A “cool-water”, non-tropical, mixed volcaniclastic–carbonate ramp from the Early Cretaceous of southern Chile (45°40’S)

Rivas, Hermann; Salazar, Christian; Stinnesbeck, Wolfgang

doi:10.1007/s10347-023-00669-4

A “cool-water”, non-tropical, mixed volcaniclastic–carbonate ramp from the Early Cretaceous of southern Chile (45°40’S)

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Published: 16 June 2023

Volume 69, article number 14, (2023)
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A “cool-water”, non-tropical, mixed volcaniclastic–carbonate ramp from the Early Cretaceous of southern Chile (45°40’S)

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Abstract

The Aysén-Río Mayo Basin was a back-arc/marginal basin developed in southwestern South America (43°–47°S) between the Tithonian–Aptian. Its sedimentary fill corresponds to the Coyhaique Group, which represents a transgressive–regressive succession. Six lithofacies and five microfacies were defined for three outcrops exposed south of Coyhaique (45°40’S). The outcrops have a mixed calcareous–volcaniclastic composition and were assigned to the early transgressive Toqui Formation, i.e., lowermost part of the Coyhaique Group. These mixed rocks comprise bioclastic–volcaniclastic conglomerate, gravelly allochemic sandstone, and gravelly–sandy allochem limestone. Bedding is sharp to amalgamated, sometimes rippled, depicting a wave- and storm-influenced, mixed inner- to mid-ramp. The ramp developed over a Valanginian, active volcanic terrain (Foitzick Volcanic Complex), source of the volcaniclastic sediments. Limestones are rich in reworked bioclasts, and controlled by calcitic organisms including gryphaeid oysters, non-geniculate red algae, and echinoid fragments, defining a heterozoan association (“maerl”-like sediments); less frequent are ahermatypic corals, serpulids, and carbonized wood. Based on their inferred paleolatitude (south of 45°–50°S), fossil assemblage (heterozoan), and kind of carbonate platform (ramp-type), these calcareous rocks of the Toqui Formation depict a “cool-water” (sensu lato), non-tropical setting. The fossil assemblage includes oysters (Aetostreon spp.), and abundant calcareous red algae attributed to Archamphiroa jurassica Steinmann (1930), a taxon previously known from the upper Tithonian Cotidiano Formation of Argentina. A. jurassica is here reported for the first time for the Lower Cretaceous of Chile, suggesting a broader upper Tithonian—Valanginian-Hauterivian? range for the species. The facies model presented here contrasts with the depositional environments depicted for correlative reefal rocks in Argentina (Tres Lagunas Formation), which reflect a “warm-water” setting. In the Aysén-Río Mayo Basin, the influence of sea-water key physical variables in the carbonate sedimentation, as well as the position and hydraulic regime of the carbonate platforms within the basin, and their interaction with the volcanism are still unclear.

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Introduction

Mixed calcareous–volcaniclastic rocks are common constituents in the Late Jurassic–Early Cretaceous of southwestern South America (Central Patagonia, 43°–47°S); these deposits represent the early transgressive phase in the Aysén—Río Mayo Basin, whose marine stage developed during the Tithonian–Aptian (e.g., Aguirre-Urreta and Ramos 1981; Ramos 1981; Scasso 1987; 1989; Bell et al. 1996; Folguera and Iannizzotto 2004; Suárez et al. 2010a, b; Fig. 1A, B). In Chile, these mixed rocks are exposed as scattered outcrops in the Los Lagos and Aysén regions (De la Cruz et al. 1996; Welkner et al., 2004) and have been referred to as the Toqui Formation (Bell et al. 1994; Suárez and De la Cruz 1994a,b; Fig. 1A, B). This unit comprises two members (sensu Rivas et al. 2021): the mixed, calcareous–volcaniclastic Manto Member conforming the basal portion of the Toqui Fm., and the volcaniclastic San Antonio Member which overlies and interfingers the Manto Member. Both members display marked lateral- and latitudinal facial changes, reflecting strong diachronism during the Tithonian–Valanginian interval (Suárez et al. 2005; 2010a, b), and the development of mixed, shallow-marine platforms over an irregular extensional topography (Scasso 1989; Hechem et al. 1993; Folguera and Iannizzotto 2004; Rivas et al. 2021).

In Argentina, the Toqui Formation has been correlated with coeval (Tithonian–Valanginian), calcareous and mixed units exposed in the area adjacent to the La Plata—Fontana lakes (e.g., Cotidiano and Tres Lagunas Formations; Sect. “Stratigraphy and correlations”; Fig. 1A). Overall, these mixed rocks represent transgressive deposits, depicted as settled in euhaline, shallow marine environments, and locally influenced by active volcanism (e.g., Ramos 1981; Ploszkiewicz and Ramos 1977; Olivero 1982; Scasso 1987; 1989; Bell et al. 1996; Suárez et al. 1996; De la Cruz et al. 2003; Scasso and Kiessling 2002; Iannizzotto et al. 2004).

Given the discrete, discontinuous, and diachronic exposure of these mixed calcareous–volcaniclastic units, they have been studied and interpreted separately (see review in Rivas et al. 2021; Fig. 1C), and several regional models for its sedimentation have been proposed (e.g., Skarmeta 1976; Aguirre-Urreta and Ramos 1981; Scasso 1989; Hechem et al. 1993; Bell et al. 1994; Townsend 1998; Folguera and Iannizzotto 2004; Suárez and De la Cruz 1994a, b). Here, a sedimentological and microfacial analysis is presented of three mixed volcaniclastic-calcareous sections, assigned to the Toqui Formation, and exposed south of Coyhaique in southern Chile (45°40°S; Fig. 1D). This study improves the understanding of the depositional environments of mixed rocks included in the Toqui Formation, and their relationship with the Argentinian exposures is also discussed. The outcrops presented here provide new insight into non-tropical or “cool-water” carbonate sedimentation as well as the development of carbonate ramps in volcanic arc settings.

Geological setting

The Aysén—Río Mayo Basin was a back-arc/marginal basin (see models without seafloor-spreading in Tarney et al. 1981; Artemieva 2023) formed at the southwestern margin of West Gondwana (proto-South America, ca. 43°–49°S) during the Late Jurassic and Early Cretaceous (Tithonian—Aptian, e.g., Aguirre-Urreta and Ramos 1981; Suárez et al. 2010a, b; Fig. 1B). Its onset is regarded as triggered by back-arc extension to the east of the coeval volcanic arc (Suárez et al. 2010a, b; Echaurren et al. 2016), which is represented by the Patagonian Batholith (Pankhurst et al. 1999; Suárez and De la Cruz 2001; Suárez et al. 2023; Fig. 1C). The basin developed unconformably over the basement, which consists of upper Paleozoic metamorphic and metasedimentary rocks as well as Lower Jurassic sedimentary units (e.g., Olivero 1982; Ploszkiewicz 1987; De la Cruz et al. 2003; Hervé et al. 2008; Fig. 1A).

The oldest units conforming the Aysén—Río Mayo basin fill, correspond to the Ibáñez Formation in Chile and the correlative Lago La Plata Formation in Argentina (e.g., Ramos 1976; Haller et al. 1981; Haller and Lapido 1982; De la Cruz et al. 1994; 2003; Fig. 1A). The Ibáñez Formation consists of calc-alkaline, subaerial- and minor submarine-settled, felsic volcanic- and volcaniclastic rocks, depicting the Late Jurassic–Early Cretaceous (Oxfordian?—early Hauterivian) extrusive counterpart of the Patagonian Batholith (Pankhurst et al. 2003; Suárez et al. 2005). These volcanic rocks syn-tectonically filled the extensional topography (Echaurren et al. 2016; Folguera and Iannizzotto 2004), whereas its uppermost strata underlie and interfinger with the shallow marine deposits of the Toqui Formation and correlative units (see Sect. “Stratigraphy and correlations” below), i.e., the lowermost part of the Coyhaique Group (e.g., Skarmeta 1976; Olivero 1982; Scasso 1989; Covacevich et al. 1994; Suárez et al. 2009; 2010a; Fig. 1A).

The Coyhaique Group is a transgressive–regressive succession of Tithonian–Aptian age (Ramos 1981; Scasso 1987; Covacevich et al. 1994; Bell et al. 1994; Suárez et al. 2010a; Suárez and De la Cruz 1994a, b), and linked to a back-arc “Pacific” marine incursion caused by tectonic-thermal subsidence (Skarmeta 1976; Haller and Lapido 1980; Aguirre-Urreta and Ramos 1981). In Chile, the Coyhaique Group includes three formations (Fig. 1A): (1) the Toqui Formation (Tithonian—Valanginian, Hauterivian?), corresponding to shallow-marine, mixed (calcareous–volcaniclastic) and tuffaceous rocks, deposited during the early transgression (De la Cruz et al. 1994; Suárez et al. 2009; Rivas et al. 2021); (2) the Katterfeld Formation (Valanginian-Barremian; Masiuk and Nakayama 1978; Aguirre-Urreta et al. 2000; Olivero and Aguirre-Urreta 2002; Kesjár et al. 2017) overlies the Toqui Fm. with a transitional or conformable contact and consists of tuffaceous and carbonaceous black mudstone, settled in offshore environments during the post-rift stage (Scasso 1989; Suárez et al. 2007; 2010a); and 3) the Apeleg Formation (Hauterivian-Aptian) overlies the Katterfeld Formation with a gradational contact; it includes heterolithic, sandy, and gravelly facies, depicting tidal, and deltaic-paralic, regressive successions (e.g., Ramos 1981; Scasso 1989; Gonzalez-Bonorino and Suárez 1995; Bell and Suárez 1997; Fig. 1A).

The Aysén-Río Mayo Basin was inverted during the Aptian, linked to compression after a fast, westwards relative movement of the South American Plate (Suárez et al. 2010a, b; Gianni et al. 2019; Ramos et al. 2019). The latter reactivated the volcanic arc, resulting in local Surtseyan eruptions (Baño Nuevo Volcanic Complex; Demant et al. 2010; Suárez et al. 2010b; Fig. 1A), and the effusion of subaerial, intermediate, volcanic rocks of the Divisadero Group (Aptian—early Cenomanian). The Divisadero Group covers the underlying Coyhaique Group with a unconformable, but locally (para-)conformable contact (e.g., Aguirre-Urreta and Ramos 1981; Ramos 1981; De la Cruz et al. 1994; Suárez et al. 2010a; Fig. 1A).

