Description . Thin-bedded heterolithic deposits of very fine to fine-grained sandstone and siltstone/mudstone with rare thicker-bedded hummocky cross-stratified sandstone compose lithofacies association 1 (LFA-1; Lithofacies 1–4; Table 1; Figures 5A,B). Towards the base of LFA-1, sandstone beds are fine-grained and thin-bedded with sharp and occasionally erosional bases into siltstone and mudstone beds (Figure 5B). Individual beds are <5 cm thick, whereas laminated heterolithic bedsets of interbedded sandstone and mudstone are on average ∼30 cm thick. Beds are tabular and commonly continuous. However, some sandstone beds pinch and swell in thickness laterally. Bioturbation index (BI; Taylor and Goldring, 1993) is low (0–2) and LFA-1 contains a low diversity of trace fossil assemblages of vertical to sub-vertical burrows (Ophiomorpha isp., Rhizocorallium isp.; Figures 6A,B). Organic debris is common between sandstone laminae (Figure 6C). Current ripple to undulating/planar lamination in fine-grained sandstone beds are the most common sedimentary structures (Figure 5B). Hummocky cross-stratified (HCS), fine-grained sandstone beds are rare in mud-rich portions of LFA-1, although undulating to long wavelength cross-stratification is common (Figures 5C,D). In zones interpreted to be more proximal to fluvio-deltaic sources (see interpretation of LFA-4), hummocky and long wavelength cross bedding become the dominant sedimentary structure, sand to mud ratios increase, and beds appear to pinch and swell from several decimeters to a few centimeters in thickness in a laterally short (<5 m) distance.

Interpretation . We interpret LFA-1 to represent deposition onto the outer shelf and lower shoreface between storm and fair-weather wave base. Individual mudstone beds represent deposition via background sedimentation of fine-grained materials suspended in the water column via storms and currents. Hummocky cross-stratified, current rippled, and planar laminated beds likely formed from storm events via combined flows from oscillatory wave and unidirectional currents (Dumas et al., 2005; Dumas and Arnott, 2006; Plint, 2010). The common occurrence of interstratified mudstones suggests storm events were not powerful enough to erode and resuspend previously deposited sediment (Plint, 2010). The higher abundance of sands making up LFA-1 at section SrC (Figure 4) suggests increased proximity to fluvio-deltaic sources, which provided coarser sediment and higher amounts of organic detritus. Pronounced lateral variability in sandstone bed thicknesses at section SrC suggest these beds may be organized in shoreface sand ridges that trended oblique to the direction of shoaling (e.g., Snedden and Dalrymple, 1999).

Description . Lithofacies association 2 (LFA-2) consists primarily of pervasively bioturbated, amalgamated white to gray, very fine to fine-grained sandstone (Table 1; Lithofacies 2, 4–5; BI = 4–6). The presence of interstratified mudstone beds reduces significantly compared to LFA-1. At outcrop scale, bed sets are tabular and laterally continuous. Individual beds commonly have gently undulating contacts with long wavelength (meter scale) scours filled with upward-flattening laminations (Figure 5E). Mud-lined sand-filled vertical to sub-vertical burrows (Ophiomorpha isp.; Figure 6D) are the only recognized trace fossils. Where bioturbation is less intense, relict sedimentary structures include undulating to planar laminations, swaley, very low-angle cross stratification, rare trough cross-bedding, coarse gravel lags defining the bases of beds, and rare soft-sediment deformation. Paleocurrent directions from cross beds are primarily southeast directed, with a lesser amount to the northeast (Figure 4). Isolated quartzite pebbles and cobbles are common. Plant debris and carbonaceous fragments are abundant throughout the lithofacies association.

Interpretation . LFA-2 represents deposition on the lower to middle shoreface of a wave-dominated, shallow-marine system. These sediments were deposited between storm and fair-weather wave bases, but at shallower depths than LFA-1 in which oscillatory energy from storm waves are powerful enough to erode muds deposited during fair weather conditions. Swaley cross-stratified, fine-grained sandstone beds that grade from heterolithic HCS are inferred to occupy the middle shoreface where aggradation rates and scouring potential is higher (Leckie and Walker, 1982; Dumas and Arnott, 2006; Plint 2010). The massive appearance of sand beds is attributed to the pervasive bioturbation throughout the unit. High BI values and overall burrowing uniformity throughout the section may suggest that physiochemical parameters for Ophiomorpha isp. burrowing biota were temporally stable and/or sedimentation rates were low enough to yield a low-stress environment for burrowers (MacEachern et al., 2010). Increased amounts of plant debris up-section and the presence of quartz pebbles and cobbles imply a proximity to fluvio-deltaic sources (Bhattacharya, 2010). We interpret stratigraphic sections with greater thicknesses of LFA-2 to reflect greater influence from wave energy (e.g., sec. RG-3; Supplementary Figure S1) whereas fluvio-deltaic influenced sections contain more distributary channel and mouth bar facies (e.g., secs. R40, CoC).

