Introduction

The formation of basins is influenced by both internal and external mechanisms, which play significant roles in sediment accumulation and preservation. The Devonian Period, known for its rich fossil records from marine environments worldwide, provides an unique insight into these dynamics. These records not only reflect the dynamic climatic fluctuations of the time, including cool intervals during the Middle and Late Devonian in addition to greenhouse conditions (e.g. Joachmiski et al. 2009), but also serve as archives of the diverse life forms that inhabited these ancient marine ecosystems.

The Paleozoic depocentres in northwestern South America, such as the Llanos Basin and the Eastern Cordillera of Colombia, have tectonic histories and sedimentation patterns making them pivotal for understanding basin evolution (e.g. Dickey 1941; Trumpy 1943). Although the specific facies setting of such depocentres may not be as prominently fossiliferous as certain environments during the Devonian, the existence of Devonian successions in Colombia aligns with the broader Appalachian-East Americas Realm (Barrett and Isaacson 1988; Dowding et al. 2022).

In the Eastern Cordillera, the Floresta Massif, consisting of a core of Precambrian rocks superseded by Paleozoic and Mesozoic successions, provides well-preserved sections allowing detailed measurements and descriptions despite Andean tectonics. Within this context, the Devonian Floresta Formation holds significant geological interest due to its fossil record that has been crucial in shaping the understanding of its stratigraphy (e.g. Caster 1939; Janvier and Villarroel 2000; Morzadec et al. 2015; Olive et al. 2019). Efforts to establish the stratigraphic age of the Floresta Formation rely on composite stratigraphic distributions based mainly on palynomorphs, brachiopods, plant megafossils, and vertebrates (e.g. Moreno-Sánchez et al. 2020; Pastor-Chacón et al. 2023). However, the lack of temperature-sensitive geothermochronometers and the occasional occurrence of pelagic fauna further add complexity to supraregional correlations with horizons, zones, and subzones in different Global Boundary Stratotype Sections and Points (GSSP).

On the other hand, the Llanos Basin has restrained areas with Devonian rocks that host organic matter. Many wells drilled into these rocks, along with other Paleozoic strata, consistently show oil and gas indications (e.g. Bogotá-Ruiz 1988; Dartora and Moretti 2014; Mora et al. 2006; Aguilera 2022). These successions have caught attention because they might have untapped potential for both generating hydrocarbons and natural hydrogen (Aguilera 2022; Tian et al. 2022).

However, despite the economic importance of Devonian successions, many fundamental questions remain unanswered. One significant challenge is the limited availability of comprehensive studies that integrate sedimentology, geochemistry, and stratigraphy to understand both the palaeoenvironmental conditions and the stratigraphic architecture of these formations. These studies are crucial for unraveling the complex relationships between environmental factors, sedimentary processes, and organic matter preservation. The integration of stratigraphic investigations is particularly vital for developing predictive models that enhance our understanding of stacking patterns, key stratigraphic surfaces, and potential cyclic sedimentation within Devonian successions.

This contribution seeks to answer the following research questions: What are the depositional environments, stratigraphic controls, and geochemical constraints of the Devonian succession? What is the quantity and type of organic matter hosted in these depositional environments? Are there specific factors, such as tectonics or depositional environments, that hint at cyclic sedimentation patterns within the Devonian sequence?

The aim of this study is to analyse sedimentological and geochemical data from an outcrop in the Floresta Massif to: 1) interpret the depositional processes, sedimentation patterns, and palaeoenvironmental conditions of the succession; and 2) describe and quantify the organic matter hosted within the succession.

Geological framework

The Floresta Massif locates in the central-northern region of the Andes in Colombia and formed by plate tectonic movements of the South American, Nazca, and Caribbean plates. Remnants of Devonian rocks can be found overlying Ordovician and Proterozoic rocks preserved in inliers, such as the Perijá Range and Macarena Range, as well as in the subsurface of foreland basins, such as the Llanos Basin (Figure 1).

Fig. 1
figure 1

Location of the study area. a Simplified tectonic setting of northwestern South America (after Pastor-Chacón et al. 2023). b Simplified geology of the Floresta Massif (modified from Mojica and Villarroel 1984, and  Pardo-Torres and Camargo 2023). Abbreviations: EC Eastern Cordillera, CC Central Cordillera, WC Western Cordillera, SNSM Santa Marta Massif, PE Perijá Range, SM Santander Massif, ME Mérida Andes, SLR San Lucas Range, CR Cocuy Range, FM Floresta Massif, QM Quetame Massif, MR Macarena Range, LLA Llanos Basin, SBF Bucaramanga Fault, PF Palestina Fault, RF Romeral Fault, BLLFS Borde Llanero Fault System, CF Caparo Fault, AF Apure Fault, GF Guayabero Fault

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Despite ongoing debates whether the Paleozoic tectonic setting of northwestern South America, which has been interpreted as a para-autochthonous terrane or a protracted active-margin setting (e.g. Ramos 2018; Spikings and Van der Lelij 2022), the Floresta Massif has a long, complex, and multifaceted history dating back to the Paleozoic. At its core, it is composed of apparent low-grade Proterozoic metapelitic rocks that have been intruded by Ordovician granitoids (Mantilla-Figueroa et al. 2012) that are unconformably overlain by Devonian, Cretaceous, and Cenozoic sedimentary rocks (Horton et al. 2010; Manosalva-Sánchez et al. 2017; Mojica and Villarroel 1984). The Soapaga Fault, located to the east of the massif, has documented extensional tectonics from the Paleozoic to early Mesozoic (Mojica and Villarroel 1984; Kammer and Sánchez 2006) with a final inversion during the Andean Orogeny (Horton et al. 2010; Tesón et al. 2013; Saylor et al. 2012; Pardo-Torres and Camargo 2023).

This area also contains well-preserved mid- to late-Paleozoic outcrops, with major sediment thicknesses from this time span in the Colombian Andes. This succession includes a complete Devonian succession comprising three well-exposed formations: (1) the continental-transitional strata of the El Tibet Formation (Pragian-Emsian), which are related to the first stages of marine transgression and basin opening (Tellez and Sotelo 1997; Mojica and Villarroel 1984); (2) the Floresta Formation (Emsian-Givetian), which includes the maximum flooding surface in the basin and shows the evolution of transitional to shallow marine siliciclastic and platform carbonates (Mojica and Villarroel 1984; Morzadec et al. 2015); (3) the Cuche Formation (Frasnian), which encompasses the shallowing of the basin and has transitional environments (Berry et al. 2000; Olive et al. 2019).

