Chapter E2 Integrated stratigraphy of the late tortonian pieve di gesso section (Romagna, Italy)

Miocene Stratigraphy: An Integrated Approach A. Montanari, G.S. Odin and R. Coccioni, eds. 9 1997. Elsevier Science B.V. All rights reserved. C h a p t e r E2

INTEGRATED STRATIGRAPHY OF THE LATE TORTONIAN PIEVE DI GESSO SECTION (ROMAGNA, ITALY) G.S. Odin, M. Cosca, E Tateo, A. Negri, G.B. Vai and J.C. Hunziker

INTRODUCTION The first quotation of a volcaniclastic arenaceous layer near the Tortonian/Messinian boundary was made by Gandolfi et al. (1983) from the Marnoso-arenacea Formation sampled near Fontanelice (Romagna, Italy). The study of these authors emphasizes the volcanic origin of one layer. G.G. Zuffa (pers. commun., 1991) informed us that several similar layers were seen at the same locality but not analysed for petrology. The present study was undertaken with the aim to characterize the potential use of those layers for integrated mostly geochronological-biostratigraphical knowledge. Similar biotite-rich layers were described by Calieri (1992) from the same province pointing to a regional significance of these layers which can be seen along fiver valleys oriented SW-NE and crossing the general NW-SE strike of the deposits (Fig. 1). Farther to the southwest a probably equivalent layer has been found by D. Cosentino (Roma) in the 'Valle del Salto' (on the border between the Umbria and Latium regions). The first results obtained from these layers sampled in the Monte Tondo and Monte del Casino sections were extremely positive (Vai et al., 1993); they were obtained from two groups of biotite-rich layers located about 5 m and about 10 to 13 m below what is considered the base of the Messinian Stage in the area; the present work intends to supplement those pioneer results. LITHOSTRATIGRAPHY The Pieve di Gesso section is located 25 km southeast of Bologna, 15 km southwest of Imola near Fontanelice (Fig. 1). The stratigraphy is generally similar to that of the Monte del Casino and Monte Tondo sections which are located 10 to 15 km farther to the east-southeast. Three to six particular layers (2 to 4 cm thick) can be observed in the Pieve di Gesso section along the 100-m-long bank of the road (Fig. 2A). The base of the regional marker bed 'Calcare di base' can be seen 40 to 50 m above the section (Fig. 2B). The Calcare di base is a characteristic Messinian feature usually located 20 to 30 m above the base of the Messinian historical stratotype of Selli (1960). This means that the layers are lithostratigraphically correlated to near and below the Tortonian/Messinian boundary. In the field, these layers differ from the marly sequence by their hardness; they can be followed easily laterally and the presence of abundant biotite flakes is diagnostic. Sedimentologically, they are locally considered to be faint turbidite layers and contain

G.S. Odin et aL

452

e

~

Bologna

Adriatic Sea

/

Pieve di Gesso ~ " 1"~~ del C a s i ~ Font~nelice " /

J

/

L

/I"

FAENZ-A("~ ......... ~

Faenza~ . ~ / ~ ~ ~n&na~.,.~

10 km

/ Fig. 1. Location of the Pieve di Gesso section in the Faenza area. The outcrop from the Valle del Salto is shown (star in the small map of Italy).

more or less abundant white mica flakes. The two layers richest in biotite were sampled for geochronological study and several samples were collected for biostratigraphy. BIOSTRATIGRAPHY Five samples from the Pieve di Gesso section were analyzed for calcareous nannoplankton biostratigraphic purposes. The samples bracket the two biotite-rich layers studied in the present work. Calcareous nannofossils generally show high abundance but poor preservation. The taxa identified are: Calcidiscus leptoporus, Calcidiscus macintyrei, Helicosphaera carteri, Discoaster pentaradiatus, Discoaster brouweri, Discoaster variabilis, Reticulofenestra pseudoumbilica, and Amaurolithus sp. The range chart for some selected species is shown in Table 1. The occurrence of Amaurolithus sp. is rare and discontinuous along the section. The first occurrence of the taxon commonly marks the boundary between subzones CN9a and CN9b of Okada and Bukry (1980) and falls in the youngest portion of the Tortonian. In the extra-Mediterranean area, the boundary between subzone CN9a and CN9b is indicated by the first occurrence (FO) of Amaurolithus primus, the first horseshoeshaped calcareous nannofossil in the Neogene. Another species appears shortly after A. primus, namely Amaurolithus delicatus. Since in the Mediterranean area, the genus Amaurolithus occurs very rarely, it appears difficult to discriminate whether or not the observed first occurrence of A. primus really predates the A. delicatus FO and it is thus