Description. Deposits of lithofacies association 3 (LFA-3) consist primarily of lower-medium to very coarse grained, cross bedded sandstone (Lithofacies 6–7) and subordinate amounts of thin-bedded rippled sandstone and siltstone (Lithofacies 1–2; Figures 5F–H). Average bed thickness is 48 cm and amalgamated bed sets reach several meters (∼5 m) in thickness. Bed geometries consist of tabular bounding surfaces of inclined crossbed sets as well as erosive, trough-shaped, and laterally discontinuous surfaces infilled with cross-bedded conglomerate and sandstone. Organic-rich mudstone beds (<1 cm thick) occasionally mantle sandy bedforms. The most common sedimentary structure is multi-directional trough cross-bedding with pebble-filled scours and current-ripple lamination as well as rare gravel oscillation ripples (Figure 5G). Cross-stratified sandstone beds display moderate to poor sorting and outsized quartzite pebbles and cobbles are common. Paleocurrent directions from cross-stratified sandstone beds are diffusely to the south and southeast, with lesser amounts radially to the north (Figure 4; secs. SpC, CC, CgC). Sandstone beds are sporadically bioturbated (BI = 0–3) with fugichnia escape-equilibrium structures associated with Rosselia isp. and subordinate amounts of Ophiomorpha isp. and Skolithos isp. (Figures 6E–H).

Interpretation . LFA-3 represents the deposits of two- and three-dimensional dunes deposited on a high-energy, coarse-grained upper shoreface. These were deposited above fair-weather wave base where the influence of shoaling wave oscillatory flows, longshore drift, rip currents, and/or geostrophic flows facilitates the migration of subaqueous dunes resulting in multi-directional trough and tabular cross-bedding (Clifton, 2006; Plint, 2010). The availability of coarse-grained sandstone and pebble conglomerate suggests proximity to fluvio-deltaic sources. Poor sorting, outsized clasts, fugichnia escape structures, and the decrease in bioturbation index and burrowing uniformity suggest that sedimentation rates were high (Clifton, 2006; MacEachern et al., 2010; Plint, 2010). Rare organic-rich mudstone drapes suggest intermittent periods of quiescence between storm-induced dune migration events (Plint, 2010).

Description . Medium-lower to very coarse-grained, occasionally amalgamated sandstone and moderately to poorly sorted pebble conglomerate primarily make up lithofacies association 4 (LFA-4; Lithofacies 6–10, Figure 5I–M; Table 1). Subordinate amounts of fine-grained, planar to current-ripple laminated sandstone, siltstone, and carbonaceous mudstone also are present (Lithofacies 1–2; Table 1). Sandstone thicknesses range from 10 to 120 cm (avg. thickness = 31 cm) with cross-bedded or massive bed sets bounded by recessive intervals or erosional surfaces reaching up to ∼3–4 m in relief (Figure 7). Both tabular and erosive geometries are present at outcrop scale. Several bounding surfaces have channel-like geometries which scour and erode adjacent beds up to ∼2 m (Figure 7). The predominant sedimentary structure is dune-scale trough and planar, unidirectional crossbedding. Bases of crossbeds commonly contain gravel lags and mudstone rip-up clasts (Figure 5M). Paleocurrent directions from crossbeds are radially southward directed (Figure 4; secs. R40 and CoC). Current-ripple lamination, planar lamination, and graded beds are also present (Figures 5I,J). Large-scale (∼1–2 m) load structures are present at the bases of thick amalgamated sandstone units into recessive heterolithic deposits (Figure 5K). Finer-grained heterolithic intervals commonly contain interstratified carbonaceous mudstone that drape fine-grained current-ripple laminations. At the base of section CoC, LFA-4 sandstone beds are in sharp, likely erosional contact with fine-grained, moderately bioturbated sandstone of LFA-2 affinity (Figure 5L). Bioturbation throughout the section is low (BI = 0–2) and decreases upsection. Burrowing intensity is non-uniform between beds. Fugichnia escape structures, Rosselia isp., Ophiomorpha isp., and Skolithos isp. ichnotaxa are present in the sandstone beds (Figures 6I,J). Ichnotaxa within carbonaceous, heterolithic fine-grained sandstones and mudstones include Ophiomorpha isp., Thalassinoides isp., Teredolites isp., and possible Arenicolites ichnospecies (Figures 6K–N).