The Devonian stratigraphy in this region is further constrained by the abundant record of rhynchonelliform brachiopods, bivalves, and trilobites, along with plant megafossils and marine vertebrates (e.g. Berry et al. 2000; Caster 1939; Grösser and Prössl 1994; Morzadec et al. 2015; Olive et al. 2019). The lower part, corresponding to the late Emsian, is marked by the presence of miospores (e.g. Camarozonotriletes sextantii and Brochotriletes robustus; Grösser and Prössl 1994), rhynchonelliform brachiopods (e.g. Eodevonaria imperialis and Leptaena boyaca; Caster 1939), and trilobites (e.g. Dechenella cf. D. boteroi and Viaphacops cristata; Morzadec et al. 2015). The succession extends into the Eifelian in the lower and middle portions, which include a diverse brachiopod and bivalve assemblage (e.g. Tropidoleptus carinatus, Actinopteria sp., Leiopteria nitida, Nuculites oblongatus, and Prothyris sp.; Morales 1965; Beltran et al. 2020). The middle part spans the Givetian, distinguished by inarticulate brachiopods and trilobites (e.g. Schizobolus pilasiensis and Dipleura cf. D. dekayi; Morzadec et al. 2015), and bivalves (e.g. Cypricardella cf. C. bellastriata; Beltran et al. 2020). Lastly, the upper part contains early Frasnian floras and marine vertebrates (e.g. Archaeopteris obtusa; Moreno-Sánchez et al. 2020).

The Mesozoic fill (Tithonian-Maastrichtian) of the massif reflects an superimposed tectonic history consisting of the opening and closure of a basin filled with transitional to shallow marine sediments (e.g. Horton et al. 2010). Finally, the Cenozoic fill (Paleogene) preserved in the footwall of the Soapaga Fault consists of continental strata, which reflect the peak of the Andean Orogeny (e.g. Horton et al. 2010; Saylor et al. 2012) (Figure 2).

Fig. 2
figure 2

Representative stratigraphic chart of the Floresta Massif. Based on geologic descriptions and tectonic events by Horton et al. (2010), Saylor et al. (2012), and Pardo-Torres and Camargo (2023)

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Materials and methods

In the southern part of the Floresta Massif (Figure 1), in the place called Vereda Las Pilas (5.8136°, -72.8974°), we measured 530m of Devonian sediments, including the El Tibet, Floresta, and Cuche Formations. These measurements included detailed sampling for petrography, X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), organic geochemistry, and Spectral Gamma-Ray (SGR) counts. The thin sections and powder samples are housed in the "Laboratorio de Caracterización Litogeoquímica" at the Universidad Nacional de Colombia, Bogotá.

Sedimentology and petrography

The lithology was described at the scale of meters in the Las Pilas section, and a detailed examination to the nearest meter was performed on the levels with shell pavements at the base of the section (5.8132°, 72.8971°). Measurements were taken directly using Jacob's staff, tape measure, and compass and were recorded using GPS, topographic maps, and satellite images. Rock samples were collected at closely but irregularly spaced stratigraphic intervals from the entire measured section (Figure 3).

Fig. 3
figure 3

Stratigraphical and sedimentological log of the Devonian succession at Vereda Las Pilas, Floresta Massif. Biostratigraphic data after Beltran et al. (2020), Grösser and Prössl (1994), and Morzadec et al. (2015) (see details in text). Abbreviations: BI Bioturbation index, As Asterosoma, Ne Nereites, Pl Planolites, Th Thalassinoides, Ph Phycosyphon, Ro Rosselia, Sk Skolithos, Ps Psilonichnus, cl clay size, sl silt size, vf very fine grain, f fine grain, m medium grain, c coarse grain, vc very coarse grain, gr granules

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For rock classification, we employed the terminology proposed by Folk (1980), and the granulometric scale of Wentworth (1922) for grain size of siliciclastic. We also utilised the thickness scale of layers and laminations of Ingram (1954), as modified by Campbell (1967). For particle selection and bioturbation characterisation, we used the methods proposed by Pettijohn et al. (1987) and Taylor and Goldring (1993). Facies characteristics were examined in detail, to characterise sedimentary structures, formulating facies associations, and interpreting sedimentary processes and environments. We described intervals of shell pavements using the terminology of Kidwell et al. (1986) and Kidwell (1991).

Forty-five thin sections were prepared, and we conducted a point count using the Gazzi-Dickinson method to ensure statistical representativeness. An Olympus BX51 microscope, Olympus DP72 camera, and Stream Essentials v 1.5.1 photography software were utilised for the counts. The thin sections were classified using the compositional and textural nomenclature of sandstones proposed by Folk (1980) and Dickinson (in Compton, 1985), while the classification of rocks with allochemical content, sandstone, and mudstone, employed Mount's (1985) hybrid rock classification.

The interpretation of sandstone provenance was determined using Dickinson et al. (1983), Dickinson (1985), Basu et al. (1975), and Tortosa et al. (1991). Since there is no significant variation in the composition and provenance characteristics between sandstones and coarse sandy siltstones, the latter were included in the compositional and provenance classification as part of the sandstone category.

Lithogeochemistry

We conducted XRD analysis of seventy samples using a Bruker D2 Phaser diffractometer with a Copper (Cu) cathode Kα=1.5406 μm, following the parameters outlined by Bonilla et al. (2011). In addition, we examined both bulk and oriented samples that were treated with glycol and calcined at 500°C to identify clay minerals. For mineral quantification, we utilised the DIFFRAC.EVA v. 4.1.1 software package.

Furthermore, we performed XRF analysis on seventy bulk samples using a Bruker IV-SD ED-XRF with a Rhodium x-ray tube. The samples were initially analysed under vacuum conditions, with a maximum voltage of 40kV and 30 μA for 30 seconds. We then applied principal components analysis (PCA), a statistical method used to identify patterns in data, between major and trace elements, focusing on the weight of the first and second components only (e.g. Craigie et al. 2016; Königshof et al. 2016).

Additionally, we took SGR measurements spaced at 1ft intervals for 30 seconds each along the stratigraphic section using a portable spectrometer RS-125, with a Sodium Iodide detector of 6.3 cu in and energy responses between 30 keV and 3000 keV, obtaining Potassium results in percentage and Uranium and Thorium results in ppm. The spacing between measures allows for comparison of the results to a borehole electric log to identify source-rock intervals and stratigraphic correlation (e.g. Slatt et al. 1995; Fertl and Chilingar 1988). Next, we selected elements and their relationships from both XRF and SGR measurements to serve as proxies for dysoxia/anoxia (e.g. Th/U ratio; Hallberg 1976; Rimmer 2004; Algeo and Liu 2020) and productivity or carbonate input (e.g. Rb/Sr ratio, K2O and P2O5 content; Königshof et al. 2016; Hong et al. 2018; Wei and Algeo 2020).

In our study, we applied the Integrated Prediction Error Filter Analysis (INPEFA) to SGR measurements. This allowed us to capture changes in such measurements that may have frequency and time implications (e.g. Li et al. 2023). We used this technique to divide the Devonian succession into base-level cycles of varying orders: (1) fifth-order cycles or parasequences not higher than ca. 0.5 m thick; (2) fourth-order cycles or parasequences of ca. 1.0 m thick; (3) third-order cycles of ca. 10 m thick; (4) second-order cycles of ca. 20 m thick; and (5) first-order cycles of ca. 200m thick. These cycles exhibit both symmetrical and asymmetrical patterns, also displaying progradation and retrogradation characteristics, and were compared against sedimentary structures and facies interpretations to ensure a comprehensive understanding of the stratigraphic framework.

