10-2021

My exploration history of the geology of Valle Farma (1) and the state of the art (2)

(Monticiano-Roccastrada metamorphic core complex, South Tuscany, Italy)

(1)
During the winter-term 1979/80 I joined the Munich Study Team Southern Tuscany. Due to the scope of the EU funded project (Dehm et al. 1983), I started in September 1980 to map a 12 km² measuring terrain at the village of Scalvaia in the western part of the Monticiano-Roccastrada area. Together with other students, I had - beside mapping - the task to find early Palaeozoic units (lithological and chronological equivalents of blackschists and porphyroids present in the Apuan Alps and in Sardinia), which were - according to the theory of time and strata bound ore deposits - seen by our supervisors as source of the nearby epizonal-hydrothermal Antimony-deposits (Greppoli, le Cetine di Cotorniano) at the eastern and northern margins of the Monticiano-Roccastrada area: thermal mobilisation of the primary synsedimentary stratiform Sb, caused by late Pliocene - Quaternary rise of the geotherms, upward migration of the Sb contained in hot fluids, chemical precipitation of the mineral content and finally deposition via replacement-processes close to the tectonic boundary separating Calcare cavernoso from Ligurian units, effected by high permeability difference between these both latter formations (Mueller & Klemm 1989). But years later it was argued that this type of Sb-deposit is very probably a recent analogon of the Nevadan Carlin-type gold deposit and that the sources of the Antimony have very probably been fluids contained in S-type plutons (Sillitoe & Broghi 2021), which ascended from magma formed in the lower crust.
During the introductory excursion in March 1980, we were instructed that the Farma Formation was deposited in a foreland flysch basin, that the strata were folded and locally overturned by compressive hercynian tectonic events and that these folded turbidites were covered via tectonic unconformity with less deformed Late Hercynian molasse and the Verrucano Group.
I started hopefully, because we were told that it would be no problem to finish the task within six weeks; in addition I remembered that during the elementary course „geological mapping for beginners“, an academic instructor said that it is no problem to map 1 km² on one single day. So I expected no problems to finish the 12 km² mapping area in the time estimated. But it came rather different:
Like all other students, I failed to find the Early Palaeozoic blackschists and porphyroids and I had problems with progress in mapping, because of the similarity of the lithologies, because of contradicting informations we got from supervisors and from what we read in papers published in geological journals: how to discriminate the „Verrucano“ from the „Preverrucano“ and how to subdivide the latter.
Neither during field-inspections nor in the geological department I got clearing answers to many of my questions; I felt overcharged. Fact has been: The supervisors never studied the terrain to develope a reliable manual/guidance for their students, how to discern the lithologies and how to map; instead they just relied on the information they read in literature; but they did not countercheck in the referring terrain, whether this information was consistent with geological reality and applicable for mapping. During the introductory excursion, we students were not guided to and trained at thoroughly selected outcrops, where e. g. stratigraphic boundaries of formations, folds or faults were well recognisable.
In the following, I made lots of measurements (predominantly s₀, but also joints) in my mapping area, but was - except the Risanguigno Fm. - unable to discriminate lithologically the formations, to fix the lithostratigraphic boundaries and to clarify the properties of the formations. I covered many kilometers, but was unable to understand stratigraphy and tectonics of my terrain. But at least I was the first among the students who found out empirically that the eastern part of the terrain is dominated by E-vergency (and probably of higher metamorphic grade), contrasting with the non-vergent tectonic style of the western part (probably less metamorphosed) of the area. Because of the presence of older formations in the eastern part, I temporarily thought that this might also be the case for the age of the tectonic deformation in this part. But I soon understood that this idea was wrong, that a tectonic unconformity equivalent to that between Palaeozoic and Permo-Triassic formations, is not present in my mapping area and that the tectonic deformations in both parts of my area occurred probably coevally in Middle-Late-Tertiary.
Numerous unsolved geological problems persisted and I finally understood that I had to find my own way. This had also to do with the fact that my supervisor was specialised in ore mineralogy, geology of mineral deposits, archaeology and economic geology, but not in stratigraphy, sedimentology, sedimentary basin formation, tectonics, metamorphism, palaeontology and palaeogeography.