Stratigraphy and correlations

In central Patagonia (43°–47°S), the early transgressive deposits of the Aysén-Río Mayo Basin (ARB) comprise a Tithonian – Hauterivian? range (e.g., Haller and Lapido 1980; Olivero 1983; 1987; Scasso 1989; Covacevich et al. 1994; Suárez et al. 2009), reflecting strong diachronism during the transgression (Suárez et al. 2005; Fig. 1A). The early transgressive rocks of the ARB have a mixed- and calcareous composition, and they depict shallow marine settings near volcanic centers (e.g., Ramos 1978; Suárez et al. 1996, 2009; Scasso and Kiessling 2002; Ramos and Lazo 2006; Rivas et al. 2021, and references therein).

In Argentina, these rocks have been treated as singular units (Fig. 1A), including: (i) the Cotidiano Formation (Oxfordian-Kimmeridgian sensu Ramos 1976; Ramos 1981; upper Tithonian sensu Olivero 1987; Bucur et al. 2009); (ii) the “Arroyo Pedregoso Beds” (upper Tithonian sensu Ramos 1981; Olivero 1982; 1987); (iii) the “Arroyo Blanco Beds” (Berriasian-Hauterivian sensu Olivero 1982; 1987; upper Valanginian sensu Olivero and Aguirre-Urreta 2002; Fig. 1A); and (iv) the Tres Lagunas Formation (at its type locality “Laguna Salada”, Valanginian-lower Hauterivian sensu Ploszkiewicz and Ramos 1977; Berriasian sensu Olivero 1983; Tithonian?—upper Valanginian sensu Scasso and Kiessling 2002; Fig. 1A).

The former three were regarded by Olivero (1987) as sedimentary alternations within the Lago la Plata Formation; this approach was followed by Covacevich et al. (1994) for some calcareous outcrops exposed north of the town of Ñireguao, including them within the Ibáñez Formation. In the Chilean side, these mixed- and calcareous outcrops were later grouped together in the Toqui Formation (e.g., Bell et al. 1994; De la Cruz et al. 1996; 2003; Suárez and De la Cruz 1994a, b). The Toqui Formation was redefined by Rivas et al. (2021; after Bell et al. 1994; Suárez et al. 1996; Suárez and De la Cruz 1994a, b), as conformed by two members: the mixed calcareous–volcaniclastic Manto Member, interbedded and overlied by the volcaniclastic San Antonio Member. In the area south of Coyhaique (45°35’S; Fig. 1D), these members appear discretely as mixed, either calcareous- or volcaniclastic-rich outcrops, depicting an upper Berriasian/lower Valanginian – Hauterivian? range (see “Age of the deposits”).

Age of the deposits

In the study area, the age of the Toqui Formation is interpreted to be late Berriasian/early Valanginian-Hauterivian? South of “Laguna Foitzick”, a late Berriasian age is based on a faunal association including Neocomitidae indet., and the similarity of this fauna with Berriasian outcrops exposed north of Ñireguao, at the “Estero La Horqueta” (Covacevich et al. 1994; Suárez et al. 1996; De la Cruz et al. 2003; Fig. 1C). In Laguna Foitzick, the Toqui Formation overlies the Foitzick Volcanic Complex, a local member of the Ibáñez Formation, with an erosion-angular unconformity, but locally displaying a peperitic contact (Bell et al. 1994; Suárez et al. 1996; Suárez and De la Cruz 1994a, b; Fig. 1A). There, rocks from the uppermost volcanic strata, below the unconformity, were dated to U–Pb 138.3 ± 1.3 Ma (Pankhurst et al. 2003), providing a “Valanginian or even younger” lower range limit for the sedimentary succession in the Coyhaique area (Suárez et al. 2005). A broader Berriasian-Hauterivian age (“Neocomian”) was inferred by Rubilar (2000) near the “Salto del Pollux”, based on a tentative new species of Aetostreon sp. In the same locality, Cecioni and Charrier (1974) reported findings of “Belemnopsis patagoniensis”, commonly found in the Lower Cretaceous of southernmost South America (Riccardi 1977; Aguirre-Urreta 2002; Ippolitov et al. 2015).

The top of the Toqui Formation is not exposed in the study area; however, in its type locality (El Toqui Mine), it pass transitionally to tuffaceous- and black mudstone of the Katterfeld Formation (Bussey et al., 2010; Rivas et al. 2021). At the Salto del Pollux locality (outcrop SRP2; Fig. 1D), the Toqui Formation is in tectonic contact with black mudstone of the Katterfeld Formation. There, the marine succession is confined by a post-depositional basaltic andesite sill dated to Ar–Ar 61 ± 0.4 Ma (“Muralla China Sill”; Petford and Turner 1996; De la Cruz et al. 2003), intruding both the Toqui- and Katterfeld Formations (Fig. 1D). Regionally, the Katterfeld Formation provides a Valanginian—Barremian upper range limit for the Toqui Formation (Masiuk and Nakayama 1978; Aguirre-Urreta et al. 2000; Olivero and Aguirre-Urreta 2002; Kesjár et al. 2017).

Materials and methods

This study comprises three sedimentary sections exposed between south of the Laguna Foitzick and the waterfall of the Pollux River (known as “El Salto”, Fig. 1D), between 8 and 11 km south of Coyhaique, capital of the Aysén Region, Chile (45°34’S; Fig. 1B–C). They are accessed via Route 7. The three outcrops define a ca. 67 m-thick composite log (outcrops LRO, MCH, SRP2; Fig. 1D), and they have been assigned to the Toqui Formation (Suárez and De la Cruz 1994a, b; De la Cruz et al. 2003). Based on their lithology, these stratified rocks are here arranged in six lithofacies (Table 2); five microfacies were additionally defined based on the petrographic analysis of fourteen thin sections (Table 1). The outcrop LRO has been previously addressed (Bell et al. 1994; Townsend 1998; Suárez and De la Cruz 1994a, b) as well as outcrop SRP2 (Katz 1961).

Table 1 Summary of the petrography and microfacies

Full size table

Grain-size categories used here follow Blair and McPherson 1999, and modal compositions of sandstone are based on Garzanti 2019. Lithological description of limestone is after Wright (1992), whereas non-genetic schemes were preferred for mixed- (Mount 1985), and volcaniclastic sediments (Fisher 1961; 1966; Fisher and Schmincke 1984; Cas and Wright 1987). A brief glossary of the volcanic terms applied here can be found as supplementary material; the term “volcaniclastic” was preferred (instead of “volcanic”), to highlight the epiclastic nature of the volcanic material (e.g., volcaniclastic sandstone). Given the mixed nature of most of rocks analyzed here, depositional models for the interpretation of siliciclastic (e.g., Clifton 2006; Plint 2010), and for carbonate environments were required (e.g., Burchette and Wright 1992; Flügel 2010). Raw lithological data taken in the field are presented as supplementary material (Tables SM-1 to SM-3).

Measurements of microfossils were registered with the software ImageJ, using micro-photographs taken with an Olympus camera, model PEN Lite E-PL7. SEM images of thin sections were taken with a microscope ZEISS WITec RISE EVO MA15 at the Universität Heidelberg, following the procedure of Munnecke et al. (2000). Calculations were made in Microsoft Excel. Chronostratigraphic categories used here correspond with the ICS International Stratigraphic Chart v2022/02 (Cohen et al. 2013; updated). Illustrations were created with Adobe Illustrator CS6. Rocks and fossils samples are kept in the Geology Department of the Universidad Mayor in Santiago, Chile, as a temporary repository.

Facies analysis

The litho- and microfacies of the Toqui Formation analysed here have a mixed volcaniclastic–bioclastic composition. Rocks were classified into six lithofacies (see “Lithofacies”; Table 2), supported by the characterization of five microfacies (Table 1), presented below.

Table 2 Summary of facies associations, litho- and microfacies, and inferred depositional environments. Lithofacies occurrences (typography): bSGm = major, bSGm = secondary, bSGm = rare

Full size table

Petrography

Microfacies comprise both ends of mixed rocks (calcareous- and terrigenous-rich), varying between gravelly–sandy allochem limestone and gravelly allochemic sandstone (sensu Mount 1985). Calcareous-rich microfacies include volcaniclastic–bioclastic, pack–rudstone, float–wackestone, and grainstone (microfacies Mf-1 to Mf-3; Table 1, Figs. 2; 3). On the other hand, terrigenous-rich microfacies consist of bioclastic–volcaniclastic, quartzo-litho-feldspathic- to quartzo-feldspathic sandstone (microfacies Mf-4, Mf-5).

Calcareous-rich microfacies

Calcareous-rich microfacies comprise mostly skeletal allochems, and scarce intraclasts (Figs. 2; 3). Bioclasts are reworked and dominated by bivalves (mostly oysters), calcareous red algae, and echinoid fragments (Figs. 2A–F; 3A). Red algae is controlled by Archamphiroa jurassica Steinmann (1930), and scarce Parachaetetes sp. and “solenoporoids” (Bucur, pers. com., 2020; Fig. 2E, F). Less abundant bioclasts include reworked serpulids (Fig. 3A), bryozoans (Fig. 3G) and, rarely, benthic foraminifers (Fig. 3F) and ostracods. Non-skeletal allochem comprise intraclasts composed of bioclastic wackestone (intraclasts type 1, Fig. 3B), or formed of “cloudy” radiaxial fibrous calcite (intraclasts type 2; Fig. 3C). Bioclasts are sub-angular- to sub-rounded, usually affected by microboring, or sometimes conforming cortoids (Fig. 2B, F).

Matrix is neomorphic (microspar), but patches of mud-peloids and micritic envelopes were also observed (Fig. 3L). Intergranular porosity was filled by thin layers of isopachous and dog-toothed calcite, followed by granular, blocky, and syntaxial cements (Flügel 2010; Figs. 2C, E, F; 3K, L).

Terrigenous-rich microfacies

The terrigenous part is dominated by oligomictic, non-vesicular volcanic clasts and isolated crystals (Fig. 2C, I-K). Volcanic rock fragments are sub-angular to sub-rounded; they have an intermediate composition (quartz-trachyte) with a trachytic-porphyritic texture (potassium feldspar phenocrysts, minor plagioclase, scarce pyroxene and quartz; Fig. 3I, J). In contrast, felsic rock fragments are less frequent (rhyolite, rhyodacite < 10%), displaying a felsitic or micropoikilitic texture (Fig. 3J). Volcanic groundmass is pale brown-colored, partly devitrified or sericitized, or locally replaced to carbonates, Fe oxides-hydroxides, silica (chalcedony), or scattered opaques (Figs. 2A, D, K; 3I, J). Discrete crystals are dominated by euhedral–subhedral feldspars (mostly sanidine, minor orthoclase and plagioclase), slightly to moderately altered (Figs. 2K; 3I). Quartz is angular to sub-rounded, sometimes displaying resorption (Figs. 2J, K; 3I). Infrequent minerals include micas (mostly biotite; Figs. 2J, 3G), and rare rounded glauconite.