Organic geochemistry

We selected thirty fine-grained samples for TOC analysis using a LECO SC 632 carbon and sulfur analyser. Seven samples with values higher than 0.5% TOC were selected for pyrolysis using a Rock-Eval Hawk pyrolyzer with an FID detector and infrared cells for CO2 and CO detection. We applied methods from Peters and Cassa (1994) to calculate hydrogen and oxygen indexes.

We selected three samples in the lower, middle, and upper parts of the section for visual analysis of kerogen and Thermal Alteration Index (TAI) determination using a Zeiss Axio Imager A2M microscope and its organic matter classification following Tyson (1995), and Pepper and Corvi (1995). Finally, we calculated the original hydrogen indices following mathematical models from Banerjee et al. (1998), Chen and Jiang (2015), and Hong et al. (2018).

Results

Facies analysis and petrography

The sedimentological analysis from the study area revealed five facies associations (FA1-5), constituted of nine major facies and five subordinate facies (Fig. 3.). Table 1 provides detailed information on their sedimentary structures, bed geometry, bioturbation index (BI), fossil content, and interpreted sedimentary processes.

Facies association 1 (FA1): Intertidal zone with tidal channels and tidal muds

This facies association (FA1) is composed of facies F1 and F2 and is present in the lower part of the sedimentary succession of the Las Pilas Section (Fig. 3.). It includes the uppermost meters of the El Tibet Formation and records the deposition of a tidal system. Facies F1 and F2 consist of erosive, fining-upwards, planar to cross-stratified, matrix-supported granular lithic conglomerates to fine-grained sublitharenites and siltstones with very fine-grained sandy lenses or wavy lamination. Bioturbation is generally low or absent in FA1, with no significant trace fossils in the finer intervals (Fig. 4.).

Fig 4
figure 4

Selected photographs of the facies and sedimentary structures of the base and middle segments of the Las Pilas Section. a Lenticular, granular lithic conglomerate beds near the base at 1 m. b Bioturbated allochem siltstones (As Asterosoma) at 8 m. c Fine-grained allochem sandstones with bioclasts (indicated by arrows) and coarse ooids at 2 m. d Fossiliferous siltstones interlaminated with shell pavements (indicated by arrows) at 21 m. e Bioturbated dispersed fossiliferous siltstones with ripples (Ne Nereites) at 23 m. f Shell pavements at 24 m (arrow points to a broken coral). g Concretion-bearing dark grey laminated shales at 69 m. (h-i) Bioturbated dark grey laminated shales (Th Thalassinoides) at 50 m and 103 m

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The sedimentary process is characterised by debris transport as bedload by traction at the base, transitioning towards suspension and settling in the high tide zone at the top. Finer materials settle and suspend on a rippled bottom in a lower flow regime, occasionally forming sand lenses during episodes of higher energy in the low tide zone. The conglomerate-sandstone beds are interpreted as evidence of tidal channels, while the siltstone intervals document tidal mud deposition.

Facies association 2 (FA2): Transgressive lag

This facies association, which includes the F3 facies, is located in the lower part of the study section. It contains the stratigraphic surface that separates the El Tibet Formation from the Floresta Formation (Fig. 3.). The F3 facies is characterised by thin beds of erosive, not laminated, coarse-grained siltstones and fine-grained allochem sandstones. These contain bioclasts and coarse to mid-grained ooids, along with para-autochthonous conglomerate, quartz-sandstone, and schist fragments. In addition, it features brachiopod molds (spiriferids, chonetids and orthids), broken gastropods, tabulate and solitary rugose corals, matrix-supported shell fragments oriented chaotically, deposited in pods and clumps with no evidence of perforations or abrasion (Fig. 4.).

The dominant sedimentary processes are high-energy turbulent flows induced by storm wave action. These flows gradually lose energy and deposit the finer fraction, transporting bioclasts and ooids from neighboring parts of the platform and para-autochthonous intraclasts from structural highs within the orogenic belt. The presence of abundant ooids suggests sedimentary environments with normal to slightly elevated salinity, warm temperature, and productivity, and organic matter content for bacterial growth. These bacteria accrete amorphous calcium carbonate laminae or phosphates around terrigenous grains, which are later moved by water agitation through oscillatory flows or turbulent flows in high-energy conditions (Diaz and Eberli 2019; Riaz et al. 2023). The wave ravinement transgressive surface is an indication of the shoreline retreat and the erosive effects of the waves on the shallow platform, which eventually led to the formation of the transgressive lag and the marine ingression into coastal areas. The deposition of the F3 facies at this surface reflects the transition from the shoreline environment to deeper marine environments.

Facies association 3 (FA3): Proximal storm-dominated lower shoreface and offshore-shoreface transition with fossiliferous siltstones and shell pavements

It includes the facies F4 and F4a, located in the lower part of the section (Fig. 3.) and containing most of the fossil-rich levels described by Caster (1939) in the Floresta Formation, consists of erosive, tabular medium to thin beds with symmetrical wave ripples and wavy lamination. Fossiliferous siltstones interbedded with centimeter to meter-stacked shell pavements make up these facies. These pavements are composed of wavy laminations of micro-erosive shell debris and dispersed fossiliferous siltstones with ripples. The matrix includes bioclasts such as strongly ornamented brachiopods, crinoids, gastropods, and corals in life position. Shelly laminations consist of disarticulated trilobites, brachiopods, and crinoid columnals, as well as broken bryozoans (Fig. 4). Bryozoans have encrusted some shells, and the matrix also displays traces of Nereites (?) and Planolites (?).

Presence of symmetric ripples indicates wave or oscillatory flows, typical of wave-dominated environments. Wavy lamination, on the other hand, suggests the reactivation of the shell beds by episodic storm events, generating combined flows with oscillatory and unidirectional components. Combined flows migrate towards shallower parts of the platform due to the transgressive trend that flooded previously exposed or shallow areas, or due to the storm-induced setup that increased the water level near the shore. This migration allows for better circulation of water and nutrients, enhancing the organic productivity and decomposition, leading to the growth of abundant strongly ornamented brachiopods. Similarly, shells and bioclasts selectively preserve mud, serving as traps or obstacles to coarser grains through winnowing currents. Nonetheless, such processes do not consistently exhibit high-energy dynamics and alternate with low-energy conditions. For example, the preservation of organisms in life position and the evidence of encrusted shells, denotes optimal light and nutrient conditions (e.g. Sanchez and Benedetto 1983), and indicate rapid burial and low-energy conditions occurred during the intervals of low sediment input (e.g. Brett and Baird 1996). Due to these sedimentary processes, facies F4 and F4a are interpreted as the lower shoreface and the transition zone between offshore and shoreface, influenced by proximal storms.