I drew a columnar section of sedimentary textures well exposed at I Canaloni, but I found nobody to give advice how to interpret the textures, to derive the depositional mechanisms and to make conclusions about the depositional area.
Despite of this problematic development, I had a successful moment, when I found in June 1982 in the alluvial plain of Ferriera di Ruota (263m) fusulinids in reddish coloured carbonate pebbles, constituents of conglomerate boulders, which eroded from outcrops of the Monte Quoio Formation. I sampled them from the same site, from the same Formation and the same clast-lithotype as described in Cocozza et al. (1975). Palaeontologist Prof. Mario Pasini from the Dipartimento di Scienze della Terra in Siena sustained me in a very friendly and patient way in species determination and age attribution (Late Carboniferous to Early Permian); thus a paper was published 1988/89 in Rivista.
In 1985 I was compelled to submit my diploma thesis about a theme, which I had by far not understood. Astonishingly, it was evaluated by my supervisor as „nearly very good“. But when I compared it in the faculty-library with other diploma theses of students supervised by other university staff, I knew that this mark was only subjective and wrong and that I had to revise and amend my work to attempt to recover lost ground to be able to keep pace with other fellow-students. It was intolerable for me having still not solved correctly this academic task. Further motivations to continue and improve my work were: responsibility; that low quality mapping in terrain abroad - or leaving work/problems undone/unsolved - are no options; that I already had invested so much energy and time that it would be an unbearable loss having abandoned prior to its completion; that I urgently wanted to solve this seemingly immense geological problem.
So I decided to continue mapping and to do the thing properly. I already knew that a revision must not cover only a part of the Farma Valley, because I noticed that in its eastern part formations different from those in the western part are present; this meant that the entire valley from Ponte di Torniella (P 325) at the western margin to Ponte di Petriolo (P 161) at the eastern margin had to be mapped. My selected terrain measured 13 km EW and 2,3 km NS; in sum ca. 30 km².
My supervisor agreed in the beginning, because I took over the position of a pro bono assistant helping other diploma students of him to do their mapping work in other regions of the Monticiano-Roccastrada area; until 1988 27 mapping areas were distributed by him to students, until the whole area of Monticiano-Roccastrada (nearly 300 km²) was covered. Unfortunately my knowledge was still insufficient to present a reliable manual/guidance to the students. Therefore conflicts arouse between us students, because of too many different opinions/positions about the lithological criteria, how to discern the formations. It was finally my task to compile these contradicting mapping results.
Because some lithologies of the Verrucano Group and the Civitella m.ma Fm. are identic - e. g. the arenaceous violet schists - , insecurity in attribution persists until today.
A severe and unsolvable conflict arouse between my supervisor and me: he highly disagreed in my mapping revision of the Valle Farma, because he said that this had been already done by other students he also had supervised a few years before and because he has checked their maps during final examinations; nobody else was allowed to change anything of the results. But as insider I knew that these maps - including my one - urgently needed amendment and so I did that work between 1986-1989, parallel to sustaining my supervisor in assisting and attempting to guide his students. Many problems, expecially in the eastern part of the Valle Farma, remained again unsolved.
During my mapping campaign, some other positive events occurred:
- When I realised that 200m south of P 190 the Carpineta-, Civitella Marittima- and Monte Quoio Formations form the overturned limb of a Tertiary E-vergent synform and that this structural element persists over 300 m difference in altitude.
- The awareness that the shallow marine strata of the Poggio al Carpino Fm. in the western part of Valle Farma must be coeval with the deeper marine deposits of the Farma- and Carpineta Formations in the eastern part and that these sediments originated probably at a continental margin. Years later I realised that this idea has already been published by Roberto Redini (1958), but for many years colleagues did not integrate it in further research. Literally citation in R. Redini (1958: 611): „L'Antracolitico....presenta due facies, le quali.....correspondono a due differenti ambienti di sedimentazione: cioè la facies occidentale, di mar sottile, litoranea, e la facies orientale, di mar meno sottile, più discosto dalla linea di spiaggia.“
- The idea that seaquakes and/or tropical storms, both capable to trigger sediment gravity flows, might have generated the formations described above.
