Post-conference fieldtrip, October 21-24, 2019: Ediacaran and Terreneuvian strata surrounding the Villuercas-Ibores-Jara UNESCO Global Geopark
Guía de campo post-congreso, 21-24 de Octubre de 2019: Ediacárico y Terreneuviense en los alrededores del Geoparque de la UNESCO Villuercas-Ibores-Jara

J.J. Álvaro¹, S. Jensen², T. Palacios²

¹Instituto de Geociencias (CSIC-UCM), Dr. Severo Ochoa 7, 28040 Madrid, Spain. Email: jj.alvaro@csic.es, ORCID ID: http://orcid.org/0000-0001-6294-1998

²Área de Paleontología, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain. Email: soren@unex.es, medrano@unex.es; ORCID ID: http://orcid.org/0000-0003-4573-9282, http://orcid.org/0000-0002-4917-0559

IntroductionTOP

The surroundings of the Villuercas-Ibores-Jara UNESCO Global Geopark (Cáceres province, Spain) comprise some key outcrops to constrain the Ediacaran-Cambrian boundary interval in the Iberian Peninsula. The stops of the International Meeting on the Ediacaran-Cambrian Boundary (IMECT), post-conference fieldtrip (October 21-24, 2019) located in the Badajoz, Cáceres and Toledo provinces, are described and illustrated below.

October 21: Lower Alcudian-Domo Extremeño SupergroupTOP

The stops of this day provide examples of sedimentary rocks, including glaciogenic diamictites, from the Lower Alcudian-Domo Extremeño Supergroup, cropping out in the Extremenian Anticlinorium; for stratigraphic setting, see Figure 1.

STOP 1. Zújar damTOP

The area to be visited is reached by driving south from Guadalupe into the northern regions of the Badajoz Province, south of Orellana la Vieja village. During this drive a vast flat area is crossed by road EX-116. Thousands of cranes pass the winters in this area.

Location and description. Road cut close to lock of Zújar dam, at (WGS 84) 38°54’51”N, 05°24’37”W. This outcrop gives a view of greywacke and shaly sediments typical of the Lower Acudian-Domo Extremeño Supergroup commonly exhibiting vertically oriented fold axes.

STOP 2. Weesenstein-Orellana post-Gaskiers glaciationTOP

(Ulf Linnemann & Agustín Pieren)

In the Cadomian orogen of the NE Bohemian Massif and of SW Iberia a ca. 565 Ma old post-Gaskiers glacial event has been detected (Linnemann et al., 2018). Such Ediacaran-aged glaciomarine deposits occur in the Weesenstein and Clanzschwitz groups of the Saxo-Thuringian Zone (Bohemia) and in the Lower Alcudian-Domo Extremeño Supergroup of the southern Central Iberian Zone (Iberian Massif). Both areas are parts of Cadomia situated in the Western and Central European Variscides. A suspect tillite exists in the Armorican Massif at Granville (Normandy; Fig. 2) (Wegmann et al., 1950, Wegmann, 1951, Graindor, 1964). Its origin is under discussion and the interpretation as glaciomarine deposit was rejected by Eyles (1990). Glaciomarine sedimentary rocks are characterized by such features as dropstones, flat iron shaped pebbles (“Bügeleisen-Geschiebe”), facetted pebbles, dreikanters and zircon grains affected by ice abrasion. For the maximum age of deposition and for provenance determination LA-ICP-MS U-Pb zircon ages were performed. The maximum age of the glaciomarine deposits within a Cadomian back-arc basin based on U-Pb analytics resulted in the youngest detrital zircon populations showing ages of ca. 565 Ma. Described glaciomarine diamictites of Cadomia (Weesenstein, Clanzschwitz and Orellana diamictites) are definitely younger than the ca. 579-581 Ma old Gaskiers glaciation in Newfoundland (Gaskiers) (Pu et al., 2016). Linnemann et al. (2018) proposed the term Weesenstein-Orellana Glaciation for the new Ediacaran glacial event, named after its most relevant regions of exposure. Palaeogeographically, these glaciomarine diamictites and related sedimentary deposits lie on the periphery of the West African craton (western peri-Gondwana), evidence provided by detrital zircon U-Pb ages and their Hf isotope composition (Linnemann et al., 2018). Correlation with similar glaciomarine deposits in the Anti-Atlas (Bou Azzer inlier; Vernhet et al., 2012) and Arabia (Vickers-Rich et al., 2012) suggests a continued distribution of post-Gaskiers glacial deposits along the Gondwana margin of Northern Africa. The Weesenstein-Orellana Glaciation correlates with the Shuram-Wonoka δ¹³C anomaly.

