New Bilbilian ( early Cambrian ) archaeocyath-rich thrombolitic microbialite from the Láncara Formation ( Cantabrian Mts . , northern Spain )

Recent palaeontological and microfacies studies carried out on the Láncara Formation (early Cambrian) provide evidence for an interesting, previously undescribed association of archaeocyaths (Salce locality) and microbialites (Salce and Barrios de Luna localities). The archaeocyathan assemblage consists of Archaeocyathus laqueus (VologdIn, 1932) and Pycnoidocyathus erbiensis (ZhurAVleVA, 1955), indicating an early Bilbilian age (Stage 4, Series 2, Cambrian) for these materials. The analysis of the upper part of the lower member has allowed differentiation of eleven carbonate facies that have been grouped into: i) non-skeletal grain packstone-grainstone, ii) fenestral mudstone-packstone, iii) heterolithic stylonodular facies, iv) microbialites, v) bioclast-intraclast packstone-grainstone. Archaeocyaths occur reworked in stylonodular facies as well as forming small archaeocyaths-thrombolitic patches (centimetre-scale). The archaeocyath-rich thrombolitic microbialites from Salce were developed in very shallow subtidal conditions surrounded by other microbialites and small lenticular intertidal bars in the inner ramp. Toyonian biostratigraphic and paleobiogeographic analyses have also been carried out. After the comparison with Toyonian archaeocyathan rich facies from Gondwana, it has become evident that the early Cambrian record from the Cantabrian Mountains provides the richest generic assemblage from Gondwana for Toyonian time.

The purpose of this paper is to: 1) analyze the litho-and biostratigraphic record of Lower Cambrian materials in Salce and its correlation with the closest Barrios de Luna section (reference section for the Láncara Formation in the Somiedo-Correcilla Subunit); 2) reconstruct the environmental setting of the archaeocyath-thrombolitic microbialites from the lower member of the Láncara Formation; 3) document the taxonomy of the archaeocyaths; 4) establish the biostratigraphic and paleogeographic correlations with other regions; and 5) compare the Toyonian archaeocyath-rich biofacies from Gondwana.