- The finding of a few outcrops displaying stratigraphic contacts between Carboniferous formations (Farma- and Carpineta Fm.) and the Civitella Marittima Fm. above, which indicate their erosive nature and which exclude hercynian folding in the older formations.
- The finding that the stratigraphic contacts between Risanguigno Fm./Poggio al Carpino Fm. and Risanguigno Fm./Farma Fm. are of erosive nature.
In 2000 - subsequent to consolidation - I resumed geological work in the Monticiano-Roccastrada area. Until ca. 2005, I mapped the area from Ferriera di Ruota northward to the type location of the Risanguigno Fm. in order to check the geological relations. Then I mapped an area to the south of the Farma Valley (between I Piloni and Podere Lanzo) and of the abandoned, small mining area near Podere San Antonio.
When I had finished lithological logs of the turbidites of the Farma Fm. at P 190, 183 and 170, I realised that it was necessary to review my old thesis map (1986-1989). This meant to clear as many failures, errors and omissions made in the past as possible and to avoid new ones.
Further positive events occurred: Finding of brachiopods and phytoclasts in the Carpineta Fm.; of conodonts in calciturbidites of the ultradistal Farma Fm.; of Neptunian dykes and olistostromes (block-in-matrix rocks: so called broken member) in distal parts of the Farma Fm.; of the stratigraphic contact between the Risanguigno Fm. and the Carpineta Fm..

(2)
State of the art:
Working area, geomorphology and mapping method:
The Farma Valley is central part of the Monticiano-Roccastrada region, situated in Southern Tuscany, Italy. The valley, displaying a general direction running straight W-E over 13 km in linear distance, forms the boundary between the provinces of Siena and Grosseto. The mountainous region is - with the exception of several fluvial terraces and few agricultural land - covered with dense mediterranean forest. Access to the terrain via unpaved roads and paths has become sometimes troublesome, since - due to industrialisation and rural exodus - local agrarian activity to maintain subsistence level has nearly been given up and the referring network of supply lines fell into decay. A few small mining sites yielding Sb, Fe, Cu, Pb, Zn and lignite were exploited until ca. 1940. Elban hematite was smeltered in Farma Valley until ca. 1950. Since then forges, mills, limekilns, farm houses, seccatoi, aqueducts and charcoal sites went to ruin. At present, local economy is restricted to mining kaolin, woodcutting, hunting, tourism and harvesting mushrooms, olives and edible chestnuts. At the eastern margin of the Farma Valley, the thermal spring Bagno di Petriolo (44°C) is exploited for balneotherapeutic purposes.
The development of the slope of Torrent Farma and of its fluvial terraces are disequilibrated: For instance, V-shaped valleys follow seemingly older U-shaped valleys downriver. This fact and the numerous deflections and displacements of the torrent's course from its overall W-E direction can be explained by subrecent differential, compartmentalised vertical tectonic throws or steeply inclined shear fractures. In many situations it is obvious that large parts of the course of Torrent Farma and of its tributaries are controlled by these tectonic faults, along which fluvial erosion was most effective because of the presence of fractured rock material, less resistive to erosion. Thus Torrent Farma incised its course diametrically to the N-S strike direction of the Monticiano-Roccastrada area, which consists predominantly of resistive rock types (quartzites, quartzconglomerates, etc.). Farma Valley originated probably antecedentally; i. e. the valley existed prior to the above-ground uplift of the geological units forming the Monticiano-Roccastrada area. Relict fluvial deposits cut off from the actual drainage system are present.