Interpretation . Facies assemblages and ichnofacies of LFA-4 are consistent with a river-dominated, coarse-grained delta-front environment reflecting high-rates of deposition from decelerating unidirectional flows within distributary channels and adjacent mouth bars. The coarse grain size, sedimentary structures, and paucity of fine-grained units suggest constant reworking of sediments by tractive currents. Sedimentation rates were high as evidenced by graded beds, variable poor sorting, load structures, and high abundance of fugichnia escape structures (Bhattacharya, 2010). Large 1–2 m thick load structures at the base of sandstone beds may be dinosaur track imprints (Figure 5K), which is consistent with the large amount of fossilized dinosaur bones throughout terrestrial units of the Austral sector of the basin (e.g., Novas et al., 2008; Lacovara et al., 2014). The majority of the distributary channels are infilled with massive sandstone beds interpreted as infill from migrating mouth bars. Other channels display lateral accretion packages, suggesting these channels were long-lived enough to migrate laterally over time (Figure 7). Trace fossil assemblages attributed to the Skolithos ichnofacies assemblage suggest an open to brackish marine environment with high levels of wave/current energy within clean, noncohesive substrates (MacEachern et al., 2010). The erosive, channel-like, and tabular cross-stratified bed set geometries are consistent with a mouth bar succession separated by terminal distributary channels (Olariu and Bhattacharya, 2006; Bhattacharya, 2010). Fine-grained, recessive heterolithic units with abundant plant debris and higher rates of bioturbation likely reflect mouth bar tops and/or levees of adjacent channels (Figure 7). Abundant plant debris, gravel deposits, and mud rip-up clasts common at bases of channels suggest a strong fluvial influence on large-scale delta morphology.

Description . The uppermost exposed strata from the La Anita Formation in the Zona Centro region (Figure 3A) consist of fine to coarse-upper sandstone to pebble conglomerate with well-segregated, pancake-shaped lenses of massive gravel interbedded with undulating laminated and cross-bedded sandstone (LFA-5; Lithofacies 11–12; Figures 5N–O). Bed thicknesses are approximately 10 cm and beds are tabular, albeit undulating, at outcrop scale. Large (up to 1.5 m in length) fragments of petrified wood, dinosaur bones (Figures 6O–P), and root structures are commonly associated with this unit. A recessive, poorly exposed mud-rich, interval overlies this unit and forms a mesa with abundant large (decimeter to meter scale) pieces of petrified wood in float.

Interpretation . LFA-5 represents deposits of a gravel-rich nearshore environment. While individual planar laminations are difficult to identify, the well-segregated, pancake-shaped bedding style may be relict laminations found in a foreshore swash zone or the uppermost shoreface environment (Clifton, 2006). The presence of large dinosaur bones, transported logs of petrified wood, and root structures suggest LFA-5 was deposited in a marginal-marine to terrestrial environment. Well-segregated layers of gravel beds may be indicative of grain size and density partitioning produced by wave swash on the foreshore (Clifton, 1969; Clifton, 2006). The poorly outcropping, muddy interval above LFA-5 is consistent with a low-energy environment such as backshore deposits of a coastal plain. Pervasive cover of this unit, however, prevents further investigation.

We present 193 new paleocurrent measurements sampled from trough and planar cross bedded sandstone and gravel conglomerate from the La Anita Formation. Rose diagrams of paleoflow directions are plotted according to which stratigraphic section the measurements were taken from (Figure 4). In general, our results demonstrate that the paleoflow direction was radially to the south and southeast. These results are consistent with the findings from other workers (Moyano Paz et al., 2018; Sickmann et al., 2019). Paleocurrent directions from sections with interpreted fluvio-deltaic influence (secs. R40, CoC, SpC) record radial dispersion of paleoflow broadly to the south while those from open shoreface and foreshore environments (secs. CC, FC, CgC, RG-3) record a southeast flow direction with subordinate paleoflow to the northeast.

We interpret the La Anita Formation to be deposited in a fluvially dominated and wave- influenced delta and storm and wave-influenced shoreface. The succession records a progradational shelf with basal offshore to lower shoreface facies (LFA-1) overlying continental slope facies of the Alta Vista Formation, which transitions into shoreface and laterally variable foreshore or delta-front environments. An increase in sediment grain size, terrestrial and marginal marine facies and sedimentary structures (e.g., coal beds, rooted horizons, distributary channels, dune-scale cross bedding), and a decrease in heterolithic interstratified mudstone beds and storm-induced event beds are consistent with a prograding delta environment. East-to-west lateral variability from gravelly delta-front mouth bars and channel complexes to wave and storm-influenced upper shoreface and foreshore deposits suggests a lateral transition in depositional environments across the upper La Anita Formation. Thus, sections of the La Anita Formation in the eastern portion of the study area (secs. CoC, R40, SrC, SpC(?)) were in closer proximity to, and likely a part of, the fluvio-deltaic point source providing the majority of the coarse-grained fraction into the basin. Whereas sections further west (e.g., secs. CC, FC, CGC, RG-3) were likely deposited on an open shoreface in which wave and current energy reworked sediments. Channelized beds, lateral accretion packages, coarse sediment size, pebble-sized conglomerate, poor sorting, large petrified logs, and carbonaceous levee deposits all suggest strong fluvial influence on the deltaic portions of the formation. Sedimentation rates were likely consistently high given the pervasive amount of fugichnia escape trace fossils and load structures present in the sandy portions of the unit.

