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INTRODUCTION
Transitional marine depositional environments characterize areas where sediment is transferred from the continent to the marine realm (Boyd et al., 1992). Much of this sediment is carried by rivers and deposited in the form of deltas (Bhattacharya, 2006, 2010). The general morphology of deltas is the result of the interaction between fluvial, tidal and wave processes that rework the sediment provided by the river, the dominance of one over the others, and relative changes in sea level (Galloway, 1975; Boyd et al., 1992; Olariu and Bhattacharya, 2006; Bhattacharya 2010; Ainsworth et al., 2011). However, the general predominance of one of the processes over the others does not imply the total absence of typical facies of secondary processes, in addition, the dominant process can change laterally and in time (Boyd et al., 1992; Orton and Reading, 1993; Bhattacharya, 2006).
Organisms interact with the substrate in response to environmental factors. Ichnological studies provide detailed information on the environmental parameters involved during sediment deposition, and therefore serve as the basis for an analysis of the facies and the sedimentary environment (Seilacher, 1964a, 1967b; Frey and Seilacher, 1980; Bromley et al., 1984; Frey and Pemberton, 1984, 1985, 1987; Bromley, 1990, 1996; Pemberton et al., 1992b; Bromley and Asgaard, 1993a; Lockley et al., 1994; Buatois and Mángano, 1995b, 2009; Genise et al., 2000, 2010a; Ekdale et al., 2007; Hunt and Lucas, 2007). Deltas represent very unstable and stressful environments for benthos (MacEachern et al., 2005; Dasgupta et al., 2016), since organisms are affected by a variety of factors like type of sediment, energy, turbidity, salinity, oxygen level, sedimentary rate and food availability. As a result, trace fossil associations are sensitive indicators of physico-chemical stresses, and could be very helpful to determinate the dominant processes in deltaic sedimentation (MacEachern et al., 2005, Buatois and Mángano, 2011)
The Lajas Formation (Middle Jurassic, Neuquén Basin) records sediments mainly deposited in transitional marine environments, which have been interpreted primarily as deltaic systems. In the literature there have been different interpretations in outcrops regarding the processes that dominate these deltaic systems: fluvial, wave and tidal processes (Gulisano and Hinterwimer, 1986; Poiré and del Valle, 1992; Zavala, 1996a, 1996b; McIlroy et al., 2005; McIlroy 2007; Rossi and Steel, 2015; Gugliotta et al., 2015, 2016a, 2016b, 2016c; Canale et al., 2015, 2016; 2020; Kurcinka et al., 2018). In contrast, there are few studies carried on in subsurface (Veiga et al., 2013), and most of them are located in the engulfment area, using seismic data (Gómez Omil et al., 2002; Freguglia et al., 2009; Brinkworth et al., 2017; Vocaturro et al., 2018. The aims of this contribution are twofold: 1) to define the sedimentary paleoenvironments (including ichnology) where Lajas Formation was accumulated using data from the subsurface in the engulfment area of the basin; 2) to describe and interpret the stratigraphic evolution of the unit.
GEOLOGICAL SETTING
The Neuquén Basin is located in the west central of Argentina, and a small fraction in central Chile (Franzese et al., 2006; Spalletti et al., 2010). It covers an area of more than 200.000 km2 (Yrigoyen, 1991). It is bounded by wide cratonic areas, the San Rafael System to the northeast and the North Patagonian Massif to the south, and by a magmatic arc on the active western margin of the Gondwanan-South American Plate (Spalletti et al., 2010) (Fig. 1). The basin has a broadly triangular shape, and three main regions are commonly recognized: the Main Cordillera to the west and north, the Patagonian Cordillera to the west, and the embayment area to the east (Ramos et al., 2011). The basin starts with a volcanic rift in the Triassic and evolved to a post-rift stage during the Jurassic, ending with a foreland stage that spans from the Late Cretaceous to the Cenozoic (Howell et al., 2005, Spalletti et al., 2005). The result is an almost continuous sedimentary record of ca. 7,000 m of marine and continental deposits, from the Late Triassic to the Paleocene (Arregui et al., 2011a) (Fig. 1).
Figure 1 Study area and stratigraphic context. a) Neuquén Basin, with marine ingressions delimited. Study area in red. (Modified from Arregui et al., 2011a). b) Stratigraphic column of the Neuquén Basin. Detail of the Cuyo group, showing the arrangement of second-order courtships (taken from Arregui et al., 2011b).
The Cuyo Group (Dellapé et al., 1978a), deposited during Sinemurian to middle Callovian, represents the first mayor marine flooding that covers the entire basin (Arregui et al., 2011b). The Los Molles Formation (Wever, 1931) is the basal unit of the Cuyo Group, and its composed of gray and dark gray shales, which alternate with fine to coarse sandstone and conglomerates, as well as limestone and gray marl (Leanza et al., 2001; Llambías and Leanza, 2005). Overlaying Los Molles Formation is the Lajas Formation (Weaver, 1931), which is mainly composed by sandstones and to a lesser extent by dark green shales with abundant plant debris and conglomerates (Zavala 1996a, b; McIlroy et al., 1999). These deposits accumulate in marginal marine settings, mainly interpreted as deltaic systems (Spalletti, 1995; Zavala, 1996a, b; McIlroy et al., 2005). The 200-900 m thick Lajas Formation is regarded as Bajocian-Bathonian in age based in ammonoid zonations (Riccardi, 2008; Dietze et al., 2012) (Fig 1). In the subsurface at the engulfment area, the Lajas Formation is covered by sandstones, conglomerates and fluvial red clays of the Punta Rosada Formation. (Digregory, 1972).