The deposits of the Weesenstein-Orellana glaciation in the Central Iberian Zone. Here we focus, due to the working area of the field workshop, on the Spanish part of the glacial deposits of the Weesenstein-Orellana Glaciation in the Orellana area (Figs. 3-4). The latter one is part of the Central Iberian Zone (Julivert et al., 1972). Ovtracht & Tamain (1970) studied the Proterozoic rocks of the Schist and Greywacke Complex in the core of the Alcudia Anticline. They subdivided the Ediacaran sedimentary rocks into “Lower and Upper Alcudian groups” (Fig. 4), subsequently integrated into the so-called “Alcudian Supergroup” (San José et al., 1990). These rocks have also been called the Extremenian or Extremeño Dome Group (Álvarez Nava et al., 1988), though, despite the name, the unit was defined in another structure further north. Therefore, the term Lower Alcudian-Domo Extremeño Supergroup is selected here (see Álvaro et al., 2019). In the surroundings of Orellana, Ediacaran and Lower Palaeozoic strata were mapped and studied in detail by Pieren (2000). The exposed Lower Alcudian-Domo Extremeño Supergroup is about 4000 m thick but its base is not exposed. The supergroup is characterized by a monotonous succession of turbidites composed of greywackes and mudstones (Fig. 4). According to García-Hidalgo et al. (1993) and Pieren (2000), the supergroup is composed, from bottom to top, of (i) shale and greywacke (“La Coronada Shales formation”), (ii) greywacke, microconglomerate and shale (“Saint Mary of Zújar chapel greywackes and conglomerates formation”), and (iii) shale, diamictite (pebbly mudstone) and greywacke (“Orellana matrix supported conglomerates formation”) (Fig. 4). Some greywacke turbidite beds of the second formation are up to 6 m thick and have been used as marker beds during the geological mapping of the region.

The Lower Alcudian-Domo Extremeño Supergroup was folded and locally cleaved during the Cadomian orogenic activity. Generally, a vertical and subvertical L₁ lineation is present, which was generated by a (in the area) non-schistose gentle folding in the late Ediacaran (“intra-Alcudian” or Cadomian deformation). In addition, a Variscan schistose F₁ exists. Most cobbles were oriented parallel to a 70º N dipping L¹ lineation. Some of them were not oriented or almost normal to the L¹ lineation (Pieren, 2000).

STOPS 3 (Orellana reservoir at “Poblado Turístico”) and 4 (road Orellana la Vieja to Zújar river reservoir)TOP

(Ulf Linnemann & Agustín Pieren)

Stops 3 (38°59’49.02”N, 5°31’49.97”W) and 4 (38°58’36.10”N, 5°31’19.14”W) are focused on diamictites from the uppermost part the Lower Alcudian-Domo Extremeño Supergroup (Figs. 4, 5A, 5F). These show distinct features indicative of a glaciomarine origin, including dropstones (Fig. 5B), rain-out sediments (Fig. 5C), flat iron shaped pebbles (“Bügeleisen-Geschiebe”; Fig. 5D), and facetted pebbles (Fig. 5E). Pebble size varies between a few millimetres and 20 cm. Glaciomarine diamictite occurs in ca. 10 to 50 m thick beds in the region of Orellana la Vieja, Orellana de la Sierra and the Orellana dam (Pieren, 2000).

