and some layers of mudstones, ending with 3.3 m of tabular, thin to medium layers of very fine-grained quartz arenites with nonvisible lamination and high bioturbation. The rocks of this interval are brownish and yellowish gray. They contain remains of fish, crabs, peloids, crab claws, horizontal burrows, benthic foraminifera, and some bivalves in porcellanites, in addition to mica as an accessory mineral. Seg. C. Segment D (99.8-243.1 m). Most layers show continuous, plane-parallel bedding and thin to very thin, discontinuous, plane-parallel lamination. Segment D starts with a 125.1-m cover (Figure 13), followed by a 9.9-m-thick stratum consisting of thick and very thick layers of mudstones, with high bioturbation. Next, a tabular, medium layer of siltstones is followed by thick to very thick layers of mudstones; thick layers of clays-tones; thin to very thin layers of mudstones; a tabular, very thick layer of very fine-grained, mature quartz arenites; a tabular, very thick layer of claystone, with nonvisible lamination; a tabular, very thick layer of mudstone; and, lastly, medium to thick layers of siltstones. Ascending stratigraphically, a 7.3-m cover is then identified in this segment, which ends with 1 m of claystones in thin to medium layers. The rocks of this interval are medium gray, brownish gray, dark gray, and light olive gray    na, 2015), as shown in Figure 14. Next we find medium, thick, and very thick layers of very fine-grained, mature quartz arenites, with medium to high bioturbation and sporadic layers of siltstones. The rocks in this interval are grayish orange, very pale orange, and greenish yellow and contain peloids, crab claws, benthic foraminifera, and muscovite as an accessory mineral.
acutilineata (Conrad, 1858), and crab remains of the Upper Campanian (Etayo-Serna, 2015). Segment E ends with a 5.2-m-thick stratum formed by medium to thick cuneiform layers of very fine-grained, pale-greenish-yellow quartz arenites with high bioturbation, followed by a thick, cuneiform layer of phosphatic, light-brown quartz arenites with peloids, then very thick layers of sandy, white siltstone with discontinuous, plane-parallel, thin lamination with bivalves and peloids, then a tabular, medium layer of porcellanite, and lastly medium to thick layers of silicious siltstones ( Figure 16) with nonvisible lamination and with some benthic foraminifera and micas.   The section was divided into four segments (Figure 18), termed, from the base to the top, A, B, C, and D. Segments A and B, with a thickness of 66 m, correspond to the top of the Conejo Formation, and segments C and D, with a thickness of 75.6 m, correspond to the base of the Guadalupe Group. In this locality, the base of the Guadalupe Group generates a ridge that contrasts with the valleys of the Conejo and Plaeners formations ( Figure 19).
In general, in this stratigraphic section, the base of the Guadalupe Group predominantly shows porcellanites and cherts with wackestone texture, with some layers of very fine-grained quartz arenites and quartz siltstones.  The layers typically show continuous, plane-parallel bedding and very thin to thin, discontinuous, plane-parallel lamination, and the rocks are highly weathered. From the base to the top, segment A starts with a 6.9-m stratum of medium to very thick layers of very fine-grained, mature, pale-greenish-yellow quartz arenites with high bioturbation and sporadic layers of very fine-grained quartz arenites with discontinuous, wavy, nonparallel lamination, followed by a 2.4-m stratum of thin layers of very light-gray mudstones, continuing with 15 m of thin layers of brownish-gray claystones. Then a 1.7-m tabular layer of very fine-grained quartz arenites is followed by a 2-m layer of pale brown mudstones, by a 0.5-m tabular layer of very fine-grained quartz arenites, and by a 1.2-m layer of light-gray claystones, ending with 5.7 m of thin, medium, thick, and very thick layers of brownish-gray clayey siltstones ( Figure  20). This interval contains planktonic foraminifera, fish remains, gastropods, crab claws, peloids, micas, and iron oxides.

Segment B (35.4-66.0 m).