Table 1 Facies and sedimentary processes of the Devonian sediments in the Las Pilas Section
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Facies association 4 (FA4): Upper offshore to lower shoreface with cyclic dark grey laminated shales and dispersed fossiliferous intervals

This facies association contains facies F5, F5a, F5b, F6, and F7 and is found in the middle part of the Floresta Formation (Fig. 3.). This facies association is characterised by the presence of sharp, locally erosive, medium-sized beds with planar, inclined, wavy, and flaser laminations and ripples. Also, it is comprised of bioturbated allochem siltstones, laminated very fine-grained sandstones, dark grey laminated shales, and locally fossiliferous very fine to fine-grained sandstones. Discrete, fossiliferous levels contain smooth, small bivalves (pectinids), articulate and inarticulate brachiopods (chonetids, spiriferids, and discinids), and small trilobites (Figs. 4 and 5.). Some levels also exhibit local micro-erosive laminaes of disarticulated brachiopods and crinoids. In addition, the bioturbated beds show evidence of Thalassinoides, Asterosoma, Skolithos, Paleophycus, Rosellia, and Planolites (?). The facies F7 is particularly notable, consisting of a medium-sized tabular bed that is sharp at the base and laminated with an accumulation of monospecific rhipidomellid brachiopods (?Aulacella sp.) embedded in a mid-grained silt matrix. This bed features internal molds, closely associated articulated specimens with no perforations, predominantly convex-up orientation, sorted, no abrasion, and no shell fragmentation (Fig. 5.).

Fig. 5
figure 5

Selected photographs of the facies and sedimentary structures of the middle and upper segments of the Las Pilas Section. a-c Monospecific brachiopod coquina (?Aulacella sp.) at 160 m, shown in cross-section (a, c) and bedding plane view (b). d Cyclic dark laminated shales to very fine-grained sandstones at 226-238 m. e Bioturbated fine-grained sandstones (Pa Paleophycus, Ro Rosselia, and Th Thalassinoides) at 251 m, seen in bedding plane view. f Fine-grained sandstones with flaser lamination (indicated by arrows) at 278 m. g Bioturbated very fine-grained sandstones (roots indicated by arrows) at 508 m. h Laminated red siltstones at 465 m. i Bioturbated very fine-grained sandstones (Ps Psilonichnus?) at 380 m

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The depositional processes in this facies association contain coarsening-upwards bedsets that reveal variations between low-energy flows depositing fine grains by settling and suspension. These flows gradually gain energy and coarser sediment near the fair-weather wave action level due to oscillatory flows. The sediment contains delicate small-sized brachiopods, trilobites, and bivalves that could thrive through low-energy conditions and nutrients. In some beds with high energy levels, the good preservation of brachiopods and crinoids suggests low transport and relatively rapid burial; at the same time, discrete siltstone and sandstone beds are deposited through the action of distal storms in the shallow part of the platform. The finer parts of the facies association suggest that the absence of currents and waves below the storm-wave action level results in low sediment accumulation rates. These processes indicate variations between the upper offshore, transition offshore-shoreface, and lower shoreface influenced by distal storms. Finally, a rising base level favoured the development of rhipidomellid communities and the formation of the maximum flooding zone of the succession with a low sediment accumulation rate.

Facies association 5 (FA5): Floodplains, distributary channels, and paleosols

This facies association, characterised by facies F8, F8a, F9, and F9a, corresponds to the upper part of the studied section within the Cuche Formation (Fig. 3.). The beds are predominantly composed of wavy, sub tabular, and medium-sized tabular beds with wavy, low-angle inclined, planar laminated, very fine-grained sandstones, laminated, and bioturbated red siltstones. The facies also include fining-upwards beds with mud drapes, sandy lenses, plant fragments, and local levels with nodules. Bioturbation is evident in the form of Psilonichnus, vertical ramified root remains cutting beds from base to top, and remains of plant stems.

The sedimentary processes recorded in these facies suggest fine-grained sedimentation through suspension and settling. For example, red-laminated and bioturbated siltstones, also indicate oxidizing environments that favour the bioturbation and precipitation of iron oxides such as hematite and goethite. Furthermore, the sedimentary processes recorded also include high-energy sedimentation, possibly from river streams, which transport sand grains as bedload by traction. However, these facies exhibit coastal indications as well through the presence of sandy lenses and wavy lamination, potentially influenced by fluvial inputs or other dynamic processes. The depositional environments are interpreted as distributary channels, flood plains, and paleosols, implying a dynamic, mixed-energy environment that varied from subaerial exposure to subaqueous conditions.

Petrography

The petrographic analysis of selected thin sections from the study area provides a detailed understanding of the sedimentary components and diagenetic processes within the mudstones and sandstones. Figure 1 shows the locations of the samples selected for analysis. The results reveal a diverse range of components and processes, as described in further detail below.

A total of seven fossiliferous mudstones, five sandstones, three muddy sandstones, eleven sandy mudstones, and four mudstones were classified (sensu Mount 1985) based on textural characteristics. In contrast, Folk's (1980) classification corresponded to two muddy sandstones, eight sandy siltstones, ten siltstones, two sandy mudstones, one silty sandstone, and seven sandstones (Supplementary Material). The petrographic analysis uncovered a wide array of sedimentary components and diagenetic processes in the mudstones and sandstones.

The eleven sandy mudstones, four mudstones, and seven allochem mudstones were characterised by a dominant presence of siliciclastic grains, including fragments of metamorphic lithics such as schists, phyllites, quartzites, quartz, and mica (Fig. 6.). The absence of feldspars and abundance of lithic grains resulted in the classification of the sandstones as metamorphic litharenites. According to Dickinson’s classification (in Compton 1985), some samples were also classified as lithic sub-quartzose and lithic semi-quartzose sandstones. Allochem fragments were identified as bioclasts, including crinoid remains, brachiopods, bryozoans, and concentric phosphate and phosphate-ferruginous ooids. Nonetheless, XRD and XRF analyses revealed that the phosphate fragment concentrations in these samples are low and do not meet the criteria to classify them as phosphorites (Fig. 6; Section "Lithogeochemistry").

Fig. 6
figure 6

Selected photomicrographs of the Devonian succession at Las Pilas Section. Location of samples in Fig. 3. a Fine-grained lithic sandstone (Facies 1, Sample B1, 4X). b Fine-grained allochem sandstone (Facies 3, Sample B1b, 10X) c Fossiliferous siltstones interlaminated with shell pavements (Facies 4, Sample B28, 10X) d Bioturbated allochem siltstones (Facies 5, Sample B37, 10X) e Very fine-grained sandstone (Facies 8, Sample B133, 10X). f Bioturbated allochem siltstone (Facies 5, Sample B107, 10X). Abbreviations: I Intraclasts, Lm Metamorphic lithics, Mca Micas, Qp Polycrystalline quartz, Qo Ondulose quartz, O Ooids, Bq Brachiopods, Br Bryozoans, Cr Crinoids

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The petrographic analysis also revealed variations in the ooids within the different sections of the sedimentary succession; notably, ooids in the lower part of the unit (FA2) exhibit fewer phosphate laminae and a more circular shape, in contrast to those in the higher parts of the section (FA4), which are characterised by increased ellipticity and a greater number of phosphate laminae (Fig. 6.). In addition, a range of diagenetic processes was identified, such as compaction, silicification, phosphatization, and bioclast dissolution, underscoring the complex nature of the mudstones. The presence of sutured quartz grains within sandstones also indicates pressure-dissolution processes (Fig. 6.).

