WALTER ALVAREZ Lamont-Doherty Geological Observatory of Columbia University, Palisades, New Yor\ 10964

Ancient Course of the River Near Rome: An Introduction to the Middle Pleistocene Volcanic Stratigraphy of Central

ABSTRACT with raised strand-line facies formed during several different high stands of sea level. The volcanic sequence exposed in the Treia Detailed chronologic information should per- Valley, 40 km north of Rome, promises to be mit an accurate determination of variations in useful in establishing the climate and chronol- erosion rates and thus a correlation with glacial ogy of the Mediterranean middle Pleistocene. activity in the high Apennines. Eventually it A change in the course of the Tiber River has may be possible to tie the volcanic chronology been a critical factor in the development of to climate indicators in the Tyrrhenian deep- this section. The Tiber makes a detour east- sea cores by means of tephra stratigraphy. ward from Civita Castellana around the vol- Thus the volcanoes of Central Italy could canic area of the Monti Sabatini before reach- prove suitable as a world type area for the ing Rome and the sea. Stratigraphic relations middle Pleistocene. exposed in the Treia Canyon show that early In this area, Blanc established the local in the Pleistocene the "Paleotiber" flowed due Italian zonation of the Pleistocene (Blanc and south from Civita Castellana to Rome, and others, 1953; Blanc, 1956). Recent work by was diverted by the first products of the Bonadonna and his colleagues has increased our Sabatini volcanoes. The former bed of the knowledge of the stratigraphy of this region "Paleotiber" can be identified, and the occur- and of the significance of the local Pleistocene rence of lake beds clarifies the way in which stages (Bonadonna, 1968; Bonadonna and damming took place. The present course of the Bigazzi, 1969). Tiber probably formed when the lake backed Because of the competition between con- into a tributary valley and overflowed into the structive and destructive processes during the headwaters of another tributary. growth of volcanoes, stratigraphic relations here are complex, with multiple channel fillings INTRODUCTION on all scales, damming of valleys, stream cap- The Pleistocene volcanoes of Central Italy ture, and the formation and filling of small have long attracted geologic attention because craters and large calderas. A full understanding of their abnormally high potash content. In of this complicated stratigraphy is essential for addition, these volcanoes promise important further progress in interpreting the climatic information on the climate and chronology of and chronological significance of the volcanoes. the middle Pleistocene of the Mediterranean Two 1:100,000 scale maps, that of Mattias and region. Widespread potassic ignimbrite sheets Ventriglia (1970) which covers the two vol- provide marker horizons which should be canic centers immediately north of Rome, and accurately datable, even though their ages fall sheet 143 "Bracciano" of the Carta Geologica near the lower limit of the K/Ar time scale. d'ltalia (Servizio Geologico d'ltalia, 1971), Between these marker beds are stratified vol- provide a valuable basis for this understanding. canic and sedimentary units including lake More recent mapping at larger scales is filling beds rich in diatoms and pollen which should in the details of certain critical regions (Alvarez, provide climatic information. Along the 1972). Tyrrhenian coast the volcanic rocks interfinger A section that promises to be useful in the

Geological Society of America Bulletin, v. 84, p. 749-758, 6 figs., March 1973 749

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study of the middle Pleistocene is on the north flank of the Monti Sabatini volcanic district, about 40 km north of Rome. Here the River Treia has cut down through the Pleistocene volcanic sequence into the underlying marine Pliocene strata. The purpose of this paper :s to introduce the potentially useful sequence of the Treia Valley and to describe its role in the history of the Tiber River. Studies in this area show that in the early Pleistocene, before the beginning of the Sabatini volcanic activity, the Tiber River flowed due south from Civita Castellana to Rome through the area of the present Sabatini volcanoes. The first volcanic products dammed this "Paleotiber" valley, forcing the river to find a new route to the sea. The new route is the present course of the Tiber, which makes a detour to the east around (Fig. 1).