The terrigenous matrix is clay-silt-sized, brown- to gray-colored (Figs. 2G, H; 3G–I), sometimes stained by Fe oxides-hydroxides (Figs. 2A, B). It bears silt-sized, angular crystals and opaque minerals, and it usually displays solution seams (Figs. 2G, H; 3H). In crossed nicols, it appears isotropic or partly sericitized, hindering its differentiation from discrete lithics and reflecting a similar composition with them (Fig. 3I, M–O). Locally, some altered-compacted lithics have been converted to pseudomatrix (Fig. 2A). Cementation is minor, including silica and clay cements (Figs. 2K; 3G), and wispy-shaped chert (Fig. 3I); but also, scattered calcite or dolomite (Figs. 2C, I–K, 3J).

Microfacies

Five microfacies were defined (microfacies Mf-1 to Mf-5); they are briefly described below and summarized in Table 1. These will be referred to in the following sections, linked to their associated lithofacies and outcrops. Detailed compositional information of each thin section can be found as supplementary material (Table SM-4).

Microfacies Mf-1: It comprises sandy–gravelly, volcaniclastic–bioclastic packstone to rudstone, with grainstone patches (Fig. 2A, B). Its fabric is poorly washed and massive, but locally imbricated (bivalves). Framework is rich in bioclasts and altered trachytic rock fragments embedded in argillaceous microspar, locally associated with mud-peloids (Fig. 3L), or clayey matrix (pseudomatrix, Fig. 2A, B). Bioclasts includes mostly oysters, followed by red algae and echinoids; serpulids (some of them attached to oysters; Fig. 3A) and intraclasts type 2 are sporadic; bryozoans and calcispheres are scarce (Fig. 3E).

Microfacies Mf-2: Formed of sandy–gravelly, volcaniclastic–bioclastic float–wackestone, with packstone patches (Fig. 2D). This microfacies is analog in fabric and allochems to Mf-1, though poorer sorted, richer in big-sized bioclasts, and poorer in volcanic lithics. Additionally, its neomorphism is stronger, with coarser microspar (pseudo-grainstone), calcite veins and solution seams (“fitted” packstone; Fig. 2D). Novel fossils include scarce inoceramid fragments.

Microfacies Mf-3: It comprises sandy volcaniclastic–bioclastic grainstone, and red algae-rich grainstone (sub-type Mf-3a; Fig., 2F), with rud-, wacke-, or packstone patches (Fig. 2E, F). Its fabric is massive or locally showing aligned bioclasts. In contrast to previous microfacies, this one is poorer in volcanic clasts, and allochems are better rounded and sorted. Bioclasts comprise mostly bivalves, red algae, and subordinate echinoids plates (Fig. 2E, F). Less frequent allochems include intraclasts type 2 (Fig. 3C), and scarce bryozoans, carbonized wood (Fig. 3D), benthic foraminifers (Fig. 3F), and ostracods. Matrix is patchy (microspar, mud-peloidal; Fig. 2F), or it occurs as micritic envelopes; cementation is analog to previous microfacies (see Sect. “Calcareous-rich microfacies”), though more abundant (Fig. 3K).

Microfacies Mf-4: This microfacies is composed of gravelly-muddy, mixed bioclastic–volcaniclastic, quartzo-feldspatho-lithic to feldspatho-lithic sandstone (Figs. 2C, I, J; 3I, M–O). Microfacies Mf-4 is poorly sorted, either rich in devitrified matrix, or bearing organic matter (Mf-4om; Figs. 2G, 3E, H). Some samples also show patches of calcitic cements (granular and poikilotopic, sub-type Mf-4c; Fig. 2C, I). Its framework is mostly composed of sub-angular to sub-rounded volcanic lithics, and angular to sub-angular crystals. It displays a subtle parallel lamination (Fig., 2I, J), noticed by aligned elongated bioclasts or micas (mostly biotite, minor white mica).

Volcanic lithics are mostly intermediate, but some samples bear considerable felsic clasts (proportion intermediate:felsic = 9:1 to 6:4; Fig. 3I). Many rock fragments concentrate opaques in the core (Figs. 2G; 3H) or, when devitrified, they are replaced by chert (Fig. 3I), sericite, or calcite (pumice? Fig. 3M, N). Bioclasts comprise mostly reworked bivalves, and minor red algae and echinoids (Fig., 2C, G, I); bryozoans and intraclasts type 1 are scarce (Fig. 3B). Matrix is argillaceous-carbonaceous, and cements were already described (see “Terrigenous-rich microfacies”; Figs. 2I, K; 3G; M–O). Solution seams are common, concentrating insoluble residues and organic matter (Figs. 2G, 3H).

Microfacies Mf-5: Formed of (muddy) volcaniclastic quartzo-feldspathic sandstone with subtle parallel lamination (Fig. 2J, K). Its framework is fine- to medium-grained, dominated by angular to sub-angular crystals (mostly feldspars, followed by quartz, and minor lithics and micas). Volcanic lithics are subordinated, usually devitrified, or altered (Fig. 2K). Argillaceous matrix is scarce. Cements include calcite/dolomite, usually spotted or altering the framework (Fig., 2K); titanite was also observed. Bioclast and micas are elongated and imbricated; grains appear fractured in a preferential direction.

Interpretation of microfacies

Calcareous-rich microfacies reflect a major carbonate sedimentation incorporating reworked, older-altered volcanic rocks, or sporadic input of fresh volcaniclastics. Carbonate mud might have an allochthonous origin (mud-peloids, bioeroded clasts, absence of fecal pellets), and a minor bacterial origin (cortoids, microbial structures). Cementation depicts a phreatic marine environment and the subsequent porosity-filling during burial diagenesis (Flügel 2010). However, intraclasts type 2 may indicate erosion of aragonite-filled cavities in a coastal setting (later recrystallized to calcite; see Kendall and Tucker 1973). Coarse-grained, rud-floatstone microfacies (Mf-1, Mf-2) reflect a near-coast reworking influenced by currents (aligned bioclasts) and a rapid settling (poorly-sorted). Grainstones (Mf-3) depict a near-coast, good hydraulic sorting (Flügel 2010).

Terrigenous-rich microfacies indicate a submarine-settled, volcaniclastic sedimentation. For microfacies Mf-4, rounded lithics and abundant shell debris reflect epiclastic transport or coastal reworking. Non-vesicular volcanic rock fragments, and their core-margin differential alteration may be related to phreatomagmatism or quenching, respectively (Fisher and Schmincke 1984; Cas and Wright 1987). Allochems are analog to the ones found in the calcareous microfacies (Mf-1 to Mf-3), they were most likely swept off- (shell debris) or eroded from near-coast areas (cemented?). Given their poor textural maturity (matrix-rich) and devitrified matrix, these microfacies are interpreted as formed after remobilization of non-consolidated deposits. However, organic-rich samples (e.g., phosphatic algal cysts in Mf-4om; Fig. 3E) indicate sedimentation in a poorly oxygenated setting or during a period of high productivity (Capelli et al. 2021). Microfacies Mf-5 depicts remobilization of crystal-rich volcaniclastics, likely fractionated during the eruption or concentrated during the epiclastic transport (see Chapter 11 in Cas and Wright 1987). Its parallel lamination, and the incorporation of bioclasts is linked to current-transport. Calcareous cements are interpreted as diagenetical, while fractured crystals and titanite might reflect burial diagenesis and circulation of high-T pore fluids.

Lithofacies

Six lithofacies were defined in the study area; they are presented in decreasing grain-size order (Table 2). The main lithology is shown in upper case (e.g., R: rudstone; SG: sandstone > conglomerate, also sandy conglomerate), whereas lower case prefixes indicate composition (e.g., b: bioclastic; v: volcaniclastic). Suffixes refer to structures (e.g., p: planar bedding; m: massive). Sedimentary logs and their associated lithofacies can be found in Fig. 4.

Mixed bioclastic–volcaniclastic, sandy conglomerate (bGSm)

This lithofacies is formed of bioclastic–volcaniclastic, sandy conglomerate, arranged in tabular beds (Fig. 5). Bedding is 0.3–0.4 m-thick, with sharp contacts, usually marked by changes in gravel content (Fig. 5I). The framework of lithofacies bGSm is matrix-supported (locally clast-supported), conformed by pebble-sized clasts (diameter: max = 2.5 cm, average = 1 cm), embedded in a coarse- to very coarse-grained, sandy–muddy calcareous matrix (Fig. 5E, I, J). Exceptionally, a singular bed of lithofacies bGSm bears cobble-sized lithics (sub-facies bGSm-g; clasts-size up to 15 cm, average 2–5 cm; Fig., 5G). Gravel is formed of massive to diffusely graded, volcanic rocks and frequent bioclasts (Fig. 5E, G, J). Lithics are poorly-sorted, rounded- to sub-angular, light-colored and mono-oligomictic. Fossils are reworked, highly fragmented, and poorly packed; bioclasts are dominated by bedding-concordant, platy oyster shells (length up to 10 cm; Fig. 5E), though some beds also bear in situ and reworked colonial corals (Fig. 5J) as well as scarce carbonized wood remains.

The matrix of lithofacies bGSm consists of sandy–gravelly, volcaniclastic–bioclastic, packstone–rudstone, with grainstone patches (microfacies Mf-1; Fig. 2A, B).

Interpretation: Based on its mixed volcaniclastic–calcareous composition, coarse-grained and poorly sorted components, and reworked sessile and free-living epifauna, lithofacies bGSm represents deposition in a relatively high-energy coastal environment (Leithold and Bourgeois 1984; Plint 2010). Coarse-grained coastal sediments are usually linked to the reworking of fluvial mouth-bar deposits (Bourgeois and Leithold 1984; Hart and Plint 1995), though the mono-oligomictic composition of the volcanic clasts suggests erosion of a local source, as observed in some volcanic islands (Felton 2002; Felton et al. 2006). The tabular beds, poor sorting, and matrix-supported fabric, may indicate a rapid settling, likely occurring during storms (Leithold and Bourgeois 1984); the latter is also supported by the packstone and rudstone microfacies (Flügel 2010). Mixture of sub-rounded and sub-angular clasts might indicate abrasion in the beachface, during storm swells (“swash area”; Hart and Plint 1989; 1995).

Oyster-rich rudstone/coquinite (bRp)

The lithofacies bRp consists of an accumulation of cobble-sized, broken bivalve shells (40%), mostly oysters. Beds are sharp-bounded and decimeter-thick. Shells are fragmentary, platy- and wedge-shaped, arranged chaotically or concordant with bedding (Fig. 6I). They are clast-supported and embedded in a light grey-colored, fine-grained calcareous matrix.

Interpretation: This lithofacies corresponds to a basal lag formed by shell debris (Collinson et al. 2006). Based on its coarse grain-size, dense packing, and bedding-concordant bioclasts, it is interpreted as a “sedimentologic concentration” deposit, conforming proximal tempestites settled over the normal-weather wave base (Kidwell et al. 1986), and likely transported by storm-induced bottom flows (Flügel 2010).

