04-28-2021

Comment to the journal-contribution:
Capezzuoli, E. et al.: Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines), Geosci., 2021, 11(2): 84, http://doi.org/10.3390/geosciences11020084

This contribution concerns the geology of the Monticiano-Roccastrada Range, a still underexplored Tertiary metamorphic core complex (MCC), compared to the Pisan Mounts and Apuan Alps. Both the latter are interpreted as their geological continuations - similar in structure and lithology - to the NNW (Mid Tuscan Ridge).
One general point of interest and importance must be declared in advance: a scientific model can not be better than the quality of the results/findings on which it stands on or from which it is derived of. This means in the case here: basic requirement for a reliable, high-quality model of the stratigraphic and tectonic evolution of the Monticiano-Roccastrada MCC is thorough geological survey, i. e. mapping work, which must be accomplished as carefully and accurately as possible. Because the complexity-grade of the geomorphologic pattern of the area is rather high, difficult geological situations are expectable and the information found needs sensitive and careful interpretation. According to the physical accessibility of the mountainous terrain - few roads and paths; lots of macchia - , the realisation of this compellingly high demand means strenuous work. According to own experience, time consuming and exhausting step by step mapping work is a basic requirement; and the detectability of geological boundaries is hindered and is interpretation complicated, because nearly all of them are covered beneath late Quaternary detritus.

The reasons for discussing this paper: not comparable mapping results from identic regions at Rio Risanguigno (Figs. 1, 2) near Monticiano and at Ferriera di Ruota (Figs. 3, 4) in the western part of the Farma Valley of different revisers (responsible persons); as well as thereof derived differring geological interpretations concerning tectonics, lithostratigraphy, palaeontology and palaeogeography.

Figure captions

The structural division of the Monticiano Roccastrada and Monte Leoni MCCs into subzones proposed by Capezzuoli et al. (2021: 05: Fig. 4) is not accepted, because the drawn in line does not separate coeval formations deposited in different palaeogeographical terrains. See also http://www.umweltgeol-he.de/discussion2.htm and Fig. 2 in Zeitschrift der Deutschen Geologischen Gesellschaft 148/3-4: 523-546. The Risanguigno Fm. is indicated in Fig. 4 of Capezzuoli et al. (2021) exclusively in the western part of the Monticiano Roccastrada MCC. However, this Formation is also present in its eastern area according to Pasini & Vai (1997) and Engelbrecht (1997).

Page 1: The statements "...the scarcity of fossils remains ....." and "... in absence of fossil records..." contradict each other.

Page 2: It is stated in the chapter about "Geological outline of the Palaeozoic Units of Tuscany" of Capezzuoli et al. (2021: 04) that the Late Tertiary exhumation of the formations has occurred via extensional detachments, which resulted in "megaboudins" and "lateral segmentation of previously stacked tectonic units".
Tectonic segmentation via normal faults, wrench faults, shear fractures - inferred also from the presence of joints and slickensides - has been observed; see Figs. 2, 4: Both areas resemble megabreccias, because they are composed of tectonic blocks, made up of folded and/or thrusted sequences consisting of Risanguigno Fm., Poggio al Carpino Fm., Civitella marittima Fm. and Monte Quoio Fm.. Inferred b-axes persist on max. 350m. Isoclinic folds were rarely observed; when present, they even occur in competent lithologies like thick layered quartzconglomerates; however, refolding of isoclinic folds in competent lithologies was not observed. Large-scale tectonic overturning could not be ascertained. Some of the strike directions of the faults bounding the segments correlate with main directions of the geographic morphology and the drainage system.
This tectonic style contrasts with that one outlined in Figs. 1, 3: it is devoid of fracture bounded tectonic blocks and consists essentially of F1 folds - inferred to persist up to 500m in strike direction - , occasionally refolded by F2 events; see also profiles in Capezzuoli et al. (2021: Fig. 5). Extensive tectonic overturning - > 500m thickness - delineated in the profiles is not sustained by data (e. g. sedimentary polarity of strata) in the geological maps. The presented tectonic structure in Figs. 1 and 3 is in logical conflict with the statements in the "Geological outline..." cited above and hardly comparable with the contents of Figs. 2 and 4. It is doubted whether Figs. 1, 3 prove the assertion that "A detailed field survey was carried out in key areas where the Risanguigno Fm is exposed." (page 9). Fig. 5b in Capezzuoli et al. (2021) does not found on the map in Engelbrecht (2008: Fig. 2), as stated in the figure caption. No bearings of fractures/faults are present in the maps of Fig. 1, 3, which can be correlated with the main directions of the geographic morphology and the drainage system. Moreover: No reasons are given, why exactly the maps of Figs. 1, 3 are devoid of faults, although mentioned as common features in the "Geological outline...". Notabene: Thesis Engelbrecht (1997a) was sent in 2001 to the head management of the Dipartimento di Scienze della Terra, Università di Siena.