U-Pb ages of detrital zircons are represented in figure 6. Because ash beds and fossils have not been found in the investigated sections, the maximum depositional age (MDA) had to be used for the stratigraphic age determination (Condon & Bowring, 2011).

The maximum depositional age for each sample was calculated from the youngest zircon population for each sandstone sample (Fig. 6). The zircons from the Orellana diamictite (sample OR1) in the Lower Alcudian-Domo Extremeño Supergroup of the Central Iberian zone yielded a maximum depositional age of 565 ± 4 Ma (Fig. 6) (Linnemann et al., 2018). The maximum depositional age for the diamictites is significantly younger than that for the Gaskiers glaciation in Newfoundland. Thus, the diamictite should represent a post-Gaskiers glacial event during Ediacaran time. Alternatively, a shifting of the ice center from Avalonia (Gaskiers glaciation, ca. 579-581 Ma) to West African peri-Gondwana (Weesenstein-Orellana glaciation, ca. 565 Ma) over time would be possible. The detrital zircon populations of the Weesenstein-Orellana glaciation support a palaeoposition marginal to West Africa (Linnemann et al., 2018).

Age constraints completely rule out a correlation of this glacial event recorded in the Cadomian metasedimentary rocks with the Gaskiers glaciation, which occurred around 579-581 Ma (Bowring et al., 2002, Thompson et al., 2014; Pu et al., 2016). Instead, a correlation with a glacial event recorded in Northwest Africa (Morocco) (Vernhet et al., 2012) and in Arabia (Vickers-Rich et al., 2012) is much more realistic and provides information about the timing of this young Neoproterozoic glacial event. At Bou Azzer (Moroccan Anti-Atlas) Vernhet et al. (2012) describe glaciomarine structures (striated pavement on rhyolites) within the Ouarzazate group. The age of the latter is constrained by published U-Pb ages of the Ouarzazate Supergroup and can be placed in a time window between ca. 570 and 562 Ma (Karaoui et al., 2015). In Arabia, another glaciomarine diamictite has been detected in Ediacaran sedimentary deposits of the Dhaiqua formation. An ash bed just below the diamictite provided a U-Pb age of 560±4 Ma (Vickers-Rich et al., 2012). Further, an Ediacaran glacial deposit (Moelv diamictite) crops out in Baltica (Bingen et al., 2005). But available age data are not sufficient to constrain an age younger than the Gaskiers glaciation (Bingen et al., 2005). Existence of this Cadomian glaciation could be the reason for the negative excursion of the δ¹³C curve (Shuram-Wonoka anomaly) (Halverson et al., 2005: fig. 18) (Fig. 7). The origin of the Shuram-Wonoka anomaly is still under discussion. In addition to all, Letsch et al. (2018) reported also equivalents of the true Gaskiers glaciation in a time window in the range of 592 to 579 Ma from the Anti-Atlas (Morocco). It seems that West Africa and adjoining terranes represents an area that was affected by several glacial events in Upper Ediacaran time.

October 22: Plunging into reefal worlds, Villarta FormationTOP

Drive to Stop 5. The picturesque village of Villarta de los Montes (province of Badajoz) is located in a hilly area, on the foothills of the Umbria Mountains. Its setting is quite far from other towns of the area, from which it is separated by a rugged orography of enormous magnificence and scenic beauty, covered with forests and scrubland. Villarta “of the mountains” is famous for the hunting parties that are organized in the Cíjara Regional Hunting Reserve, and the fishing activities that take place in the neighbouring Cíjara dam. The northern shore of the dam has represented, for decades, the target of Ediacaran palaeontological research: along a W-E transect of about 4 km, the hills offer some key exposures of the Castañar and Villarta formations. The latter are unconformably overlain by the Lower Ordovician “Purple Series” and Armorican Quartzite, which form the mountainous crests.