Mixed bioclastic–volcaniclastic, gravelly sandstone (bSGm)

This lithofacies resembles the mixed conglomerate facies (bGSm), but it is finer grained. It consists of bioclastic–volcaniclastic, gravelly sandstone to granule-pebble-sized, sandy conglomerate (Fig. 5I). Strata of lithofacies bSGm are decimeter-thick, crudely bedded, amalgamated or grading from the gravel-rich ones (bGSm), though some sharp bed-boundaries are sinuous (Fig. 5C, D). Internal structures were not observed; they were likely effaced during weathering (Fig. 5D). Sandstone is medium- to very coarse-grained, whereas conglomerate is granule- to pebble-sized (clasts-size up to 1–2 cm, average < 1 cm), and usually sandy matrix-supported. Its framework is rich in volcanic lithics and fragmented oyster shells (length < 2 cm), and minor corals in some strata (Fig. 5I).

Under the microscope, the matrix of lithofacies bSGm varies between sandy, volcaniclastic–bioclastic limestone (microfacies Mf-1; Fig. 2B), and gravelly, bioclastic–volcaniclastic, quartzo-feldspatho-lithic sandstone (microfacies Mf-4c; Fig. 2C).

Interpretation: Analog to bGSm, lithofacies bSGm depicts a subtidal environment. Better sorting of clasts and oriented shell debris reflect current-influenced hydraulic sorting. Sinuous-topped, amalgamated beds depict periodic erosion–deposition pulses (Collinson et al. 2006), and subsequently wave- or current reworking (Walker and Plint 1992; Clifton 2006); whereas sharp contacts might represent reactivation surfaces. These tabular beds could represent the formation of gravelly–sandy low relief bedforms linked to alongshore currents (similar to Fig. 14 in Hart and Plint 1995; also “bedload-sheets” sensu Carling 1999; or “carbonate sandbodies” sensu Wright and Burchette 1996). These bedforms are typically developed on the shoreface (Hart and Plint 1995).

Rippled, bioclastic (volcaniclastic) limestone (bLr)

The lithofacies bLr is composed of gravelly–sandy, bioclastic limestone, with a minor volcaniclastic content. It is arranged in tabular layers with sinuous bed boundaries, usually amalgamated (Fig. 6C–E). Based on the grain-size and scale of ripples, two subtypes were defined: (i) bLr-m: “mega-rippled” (sensu Swift et al. 1983), or coarse-grained-rippled (sensu Leckie 1988), gravelly, bioclastic limestone (Fig. 6D, E); and (ii) bLr-o: oscillation-rippled, sandy, bioclastic limestone (Fig. 6F, G).

The mega-rippled subtype (bLr-m) consists of sandy–gravelly float–rudstone, bearing pebble- to cobble-sized bioclasts and volcanic lithics (1–2 cm-length, max = 9 cm-length in broken-disarticulated oyster shells). The framework is massively arranged, but, locally, it displays a sub-parallel arrangement of shells (Fig. 6E). Limestone is sharply bedded, arranged in decimeter to meter-thick bedsets with sinuous contacts (Fig. 6D). Internally, the sub-type bLr-m displays symmetric large-sized ripples (wavelength ≈ 50–90 cm; height ≈ 15–20 cm; Fig. 6D). On the other hand, the oscillation-rippled subtype (bLr-o) is formed by mixed, gravelly–sandy calcarenites (grain size 1–5 mm, max: 1.7 cm), displaying smaller and regular symmetric ripples, and subordinate asymmetric ripples (wavelength ≈ 20–30 cm; height ≈ 10 cm; Fig. 6F, G).

The matrix of lithofacies bLr-m consists of sandy–gravelly, volcaniclastic–bioclastic float–wackestone with packstone patches (microfacies Mf-2; Fig. 2D), whereas sub-type bLr-o is composed of sandy, bioclastic rudstone–grainstone, and red algae-rich grainstone (microfacies Mf-3, Mf-3a; Fig. 2E, F).

Interpretation: This lithofacies depicts a shallow marine environment emplaced above the fair-weather-wave base (Burchette and Wright 1992; Flügel 2010). Mega-ripples (bLr-m) have been regarded as the equivalent of hummocky cross-stratification for coarse-grained sediments, which are typically linked to storm-surge deposits (Leckie 1988; Cummings et al. 2009). Based on the previous, and on their poorly sorted microfacies (Mf-2), they are interpreted here as calcareous proximal tempestites (analog to “Facies A” of Pérez-López and Pérez-Valera 2012). On the other hand, given its grain size, better sorted microfacies (Mf-3), and symmetrical rippled-beds, lithofacies bLr-o reflects an oscillatory flow typical of fair-weather wave action, and minor influence of near-coast unidirectional currents (Collinson et al. 2006; Plint 2010).

Parallel-bedded, bioclastic–volcaniclastic, muddy sandstone (bSp)

Lithofacies bSp crops out as sandy, bioclastic mudstone to calcareous muddy sandstone; it is dark grey-colored, and it expels a fetid odor when crushed (Fig. 7C, E). Its bedding is arranged in decimeter-thick layers with diffuse, flat-amalgamated contacts (10–40 cm-thick; Fig. 7A-C, E, H). Beds are commonly friable; however, some better cemented strata serve as guide-beds, these have sharp margins with a rippled top or base (sub-facies bSp-c; Fig. 7B–D, H). Bedding is internally massive, but some beds display a coarse-tail grading of bioclasts (i.e., muddier tops, or diffuse amalgamated ripples (Fig. 7D). Shell debris are abundant, regularly to well-sorted, and highly fragmented (ca. 20–30% bioclasts, 2–3 mm-length; Fig. 7E); they comprise mostly bed-concordant bivalves, and some echinoids fragments. Other complete macrofossils are scattered, including reworked oysters and belemnites (Fig. 7F, G, I, J).

Microscopically, lithofacies bSp consists of gravelly–muddy, mixed bioclastic–volcaniclastic sandstone (microfacies Mf-4, Mf-5 subordinate).

Interpretation: Based on its amalgamated tabular beds, parallel lamination (microfacies Mf-4, Mf-5), sandy framework, bimodal bioclasts (fragmented-complete), and grading, this lithofacies reflects waning flows with an intermediate character between proximal (i.e., coarse-grained, bioclast-dominated; similar to “Facies B” of Pérez-López and Pérez-Valera 2012) and distal tempestites (i.e., interbedded with mud; Flügel 2010). The latter are usually difficult to differentiate from turbidites (e.g., “turbidite-like tempestites” sensu Myrow 1992; 2005), however, turbidites are finer grained (mud to fine-sand sensu Shanmugam 1997, 2002, 2016). Its volcaniclastic composition reflects remobilization of non-consolidated deposits to relatively quiet, soft-bottom subtidal settings.

Laminated, tuffaceous muddy sandstone (tSp-b)

This lithofacies is formed of bioclasts-bearing, medium-to-coarse-grained, muddy sandstone (Fig. 7C, H, I). Sandstone is friable, light grey-colored, slightly calcareous, and partly altered to phyllosilicates. Bedding is arranged in centimeter-thick layers with parallel bedding (Fig. 7C, H), and its framework is rich in white-colored, sand-sized rounded clasts, interpreted as altered pumice (Fig. 7I). Subordinately, it bears sand-sized micas and glauconite. Its fossil content includes reworked left valves of gryphaeid oysters (Fig. 7I).

Interpretation: Based on its composition and alteration, this lithofacies is interpreted as a submarine-settled, tuffaceous sandstone (remobilized tuffaceous deposits sensu McPhie et al. 1993). Given its grain size, lamination, and fossil content, it may reflect sedimentation by currents.

Sedimentary logs and facies associations

This section describes three sedimentary logs (Fig. 1D) as well as their facies associations and paleoenvironmental interpretation. Outcrops have mixed composition varying from calcareous-rich (LRO, MCH) to volcaniclastic-rich lithofacies (SPR2); they are described from north to south. The detailed description of each sedimentary log and their beds, compiled in the field, has been provided as supplementary material (Tables SM-1 to SM-3).

La Rosita Section (LRO)—gravelly, mixed inner ramp

This locality comprises at least three grey-colored, wedge-shaped small outcrops, exposed in a ca. 1 km radius, about 8 km to the south of Coyhaique (Figs. 1D; 5B–D; 4A). The central outcrop, here described (LRO), is better exposed and consists of a 17-m-thick succession slightly inclined to the SSW (strike/dip = 105/13; Figs. 5A–D). The outcrop displays several overhangs with diffuse contacts, after amalgamation and differential weathering of the strata (Fig. 5D). These rocks include mixed, volcaniclastic–bioclastic conglomerate and sandstone (lithofacies bGSm; bSGm; Fig. 5E, G, H–J), and subordinate bioclastic–volcaniclastic sandstone (bSp, Fig. 5I).

At the base, mixed rocks overlie meter-sized, volcanic boulders of the Foitzick Volcanic Complex with an erosive unconformity (De la Cruz et al. 1994, 2003; Fig. 5F). In addition, an angular unconformity, and local peperitic contact have been reported from adjacent, correlative outcrops (Bell et al. 1994; Suárez et al. 1996; Suárez and De la Cruz 1994a, b). The top of the LRO section is not exposed (Fig. 5B–D).

In the LRO section, the mixed succession is arranged in decimeter-thick, tabular beds, commonly amalgamated (avrg. 0.3–0.4 m, max: 0.8–1.0 m, probably fused beds; Fig. 5C–D). Upsection, sediments are better sorted and their bedding is clearer (Fig. 5C). Coarsest, poorly sorted gravelly beds are usually thicker and more common in the lowermost part of the outcrop (bGSm; bGSm-g, Fig. 5E, G, J), changing to a gravelly–sandy alternation in the central part (bGSm; bSGm; Fig. 5C, D). The last third of the outcrop displays a thinning- and fining-upwards trend, dominated by gravelly sandstone with diffuse- and sharp, sinuous contacts (lithofacies bSGm; bSp; 0.3–0.5 m-thick; Fig. 5C); some beds are richer in volcaniclastics (microfacies Mf-4c, Table 1).

Macrofossils are reworked and fragmented, dominated by reworked oysters (rarely big-sized specimens of Aetostreon sp.1; Fig. 5H), but colonial corals were also found in the lowermost beds, some of them in life position (Bell et al. 1994; Townsend 1998; and here, see Fig. 5J); belemnites are scarce and broken. Articulated- and encrusting oysters in life position have also been reported from this outcrop (“Exogyra” sp. sensu Bell et al. 1994; Townsend 1998), but they were not observed during this work.