Page 4: The elaborate forms of arrows marking geographic locations in Fig. 3 in Capezzuoli et al. (2021) are not necessary.

Page 6: "...the base of the Risanguigno Fm. is never exposed...". The exclusiveness of this statement is risky, because it takes for granted that the working group has thoroughly and completely investigated the whole terrain (ca. 280 km²), that it has never errored, and because it may occur that new outcrops manifest the contrary. The possibility of new data from drillings in future was not taken into consideration. The text continues: "... although [20] (Engelbrecht 2008: 292) postulated the presence of a basal stratigraphic unconformity separating the Risanguigno Fm from the underlying Variscan deformed units." This statement was given in the chapter "Interpretation of depositional setting". It is logic that this postulation aimed not at the actual situation, but.at the depositional state prior to the alpine orogenic cycle, as e. g. outlined in Cassinis et al. (2018: 483, 491). Concerning the actual state, Engelbrecht (2008: 283) wrote that "...the stratigraphic base of the Risanguigno Fm. is not exposed."

Page 7: "...a chert subsequence, up to 4.5 m thick and intercalated with fine grained clastics, was also recognized in close outcrops by Pasini & Vai (1997), and later correlated with the small chert sequence present also in the Risanguigno type locality (Engelbrecht 2008). Pasini & Vai (1997: 188) did not estimate the thickness of the chert subsequence present in the eastern part of the Farma Valley. Engelbrecht (2008) did not correlate this chert subsequence with that one of the type locality, because this seemed impossible due to intense tectonic deformation at P 170 in the eastern part. Instead Engelbrecht (1997b, 2008) attempted to correlate the two chert- and lydite-key-horizonts pesent at Ferriera di Ruota and at the type locality via lithological logs.
The text continues in the same paragraph with a debate about the presence of anhydrite in silicified limestones of the Risanguigno Fm.; immediately followed by the statement that post-tectonic Chd blasts were reported in siliciclastic subsequences. This kind of mixing of unrelated geological themes (depositional milieu of Palaeozoic carbonates - Tertiary metamorphic blastesis in siliciclastics) is confusing to the reader.

Page 7: The authors state that the Risanguigno Fm. has preliminarily been attributed to the Tournaian-Visean based on radiolaria found by Pasini & Vai (1997) in the chert subsequence. But Pasini & Vai (1997: 188) did not mention radiolarian species; they just refer to "cherts: lyditic radiolarites" of probably Dinantian age. They clearly distinguish this chert subsequence from the Risanguigno Fm., which they definitely attributed to the Devonian. Puxeddu et al. (1979: 241) also mention radiolaria, but do not refer to radiolarian species; they too attribute the Risanguigno Fm. to the Devonian.

Page 7: The statements "Rocks are strongly deformed, making the stratigraphic reconstruction difficult." and "...the complete stratigraphic reconstruction is prevented by an intense folding..." on page 9 contradict each other. Notabene: The Risanguigno Fm. at Ferriera di Ruota is tectonically less deformed than the sequence at the type locality, so that Engelbrecht (1997a: 28-33) attempted to set up a lithological log.

In table 1 on page 8 the spelling of "lidyte" in the lines of the description of the samples RIS 5, 6 is incorrect.

Page 9: it is stated that "Along the Farma River ... close to Ferriera ... the 4.5 m thick chert sequence evidenced by Pasini & Vai (1997) constitutes the main lithological variation...". This is confusing, because Pasini & Vai (1997) refer to the outcrops at P 170 in the eastern part of the Farma Valley. The thicknesses of both chert-lydite subsequences were estimated by Engelbrecht (1997a, b) in the western part at Ferriera di Ruota and at the type locality. The following statement that "...the chert beds are positioned at the top of the succession, immediately below the Poggio al Carpino Fm." is incorrect: see profile in Engelbrecht 1997b: 536): the amount of chert layers successively diminishes towards the top of the formation and siliciclastics begin to prevail (Engelbrecht 1997a: 28).