The Lajas Formation in subsurface
In subsurface, the Lajas Formation constitute one of the traditional oil reservoirs along the Huincul Ridge, and it has a great potential as a tight-sand gas reservoir (Arregui et al., 2011b, Giusiano, et al., 2011). Most of the studies on subsurface of the Cuyo Group are focused on the southern sector of the basin, mainly in the engulfment area and the Huincul ridge and uses mainly seismic data (Gómez Omil et al., 2002, Freguglia et al., 2009, Brinkworth et al., 2017; Vocaturro et al., 2018) (Fig. 1). In these studies, different number of sedimentary sequences have been proposed: four (Gómez Omil et al., 2002), nine (Freguglia et al., 2009; Brinkworth et al., 2017) and ten (Vocaturro et al., 2018). The different number of sequences recognized in the previous works puts in evidence that the evolution of the progradation of clinoforms is directed in an East-West direction, which is influenced by changes in subsidence caused by tectonic activity related to the Huincul Ridge. In the Sierra Barrosa area, the informal division proposed by Freguglia et al. (2009) is followed in subsurface. In this model, nine depositional sequences are recognized, grouped in three stratigraphic intervals: Upper Cuyo Group (sequences 1, 2, 3), Middle Cuyo Group (Sequences 4, 5, 6), and Lower Cuyo Group (sequences 7, 8, 9). In the study area these divisions are practically ascribed to the Lajas Formation, and, therefore, the previously mentioned intervals are regarded as “Lower Lajas”, “Middle Lajas” and “Upper Lajas”. In this sector, the Lajas Formation has the greatest thickness, and the Los Molles (lower) and Punta Rosada (upper) Formations, are only thin strata in sequences 9 and 1 respectively (Fig. 2).
MATERIALS AND METHODS
Eight core samples of the Lajas Formation were studied in the Huincul Ridge and the engulfment area. The wells are located in the Loma La Lata-Sierra Barrosa exploration block and the Dadin 1 block (Fig.2). In this area, the Lajas Formation is 800 m thick, and the total length of the studied core samples is 346 m. For confidentiality reasons, the names of the wells have been altered. The core samples studied are: for the “Lower Lajas” interval: Aguada Toledo 1 (AgTo-1), Barrosa 1 (Ba-1), Aguada Toledo 2 (AgTo-2), Barrosa Norte 1 (BaN-1) and Huincul Norte 1 (HuN-1); from “Middle Lajas”: Barrosa 2 (Ba- 2), Aguada Toledo 3 (AgTo-3) and Aguada Toledo 4 (AgTo-4); for the upper section Aguada Toledo 4 (AgTo-4) is considered “Upper Lajas” (Table 1).
A facies description was made, using the methodology proposed by Miall (1978). In order to make an interpretation of the environmental evolution of each of the core samples, the described facies were grouped according to their spatial relationships and the interpretation of the processes that originated them, following the criteria of Collinson (1969) (Table 2). The methodology for ichnological analysis employed in this study follows conventional practices (Pemberton et al., 1992, 2001; Gerard and Bromley, 2008; Knaust, 2017). Bioturbation intensity was recorded at intervals of 10 cm, using the bioturbation index (BI) of Taylor and Goldring (1993). The trace fossils were identified using ichnotaxobases (Bromley, 1990, 1996). The ichnodiversity is the number of ichnotaxa observed in 10 cm intervals. The sedimentological and ichnological analysis were performed on the 1/3 of the core slabbed, using a microscope Leica S8APO, with a Leica MC170camera.The images were processed using Leica LAS EZ software.
SEDIMENTOLOGY AND ICHNOLOGY
Facies Association 1: delta front mouth bars
Sedimentology and ichnology description. The FA1 consists mainly of up to 5-10 m thick amalgamated sandstone bodies with subordinate participation of conglomerate, heterolithic deposits and siltstones. The different lithologies displays vertically forming clear coarsening upward units (0,6-2 m thick). Sandstone beds show irregular erosive bases and are typically structureless (Sm), pervasively bioturbated (Smb) or show trough cross-bedding (Set and SGt). Conglomerates are structureless or show trough cross-bedding (Gm and Get). Silstone beds are typically structureless and the heterolithic deposits display wavy and lenticular bedding (Fm, Htw, Htl). Terrestrial plant remains and organic particles (phytodetritus) are abundant in this facies association. In general, the arrangement of this facies association is usually coarsening upwards strata stacking patterns, with fine sediments strata (Fm, Htw; Htl) at the bases of the successions, and sandy and conglomerate bodies upwards. This FA could be founded on top of FA5, and it could be interbedded with FA2 (Fig. 3).
Figure 3 Facies Association 1 (FA1), delta front mouth bars. Trough cross-bedding sandstone (Set), massive sandstone (Sm), heterolithic wavy (Htw), heterolithic flaser (Htf), massive siliciclastic mudstones (Fm), laminated siliciclastic mudstones (Fl). a, b) Skolithos/Rosselia ichnofacies: Op: Ophiomorpha irregulaire, Gy: Gyrolithes isp., Hz: Haentzchelinia isp., Phz: Parahaentzschelinia isp., Th: Thalassinoides isp., Sk: Skolithos isp., c) White arrow pointing the siphon structure in Parahaentzschelinia isp.
This FA is characterized by trace fossils of complex composition, of varied three-dimensional structures, with vertical and horizontal components. The diversity of trace fossils is highly variable, from low to high (1 to 6 ichnogenera), with also variable bioturbation index values (BI 1-6), being generally medium (3-4). It is dominated in order of abundance by Ophiomorpha irregulaire, Gyrolithes isp., Haentzschelinia isp., Parahaentzchelinia isp., Lockeia isp. and subordinate Chondrites isp., Thalassinoides isp. Planolites isp., Rhizocorallium isp. Skolithos isp., and Arenicolites isp. There are also cryptobioturbation in some levels. Ophiomorpha irregulaire and Gyrolithes isp. are the dominant traces; “elite” traces, sensu Bromley (1990, 1996). This concept not only implies that they are the most abundant traces and the most noticeable, but also due to their penetration capacity, they can obliterate other trace fossils and bioturbate deep levels below the water-sediment interface. As an example, O. irregulaire has been found even at conglomerate bar bases. They are also one of the largest traces, both in size of the gallery and the general structure of the trace. Most of the members of this association of trace fossils are interpreted as detritivores, with a low representation of suspensivorous animals (Fig 3 a, b, c).