Geological Setting and Stratigraphy of the Láncara Formation
The analyzed occurrence is located in the Cantabrian Zone of the northwestern Iberian Peninsula, in the most external position in the northeastern part of the Iberian Massif (Lotze, 1945) (Fig. 1A).The Cantabrian Zone corresponds to the foreland-and-thrust belt of the northwestern Iberian Variscan Orogen (Julivert, 1971).Orogenic deformation during Carboniferous time resulted in a characteristic thin-skinned tectonic style in the Can-Cantabrian Zone.The Cantabrian Zone has been divided into different tectonostratigraphical units (Vera, 2004;Julivert, 1967;1971).The present work is focused on Cambrian limestone from the Láncara Formation in the Somiedo-Correcilla Subunit (Julivert et al., 1968), which is part of the Unidades Occidentales y Meridionales [Región de Pliegues y Mantos according to Julivert (1967Julivert ( , 1971))], where practically the whole Palaeozoic succession is present.The meridional tract of the Somiedo-Correcilla Subunit (Fig. 1B) comprises the Narcea-Mora, Herrería, Láncara and Oville Formations (Proterozoic to middle Cambrian record, Fig. 2).
Archaeocyaths have been found at the Salce locality (Perejón et al., 2007) near the Barrios de Luna locality.However the Láncara Formation at Salce is not as well exposed and shows significant differences from the Barrios de Luna section (Fig. 3).To the north of Salce, the Proterozoic and Cambrian rocks are tectonically bounded between two NE-SW faults into a NW-SE nappe structure (MAGNA Sheet 102;Rodriguez Fernández et al., 1990).Along the slopes of Cerro de Valdemarzón, the geological record begins with the siliciclastic rocks of the Narcea-Mora Formation (Neoproterozoic).This is unconformably overlain by sandstone of the Herrería Formation (lower Cambrian), followed by the Láncara Formation carbonates which are in turn conformably overlain by shale of the Oville Formation.
The Láncara Formation was informally divided into two members (lower and upper) by Zamarreño (1972) and displays five distinct units (A-E in Fig. 2), various constituents of which have been named and described by several authors (Zamarreño, 1972(Zamarreño, , 1978;;Álvaro et al., 2000b;Wotte, 2009).The lower member has a variable thickness from 117 m up to 223 m whereas the upper member ranges from 11 m to 48 m (Aramburu et al., 2006).The lower member exhibits two or three units (A-C in Fig. 2), which according to Zamarreño (1972) correspond to: i) yellow dolostone; ii) grey bedded limestone with birdseyes; and, in some localities, iii) an upper detrital interval up to 12 m in thickness composed of ooid limestone, sandstone and nodular limestone with archaeocyaths.The upper detrital interval has been recognized in the Esla nappe and at northward of the Esla nappe (Zamarreño, 1978) but has been never described in the Somiedo-Correcilla Subunit.The upper member corresponds to encrinitic-glauconitic limestone and nodulargriotte limestone respectively termed Beleño and Barrios facies by Zamarreño (1972) (D-E in Fig. 2).The present paper describes the first recorded occurrence of archaeocyaths in the Somiedo-Correcilla Subunit at Salce and their lateral equivalents in the nearby Barrios de Luna section; thus carbonate facies descriptions are focused on the upper levels of the lower member of the Láncara Formation.
The top of the Láncara Formation shows three discontinuities (D1-D3 according to Álvaro et al., 2000 b) in the Crémenes and Valdoré sections (Esla nappe).The Salce section records these discontinuities as well, in fact, the archaeocyaths occur in the lenticular limestone (Level 9, Fig. 3) that are bound by the erosive surfaces D1 and D2.The lenticular limestone is succeeded by encrinitic white limestone with trilobites, which marks In this paper the facies analysis is focused on unit B in Barrios de Luna (see Fig. 2 and Table 1) and units B and C in Salce.
Spongiostromate-oncoid packstone-grainstone (a2) with poorly sorted fabric is conspicuous (Fig. 4B, facies a2 in Fig. 3 and Fig. 6).Spongiostromate oncoid reach up 15 mm size with a poorly to unlaminated spongy micritic cortex.The term spongiostromate oncoid is used for micrite oncoids possessing a laminated dense micritic or the transition from the lower to middle Cambrian.The third discontinuity (D3) occurs between the encrinitic and griotte limestones and is related to a succession of tectonic pulses that have been recognized in other areas in southwestern Europe (Bechstädt et al., 1988;Bechstädt and Boni, 1989;Aramburu et al., 1992;Russo and Bechstädt, 1994;Álvaro and Vennin, 1996).However, in the Barrios de Luna section we have only recognized discontinuities D2 and D3 (Fig. 3).
The upper part of the lower member of the Láncara Formation displays irregular laminoid fenestral fabric type b1 and b2 (Tebbutt et al., 1965;Müller-Jungbluth and Toschek, 1969).The type b1 occurs in mudstone (facies b1), while the type b2 is characteristic of wackestonepackstone (facies b2).These two depositional textures are vertically arranged in centimetre scale cycles (1-5 cm) in metre scale beds and occur interstratified with microbialitic facies.The fenestral mudstone-packstones are partially dolomitized, forming coarsening upwards cycles from fine-grained to grain-supported fabrics.Larger fenestrae occur at the base, however, the connectivity between fenestrae increases towards the top together with the degree of dolomitization.

