Sedimentary facies in an extensional and deep-lacustrine depositional system: the Pliocene Tiberino Basin, Central Italy PDF

Preview

Summary

Sedimentary Geology ELSEVIER Sedimentary Geology 109 (1997) 73-94 Sedimentary facies in an extensional and deep-lacustrine depositional system: the Pliocene Tiberino Basin, Central Italy Giorgio Basilici * Dipartimento di Scienze della Terra, Universitg~ di Torino, Via Accademia delle Science, 5, 10...

Description

Accelerat ing t he world's research.

Sedimentary facies in an extensional and deep-lacustrine depositional system: the Pliocene Tiberino Basin, Centr... Williams Themison Sedimentary Geology

Cite this paper

Downloaded from Academia.edu 

Get the citation in MLA, APA, or Chicago styles

Related papers

Download a PDF Pack of t he best relat ed papers 

T he last int erglacial pedocomplexes in t he lit ho- and morpho-st rat igraphical framework of t h… Mauro Colt ort i

2006 pedocomplex Pierluigi Pieruccini T he Pleist ocene glaciolacust rine sediment s in t he Bełchat ów mine (cent ral Poland): Endogenic and e… Beat a Gruszka

Sedimentary

Geology ELSEVIER

Sedimentary Geology 109 (1997) 73-94

Sedimentary facies in an extensional and deep-lacustrine depositional system: the Pliocene Tiberino Basin, Central Italy Giorgio Basilici * Dipartimento di Scienze della Terra, Universitg~ di Torino, Via Accademia delle Science, 5, 10123 Turin, Italy Received 22 January 1996; accepted 16 July 1996

Abstract

The Tiberino Basin was formed during Plio-Pleistocene extensional tectonic activity in Central Italy. The Pliocene sediments that fill this structure consist predominately of fine-grained clastic lithofacies deposited in a lacustrine system; four facies associations have been distinguished. Facies association A was deposited in a deep-offshore lacustrine environment and consists of massive, laminated, bluish-gray marly clays that lack benthonic fauna but have preserved organic matter (leaves, wood). This facies association is the main lacustrine deposit, whereas the other three represent marginal facies. Facies association B is interpreted as a delta system deposit; it has Gilbert-type sandy gravel bodies with foresets up to 50 m high and prodelta bodies consisting of laminated to massive marly clays that alternate with sandy and gravelly mud strata. Association C corresponds to a coastal environment, where two sub-environments are recognized: (1) a wave-dominated coastline characterized by interbedded muddy and sandy strata with sedimentary structures attributed to hummocky cross-stratification; and (2) clayey silts and lignites interpreted &s a coastal wetland; a spectacular fossil forest is preserved in situ within this unit. Association D, formed on the distal part of a muddy alluvial fan, consists of sheet-flood clayey sandy silts, that alternate with incipient paleosols; rare, ribbon-channel fills are also present. Paleoenvironmental reconstruction indicates that the Tiberino Basin was occupied by a narrow, deep meromictic lake during the Pliocene. The size, shape, depth and sub-environment distribution of this lake were controlled by tectonic phenomena, such as the high subsidence rate, which caused the lake to be large and deep. Only in the few coastal areas that had high sedimentary input were wetland or alluvial fan deposits formed.

Keywords: Central Italy; Extensional basin; Lacustrine sediments; Pliocene

1. I n t r o d u c t i o n Few articles exist in the international literature on ancient lacustrine sediments within the Italian Peninsula. Notable exceptions include Giraudi (1989), Billi et al. (1991), Caggianelli et al. (1992) and * Present address: Dipartimento di Scienze della Terra, Universita' di Perugia, Piazza dell'Universita" 1, 06123 Perugia, Italy. Tel.: +39 (75) 585-3220; Fax: +39 (75) 585-3203.

Sagri et al. (1994), who considered Miocene to Quaternary lacustrine deposits and Ori et al. (1986), who considered Permian lacustrine systems. This paper describes the characteristics and organization o f the Pliocene lacustrine lithofacies within the continental Tiberino Basin in Central Italy (Fig. 1), with the goal o f reconstructing the depositional system, paleogeography and paleoenvironmental features o f this lake, as well as analysing the tectonic aspects controlling sedimentation. This study con-

0037-0738/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 6 ) 0 0 0 5 6 - 5

G. Ba;ilici/Sedimentary Geology 109 (1997) ZL.94

74

the entire basin utilizing other smaller outcrops, drill cores, geologic surveys and gravimetric data. A total of 25 outcrop sections and 20 cores were analysed.