My geological mapping methods consist in a combination of classical and modern techniques. The observed complexity of the areal distruibution of geological formations necessitated an elevated density of observation points: outcrops, at which geological information was available (rock type, sedimentology, dip and strike of strata, b-axes, schistosities, joints filled with quartz mobilisates, fault planes with slickensides, stretching linears of minerals, etc.). But outcrops were often scarce because covered with recent detritus and soil; except these parts in creeks, where erosion dominated over accumulation of talus. At many outcrops it was rather difficult to decide, whether the strata were in original position or had already been tilted by gravitation. Often the only geological information found consisted in floatstones scattered on the ground. Dense forest restricted not only viability of terrain, but also range of sight and often hindered exact localisation of outcrops. This was problematic before GPS-technique: steps had to be counted along the lines between fixed points. But because GPS-signals may blur/fade close to steep flanks or below dense and humid canopies of leaves, step counting is sometimes additionally necessary to approximate localisation.
The types of geological boundaries - e. g. stratigraphic contact; fault plane - are directly observable only in very rare cases and only over short distances of maximum a few meters. Because most of the contacts are covered with detritus, the type of contact must be determined indirectly: by means of the measured planes of separation and their bearing. Mapping technique is well explained in Compton (1985).
Geological setting of the Monticiano-Roccastrada area: It is situated at the eastern border of the internal, metamorphosed zone of the Northern Apennines fold and thrust belt. Mentioned zone originated as consequence of
- Triassic to Early Cretaceous divergence - rifting, followed by spreading - between the microplates Corsosardinia (part of European continent) and Adria (part of Gondwana), followed by
- Middle Cretaceous to Paleocene convergence of the microplates and subduction of the Adria slab beneath Corsosardinia,
- Eocene collision of the microplates with formation of accretionary wedges at the continental margins of Corsica and Adria, followed by
- Oligocene subduction-related tectonic burial of Tuscan units to middle-lower crustal niveau at 350-400°C (burial metamorphism: blueschist - Glaucophane-schist - facies) and
- Miocene to Recent delamination and detachment of the lower part of the Adriatic lithosphere and slab, causing advection of hot asthenosphere, eastward migration of magmatism- and deformation-fronts, rise of the geothermal gradient, orogenic collapse of the stack of tectonic nappes, uplift, consecutive thinning of the crust and back arc basin formation (opening of the North Tyrrhen. sea).
Dilatation of the crust also generated the so called metamorphic core complexes (Ring 2014, Giuntoli & Viola 2021), which occur as discretely appearing, oval- and dome-shaped geomorphological elements, which delineate the Mid Tuscan Ridge. They mark extensional centres, where dilatational deformation is localised and metamorphosed rocks were exhumed from lower and mid crustal levels: unroofing of large parts of the hangingwall via low angle normal faults caused isostatic disequilibrium, when the hangingwall slid aside and subsided - forming the „Serie Toscana completa“ - and the tectonically uncovered and buoyant footwall began to rise into shallower crustal levels, forming finally the above-ground uplifted MCCs. Unroofing was enabled in that regions, where Late Triassic evaporites above the metamorphosed footwall effected the development of low angle normal faults; actually they form un- to anchimetamorphosed tectonic breccias (Calcare cavernoso, Tocchi Formation) immediately atop the footwall („Serie Toscana ridotta“). Unroofing was fostered in the MRMCC probably by a peculiarity characterising the stratigraphy of the footwall: the presence of a Carboniferous lineament separating coeval shelf- and basin-sediments deposited at an ancient continental margin: This Palaeozoic normal fault, having acted as zone of crustal weakness during later geological development, was reactivated during Tertiary compressive and dilatative tectonic events.
An important point probably is the occurrence of magmatic manifestations at the western and eastern border of the Farma Valley, where major faults intersect at high angles: The Tertiary-Quarternary Tuscan Magmatism was controlled by eastward propagating tear fractures; intrusions occurred at those points, where faults with different strike directions intersected (pers. comm. with Dr. Matteo Lupi, Univ. Geneva). Therefore the Farma Valley fault might be one of these tear fractures, which got exhumed and which connects the emptied magma reservoir of the extinct rhyolithes of Roccastrada-Torniella (2,3 Ma) with the magma chamber below the active geothermal area of Bagno di Petriolo.