Heterolithic stylonodular facies (pelletoid grainstone; archaeocyathan wackestone and dolosparitic nodules)
This lithofacies assemblage occurs at the top of the lower member of the Láncara Formation and displays massive to stylonodular structure with irregular anastomosing spongy fabric without visible filaments (Flügel, 2004).The oncoid nuclei are absent or partially replaced by dolospar mosaic with high content in framboidal pyrite.Oncoids have spheroidal to ellipsoidal shapes and their surfaces are partially eroded.Spongiostromate oncoids represent 25-45% of rock volume, whereas bioclasts, mainly brachiopods, echinoderms, trilobites and small shelly fossils (SSF), are around 10%.The content of micritic intraclasts fluctuates between 5-10% of the rock volume.
Pelletoid grainstone (c1) displays massive fabric at the base and are mainly composed of very well sorted, round to elongated, recrystallized micritic grains or pelletoids (Flügel, 2004, 112 pp) with vague residual internal structures and diffuse margins, and sizes between 200 µm and  greater proportion of bioclasts shows an irregular distribution of orientations that is linked to the presence of intraclasts, cortoids and reworked spongiostromate oncoids.
The vertical arrangement of this lithofacies assemblage starts with massive pelletoid grainstone beds which grade into interbedded stylonodular levels where the nod-ules are composed of pelletoid grainstone, achaeocyath wackestone and dolospar.Detrital quartz grains (sand to silt size) occur as accessory to minor component dispersed within nodules and also concentrated in the stylolaminated intervals

Microbialites
Microbialite is taken here sensu Burne and Moore (1987, p. 241-242) as organosedimentary deposits that have accreted as a result of benthic microbial community trapping and binding detrital sediment and/or forming the locus of mineral precipitation.The microbialites in the upper part of the lower member of the Láncara For-  mation show three different mesostructures (scales of observation following Shapiro, 2000): i) cryptic or massive microbialite; ii) digitate thrombolite (sensu Aitken, 1967); and iii) archaeocyath-rich thrombolitic microbialite and spongiostromate-oncoid peloidal microbialite.
Digitate thrombolite ( d2) is composed of minicolumnar mesoclots 1-3 mm wide and up to 20 mm high (Fig. 5D, facies d2 in Fig. 3 and Fig. 6).The microstructure of mesoclots corresponds to massive to crudely laminated microspar.The intercolumnar space is filled by massive dense to peloidal micrite intervals covered by finely laminated intervals that laterally link the minicolumnnar mesoclots.These laminated intervals are composed of alternating microsparitic and dense micritic laminae.In some cases, the microstructure of mesoclots is well preserved, showing their peloidal character and finely laminated dense micrite.The digitate thrombolites occur associated with spongiostromate-oncoid packstone (a2) and commonly with cryptic or massive microbialite (d1).
Archaeocyath-rich thrombolitic microbialite (d3) comprises small lenticular patches (up to 15 cm high and around 20 cm wide) formed by densely packed dark mesoclots (up to 40% of rock volume) surrounding small branched colonies of archaeocyaths, which constitute 25% of the rock volume (Fig. 5E, facies d3 in Figs. 3 and  6).The main genus is Archaeocyathus.The microstructure of mesoclots is partially recrystallized, but still displays branching, shrub-like forms of Epiphyton.Dense patches of mesoclots are the dominant fabric and the occurrence of hyoliths is sporadic.Cavities do not exceed 10% and show stromatactoid shapes with flat bases and irregular roofs.They are about 5 mm wide and are filled by internal sediment and prismatic and equant calcite cements, now partially recrystallized (Fig. 5F).Clusters of mesoclots also occur pendent from cavity roofs and encrusting the outer walls of archaeocyaths.The encrustations around them are asymmetric, showing a preferential growth direction, indicating current influence during accretion.Intermesoclot spaces are filled by peloidal mic-rite (10-20%) and recrystallized, partially dolomitized micrite with quartz silt.There are pockets of bioclast packstone with eocrinoid arm plates (up to 5%), plus brachiopod and trilobite fragments (Fig. 5G).