Interpretation: The La Rosita outcrop depicts a relatively shallow marine environment, placed above the fair-weather wave-base. Its sub-environments comprise a deepening-upwards succession from a storm-influenced, gravelly upper shoreface towards a sandy lower shoreface (Hart and Plint 1995). These are here interpreted to represent a mixed volcaniclastic-carbonate inner ramp (sensu Burchette and Wright 1992; Fig. 14.3 in Flügel 2010). The fragmented fossil content, including sessile and free-living epifauna, reflects the development of local communities, though not forming reefs (“ahermatypic corals” sensu De la Cruz et al. 2003), and rather grew as discrete small crusts over the coastal gravel deposits.

These gravelly deposits represent the marine erosion over a rocky shoreline (coastal cliff), eroding an older volcanic terrain (Suárez et al. 1996). Based on the inferred “onlap” contact of these rocks above the volcanic complex (illustrated in Fig. 5 in Suarez and De La Cruz 1994a; also in Fig. 3.10 in Townsend 1998; Cover Photo in De la Cruz et al. 2003; and Fig. 5B here), also reported as locally peperitic (Bell et al. 1994; Suarez and De La Cruz 1994a, b), these rocks might reflect transgressive deposits settled over a volcanic flank (De la Cruz et al. 1994; Bell et al. 1994; Suárez et al. 1996; Suárez and De la Cruz 1994a, b). In particular, the gravelly facies (bGSm) may represent transgressive conglomerates (Suárez et al. 1996).

Muralla China Section (MCH)—wave- and storm-dominated inner-ramp

This section is located 2.8 km to the south-southeast of section LRO, near the northern end of a sub-horizontal sill, locally known as “Muralla China” (Figs. 1D; 4B; 6B–C). The succession comprises a wedge-shaped, steep outcrop formed of ca. 34 m of gravelly–sandy, bioclastic limestone dipping to the southeast (strike/dip = 060/20; Fig. 6A). The limestone conformably overlies pale grey-green-colored, diffusely bedded, volcanic rocks of the Foitzick Volcanic Complex (sensu Bell et al. 1994; Suárez and De la Cruz 1994a, b, see Fig. 6A). Gravel-sized volcanic clasts found within the calcareous beds suggest erosion over this basal unit. The top of the MCH succession is not exposed (Fig. 6A).

In the MCH outcrop, the lower and uppermost beds are coarser grained, conformed of volcaniclastic–bioclastic, mixed gravelly–sandstone (lithofacies bSGm; Fig. 6E), and mega-rippled bioclastic limestone (lithofacies bLr-m; Fig. 6D, F, G). The central part of the outcrop is relatively homogeneous and consists of well-sorted, wave-rippled bioclastic limestone (bLr-o; Fig. 6C, F, G). Bedding is tabular to wedge-shaped, displaying a subtle, fining- to coarsening-upwards trend (Fig. 6B). Bedding planes are commonly amalgamated, sinuous, and only recognized by tafoni superimposed on ripples (Fig. 6C, F). Sharp boundaries are subordinate, these are interpreted as erosive or reactivation surfaces (Fig. 6C).

The fossil content is fragmentary, though reworked, but articulated big-sized oysters (Gryphaeidae indet; Aetostreon sp.1.; Fig. 6M, N) and other small pectinids (Entolium? sp.; Fig. 6K), as well as bulb-shaped fish teeth (Pycnodontidae indet.), were found scattered in the middle part of the section (Fig. 6A, C). Near the top, complete, small- and medium-sized, oyster left-valves assigned to Aetostreon sp. and A. sp.2 (Fig. 6H) are abundant in mixed, gravelly floatstone beds. The latter were also found as small clusters, but detached from its original bed (ex situ; Fig. 6J). Exceptionally, some lenses of carbonized wood were observed (Fig. 6L) and, at the northern end of the outcrop, a lenticular bed of oyster-rich rudstone/coquinite was found (bRp; Fig. 6I).

Interpretation: Based on wave ripples (Walker and Plint 1992), mostly comminuted shells, and packstone–grainstone microfacies, these sediments suffered wave-reworking above the fair-weather wave-base, i.e., in the inner-ramp (Burchette and Wright 1992; Flügel 2010). This mixed inner ramp was affected by periodical storm-surges, as shown by mega-rippled limestone, coarse-grained coquinites, and layers with a bimodal distribution of bioclasts interpreted as tempestites (Kidwell et al. 1986; Flügel 2010; Fig. 6H). Storm-currents were strong enough to remobilize and rapidly bury big-sized, reclining oysters (Aetostreon spp.). These energetic events were periodic as shown by amalgamated layers and pinched-out bed-sets, enclosed by erosive surfaces (Fig. 6C).

This outcrop is described here for the first time. Regarding composition, thickness, and inferred age (Sect. “Stratigraphy and correlations”), the Muralla China limestone corresponds to one of the most remarkable Lower Cretaceous calcareous outcrop reported from the Aysén-Río Mayo Basin.

Salto Río Pollux Section (SRP2)—mixed, distal mid-ramp

This outcrop is located about 1 km to the south of the Muralla China Section (MCH), at the northern hand of Route 7, just before reaching the Pollux River (Fig. 1D). It is composed of ca. 9 m of dark-colored, bioclastic muddy sandstone dipping towards the southeast (strike/dip = 041/10; Figs. 4C; 7A, B). Its base and top are not exposed; however, the outcrop is capped by the Muralla China sill (see 1.2; Fig. 7A-C) and, about 100 m to the east, this outcrop is separated from the black mudstones of the Katterfeld Formation by an inferred fault (Fig. 1D).

The Salto Río Pollux outcrop comprises bioclastic–volcaniclastic, muddy sandstones (lithofacies bSp), alternated with calcareous sandy beds (bSp-c; Fig. 7A). The former (bSp) are more abundant in the lower half of the outcrop, exposed as poorly defined, amalgamated beds (Fig. 7C, H). Some of them are rich in organic matter (microfacies Mf-4om; Fig. 7E). In contrast, calcareous beds (bSp-c), stand out as weathering-resistant guide beds (Fig. 7B–D). Bed-boundaries between these both lithofacies are sharp and sinuous (Fig. 7C, D). The middle point of the outcrop is marked by a 0.3 m-thick, fine interbedding of fossil-bearing, mixed sandstone (bSp) and tuffaceous sandstone (lithofacies tSp-b; Fig. 7C, H, I).

Bioclasts are abundant, dominated by bedding-concordant shell debris (mostly oysters; Fig. 7E). Complete macrofossils include scattered belemnite rostra (Belemnopsis sp.), and left valves of Aetostreon sp.2 (“Aetostreon sp. nov.” Rubilar 2000; Fig. 7F, I, J here), the latter also found conforming small clusters in life position (Fig. 7J).

Interpretation: Based on the matrix-rich, mixed sandstone (microfacies Mf-4), its bimodal fossil distribution (comminuted-complete), the amalgamation of its beds, and its sedimentary structures, these deposits reflect “event sedimentation” or “eventites” (Flügel 2010). They are interpreted as mixed calcareous–volcaniclastic tempestites settled in a mixed, distal mid-ramp (Wright and Burchette 1996; Flügel 2010), i.e., between the fair-weather- and the storm wave-base (Burchette and Wright 1992; Ramalho et al. 2013).

These tempestites show grain size differences between gravelly-grained shelly fragments and sandy-sized volcaniclastics, indicating a turbulent hydraulic sorting (Kidwell and Bosence 1991; Immenhauser 2009; Figs. 3I; 7E). The abundant comminuted shelly fragments were most likely swept off from the breaker- or swash area and were offshore-transported during storm-surges (Flügel 2010). As shown from the muddy matrix and well-preserved complete fossils, storm currents interrupted the relatively quiet background sedimentation, rapidly burying (or mildly reworking) the small, thin-shelled, recliner oysters in life position (Aetostreon sp.2; Fig. 7F, I–J) as well as other corporal fossils (e.g., belemnites; Fig. 7G). These small-sized Aetostreon sp.2 likely proliferated in a water–sediment interface affected by sporadic events of eutrophication, inferred from its organic matter content (Nori and Lathuilière 2003).

The high terrigenous content of the Salto Río Pollux beds reflects a major volcaniclastic supply (Table 1; Figs. 2G–K; 3M–O). Given the interbedding with tuffaceous beds (lithofacies tSp-b; Fig. 7C, H), and the similar composition of volcanic lithics and sedimentary matrix (Microfacies Mf-4; Fig. 3I, M–O), a remobilization of non-consolidated volcaniclastic deposits is inferred (syneruptive?, McPhie et al. 1993; Schneider 2000). Previously, these volcanic deposits were classified as “volcanic coquinas” by Katz (1961). This author inferred a poorly-oxygenated, marine environment for these rocks, linked to a coeval, subaerial- and submarine volcanic eruptions. The former is supported by the fetid-odor and organic matter content of these rocks (microfacies Mf-4; O’Brien and Slatt 1990; Ulmer-Scholle et al. 2014), and it could be linked to settling in a restricted environment or during a period of high biological productivity (Capelli et al. 2021). Regarding the volcanism, there is not enough evidence to support a subaqueous source.

Systematic paleontology

Oysters and red algae are the most abundant fossils found in the outcrops analyzed here. This section presents a detailed description of newly found fragments of the calcareous red alga Archamphiroa jurassica, identified here for the first time in the Lower Cretaceous of Chile (see “Calcareous-rich microfacies”; Fig. 2E, F; 8A–I). In addition, some oysters of the Aetostreon genus are also described (Fig. 9).

Archamphiroa jurassica

Class Rhodophyceae Rabenhorst 1863

Order ?Corallinales Silva and Johansen 1986

Genus Archamphiroa Steinmann 1930; emend. Bucur et al. 2009

Archamphiroa jurassica Steinmann 1930 (Pl. 1, Figs. 1–9]); emend. Bucur et al. 2009 (neotype, Figs. 2–3).

Type material: The neotype was designated by Bucur et al. (2009) in algal wackestone of the Cotidiano Formation (see Fig. 1A, C). According to these authors, a neotype was required given the loss of the original material, in which no holotype had been defined (see Steinmann 1930).

Type locality: Puesto Cotidiano (44°50’S; 71°39’W), at the isthmus connecting the La Plata and Fontana lakes in the Chubut province of Argentina (Fig. 1C). Originally defined by Steinmann (1930) at the Arroyo Negro, tributary of the Malargüe River in the Neuquén province of Argentina (ca. 35°S).

Material: Dozens of thallus fragments from the La Rosita (LRO, five thin sections; Fig. 4A) and Salto Río Pollux outcrops (SRP2, four thin sections; Fig. 4B); hundreds of thallus fragments from the Muralla China beds (MCH, six thin sections; Fig. 4C).