Sedimentology and ichnology interpretation. The coarsening upwards arrangement, the sedimentary structures that indicate unidirectional currents, as well as its relationship with FA2 (see below) suggest that FA1 was accumulated in a situation of mouth bars in a delta front depositional environment (Bhattacharya 2010; Schomacker et al., 2010). These deposits are interpreted as a succession of sand bodies representing mouth bars interbedded with finer deposits (heterolithic) identified as bar foot areas or interbars. These bars were formed by fluvial tractive flows that decelerate when the distributary channels enter the sea, causing accumulations of sand at their mouths (Ainsworth et al., 2016; Kurcika et al., 2018; Van Yperen et al., 2020). The different facies present refer to different depositional processes, being the Sm, Smb, Set, SGt, Gm, Gmpi facies deposited by fluvial-derived flows of variable density. Of the previous facies, the Gmpi, Gm, SGt facies present the greatest evidence of high-density fluxes since they have variable energy episodes in a single rock body. Facies Smb is interpreted as a product of fluvial deposition that is affected by marine processes (marine bioturbation). The heterolithic deposits, represented by the Htw, Hts facies mark episodes of lower energy, which alternate deposition by settling, with deposition by traction and / or traction settling of thicker sediments originating in minor fluvial avenues. These deposits are interpreted as interbar or distal fringe, since they are usually found between successions of bars, and do not necessarily show a deepening of the system, which would lead to interpret them as deposited in a prodelta environment (Ainsworth et al., 2016; Van Yperen et al., 2020).
The foreset of the bars present the coarsest grain sizes, and sedimentary structures that suggest a rapid deposition produced by the influences of rivers, while the bar foot is where the finest sediments accumulate, as they are the areas of lower energy (Enge et al., 2010). Sand is deposited during highenergy events, and when the event loses energy, the finest material is deposited by settling from suspension (Gugliotta et al., 2015, Kurcinka et al., 2018). The scarce representation of wave- and tidalgenerated structures suggest a deposition in a fluvialdominated environment (Canale et al., 2015, 2016). The dominance of trace fossils assignable to detritivore strategies would lead to attribute them to Cruziana’s ichnofacies, however, the fact that most of these structures are preferably vertical, and that overall energy of the system is high leads to think that it actually could be and expression of the recently prosposed for deltaic environments Rosellia ichnofacies (MacEachern and Bann 2020; Moyano- Paz et al., 2022). This trace fossil association has its suspensivorous components limited by the amount of sediment in suspension, since only in a few places the presence of typical Skolithos ichnofacies components is observed. This may be due to the suppression of the suspensivorous component of the Skolithos ichnofacies product of the high amount of suspended sediment, a common situation in deltaic environments (MacEachern, et al., 2005, 2007, MacEachern and Bann 2020, Buatois and Mángano, 2011, Moyano-Paz et al., 2022).
Facies Association 2: Distributary channels
Sedimentology and ichnology description. the FA2 is composed mainly by interbedded conglomerates (Gm, Gmpi, Get) and sandstones (SGt, Sm, Set), with a highly variable thickness, ranging from 1 m up to 10 m. These coarse-grained bodies are disposed vertically forming fining upwards strata stacking units. The strata are dominated mainly by massive structures (Gm, Sm) sometimes with rip up mud clasts (Gmpi). Trough cross-bedding structures are less represented (Get, SGt, Set), and sometimes they become diffuse. The bases of these units are mainly erosive, but locally net and even transitional contacts have been observed. This FA2 usually erodes deposits from FA3 and also FA 1. The FA2 never show bioturbation of any kind (Fig. 4).
Figure 4 Facies Association 2 (FA2), distributary channels. Massive conglomerates (Gm), trough cross-bedding conglomerates (Get), trough cross-bedding sandstone to conglomerates (SGt), trough cross-bedding sandstone (Set), massive sandstone (Sm). References as in figure 3.
Sedimentology and ichnology interpretation. The presence of erosive bases, the general fining upwards strata stacking pattern trend and variations in grain size within the same bed suggest deposition within channels under high-energy conditions, showing typical fluctuations of river discharges (Bhattacharya, 2006). The high energy and high deposition rates prevent the development of benthos, which is the reason why they are never bioturbated (MacEachern et al., 2005). Commonly during periods of low river discharge, tides can move inland through these types of channels and favoring conditions for colonization by benthic organisms (e.g., Moyano-Paz et al., 2020). This doesn’t happen in this case, so these channels were not affected by the action of tides. These deposits are interpreted as the infill of distributary channels in the deltaic plain, sometimes reaching the proximal delta front, eroding the mouth bar deposits.
Facies Association 3: Interdistributive plain
Sedimentology and ichnology description. These deposits show a great variety of facies, from finegrained muddy facies (Fm, Fl, Fmb), heterolithic deposits (Htw and Hts), to sandstone facies (Sr and Sm). The muddy and heterolithic facies are the most abundant, with variable thickness, ranging from 10 cm to 3m, with transitional, net and erosional limits. Sandstone bodies are arranged displaying a fining upwars grain tendency, with net and erosional limits, with variable thickness ranging from 10 cm to 2 m. They also show in some levels evidence of pedogenic processes (pedogenetic mottling, soil peds, slickenside structures, pedogenic carbonate). It can present load deformation structures. This FA3 is usually eroded by the channel of the FA2 (Fig. 5). Considering ichnology, in this FA two different trace fossils associations are present. The first one is characterized by a dominance of horizontal traces, relatively simple, shows low ichnodiversity (typically 1 to 3 ichnogenera), but in some levels it could reach 6 ichnogenera. Bioturbation index is also variable (BI 1-6), being generally low (BI 2-3), but in some localized sections it could reach BI 6. The dominant trace fossils are Planolites isp., Teichichnus isp., and mantle & swirl structures (Fig. 5 a). The second ichnoassociation presents root structures (rhizoliths), burrows with backfilling attributable to Taenidium isp., and very scarce Planolites isp. This trace fossil association has a very low ichnodiversity (from 1 to 3), and also generally presents low bioturbation intensity (BI 1-2), reaching only in some localized sectors a maximum of 4 (Fig. 5 B).