STOP 5. Visiting the Hermitage of Santa María de la AntiguaTOP

On the mountainous heights, after overpassing the bridge that cross the Cíjara reservoir, stands the hermitage of “Nuestra Señora de la Antigua” (Our Lady the Old), protector of Villarta de los Montes. The basement of the hermitage is the stratotype of the so-called La Antigua Conglomerate Member (Castañar Formation). The unit, up to 100 m thick, consists of clast-supported heterolithic granule-to-cobble clasts embedded in an unsorted coarse- to fine-grained litharenite matrix. Clasts are composed of greywacke, sandstone, shale, chert and vein quartz. The member unconformably overlies the Lower Alcudian-Domo Extremeño greywackes.

STOP 6. Sequence analysis of Cloudina-microbial reefsTOP

Following a pathway that follows the northern shore of the Cíjara dam, we will enter a private area for livestock farming. A sheep, pig and cattle farm, known as “la Majada del Andaluz” (the Andalusian sheepfold), marks the beginning of this private pastureland. We advise against any furtive sampling of rocks: if you want to visit the area on your own, please, request permission at the town hall before entering this private area.

The “Majada” lies at the mid-point of a stratigraphic log that begins at the Cíjara dam and reaches the overlying Lower-Ordovician “Purple Series” on the crest of the hill: the stratotype of the Villarta Formation. The latter displays three distinct members: (i) an alternation of lenticular and bedded carbonates embedded in greenish shales (Figs. 8, 9A-D); (ii) a sandstone-dominant unit passing upward to sandy dolostones (Fig. 9E-F); and (iii) an alternation of shales and bedded carbonates (Fig. 9G-H).

Stop 6 represents the stratotype of the formation (GPS: N39º13’52.86’’, W39º13’52.86’’), where complete lower and middle members are well exposed (Fig. 8). The lower member consists of a superposition of shale-marlstone-limestone cycles, up to 2.4 m thick. The uppermost part of each cycle is marked by Cloudina-microbial patch-reefs and/or biostromes, dominated by thromboid textures and punctuated by Cloudina shells preserved both upright and lying on the surface. Cloudina-thromboid-stromatoid buildups 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). The heterolithic middle member consists of conglomerate, sandstone and shale beds. Sandy litharenites show common trough cross-stratification sets punctuated by channels. Finally, the upper member comprises limestone/shale alternations, the latter with trace fossils of an appearance consistent with a Cambrian age (Álvaro et al., 2019).

STOP 7. Geometrical relationships of Cloudina-microbial reef complexesTOP

This stop is located some 500 m SE of stop 6. Frame reefs display two main geometries, core and flank beds, which stand out in contrast as lenticular and bedded masses of beige resistant limestones encased in thinly bedded, greenish shales and marlstones (inter-reef sediments; Fig. 10A-B). The core geometry ranges from patches or isolated pillow-shaped masses up to 0.8 m thick, to bioherms or lenticular masses of larger dimensions, less than 1.8 m thick, where well-developed flank beds commonly intercalate with inter-reef sediments. Locally, these individual geometries are arranged into stacked patches becoming both vertically and laterally aggregated into reef complexes, up to 2.8 m high and 6 m wide.

Texturally, the reef-core facies consists of Cloudina-thromboid boundstones and floatstones with local development of peloidal and intraclastic pockets, punctuated by domal stromatolites. Due to intense neomorphism, the calcified microbial communities are heterogeneously preserved, with clotted textures occurring directly encrusting both microsparitic substrates and shelly walls. The microbial framestone is dominated by clotted, peloidal, fenestral textures, and subsidiary stromatolitic microsparite/sparry textures.

Flank beds form the margin of some bioherms and reef complexes. Proximal flanks consist of well-bedded, intraclastic packstone-to-wackestone and nodular limestones rich in breccia intraclasts and oncoidal lags. Proximal flanks grade laterally into thinly bedded (0.1-0.2 m) limestones and marlstones that finally alternate with shales bearing centimetre-thick nodules (inter-reef sediments). Flank beds thin away from the core passing laterally into debris beds or flat bedded inter-reef sediments. Inter- and off-reef sediments are dominated by bedded marlstones and shales. Some flanks are slumped and contain reef blocks and slope intraclast beds (Fig. 10D). Inter-reef, thinly laminated shales and marlstones display contorted and convolute folds (laterally extensive series of folds confined between two undeformed layers) and slumps bearing scattered shells preserved as moulds. Larger bioherms and reef complexes contain Neptunian dykes (Fig. 10C), up to 30 cm in width. These were infilled with angular calcite and dolomite clasts, up to 5 cm long, embedded in a microsparitic matrix with both calcite and dolomite cements.