Description: Because of their small size, fragmentary preservation, strong cementation, and similar color to that of the calcareous matrix-cement, thalli of A. jurassica are difficult to identify in hand samples. The following characteristics were used here for the assignation in thin section (based on Steinmann 1930; Bucur et al. 2009).

External: Thallus fragments are singular or frequently bifurcated, with rounded margins (Fig. 8A, C, E, F–I). Their cross-sections are circular, or ellipsoidal when obliquely cut (Fig. 8E, G, I); longitudinal views are ellipsoidal or sub-rectangular (Fig. 8A–C, H). When bifurcated, thalli are lambda-shaped, branches appear parallel-arranged, showing no hinge-like structure or constriction at the point of bifurcation (i.e., non-geniculate; Fig. 8F–H). Given their size and “dotted”, internal cell arrangement, thalli fragments might be confused with echinoids, though red algae display a clear fan-like extinction (Fig. 8H, I).

Internal: Thalli of A. jurassica are two-layered, with a central hypothallium covered by an outer perithallium (Fig. 8A, C, F–I). The hypothallium is multilayered coaxial, conformed of an alternation of dark- and pale-colored arched cellular layers, convexly-oriented towards the apical part (Fig. 8A, C, F). Internally, the arches comprise elongated filaments, which diverge in an acute fan-shaped fashion (Fig. 8F, H). Filaments of the perithallium are oriented perpendicular to the thallus margin (Fig. 8A, E–G, I). In cross-section, the hypothallium appears darker or with a structureless cell arrangement, contrasting with the radial pattern observed in the perithallium (Fig. 8E, G, I).

In order to confirm peripheral cellular fusion, regarded as a diagnostic feature of this taxon, samples were observed using SEM, but without successful results. This may be linked to diagenetical alteration or incorrect preparation of the samples (see SEM-images as supplementary material). Nevertheless, the assignation was confirmed by the Prof. Dr. Bucur (pers. com., 2020), with whom an algae-rich sample was shared.

Dimensions: The main size parameters for A. jurassica are summarized in Table 3 (n = 128, single measures are presented in Table SM-5 as supplementary material):

Table 3 Measures of Archamphiroa jurassica taken in standards thin sections (summary)

Full size table

Remarks:

Reproductive structures are absent. However, many thallus fragments show occasional circular, club-shaped, and irregular cavities as well as small, marginal vermicular borings, caused by macro- and microborers (Fig. 8A–B, E).

Based on its cylindrical thallus shape and non-geniculate bifurcation, a fruticose to arborescent growth form is inferred for Archamphiroa jurassica (sensu classification of Woelkerling et al. 1993; see Fig. 10A, A’).

Stratigraphic range: An early Callovian age was inferred for the beds bearing Archamphiroa jurassica in Central Argentina by Steinmann (1930), based on its association with Upper Jurassic brachiopods (“Rhynchonella acuticosta”) and corals (Thamnasteria sp.). In Southern Argentina, a late Tithonian age was suggested for the Cotidiano Formation by Bucur et al. (2009), unit bearing the A. jurassica neotype, type specimen defined by the same authors. The latter was based on the presence of Steinmanella (Macrotrigonia sp.) and Megatrigonia cf. fontanaensis, and on the close similarity of this fauna with the one found at Arroyo Pedregoso, which has been ammonite-dated to late Tithonian by Olivero (1982; based on Berriasella sp. and Micracanthoceras ruedai).

Outside southern Argentina, Archamphiroa jurassica Steinmann (1930) is now described from Coyhaique, Chile. This red alga is abundant in the mixed-calcareous outcrops here presented (see Sects. “La Rosita Section (LRO) - gravelly, mixed inner ramp”–“Muralla China Section (MCH) - wave and storm-dominated inner-ramp” above). The age of these outcrops is restricted to the late Berriasian/early Valanginian -Hauterivian? (see “Age of the deposits”). Therefore, this extends the range of A. jurassica to the Early Cretaceous, in particular, between the Tithonian-Valanginian (Hauterivian?).

Aetostreon spp.

Superfamily Ostreoidea Rafinesque 1815

Family Gryphaeidae Vialov 1936

Subfamily Exogyrinae Vialov 1936

Tribe Nanogyrini Malchus 1990

Genus Aetostreon Bayle 1878

Type species: Gryphaea latissima Lamarck 1801, p. 399.

Diagnosis (modified from Stenzel 1971; Cooper 1995; Toscano and Lazo 2020): Medium- to large-sized, thick-shelled, inequivalve. Opisthogyrate umbo, gyrostreoid or exogyroid ligament area. Left valve: convex and deep; right valve: flat or slightly concave. Left valve thickened in the crest zone of umbonal ridge, displaying a keel in the posterior third. Keel is rounded to acute, commonly surmounted by knobs. Shallow groove parallel to the keel separates a slightly more convex posterior flank. No ornamentation except for growth lines or occasionally radial wrinkles. Internally, adductor muscle scar usually subtruncate antero-dorsally, but sometimes rounded; paradontal apparatus well developed; chomata lacking.

Description: Oysters assigned to Aetostreon in this study display a marked morphological variation, interpreted here as evidence of the occurrence of at least two different species. Morphs are discernable based on their size and general shape of the valves, but specific relevant taxonomic information is unfortunately not preserved. Shells of Aetostreon sp.1 are big-sized, thick, with a subtrigonal outline and a keel more pronounced in the dorsal third (Figs. 5H; 9A-B); internally, lack of an umbonal cavity, and the rounded adductor muscle scar is dorsally truncated (Fig. 9B). Overall, Aetostreon sp.1 resembles A. latissimum from the northern Neuquén Basin in Argentina (e.g., Lazo 2007; Rubilar and Lazo 2009; Aguirre-Urreta et al. 2011; Toscano and Lazo 2020). The lack of key diagnostic traits (i.e., ligament area, umbo), presently demands the use of an open nomenclature for them.

Specimens of Aetostreon sp.2 are smaller, and thinner than A. sp.1, their left valves are globular with a subovate outline, and the umbo is opisthogyrate, with a small- to big-sized attachment area (Fig. 9J-R), and well-preserved growth lines (Fig. 9M, O, P). The keel is more pronounced in the umbonal area, changing to a slightly more convex posterior flank in the rest of the valve (Fig. 9L-R). Internally, Aetostreon sp.2 displays a moderate umbonal cavity (Fig. 9L, R), a narrow, linear paradontal recess, a subcircular adductor muscle scar, and the lack of chomata (Fig. 9L, R); these features are very similar to “Aetostreon sp. nov.” of Rubilar (2000). A third type of Aetostreon displays intermediate size between both Aetostreon sp.1 and A. sp.2, with left valves elongated in a ventral or ventro-posterior direction (Fig. 9C, E), a subovate- to subtriangular outline (Fig. 9C, E, G), and a keel in the posterior third, more pronounced near the opisthogyrate umbo (Fig. 9D–E, G). Given their poor preservation and sediment-coverage, it is not clear whether these oysters represent juveniles of Aetostreon sp.1, bigger morphotypes of Aetostreon sp.2, or a third different species. Therefore, they are presented conservatively as Aetostreon sp. (Fig. 9C–I).

Stratigraphic range: Globally, Aetostreon have been reported from the Oxfordian-Albian, but mainly from the Berriasian-Aptian range (Cooper 1995; Toscano and Lazo 2020, and references therein). In the Neuquén Basin of Argentina, several species of Aetostreon have been described in early-mid Berriasian (A. subsinuatum), late Berriasian-early Valanginian (A. latissimum; Toscano and Lazo 2020), late Valanginian (A. pilmatuegrosum; Rubilar and Lazo 2009), and late Valanginian-early Hauterivian units (Aetostreon sp.; Lazo 2007).

Discussion

Paleogeography

In the Aysén-Río Mayo Basin, several mixed- and calcareous outcrops have been identified (e.g.,Ploszkiewicz and Ramos 1977; Ramos 1978; Olivero 1983, 1987; Scasso 1987; Covacevich et al. 1994; Bell et al. 1994; Suárez et al. 1996; Townsend 1998; Ramos and Lazo 2006; Suárez and De la Cruz 1994a, b, among many others). However, an unequivocal correlation of these deposits has been hindered, due to their discrete and discontinuous exposures, rapid lateral and vertical facies changes (Scasso 1989; Scasso and Kiessling 2002), and diachronism (Suárez et al. 2005, 2010a). The latter is linked to a marine incursion over an irregular topography, affected by extensional tectonics and active volcanism (Ramos 1981; Scasso 1987; 1989; Folguera and Iannizzotto 2004; Suárez et al. 2009, 2010a). In addition, during the Cenozoic, strong strike-slip tectonic disruption as well as Neogene glacial events have affected the Coyhaique area (Scalabrino 2009). This is evident in the study area, where the outcrops are disconnected, incomplete, and located at different elevations (Figs. 1C, D; 5B, 6A, 7B).

Nevertheless, based on composition, common fossil assemblages, inferred sedimentary environments, and contact relationship with the volcanic units, the outcrops La Rosita (LRO) and Muralla China (MCH) appear to be laterally correlative, and part of the same carbonate-rich, mixed inner ramp setting (Figs. 4A, B; 10A–A’). Volcaniclastic-rich strata from the uppermost LRO outcrop show a sporadic clastic input, whereas thicker bedded limestones of the MCH outcrop reflects a relatively stable carbonate sedimentation, either located in a more distant setting from the coastal volcaniclastic source, or deposited during a phase of quiescent terrigenous supply (Nelson 1988; James 1997; Dorobek, 2008; Fig. 10A). Both outcrops are carbonate-rich and assigned here to the Manto Member of the Toqui Formation (sensu Rivas et al. 2021).

As shown in Sects. “La Rosita Section (LRO) - gravelly, mixed inner ramp”–“Muralla China Section (MCH) - wave- and storm-dominated inner-ramp”, this ramp was developed during the marine transgression above a volcanic terrain (Fig. 10A, B), interpreted by several authors as an insular volcanic flank (e.g., Bell et al. 1994; De la Cruz et al. 1994; Suárez et al. 1996; Townsend 1998; De la Cruz et al. 2003; Suárez et al. 2023). The latter may explain the preservation of gravel-rich facies, common in volcanic coasts affected by rapid shoreline translation (Felton 2002; Ramalho et al. 2013).

The relationship between the previous outcrops (LRO, MCH) and the Salto Río Pollux (SRP2) section is unclear (Fig. 4B, C), given their contrasting lithologies, structural attitude, and inferred tectonic contacts. Although they share the same fossil content (but in different proportions, see Table 1), which may support a lateral correlation, the SRP2 succession bear intraclasts type 1 (reworked bioclastic limestone; Fig. 3B), and its composition reflects a deeper setting and a major volcaniclastic supply. Alternatively, based on the volcaniclastic-rich facies of the SRP2 outcrop, more compatible with the San Antonio Member of the Toqui Formation (sensu Rivas et al. 2021), it might represent a vertical transition in the facies, with sediments deposited in a (younger) period controlled by a terrigenous and probably syneruptive input (tuffaceous beds of lithofacies tSp-b; Fig. 10B–B’). An increased clastic supply may also be related to enhanced run-off, triggered by the change from dry to humid (and hotter?) conditions (Capelli et al. 2021).