Integrated stratigraphy of the late Tortonian Pieve di Gesso section

B

453

LITHOLOGY Composite Section "La Pieve di Gesso"

Evaporites

I

III

T185

4:

"Calcare di Base" I

Dark bituminous mudstone

4~

Dark grey slightly bituminous mdst r'--!

Grey mudstone

----

Turbidite with :1: biotite

44

45

46

•

?

T z

< z O I--

cn .,90

-

..r

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,

_

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~

48-

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-

50-

:

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Level of the Road

T186

,...I

Fig. 2. (A) Photograph of the Pieve di Gesso section. The person to the bottom right of the figure (R. Calieri) stands on the lower sampled layer; the person to the top with a 1-m scale, is on the upper sampled layer. (Photo by GSO, 1992). (B) Schematic section at 'La Pieve di Gesso'. (Simplified from field observation and section by Ferretti, 1993). Note that the turbidite layers may or may not correspond to biotite-rich sediments. The section may be compared to the precise ones proposed by Vai et al. (1993, and in Chapter E3).

454

G.S. Odin et aL

Table 1 Calcareous nannofossils from the Pieve di Gesso section (original data from A.N.) Taxa T185

Amaurolithus sp. Calcidiscus leptoporus Calcidiscus macintyrei Coccolithus pelagicus Dictyococcites spp. Discoaster 5 rays spp. Discoaster 6 rays spp. Helicosphaera carteri Reticulofenestra spp. Sphenolithus moriformis

PG1

PG2

+-- young Samples old --+ PG3 PG4

R

T186

PG5 R

R R C

R R C

F R C

F C

F F C

A R F

A C C

A R C

A R R

A R C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

A = abundant, 1 or more specimen every field of view; C = common, 1 specimen every 2-10 fields of view; F = few, 1 specimen every 11-50 fields of view; R = rare, 1 specimen every 51-200 fields of view.

preferred to use the FO of the Amaurolithus group as a diagnostic key marker. The FO of A. primus is used in the extra-Mediterranean area, to approximate (by the old side) the Tortonian/Messinian boundary. The planktonic foraminiferal study in the section (Colalongo, in Ferretti, 1993) points out the absence of the Messinian taxon Globorotalia conomiozea and the occurrence of Globorotalia humerosa and Globorotalia praemargaritae. The accepted chronology from older to younger nannofossils and Foraminifera (Colalongo et al., 1979a) within this interval is as follows: FO of G. humerosa; FO of Globorotalia suterae; FO of A. primus followed by the FO of A. delicatus; and finally the Tortonian/Messinian boundary, contemporaneous or immediately followed by FO of G. conomiozea. Thus the dated section is slightly older than the Tortonian/Messinian boundary in terms of biostratigraphic control. In summary, the layers collected from the Pieve di Gesso section may be contemporaneous with either the lower first cluster or the upper second cluster of those previously studied for integrated stratigraphy in the Monte Tondo and Monte del Casino sections (Vai et al., 1993). GEOCHRONOLOGY Petrography and mineralogy of the geochronometers The volcanic origin of the material from the layers sampled was deduced from their distinctive nature compared to all neighbouring sediments (Gandolfi et al., 1983). These authors quote the presence of 100% of acidic volcanic elements in the rock fragments extracted from the sediment. For these authors, the spectrum of components is diagnostic with the total sand-sized sediment made of 25% of plagioclase and 34% of mica flakes (biotite); heavy minerals extracted following acid treatment, show a volcanic composition with 97% of opaques and, amongst transparent minerals, a unique assemblage of zircon (75%) and allanite (18%). The regional extent of