Pages 10-11 and 15-16: Despite the fact that the palynofloral assemblage was "... strongly degraded..." - because of subduction-related (HP-LT) metamorphic peak conditions of 350-400°C and 10³ MPa - and "characterized by pseudosculpture induced by deposition of pyrite crystals", species attributions of many individuals were given in detail; even type species. However, systematics, specifica and characteristics of the microfloral specimens are lacking. Species attributions of some specimens in Fig. 7 are doubted: The terms Spelaeotriletes balteatus (Playford) Higgs 1975 and S. pretiosus (Playford) Neves and Belt 1970 were considered by Brittain & Higgs (2007: 113-114) as "Combination invalid" and the specimens of Fig. 7.9 and 7.12 on page 11 differ significantly from the referring specimens in Fig. 3 on page 112 in Brittain & Higgs (2007). The latter also accounts for the specimen termed Perotrilites magnus Hughes and Playford 1961, when compared with Hughes, N. F. & Playford, G. (1961): fig. 5, 6 on plate 2 in Micropaleontology, 7/1: 27-44: Palynological reconnaissance of the Lower Carboniferous of Spitzbergen. The same accounts to Claytonispora distincta Playford and Melo 2012 in Fig. 7.1, when compared with G. Playford, G. & Melo, J.H.G.: Miospore palynology and biostratigraphy of Mississippian strata, Amazonas Basin, Brazil (Part One), American Association of Stratigraphic Palynologists, Contributions Series No. 47, p. 74-75: plate 6, fig. 17; plate 7, fig. 1-2, p. 76-77, Dallas, USA.

Page 13-14: "Mica domains are mainly composed of fine grained white mica and biotite...": Redundant content.

Page 16: It is stated that the dolostones sampled for conodonts were barren and that "...the younger aged rich microflora, suggests that the previously reported conodonts were reasonably reworked fossils." Only five samples were investigated in order to test the reproducibility of the results in Bagnoli & Tongiorgi (1979); the amount of material processed therefore and accompanying finds of other microfossils were not mentioned. No argument was given and no sedimentological investigation described, which proves the postulated redeposition of the microfauna described by Bagnoli & Tongiorgi (1979). Distinctive sediment-textural marks of redeposition would have even then been recognised by and evaluated by Bagnoli & Tongiorgi (1979). Reworked palynofloral assemblages of Devonian age and non-redeposited - in situ - microfossils of Early Carboniferous age were not described.
"... the presence of ornamented spores and tetrads suggest a proximal depositional environment since the spores were selected according to their hydrodynamic equivalence": regarding the poorly preserved state of the specimens caused by synsedimentary pyritisation and medium grade (HP-LT) metamorphism, it is doubted whether it is possible to estimate reliably the proximality of the depositional area of the spores in relation to the degrees of integrity-defects caused by aquatic transport.
"... we interpreted the Risanguigno Fm as being deposited during the Middle Mississippian in a shallow-marine epicontinental setting, characterized by starved, anoxic condition in its lower portion and progressively evolving to carbonate-radiolarite platform." This palaeogeographic interpretation is not justified, because sustaining sedimentological and geochemical data, as given in detail in Randon & Caridroit (2008), are not present. Siliceous sediments as major components of epicontinental platform deposits are unknown. The outcropping thickness of the Risanguigno Fm. was estimated at min. 40m (Engelbrecht 2008); but this value is by far not enough to characterise it as epicontinental platform deposits (see below).

Page 17: The age attributions for subunits 2 and 3 in Fig. 11 are doubted; see Engelbrecht (2008) and discussions 1-3.
"... presence of middle Carboniferous (late Visean - early Namurian...) clasts, bioclasts and olistoliths embedded within the middle-late Permian Farma and Carpineta formations (70)": This has never been stated by Engelbrecht (2000): mentioned formations were attributed to the Carboniferous and the statement of olistolithic origin of the San Antonio- and Botro al Confine Lst. deposits was rejected.