The sandstone provenance was evaluated based on the proposals of Basu et al. (1975), Dickinson et al. (1983), Dickinson (in Compton, 1985), and Tortosa et al. (1991), and the results helped to address the origin of the detrital components in the thin sections studied (Fig. 7.). In the lower levels of the succession (FA1), a higher proportion of metamorphic lithics was observed This observation indicates that igneous bodies and areas with a higher metamorphic grade of the orogenic belt were gradually exhumed and eroded. The sorting is better in higher stratigraphic levels (FA4 and FA5), with a predominance of low-grade metamorphic and sedimentary lithics. Finally, sedimentary provenance classifications showed that most of the provenance areas correspond to the orogenic recycling of metamorphic belts.

Fig. 7
figure 7

Ternary textural and compositional classification, and sandstone provenance diagrams of the Devonian succession at Las Pilas Section (Folk 1980; Mount 1985; Dickinson et al. 1983; Dickinson 1985; Dickinson in Compton 1985)

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Organic geochemistry

The results from Rock-Eval pyrolysis and total organic carbon content (TOC) within the studied section revealed the quantity and quality of the organic matter in the fine-grained rocks. This aided in the determination of potential hydrocarbon source rocks and gave additional insights into the chemistry of the depositional systems. For example, key parameters such as Tmax (temperature at which the S2 peak forms in Rock-Eval pyrolysis) and hydrogen index (HI, a measure of the quantity of hydrocarbons generated per unit of organic carbon), indicate the type and maturity of the organic matter.

Thirty samples along the sedimentary succession were selected for TOC estimation, and their values are included in Supplementary material 2. The values range between 0.027 wt% and 1.75 wt%, and the FA4 (particularly in the F6) contains the higher values in the studied section (1.75 wt%). According to Peters and Cassa (1994), these TOC values classify the samples as acceptable to good in terms of their organic matter quantity (Fig. 8).

Fig. 8
figure 8

Quality of the organic matter within the Devonian succession at Las Pilas Section. a Photomicrograph of tabular phytoclasts at 2 m. b Photomicrograph of amorphous organic matter at 275 m. c Relationship between Tmax and hydrogen index (after Peters and Cassa 1994)

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Furthermore, seven samples with TOC values higher than 0.5% were selected for Rock-Eval pyrolysis analyses, and their values are included in Supplementary material 2. The first data visualization included the relationship between the Tmax and the hydrogen index (IH), which shows that the samples have Tmax values ranging from 550 °C to 600 °C and IH values ranging from 10mg HC/g TOC to 95mg HC/g TOC. Also, the samples have thermal maturity corresponding to the dry gas window. (Fig. 8).

Finally, three samples along the succession were selected for kerogen visual analysis (Fig. 8) and revealed tabular phytoclasts corresponding to tracheids and translucent palynomorph fragments (only on FA2 and FA5; D/E organofacies sensu Pepper and Corvi 1995), and amorphous organic matter (FA2, FA4, and FA5; B organofacies sensu Pepper and Corvi 1995). Also, in all samples, the TAI values are 4 (Fig. 8).

The current hydrogen index values might not accurately represent the original conditions of the organic matter. Applying models by Banerjee et al. (1998) and Chen and Jiang (2015) through non-linear regressions, we estimated original hydrogen index (HIo) values at 300 mg HC/g TOC (Fig. 8., model B) and 250 mg HC/g TOC (Fig. 8., model A), respectively. We also estimated HIo values at 680 mg HC/g TOC based on new XRF datasets (section "Lithogeochemistry".), considering the behaviours of elements like Cu, Ni, or Mo during thermal evolution (Hong et al. 2018).

However, these estimations seem high given previous sedimentology and geochemistry constraints. The well-oxygenated sedimentary environments (section "Facies analysis and petrography".) could have hindered organic matter preservation and hydrocarbon generation. The HIo values from regression models are thought to represent pre-maturation organic matter conditions. But these values should be interpreted with caution as they depend on mathematical models and may not accurately reflect actual source rock conditions. Further research into the Devonian succession’s immature thermal conditions could refine these models.

Lithogeochemistry

The XRD, XRF, and SGR analyses of samples from the studied section provide further insight into the chemistry of the depositional systems. For instance, the presence of certain elements or minerals can indicate the salinity, the redox, or the temperature of the water (e.g. Königshof et al. 2017).

XRF analyses were conducted on mudstones and sandstones in the studied section (Supplementary Material), and the results were analysed using Principal Component Analysis (PCA) (Fig. 9). The first principal component (PC-1) revealed positive weights for elements associated with indications of sedimentary condensation and clays, while negative weights were observed for elements related to micas, heavy mineral-rich sandstones, and samples with high organic matter content.

Fig. 9
figure 9

Principal component analysis (PCA) for the PC-1 and PC-2 for the individuals and variables in samples along the Devonian succession at Las Pilas Section

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The second principal component (PC-2) showed positive weights for iron-rich minerals and negative weights for sulfur-rich minerals. When facies association was included in the PCA, PC-1 grouped FA5 with higher weights, indicating its separation from other associations and highlighting the presence of higher mica content and heavy minerals in the upper segment of the sedimentary succession (Fig. 9.). On the other hand, the higher positive weights in FA4 from PC-2 might indicate conditions associated with sulfide minerals or organic matter (Fig. 9.).

The XRF results (Fig. 10), revealed element trends in the studied section. Notably, the upper segment (FA5) shows an increased content of potassium oxide (K2O), typically associated with the alteration of feldspar minerals under relatively warm climates. Concurrently, the heightened levels of titanium oxide (TiO2) and ferric oxide (Fe2O3) suggest oxidizing conditions. Conversely, increased Molybdenum content, Cu+Mo/Zn ratio, and sulfur trioxide (SO3) content (Hallberg 1976; Königshof et al. 2016) in FA4 indicate dysoxia and high organic matter content, suggesting a relatively cool and reducing depositional environment. Additionally, increased content of phosphorus pentoxide (P2O5) in FA3 and FA4 points to an increase in productivity in the sedimentary environment.