Figure 5 Facies Association 3 (FA3), interdistributive plain. Massive siliciclastic mudstones (Fm), laminated siliciclastic mudstones (Fl), trough cross-bedding sandstone (Set). a, b) Impoverished Cruziana ichnofacies: M&S: Mantle & swirl structures, Tei: Teichichnus isp. c, d) Scoyenia ichnofacies: RT: Root traces, Tae: Taenidium isp. References as in figure 3.
Sedimentology and ichnology interpretation. the fine-grained sediment facies are deposited in interdistributary plains between distributary channels, mainly by suspension settling processes in low-energy environments. Interdistributary plains are characterized by brackish water conditions, influenced by fluvial and marine processes, but predominantly low-energy situations (Elliott 1974; Bhattacharya 2006; Gugliotta et al., 2015). The facies Hts, Sm and Sr are interpreted as overflow lobe deposits (crevasse-splays) in the interdistribute plain. Sm, Fm facies are interpreted as plain deposits affected by pedogenetic processes (Buatois and Mángano, 2011). The dominant ichnological components of the first ichnoassociation are horizontal structures, interpreted as feeding structures of. vermiform animals, mainly with a detritivore feeding strategy. The presence of mantle & swirl structures indicates a sediment with very little cohesion (soupground) (Lobza and Schieber 1999). Due to the aforementioned characteristics, this trace fossil association is assigned to the Cruziana ichnofacies, but due to the low ichnodiversity that exhibits, it is interpreted as an impoverished Cruziana ichnofacies (MacEachern, et al., 2005, 2007; Buatois and Mángano, 2011). The second ichnoassociation, is dominated by root traces and Taenidium isp. Although Taenidium isp. is a trace fossil that occurs in a great variety of environments from marine to continental, in this case it is interpreted as a product of the activity of insect larvae in continental environments by the association with rizoliths. Therefore, this association of trace fossils is assigned to the Scoyenia ichnofacies, which is typical of continental environments, but impoverished, which is characteristic of transitional marine-continental environments. The poor and saltuary development of this association of trace fossils points to a stressed environment, due to large variations in energy, salinity and water level. In the studied sections, it is found in deposits interpreted as flood plain and / or overflow lobes (crevasse splays) developed in a delta plain with aerial exposure (MacEachern et al., 2005, 2007; Buatois and Mángano, 2011) (Fig. 6. G, H).
Figure 6 Facies Association 4 (FA4), wave affected bars. Wave affected bars: trough cross-bedding sandstone to conglomerates (SGt), trough cross-bedding sandstone (Set), massive sandstone (Sm), bioturbated massive sandstone (Smb), hummocky crossstratification sandstone (Shcs). A) Macaronichnus suite Op: Ophiomorpha irregulaire, Ma, Macaronichnus isp. References as in figure 3.
Facies Association 4: Wave affected bars
Sedimentology and ichnology description. This facies association is composed mainly by amalgamated sand bodies (Sm, Smb, Shcsb, Srw SGt), with very little representation of heterolithic and muddy fine-grained facies (Htw, Fl). Sandstones with upward convex stratifications (i.e. hummocky cross-stratification; Hcs) and parallel lamination show net to erosive contacts at the bases and net limit at the top. The thicknesses vary from tens of centimeters to a maximum of 1 meter. The transition between different granulometries may show erosive contacts. Srw is composed by very fine to fine sandstones, with fine clayey sandstones with subordinate carbonaceous vegetal debris. They display small scale wave ripple lamination, and the thickness of this bodies never exceed 30 cm. This FA4 is similar to the FA1 previously described, in terms of the facies that comprise it. However, in this facies association, the proportion of sandstone vs fine-grained facies is higher than in FA1. In addition, there are two sandstone facies that are very abundant in this FA that does not appear in FA1, Shcsb and Srw, the contacts between the different sand bodies are usually sharp, and the presence of phytodetritus is less abundant than in the FA1 (Fig. 6). In this FA4 the dominant trace fossil is Macaronichnus isp., sometimes conforming a monospecific association. Eventually O. irregulaire and Gyrolithes isp., and rarely Planolites isp., Thalassinoides isp. and Teichichnus isp. could be observed. The ichnodiversity is usually very low (1- 3), and the bioturbation index could vary from BI 1-4. (Fig. 6a).
Sedimentology and ichnology interpretation. This FA4 is characterized by deposits accumulated in areas located in zones between the fair-weather waves level and storm waves level. Sands accumulated in bar and dune systems result from the action of unidirectional flows, while the presence of wave and hummocky ondulitic lamination-type structures are linked to development of combined unidirectional and oscillatory currents generated during storms (Duke, et al., 1991). The lower concentrations of organic matter in FA4 suggests a deposition environment with less influence of a fluvial input, being more associated with a siliciclastic coastal geometry (shoreface), where wave action is the dominant process. This FA4 is interpreted as formed by tractive processes in high to very high energy environments, showing evidence of subsequent rework due to fair-weather and storm wave processes (Plint, 2010; Ainsworth et al., 2016). Because wave lamination and storm wave structures (HCS) are preserved, it is interpreted that has been accumulated in beach environments in medium shoreface to offshore transition positions (Walker and Plint, 1992, Plint 2010). These sand bodies are interpreted as littoral bars, developed in a shoreface position, or strandplains related to mouth bars dominated by wave and storm action. Macaronichnus isp. is produced by intrastratal deposit-feeding of opheliid polychaetes (Clifton and Thompson 1978; Seike 2008) feeding on sand grains. It is indicative of very high energy environmental conditions, and it has been proposed as an ichnosubfacies of the ichnofacies of Skolithos (Pemberton et al., 2001), which would be characteristic of foreshore environments. However, in the studied cores, although it follows this trend, and represents high-energy conditions, in many sectors it is accompanied by storm-generated structures (HCS). In addition, there is no other element that indicates foreshore conditions. Other associations of Macaronichnus isp. have been described for different environments, some deeper than shoreface positions (Nara and Seike 2004; Seike 2007, Seike et al., 2011; Bromley et al. 2009; Quiroz et al. 2010, Rodríguez-Tovar and Aguirre 2014). The levels that contain Macaronichnus in FA4 are always storm levels, which indicates that the organism that produces this structure would have behaved as an opportunistic colonizer in these environments. The first traces formed in this environment are Macaronichnus isp., while the other traces present are colonizers of overlying sediments developed in fair-weather conditions that are not preserved as a result of the different storm events (Pemberton et al., 2001; Seike, 2008, 2009). Due to all these evidences, this association is interpreted as developed in shoreface conditions, which can vary from middle to lower shoreface.