EVIDENCE The evidence for this ancient river course is of four kinds: stratigraphic relations from east to west; facies changes from north to south, the map pattern of volcanic and sedimentary units which filled the ancient valley; and strati- graphic details that make it possible to identify the actual bed of the "Paleotiber." Figure 1. Location map, Rome and the Sabatini East-West Stratigraphic Relations volcanic area. Figure 2 is a cross section passing generally eastward from Castel Sant'Elia to Monte Soratte, as shown in Figure 4. This section A layer of gravel several meters thick, over- clearly shows the valley of the "Paleotiber." lain by a similar thickness of silt, covers the Monte Soratte is an isolated ridge of miogeo- bottom of the valley. On these sediments rest synclinal carbonate rocks, chiefly the Liassic volcanic units. Similar gravel and silt units Calcare Massiccio and Corniola (Servizio exist as terraces at various levels up the sides of Geologico d'ltalia, 1970), formations typical the valley. These sediments represent river of the autochthonous Tuscan and Umbrian gravel and flood-plain silt resulting from lateral sequences (Bortolotti and others, 1970). Eijht erosion by the "Paleot.ber," and the higher kilometers west of the section, near Sutri, out- levels are fluvial terraces marking stages in the crops of the Upper Cretaceous to Eocene Tolfa down-cutting of the river. Although faulting Formation form part of the allochthonous may have played a role in determining the path eugeosynclinal Calvana Supergroup (Abbate of the valley, faulting alone does not explain and Sagri, 1970). the pattern of terraces; tne valley is not simply Between these high areas is a depression a complex graben with gravel marking the tops filled with Pliocene to (?) Calabrian marine of fault blocks. sediments (blue and gray clay in the lower part Along the line of the cross section, five vol- and yellow sand toward the top), which are canic units compr.se the filling of the ancient probably faulted. The highest Pliocene out- valley; these include three ash-flow tuffs, crops are at 225 m (Filissano) and the lowest separated by two stratified air-fall tuff units are at 100 m above sea level. This relief on t he (Mattias and Ventriglia, 1970; Alvarez, 1972). top of the Pliocene outlines a former val ey Stratigraphic details are complicated, and each that is interpreted as that of the ancient Tiber, of the three ash-flow tuff units rests on a surface and that is presently filled with more than 100 showing erosional topography. The lowest unit, m of volcanic deposits. the Tufo Giallo della Vk. Tiberina, is confined

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/3/749/3433335/i0016-7606-84-3-749.pdf by guest on 24 September 2021 Figure 2. Cross section from Castel Sant'Elia to della Via Tiberina (yellow ash-flow tuff). 5. Tufi Storta (stratified air-fall tuffs). 8. Tufo Giallo di Monte Sorattc, showing the "Paleotiber" valley. 1. Stratificati Varicolori di Sacrofano (stratified air-fall Sacrofano (yellow ash-flow tuff). Nomenclature from Mcsozoic limestones and slope debris. 2. Pliocene tuffs). 6. Tufo Rosso a Scorie Nere (red ash-flow tuff Mattias and Ventriglia (1970). sediments. 3. Paleotiber gravel and silt. 4. Tufo Giallo with black scoriae). 7. Tufi Stratificati Varicolori de' La

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Civita Mazzano Calcata Castellana

Figure 3. North-south stratigraphie relations be- Paleotiber gravel and silt. 4. Tufo Giallo della Via tween Mazzano and Civita Castellana after damming of Tiberina. 5. Mud-flow deposits. 6. Lake beds, the Paleotiber. 1. Pliocene clays. 2. Pliocene saads. 3.

