The Lower Alcudian-Domo Extremeño Supergroup represents both (i) the “Lower Alcudian” Group, Stage and megasequence of Ovtracht & Tamain (1970), Herranz et al. (1977) and San José et al. (1990); and (ii) the lower part (underlying the Cadomian gap) of the widely used “Domo Extremeño” Group of Álvarez Nava et al. (1988). Both terms have been widely used and their conceptual modifications in time are not too significant. We propose the Alcudia valley as type area for the supergroup. Its base is not identified and its thickness is broadly estimated to be more than 6 km. The supergroup mainly consists of monotonous shales and greywackes with subsidiary sandstone (lower part) and includes clast- and matrix-supported conglomerate and breccia interbeds (upper part), and scattered centimetre-scale carbonate nodules and layers. Clasts from these conglomerates and breccias are polymictic and include intraformational (mainly greywacke and shale) and exotic (e.g., lydite, quartzite, gneiss and granite) clasts, locally associated with debris derived from pegmatite and vein-quartz remains; many exotic clasts carry internal deformation fabrics.

The lower part of the Lower Alcudian-Domo Extremeño Supergroup corresponds to both the La Coronada and Sta. María de Zújar formations of García Hidalgo et al. (1993) and Pieren Pidal (2000), and to the Guadiana, Botija, Monroy and lower part of the Orellana formations sensu Palacios et al. (2010, 2013). Due to their lateral equivalence, the Guadiana Formation should be considered as a group subdivided into the La Coronada and Sta. María de Zújar formations. The upper part of the supergroup includes the Orellana Formation of García Hidalgo et al. (1993) and Pieren Pidal (2000), which corresponds to the upper part of the “Orellana” Formation of Palacios et al. (2010, 2013). In order to avoid the conceptual changes associated with the Orellana toponymy, we propose that the Campanario Group should be considered as the upper part of the supergroup, subdividing it into the Botija, Monroy and Orellana formations.

The Ibor Group is mainly recognized in the Sierra de la Zarzuela and the surroundings of the Orellana dam, the Ibor, Navalpino and Alcudia anticlines, and the Abenójar-Tirteafuera domes. It is subdivided below into the Castañar, Villarta and Arrocampo formations.

2A. The Castañar Formation (“Castañar Siltstones” of Álvarez Nava et al. 1988; Nozal Martín, 1988a, b, c; Palacios et al., 2010), up to about 400 m, consists of shales and greywackes with minor coarse-grained sandstone-to-conglomerate interbeds, and carbonate nodules and thin layers. In the surroundings of Guadalupe town, the base of the formation is marked by distinct arkosic sandstones (Palacios et al., 2013). The stratotype lies along the road that links Robledollano and Castañar de Ibor (GPS: N39º37’30’’, W5º25’46’’). The shale interbeds contain both Vendotaenids and simple horizontal ichnofossils. Close to Navalvillar de Ibor, Hufnagel (2008) described a greater diversity of macrophytes: “club-shaped” forms were tentatively compared with Longfengshania, although substantially smaller; a spindle-shaped form was described as Salobrigia guadalupensis and a club-shaped form as Ibora sdzuyi. This site has also yielded spirally coiled forms and rare forms branching into three.

In the Sierra de la Zarzuela and the surroundings of the Orellana dam, Pieren Pidal (1990) subdivided the strata unconformably overlying the Lower Alcudian-Domo Extremeño Supergroup into several informal units, which are proposed in this study as members of the Castañar Formation. These include, from bottom to top, the Orellanita conglomerate (Fig. 7A) and the Cogolludo sandstone (Fig. 7B) members: the former, up to 50 m thick, consists of amalgamated quartzarenitic conglomerates grading upward into sandstone-dominant sandstone/shale alternations, easily recognized by their variegated colours, which have been interpreted as the onset of alluvial-to-fluvial environments (Pieren Pidal et al., 1991).

In the Navalpino Anticline, the Castañar Formation has been described as the “Parrales Claystones” (San José, 1984), and the base of the Ibor Group is represented by the La Antigua Conglomerate (Fig. C), which is here proposed as another member.

2B. The Villarta Formation (San José, 1984; López Díaz, 1994) is a heterolithic succession, ranging from 100 to 250 m in thickness. In its type area, the northern shore of the Cíjara dam, close to Villarta de los Montes village, the Villarta Formation can be subdivided into three members, from bottom to top: (i) an alternation of lenticular and bedded carbonates embedded in greenish shales (Fig. 8B-C); (ii) a conglomeratic to sandstone-dominant unit passing upward to sandy dolostones (Fig. 8F); and (iii) an alternation of shales and bedded carbonates (Fig. 8H). The stratotype of the formation is located on the northern shore of the Cíjara dam, at the La Majada del Andaluz hut (GPS: N39º13’52.86’’, W39º13’52.86’’).