The abrupt contact between fine-grained, pervasively bioturbated sandstone with massive, channelized medium–coarse grained sandstone may represent the contact between the prograding delta front and prodelta (Figure 5L). Paleocurrents are generally to the east-southeast and southeast. Longshore drift and currents generated on the western shoreface may have translated sediment laterally to its deltaic counterparts, producing a mixed river and wave influenced, asymmetric delta system. The ichnofacies present are broadly consistent with a shoaling shallow-marine to marginal marine environment in a moderately physiochemically stressed environment due to high rates of sedimentation (MacEachern et al., 2010; Moyano Paz et al., 2020). Facies analysis by Moyano Paz et al. (2018) proposed that the La Anita Formation should be divided into two informal units bounded by a regional surface of erosion marked by a prominent (ca. 5 m thick) coal bed. From this, those authors argued that the La Anita Formation is composed of two genetically unrelated deltas—with a lower, wave-influenced unit and upper fluvial-influenced unit—driven by a sea-level regression. Stratigraphic sections from this study did not identify any evidence that suggests more than a single prograding facies succession making up the La Anita Formation. Maximum depositional age data from these sections also support coeval deposition of these strata (see Section 4.4). We therefore interpret the La Anita Formation as one progradational unit with lateral facies variability producing mixed input from river and wave activity across an asymmetric delta system. Sections from this study are north of most of the sections reported by Moyano Paz et al. (2018), which may only capture the wave-dominated, fluvial-influenced deltaic lower interval described by those authors.

A total of 20 sandstone samples were collected from the Upper Cretaceous Alta Vista (N = 4; n = 1,159) and La Anita Formations (N = 16; n = 4,060) for U-Pb detrital zircon analysis (Table 2; Supplementary Table S1; Figure 8). We also present five new zircon U-Pb ages of volcanic ashes from a ca. 250 m thick section of the Alta Vista Formation consisting of interbedded fine-grained sandstone and mudstone (sec. AV-1; Figure 3A; Table 2; Supplementary Table S1). Fine to medium-grained, well sorted sandstones were sampled to minimize the potential effects of grain size bias on provenance signatures. Sandstone and ash samples were crushed and zircons were isolated using common mineral separation techniques including a Gemini water table, magnetic separation, and heavy-liquid separation procedures (e.g., Gehrels, 2000; Fedo et al., 2003; Gehrels et al., 2008) at Stanford University’s Earth Materials Preparation Lab. Measurements of U-Th-Pb isotopic ratios from detrital zircon were made using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with either a Nu or Element 2 mass spectrometer at the University of Arizona LaserChron Center following standard analytical methods (e.g., Gehrels et al., 2008; Gehrels and Pecha, 2014). Individual zircons were ablated using Photon Machines Analyte G2 excimer lasers that delivered a 30 μm beam to hand-selected crystal locations. To prevent biasing of differential grain size, zircon spots were selected by choosing grains that were the minimum distance to a spot along an evenly spaced grid of spot locations. Standardized zircon reference material SL-2 (563 Ma; Gehrels et al., 2008) was used to calibrate analyses between every 5 unknown analyses. The Plešovice zircon (337.71 ± 0.37 Ma; Sláma et al., 2008) was used as a secondary standard to assess machine drift at the beginning and end of each run. Approximately 300 grains were targeted per sample. U-Th-Pb ratios that had >10–15% precision, >30% discordance, >5% reverse discordance or >200–400 Pb counts per second were excluded from age interpretation. Due to precision cutoffs in isotopic chronometers, all reported ages <1 Ga are ²⁰⁶Pb/²³⁸U measurements while >1 Ga grains are ²⁰⁶Pb/²⁰⁷Pb ratios with 1σ errors. Data were reduced at the Arizona LaserChron Center using Iolite v. 2.31 software in WaveMetrics Igor Pro following Gehrels et al. (2008) and Gehrels and Pecha (2014).

Kernel density estimates and cumulative frequency diagrams of detrital zircon data, weighted mean averages for depositional age analysis, and multidimensional scaling plots were created using both the detritalPy Python module (Sharman et al., 2018) and DZStats/DZ MDS softwares (Saylor and Sundell, 2016). Individual kernel density estimate plots (bandwidth = 5) were plotted for each individual sample (Figure 8). Plots were colored according to known source locations with distinguishable age ranges, which are also represented as bar graphs for each sample (Figure 8). Samples from the La Anita and Alta Vista Formations were subgrouped by interpreted lithofacies associations (see Section 3) in order to assess the relationship between lithofacies and provenance signatures (Section 5.1).