Paleoenvironmental interpretation

“Cool-water” carbonate setting

Following the “cool-water” criteria of James (1997), and based on paleolatitude (non-tropical), platform morphology (ramp-type), and mode of life of benthic organisms (heterozoan association), a non-tropical, “temperate” or “cool-water” (sensu lato, Schlager 2003) setting is inferred for the study area, i.e., an open marine environment with water temperatures of < 18°–20 °C (“temperate-type” sensu Simone and Carannante 1988; James 1997; Schlager 2005). However, since factors other than temperature may strongly affect carbonate ecosystems and their grain associations (e.g., terrigenous input, trophic conditions, water chemistry, CO2 saturation, among others, Kindler and Wilson 2010; Westphal et al. 2010; Michel et al. 2018), a “cool water” setting is used here in a broad sense (see contrasting examples in “Regional comparison”). Criteria supporting a cool-water setting for the rocks studied here are explained below.

As shown in “Age of the deposits”, the mixed rocks studied here appear to be restricted to the Lower Cretaceous. For that period, paleotectonic reconstructions of South America place the study area to south of 45°–50°S (e.g., Leinfelder et al. 2002; Scasso and Kiessling 2002; Seton et al. 2012; Matthews et al. 2016), i.e., in a non-tropical, high paleo-latitude (Scasso and Kiessling 2002).

Facies, sedimentary structures and reworked bioclasts depict a shallow-marine environment for the mixed outcrops of the Toqui Formation (refer to Sects. “Sedimentary logs and facies associations”). Wave- and storm-related structures reflect a relatively high-energy and a hydrodynamic control (Figs. 5C, 6C-G; 7D, E) regarded as typical for open shelves and ramp-type carbonate platforms lacking physical barriers (i.e., reefs, sand shoals; Burchette and Wright 1992; Wright and Burchette 1996; James 1997; Pedley and Carannante 2006; Flügel 2010).

Tropical carbonates usually accumulate in low latitudes (between ca. 30°N and S of the equator, Schlager 2003), in waters with temperatures higher than 20°–22 °C (Nelson 1988; James 1997; Schlager 2003). They are characterized by photo-autotrophic organisms with a dominant aragonitic mineralogy (e.g., green and red algae, hermatypic corals, and photosymbiotic interactions), and a biotic-abiotic precipitation of carbonate mud, cements, and non-skeletal allochems (Lees and Buller 1972; Nelson 1988; Schlager 2003; Kindler and Wilson 2010 and references therein). In contrast, limestones documented here are poor in carbonate mud and non-skeletal allochems (only sporadic intraclasts), and the benthic fauna is dominated by calcareous red algae and calcitic-shelled heterotrophic organisms (see next Sect. “Paleoecology”), conforming a heterozoan association (sensu James 1997; Clarke 2009; Kindler and Wilson 2010).

Paleoecology

The fossil assemblage studied here is mostly composed of sessile organisms, including encrusters (red algae; serpulids; bryozoans; ahermatypic corals, oysters; Figs. 2; 5E, H, J), soft-bottoms recliners (gryphaeid oysters; Figs. 6J, M, N; 7F, J), vagile epifauna (echinoids, pectinids; Fig. 6K), and nektonic animals (fishes, belemnites; Fig. 7G). Most of these organisms are stenohaline and support an euhaline and shallow, open marine environment (e.g., Wray 1977; Scholle and Ulmer-Scholle 2003; Stenzel 1971; Toscano et al. 2018; Cawley et al. 2021). However, some of these organisms may also have an euryhaline physiology (e.g., bryozoans, oysters, red algae, serpulids sensu Scholle and Ulmer-Scholle 2003), thus, local brackish areas or zones with a variable salinity may also be feasible (Townsend 1998). This has been commonly reported from some volcanic coastlines with an alluvial influence (Ramalho et al. 2013).

Red algae are the most remarkable photo-autotrophic organisms found in these rocks of the Toqui Formation (Fig. 8); they might represent the main source of carbonate (Wray 1977; Flügel 2010; abraded-bioeroded algae in Fig. 8B, D). Algal content is reworked and found as broken branches or rounded bioclasts, abundant in wave-influenced sediments (inner ramp; lithofacies bLr). An original shallow-water setting in the photic zone is inferred for them (Michel et al. 2018), encrusting submerged rocky blocks (“solenoporoids”), or conforming semi-protected?, low-relief meadows or “pavements” in coastal areas (see Fig. 5 in Henrich et al. 1995; Fig. 10A, A’ here). The latter, rich in ramified non-geniculate algae (A. jurassica), is considered as wave-reworked and offshore-transported (Fig. 10A, A’). Red algae-rich grainstones like the ones described resemble the current “maerl-type” deposits, conformed by broken branches of non-geniculate, unattached rhodoliths, typically found today in high-latitude, shallow and cool-water settings of the North East Atlantic (Henrich et al. 1995; Ehrhold et al. 2021).

Ahermatypic corals flourished during the incipient transgression, encrusting subtidal gravel in a shallow-water setting; they disappeared upsection likely due to sediment stress (Risk and Edinger 2011; see Fig. 10A, A’). Echinoid plates and spines are rich in wave- and storm-reworked sediments. An epifaunal or semi-infaunal habit in the sandy sediments, grazing on the algal meadows is commonly recognized for them (Nebelsick and Bassi 2000; Kroh 2003; see Fig. 10A, A’).

In the Aysén-Río Mayo Basin, oysters are mostly represented by exogyrids of the genre Aetostreon (Fig. 9), regarded as a homeomorph of Gryphaea (Stenzel 1971). Gryphaeids could live attached or free-lying (MacKerrow, 1981) in subtidal settings with soft-bottom and euhaline quiet water (Stenzel 1971; LaBarbera 1981). Analogous to Gryphaea, Aetostreon is also depicted as a soft-bottom recliner (Lazo 2007) living in shallow, low-energy inner-shelf settings with sandy–muddy, soft-bottoms (Cooper 1995; Lazo et al. 2005; Rubilar and Lazo 2009); in intertidal shelly mudstones (Cooper 1995); or forming biostromes in micritic mid-ramp settings (Rivas et al. 2021). In this study, big-sized articulated Aetostreon sp.1 from the LRO and MCH outcrops are found reworked in sandy limestones (lithofacies bLr), reflecting a local provenance (Figs. 5H, 6M-N, 9A–B). It is unclear if the oyster coquinites (lithofacies bRp) from the MCH outcrop may correspond to the same species (Fig. 6I), while in the LRO outcrop, Townsend (1998) reported life-positioned oysters encrusted in the basal volcanic blocks, but without providing a taxonomical assignation.

Smaller oysters from the MCH and SRP2 outcrops were tentatively classified as different species (Aetostreon sp. and A. sp.2; Fig. 9), though subtle morphological differences between specimens may represent morphotypes (Rubilar 2000). For example, the uppermost beds of the MCH outcrop bear a mixture of medium-sized (Aetostreon sp.; Fig. 9C–I) and small-sized oysters (Aetostreon sp.2; Fig. 9J–L); both appear as reworked and disarticulated valves, embedded in sandy limestone (Fig. 6H, J). In contrast, in the SRP2 outcrop, only small specimens were observed (Aetostreon sp.2 = “Aetostreon sp. nov.” Rubilar 2000, ex “Gryphaea balli” sensu Cecioni and Charrier 1974; Fig. 9M-R), in life position and reworked, embedded in shelly, muddy sandstone (lithofacies bSp; Fig. 7F, I–J).

In Gryphaea, their relatively heavy shells are unlikely to be distantly transported, serving as a good paleoecological indicator (Bayer et al. 1985). In addition, wider morphotypes may reflect an ecophenotypic adaptation to firmer substrates (e.g., sand-rich); this is also supported by the presence of bioeroded and reworked shells, more exposed to erosional events (Bayer et al. 1985). Therefore, in the MCH outcrop, small- and mid-sized Aetostreon spp. were likely transported from low-energy, muddy–sandy shallow-marine settings (see paragraph above), or they were adapted to bioclastic sandy bottoms of the carbonate inner ramp (Fig. 10A, A’). In the SPR2 beds, dominant small-sized Aetostreon sp.2 thrived in mud-rich soft-bottoms (LaBarbera 1981), with tendency to eutrophication (rich in organic matter; Nori and Lathuilière 2003), interpreted here as part of the mid-ramp (Fig. 10B, B’).

Based on the previous, the fossil assemblage described here depicts an autochthonous-para-autochthonous assemblage in the inner-ramp (outcrops LRO, MCH; Fig. 10A, A’), but an autochthonous-allochthonous in the mid-ramp (outcrop SPR2; Fig. 10B, B’).

Paleoclimate

The calcareous sedimentation in the Aysén-Río Mayo Basin was strongly diachronic, likely ranging from the Tithonian–Valanginian, or even Hauterivian (see Sect. “Stratigraphy and correlations”). Paleoclimate models for the Jurassic-Cretaceous transitions are usually contrasting. Some authors infer an equable warm climate, with a lower temperature gradient and absence of polar ice caps (Hay 2008; Föllmi 2012), while others depict episodic cooling or “cool-snaps” (Price 1999; Kessels et al. 2006; Donnadieu et al. 2011), or even seasonal growth of polar ice (Price 1999, 2009; Price et al. 2000; McArthur et al. 2007; Mutterlose et al. 2009; Hay and Floegel 2012; Tennant et al. 2017 among many others). Globally, the most remarkable J-K cool-spans are depicted as occurring during the late Tithonian, the mid-Valanginian, and across the Aptian-Albian boundary (Price 1999), though they have being called into question in more recent studies (Föllmi 2012; Jenkyns et al. 2012). This paradox is also extended to southern South America, where some models infer warm (> 25 °C), sea-surface temperatures in the Southern Ocean during the J-K transition (ca. 52°S; Jenkyns et al. 2012; Vickers et al. 2019); while other propose episodic cool intervals in the late Tithonian–earliest Berriasian, Valanginian, earliest Hauterivian, late Barremian, Aptian, and earliest Albian (Salazar and Stinnesbeck 2015; Brysch 2018).