Cloudinid-microbial buildups are recorded in the lower member of the Villarta Formation (Fig. 8B). These have yielded, after acid etching, specimens of Cloudina (C. carinata, C. hartmanae and probably C. xuanjiangpingensis), Sinotubulites (S. baimatuoensis) and Protolagena sp. (a problematic flask-shaped microfossil; Cortijo et al., 2010, 2015) (Fig. 9A-E). Vendotaenids are relatively common in the shale interbeds of the same member (Fig. 8G), whereas Sabelliditids are abundant in the shale interbeds of the middle member. Simple ichnofossils are scattered, except in the upper member, where they are relatively common.

The Villarta Formation is interpreted to have formed on rimmed and unrimmed carbonate platform-blocks with localized ooidal shoals (García Hidalgo, 1984; Calvet Alonso & Salas, 1988), episodically recording phosphogenic episodes that are commonly recorded as phosphoritic crusts and clasts (Álvaro et al., 2016b). Diagenesis (mainly related to neomorphism and replacement to dolomite and magnesite) of the lower member limestone interbeds has been geochemically described and interpreted in Alonso-Zarza & Martín-Pérez (2008), Martín-García et al. (2009, 2011, 2019), Martín-Pérez et al. (2012) and García-Guinea et al. (2013). These carbonates were the main source for the material incorporated into the megabreccia beds described below.

In the Sierra de la Zarzuela and the surroundings of the Orellana dam, Pieren Pidal (1990) distinguished a heterolithic, sandstone/shale succession, overlying the above-reported Cogolludo Sandstone Member, which comprises two distinct limestone interbeds, up to 2 m thick, named by the author “Loma de la Calera” (Fig. 8A) and “Collado de la Liebre” limestone interbeds, in some cases passing laterally into centimetre to decimetre-scale nodules of carbonate embedded in a monotonous shale. The shales between both carbonate marker beds have yielded ichnofossils, such as Treptichnus isp. The lower limestone interbed grades laterally into the Talarrubias Dolostone Bed, up to 6 m thick, rich in Sabellidites and treptichnids, and represents the lower part of the Villarta Formation in the area. The limestones of the Abenójar Dome have also yielded tubular skeletal fossils including Cloudina (Vidal et al., 1994b; Zhuravlev et al., 2012; Simón, 2018).

2C. The Arrocampo Formation (new) is proposed to solve numerous structural problems, reported in previous works. Several formations have been defined in the literature seemingly overlying the Villarta Formation, but all of them are strongly affected by structural folds and faults that preclude any correct stratigraphic setting. As a result, we propose the Arrocampo Formation as a shale-dominated unit with subsidiary sandstone interbeds increasing upwards, and local presence of centimetre-scale carbonate nodules parallel to stratification. This unit is up to 350 m thick, and conformably overlies the Villarta Formation. Its stratotype lies along the right bank of the Tagus River (GPS: N39º46’51.13’’, W5º44’48.71’’), close to the Almaraz nuclear power plant, named Arrocampo by García-Hidalgo (1985). The monoclinal outcrops of this formation are unconformably overlain by either the San Lorenzo Formation or the Lower Ordovician conglomerates and quartzites. The base of the formation contains the earliest ichnofossils assigned to Treptichnus pedum, so tentatively marking the base of the Terreneuvian.

2D. The Tamujar, Hinojosas and Cabezarrubias formations are exclusive of the Alcudia Anticline. They are laterally and lithologically equivalent to the triad Castañar - Villarta - Arrocampo formations and are maintained here for practical reasons, with the aim of simplifying the stratigraphic framework. The Tamujar Formation (stratotype of “Tamujar beds” along the homonymous ravine, in the vicinity of Hinojosas village; Peláez Pruneda et al., 1988; Pieren Pidal & Herranz Araújo, 1988; San José et al., 1990; Pieren Pidal, 2009), which unconformably overlies the Lower Alcudian-Domo Extremeño Supergroup, is up to 50 m thick and dominated by amalgamated sandstones with subsidiary conglomerate and shale interbeds (Pieren & García Hidalgo, 1999) (GPS: N38°36’8”, W4º12’23”). The Hinojosas Formation (lower part of Bouyx’s “Hinojosas Series”), up to 80 m thick, is a heterolithic unit with carbonates, sandstones, shales and subisidiary conglomerate interbeds (GPS: N38°36’33”, W4°11’43”). The Cabezarrubias Formation (“Lower Shaly Formation” of Palero, 1991, 1993; Pieren & García Hidalgo, 1999) is a monotonous shale-dominated succession with a maximum thickness, to the South of the homonymous village, of about 120 m thick (GPS: N38°36’51”, W4°11’35.40”).