Facies Association 5: Prodelta
Sedimentology and ichnology description. This FA5 is dominated by fine-grained facies (Fm, Fl) and sandstone facies (Sm, Set, SGt). This FA includes massive fine-grained sediments with thickness from 40 cm to 2 m, with transitional bases. Heterolithic deposits with deformation structures as slumps or convolute beddings show variable thickness (from 30 cm to 2 m). Sandstones and conglomerates have erosive bases, dominated by massive structures and fining upward trends, with thickness from 1 to 5 m. The contacts between facies of dissimilar granulometries are usually erosional. The content of particulate organic matter is very abundant, as well as the development of syneresis cracks. This FA5 shows coarsening upwards strata stacking trends, as the fine sediments decrease (Fig. 7). The ichnological content is very scarce, but two trace fossil associations can be differentiated. One is represented by specimens of Planolites isp., Teichichnus isp, Thallasinoides isp., Chondrites isp. and cryptobioturbation, with low ichnodiversity (ranging from 1-4) and also low bioturbation intensity (BI 1-3) (Fig. 7a). The other ichnoassociation is even more scarce and it is represented by Zoophycos isp. and Planolites isp., the ichnodiversity is very low, being typically 1-2. Bioturbation intensity is also low (1-2) (Fig.7 b).
Figure 7 Facies Association 5 (FA5), prodelta. Trough cross-bedding sandstone (Set), massive sandstone (Sm), bioturbated massive sandstone (Smb), heterolithic wavy (Htw), heterolithic flaser (Htf), massive siliciclastic mudstones (Fm), laminated siliciclastic mudstones (Fl). a) Cruziana ichnofacies: Thalassinoides isp., Pl: Planolites isp., Esc: Escape burrows (Fugichnia). b) Zoophycos ichnofacies: Zoo: Zoophycos isp. References as in figure 3.
Sedimentology and ichnology interpretation. This FA5 is interpreted as representative of low-energy environments, where settling processes dominated the sedimentary background, with sporadic high energy discharges, and also synsedimentary deformation due to rapid deposition is observed. In this environment, however, there are occasional arrivals of much more energetic hyperpycnal flows, which deposit the sandy facies tractively. Unburrowed to weakly burrowed mudstone intervals with abundant presences of phytodetritus are interpreted to represent rapid deposition of mud and the development of soupground substrates, due to bouyant plumes during post-storm river floods (Plint, 2014; MacEachern and Bann, 2020; Moyano- Paz et al., 2022). These levels of fluid mud inhibit the development of infaunal communities as suggested by MacEachern et al. (2005). The recurrent presence of syneresis cracks indicates changes in salinity produced by the supply of fresh water from a nearby river system (e.g., MacEachern et al., 2005). All these evidence of fluvial inflows in a distal position indicate that these sediments accumulated in a prodelta environment to which high-energy fluvialderived flows still eventually arrive (Asquith, 1974; Bhattacharya, 2010).
The first trace fossil association, interpreted as Cruziana ichnofacies, is typical of low energy environments, and the scarcity of it is typical of the delta front-prodelta environment, due to the physicochemical stress of this environment (MacEarchern et al., 2005; Buatois and Mangano, 2011). Zoophycos isp. has been interpreted as a deposit-feeding behavior of a vermiform animal but also alternative interpretations, has been made (Löwemark 2012). Zoophycos isp., is a trace that usually develops at sea environments with marked dysoxia. Therefore, is the second trace fossil association is interpreted as belonging to the Zoophycos ichnofacies of low energy environment, with some oxygen restriction. These conditions can occur in deltaic environments in the prodelta positions, which are the most distal of the system (MacEachern, et al., 2005; MacEachern and Bann, 2020; Bhattacharya et al., 2020; Moyano-Paz et al., 2022). An association of trace fossils with the presence of Zoophycos has been described for offshore-proximal and offshore distal to shelf environments in coronas cores of the Bardas Blancas Formation, which is homologous to the Lajas Formation north of Neuquén Basin (Veiga et al., 2013).
Glossifungites surfaces
There are two levels that present quite particular bioturbation, and for that reason they are described separately. In the AG-TO2 core samples, at a depth of 3,080.71 m, a 30 cm thick level of massive bioturbated mudstones (Fmb) is observed, overlaying with erosive contact a coarse sandstone/ conglomerate body with trough cross-bedding (SGt). The bioturbation is dominated by Thallasinoides isp., of approximated 1-2 cm in cross section, with sharp walls, infilled with medium massive sandstone (Fig. 8a). In AG-TO4 core sample, at 2,531 m depth, a level of 6 cm of alternating silt and very fine sand, with wavy lamination (Htw) shows Rhizocorallium isp., with sharp walls, infilled with fine sandstone. These burrows are arranged in the last centimeters of the stratum, which shows evidence of erosion at the top, and a sharp basal contact with an underlying sand body with trough cross-bedding (Set) (Fig. 8 B).