In the Aysén-Río Mayo Basin (Fig. 1B), the paleoclimatic evidence for the existence of cold snaps is inconclusive. Overall, warm and humid conditions have been inferred for the deposition of the Coyhaique Group (Tithonian–Aptian), but only based on the presence of limestone (Skarmeta 1976; Bell et al. 1994, 1996; Townsend 1998). However, as presented in “Paleoecology”, mixed rocks of the Toqui Formation exposed south of Coyhaique, and assigned to the late Berriasian/early Valanginian-Hauterivian? (see “Age of the deposits”; “Stratigraphy and correlations”), display typical features of “cool water” carbonates (see ““Cool-water” carbonate setting”).

Two events of global cooling are known from this period and are supported by evidence from Patagonia (Brysch 2018). The first one, across the Berriasian-Valanginian boundary, is marked by a glacioeustatic, long-term sea-level fall (Pucéat et al. 2003; Haq 2014). The second, in the Valanginian (Weissert and Erba 2004; Cavalheiro et al. 2021), corresponds to a positive carbon isotope excursion (“Weissert Event”; e.g., van de Schootbrugge et al. 2000; Meissner et al. 2015; Price et al. 2018), linked to a high rate of organic carbon burial (Erba et al. 2004; Price et al. 2018). Alternatively, these “episodes of environmental change” (sensu Föllmi 2012), among others during the Early Cretaceous, may represent short-termed humid periods alternated with long-termed arid greenhouse conditions (Föllmi 2012). For example, to the north, in the Neuquén basin (ca. 37°S), a shift to more humid conditions have been inferred during the early Berriasian–latest early Valanginian, prior to the onset of the Weissert Event (Capelli et al. 2021). There, humid conditions would have led to an enhanced runoff, triggering a shift from calcareous-rich to clastic-rich sedimentation (Capelli et al. 2021) as observed here (see ““Cool-water” carbonate setting”).

However, given the several unresolved problems regarding the global J-K paleoclimate models (Hay 2017), and the ambiguous, broad age range inferred for these outcrops of the Toqui Formation, more analytical data are required, to incorporate them in the global paleoclimatic scheme.

Regional comparison

The Toqui formation has been correlated with the upper Tithonian Cotidiano Formation, and with the Tithonian? – Hauterivian Tres Lagunas Formation from Argentina (45°S; see Sect. “Stratigraphy and correlations”). Archamphiroa jurassica has been reported from both the Cotidiano Fm. and from the outcrops presented here (Toqui Fm.), though both units differ in their carbonate depositional settings and inferred ages (see “Age of the deposits”). The Cotidiano Formation consists of biohermal limestone (“coral-stromatoporoid patch reefs”; Ramos 1978), analog to the “Photozoan Association” of James (1997), likely settled in a tropical–subtropical environment (T > 22 °C sensu Ramos 1976; 1978), or in a warm-water ocean (T > 18 °C sensu Kiessling et al. 1999; Leinfelder et al. 2002; Leinfelder et al. 2005). This contrast may be explained by the marked temporal gap between both deposits (Tithonian versus upper Berriasian-Hauterivian?), and potential deposition under differing paleoenvironmental conditions.

A similar problem arises when comparing the present outcrops of the Toqui Formation with those from the Tres Lagunas Formation. Even though both units display a mixed carbonate-clastic composition and partly coeval ages, their calcareous facies and depositional environments are clearly different. At its type locality (Laguna Salada), the Tres Lagunas Formation includes conglomerates, limestones (coquinites and coral-bioherms), tuffaceous beds, and sandstone–mudstone alternations (some calcareous or bioclastic); these rocks are discretely exposed showing abrupt vertical and lateral facies changes (Scasso 1987; 1989; Scasso and Kiessling 2002). Carbonate facies of the Tres Lagunas Fm. have been interpreted as coral patch-reefs and associated fore-reef deposits (Olivero 1983; Scasso 1987; 1989), likely developed in a warm-water setting during the Berriasian-Valanginian (Scasso 1989; Scasso and Kiessling 2002). These rocks depict a rimmed carbonate shelf (bioherms), contrasting with the wave- and storm-influenced, open-marine ramp deposits presented here (see Sects. 4, and 6.1).

The cause behind these opposing depositional environments is not clear. Paleolatitude and temporality do not seem to be the controlling factors, and other variables must be considered; for example, their position within the basin (see paleo-reconstructions in Scasso 1989; Folguera and Iannizzotto 2004), but also coast morphology and hydraulic settings (rimmed shelf vs ramp profile), coeval volcanism (Scasso and Kiessling 2002), and terrigenous input or trophic conditions (e.g., coastal upwelling; Michel et al. 2018). The latter could have played a role in reef growth (Scasso and Kiessling 2002), as observed repeatedly in volcanic islands (Ramalho et al. 2013). Several other factors may favor the development of photozoan versus heterozoan benthic communities (e.g., temperature, salinity, water depth, CO₂ concentration, etc.; Nelson 1988; Pomar et al. 2004; Kindler and Wilson 2010; and references therein); their role in the study area is still unclear and needs to be addressed in future studies.

Conclusions

Three sections assigned to the Lower Cretaceous Toqui Formation, the lowermost member of the Coyhaique Group, are here investigated from an area south of Coyhaique in southern Chile (45°40’S). These rocks overlie the volcanic Ibáñez Formation, regarded to represent the early Valanginian in the study area, and their top is not exposed. In the study area, the sedimentological and microfacial analysis of the Toqui formation reveals a mixed calcareous–volcaniclastic composition, rich in reworked bioclasts conforming a heterozoan-type fossil assemblage. Mixed lithofacies and associated sedimentary structures reflect the development of a wave- and storm-influenced mixed ramp. Based on their lithologies, two outcrops were assigned to the Manto Member (calcareous-rich LRO, MCH sections) and one to the San Antonio Member of the Toqui Formation (volcaniclastic-rich SRP2 section). The calcareous sedimentation is depicted as part of a retrograding coast above an active volcanic terrain with sporadic activity.

Bioclastic, mixed calcareous rocks of the Toqui Formation conform a “heterozoan” association (heterotrophic organisms + red algae), bearing gryphaeid oysters (Aetostreon spp.), non-geniculate calcareous red algae, and echinoids as major constituents. Red algae are controlled by Archamphiroa jurassica, identified here for the first time in the Lower Cretaceous of Chile and previously reported only from the Tithonian of Argentina.

Based on the inferred high paleolatitude of these rocks (south of 45°–50°S), their heterozoan fossil assemblage, and depicted depositional setting (open-marine, ramp-type), a “cool-water” non-tropical setting is inferred for the Aysén-Río Mayo Basin in the Coyhaique area during the late Berriasian/early Valanginian—Hauterivian? This contrasts with the depositional environments inferred for correlative reefal rocks in Argentina (Tres Lagunas Formation), which reflect “warm-water” settings. In the Aysén-Río Mayo Basin, the influence of sea-water key physical variables in the carbonate sedimentation, as well as the position of the carbonate platforms within the basin, and their interaction with the volcanism are still unclear.

Data availability statement

The data that support the findings of this study are available as supplementary material of this article. This additional information includes: i) description of each sedimentary log and their beds, compiled in the field (Table SM-1 to SM-3); ii) description and modal composition of each thin section (Table SM-4); measurement of exemplars of Archamphiroa jurassica under the microscope (Table SM-5).

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Acknowledgements

We gratefully acknowledge Prof. Dr. Ioan Bucur (Babes-Bolyai University), and Dr. Stephen Kershaw (Brunel University London) for their help in the determination of microfossils. We appreciate the guidance of the Prof. Dr. Axel Munnecke (Universität Erlangen-Nürnberg), and Dr. Alexander Varychev (Universität Heidelberg) during the acquisition of SEM images. The authors would like to thank Valentina Maldonado, Benjamín Aldridge and Rayen Álvarez for their support during field work. We are grateful to Dr. Roberto Scasso (Universidad de Buenos Aires) and one anonymous reviewer for their careful revision and constructive remarks on the manuscript, and Enrique Bostelmann for his thoughtful observations.

Funding

Open Access funding enabled and organized by Projekt DEAL. Financial support to this project was provided by the Chilean National Fund for Scientific and Technological Development (FONDECYT de Iniciación 11140176), by the Chilean National Agency for Research and Development (ANID/DOCTORADO BECAS CHILE/2016–72170384), and by the German Academic Exchange Service (DAAD, STIBET/ “Studienabschlussbeihilfe Stipendium”).

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Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234, 69120, Heidelberg, Germany
Hermann Rivas & Wolfgang Stinnesbeck
Escuela de Geología, Facultad de Ciencias, Universidad Mayor, Manuel Montt 367, 7500994, Providencia, Chile
Christian Salazar

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Hermann Rivas
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Christian Salazar
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Wolfgang Stinnesbeck
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HR: software, formal analysis, investigation, data curation, writing—original draft, writing—review and editing, visualization. CS: conceptualization, methodology, writing—review and editing, supervision, funding acquisition. WS: resources, writing—review and editing, supervision, project administration.

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Correspondence to Hermann Rivas.

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Rivas, H., Salazar, C. & Stinnesbeck, W. A “cool-water”, non-tropical, mixed volcaniclastic–carbonate ramp from the Early Cretaceous of southern Chile (45°40’S). Facies 69, 14 (2023). https://doi.org/10.1007/s10347-023-00669-4

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Received: 07 February 2022
Accepted: 07 May 2023
Published: 16 June 2023
DOI: https://doi.org/10.1007/s10347-023-00669-4

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A “cool-water”, non-tropical, mixed volcaniclastic–carbonate ramp from the Early Cretaceous of southern Chile (45°40’S)

Abstract

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Introduction

Geological setting

Stratigraphy and correlations

Age of the deposits

Materials and methods

Facies analysis

Petrography

Calcareous-rich microfacies

Terrigenous-rich microfacies

Microfacies

Interpretation of microfacies

Lithofacies

Mixed bioclastic–volcaniclastic, sandy conglomerate (bGSm)

Oyster-rich rudstone/coquinite (bRp)

Mixed bioclastic–volcaniclastic, gravelly sandstone (bSGm)

Rippled, bioclastic (volcaniclastic) limestone (bLr)

Parallel-bedded, bioclastic–volcaniclastic, muddy sandstone (bSp)

Laminated, tuffaceous muddy sandstone (tSp-b)

Sedimentary logs and facies associations

La Rosita Section (LRO)—gravelly, mixed inner ramp

Muralla China Section (MCH)—wave- and storm-dominated inner-ramp

Salto Río Pollux Section (SRP2)—mixed, distal mid-ramp

Systematic paleontology

Archamphiroa jurassica

Remarks:

Aetostreon spp.

Discussion

Paleogeography

Paleoenvironmental interpretation

“Cool-water” carbonate setting

Paleoecology

Paleoclimate

Regional comparison

Conclusions

Data availability statement

References

Acknowledgements

Funding

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Contributions

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Supplementary file1 (DOCX 15438 kb)

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