More about geological overview and setting, history of geological exploration and some thoughts about lithostratigraphic subdivision, metamorphism, tectonics and palaeogeographic development of the Palaeozoic to late Triassic units of the MRMCC can be read in Engelbrecht et al. (1989) and Engelbrecht (1997, 2000, 2008, 2014, 2016, 2019). Important: please consider additionally the views of Italian authors about the geology of that area: They study this terrain since at least 100 years and their contribution is much more than mine.
Notabene:
Worth and benefit of geological mapping:
Geological research always starts with mapping of unknown terrain. The basics of all geological and geotechnical work is exact and objective registration of the spatial distribution of geological formations and structures as well as their representation in map and profile. Geoscientific research in the following and geotechnical as well as mining projects rely on the quality of this information.
Each geological survey is strenuous; work in terra incognita may stress the geologist to the limits of physis, intellect, psyche, patience and perseverance. But these efforts will be rewarded with a deeper knowledge of causal contexts between the morphological forms of the Earth's surface and the geological structures controlling them from beneath. This recognition is for each geologist a precious gift, but the long way towards terra cognita is unpaved.
Decoding of lithostratigraphic sequences, of metamorphic zonation, or of tectonic structures: it makes no difference: mapping can be compared with carefully raising a subterranean treasure or with cautiously opening up a natural stone archive: ever more new, aesthetic forms, patterns and structures - often cyclically organised in different hierarchies - is revealed to the human intellect. Romanticists might state: “Lithified symphonies get liberated from their eternal dungeons“.
Mapping new terrain means to fill a „blank map“ with correct geological information; this requires lots of courage, carefullness and pioneer work. This situation has been described by a famous alpinist by saying „always break new ground“. The stimulus towards endless experimenting is valid especially for the information society: only in this way it is possible to make new discoveries, inventions, to secure lead in science and to gain competence. It is clear that often immense effort and tenacity are necessary to follow the new path to its end, after having left the beaten trails of epigons.
Progress in mapping is controlled by geogene and anthropogene factors: 1. accessibility and development of the terrain; 2. elevation a.s.l., climate and vegetation; 3. complexity degree of geology; 4. quality of available topographic maps and of previous work; 5. objective of the study (general view or detailed work). Points 3 and 4 are the least of all accountable factors, because often no or barely reliable information is present for assessment in advance. Therefore it's impossible to meet any schedule.
Adalbert Stifter about the impact of landscape on human spirit: „It was a huge stimulus for mind - and intellect (added by HE) - , to perceive the unnamable, hidden in the objects seen by me; and the more I attempted to perceive the unspeakable, the more it became a feast for my mind's eye“.

References
Brogi, A. & Giorgetti, G. (2012): Tectono-metamorphic evolution of the siliciclastic units in the Middle Tuscan Range (inner Northern Apennines): Mg-carpholite bearing quartz veins related to syn-metamorphic syn-orogenic foliation.- Tectonophysics 526-529: 167-184, Elsevier, Amsterdam.
Capezzuoli, E. et al. (2021): Reconsidering the Variscan Basement of Southern Tuscany (Inner Northern Apennines), Geosci. 11(2): 84, http://doi.org/10.3390/geosciences11020084
Casini, G., Decandia, F. A. & Tavarnelli, E. (2007). Analysis of a mesoscopic duplex in SW Tuscany, Italy: implications for thrust system development during positive tectonic inversion. In: Ries, A. C., Butler, R. W. H. & Graham, R. H. (eds.). Deformation of the Continental Crust. The Legacy of Mike Coward. Geological Society, London, Special Publications, vol. 272: 437-446. https://doi.org/10.1144/GSL.SP.2007.272.01.22.
Cocozza, T., Lazzarotto, A., Pasini, M. (1975): Segnalazione di una fauna Triassica nel conglomerato di Monte Quoio (Verrucano del Torrente Farma - Toscana meridionale).- Riv. Ital. Paleont. 81/4: 425-436, Milano.
Cocozza, T. & Vai, G.-B. (1974): Flysch e Molassa ercinici del Torrente Farma (Toscana).- Boll. Soc. Geol. It. 93: 115-128, Roma.