Segment B predominantly shows thin, medium, thick, and very thick layers of dark-yellowish-brown, brownish-gray, medium-brown, and medium-darkgray mudstones with discontinuous to solid plane-parallel lamination and moderate to high bioturbation; sporadic thin, medium, and thick layers of dark-brown claystones; and tabular, medium to very thick layers of very fine-grained, mediumdark-gray quartz arenites with high bioturbation (Figure 21). This segment is characterized by the presence of ammonites such as "Cocuyites cocuyensis (Etayo-Serna, 1985), Paratexanites sp. inc., Placenticeras sp.? and the Upper Santonian bivalves Meretrix eufaulensis? (Conrad, 1860) and Platyceramus ex gr. P. cycloides (Wegner, 1905)" (Etayo-Serna, 2015. Some of these fossils are illustrated in Figures 21 and 22. Segment B also contains iron oxide concretions, siliceous nodules, planktonic foraminifera, crab claws, fish remains, organic matter, and micas.  (Figure 23). Segment B is characterized by continuous plane-parallel bedding and by very thin to thin, discontinuous, plane-parallel lamination. The lower part of segment C starts with 1.7 m of thin to medium layers of siliceous siltstones, followed by 7.7 m of stratigraphic thickness with the following components: thin layers of mudstones and claystones ( Figure 24) with siltstone nodules, followed by medium to thin layers of siliceous siltstones, with Lower Campanian ammonites such as "Submortoniceras sp. cf. uddeni (Young, 1963)" (Etayo-Serna, 2015, interspersed with tabular, thin layers of mudstones ( Figure 25). Ascending stratigraphically, 1.5 m of thin to medium layers of porcellanites and cherts are followed by 5.7 m of the following lithology: medium layers of siltstones, followed by thin to medium layers of somewhat fossiliferous, very fine-grained quartz arenites with tabular geometry, and then thin to medium layers of siltstones with sporadic layers of porcellanites. Subsequently, 3.4 m of medium to thin layers of porcellanites containing quartz arenite nodules are followed by layers with chert nucleation, by a very thin layer of ferruginous grayish red siltstones, by a 0.9-m-thick stratum of thin to medium layers of siltstones, and by a 1.8-m-thick stratum of thin to medium layers of porcellanites and chert with foraminifera and peloid wackestone texture ( Figure 26). These are interspersed with sporadic layers of quartz arenites and siliceous siltstones. Most rocks of this interval show the following colors: very light gray, brownish gray, pinkish gray and pale greenish yellow.
In the upper part of segment C, a fossil level of 1.2 m (Figure 27) consists of medium to thick layers of yellowish-gray mudstones, with impressions of ammonites such as Submortoniceras uddeni? (Young, 1963), suggesting the Lower Campanian (Etayo-Serna, 2015), and, as shown in Figure 27, fish and crab remains and peloids, followed by subtabular, thin to medium layers of sandy mudstone with high bioturbation (Figure 28). The upper part of segment C continues with a 7.8-m-thick stratum with the following structure: a thick and somewhat fossiliferous layer of quartz arenites, with nonvisible lamination, followed by thin layers of porcellanites with continuous, plane-parallel bedding and sporadic layers of chert; above, tabular, thin to medium layers of siliceous siltstones interspersed with a tabular, medium layer of chert with peloid wackestone texture ( Figure 29) and nonvisible lamination, which are followed by thin to medium layers of porcellanites and by a tabular, medium layer of phosphatic porcellanite with peloid wackestone texture and nonvisible lamination. In this interval, the rocks are very light gray and pale greenish yellow. The upper part of segment C also contains benthic foraminifera, ammonite impressions, crabs, fish remains, peloids, micas, and iron oxides.      Segment D (100.8-141.6 m). The outcrop generally presents continuous, plane-parallel bedding, and the layers show very thin to thin, nonvisible, discontinuous, plane-parallel lamination, predominantly with porcellanites ( Figure 30). From the base to the top, the following sequence is identified: First, thin layers of porcellanites are interspersed with sporadic layers of chert and three wavy, intercalated, very thin layers of ferruginous, grayish-red siltstone. Then, thin to medium layers of siltstones are followed by thin to medium layers of porcellanites with tabular geometry, with sporadic layers of chert, siliceous siltstones, and two very thin layers of ferruginous siltsto-nes. Next, a tabular, medium layer of fossiliferous porcellanite shows a peloid wackestone texture. At the top of the segment, tabular, thin to medium layers of fossiliferous porcellanite with a benthic foraminifera wackestone texture, interspersed with layers of chert, are followed by a covered zone with soft morphology that belongs to the Plaeners Formation. In this interval, the rocks show the following colors: very light gray, white, pinkish gray, and bluish white. The top of segment D also contains benthic foraminifera, fish remains, peloids, micas, and iron oxides.