to the narrow, bottom part of the valley. The near Civita Castellana contained 20.3 percent overlying Tufi Stratificati Varicolori di CaC03; the remainder was very fine-grained Sacrofano and Tufo Rosso a Scorie Nere fill sanidine, biotite, leucite, and analcime (Mario the much wider upper part of the valley. The Fornaseri, 1971, oral commun.). Tufi Stratificati Varicolori de'La Storta spread These relations show that the "Paleotiber" over the entire area because of their air-fall valley was dammed by the Tufo Giallo della origin, and because by this time the ancient Via Tiberina. This ash-flow tuff is a product of valley was nearly filled in. The final ash-flow the Sacrofano volcano, which grew directly in tuff, the Tufo Giallo di Sacrofano, is confined the path of the ancierit Tiber (see below), and in this area to stream valleys eroded into the thus the unit thins northward up the ancient earlier volcanic sequence. Minor faulting valley. After emplacement of the Tufo Giallo affected the area during the deposition of the della Via Tiberina, the waters of the "Paleo- volcanic units. tiber" were ponded between the northward slope of the ash-flow tuff and the southward North-South Stratigraphic Relations slope of the river valley. This depression was Exposures along the present Treia Valley filled at first by mud flows in its southern part between Mazzano and Civita Castellana show and later by lacustrine sediments, both deriving stratigraphic changes that indicate the manner their material largely from erosion of the ash- in which the "Paleotiber" valley was blocked flow tuff. Eventually the lake dried up as a by volcanic products (Fig. 3). The channel result of filling by sediments and draining as gravel and flood-plain silt of the "Paleotiber" the Tiber founci a new route to the sea. Sub- can be seen, along the entire distance. lit the sequently the air-fall tuff layers of the Tufi Mazzano area, the gravel and silt are overlain Stratificati Varicolori di Sacrofano were by about 40 m of Tufo Giallo della Via Ti- deposited. berina, which is overlain by air-fall tuffs of the Tufi Stratificati Varicolori di Sacrofano. At Course of the "Paleotiber" Valley Calcata, the Tufo Giallo della Via Tiberina is The Tufo Giallo della Via Tiberina and the thinner (30 to 35 m) and is overlain by mud- lake beds were deposited in the bottom of the flow deposits and lake beds, both of which are "Paleotiber" valley, so the distribution of these composed of material apparently derived from units shows the extent of the ancient valley the underlying ash-flow tuff. The lake beds (Fig. 4). Figure 4 shows the course of the are overlain by air-fall tuff like that at Mazzano. "Paleotiber" valley from Civita Castellana to Between Calcata and Civita Castellana, the Prima Porta, how the Sacrofano volcano grew Tufo Giallo della Via Tiberina and the overly- within the ancient valley, and how the lake ing mud-flow deposits gradually diminish in beds overlap the northern slope of the Tufo thickness and die out, so that at Civita Castel- Giallo della Via Tiberina. lana the lake beds lie directly on "Paleotiber" Blanc and others (1953, p. 24) mention gravel and silt. One sample from the lake beds briefly that the volcanoes of the Bracciano

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/3/749/3433335/i0016-7606-84-3-749.pdf by guest on 24 September 2021 Figure 4. The ancient and modern Tiber valleys between Civita Castellana and Rome. a. Floodplain of the modern Tiber, b. Extent of lake beds deposited after damming of the Paleotiber. c. Extent of the Tufo Giallo delta Via Tiberina. d. Mesozoic limestones, e. Cross sections. (1) This paper, Figure 2. (2) Bonadonna (1968, Fig. 10), width of the buried valley near Prima Porta (one portion of a longer cross section).

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Figure 5. The bed of the Pale- otiber, reconstructed from outcrops along the Mazzano-Calcata road. 1. Pliocene sediments. 2. Channel gravel of the Paleotiber. 3. Paleotiber flood-plain silt with terrestrial gastro- pods. 4. Tufo Giallo della Via Tiberina, basal water-laid facies. 5. Tufo Giallo della Via Tibernia, normal facies.