The Hinojosas Formation has yielded numerous ichnofossils, such as treptichnids (reported as Hormosiroidea cf. canadensis), Monomorphichnus lineatus (García-Hidalgo, 1993a, b) and, from a section probably attributable to this unit, bilobed trace fossils with a circling motion (Pieren Pidal, 2000), assigned to Taphrhelminthopsis circularis by Fernández Remolar et al. (2005). In the Cabezarrubias Formation, García-Hidalgo (1993b) reported the presence of Bergaueria aff. langi and Planolites sp. Simón (2017) reported additional trace fossils from the Hinojosas and Cabezarrubias formations including a centimetre-wide trace fossil from the Hinojosas Formation that was tentatively compared with Psammichnites.

The Cíjara Formation was initially described by McDougall et al. (1987) as a siliciclastic “member” unconformably overlying the Lower Alcudian-Domo Extremeño Supergroup, but including the lower part of the unconformably overlying Lower-Ordovician Skolithos-bearing “Purple Series”. Subsequently, Palacios Medrano (1989) proposed the restriction of the Cíjara Formation to the pre-Ordovician part. According to the author, the heterolithic formation, up to 1400 m thick, consists of conglomerates, sandstones and subsidiary shales, phosphatic crusts and clasts, which commonly infill interbedded channelized conglomerates (Álvaro et al., 2016b) (Fig. 10A-H). Its stratotype is proposed along the Estenilla River (GPS: N39º23’48.58’’, W4º51’49.78’’), where both the base and top are observable.

The Estenilla Formation, defined by Palacios Medrano (1989) in the geographically upper part of the homonymous valley, represents two units: (i) the lower “member” displays the lithology and facies associations characteristic of the above-reported Orellana Formation, and is not retained in this work; whereas (ii) the upper part comprises the shales that mark the top of the Orellana Formation and the basal part of the Cíjara Formation, in the sense proposed here. An analysis of sulphur isotopes made in the Estenilla Formation by Strauss (2002) yielded organic carbon contents between <0.1 and 1.0 wt% (n=67) and sulphide sulphur abundances between <0.1 and 5.9 wt% (n=83). Resulting S/C ratios range from 0.1 to 7.1, with a few values substantially higher. DOP values are <0.40. δ³⁴S values between -17.4 and +49.3‰ (n=14) characterize pyrite between +32.4 and +49.3‰. A most prominent feature is the positive correlation between organic carbon and sulphide sulphur, and high S/C ratios pointing to overabundance of sulphur over organic carbon.

The Cíjara Formation has yielded bacterial acritarchs attributed to Bavlinella faveolata (=Sphaerocongregus variabilis Moorman, 1974) and Palaeogomphosphaeria cauriensis Palacios Medrano (1989). Ichnofossils are represented by simple horizontal forms such as Gordia marina and Helminthoidichnites (for discussion on identification, see Jensen & Palacios, 2016). Possible treptichnids have been reported from this unit (Vidal et al., 1994b; Jensen, 2003), but it remains unclear if these represent trace fossils with multiple outlets or a vertical sinusoidal movement (cf. Jensen et al., 2006).

The Cíjara Formation includes numerous slope-related event deposits characterized by the presence of tool, bounce and flute marks, and common channelized deposits associated with slumps and olistostromes. The presence of limestone clasts, reflecting episodes of carbonate production in laterally equivalent areas, suggests the contemporaneous deposition of the Villarta Formation during the sedimentation of parts of the Cíjara Formation. The abundance of coeval fracturing and fissuring in a substrate submitted to episodic uplift, tilting and rotation of blocks is highlighted by the abundance of hydrothermal veining and reworking of their vein quartz clast counterparts.

The Cíjara/Pusa contact is locally marked by the onset of megabreccia beds: the Fuentes Bed in the Valdelacasa Anticline (Cortázar, 1878a, b), and the Navalpino and Membrillar Beds in the Navalpino Anticline (Moreno Serrano, 1974; Moreno, 1975, 1977a, b; López Díaz, 1992, 1994). The beds, up to 200 m thick, consist of clast-supported, unsorted conglomerates and gravel sandstones that display drastic changes in thickness (Fig. 11A-D). Their blocks, which can reach 2 m in size, are composed of limestone, dolostone, sandstone, shale and conglomerate, and are crosscut by networks of quartz veins. The larger blocks are commonly subrounded, whereas the smaller ones are (sub)angular in shape.