Bioclast-intraclast packstone-grainstone
Bioclast-intraclast packstone-grainstone (e) occurs surrounding the patches of archaeocyath-rich thrombolitic microbialite (Figs. 3 and 6).It is characterized by a poorly sorted fabric with high skeletal content, up to 25% of rock volume with remains of brachiopods, echinoderms, trilobites and archaeocyaths.Intraclasts are conspicuous and their internal fabric shows clotted textures resembling those observed in mesoclots from archaeocyath-rich thrombolite.Pelletoids can attain up to 10-15% of rock volume and spongiostromate oncoids are accessory components.

Environmental setting of archaeocyath-rich thrombolitic microbialites
The lower member of the Láncara Formation shows sedimentary and paleontological features linked to tidal plain environments (Zamarreño, 1972(Zamarreño, , 1975;;Aramburu et al., 1992) developed in a homoclinal ramp (Aramburu, 1989).The upper part of the lower member is characterized by the occurrence of non-skeletal grain-rich facies, fenestral mudstone-packstone and microbialites (Fig. 6), whereas skeletal-rich facies are minor deposits.The microbial activity was significant and widespread, forming several types of structures (massive and microlaminated peloidal fabric, stromatolites, thrombolites, calcimicrobial remains), and was also linked to the formation of such non-skeletal grains as spongiostromate oncoids and microbial peloids (Flügel, 2004).Spongiostromate oncoids occur mostly in lacustrine and transitional marine environments and they are commonly associated with stromatolites in settings with a relatively fast rate of deposition (Peryt, 1981).Spongiostromate oncoids exhibit a great variety of microstructures but cyanobacterial remains are unrecognizable because of their rapid transformation (Krumbein and Cohen, 1977).Grain-dominated lithofacies are local in the upper part of the lower member (Fig. 6) and correspond to: i) poorly sorted fabrics generated by high-energy sedimentation as intraclast-oncoid shoals (a1-a2), ii) graded storm deposits (a3), and iii) very well sorted, pelletoid-rich, lenticular (centimetre scale), intertidal bars (c1).On the contrary, the typical and most extended facies assemblage is formed by massive to fenestral mudstone-packstone (b1 and b2) and microbialites (d).Fenestral fabrics are characteristic structures in peritidal environments and they have been related to degassing of decaying organic matter, gas bubbles, burrowing, soft-deformation, wetting and drying of carbonate mud in supratidal environments (Shinn, 1968), and drying of cyanobacterial mats (Davies, 1970) as they commonly occur in association with microbial mats and microbialites.
Diagnosis amended.Species of genus Archaeocyathus usually with a modular habit, forming branching colonies by budding.Central cavity narrow, sometimes non-existent and occupied by vesicular tissue and thickened interval elements.The presence of vesicular tissue determines the thickening of the taeniaes (stereoplasma).
Description.Cups small, solitary or modular with a variable number of individuals.In the youngest branches and basal areas of the cups the outer wall is imperforate, and in adults it has centripetal porosity.Intervalllum occupied by warped taeniae with large pores, sometimes thickened and linked by synapticulae and vesicular tissue, which can be very abundant.In many sections of small diameter, the interval presents alveolar structure and only in cups of larger diameter do taeniae have a clearly radial development.The inner wall has one pore per intertaenia, bearing a projecting short tube, although in many sections this wall is not well defined.The central cavity is small and in many cups does not exist, in these cases the space is occupied by intervallar elements and vesicular tissue.In some sections exocyathoids buttresses are developed on the outer wall.
Discussion.The abundant material is assigned to the species A. laqueus (Vologdin) on the basis of the similar structure of the cups in which the dimensions and ratios are consistent with the limits of variability of the species described in other similarly-aged locations in the Cantabrian Mountains.For further observations and a more complete discussion see Perejón and Moreno-Eiris (2003).
Diagnosis.Cups with centripetal outer wall; inner wall with one row of pores per intersept, bearing straight, upwardly projecting pore tubes; coarsely porous taeniae linked by synapticulae at the base, taeniae becoming progressively less porous, more planar and without synapticulae (Debrenne et al., 2002).
Discussion.The studied specimen is assigned to the genus Pycnoidocyathus based on the structure of the walls and intervallum.Due to its size, structural characteristics and coefficients, the specimen is included in the species P. erbiensis (Zhuravleva, 1955, Fig. 4), although the diameter of the present cup is smaller and the outer wall is corrugated, though not the inner wall.We also assign to this species the material from Tuva, described by Zhuravleva et al. (1967)