Calculations of maximum depositional ages (MDA) are reported in Table 2. We present results from the youngest single grain (YSG) per sample, and weighted mean averages from both the youngest 2+ grains within 1σ uncertainty (YC1σ(2+)) and the youngest 3+ grains within 2σ uncertainty (YC2σ(3+); Dickinson and Gehrels, 2009). We opt to use the more conservative YC2σ(3+) MDA method as estimates for depositional ages to avoid estimates that may be younger than the true depositional age (cf. Coutts et al., 2019; Sharman and Malkowski, 2020). Depositional age calculations for ashes were determined using the Youngest Statistical Population (YSP) method (i.e., Coutts et al., 2019; Table 2). This method calculates a maximum depositional age as a weighted average of the subset of ≥2 zircon grains that yield a mean square weighted deviation (MSWD) of approximately 1. We utilize multidimensional scaling (MDS) plots (e.g., Vermeesch, 2013; Vermeesch, 2018) to assess multi-sample detrital zircon age spectra dissimilarities across all samples from the La Anita and Alta Vista Formations. These plots represent a “map” onto a dimensionless plane or space wherein samples that are more similar to each other—according to the comparative metrics—plot closer to each other. Conversely, dissimilar samples plot further away from each other. This transformation is achieved through an iterative process that attempts to minimize the misfit, also known as the “stress” value, between degree of correspondence of MDS map distances and the comparative matrix selected by the user. High stress values (>0.1; Kruskal, 1964) indicate a poor fit wherein the dimensional “flattening” of data onto a plane or space yields distortions unrepresentative of the actual comparative metrics. We use both the V value of the Kuiper test and the D value of the Kolmogorov-Smirnov (K-S) test, as well as a metric-squared stress criterion for transforming matrices onto an MDS plane while maintaining the lowest possible stress values. Resulting V and D value MDS plots yield stress values of 0.14 and 0.08 respectively (Supplementary Figure S2). These reasonable stress values indicate that distances represented on these maps are reliable for multi-sample dissimilarity comparisons across the Alta Vista and La Anita Formations.

Interstratified tuffaceous rocks from a 250 m thick section (section AV-1; Figure 3A; Figure 9A) of mudstone and thin-bedded turbidites were sampled from outcrops of the Alta Vista Formation approximately 3 km west of the primary study location in Zona Centro (Figure 3A). We present calculated weighted mean ages using the Youngest Statistical Population method (see Section 4.2) of these ash beds as well as a detrital zircon maximum depositional age from one interstratified fine-grained sandstone from this section (sample RG-01). We also calculate maximum depositional ages from three samples collected from ca. 600 m thick exposure of the Alta Vista Formation near Cerro Mora (sec. CM; Figure 3A). Zircon U-Pb ages from ash beds range from 90.43 to 86.30 Ma while detrital zircon maximum depositional ages are between 88.2 and 79 Ma (Table 2). Ash bed ages bracket the depositional age of the Alta Vista Formation between the late Turonian and early Santonian while a maximum depositional age from the uppermost exposed portion of the Alta Vista Formation extends its potential maximum age to the middle Campanian (Figure 3B).

Previous studies bracketed the depositional age of the Alta Vista Formation between the late Santonian and middle Campanian (Macellari et al., 1989; Arbe, 2002; Sickmann et al., 2018). Based on these new ash age results, we suggest that deposition of the lower Alta Vista Formation onto the continental slope may have been active as early as the late Turonian. Basin shoaling and coeval deposition of shallow-marine deposits of the La Anita Formation occurred by ca. 86 Ma based on contemporaneous Alta Vista ash ages and the maximum depositional ages from the La Anita Formation from both this study and others (Sickmann et al., 2018; Ghiglione et al., 2021; Table 2).

Calculated MDAs from the La Anita Formation range from 85.9 to 75.3 Ma, with all but one sample bracketed between ca. 86–80 Ma (Table 2). This places the age of the La Anita Formation between late Santonian and early Campanian, which is consistent with MDA estimates presented in previous studies (Sickmann et al., 2018; Ghiglione et al., 2021). These results are also in partial agreement with Campanian assignments of ammonite fossils from the lower La Anita Formation (Arbe and Hechem, 1984; Macellari et al., 1989; Kraemer and Riccardi, 1997; Arbe, 2002). We argue that the MDAs are, in general, apt approximations of a true depositional age due to 1) the broad agreement between these age data and biostratigraphic constraints, 2) the proximity of the active Andean arc (Hervé et al., 2007), which provided the ca. 60% of all analyzed zircons from this study, and 3) consistency with results presented from other Cretaceous formations in the Magallanes-Austral Basin (e.g., Bernhardt et al., 2012; Malkowski et al., 2017a; Malkowski et al., 2017b; Schwartz et al., 2017; Daniels et al., 2018; Malkowski et al., 2018) that have demonstrated age agreement between calculated MDAs and other geochronological methods such as biostratigraphic data and volcanic ash U-Pb geochronology.