The megabreccia beds have been previously interpreted as gravitational slides triggered by “seismoevents” (e.g., Moreno, 1975, 1977a, b; Santamaría Casanovas & Remacha Grau, 1994). Although these chaotic deposits have been traditionally interpreted as “olistostromes” (a term that refers to accumulation as a semifluid body by submarine gravity sliding or slumping of unconsolidated sediments), they represent indeed submarine fan and slope-apron deposits whose sub-units can show overlapping geometries.

The age of these beds is constrained by the fossiliferous content of their allochthonous blocks. In the Membrillar Bed, Palacios (1983) reported the presence of the acritarchs Bavlinella faveolata Shepeleva, 1962 and Trachysphaeridium laufeldi? Vidal, 1976. Subsequently, Palacios (1983) and Palacios Medrano (1989) dismissed the presence of T. laufeldi? and reported the presence of the acritarch Bavlinella faveolata Shepeleva, 1962, whereas Brasier et al. (1979) and Brasier & Cowie (1989) highlighted the occurrence of Planolites isp. in shale interbeds. In the Membrillar Bed of the Valdelacasa Anticline, Palacios Medrano (1989) reported the presence of Cloudina shells within reworked blocks of the megabreccia (Fig. 11E). The mixture of heterolithic clasts sourced from different Ediacaran-bearing fossiliferous sources point to the incorporation of polyphase blocks, sourced from underlying formations, in slope-apron deposits.

The Pusa Formation, up to 3500 m thick, is a heterolithic unit dominated by shale strata but episodically containing breccia, conglomerate and mélange interbeds, sandstone/shale alternations, local carbonate interbeds and phosphorites. Its stratotype lies along an abandoned railway section, about 4 km to the west of La Nava de Ricomalillo village (GPS: N39°39’43’’, W5°2’2”; see Brasier et al., 1979). A parastratotype lies along the neighbouring Huso River, close to the same village. The formation was subdivided by Gabaldón López & Hernández Urroz (1989) into six sequence units, but only three are mappable (e.g., Vidal et al., 1994b). These have been traditionally considered as members: a conglomerate-bearing member, with high modifications in phosphate content, sandwiched between two shale-dominant ones.

Until the 1970s, the age of the Pusa Formation was tentatively assigned to the Vendian. Brasier et al. (1979) reported the presence of Beltanelloides (“Vendian”) and Chuaria (“upper Riphean-Vendian”) associated with ichnofossils, such as Monomorphichnus, which suggested the onset of a superposition of distinct Vendian and Cambrian fossil remains. San José (1983) suggested placing the base of the Cambrian at an indeterminate horizon ranging from the base of the megrabreccias to the top of the Pusa Shales.

5.1. The lower member, about 1100 m thick, shows abundant slumpings, breccia levels and contorted beds at its basal part (Figs. 11F, 12A). Basal clast- and matrix-supported breccias and contorted beds, rich in abundant hydrothermal veins and clasts, grade upward into monotonous shale (Fig. 12B) with interbedded greywacke sandstone. Phosphatic crusts are locally abundant. The only body fossils known from this member are circular (now tectonically elongated) carbonaceous compressions that have been reported under various names (see above) but are now generally attributed to Beltanelliformis. (e.g., Jensen et al., 2006; Ivantsov et al., 2014) (Fig. 11C). Their stratigraphic implications will be discussed below. Trace fossils include a range of ichnotaxa (e.g. Vidal et al., 1994b; Jensen et al., 2006; Gámez Vintaned & Liñán, 2007), such as Monomorphichnus lineatus Crimes et al. (1977), Treptichnus bifurcus Miller, 1889 from the middle part of the member. Liñán et al. (1993) proposed as stratotype section for the base of the Cordub(i)an regional Stage the aforementioned disused railtrack section, close to the La Nava de Ricomalillo. Talavera et al. (2012) reported youngest concordant detrital zircon ages of 536±13 Ma and 533±17 Ma from the approximate mid-portion of this unit; additional detrital zircon ages from the lower member would help constraining the maximum depositional age of this unit and the formation of the Fuentes Bed.