STOP 8. When reefs nucleated along the border of an unstable blockTOP

In the easternmost outcrops of the visited W-E transect along the northern shore of the Cíjara dam, the lower member consists of carbonate/shale alternations. Carbonates, up to 1.4 m thick, display lower shelly-microbial frame-building textures commonly scoured by upper chaotic breccias (Fig. 10E-G). The latter, up to 30 cm thick, are unsorted, polymictic and rich in angular to subrounded, pebble- to sand-sized clasts. Evidence of rebrecciation is observed as larger angular clasts are composed of polyphase breccia generations. In contrast, smaller clasts exhibit some degree of rounding and sorting. The breccia is dominantly clast-supported, with a matrix rich in illite locally punctuated with framboidal pyrite. Polymictic clasts consist of clotted limestone and partly or wholly dolomitized limestone, and subsidiary shale, chert and phosphate. Steeply dipping fissures and veins occur crosscutting the carbonate beds and their clast counterparts. Veining, associated with hematite and barite precipitation, is conspicuous both in the encasing limestone and clasts. Some fissured breccia lags are sealed by patch-reefs, up to 60 cm high and 80 cm in basal diameter, which have no distinct flanks. This outcrop represents the easternmost exposure of the Villarta Formation in the area. To the east, the Navalpino megabreccia Bed unconformably overlies the Castañar Formation (the latter known as the “Torilejo conglomerates” in regional geology).

October 23: Traditional outcrops since the 1970sTOP

During this excursion will be examined several sections in the area of La Nava de Ricomalillo, Toledo Province, which are of both historical and contemporary interest for questions on the Ediacaran-Cambrian transition in the Central Iberian Zone. Sections west of La Nava de Ricomalillo provide some of the most important outcrops of the Cíjara and Pusa formations. An uncompleted railway trench that runs approximately in a S-to-N orientation provides exposures of the Cíjara and Pusa formations, which it cuts obliquely. This track, which is now “Vía Verde”, a recreational “green” path, runs close to the Huso (or Uso) river, also provides important outcrops. Additional sections are found in road cuts. Three stops will be visited during this field trip (Fig. 11).

STOP 9. Denudation of a Cadomian Arc along the Cubilar streamTOP

Location.- The section is located along the Arroyo de Cubilar stream which can be accessed by dropping down from road TO-1195 about 6.5 km from La Nava de Ricomalillo. This section exposes the lower part of the Pusa Formation, the Fuentes megabreccia Bed and the Cíjara Formation, including good exposures of channelized conglomerates in the latter.

Description. The first part of the section shows intermittent outcrop continuity of the lower part of the Pusa Formation. Trace fossils are present, but hard to find, in the first slope including Treptichnus isp. (Jensen et al., 2007: fig. 2a, f). The Fuentes megabreccia is reached after a 20 minutes’ walk during which intermittent outcrop, showing typical examples of laminated siltstone and fine sandstone of the Pusa Formation are exposed. At outcrop scale, the Fuentes megabreccia Bed is composed of two principal levels separated by laminated sandstone (Fig. 12A). Outcrops on the NE side of the same hill show clearly the channelized nature of the megabreccia lobes (Fig. 13). The breccia contains a variety of clasts including greywacke, shale and carbonate clasts (Fig. 12C). Carbonate blocks in the lower interval have yielded examples of silicified Cloudina shells. The material is typically poorly preserved (Fig. 12B), but clear examples of Cloudina carinata have been encountered. Following Arroyo de Cubilar in a southerly direction, sandstone and minor mudstone beds of the Cíjara Formation are seen (Fig. 12D). Polarity is opposite to that of the Pusa Formation, and the contact here between the Cíjara Formation and the Fuentes megabreccia is faulted. Sandstone beds often show graded bedding and ripples. After a further 10 minutes’ walk, channelized conglomerates with abundant hydrothermal quartz clasts are encountered (Fig. 12E). The conglomerates are mainly clast-supported but other levels further along the Arroyo Cubilar are matrix-supported. Other sedimentary structures in the Cíjara Formation include flute, tool and frondescent marks.