Page 18: "... fairly narrow continental or epicontinental domains. These depositional features could have favoured the low energy, anoxic environments." This statement remains unclear, because no reasons for elevated influx of nutrients having caused eutrophication are mentioned.
"... development to carbonate platform (Calcare di S. Antonio Fm)": The thickness of that formation is estimated to a few tens of meters (< 35m) only and cannot be termed as carbonate platform like the Early Cambrian Gonnesa Limestone, the Middle Permian Capitan Reef, the Late Triassic Dolomia Principale or the Early-Middle Cretaceous Urgonian Lst.. Engelbrecht (2008: 297) interpreted the S. Antonio Lst. as small scale biostromal accumulation, which developed in the outer shelf area.
"This setting, localised in the Variscan foreland, evolved to open marine during the Pennsylvanian-Permian without any involvement in the Variscan Orogenesis". This statement is illogic, because the sedimentary piles of depositional systems developed in forelands are transformed to structural components like tectonic fragments or wedges in fold-thrust-belts of each orogen. The development of basin geometry and dynamics (depth, thickness of pile, detritus flux, compressional, transpressional or dilatative regime) is controlled by orogenesis.
"... Tuscan Palaeozoic-Triassic sedimentary succession (Phyllite-Quartzitic Group ...), classically considered as "post Variscan" and now comprising the Middle Mississippian Risanguigno Fm, is no more to be related to the Variscan Orogenesis." If this statement would be correct, the depositional area of the Risanguigno Fm would have established in that parts of the Gondwana continent not affected by the Variscan orogenesis; e. g. in the Devonian-Carboniferous parts of the palaeozoic Amazonas Basin developed in SW-Gondwana.

The expressions "isolated ... formation", "scattered fossils", "determnination of Variscan formations", "mostly exposed", "close outcrops", "supplementary outcrop", "never described before", "bigger crystals", "symmetric or asymmetric microfolds", "scenario with fallouts", "reasonably reworked", "up to lower Permian", "... older cropping out deposits..." are not present in the list of termini technici.

According to the explanations and comments above, the proposal is made to better withdraw that publication to prevent further confusion and trivialisation in academia. I am worried about the academic niveau of that paper.

Hubert Engelbrecht

References cited:
- Bagnoli, G. & Tongiorgi, M. (1979): New fossiliferous Silurina (Mt. Corchia) and Devonian (Monticiano) layers in the Tuscan Paleozoic.- Mem. Soc. Geol. It. 20: 299-311, Roma.
- Cassinis, G. et al. (2018): Post-Variscan Verrucano-like deposits in Italy, and the onset of the alpine tectono-sedimentary cycle, Earth-Sci. Rev. 185: 476–497, http://doi.10.1016/j.earscirev.2018.06.021
Engelbrecht, H. (1997a): Zur Geologie der Zone von Monitciano-Roccastrada, Südtoskana, Italien); thesis, 236 pages, Utz Verlag, Munich.
- (1997b): From Upper Palaeozoic extensional basin fill to late Alpine low grade metamorphic core complex: preliminary note on the sedimentary and tectonic development on the Monticiano-Roccastrada-Zone (MRZ; Southern Tuscany, Italy).- Z. dt. geol. Ges.; 148/3-4, pp. 523-546, Stuttgart.
- (2000): Tempestite deposition in the eastern Rheic Strait: Evidence from the Upper Palaeozoic in Southern Tuscany.- FACIES 43:103-122; Erlangen.
- (2005): Unpublished geological map unraveling the geological context between the type localities of the Poggio al Carpino- and the Risanguigno Formations (Farma- and Risanguigno Valleys, Monticiano area, Southern Tuscany, Italy). Archive H. Engelbrecht.
- (2008): Carboniferous continental margin deposits in Southern Tuscany, Italy: results from geological mapping of the geotopes Farma Valley and San Antonio Mine area.- Geological Journal, special issue, vol. 43/2-3: 279-305, Chichester, UK.
- Pasini, M. & Vai, G.-B. (1997): Review and updating of the Moscovian to Artinskian marine rocks in peninsula Italy. In: Tethys: Stratigraphic Correlations; Crasquin-Soleau, S., De Wever, P. (Eds.), Geodiversitas: Paris, France, 1997; Volume 19/2: 187-191.
- Puxeddu, M.; Raggi, G.; Tongiorgi, M. (1979): Descrizione di alcuni sondaggi e osservazioni geologiche nel Paleozoico della zona di Monticiano (Siena). Mem. Soc. Geol. Ital. 20: 233-242, Roma.
- Randon, C. & Caridroit, M. (2008): Age and origin of Mississippian lydites: Examples from the Pyrénées, southern France. Geological Journal, http://dx.doi.org/10.1002/gj.1101, Chichester, UK.