5.2. The middle member contains conglomerates rich in quartz with subsidiary sandstones, shales rich in disseminated apatite and phosphatic crusts. The member is characterized by a finely developed lamination. In the Navalpino Anticline, the phosphorites can reach 500 m in thickness, and form an amalgamation of channels rich in lithoclasts, oncoids and phosphatic hardground-derived clasts (Gabaldón López et al., 1987; Santamaría et al., 1987a, b; Santamaría, 1988, 1996; Picart Boira, 1988; Gabaldón López & Hernández Urroz, 1989; Santamaría i Casanovas, 1995; Álvaro et al., 2016b).

Ore deposits of economic importance are lithostratigraphically represented by the Fontanarejo Bed (Nozal Martín et al., 1988a, b, c; Perconig et al., 1983, 1986; Gabaldón López et al., 1987; Santamaría et al., 1987a; Picart Boira, 1988; Santamaría, 1988, 1996; Gabaldón López & Hernández Urroz, 1989; López & Hernández Urroz, 1989) in the Valdelacasa and Navalpino anticlines. Notable occurrences include those at Robledo del Mazo and Horcajo de los Montes in the Valdelacasa Anticline and at Fontanarejo in the Navalpino Anticline (level Ic of Perconig et al., 1983). Channel infill consists of centimetre to decimetre-scale conglomerate layers punctuated by sandstone and shale beds displaying normal grading, convolute bedding, local soft sediment slumping and disrupted bedding. Conglomeratic units are composed of subrounded and moderately sorted, litharenite cobbles and pebbles embedded in an arkosic matrix. Most of the units are clast-supported. Channel infill consists of homogeneous (amorphous to microcrystalline francolite) and polymictic clasts, phosphatized oncoids (locally up to 75% in volume), with moderately sorted sericite, quartz and dolomite grains, and scattered pyrite, siderite, zircon, tourmaline and rutile (Álvaro et al., 2016b). Sponge sclerites, probably hexactinellids and demosponges (Fig. 12E-F), are scattered and commonly embedded in a thromboid texture (Reitner et al., 2012).

5.3. The upper member, 1900-2000 m thick, is composed of shale and greywacke sandstone with minor quartz-arenite interbeds and rare carbonates. Depositional sedimentary structures are well developed and soft-sediment slumping is common. In comparison to the underlying members of the Pusa Formation, the upper member represents relative shallower deposits, which grade upward into the conformable Azorejo Formation. Body fossils from the upper part of the member occur near Robledo de Buey, where carbonate-cemented siltstone and partly silicified nodules have yielded trilobites (cf. Abadiella bourgini) and microfossils (Cupitheca, Pelagiella and unidentified hyolithids) (Jensen et al., 2010). This level has also yielded poorly preserved archaeocyaths, amongst which A. Zhuravlev (pers. comm. in Liñán et al., 2015: p. 1130) identified Capsulocyathus sp. Ichnofossils were reported by Jensen et al. (2010), such as Dactyloidites, Psammichnites gigas (Fig. 12D), Teichichnus and the lowest recorded occurrence of Rusophycus in the Pusa Formation. Scenella sp. has been reported across the Member 2-3 transition in the area of La Nava de Ricomalillo.

These units were defined in the Alcudia Anticline and are considered as lateral equivalents of the lower and middle members of the Pusa Formation, respectively. Due to the presence of reddish channelled conglomerates and sandstones in the neighbouring Abenójar and Tirteafuera domes, as well as variegated sandstone/shale alternations capping the Arrocampo Formation in the Ibor and Navalpino anticlines, both formations are proposed as mappable units.

The San Lorenzo Formation, up to 235 m thick, consists of reddish amalgamated conglomerate and sandstone beds and channels overlain by variegated and partly burrowed, sandstone/shale alternations. Its type area is located in the eastern part of the Alcudia Anticline, in the vicinity of San Loreno de Calatrava town (GPS: N38º27’57’’, W3º48’3’’). Its top is commonly eroded beneath the Armorican Quartzite but, in the eastern edge of the anticline, the sandstones are conformably overlain by a monotonous succession of shales (“Upper Shaly Formation” sensu Pieren & García-Hidalgo, 1999), up to 80 m thick. The stratotype of the Fuentepizarra Formation (corresponding to the informal “Upper Shaly Formation”) is located along its homonymous ravine (GPS: N38º28’6’’, W3º47’57’’), close to San Lorenzo de Calatrava town. In the Alcudia Anticline and the Abenójar Dome, the Fuentepizarra Formation contains several distinct phosphorite (wt.% P₂O₅ >18%) and phosphatic carbonate interbeds (Fig. 13C-D), up to 30 cm thick, which have yielded microfossils after acid etching (Fig. 13E-F), such as helcionellids (Lorenzo Álvarez & Solé, 1988; Vidal et al., 1995; Pieren & García-Hidalgo, 1999), identified as Anabarella plana Vostokova (Gubanov in Vidal et al., 1999; Gubanov & Peel, 2003) (Fig. 13G). Anabarella may be late Nemakit-Daldynian or early Tommotian in age (Vidal et al., 1999).