(Sabatini) group obstructed the valley of the present study (Fig. 4). On this basis, the "Paleotiber," forcing it to swing east, while the "Paleotiber" gravel exposed in the Treia formation of the volcanoes south Valley near Civita Castellana and Mazzano is of Rome diverted the "Paleotiber" westward considered to be cc Cromerian age. to the sea. They also show the situation in the Prima Porta-Rome area. In this region they Bed of the Paleotiber show the "Paleotiber" valley separated from Where the road from Mazzano to Calcata the Tyrrhenian Sea by the Monte Mario crosses the Treia F.iver, there are exposures of Ridge, an uplift of Pliocene and Calabrian "Paleotiber" grav;l and silt. Several features sedimentary rocks which is now buried by of these sections show that the actual bed of the pyroclastic deposits except at its southeastern "Paleotiber" passed through here at the time end within the cit y of Rome (Fig. 1). when the river was dammed by the Tufo Giallo Bonadonna (1968) has also studied the stratig- della Via Tiberina (Fig. 5). raphy of the Prima Porta area. In Figure 10 of The normal sequence of fluviatile sediments that paper, Bonadonna shows a cross section found throughout the area is shown at the east that begins at Rome and extends 25 kn north- end of the section: 7 m of channel gravel are northeastward along the right bank of the overlain by 5 m af flood-plain silt with ter- Tiber. The most conspicuous feature on this restrial gastropods, on which rests the Tufo section is a valley cut into the Plbcene to Giallo della Via T:.berina. The sequence is dif- Calabrian marine sediments and fil.ed with ferent at the west end of the section where there Cromerian1 fluviatile gravels and younger are only 3 m of gravel and no silt, which sug- volcanic rocks. The portion of the cross section gests that the thin sequence at point A (Fig. 5) corresponding to this valley is indicated on represents the bed of the river. This conclusion Figure 4. This feature is the same as the is strengthened by the water-laid characteristics "Paleotiber" valley of Blanc and others (1953), of the overlying Tufo Giallo della Via Tiberina and although it is buried beneath the Sabatini (Alvarez, 1972). In its normal development, volcanoes in the area of the Sacrofano Crater, this unit is a pale, yellowish ash-flow tuff with it is clearly the same as the ancieit Tiber abundant light-colored pumice of centimeter Valley identified farther to the north during the size. In cliff faces h can be seen that the unit is broken by sharp horizontal planes into several •The Cromerian date is based on an unpublished massive divisions, probably representing several description of a mandible of Megaceros (Mcgaceroides) flows that followed each other in quick succes- verticornis, "the most typical representative of the sion. Within each flow, however, there is no European Cromerian forms" (Bonadonna, 1968, p. 264, compositional stratification. On the other hand, 309).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/3/749/3433335/i0016-7606-84-3-749.pdf by guest on 24 September 2021 Figure 6. Evolution of the Tiber drainage system Tiber system by the Arno, disappearance of most of the (based largely on Merla, 1944). a. Pliocene paleogeo- intermontane lakes, and diversion of the lower Tiber graphy, modern coastline dotted for comparison, b. The by volcanic products. The Treia drainage system is early Pleistocene, after formation of the intermontane shown in the angle between Lago di Vico, Lago di lacustrine basins and emergence of the lower Tiber Bracciano, and the Sacrofano Crater. region, c. Present situation, after capture of part of the