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9

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Fig. 3. Geochemical homogeneity of the biotite flakes purified from Pieve di Gesso (T186 lower layer, T 1 8 5

upper layer). Note that the upper layer may be qualified as homogeneous but a few flakes from the lower layer have slightly less potassium than the others. This may be compared to the fact that the step-heating age spectrum for the upper layer shows a good plateau, while there is only a pseudo-plateau for the lower layer. (Original data by FT.)

monitored according to systematic and synchronous X-ray diffraction analyses for recognition of the most promising phases. The two samples comprise a variety of pyroclastic minerals including diversely preserved biotite crystals (up to 90%o versus whole-rock, WR), abundant plagioclase, rare and elongated apatite and zircon crystals suggesting a restricted submarine transportation (if any). The sampled sediments also contain muscovite, internally pyritized microforaminifers, and organic phosphate (including a fish tooth in T186) pointing to a triple origin (volcanic, detrital, and marine). In addition, diagenetic pyrite is also common. The biotite fraction heavy in bromoform was first gently crushed in acetone and sieved (mesh 0.16 mm), then purified by hand-picking especially for sample T185 where a few % of the idiomorphic hexagonal flakes with a red colour was removed from the generally dark brown ones. X-ray diffraction patterns denote absence of alteration and well closed structure with (001) reflection at 10.03 and 9.99/~ for samples T185 and T186, respectively. Due to the possibly mixed origin of the sediment (presence of suspected muscovite), a study of the geochemical homogeneity of the biotite flakes was undertaken. We

Integrated stratigraphy of the late Tortonian Pieve di Gesso section

457

used the electron dispersive microprobe technique (Philips' SEM-EDS instrument) on polished surfaces and adopted the approach pioneered by Montanari (Montanari, 1988; Odin et al., 1991). The results (Table 2 and Fig. 3) for T185 are (1) that K content is large (9.66% K20) and homogeneous with no alteration (K loss), and (2) that Mg, Fe and Ti contents are reasonably homogeneous. For biotite T186, K content is even greater as a mean (9.66% K20) but two flakes of 15 are very slightly altered; Fe and Mg contents are more homogeneous than in T185 but the Ti content is significantly greater for three flakes of 15 in this sample. If one removes those three flakes from T185, the mean Ti content becomes similar to that in T186. The biotite phase from the upper layer T185, shows a FeO/MgO ratio (1.45) smaller than the one from the lower layer T186 (1.63) suggesting two distinct eruptive events. However, because these ratios are near each other, taking into consideration the analytical error bars, this suggests a common, slightly evolving source compared to biotite phases from other Miocene volcaniclastic layers (the FeO/MgO ratio is 1.03 in the Serravallian Sicilian biotite from Monte Giammoia (Chapter D3) and 3.52 in the late Messinian biotite from Maccarone in the Marche Province (Chapter El0). The overall geochemical homogeneity of the biotite selected for dating suggests both a volcanic origin of the mineral and, furthermore, a single volcanic event feeding the depositional unit in T185 and a possibly less obvious quality for T186. The plagioclase fraction of T186 was selected using a series of bromoformacetone mixtures and ultrasonically cleaned for 20 min using a nitric-acetic acid mixture. The obtained crystals were mostly transparent and about 0.1 mm in size. X-ray diffraction patterns indicate only a small proportion of quartz and a dominantly anorthitic composition. 4~