The base of the Ibor Group and the Cíjara Formation represents an intra-late Ediacaran paraconformable to high angular discordance that separates two sedimentary packages with different styles of deformation. The Lower Alcudian-Domo Extremeño Supergroup exhibits steeply plunging cleavage-free folds. The palaeorelief formed as a result of compressive deformation is onlapped by either the Ibor Group or the Cíjara Formation. The deformation phase, named “Oretanian Phase”, among others, by San José (1984) and Nozal Martín et al. (1988a, b, c), was firstly recognized in the Alcudia valley (Redlin, 1955; Crespo & Rey, 1972), although assigned to different ages. At present, it is considered as representative of a “late Cadomian folding event” (Talavera et al., 2015). No metamorphism is associated with this folding event.

Another interpretation for the onset of these steeply plunging folds was proposed by Vidal et al. (1994b), who suggested development of synsedimentary contorsion, and slumping of unconsolidated, kilometre-scale flysch deposits, in a continuous turbiditic-to-basinal infill with lack of shallowing and subaerial exposure. However, this interpretation did not take into account the uplift related to the sedimentation of alluvial-to-fluvial strata onlapping an inherited palaeorelief (e.g., the Orellanita Member of the Castañar Formation in the Sierra de la Zarzuela; Pieren Pidal et al., 1991). In addition, the authors considered the unconformity that marks the top of the Lower Alcudian-Domo Extremeño Supergroup as the same than that marking the base of the Pusa Formation, which is described below.

The base of the Pusa Formation is rarely (para)conformable, but represented by the onset of either (i) channelized megabreccias (e.g., Fuentes, Membrillar and Navalpino Beds), infilled with blocks reworked from underlying formations; or (ii) mélange units reworking consolidated and unconsolidated, intraformational shales and sandstones of the basal Pusa Formation. All event beds are affected by common hydrothermal veininig, whose derived counterparts occur as ubiquitous quartz vein clasts.

This intra-Terreneuvian paraconformable to angular discordant contact reflects the episodic record of extensional perturbations of the basement, leading to the erosion and redeposition of detritus derived from underlying formations, such as the Cloudina-bearing carbonate clasts of the Villarta Formation. This contact can be interpreted as the turnover from Cadomian-related compressive geodynamics (responsible for the input of unsorted siliciclastic clasts and preservation of high-angle substrates) to extensional conditions. In the neighbouring Ossa-Morena Zone, this intra-Terreneuvian episode is represented by the beginning of rifting conditions characterized by massive emplacement of rift-affinity igneous rocks (e.g., Sánchez-García et al., 2019).

Unquestionable Ediacara-type fossils are not known from the Central Iberian Zone although it undoubtedly contains rocks of appropriate age. The absence of such soft-bodied metazoans is likely due to the persistence of high rates of sedimentation of immature sediments in a turbiditic complex, under generally deep-water depths with clayey background sedimentation with little sediment difference for the preservation on bedding-planes. Morphologically simple trace fossils are moderately common in the Cíjara Formation. The lower part of the Ibor Group contains abundant Vendotaenids in the Ibor and Valdemanco anticlines and the Abenójar Dome. Neither the trace fossils nor the Vendotaenids allow a precise chronostratigraphic assignment within the late Ediacaran. Skeletal fossils within carbonates of the Ibor Group, in particular Cloudina, have been considered a solid evidence for a late Ediacaran age. Although this datum remains the most parsimonious interpretation, recent papers suggest the overlap of cloudinids and what is traditionally considered earliest Cambrian small shelly fossils (Yang et al., 2016; Zhu et al., 2017). The survival of Cloudina in Cambrian times cannot be discarded (Linnemann et al., 2019).

In the Valdelacasa Anticle, the presence of carbonaceous Beltanelliformis (in the lower member of the Pusa Formation), a short distance stratigraphically below or slightly overlapping the appearance of distinct Cambrian-type trace fossils have been traditionally used to mark the Ediacaran-Cambrian boundary interval (Brasier et al., 1979; Liñán et al., 1984, 1993). This level has remained the favoured placement by some workers (e.g. Gámez Vintated & Liñán, 1996, 2007). However, this is placed in doubt by the occurrence of both Monomorphichnus and Treptichnus traces from a putatively stratigraphically lower level, within the lower member, in a nearby section (Vidal et al., 1994b; Jensen et al., 2010). Fossils morphologically similar to the Beltanelliforms from the Pusa Formation have been reported from the lower part of the Soltanieh Formation in Iran (e.g. Stöcklin et al., 1964) and occur in the underlying Cíjara Formation. Shahkarami et al. (2017) discussed the stratigraphic implications of the Iranian material and comparable material from South China, and concluded that they are not reliable indicators of pre-Cambrian age.