STOP 10. Historical Ediacaran-Cambrian boundary reference section along an abandoned railway lineTOP

Location. This stop is reached by entering a dirt road that leads off road TO-1195, some 6 km from La Nava de Ricomalillo.

Description. Outcrop along the abandoned railtrack in both directions from the derelict Nava-Fuentes station house exposes portions of the lower member of the Pusa Formation. This is dominated by clayey sediments with minor sandstone interbeds, which has been interpreted as deposited in an offshore-dominant setting. Outcrop on either side of the station house have been given as being about 250 m above the base of the lower member. This is the principal section from which trace fossils were described from the lower part of the Pusa Formation by Brasier et al. (1979), Palacios Medrano (1989) and Vidal et al. (1994). Bedding dips steeply and a peculiarity of the succession is that trace fossils are generally seen only on upper bedding surfaces whereas bed bases are barren. Bedding is normal in this part of the section and observation will be made on the western face of the track. During this field trip observations will concentrate on two stretches of outcrop (Fig. 14).

Beltanelliformis level. An outcrop some 250 m SE from the station building contains upper bedding surfaces with densely packed organic fossils, now elongate but almost certainly originally circular (Fig 14A). These have been referred in the literature to as Chuaria, Beltanelloides and Beltanelliformis. The outcrop has become degraded by natural collapse but also by the activity of collectors. There are reasons to believe that these occurrences originally covered several square metres. Trace fossils have not been recovered from this part of the section, but very rare filaments have been found.

Cambrian-type ichnofossils. Passing the station houses in a NW direction, outcrops on the left hand side after passing a short tunnel, contain surfaces with abundant trace fossils (Fig. 15A). Most common are horizontal simple millimetric to sub-millimetric trace fossils of Helmimthoidichnites type. Also seen are larger irregularly winding horizontal forms. Favourable light conditions are essential for a good view of these trace fossils. A short distance further north, more complex trace fossils are seen: these include examples of Treptichnus pedum (Fig. 15B-C) and probes (Fig. 15D). Faintly developed Monomorphichnus have also been observed.

Discussion. In several publications, the transition between the two aforementioned sections has been suggested to represent the Precambrian-Cambrian boundary interval and proposed as a regional Ediacaran-Cambrian transition reference section and the base of the regional Cordub(i)an Stage (e.g. Liñán et al., 1993, 2002; Gámez Vintaned, 1996; Gámez Vintaned & Liñán, 1996, 2007; for a critical reappraisal, see Álvaro et al., 2019). It is, however, likely that the succession of strata observed here are younger than the currently used age for the base of the Cambrian at about 539 Ma. Treptichnus and Monomorphichnus have been recorded (Brasier et al., 1979; Jensen et al., 2007) from a nearby location that probably is at a lower stratigraphic level. The Fuentes megabreccia contains allochthonous Cloudina shells, including Cloudina carinata, suggesting a late Ediacaran age for the involved carbonate blocks, encased in a younger megabreccia bed. As a result, the pre-Cordub(i)an part of the Pusa Formation, underlying the lowest record of Treptichnus, should be Terreneuvian in age. Some support for this comes from a detrital zircon date of 533 Ma recorded by Talavera et al. (2012) from the middle portion of the lower Pusa Member from a nearby section. More precise radiometric dating is needed to confirm this age.

STOP 11. Simple trace fossils from the Cíjara FormationTOP

Location. Road cut along road CM-411, at 39°37’36”N, 05°01’ 45”W. IMPORTANT: this is a regularly trafficked road with speed limit of 90 km/h. Take care when crossing and avoid entering pavement.