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Compton, R. R. (1985): Geology of the field. Wiley, New York, USA.
Dehm, R. M., Klemm, D. D., Mueller, C., Wagner J., Weber-Diefenbach, K. (1983): Exploration for antimony deposits in southern Tuscany, Italy. Mineral Deposita 18: 423-434. https://doi.org/10.1007/BF00206490.
Engelbrecht, H., Klemm, D.-D. & Pasini, M. (1989): Preliminary notes on the tectonics and lithotypes of the "Verrucano s.l." in the Monticiano area (Southern Tuscany, Italy) and the finding of fusulinids within the M.te Quoio Fm.(Verrucano Group). Riv. Ital. Paleont. Strat. 94/3:361-382; Milano.
Engelbrecht, H. (1997): From Upper Palaeozoic extensional basin fill to late Alpine low grade metamorphic core complex: preliminary note on the sedimentary and tectonic development of the Monticiano-Roccastrada-Zone (MRZ; Southern Tuscany, Italy). Zeitschrift der Deutschen Geologischen Gesellschaft 148/3-4: 523-546; Stuttgart.
- (2000): Tempestite deposition in the eastern Rheic Strait: Evidence from the Upper Palaeozoic in Southern Tuscany. FACIES 43: 103-122; Erlangen.
- (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, UK.
- ( 2014): The Farma Basin (Tuscan Palaeozoic): its palaeotectonic setting and new stratigraphical data.- Conference volume of GeoFrankfurt2014 (topic: earth systems dynamics), theme "D02: Geodynamics of the Variscides". Schriftenreihe der Deutschen Gesellschaft für Geowissenschaften 85: 603, Hannover.
- (2016): Considerations on the age of the Verrucano Group of Southern Tuscany, Italy.- Abstract volume of GeoTirol2016, page 61, https://www.uibk.ac.at/geologie/geotirol2016-pangeo/images/abstractvolume.pdf.
- Engelbrecht, H. (2019): Revision of geological units of the Carboniferous Farma Basin at the Southern Convergence Front of the westmediterranean Hercynides.- Abstract volume of the 19^th International Congress on the Carboniferous and Permian, eds.: Hartenfels, S., Herbig, H.-G., Amler, M. R. W., Aretz M.; Kölner Forum für Geologie und Paläontologie, 23: 89-90, Universität zu Köln 2019.
Giuntoli, F. & Viola, G. (2021): Cyclic brittle-ductile oscillations recorded in exhumed high-pressure continental units: A record of deep episodic tremor and slow slip events in the Northern Apennines.- Geochemistry, Geophysics, Geosystems 22/9, https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GC009805.
Mueller, C. & Klemm. D.-D. (1989): Sb and Hg in lower paleozoic schists and blackschists - the basement of Sardinia and North Tuscany and their relationship to the epigenetic Plio-Pleistocene Sb-deposits in South Tuscany.- Abstracts of the 79^th annual meeting of the Geologische Vereinigung (mineral deposits), Leoben, Austria, E. F. Stumpfl (ed.), 15.-18.02.1989; Enke Verlag, Stuttgart, 1990.
Pasini, M. (1991): Residual evidences of Permian carbonate platform within the Apennine sequences (Italy). Boll. Soc. Geol. It. 110: 843-848, Roma.
Redini, R. (1958): Su varie questioni geologico-paleontologiche della Catena Metallifera Toscana. Sull' età Neotriassica della fauna del M. Rotondo, del M. Pisano e della fauna di Poggio Troncone, nelle Alpi Apuane.- Boll. Serv. Geol. d'It. 79/3-5: 593-744, Roma.
Ring, U. (2014): Metamorphic Core Complexes. Encyclopedia of Marine Geosciences, http://dx.doi.org/10.1007/978-94-007-6644-0_104-4.
Sillitoe, R. H. & Broghi, A. (2021): Geothermal systems in the Northern Apennines, Italy: Modern analogues of Carlin-style gold deposits. Economic Geology 116 (7): 1491-1501, https://doi.org/10.5382/econgeo.4883.

Hubert Engelbrecht

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