While the placement of the Ediacaran-Cambrian boundary is somewhat uncertain within the Valdelacasa Anticline, a richer association of fossils with biochronological potential occur in the Ibor Group. Skeletal fossils, in particular Cloudina, within carbonate levels of the Ibor Group, are indicative of a late Ediacaran age. The upper part of the Ibor Group has yielded sabelliditids, including probable Sabellidites cambriensis (Vidal et al., 1994b). Treptichnus pedum occurs at the basal part of the Arrocampo Formation, representing the earliest occurrence of this index-fossil in the Central Iberian Zone.

Terreneuvian skeletal fossils are rare in the Pusa and Fuentepizarra formations. They are only known from a handful of locations where they nevertheless may be abundant. The Fontanarejo Bed (middle member of the Pusa Formation) has yielded sponge spicules embedded in thromboid substrates (Fig. 12E-F) (Perconig et al., 1986; Reitner et al., 2012). In the vicinity of La Nava the Ricomalillo, some levels close to the middle-upper member transition have yielded cap-shaped fossils similar to Scenella (Martí Mus et al., 2008) associated with scarce acritarchs (Fig. 14C-F), comprising the only know record of non-bacterial acritarchs from the Pusa Formation in the Valdelacasa Anticline. Remarkable in this context is the occurrence in the Salamanca area of process-bearing acritarchs preserved in carbonate nodules in strata correlatable with the Pusa Formation (Díez Balda & Furnier Vinas, 1981).

Despite the absence of the helcionellid Watsonella crosbyi, a chronostratigraphically significant biomarker for the future selection of the GSSP marking the Fortunian/Second Cambrian Stage boundary, the occurrence of the helcionellid Anabarella plana Vostokova (Gubanov in Vidal et al., 1999; Gubanov & Peel, 2003) in the Fuentepizarra Formation of the Alcudia Anticline allows identification of a biomarker close to the lower occurrence (LO) of W. crosbyi. In fact, the stratigraphic range of A. plana in the Anabar Uplift of the Siberian Platform (Kouchinsky et al., 2017) allows identification of a time span including the pre-trilobite Purella cristata and Watsonella crosbyi Zones in Siberia.

The “Slate and Greywacke Complex” of Carrington da Costa (1950) and Teixeira (1954, 1955) was subdivided by Herranz et al. (1977) into three (mixed) litho/chronostratigraphic units, from bottom to top, the B[o]eturian (a supposed Grenvillian basement that refers to the Serie Negra, the Ediacaran basement of the neighbouring Ossa-Morena Zone; San José, 1983), the Alcudian and the Pusian. Originally, the Alcudian was considered as a broad monotonous “flysch” lithosome (Bouyx, 1970; Parga, 1970; Vegas, 1971, 1978; Tamain, 1972, 1975; Moreno Serrano, 1974), up to 10 km thick, which included a heterolithic upper part with conglomerates and limestones. This upper part was considered “molasse” deposits by Maas (1963). Herranz et al. (1977) separated the upper heterolithic part from the lower monotonous “flysch” lithosome naming them “Lower Alcudian” and “Upper Alcudian”. Originally, both terms represented mixed litho-chronostratigraphic units (groups and stages) that were separated by a distinct angular discordance. Vilas et al. (1979) and San José (1983) described the “Upper Alcudian”, up to 3700 m thick, as a complex succession of fluvio-tidal deposits influenced by terrigenous and mixed platforms. Due to the presence of microbial structures and imprecise ichnofossils, the “Upper Alcudian” was dated as “Riphean-Vendian” (San José et al., 1974; Brasier et al., 1979).

The term “Pusian” was coined by San José (1983, 1984) as another mixed litho-chronostratigraphic lithosome that included the Precambrian-Cambrian boundary, and was sandwiched between the “olistostrome” megabreccias that unconformably overlie the “Upper Alcudian”, and the sandstone packages (Azorejo Formation) that traditionally marked the base of the Cambrian due to the presence of characteristic ichnofossils, such as Astropolithon and Psammichnites (Brasier et al., 1979). The Pusian comprised ichnofossils such as Monomorphichnus and Planolites, and soft-bodied impressions such as Beltanelloides and Chuaria.