Figure 8 Diagram of the evolution of transitional marine systems in the Sierra Barrosa sector, Lajas Formation. a, b, c) Lower Lajas; d, e) Middle Lajas; f) Upper Lajas. a) Fluvial-dominated deltas develop in the Huincul Norte sector, while in the Sierra Barrosa area there is an alternation of fluvial and wave processes. b) River systems in the Huincul Norte sector, while in the Barrosa Norte sector deltas are developed with a dominance of wave processes. c) The fluvial influence is larger in Sierra Barrosa, while in Huincul Norte an alternation of fluvial and marine conditions (tides?) is observed. d) In Sierra Barrosa a progradation of fluvio-dominated deltaic systems is observed. e) New progradation of fluvio-dominated deltaic systems in Sierra Barrosa. f) A deepening of the system is observed in the Sierra Barrosa sector (transgression?).
This two bioturbated levels are interpreted as belonging to the Glossifungites ichnofacies. This ichnofacies is characterized by the presence of trace fossils with varied morphologies, being common branched excavations and vertical, cylindrical, drop-shaped or U-shaped structures, with welldefined and clear limits (Seilacher, 1967; Frey and Seilacher, 1980; Frey and Pemberton, 1984, 1985; Pemberton et al., 1992a, 1992c; MacEachern et al., 1992; Uchman et al., 2000; Carmona et al., 2006). Generally, the filling of biogenic structures is passive, with a texture different from that of the host rock and similar to that of the overlying strata. The most common ichnotaxa in this ichnofacies correspond to the ichnogenera Diplocraterion, Skolithos, Arenicolites, Gastrochaenolites, Thalassinoides, Spongeliomorpha, and Rhizocorallium (Buatois and Mangano, 2011). The trace fossils that characterize this ichnofacies belong mostly to suspensivorous organisms, and the ichnodiversity is generally low, although the abundance of specimens of each ichnotaxon can be high.
DISCUSSION
Paleoenvironmental evolution of the Lajas Formation in subsurface in the Sierra Barrosa area
Considering that all the core samples are from a restricted area it is possible to reconstruct the environmental evolution of the depositional systems according to its depth and location. As discussed before, the Lajas Formation has been informally divided into Lower, Middle and Upper in the Sierra Barrosa area in subsurface, and that order is followed for the paleoenvironmental evolution (Fig. 2).
Lower Lajas. The five core samples assigned to Lower Lajas show mainly delta front environments (FA1), with deposits dominated by fluvial processes, and delta fronts with some evidence of wave action (FA4). Also, to a lesser extent, delta plain (FA2, FA3) and prodelta environments were identified (FA5). In the basal sector, there are located the core samples AgTo-2, BaN-1 and the lower part of HuN-1. The HuN-1 core sample begins with deposits interpreted as prodelta to distal delta front, while the AgTo- 2 core sample begins with fluvial-dominated delta front environments, which passes upward into a delta front environment with wave influence, by the evidence of bioturbation and of wave and storm structures (Figs. 2, 8a). Then, the HuN-1 core shows distributary channel environments (FA2), while in BaN-1, the northernmost core sample, evidence of wave actions is observed (FA4), accompanied by a very well developed marine bioturbation that indicates normal salinities and high energy. This leads to interpret this core sample as a delta front with less fluvial influence and a dominance of wave processes. Therefore, the fluvial processes had a preponderance in the southern sector, while towards the north these processes were alternated with wave processes (Figs. 2, 8b). In the upper stratigraphic interval of the Lower Lajas Formation, we see the final portion of the HuN-1 core sample, and in the northernmost sector of Sierra Barrosa area the Ba-1 and AgTo-1 core samples. The HuN-1 shows fluvial-dominated deltaic front with some evidence of waves (FA1, FA4) that pass to a delta plain environment (FA3) with evidence of marine bioturbation (possible tides?). However the dominance of fluvial processes is still evident, therefore the subenvironment is interpreted as a fluvial-dominated deltaic plain with tidal modulation, similar to those described by Gugliotta et al. (2015). Meanwhile, to the north, in the Sierra Barrosa sector, the Ba-1 core sample begins with a fluvio-dominated delta front (FA1) that passes into a fluvio-dominated deltaic plain (FA3), while the AgTo- 1 core sample represents only river-dominated deltaic front environment (FA1). In summary, it is interpreted that the system evolves towards a situation where the fluvial contribution, although it continues in the south, becomes more important towards the Sierra Barrosa sector at north (Figs. 2, 8c).
Middle Lajas. The three core samples corresponding to the Middle Lajas (Ba-2, AgTo 2 and the base of AgTo-4) have less preponderance of coarsegrained intervals, and a larger representation of fine sediments. In none of these cores a delta front environment is interpreted. The Ba-2 core sample shows mainly distributary channels facies (FA2) and less interdistributary plain (FA3), with little influence of bioturbation, therefore it is interpreted as a distal fluvio-dominated deltaic plain environment. In the delta plains dominated by fluvial processes, the presence of the dendritic network of channels becomes more important in the distal sector, therefore, the presence of abundant levels interpreted as channels could indicate this subenvironment (Olariu and Bhattacharya, 2006; Canale et al., 2015, 2016). Furthermore, the typical bioturbation of the delta plain with subaerial exposure, with evidence of root and insect, is poorly developed in this core sample, which is another evidence that the environment is a subaqueous distal plain (MacEachern et al., 2005; Buatois and Mángano, 2011) (Figs. 2, 8d). The AgTo-3 core sample shows marked evidence of a deepening of the system, recording prodelta and distal delta-front deposits (FA5). This core is located between the Ba-2 (bottom) and AgTo-4 (top) core in a vertical position, therefore it would indicate a marine transgression, or a lobe avulsion. It is noteworthy to emphasize that prodelta environments has been interpreted in regional schemes from seismic studies from Wrinkworth et al. (2018) and Vocaturro (2018), that show a possible trasgresion of the system. The interpretation of the AgTo-3 core sample could corroborate this hypothesis (Figs. 2, 8).
