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1. THE GEOLOGIC CONTEXT

In this field trip we are going to visit some chert outcrops located in the Montsant Massif.

The Montsant Massif is located in the southern margin of the Catalan Central Depression (Fig.

1) formed up by Tertiary aged materials, and is adjacent to the southwestern limit of the

Prades Range (Catalanids), constituted by Paleozoic and Mesozoic materials.

Figure 1. Geographic location of the trip area, and different geological formations with silicifications

(Soto et al., 2014).

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The Montsant Massif shows a depositional sequence of three geological formations:

- A Paleozoic basement, formed up by slates and grey sandstones.

- A Mesozoic substratum, formed up by Triassic materials and composed by limestones,

conglomerates and sandstones and dolomitic materials, where cherts are scarce but continued

distributed.

- A Cenozoic coverage, irregulary represented and formed up by red lutites, sandstones,

evaporites and carbonates levels (Colombo, 1986).

This Cenozoic coverage (Fig. 2) presents a stratigraphic sequence named as the Cornudella

Group, where the Ulldemolins Complex and the La Morera Fm could be identified, and which

presents numerous chert outcrops. These define a rich and predictable landscape for lithic

procurement, favoring dense and continuous prehistoric occupation of this territory (Fig.3).

Figure 2. Detail of the geologic map of the Monstant Massif (1:5000) where orangish colors represent

the Cenozoic coverage formation (Cornudella Group).

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Figure 3. Image of the numberous archaeological sites located in the area (Vilaseca, 1953).

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Figure 4. Itinerary of “The Silica Road” trip.

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2.1. FIRST STOP: THE MONTSANT OVERLOOK

This place (Fig. 5) offers ( a beautiful vision of the western sector of the Montsant Massif and

its contact with the Cenozoic coverage and the Prades Mountains.

Figure 5. Location and general view of the Montsant Overlook.

The general disposition of these mountains let to identify an irregular downdip (Figs. 6 and 7)

of their conglomerates levels, in contact with the underlying levels by an angular discordance

with tectonic movements (Roca Llaurada) (Colombo & Vergés, 1992).

Figure 6. Levels distribution of the Montsant Fm and the Roca Llaurada discordance

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Figure 7. Panoramic views from the Montsant Overlook.

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2.2. SECOND STOP: ULLDEMOLINS CHERTS (Ulldemolins)

Figure 8. Second stop prospecting area

Abundant nodular cherts could be found in this point (Fig.8). Gypsiferous red lutites and

secondary gypsums with frequent anhydrite are the enclosing rocks of nodular cherts. These

are product of an early diagenesis where microquartz and positive elongated fibrous quartz

replace the gypsiferous facies.

Along the Ulldemolins Complex nodular cherts could be also found enclosed in gypsiarenites

and gypsilutites and primary lenticular gypsums (Fig. 9).

The definition of the enclosing lithofacies (Fig. 10) and its depositional models help to define

the different source-areas frequented by the prehistoric population, due to the high

abundance of lithic resources.

Figure 9. Images of the Ulldemolins cherts thin sections.

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Figure 10. Chert outcrop and detail of the silicifications found in Ulldemolins.

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2.3. THIRD STOP: THE ULLDEMOLINS COMPLEX (Cornudella del Montsant)

Figure 11. Third stop prospecting area

This geologic Complex dated on Lutetian (Lower-Upper Eocene) and overlying the Mediona Fm

(formed up by lutites and sandstones) is composed by the alternation of red lutites,

sandstones, grey marls and evaporites (Anadón et al., 1989).

This point (Fig. 11) presents a lithostratigraphic sequence (Fig.12) composed from base to top,

by: 1.Red lutites; 2. Massive gypsum (Fig. 13); 3. Lutites and nodular gypsum; 4. nodular

carbonates; 5. dolomitized lacustrine carbonates; 6. Sandy grey lutites; 7. Sandstones; 8.

Lutites. This sequence is related to centered and marginal sections of shallow saline lacustrine

environments (sabhka type) in arid climates, with frequent lateral variations due to quick

progressive and retrogressive events.

Figure 12. Lithostratigrahic units of the Ulldemolins Complex (Colombo, Barbé, Escarré, 1995).

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Figure 13. General view and detail of the silicifications in Cornudella del Montsant.

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BIBLIOGRAPHY

Anadón, P., Cabrera, L., Colldeforns, B. y Sáez, A. (1989). "Los sistemas lacustres del Eoceno superior y Oligoceno del sector oriental de la Cuenca del Ebro." Acta Geologica Hispanica 24: 205-230.

Colombo, F. (1986). "Estratigrafía y sedimentología del Paleógeno continental del borde meridional occidental de los catalánides (Provincia de Tarragona, España)." Cuadernos de Geología Ibérica 10: 55-115.

Colombo, F. y Vergés, J. (1992). "Geometria del margen S.E. de la Cuenca del Ebro: discordancias progresivas en el Grupo Scala Dei. Serra de La Llena. (Tarragona)." Acta Geologica Hispanica 27 (V-2 Homenaje a Oriol Riba Arderiu): 33-53.

Colombo, F., Barbé, D. y Escarré, V. (1995). "Controles alocíclicos en el relleno sedimentario de una cuenca paleógena: arquitectura deposicional del Grupo Cornudella, Cuenca del Ebro (Tarragona)." Geogaceta 17 (31-34).

Soto, M., Gómez de Soler, B., Vallverdú, J. y Vaquero, M. (2014). "Potential Siliceous Sources during Prehistory: Results of prospecting in the East Margin of the Ebro Basin (NE of Iberian Peninsula)." Journal of Lithic Studies 1(1).

Vilaseca, S. (1953). Las industrias del sílex Tarraconenses. Madrid, Consejo Superior de Investigaciones Científicas. Instituto Rodrigo Caro.