Maximum depositional ages across measured sections are commonly in the same age ranges within uncertainty. An exception to this observation is from section SpC, which yielded two younger, middle Campanian MDAs (79.5 ± 0.93 Ma and 75.3 ± 1.29 Ma; Table 2). While this section is along-strike from adjacent measured sections across the study area, section SpC is located near the axis of a shallowly dipping (<5° dipping limbs) syncline (Ghiglione et al., 2014). This structural low may have produced a zone of increased stratigraphic preservation in the region, which could have sheltered younger strata within a zone of reduced uplift and erosion.

Using ash ages and detrital zircon MDAs from the Alta Vista and La Anita Formations, we compare the stratal ages from the Austral (Argentine) portion of the basin to those in the Magallanes (Chilean) portion of the basin using a recently updated chronostratigraphy compiled by Daniels et al. (2019) (Figure 3B). New age data presented herein provide the most detailed chronostratigraphic constraints of the Alta Vista and La Anita Formations, which places their depositional timing between 90.43–79 Ma and 85.9–75.3 Ma, respectively. These age ranges have the most overlap with the abyssal plain facies and deep-water coarse clastic sediments of the Cerro Toro Formation (ca. 90–80 Ma; Daniels et al., 2019; Figure 3B). The Alta Vista Formation coincides mostly with the lower siltstone prone member of the Cerro Toro Formation (ca. 90–86 Ma) whereas the La Anita Formation has the most overlap with the Lago Sofia conglomerate defining the upper portion of the Cerro Toro Formation (ca. 88–81 Ma). One sample from the Alta Vista Formation produced an MDA (sample CM-04 YC2σ(3+) MDA = 79.0 ± 0.19 Ma; Table 2) that comports with the earliest slope mass transport deposits of the lower Tres Pasos Formation (80.5–77.4 Ma). One sample from the La Anita Formation produced an MDA (sample SpC-02 YC2σ(3+) MDA = 75.3 ± 1.29 Ma; Table 2) that overlaps with the shelf-slope clinoform deposits of the upper Tres Pasos and Dorotea Formations (77.4–67.1 Ma). This may suggest that the progradation of these units continued into the late Campanian. However, given that all but two samples coincide with the timing of the Cerro Toro Formation deposits, we argue that the Cerro Toro Formation is the most likely candidate as a genetically related southern counterpart to the Turonian through middle Campanian deposits of the northern depocenter. Further sampling of the uppermost Alta Vista and La Anita Formations is necessary to better constrain the latest depositional ages for these units. The co-appearance of deep-water conglomeratic facies from the upper Cerro Toro Formation with coarse-grained, shallow-marine deposits of the La Anita Formation may signal a synorogenic sedimentation event controlled by the progressive easterly advance of the orogenic front in the Austral and Magallanes portions of the basin (Ghiglione et al., 2021). Late Coniacian through early Santonian progradation of the Austral continental slope occurred in concert with the deposition of the deep-water Lago Sofia conglomerate, although deposition of lower Alta Vista Formation (sometimes grouped with the Cerro Toro Formation; e.g., Ghiglione et al., 2014; Sickmann et al., 2018; Sickmann et al., 2019) occurred as early as the late Turonian (ca. 90 Ma).

Three primary detrital components make up the provenance spectrum recorded in siliciclastic units of the Magallanes-Austral Basin: 1) pre-Jurassic metamorphic complexes representing uplift of basement rock via fold-and-thrust belt deformation to the west (>200 Ma), 2) Jurassic volcanic and volcaniclastic rocks associated with synrift volcanism as a result of the breakup of Gondwana (ca. 188–142 Ma), and 3) latest Jurassic and Cretaceous volcanic and magmatic materials derived from the Patagonian arc and batholith (ca. 157–75 Ma). These sectors can be delineated by their unique assemblage of geochronological ages in magmatic and recycled zircons from both primary and recycled sources. The following sections provide detail for each source area.

Zircon age spectra from the Alta Vista Formation (N = 4; n = 1,159) are consistent with derivation from known terranes that sourced the Magallanes-Austral Basin during the Cretaceous. These include Paleozoic metamorphic and metasedimentary basement complexes and recycled Upper Jurassic V3 extensional volcanic rocks that represent exhumation of the basement domain via tectonic development of the fold-and-thrust along the eastern flank of the South Patagonian batholith (48% of total analyzed grains; 157–142 Ma; 310–260 Ma and older Paleozoic ages; Faúndez et al., 2002; Pankhurst et al., 2000; Hervé et al., 2003; Hervé et al., 2008; Hervé et al., 2010; Figure 10), Jurassic synrift volcanic rocks associated with distal sources of the Deseado and North Patagonian Massifs to the north and northeast (2% of total analyzed grains; V1–V2: 188–162 Ma; Gust et al., 1985; Pankhurst et al., 1998; Pankhurst et al., 2000; Calderón et al., 2007; Figure 10), and Patagonian arc/batholith rocks that occupied the western Patagonia convergent margin (50% of total analyzed grains; 144–75 Ma; Hervé et al., 2007; Figure 10).