Biostratigraphic and paleobiogeographic correlation with other Toyonian localities
Archaeocyath-bearing microbialites had a wide distribution through the early Cambrian with a maximum development during the Atdabanian and Botomian.In the early Toyonian, all but a few species of archaeocyaths vanished, reducing the diversity dramatically (Perejón and Moreno-Eiris, 2006b).Such low-diversity assemblages are also recorded in the Spanish Toyonian archae-ocyathan buildups from the Cantabrian Mountains, where the archaeocyathan assemblage comprises four genera: Archaeocyathus, Pycnoidocyathus, Okulitchicyathus and Polythalamia.This assemblage characterizes the Spanish Zone X of Bilbilian age (Spanish stage), equivalent to the Toyonian age (Toyonian 1-2, Russian stage) according to Perejón and Moreno-Eiris (2006a), which corresponds to the Stage 4 within the Cambrian Series 2 (ICS, 2010).However, the first occurrence of Okulitchicyathus is in Zone I (early Ovetian age, Spanish stage).The Toyonian Iberian species are Archaeocyathus laqueus, Pycnoidocyathus erbiensis, Polythalamia sp., and Okulitchicyathus valdorensis (Debrenne and Zamarreño, 1970;Perejón and Moreno-Eiris, 2003).
During the Toyonian, the global maximum generic diversity was recorded in western Newfoundland (six genera), whereas the maximum diversity within Gondwana corresponds to the Cantabrian Mountains record (four genera).Only the ubiquitous genera Archaeocyathus and Pycnoidocyathus show a broad distribution in Laurentia, Siberian Platform and Gondwana in this age.