The AgTo-4 core sample shows abundant evidence of delta plain environment with subaerial exposure (FA3) and does not show evidence of channels. Consequently, it is interpreted as developed in a proximal deltaic plain, mainly subaerial. In a fluviodominated deltaic plain, the distributary channels follow a dendritic development pattern, which makes it much easier to find a channel in a more distal sector, than upstream where the number of channels decreases markedly (Olariu and Bhattacharya, 2006; Canale et al., 2015, 2016). In addition, bioturbation is very abundant, and is represented by a trace fossil assamblege corresponding to Scoyenia ichnofacies, typical of proximal deltaic plain environments (Buatois and Mángano, 2011). The absence of channels and the evidence of much lower water level allows us to interpret the AgTo-4 core sample as developed in a proximal delta interdistributary plain environment (Figs. 2, 8e). It is notable that the Glossifungites surface that separates both sections in AgTo-4, although it shows two successive clinoforms, they develop in a similar subenvironment (see Upper Lajas and discussion of Glossifungites surfaces).
Upper Lajas. The only core sample that has a representation of Upper Lajas is AgTo-4, which has the boundary between the so-called Middle Lajas and Upper Lajas. This limit, which was defined by the seismic used in the reservoir by YPF (Figs. 2, 8f), occurs in the lower section of the core sample and is represented in the core not only by a change of lithologies, but also by an omission surface characterized by an Glossifungites surface, with Rhizocorallium isp. After this limit, the core sample has a very similar aspect to its lower section, for that reason the environment is still interpreted as a proximal delta interdistributive plain environment. Being the only core sample representing Upper Lajas, we cannot claim that this environment is general throughout the entire section. However, it coincides with the environmental interpretations of the regional schemes from seismic studies carried out by Brinkworth et al. (2017) and Vocaturro et al. (2018) (Figs. 2, 7, 8).
Animal-substrate interactions
River-dominated deltas are the most stressful delta environments for marine biota (MacEachern et al., 2005; Buatois et al., 2011). In the core samples analyzed, the ichnofacies recognized are impoverished Skolithos, or the recently erected Rossellia, on the proximal delta front, Cruziana with Skolithos elements on the proximal delta front to the distal delta front, Zoophycos and Cruziana on prodelta environment and Scoyenia on the deltaic plain. The low diversity, low abundance, low bioturbation index in general and the simple tiering observed are interpreted as the result of short-time colonization windows that reflect suitable environmental conditions for the development of benthos only for very short periods. In facies that show more direct fluvial influence (FA1 of mouth bars, and mainly FA 3 of channels) trace fossils are almost absent. The environmental parameters that control benthos are salinity, turbidity, and hydrodynamic energy (MacEachern et al., 2005; Buatois and Mángano, 2011; Dasgupta et al., 2016). Therefore, it is interpreted that only when these parameters are not so stressful, it the colonization windows occur. The distribution of trace fossils in a delta dominated by a river is conditioned by physical-chemical stress factors related to their discharges (MacEachern et al., 2005; Dasgupta et al., 2016; Canale et al., 2015, 2016; Gugliotta et al., 2015, 2016a; Arregui et al., 2019). These stress factors act differently in different sub-environments. In the proximal delta plain, the fluvial influence is mainly present into the distributary channels, and to a lesser extent, in the interdistributary plain. The main physicalchemical stress factors in this sector are related to the alternation of periods of aerial exposure with periods of flooding. In this area, the dominant trace fossils belong to the Scoyenia ichnofacies, but presenting less diversity than the typical continental Scoyenia ichnofacies due to the previously mentioned factors. In the distal delta plain, the aerial exposure is not as important, while the marine influence becomes more noticeable. Therefore, in this sector there is an alternation between periods of fluvial influence and periods of marine influence, which causes great variations in salinity. The dominant trace fossils of this subenviroment correspond to an empoverished Cruziana ichnofacies (Fig. 9).
The mouth bars described in the lower Lajas core samples in the Sierra Barrosa area are usually grouped in amalgamated sand bodies (Arregui et al., 2019). They can be differentiated into non-bioturbated and bioturbated sand bodies. These two types of sand bodies are interpreted as two successive stages of development. High river discharge and rapid deposition, high energy and mobility of sediments, as well as significant freshwater discharge inhibit bioturbation during the main construction phase of the mouth bars. These bars are characterized by having sand-sized lithologies, and in some sectors they can even have gravels, with massive or throughcross bedding structures, with abundant remains of scattered phytodetrites or, more commonly, on top of the through-cross sets. During periods of reduced river discharge, the deposits are easily reworked by coastal processes (i.e., tides and waves), which facilitates the settlement of trace-makers, generating the bioturbated bars, which would constitute the next phase of development of the bar (Arregui et al., 2019). The most common traces found in the mouth bars are those corresponding to the Skolithos ichnofacies, but without suspensivorous component, that could be the recently erected Rossellia ichnofacies. These bioturbated bars show grain sizes similar to the previous ones, but they usually exhibit a massive structure, which is the product of the activity of burrowing animals. The decrease of river discharge may be due to a channel avulsion, which is very common in deltaic environments (Olariu and Bhattacharya, 2006). Alternatively, it may be related to the episodic nature of river discharges resulting from seasonal floods (Plink-Bjorklund 2015; Gugliotta et al., 2016b) (Fig. 9).
In the interbar or distal fringe of the mouth bars, and in distal delta front, the trace fossils found correspond to an impoverished Cruziana ichnofacies, that could be similar to the trace fossil association described for distal delta plain environments but showing more ichnodiversity (Coates and MacEachern, 2009). In prodelta, two trace fossils associations were described, the most abundant belonging to an impoverished Cruziana, and the least frequent to the Zoophycos ichnofacies (Coates and MacEachern, 2009; Buatois et al., 2011; MacEachern and Bann, 2022) (Fig. 9).