Fig. 10.
figure 10

Sedimentological, stratigraphical, mineralogical, and geochemical trends along the Devonian succession at Las Pilas Section. Details in text

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XRD analyses were performed on bulk, glycolated, and calcinated fractions from both mudstones and sandstones in the studied section. The results were then compared with petrographic descriptions (Supplementary Material, Fig. 10.). The Las Pilas Section revealed proportions of quartz (≥60%), illite-muscovite (≥30%), and goethite (≥10%) among mineral groups (Fig. 10). Conversely, the upper parts of the Cuche Formation displayed an increased presence of pyrite (≥5%), hematite (≥10%), goethite, plagioclases (2%-5%), micas, and heavy minerals. These findings closely correspond to those in previous studies (e.g. Cardona et al. 2016), while the proportions of quartz and kaolinite decreased. The limited occurrence of calcite and siderite in samples with bioclasts (e.g. FA3) is attributed to silicification and bioclast dissolution processes, as confirmed by petrographic observations. The presence of phosphate ooids in facies associations (e.g. FA2, FA3) is associated with higher phosphate concentrations, peaking within FA4. Petrographic descriptions reveal that the principal clays identified in XRD across the entire succession are kaolinite, which exhibits a massive texture and comprises ≥10% of the clay content, and illite-muscovite, which has a leafy and laminated texture and comprises ≥30% of the clay content. However, further research, such as SEM analysis, may be necessary to distinguish the proportions of muscovite and illite within the illite-muscovite group, as its diffraction pattern was similar across all three fractions (bulk, glycolated, and calcinated).

Figure 10 illustrates the SGR trends in the studied section, where total counts per second (cps) typically exceed 150 cps. Throughout the sedimentary succession, uranium (U) content modestly varies from 3 to 6 ppm, thorium (Th) content exhibits slight fluctuations ranging from 20 to 30 ppm, and potassium (K) content maintains a relatively stable range between 3 to 4%. Additionally, the Th/U ratio fluctuates between 5 and 8, while the Th/K ratio falls within the range of 6 to 9 ppm/percent. The overall consistency in the ranges of the Th/K and Th/U ratios, coupled with the aforementioned results, suggests a sedimentary environment with subtle variations such as changes in redox conditions, biological activity, sediment sourcing, or weathering intensity, rather than pronounced trends. Despite discrete intervals with minimal influence from oxidation-reduction processes (e.g. Fig. 10 at 150 m), it is important to note that high organic matter contents are not expected in the overall sedimentary environment.

Lastly, we present the results of the INPEFA analysis applied to SGR data throughout the studied outcrop (Fig. 10). The results reveal a series of well-defined cycles corresponding to variations in sediment composition, for example, aligning with coarsening-upward facies associations (see Fig. 10 at 180 m). These cycles correlate well with the observed variations in U, Th, and K contents, suggesting a relationship between sedimentary and geochemical processes. The identification of these cycles through the INPEFA technique provides a fresh perspective for interpreting the stratigraphy of the studied outcrop (see section "Stacking patterns and stratal architecture".).

Discussion

Geochemistry and dynamics of the depositional setting

The Rock-Eval pyrolysis results provided limited insights into the organic geochemistry of the sedimentary succession. Despite this, the identification of type D/E organofacies (Pepper and Corvi 1995) in FA1, FA4, and FA5 supports our interpretations. The visual analysis of kerogen corroborates the presence of distinct organic matter types within the samples. FA4 predominantly contains amorphous organic matter, while FA1 and FA5 exhibit higher proportions of vitrinite and exinite, with a lesser contribution of amorphous organic matter. The sedimentological analysis reinforces these findings, indicating well-oxygenated, shallow-marine environments for FA4 and coastal to terrestrial environments for FA1 and FA5.

The observed high Tmax values exceeding 500°C may initially hint at significant thermal evolution within the sedimentary succession, potentially indicative of metagenesis or low-grade metamorphism. However, petrographic analyses primarily reveal processes associated with high temperature diagenesis, and the thermal alteration index (TAI) values of 4, align more closely with advanced diagenesis than metamorphism (e.g. Hartkopf-Fröder et al. 2015). Nevertheless, these diagenetic processes (e.g. pressure dissolution, silicification, and compaction) have likely contributed to the reduction of interstitial water and the overall decrease in total organic carbon (TOC) content (e.g. Spötl et al. 1996).

Similarly, the elemental trends observed in the XRF analysis and SGR measurements corroborate the conclusions drawn from the Rock-Eval pyrolysis and kerogen visual analysis. The higher concentrations of potassium oxide (K2O), titanium oxide (TiO2), and ferric oxide (Fe2O3) in the upper segment (FA5) are consistent with relatively warm and oxidizing conditions during deposition. This aligns with the well-oxygenated shallow-marine environment inferred from the sedimentological analysis. In contrast, FA4, with its elevated levels of molybdenum (Mo), copper-plus-molybdenum-to-zinc (Cu+Mo/Zn) ratio, and sulfur trioxide (SO3), points to a cooler and reducing depositional environment. Such an environment, often linked to low-energy conditions, favours the preservation of organic matter, resulting in a higher organic matter content. The presence of type D/E organofacies in this facies association further reinforces this. It is important to note that while these proxies might suggest a cool and reducing environment, depositional environments can be dynamic, and conditions may vary with depth. The facies descriptions in our study primarily reflect conditions from the shoreface transition to offshore and lower shoreface (section "Facies analysis and petrography".), and the interpretation of proxies should consider the potential variations associated with changing depths and environments within this range.

Additionally, the increased content of phosphorus pentoxide (P2O5) in FA3 and FA4 may suggest enhanced productivity in the sedimentary environment (e.g. Algeo and Liu 2020), which may have been related to processes fostering the attachment of phosphates to coated grains. The attachment of phosphates to coated grains could be influenced by agitation, and through bacterial growth with optimal salinity, temperature, and productivity conditions (e.g. Diaz and Eberli 2019; Riaz et al. 2023). However, in the context of the depositional processes of the succession, especially in FA3 (see section "Facies association 3 (FA3): Proximal storm-dominated lower shoreface and offshore-shoreface transition with fossiliferous siltstones and shell pavements".), the phosphate grains may become enriched by frequent and prolonged storm reworking (e.g. Dattilo et al. 2019). While the exact mechanisms require further investigation, the potential processes could be indicative of dynamic sedimentary conditions.

Finally, although the assertion of hydrocarbon expulsion of the studied succession requires additional geochemical evidence and thermodynamic modeling, a viable oil source rock seems improbable considering additional factors such as 1) the restrained lateral distribution of the Devonian succession in the subsurface with expected terrestrial or coastal facies implying high proportions of D/E organofacies (e.g. Pastor-Chacón et al. 2023); 2) findings from previous mass balance studies conducted on global source rocks with low present-day TOC values (≤ 1.0% TOC), which have demonstrated that such rocks are generally incapable of generating sufficient hydrocarbons for migration (e.g. Pepper and Dodd 1995).

Stacking patterns and stratal architecture

The analysis of stacking patterns and stratal architecture enhances the understanding of depositional processes and basin dynamics during the sedimentary succession. The identification of transgressive surfaces (TS) and regressive surfaces (RS) has enabled the recognition of distinct parasequences within the studied section. As previously stated (see above section "Geological framework"), the Las Pilas section spans from the late Emsian to the early Frasnian with no indications of breaks or unconformities, although the temporal boundaries depicted in Figure 3 and Figure 10 are approximate and may be subject to refinement in future studies. Furthermore, it is possible that in other locations near the study area, levels extending into the late Frasnian may be preserved (e.g. Potrero Rincón locality from Berry et al. 2000); however, in the Las Pilas section, only a portion of the early Frasnian succession is present due to erosion at the base of the Cretaceous unconformity.