ITALY

2. Geographical and geological setting

. ~..~'

Sea a~'-

~

o

Citt~i di Castel

Tiberino B a s i 1t -,~-...'. "~G~ '~

%

study

t M

42 ° 47' N

I'N

'

~"

tO

Fig. I. Location of the Tiberino Basin. The study site is outlined with horizontal lines. The principal extensional basins and thrust fronts of Central Italy are shown in the upper sketch.

tributes to the sedimentological knowledge of deeplacustrine basins in extensional tectonic regimes. Despite the limited exposure it was possible to achieve a realistic basin analysis by extrapolating point data. Facies analysis was initially applie# to the best outcrop, and then subsequently expanded to

The Tiberino Basin is located in Umbria (Central Italy). The basin outcrops from Cittb, di Castelio to Perugia, where it divides into a western part that reaches Terni and an eastern part that extends south to Spoleto (Fig. l). In plan view the Tiberino Basin assumes an 'overturned Y' shape that covers approximately 1800 km 2. This paper describes the lacustrine fill sediments within the basin's southwestern branch, between the towns of Marsciano and Terni. The Tiberino Basin is an extensional basin located at the western margin of the Apennine Chain. These mountains developed following the collision of the European and African Plates from the Oligocene to present, and consist of stacked thrust sheets and nappes that have a tectonic vergence towards the northeast and east (Boccaletti et al., 1990). The compressional building phase of the Apennine Chain, which terminated in the study area during the Late Messinian, was followed by extensional tectonic regime, which began during the Pliocene and caused the fragmentation of the chain and the formation of extensional basins. According to Lavecchia (1988) the genesis of these basins is linked to the opening of the Tyrrhenian Sea as it prograded eastwards. Since the Late Miocene the Tyrrhenian Sea has been characterized by extensional tectonics which formed NW-SE-trending, en e c h e l o n grabens on the western part of the Italian Peninsula (Fig. 1). Most of the extensional basins consist of half-grabens, with a west-dipping master fault on the eastem margin and antithetical minor faults delimiting the western margin (Martini and Sagri, 1993). These authors subdivided these basins into two major groups, classitied according to their geophysical characteristics, evolutionary history and geographic position: basins located near the Tyrrhenian Sea are called 'central basins' while those located near the Apennine Chain are called 'peripheral basins'. According to this classification the Tiberino Basin is a peripheral basin. The Tiberino Basin is a complex system, formed by the coalescence of smaller basins which show different tectonic and depositional behaviours (Basilici,

G. Basilici / Sedimentary Geology 109 (1997) 73-94

1994, unpubl, data). Basin analysis, however, indicates that a uniform depositional history exists in the study area (the southwestern branch of the Tiberino Basin). Approximately 450 m of sediments are exposed in the study area, but gravimetric surveys (Ambrosetti et al., 1993) identify a depocentre to the east of the basin axis with a maximum sediment thickness of 2300 m near Collevalenza (Fig. 2); the western and eastern margins are significantly thinner (up to 120 m). Towards the north, immediately outside the study area, a seismic profile shows a maximum thickness of 500 m (Barchi et al., 1991).

3. Stratigraphic framework The succession consists mainly of clastic sediments with calcareous deposits appearing only in the upper section. Biostratigraphic, biochronologic and magnetostratigraphic data indicate a Middle Pliocene to Early Pleistocene age. The four lithostratigraphic units identified in the southern portion of the Tiberino Basin (Figs. 2 and 3) are denoted according to international nomenclature, but it should be noted that local genetic aspects were important for their definition and recognition. These lithostratigraphic units, considered as formations, are (Fig. 3): the Fosso Bianco Formation (FBF), the Ponte Naja Formation (PNF), the S. Maria di Ciciliano Formation (SMCF) and the Acquasparta Formation (AF) (Basilici, 1992).

3.1. The Fosso Bianco Formation (FBF) The Fosso Bianco Formation is up to 250 m thick, it unconformably overlies the pre-Pliocene units and is characterized by bluish-gray, marly silty clays and (to a lesser extent) by gravels, sands and clayey gravels. This unit was formed in a large lake and displays a variety of facies associations which identify offshore lacustrine, deltaic and coastal environments. Botanical (pollen, seeds and trees) and malacological (gastropods) remains indicate that this unit is of Middle-Late Pliocene age (sensu Rio et al., 1994).