Glossifungites surfaces: autogenic vs allogeneic processes
The recognition of surfaces assignable to the Glossifungites ichnofacies has been widely used in the context of sequence stratigraphy, basically because it indicates omission surfaces or unconformities (MacEachern et al., 1992a; Pemberton et al., 1984, 1995, 2004; Schwarz and Buatois, 2012; Dasgupta et al., 2016). These surfaces coincide many times with surfaces of stratigraphic importance, for example, developed during the incision of fluvio-estuarine valleys (Lowstand unconformities), incision of underwater canyons (Lowstand unconformities), erosive displacement of the shoreline towards the continent during transgressions (transgressive erosion surfaces) and towards the sea during forced regressions (regressive surfaces of marine erosion) (MacEachern et al., 1992, 2007; Pemberton et al., 1992, 2004). All these processes that delimit areas of stratigraphic importance in the sequence stratrigraphic models imply sea level changes in response to allogenic controls. In contrast, the lateral migration of fluvial or tidal channels and the avulsion of lobes in deltaic contexts can generate erosive surfaces of discordance with trace fossils assignable to the Glossifungites ichnofacies, but they are inherent processes of the system (autogenic), therefore, not they are related to variations in sea level (Gingras et al.,1999; MacEachern et al., 2007). Recently, Villegas- Martín et al. (2020) described two types of surfaces of Glossifungites, to which they assign different sequence stratigraphic meanings. Glossifungites surfaces developed in mudstone firmgrounds of estuarine deposits are interpreted as autogenic, while Glossifungites surfaces developed in stiffgrounds in shoreface deposits, with higher frequency of larger galleries is interpreted as allogeneic. In the analyzed core samples, there are two records of surfaces assignable to Glossifungites, with two opposing interpretations. The Glossifungites surface observed in the AgTo-1 core sample is composed of galleries assigned to Thalasinoides isp., developed in mudstones, with a passive sandstone fill (Fig. 7). The inferred environment for the infra and overlying sections of this Glossifungites surface is a delta front environment. This surface is interpreted as a product of autogenic processes of the system (migration of lobes or deltaic channels). These types of erosive surfaces are very common in deltaic front settings, which is where the distributary channels arrive, and the avulsion processes occur (Olariu and Bhattacharya, 2006). Glossifungites surfaces with similar interpretations have been described for the Lajas Formation (Canale et al., 2015).
On the other hand, the Glossifungites surface defined for the lower sector of the AgTo-4 core sample coincides exactly with the limit that divides two of the three divisions of the Lajas Formation in subsurface (Middle from Upper Lajas), according seismic and well data from the oil industry (Fig. 2). This suite is characterized by Rhizocorallium isp. which show passive sandstone fill (Fig 10a). These trace fossils are typical of marine environments (Knaust, 2013). The environment below and above the of this Glossifungites surface is proximal deltaic plain. In these environments, channel avulsion is less frequent than in a distal deltaic plain or delta front environments. Hence, this omission surface would imply an erosion due to a sea level fall and a subsequent rise that allows the colonization of the stiffground by the producers of Rizocorallium isp. Therefore, this erosive unconformity could represent an allogenic variation in sea level. This level could also be interpreted as a surface of stratigraphic importance, that is coincident with the limit between Middle and Upper Lajas according to seismic interpretations (Fig. 10b).
Figure 10 Glossifungites surfaces developed in different scenarios. a) Glossifungites related to lobe avulsion, in a delta front that alternates fluvial and wave conditions (Ag-To 2), b) Glossifungites related to sea level changes, in a delta plain environment (Ag- To 4). Th: Thalassinoides isp., Rz:Rhizocorallium isp. References as in figure 3.
CONCLUSIONS
A detailed sedimentological and ichnological study on core samples allows to define the main processes that accumulated the deposits of theLajas Formation in the Sierra Barrosa area. The definition of the dominant process was posible by the recognition of the main physico-chemical stress factors that controlled the distribution of the bentos, inferred by their trace fossils.
The colonization phases of the delta front mouth bars affected by river were identified, with a first constructive phase related to fluvial input, and a second phase with a decrease in fluvial influence and colonization by benthos in a normal saline marine environment. The main limiting factors for colonization by marine biota during the stage dominated by fluvial inputs would be mainly salinity, high energy and water turbidity. Dominance of one or another stage of the bars implies the permanence of conditions of fluvial influence in non bioturated bars or of a marine environment in bioturbated bars. An environmental evolution model is proposed in which, for Lower Lajas, in the most basal sector, it is interpreted that the fluvial processes had a preponderance in the southern sector, with development of a deltaic front dominated by fluvial processes, while to the north these processes alternated with wave processes, also developed in sub-environments of a deltaic front. Then, it is interpreted that the system evolves towards a situation where the fluvial contribution, although it continues in the south, becomes more important towards the Sierra Barrosa sector, with the recognition of river-dominated deltaic fronts.
For Middle and Upper Lajas it is interpreted as developed mainly in a fluvio-dominated deltaic plain environments, with abundance of channels with great mobility and cannibalization among themselves in the distal deltaic plain (Middle Lajas), and with less preponderance in proximal delta plain (Upper Lajas). However, prodelta evidence may be indicating a marine transgression in the Middle Lajas, which has implications in the sequential stratigraphic study of the region.
Two Glossifungites surfaces (AgTo-2 and AgTo-4) were recognized, representing unconformities formed during erosive events and subsequent colonization. In the first case, it would correspond to autogenic changes typical of deltaic systems, developed in delta front settings (migration of deltaic lobes and avulsion of channels). In the second case, it is interpreted as a surface of stratigraphic importance, developed in between two proximal delta plain settings of two successive deltaic clinoforms, related to the variation in sea level, and it marks the limit between the Middle Lajas and the Upper Lajas. Therefore, the entire section studied in the Sierra Barrosa and Huincul area is interpreted as the result of the progradation of deltaic clinoforms that, although they show a preponderance of fluvial processes, in lateral settings, the influence of marine processes could be observed. This study shows that integrated sedimentology and ichnology analysis allows to achieve paleoenvironmental models of greater precision on deltaic successions, unraveling the complex interaction of fluvial, wave and tidal processes that affect these environments.