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above the thin fluviatile sections a peculiar south from Civita Castellana to Prima Porta basal facies is developed, in which there and thence to the sea, which at that time are distinct stratified layers of size-sorted extended well inland fi am the present coast. pumice that strongly suggest deposition in At Rome, the Cromeria.n river gravel is sepa- water. These layers are disrupted and deformed, rated only by the narrow Monte Mario Ridge indicating that the water was periodically from contemporaneous deltaic beds and the disturbed. The basal facies passes gradually marine Sicilian Ponte Galeria Formation upward into normal Tufo Giallo della Via (Bonadonna, 1968). Tiberina at a level somewhat below the ad- During the Pleistocene, the intermontane jacent flood plain (Fig. 5). lakes gradually dried uo, and regional tilting In summary, the presence of an unusual led to capture of part of the Tiber drainage by water-laid facies of the Tufo Giallo della Via the Arno. Tiberina, in places where the channel gravel is With the first eruption of the Sacrofano abnormally thin and the flood-plain silt is volcano and emplacement of the Tufo Giallo missing, provides strong evidence for the della Via Tiberina, the ' Taleotiber" valley was identification of the former river bed. There obstructed and a lake formed in the vicinity are not enough exposures to map the course of of Civita Castellana (Fig'. 4). The emplacement the river bed, but by analogy with the present of the Tufo Giallo della Via Tiberina can now Tiber it can be assumed that it meandered only be dated as post-Cromerian and prior to back and forth across the floor of its valley. emplacement of the younger Tufo Rosso a Scorie Mere (430.000 y.rs B.P., Evernden and EVOLUTION OF THE DRAINAGE Curtis, 1965). At the present time 'Figs. 4, 6c) the Tiber The observations described here add one turns southeast near Civita Castellana and more chapter to the complicated story of the detours around Monte Soratte before rejoining development of the Tiber (Merla, 1944; its former course at P::ima Porta. It is likely Sestini, 1950; Blanc and others, 1953; Blanc, that the two segments of this new course were 1956; Ghelardoni, 1958, 1961; Albani, 1962; formerly tributaries of the "Paleotiber" with a Brandi and others, 1970). The following sum- low watershed between them at Torrita Ti- mary is based mainly on the work of Merla berina (Fig. 6b). When the ancient Tiber Valley (1944). was dammed by the Tufo Giallo della Via During the Pliocene, all the region of the Tiberina the resulting lake backed up the lower Tiber, including the area described here, northern tributary as far as Torrita Tiberina, was below sea level (Fig. 6a). Only the present after which the divide was breached by head- left-bank tributaries (the upper Tiber, the ward erosion of the second tributary and by Topino, the Nera, the Velino, and the Anio) spillover from the lake. The present course of were yet in existence. They formed indepen- the river was now complete, and the lake was dent trunk streams with separate mouths along gradually drained. Evidence for this hypothesis the west coast of a narrower Italian Peninsula. is twofold: at Torrita Tiberina the Tiber Valley At this time, the shoreline east of the area under makes a sharp bend :"rom east-southeast to discussion was along the front of the Sabine south, and at this point it is deeply incised Apennines. Several islands were nearby, the into the narrowest par': of its valley between closest being Monte Soratte. The nearest river Civita Castellana and I: he sea. Lake beds were mouths were 20 to 30 km to the east so that presumably deposited between Civita Castel- coarse clastic sediments did not reach the lana and Torrita Tib:rina, but have been present Treia area. eroded by the modern Tiber. A series of intermontane lake basins formed While an old tributary was thus reversed to in the Apennines in the late Pliocene and early become the new Tiber, the old Tiber was re- Pleistocene (Sestini, 1970), and uplift :ook versed to become a new tributary, the Treia. place probably after the Calabrian. With this Successive ash flows have accumulated in the uplift the left bank tributaries were linked old valley, and ash and lapilli have rained down by a new trunk stream—the lower Tiber (Fig. onto it, but they have never completely filled 6b). The path of this Cromerian "Paleotiber" in the depression, This remains the lowest part seems to have been generally the same as that of the plateau surface on an east-west profile, of the present Tiber, except that it flowed due and successive generations of the Treia have