The parasequences observed in the sedimentary succession exhibit variations in thickness, lateral extent, and internal architecture (Figure 3). The first type of parasequences (ca. 1–3 m; type 1 Fig. 11 and photo D Fig. 5) with lateral continuity extending for hundreds of meters, indicate relatively stable depositional conditions and gradual accommodation space filling. In our study, these thicker parasequences correspond to facies associations FA3 and FA4, characterised by simple coarsening-upward allochem siltstones to fine-grained sandstones. The presence of mudstones and siltstones within FA4 further suggests deposition in quieter, lower-energy environments at the bases of the parasequences. Additionally, a second type of parasequences (ca. 0.1–1 m; type 2 Fig. 11 and photo D Fig. 4) within FA3 exhibit thinner thicknesses and include fossiliferous siltstones with centimeter to meter-stacked shell pavements. Conversely, the thinner and discontinuous parasequences (ca. 0.5 m; type 3 Fig. 11 and photo G Fig. 5), corresponding to facies association FA5, represent intervals of higher sediment supply, likely associated with increased shoreline progradation or episodic sediment influx.

Fig. 11
figure 11

Integrated depositional model from sedimentology, stratigraphy, and geochemistry of the Devonian succession at Las Pilas Section. Details in text

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The observed stratal architecture within the parasequences provides more in-depth insights into depositional environments and their evolution. Channelized sandstone bodies with erosional bases and lateral accretion surfaces within FA1 imply deposition in fluvial or tidal channel systems. Interbedded mudstone and flaser bedded siltstone layers within FA4 indicate deposition in distal marine or bay environments. The fining-upward sequences within FA5, consisting of bioturbated sandstones, imply deposition in nearshore or terrestrial settings.

Vertical stacking patterns of the parasequences reveal cyclic alternations between larger-scale transgressive and regressive phases. These larger cycles could be attributed to changes in relative sea level, sediment supply, or tectonic influences. Transgressive phases, characterised by retrogradational stacking patterns (e.g. FA4 at 160 m, Figure 3), and containing more prominent and thicker sandstones, signify periods of generally rising sea levels and shoreline transgression. Regressive phases, marked by progradational stacking patterns (e.g. FA5 at 460m, Figure 3), containing a gradual decrease in the size and thickness of sandstone beds compared to previous cycles, indicate falling sea levels and gradual shoreline progradation. The absence of distinct evidence such as continuous subaerial exposure or other markers indicating abrupt changes in depositional environments suggests a more gradual transition between phases (section "Facies analysis and petrography".).

Further insight into depositional dynamics and basin evolution is gleaned from the potential duration of these cyclic alternations between transgressive and regressive phases. The smallest recognised cycles are less than 0.5 m thick, and the largest cycles are represented by sequences (ca. 200–300 m thick). The smallest cycles or parasequences (sensu Banerjee and Kidwell 1991; Fürsich and Pandey 2003; Zecchin and Catuneanu 2013) are characterised by condensed shell beds at the transgressive base, and the overlying maximum flooding zones may contain black shales (e.g. F7). These cycles correspond to the shorter term INPEFA-GR curves, which reflect the high-frequency variations of the gamma ray log (e.g. Li et al. 2023).

Moreover, two larger-scale cycles are recognised, each containing several parasequences and parasequence sets (e.g. Figure 10). These cycles exhibit considerable thickness, with the lower cycle spanning from the late Emsian to the late Givetian and the upper cycle extending from the late Givetian to the early Frasnian. The lower cycle begins with a sharply based, thin, marine coarse-grained siltstones and fine-grained allochem sandstones, inferred as a transgressive lag and corresponding to a large-scale transgressive systems tract. However, the upper cycle commences with a sharply based, thin, very fine-grained sandstones corresponding to a regressive or falling stage systems tract. These larger-scale cycles align with the long term INPEFA-GR curve set, which collectively reflect the low-frequency variations of the gamma ray log and can be used to identify sequence boundaries (e.g. Li et al. 2023; Fig. 10). However, it is important to note that for the upper cycle, INPEFA-GR curves are not available (see section "Limitations and future research".).

A coherent lateral facies development of a shallow marine platform in northwestern South America was proposed previously, hinting at correlations to global sea level curves (e.g. Moreno-Sánchez et al. 2020; Pastor-Chacón et al. 2023, Villarroel and Mojica 1987). We propose that the cyclicity recorded in the Las Pilas Section is related to Devonian coastal onlap curves, primarily influenced by eustatic sea level changes (e.g. Becker et al. 2020; Brett et al. 2011; Johnson et al. 1985). Specifically, the stratigraphic surfaces identified in Figures 3 and 10 predominantly represent the boundaries of second-order cycles, following a similar pattern to the Devonian coastal onlap of Euroamerica. For example, the Ic cycle (sensu Johnson et al. 1985; Brett et al. 2011) appears to be observed in the Las Pilas Section at meter 15, initiating at the base of the storm-influenced fossiliferous siltstone set (Fig 3.). Although other correspondences may exist along the section, caution is required in its interpretation, as the temporal boundaries are approximate and are subject to refinement.

The sedimentation rate estimated from the studied succession, although not exceptionally low (ca. 0.23 m/Ma), is comparable with other basins (e.g. Northern Appalachian Basin, Brett and Baird 1996). This rate may suggest a reasonably consistent sediment supply and accommodation space extending from the late Emsian to the early Frasnian. Although the overall sedimentary input to the basin is typically consistent, it is crucial to recognise minor fluctuations in sedimentation rates, which are evident in cycles of varying thicknesses and sedimentary condensation. Additionally, it is important to highlight the stability of the basin over time, with no apparent lateral variations in sedimentary environments within the study area or interruptions in sedimentation. Similarly, our findings from petrography (see Section "Petrography".) and previously published U/Pb detrital data hint towards two distinct source areas that also seem stable in time: cratonic areas and erosion from established and partially denuded orogenic belts (e.g. Horton et al. 2013; Cardona et al. 2016).

This observation aligns with previously proposed Devonian tectonic models in northwestern Gondwana, which suggest a transtensional regime resulting in a half-graben basin with oblique faults and a waning arc showing restrained magmatic activity (e.g. Pastor-Chacón et al. 2023). However, the thickness variations observed within the basin cannot be explained solely by the effects of the sediment supply and the basin architecture. This suggests that other factors, such as the proximity to basement highs and the timing of tectonic events, played a role in the sediment accumulation and preservation (e.g. Stets and Schäfer 2011).

Finally, the presence of miospore species identified in other sections of Colombia (e.g. Boinet et al. 1986) could assist in delineating the position of the Choteč, Kačák, and Taghanic events, which are not ruled out from occurring in the Las Pilas Section. Of particular interest is the potential presence of the Choteč event, which should be registered within the Ic cycle in the early Eifelian (Fig. 3).

Integrated depositional model

The multidisciplinary approach of this research has provided a robust framework for deciphering the complex depositional history of the studied sedimentary succession. Figures 10 and 11 present the integrated depositional model derived from a synthesis of sedimentological and geochemical observations.