3.2. The Ponte Naja Formation (PNF) The Ponte Naja Formation has a limited distribution (6 km 2) around the Todi hill, where a quarry

75

exposes a stratigraphic section of 140 m. Clayey sandy silts and silty clays constitute most of the deposits, while the remainder is made up of gravels or gravelly sands. Facies analysis shows that the PN-F was formed on the distal part of an alluvial fan located at the lake margin. Some mammalofauna remains (Pseudodama lyra, Equus livenzovensis) and gastropod associations indicate a Late Pliocene age.

3.3. The S. Maria di Ciciliano Formation (SMCF) The S. Maria di Ciciliano Formation is made up of 150-200 m of silty clays, or clayey silts, which altemate with sandy bodies. In the southern part of the study area the SMCF unconformably overlies the FBE The sedimentary structures of the sandy lithosome suggest deposition in meandering fluvial channels, whereas the fine-grained lithologies constitute normal humid flood-plain sedimentation. Vertebrate, malacofauna and vegetal remains date the SMCF as being Early Pleistocene.

3.4. The Acquasparta Formation (AF) The Acquasparta Formation consists of carbonate lacustrine deposits and travertines. This unit is present only on the eastern margin of the Tiberino Basin (along the Martani Mountains) and varies in thickness up to 50 m in the study area. Sedimentation is continuous between the SMCF and the AF; however, the contact between the two units is very abrupt. Sedimentary structures and fossils suggest deposition within shallow-lake and wetland environments. Gastropod associations indicate a Early Pleistocene age. The sedimentary facies of the Fosso Bianco and Ponte Naja Formations are analysed in greater detail below.

4. Lacustrine facies associations Three major facies associations (A, B, C) have been identified in the Fosso Bianco Formation and one (D) in the Ponte Naja Formation on the basis of lithology, bed geometry and continuity, sedimentary structures, paleontological records and geographical distribution. These associations mainly consist of clastic deposit formed in open-lake (association

76

G. Basilici/Sedimentary Geology 109 (1997) 73-94

../-

4 2 ° 47' N

Collevalcn~'a

LEGEND recent alluvial deposits alluvial fan and F~" ' . talus deposits (Late Picismccnc) [~i~ Acquasparta Format ion (Early Pleistocene) I]-[l I s. Maria di Clcilial~o Fornlation (Eady Pleistocene) [ ~ Pontc Naja Formation (Late Pliocene) I.....

Dunarobba:

Fosso Bianco Formation (Middle-l.ate Pliocene)

F"]

pre-Pliocene substratum

faults (principaland sccondary) inclined strata horizontal strata

m Gemini"~.

cross-section trace k"

12 ° 24' E

(~

3Km

SW

~-Ollev/denza~

•, = ' ~ .

.t:E~

t""

NE

" "" "

"

"

"

" ; : ; : ' , : ~ "" ~=

\,

/

/

.

'

t

"

~

~

"

~

- 5on., -

lO00

nl

- 1500 m -

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

](X)O

m

.

Fig. 2. (A) Geological map of the southwestern branch of the Tiberino Basin. (B) Schematic cross-section of the Tiberino Basin (location and legend in (A). The surface distribution of the lithostratigraphic formations was derived from geological surveys, whereas the position of the pre-Pliocene substratum was defined using gravimetric data (Ambrosetti et al., 1993).

G. Basilici / Sedimentary Geology 109 (1997) 73-94

77

THICKNESS

500 m 400m 300 m 200 m

Z,o,; .....

100 m 0m

I

km

W

I

3 kzm

I

6 km

.-.

I

9 km

12 Ion

15kin

DISTANCE

Fig. 3. Schematic stratigraphic framework of the Tiberino Basin between Todi and San Gemini. PS = pre-Pliocene substratum; FBF = Fosso Bianco Formation ( M i d d l e - L a t e Pliocene); PNF -- Ponte Naja Formation (Late Pliocene); SMCF = S. Maria di Ciciliano Formation (Early Pleistocene); AF = Acquasparta Formation (Early Pleistocene).

A), delta (association B), coastal lacustrine margin (association C) and alluvial fan (association D) environments (Fig. 4).