Results from XRD and XRF analysis
Within the stratigraphic interval that corresponds to the base of the Guadalupe Group, in the two stratigraphic sections measured in this study, systematic sampling was performed as follows: seventeen samples were collected in Vereda Salitre and nineteen in Alto del Gavilán. These samples were sent to the Laboratory of the SGC, where they were characterized mineralogically and geochemically by XRD and XRF. The results are outlined in Tables 2, 3, 4, and 5. The dashes shown in some of the cells of these tables indicate values below the detection limits of the diffraction and fluorescence equipment. From the batch of samples collected at the base of the Guadalupe Group, in Vereda Salitre (Tables 2 and 3), the mineralogical analyses determined that eight samples are siliceous rocks (cherts and porcellanites) and that nine samples are siliciclas-tic rocks (claystones, mudstones, and quartz arenites). They have (detritic and silica) quartz contents ranging from 66.7 to 99.8%, as corroborated by geochemical data, which shows that silica (SiO 2 ) ranges from 62.6 to 95.7% (Figure 31). In conclusion, although the texture determined by petrographic analysis is typical of calcareous and phosphatic rocks (wackestone), minerals with calcium and phosphorus, such as hydroxyapatite and carbonatofluoroapatite, are only present in two samples. In general, throughout the stratigraphic interval, the values of calcium oxide (CaO) are lower than 4.0% (Figure 31), and the values of phosphoric oxide (P 2 O 5 ) range from 0.2 to 5.4%.
Kaolinite is also detected, with values ranging from less than 2.0 to 21.3%, in line with the values of aluminum oxide (Al 2 O 3 ), which range from 2.0 to 14.4%, as possible alteration products of aluminum silicates ( Figure 31).   Figure 32). Calcium oxide (CaO) is present in amounts lower than 2.4%, and phosphoric oxide (P 2 O 5 ) ranges from 0.04% to 15.1% (Figure 32).

dIscussIon
The stratigraphic nomenclature corresponding to the Guadalupe Group, in the Tunja area of Colombia, established by Renzoni (1981) and by Renzoni et al. (1976), divided the Guadalupe Group into the Plaeners Formation (kg2) in the base and the Labor-Tierna Formation (kg1) in the top. These authors thought that the Arenisca Dura Formation (kg3) of the Bogotá savanna would be included in the layers of sandstones of the top of the Conejo Formation of this locality. However, the base of the Guadalupe Group, the object of this study, mostly dates back to the Lower Campanian (Etayo-Serna, 2015) and corresponds to the Plaeners Formation (kg2) of Renzoni (1981). In addition, stratigraphically, above the base of the Guadalupe Group, a soft unit of the Upper Campanian (Etayo-Serna, 2015) is overlaid by a set of quartz arenites representing the Plaeners and Arenisca Tierna formations of the Bogotá savanna, included by Renzoni in unit kg1, in this part of the eastern Andes of Colombia (see Figure 1). In the Guadalupe Group of Tunja and its surrounding area, three clearly distinguishable and easily recognizable units can be lithologically and geomorphologically identified. The base of these units (described in this article) differs substantially from the type section in the Eastern Hills of Bogotá. To avoid confusion with the nomenclature proposed by Renzoni (1981), for the moment, the unit in question should be informally designated as "base of the Guadalupe Group" while deciding whether to maintain the nomenclature of Montoya and Reyes (2003 a, b;2005 a, b;2007) for this area of the Eastern Andes of Colombia. The petrographic analysis performed by Martínez (2018) at the base of the Guadalupe Group, in the two stratigraphic sections described in this study, showed that most cherts and porcellanites with wackestone or mudstone texture with allochems replaced by microcrystalline quartz and chalcedony, with microcrystalline quartz support, correspond to calcareous or phosphate rocks (biomicrites and biopelmicrites), which underwent a process of replacement of the calcareous mud (micrite) and pre-existing allochems by silica, possibly during the final stage of diagenesis (compaction).

conclusIons
In the Tunja area of Colombia, the Guadalupe Group has morphologically and lithologically differentiated into three units: two hard units that originate ridges corresponding to the base of the Guadalupe Group and the Arenisca Tierna Formation, respectively, with a soft unit in the middle that generates a characteristic valley in the Plaeners Formation.
The paleontological analyses performed on the two stratigraphic sections measured in this study (Etayo-Serna, 2015) indicate that the base of the Guadalupe Group partly represents the Upper Santonian and mostly the Lower Campanian.