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continued to mark the site of the ancestral Bortolotti, V., Passerini, P., Sagri, M., and Sestini, valley of the Tiber. G., 1970, The miogeosynclinal sequences, in Sestini, G., ed., Development of the Northern ACKNOWLEDGMENTS Apennines geosyncline: Sed. Geology, v. 4, no. 3/4, p. 341-444. I thank John B. Ward-Perkins for pointing Brandi, G. P., Cerrina Feroni, A., Decandia, F. A., out the problem of the "Paleotiber" to me, Giannelli, L., Monteforti, B., and Salvatorini, Sheldon Judson, and T.W.J. Potter for their G., 1970, Il Pliocene del bacino del Tevere fra continuing interest in this work, and Mario Celleno (Terni) e Civita Castellana (Viterbo). Fornaseri, P. P. Mattias, and F. P. Bonadonna Stratigrafia ed evoluzione tettonica: Soc. for their very helpful discussions in the field. Toscana Sci. Nat. Atti., ser. A, v. 77, p. 308- My wife assisted in the field and drafted the 326. illustrations. S. Judson, R. W. Fairbridge, D. Evernden, J. F., and Curtis, G. H., 1965, The B. Ericson, and D. R. Horn offered useful potassium-argon dating of late Cenozoic rocks in east Africa and Italy: Current Anthropol- suggestions on the manuscript. This work was ogy, v. 6, no. 4, p. 343-364. supported by a North Atlantic Treaty Or- Ghelardoni, R., 1958, Spostamento dello spartiac- ganization Postdoctoral Fellowship, by the que dell'Appennino Settentrionale in con- Standard Oil Company of California, and by seguenza di catture idrografiche: Soc. Toscana the Industrial Associates of Lamont-Doherty Sci. Nat. Atti, ser. A, v. 65, p. 25-38. Geological Observatory of Columbia Univer- 1961, Evoluzione e spostamento dello spartiac- sity. que appenninico tra il M. Fumaiolo e Gualdo Tadino: Soc. Toscana Sci. Nat. Atti, ser. A, v. REFERENCES CITED 68, 25 p. Abbate, E., and Sagri, M., 1970, The eugeosynclinal Mattias, P. P., and Ventriglia, U., 1970, La regione sequences, in Sestini, G., ed., Development of vulcanica dei Monti Sabatini e Cimini: the Northern Apennines geosyncline: Sed. Geol. Soc. Ital. Mem., v. 9, p. 331-384. Geology, v. 4, no. 3/4, p. 251-340. Merla, G., 1944, Il Tevere. Geologia e permeabilità Albani, A., 1962, L'antico Lago Tiberino: L'Uni- dei terreni del bacino: Rome, Pubbl. 22, Serv. verso, v. 62, no. 4, p. 731-750. Idrogr. Alvarez, W., 1972, The Treia Valley north of Rome: Servizio Geologico d'Italia, 1970, "Palombara Volcanic stratigraphy, topographic evolution, ": Sheet 144 of the geological map of and geological influences on human settlement: Italy, scale 1:100,000: Rome, Servizio Geol. Romana (in press). Geologico d'Italia. Blanc, A. C., 1956, Sur le Pleistocène de la région 1971, "Bracciano": Sheet 143 of the geological de Rome. Stratigraphie-Palaeoéchologie- map of Italy, scale 1:100,000: Rome, Servizio Archéologie préhistorique: Actes du IVe Geologico d'Italia. Congrès International du Quaternaire, Rome- Sestini, A., 1950, Sull'origine della rete idrografica Pise, 1953, v. II, p. 1097-1118. e dei bacini intermontani nell'Appennino Blanc, A. C., Tongiorgi, E., and Trevisan, L., centro-settentrionale: Riv. Geografi Italiana, 1953, Le Pliocène et le Quaternaire aux alen- v. 57, no. 4, p. 249-256. tours de Rome: Livret-guide du IVe Congrès Sestini, G., 1970, Postgeosynclinal deposition, in International du Quaternaire, Rome-Pise, 36 p. Sestini, G., ed., Development of the Northern Bonadonna, F. P., 1968, Studi sul Pleistocene del Apennines geosvncline: Sed. Geology, v. 4, V. La biostratigrafia di Monte Mario e no. 3/4, p. 481-520. la "Fauna malacologica mariana" di Cernili Irelli: Geol. Soc. Ital. Mem., v. 7, p. 261-321. Bonadonna, F. P., and Bigazzi, G., 1969, Studi sul MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY Pleistocene del Lazio. VII-Età di un livello 17, 1972 tufaceo del bacino diatomitico di Riano REVISED MANUSCRIPT RECEIVED MAY 5, 1972 stabilita con il metodo delle tracce di fissione: LAMONT-DOHERTY GEOLOGICAL OBSERVATORY Geol. Soc. Ital. Boll., v. 88, p. 439-444. CONTRIBUTION No. 1838

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