Similar to the Alta Vista Formation, detrital zircon data from the La Anita Formation (N = 16, n = 4,060) contain age ranges consistent with known source terranes for the Magallanes-Austral Basin (Figure 8). These include grains from recycled metamorphic basement complexes and overlying V3 extensional volcanic rocks (35% of total analyzed grains; Faúndez et al., 2002; Hervé et al., 2003; Hervé et al., 2008; Hervé et al., 2010; Figure 10), Jurassic extra-basinal volcanic terranes (4% of total analyzed grains; 188–162 Ma; Gust et al., 1985; Pankhurst et al., 1998; Pankhurst et al., 2000; Calderón et al., 2007; Figure 10), and the Patagonian arc and batholith (61% of all analyzed grains; 144–75 Ma; Hervé et al., 2007; Figure 10). General age distributions are in agreement with detrital zircon distributions from age equivalent strata analyzed by other workers (Sickmann et al., 2018; Sickmann et al., 2019; Ghiglione et al., 2021).

Detrital zircon from the La Anita Formation was primarily derived from the Cretaceous Patagonian volcanic arc and batholith, with secondary contributions from recycled grains from the foreland fold-and-thrust belt and Jurassic volcanic rocks (Figure 8). Jurassic zircons are likely sourced from V3 synrift volcanic units exposed along the Patagonian fold-and-thrust belt, as well as V1–V2 extensional volcanic rocks from the North Patagonian and Deseado Massifs (Pankhurst et al., 2000; Calderón et al., 2007). Paleozoic basement underlies the North Patagonian and Deseado Massifs, which may also provide some amount of sediment into the basement (Pankhurst et al., 2006; Chernicoff et al., 2013; Moreira et al., 2013). However, the co-appearance of Late Jurassic V3 grains with Paleozoic material suggests that the more proximal Paleozoic basement exposed along the fold-and-thrust belt is a more likely source for this age peak. Moreover, V1–V2 age peaks are in greater abundance in samples with a lesser amount of Paleozoic grains, suggesting it is less likely they are derivative of the same source area (e.g., LFA-5 samples, Figure 8). The presence of these age signals within the La Anita Formation suggests catchment sizes may have been large enough to source distal (hundreds of kms) sediments into the basin by Santonian–Campanian time, a finding observed in other Cretaceous, shallow-water strata in the region (e.g., Malkowski et al., 2017a; Schwartz et al., 2017; Sickmann et al., 2019). The presence of a distinct Early Jurassic peak at ca. 180 Ma unique to extra-basinal sources from the North Patagonian Massif indicates that some proportion of Jurassic sediment is far traveled (Ghiglione et al., 2015; Navarro et al., 2015). Alternatively, Early Jurassic ages may have been recycled from Aptian–Albian fluvial deposits that also contain 180 Ma grains located north of the terminus of the basin and west of the Deseado Massif, and north of the Golfo San Jorge Basin (Figure 10; Ghiglione et al., 2015).

To examine the relationship between lithofacies and detrital zircon spectra, we plot cumulative frequency and multidimensional scaling plots of the Alta Vista and La Anita Formation samples grouped by assigned lithofacies association (Figure 11). Binning these data according to interpreted depositional environment yields a provenance trend of increasing arc and distal massif-derived volcanic rock components and decreasing contributions from the recycled material derived from the exhumation of basement within the fold-and-thrust belt that correlates with shoaling of depositional environments (Figure 11A). Zircon spectra from LFA-1 and LFA-2 tend to have a lesser contribution from the Cretaceous Andes (LFA-1: 54%, LFA-2: 53%) and a moderate contribution from Paleozoic basement and overlying V3 volcanic rocks (LFA-1: 45%, LFA-2: 43%). Age distributions for LFA-3/4 and LFA-5, however, have a higher signal strength from the Cretaceous Andes (LFA-3/4: 64%, LFA-5: 73%) and a lesser contribution from the adjacent fold-and-thrust belt (LFA-3/4: 31%, LFA-5: 22%). We also note a 1–3% increase in the amount of V1–V2 peripheral volcanic sources from LFAs-3–5 when compared to LFA-1–2 (Figure 11). This trend is apparent in both the variation in compiled lithofacies datasets as cumulative frequency distributions and in the grouping of lithofacies associations in multidimensional scaling space (Figures 11A,B). We interpret this correlation as evidence for dynamic, competing erosion and transport processes and/or downstream dilution along the continental slope, shelf and shoreline, which yielded intraformational heterogeneity in detrital zircon sample age distributions.