4.1. Facies association A (offshore lacustrine deposits) This facies association consists of massive and laminated, bluish-gray, marly silty clays (Fig. 4), has a total thickness of about 140 m and constitutes more than 80% of the Fosso Bianco Formation (Fig. 5). Beds of the massive, bluish-gray, marly clays (up to 30% calcium carbonate) are very abundant, sometimes interbedded with silty (or very fine sandy) and dark bluish-gray clayey laminae (Fig. 6A). The coarse laminae are graded and have abrupt, rarely erosive, lower contacts whereas the dark bluish-gray clayey laminae have gradational contacts and smell fetid on fresh cuts; both types are planar and parallel, have an average thickness of 5 mm and are laterally continuous up to 50 m in well exposed sections. These lithofacies contain poor fossil remains, the most common being intact leaves (Fig. 6B) on strata surfaces, freshwater ostracods and rare freshwater gastropods concentrated within the coarser laminae. Rare bioturbation includes up to 2 mm diameter cylindrical traces that may be attributed to Tubificidae or Chironomidae grubs. Siderite nodules are concentrated in some strata. The laminated marly clays (Fig. 6C) are made up

of alternating planar, parallel and continuous laminae of different grain sizes or composition and have been interpreted as rhythmites. This lithology forms only 8% of facies association A and is strictly found towards the basin margins, at the transition to facies associations B and C. Two principal kinds of rhythmite sequences can be described in association A: (1) common alternating laminae of different grain sizes (silt/marly clay) that lie stratigraphically below facies association B in coarsening-upward sequences; (2) alternating silty limestone and marly clay, sometimes located below facies association C. Facies association A was formed in a deep-offshore lacustrine depositional environment, as shown by its fine grain size, preservation of fine lamination, presence of freshwater fossils, laterally continuous strata, geometry and relationships with the other facies associations. The massive bluish-gray marly clays represent homogeneous and continuous sedimentation in still waters. The deposits are massive due to this sedimentary process, and not to biological activity as shown by the lack of benthonic remains, the scarcity of bioturbation traces ( 50 m. The offshore, deep-water lacustrine sediments, formed in an anoxic, aphotic and abiotic bottom, suggest a meromictic lake. (4) Paleocurrent data (i.e., channel flows, prograding delta foresets, sediment gravity flows) are perpendicular to the main tectonic alignments, indicating that tectonic movements exerted a strict control on sedimentation. The majority of the lacustrine basin has undergone a subsidence rate higher than the sedimentation rate, resulting in a wide and deep lake. As the drill core in Fig. 8A shows, the subsidence rate was very high during the first phase of lake evolution, bringing about a rapid drowning of the emergent and wetland areas. Subsidence was balanced by deposition only in those coastal areas where a strong sedimentary input existed. Wetland and alluvial fan coastlines exhibited high sedimentation rates (approximately 1.5-3 mm/year) with respect to the deep-offshore lake (approximately < 1 mm/year).

Acknowledgements This work was financially supported by grants from the Ministero della Pubblica Istruzione, the

Soprintendenza Archeologica per l ' U m b r i a and the European Community. ! would like to thank my Ph.D. advisors, E Ricci Lucchi and G.G. Ori, for their advice. Moreover I wish to thank Dr. P. Anadon, Prof. LI. Cabrera, Prof. EI. Martini and Prof. M. Sagri lor the first revision of the paper. I am also grateful to Prof. P.A. Hamblin and an anonymous reviewer for constructive observations.

References Ambrosetti, IZ, Barbieri, M., Basilici, G., Bozzano, tL, De Pari, E, Di Filippo, M., Di Maio, R., Duddridge, (i., Etiope, G., Gambino, E, Graingcr, P., Lombardi, S., Mottana, A., Patella, D., Pennacchioni,E., Ruspandini,T., Scarascia Mugnozza, G., Sordoni, G., Tazioli, S., Toro, B., Valentini,G. and Zuppi, G., 1993. Analysis of geoenvironmentalconditions as morphological evolution factors of the sand-clay wries of the Tiberino valley and Dunarobba Forest preservation. Proceedings of the Progress Meeting on the Mirage project, 3rd phase, Bruxelles, 7-8 October 1993, 17 pp. Anadon, E, Cabrera. LI., Julia, R. and Matzo, M., 1991. Sequential arrangement and asymmetrical fill in the Miocene Rubielos de Mora Basin (northeast Spain). In: P. Anadon, LI. Cabrera and K. Kelts (Editors), Lacustrine Facies Analysis. Int. Assoc. Sedimentol.,Spec. Publ., 13: 257-275. Anderson, R.Y. and Dean, W.E., 1988. Lacustrine varve formation through time. Palaeogeogr., Palaeoclimatol., Palaeoecol., 62:215-235. Barchi, M., Brozzetti, E, and Lavecchia, G., 1991. Analisi strutturale e geometrica dei bacini della media Valle del Tevere e della VaIle Umbra. Boll. Soc. Geol. It., 110: 65-76. Basilici, G., 1992. II Bacino continentale Tiberino (Plio-Pleistocene, Umbria): analisi sedimentologicae stratigralica. Ph.D. thesis, Universityof Bologna, 323 pp. (unpublished). Basilici,...