Comparison with Toyonian Archaeocyath-rich facies from Gondwana
As mentioned above, the archaeocyath-rich facies in the Cantabrian Mountains are low-diversity, centimeter-scale thrombolitic microbialite generated in a peritidal environment (Salce) and moderate-diversity calcimicrobialarchaeocyathan mounds (meter-scale) in a back-shoal environment (Esla nappe).The calcimicrobes were the main framebuilders of Toyonian bioconstructions, where Archaeocyathus could play a significant role forming branching modular framework.However, archaeocyaths were not only framebuilders, as they also colonized muddy environments, where they were subject to encrustation by calcimicrobes without producing true bioconstructions.In the Cantabrian Mountains, Pycnoidocyathus erbiensis occurs as solitary cups in the muddy deposits surrounding the small calcimicrobial mounds; however the isolated cups were not encrusted by calcimicrobes.On the other hand, in Sardinia, P. erbiensis appears in Renalcis boundstone and oncoid-bioclast grainstone, and they grew in a humid tropical Bahamian-type platform, though not forming bioconstructions or meadows according to Debrenne and Gandin (1985).
The archaeocyaths from the Chinese Tianheban Formation appear as small branching colonies and solitary cups.The colonies of stick-shaped cups surrounded by Epiphyton, Renalcis, Girvanella and Praulopora form small calcimicrobial mounds, whereas solitary cups occur in fine-grained sediments around the small mounds.The small mounds were developed in low-energy conditions, in a shallow water continental shelf in a warm climate (Debrenne et al., 1991).Gandin and Luchinina (1993) described the observed archaeocyath-calcimicrobe relationships in the Tianheban Formation.They detailed how the solitary archeocyath cups that occurred in wackestone facies are encrusted by Epiphyton, Renalcis and Girvanella (ERG assemblage) in the Huangshangong section, whereas the ERG calcimicrobial bioconstruc- The diverse occurrence of archaeocyath-rich bioconstructions in different sub-environments from the Cambrian record in the Cantabrian Mountains is adding new information to future paleogeographic reconstructions.In Salce, the patches grew in a peritidal environment in very shallow subtidal conditions and surrounded by cryptic massive microbialites and small lenticular pelletoid-rich intertidal bars, whereas at the Esla nappe, the archaeocyath-bearing microbialites formed larger patch reefs and grew between ooid shoal complexes.In both localities the archaeocyath-bearing microbialites were dominated by branching colonies of Archaeocyathus laqueus (Vo-logdIn, 1932), although archaeocyathan diversity was higher in the adjacent Esla nappe.On the other hand, Salce record resembles in part the lithofacies assemblage described from other localities (unit C in Fig. 2, and Table 1) but without either ooid and bioclastic shoal complex development or channelized siliciclastic deposits.
This new find increases the number of known archaeocyath localities in the upper member of the Láncara Formation in the Cantabrian Zone and allows us to assign an age of early Bilbilian (Spanish archaeocyathan Zone X), Stage 4, Series 2, Cambrian.
When the archaeocyaths decreased dramatically during the Toyonian, the maximum diversity was recorded in Laurentia.In Gondwana, the Cantabrian Mountains and Sardinia record the greatest numbers of genera (four and three respectively), and both areas have in common the occurrence of Archaeocyathus laqueus and Pycnoidocyathus erbiensis.In Gondwana, the archaeocyaths occurred as solitary cups and secondary framebuilders in low-diversity calcimicrobial-archaeocyathan bioconstructions (from centimeter-scale thrombolitic patches to large bioherms).tions with colonial archaeocyaths occurred in the Huangling section.Recently, Gandin & Debrenne (2010) classified the small mounds from the Tianheban Formation as Type 2: "calcimicrobial thrombolitic framestone composed mainly of dominant Renalcis meadows associated with low diversity clusters of small regular or modular archaeocyaths".They described the Type 2 mounds associated with high-energy ooid and skeletal/ooid shoal complexes, where they formed as "isolated patch reefs or laterally continuos biostromal bodies in rather restricted back-shoal settings".
The Toyonian archaeocyath-bearing bioconstructions in the Wirrealpa Limestone in Australia are cyanobacterial-archaeocyathan-radiocyathan bioherms and cyanobacterial-archaeocyathan bioherms.These can attain up 3 m thick and 36 m in length and, in both cases, the primary framework corresponds to Epiphyton thrombolitic stromatolite framestone (Kruse, 1991).These bioherms were developed in subtidal, open marine but calm and shallow waters (Kruse, op. cit.).
Summarizing, the development of the Toyonian archaeocyath-rich facies in Gondwana was limited to small and low-diversity calcimicrobial-archaeocyathan mounds or solitary archaeocyaths in muddy sediments, with the exception of the Australian case, where meter-sized calcimicrobial-archaeocyathan bioherms were well developed.Regarding the palaeoenvironmental conditions, the archaeocyath bioconstructions colonized from peritidal environments to shallow subtidal open marine environments as well as in protected back-shoal settings.

Conclusions
The upper part of the lower member of the Láncara Formation shows a varied assemblage of microbial and grain-dominated facies that were deposited in an inner ramp during early Cambrian times.The water-sediment interface was prolifically colonized by microbial benthic communities that built up a variety of micro-and mesostructures such as microbial peloids, calcimicrobes, spongiostromata oncoids and a diversity of microbialites (massive and laminated peloidal fabrics, stromatolites and thrombolites).

Table 1
.-Lithofacies assemblages in the lower member of the Láncara Formation according to several authors.Tabla 1.-Asociaciones de facies del miembro inferior de la Formación Láncara según diferentes autores.
as P. cf. erbiensis, although the central cavity is filled with secondary skeletal elements.