During the late Emsian (meter 2 at Fig. 3; Fig. 11), the beginning of the lower sequence registers a transgression event, which includes the deposition of FA2 onlapping FA1. Sedimentary processes during this stage were driven by turbulent flows resulting from wave action, gradually losing energy, and depositing finer sediment fractions. Higher Rb/Sr and Th/U ratios suggest bottom circulation, enhanced terrigenous input, and reduced bioturbation activity (including ichnotaxa as Asterosoma). Similarly, the presence of parautochthonous coated grains implies water agitation, adequate salinity, temperature, and productivity conditions for the proliferation of phosphate-fixating organisms around terrigenous grains in the shallow parts of the platform (Diaz and Eberli 2019; Riaz et al. 2023). The main clay proportions correspond to kaolinite (70%) and lesser proportions of illite/muscovite (30%), and other mineral species include goethite (10%).

Progressing up in the lower sequence (late Emsian - early Eifelian; meter 10 at Fig.3), it records the build-up of a storm-influenced, rimmed platform with intermittent mixing, continuing the transgressive trend, including the deposition of FA3. The sedimentary processes were interpreted as evidence of low sediment input or sediment starvation, although they occurred at the sub-wave base level (containing ichnotaxa as Nereites, Planolites or Thalassinoides), favouring the stacking of shell pavements. Although the Th/U ratios are high, the Rb/Sr proportion decreases as a result of the marine contribution. Similarly, the establishment of tabulate corals and bryozoans in life positions denotes soft substrates, high nutrients, and low water turbidity despite water agitation (e.g. Sanchez and Benedetto, 1983).

During the late Eifelian and the early Givetian (meter 160 at Fig. 3; Fig. 11), in the lower sequence, the transgressive trend peaks recording the maximum flooding of the succession (Figure 3, F7, monospecific brachiopod coquina), including the deposition of FA4. Sedimentary processes suggest the deposition of grains through settling and suspension, occasionally punctuated by low-energy episodes resulting in sand lenses, microerosive shell laminations, and coarsening-upward bedsets containing ichnotaxa such as Rosselia, Paleophycus, Thalassinoides, and Skolithos. The Th/U ratio decreases, revealing poor oxygenation in the maximum flooding zone, but gradually increases upward proportionally with the Ba/Sr ratio in the upper part of the lower sequence, indicating limited freshwater input. This input, however, is not significant enough to suggest brackish conditions given the presence of normal marine faunas. In the same way, the presence of small-sized trilobites, pectiniids, chonetiids, spiriferids, and disciniids within the finer lithologies, along with higher water turbidity, suggests a dysoxic setting with limited water circulation (e.g. Racheboeuf 1990).

Finally, during the late Givetian and the early Frasnian (meter 360 at Fig.3; Fig. 11), the upper sequence exhibits a regressive trend, indicating a shoreline progradation and including the deposition of FA5. Sedimentary processes during this stage predominantly reflect low-energy conditions, characterised by fine grained sediment settling. Nevertheless, episodes of high-energy flow created distributary channels containing ramified root remains and ichnotaxa like Psilonichnus. The relatively warm and oxidizing conditions promote bioturbation processes, the precipitation of fine-grained hematite and goethite, and increase the proportions of TiO2, Fe2O3, and Th/U ratios. The increased proportions of heavy minerals and illite-muscovite minerals correspond to their higher mineralogical stability (e.g. Cardona et al. 2016), facilitating reworking by fluvial action, while fine-grained kaolinite was removed from sediment. Similarly, the transition to overall oxidizing conditions corresponds to environmental changes indicative of terrestrial conditions, leading to the limitation of marine invertebrates such as brachiopods, bivalves, and trilobites.

Limitations and future research

This study has provided valuable insights into the sedimentary history and geochemical characteristics of the examined succession. However, it is essential to acknowledge its inherent limitations. The handheld portable XRF device utilised for geochemical analysis has detection limits, restricting its capabilities to certain elements such as sodium (Na), carbon (C), fluorine (F), and trace elements like vanadium (Va), cobalt (Co), yttrium (Y), and niobium (Nb). The absence of data for these elements may introduce uncertainties in the overall geochemical interpretations.

Additionally, the study faced logistical challenges that resulted in the absence of spectral gamma-ray data from the uppermost parts of the succession. This data gap hinders a more precise characterisation of compositional changes and sedimentary environments, as well as a more reliable correlation with well log data. Future studies could benefit from conducting spectral gamma-ray measurements at more comprehensive and representative intervals throughout the succession under investigation.

The relatively low resolution of our sampling strategy is another significant constraint. More detailed and closely spaced sampling intervals could enhance our ability to precisely delineate anoxic peaks and capture subtle variations in the sedimentary record. Lastly, the study’s reliance on original hydrogen estimations is constrained by the limited availability of data from immature samples. To improve predictions and enhance our understanding of source rock kinetics, future research would greatly benefit from a larger and more diverse database encompassing a broader range of sample maturities.

Furthermore, future research efforts should focus on enhancing the analysis of specific fossil associations, their taphonomy, and inferred palaeoecology, building upon the preliminary descriptions provided in the present study. Additionally, there is a need for more detailed investigations into the timing of events and cycles to provide a comprehensive understanding of the sedimentary dynamics.

Conclusion

The Las Pilas Section contains a well-preserved ca. 530 m-thick late Emsian-early Frasnian succession, enabling the interpretation of diverse depositional environments and geochemical constraints. Most of these environments within the succession are characterised by mixed-marine conditions, spanning from the lower shoreface to the offshore-shoreface transition. Certain sections also correspond to nearshore or terrestrial environments. Supported by geochemical proxies, these depositional processes indicate conditions conducive to organism proliferation, including factors such as water circulation, terrigenous influx, appropriate salinity, nutrient availability, temperature, productivity, and low water turbidity.

The analysis of stacking patterns and stratal architecture has revealed significant transgressive and regressive surfaces associated with small and larger cyclic alternations. These alternations provide evidence of a relatively stable sedimentary input, consistent with previous tectonic models that propose an active margin setting in northwestern South America. While the sedimentary input to the basin can generally be considered stable, it is important to note subtle variations in sedimentation rates, as evidenced by cycles with varying thicknesses and sedimentary condensation.

In summary, the Las Pilas Section offers valuable insights into past environmental conditions and organic matter preservation. Despite thin, limited intervals exhibiting minimal influence from oxidation-reduction processes, the overall sedimentary environment makes organic matter preservation difficult, affecting its distribution and quality. Specifically, this suggests that the sedimentary succession preserved in the Las Pilas Section is unlikely to serve as a viable oil source rock.

Future research endeavors are essential to address several critical aspects. These include not only investigating the lateral extent of the proposed sedimentary environments and capturing subtle variations in the sedimentary record to precisely delineate hypoxic peaks, but also conducting more careful study and understanding of the fossils and their preservation, along with their palaeoecological indications. Additionally, refining our understanding of source rock kinetics will be essential for advancing our knowledge of this geological system.