Descripton. The road cut on the eastern side contains several surfaces of overturned Cíjara Formation strata (Fig. 16A) with several bed soles showing simple trace fossils typical of the Cíjara Formation (Fig. 16B). Trace fossils from this outcrop have been figured in Jensen et al. (2007: fig. 2e) and Jensen & Palacios (2016: fig. 3e). The western road cut shows channelized conglomerates -containing phosphate clasts. Also note well-developed folds typical for the upper part of the Cíjara Formation in this area.

Some 200 m north along Vía Verde, a bed sole contains a structure consisting of two apparently joined sets of paired ridges or lobes (Fig. 16C). This shows resemblance to bilobed trace fossils, which, with a width of some 15 mm, would be unusually large for a late Ediacaran trace fossil. As this is the only known occurrence of this type of structure in the Cíjara Formation, its interpretation is better left open.

October 24: Treasures hidden under the Tagus riverTOP

Several sections of the Tagus river are dammed along Spain and Portugal. Upstream of the Almaraz nuclear power plant, the Valdemoreno and Valdecañas reservoirs attempt to maintain the river’s flow during the months of drought. The mouth of the waters that cool the nuclear reactors of the Almaraz plant marks the beginning of García Hidalgo’s (1985) Arrocampo log. Along the following 2 km on the right bank, the log represents what is here proposed as the “ichnological” Ediacaran-Cambrian boundary stratotype of the Iberian Massif. This ribbon exposure is only accessible when the upstream reservoirs are closed: between September and March. Outside those months, the outcrops are partly covered by water and the visit is discouraged.

STOP 12. The “ichnological” Ediacaran-Cambrian boundary stratotypeTOP

Due to the influence of the Cadomian orogen, the denudation of uplifting Arc areas led to deposition of coarse-grained siliciclastic sediments infilling the back-arc basin that we know now as the Central Iberian Zone (Álvaro et al., 2019). The poisoning effect on carbonate factories of this stepwise denudation was coeval with the breakdown of the back-arc basin into uplifting, rotating and collapsing platform-blocks. As a result, each block recorded diachronous processes of carbonate production and terrigenous input. The limits of the Villarta Formation are not isochronous throughout the visited stops, and the members defined in its stratotype (Villarta de los Montes) cannot be easily followed laterally.

This is the case of the Arrocampo log. This carbonate-dominant succession is here interpreted to represent the upper member of the Villarta Formation, whereas the lower member, rich in Cloudina-bearing limestones, is poorly preserved here. The upper member of the Villarta Formation shows, in ascending order, a succession of trough cross-bedding sets and beds with low-angle to parallel laminae, locally punctuated with centimetre-thick stromatolites (Fig. 17A-D) and possible Beltanelliformis moulds (Fig. 17E) A distinct level marks a sharp difference in environmental conditions; it is characterized by the presence of herringbone cross-laminated beds (Fig. 17F) unconformably overlain by a decimetric breccia bed related to platform-block breakdown (Fig. 17G). The uppermost part of the member displays, centimetre-to decimetre-thick, bedded to wavy strata (Fig. 17H). The member consists of impure limestones due to the conspicuous presence of silty grains of quartz and feldspar, which reflect the episodic influence of an actively denudating Cadomian Arc situated to the SW.

The Villarta Formation is conformable overlain by a monotonous shaly succession, which represents the stratotype of the Arrocampo Formation (Álvaro et al., 2019).The stratotype is a shale-dominated unit with subsidiary sandstone interbeds (Fig. 18A) increasing upwards, and local presence of carbonate nodules parallel to stratification. 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 base of the formation contains the earliest ichnofossils assigned to Treptichnus pedum (Fig. 18B-C), so marking the chronostratigraphic base of the Terreneuvian, although the appearance coincides with the occurrence of siliciclastic sediments and so is clearly facies controlled.

ACKNOWLEDGEMENTSTOP

The authors thanks J. Colmenar (Madrid) and J. Esteve (Bogotá) for revisions that helped to improve a previous version. This paper is a contribution to Spanish Project CGL2017-87631-P.

ReferencesTOP