Subsequently, San José et al. (1990) considered the terms “Lower Alcudian” and “Upper Alcudian-Pusian” as two megasequences displaying the lithostratigraphic significance of a supergroup. These authors still placed the base of the Cambrian in an indeterminate position within the “Pusian”.

The Cordubian or Corduban Stage was formally defined by Liñán et al. (1984) to include all strata of the Iberian Peninsula bearing arthropod-like ichnofossils and underlying the earliest trilobite record “in order to separate it from the Ovetian Stage that was defined using trilobite faunas” (p. 824). The authors proposed the Sierra de Córdoba as a type area for the stage and the (H)uso river section of the Central Iberian Zone (Brasier et al., 1979; Palacios Medrano, 1989) as stratotype for its base, marked by the lowest occurrence (LO) of the ichnogenus Monomorphichnus. Other (para)stratotypes were selected by Liñán et al. (1993) at Barrios de Luna (Cantabrian Zone; Crimes et al., 1977) and Concha de Artedo (West Asturian-Leonese Zone; Crimes et al., 1977) sections, where the Ediacaran/Cambrian contact is marked by an angular discordance, and in the La Rinconada section (Central Iberian Zone; Corrales et al., 1974). The authors also suggested the LO of the ichnofossil Astropolichnus hispanicus as the top of the Corduban in the Tamames Sandstone of the Central Iberian Zone, but this is in contradiction with the base of the overlying Ovetian Stage, defined at the LO of dolerolenid trilobites (Sdzuy, 1971; Liñán et al., 1993).

Subsequently, Liñán et al. (1993) and Gámez-Vintaned & Liñán (1996) summarized and completed a list of ichnofossils characteristic of the Corduban. Gámez-Vintaned & Liñán (1996) suggested that the LO of Monomorphichnus “nearly coincides” with that of “Phycodes” pedum, as a result of which they proposed the LO of the M. lineatus-T. pedum ichnofossil assemblage as the Precambrian-Cambrian boundary in Spain and as new base for the Corduban. In their figure 3, the base of the upper Corduban Substage is marked by the LO of Rusophycus, predating that of Cruziana.

New findings of shelly fossils and acritarchs somewhat completed this sketch. Palacios & Vidal (1996) and Vidal et al. (1999) reported the presence of Cloudina in the Ibor Group, and of Anabarella, Aldanella, hyolithids (circothecids and orthothecids), aff. Mongolitubulus and chancelloriids in the Pusa and Fuentepizarra formations; bigotinid trilobite and archaeocyathan moulds were also found in these shales, as a result of which, the authors suggested that part of the Pusa Formation would be Nemakit-Daldynian to mid-Tommotian in age.

Finally, Liñán et al. (2002) subdivided again the Corduban into two substages, the bases of which were selected at the LO of M. lineatus + T. pedum and Rusophycus avalonensis, respectively. However the authors dramatically modified the concept of the stage: Liñán et al. (2002) selected the indeterminate bigotinids reported by Palacios & Vidal (1996) and Vidal et al. (1999) to characterize the upper part of the lower Corduban. This broke into pieces the original concept of the Corduban as a Cambrian stage underlying the LO of trilobites, originally suggested as correlatable with the Nemakit-Daldynian and Tommotian stages of Siberia.

In addition, Liñán et al. (2002) established an ichnofossil-based zone to mark the topmost of the Ediacaran: the Torrowangea rosei Zone. A single specimen of the ichnotaxon Torrowangea aff. rosei from the Saviñán Formation of the Paracuellos Formation, Iberian Chains, was illustrated and described by Liñán & Tejero (1988); however, this specimen was subsequently reconsidered as questionable by Jensen et al. (2007: Table 1) because “the published information does not allow for a critical evaluation”. The presence of T. rosei (without “aff.”) in the Pusa (Liñán et al., 2002: p. 19-20) and Cíjara (Gámez-Vintaned, 1996: fig. 9) formations of the western Toledo Mountains has not yet been supported with any illustration.

Based on the presence of T. pedum at the basal part of the underlying Arrocampo Formation, the proposal of the base of the Corduban in the Pusa Formation as the regional Ediacaran-Cambrian boundary should be abandoned. The boundary is identified, at least, at the basal part of the Arrocampo Formation, below the major unconformity that marks the base of the Pusa Formation, and below the base of the regional Corduban Stage sensu Liñán et al. (1993) and Liñán et al. (2002).