Samples from the shallowest environments of the La Anita Formation (i.e., upper shoreface, foreshore, and delta front lithofacies) are the most arc-dominant (avg. ∼69%; 144–75 Ma) and all contain ages associated with Early Jurassic V1–V2 volcanic rocks from extra-basinal terranes. Large, well-developed regional watersheds likely provided sediment from the nearby volcanic arc edifice and structural highs tapping far-traveled sediment from the Deseado and North Patagonian Massifs (Figure 10). Arc-derived zircon abundance decreases noticeably in samples from further offshore environments (i.e., lower shoreface/inner shelf and middle shoreface lithofacies; LFA-1, 2; avg. ∼54% of all analyzed grains; Figure 11). Likewise, the amount of Paleozoic and Late Jurassic grains interpreted as being derived from the adjacent fold-and-thrust belt increases to an average of 44%. The progressive decrease in arc material and increase in fold-and-thrust belt zircon imply that enhanced shallow-marine erosive processes transporting local sediment and the increased distance from that La Anita delta, which provided regionally derived zircon, led to progressive downdip mixing between local and regional sources (Figure 12). Paleocurrent directions from lithofacies representative of offshore depositional environments and from regions more laterally distant from interpreted fluvio-deltaic facies are predominantly to the southeast, with subordinate amounts to the northeast. Fluvio-deltaic facies record radial dispersion to the southwest, southeast and south (Figures 4; 12). The bimodal dispersal pattern of further downdip facies may be indicative of offshore, marine currents redistributing sediment via wave or tidal actions whereas radial patterns broadly southward likely represent deposition via distributary channels of a delta (Selley, 1968; Miall, 1978; Figure 7). Offshore facies may have been progressively susceptible to reworking via marine currents that delivered local, fold-and-thrust belt derived sediment from the northwest into the basin. Thus, while these lithofacies were deposited contemporaneously and in proximal distance to each other, the relative source abundances varied markedly as a function of competition between fluvio-deltaic transport and subsequent reworking via shallow-marine transport processes.

The proximity to La Anita delta facies appears to also contribute to detrital zircon variability in continental slope facies of the Alta Vista Formation. Sample RG-01 from the Alta Vista Formation is a part of a mud-rich section of interstratified mudstone and thin-bedded, fine-grained turbidite deposits (Figure 9A). No channelized units, major erosive beds, thick-bedded sandstone beds, or channel-levee systems were noted in this region. Calculated aggradation rates from ash ages of this section were relatively low (34 m Myr⁻¹; see Section 4.3.1). We interpret section AV-1 as being off-axis from the locus of a sedimentation, such as an unincised portion of the continental slope (Figure 12). Conversely, the samples from section CM contain large, thick-bedded sandstone beds, which are amalgamated and several meters thick towards the top of the section (Figure 9D). These units are locally isoclinally folded, which we interpret as synsedimentary folds due to slumping and sliding on oversteepened portions of the continental slope (Figures 9C–E). We therefore argue that section CM represents a portion of continental slope proximal to, or a part of, an on-axis, deep-water channel system or shelf-edge delta front that received high amounts of sediment from the La Anita delta. Sample RG-01 from section AV-1 is primarily made up of Paleozoic basement and Late Jurassic grains derived from the fold-and-thrust belt (∼73% of all analyzed grains; Figures 8, 12). This suggests that the primary source for sample RG-01 was from local, small-area drainages tapping the adjacent uplifted basement within the Cretaceous fold-and-thrust belt. Notably, this sample contains no zircon grains attributed to V1–V2 distal massif sources. This indicates that the La Anita delta may not have contributed sediment to this portion of the continental slope. Instead, “off-axis” portions of the slope received sediment from slope failures that only contained locally derived sediment. Conversely, the samples from section CM (Figure 2; Figures 9B–E) have zircon distributions most similar to the shallow-water environments from the La Anita Formation (Figure 8; Figure 11). This marked variation between the sandier and muddier portions of the Alta Vista Formation highlights the impact of fluvio-deltaic connectivity to the continental slope on resulting provenance signatures. Given the large number of amalgamated sand beds making up section CM, we argue that this section was likely proximal to the sand-rich La Anita shelf-edge delta or deposited near a submarine canyon system (Figure 12). In either scenario, these samples support the hypothesis that portions of the continental slope and shelf that were down paleoslope of the primary depositional pathway of the La Anita fluvio-deltaic system had direct connectivity to its onshore counterparts via canyon incision onto the continental shelf (e.g., Monterey Canyon, California; Paull et al., 2011) or progradation of the La Anita shelf-edge delta (e.g., Dorotea-Tres Pasos Formations; Hubbard et al., 2010; Schwartz and Graham, 2015). An alternative explanation for the systematic variation in detrital zircon spectra along depositional gradient is grain-size biasing (e.g., Lawrence et al., 2011; Ibañez-Mejia et al., 2018; Cantine et al., 2021). Although grain-size dependent processes may influence resulting detrital zircon deposition, we have mitigated this potential complication via sampling sandstone beds ranging from fine to lower-medium grained sand. Moreover, recent work by Leary et al. (2020) demonstrates that grain size and detrital zircon spectra are not significantly correlated. Instead, the relative input between distally derived sources and local uplifts appeared to exert a stronger control on resulting zircon distributions. A more detailed study that incorporates grain size analysis with detrital zircon work is necessary to further detail this potential complication.