isotopic study

Two biotite and one plagioclase separates were analyzed by the 4~ technique. The samples were irradiated in the Triga reactor in Denver together with a dozen of subsamples of the biotite HD-B 1 monitor. This monitor has a recommended K-Ar age of 24.21 Ma according to Hess and Lippolt (1994). The isotopic ratios were measured using the usual procedure developed in Lausanne (Cosca et al., 1992). Sample T186 biotite (3.48 mg) was heated for seven independent heating steps following preliminary heating in vacuum at 450~ for 10 min. The calculated integrated age is 7.34 Ma. The age spectrum indicates (Fig. 4 and Table 3): (1) younger apparent ages at low and high temperatures; (2) a small but significant (95% confidence level) difference of age between steps at 1000~ and 1050~ versus those at 1100~ and 1200~ pointing to an absence of a true plateau especially due to the heating step at 1100~ with a comparatively old age at 7.58 Ma; (3) a weighted mean age of 7.47 +0.08 Ma (2or) for the four main steps (a near plateau) which includes 81.8% of the total 39Ar released. Sample T186 plagioclase (32.4 mg) was degassed for 10 heating steps (Table 4) following heating under vacuum at 600~ for 25 min. The resulting age spectrum (Fig. 5) shows a series of apparent ages between 8 Ma and 9 Ma followed by increasing apparent ages at higher temperatures (1250~ and 1560~ The calculated integrated

458

G.S. Odin et al.

P i e v e di G e s s o t~ +1

Oo

8.0- ~

1000

900

1050

1200

1100 ~.-

.....

i

o

t~ 7.0.

_.

pseudo-plateau: 81.8 %

1=

o t~ D. 0- 6.0 ,< 1._

I

GS O. 94

10

I

20

Fig. 4. 4~

!

I

I

30

40

T186 biotite Total gas age : pseudo-plateau age:

3.48 mg 7.34 Ma 7.47 + 0.08 (2~)

K20 %

9.66

=

I

I

60

70

80

39Ar release Cumulative

percent

50

I

!

90

1

age spectrum for sample T186 biotite.

Table 3 Results of 4~ Lausanne)

step heating measurements on biotite T186 (3.48 mg) from Pieve di Gesso (by GSO in

~

Apparent age (Ma)

4-2o-

%39Ar

%rad.

K/Ca

750 900 1000 1050 1100 1200 1350

5.7 7.06 7.37 7.33 7.58 7.51 7.1

1.2 0.16 0.12 0.12 0.10 0.10 1.0

2.9 13.6 22.5 16.4 19.8 23.1 1.8

4 40 50 60 75 70 8

6.2 20 30 32

~ = heating temperature. Irradiation ULRD 5, preheating 450~ for 10 min.

apparent age at 10.2 Ma (analytically similar to an age measured using the conventional isotope dilution technique) is higher than the biotite age and of no geological meaning. Sample T185 biotite (92.6 mg) was degassed in twelve heating steps (Table 5) following heating under vacuum at 450~ for 10 min. The age spectrum (Fig. 6) is essentially flat for temperatures above 750~ A good plateau age (more than three successive heating steps with similar analytical ages using 2o- error bars) can be calculated at 7.37 4-0.06 Ma (2o); this is the weighted mean age of eight steps including 87.7% of the 39AT released. That plateau age is near but slightly higher than the integrated (total gas) age at 7.20 Ma (error bar at about -I-0.05 Ma). DISCUSSION AND CONCLUSIONS Lithostratigraphic criteria useful for correlating the incompletely exposed Pieve di Gesso section with those already studied to the east (Vai et al., 1993) are: (1) the spacing of the dysoxic to anoxic layers; (2) the stratigraphic distance from the 'Calcare di base' datum level; and (3) the first occurrence of a biotite-rich horizon. The

459

Integrated stratigraphy of the late Tortonian Pieve di Gesso section Table 4 Results of 4~ by GSO Lausanne)

step heating measurements on plagioclase T186 (32.4 mg) from Pieve di Gesso (data

~

Apparent age (Ma)

-t-2cr

%39Ar

%rad.

K/Ca

750 900 1013 1085 1175 1250 1350 1440 1500 1560

7.4 8.5 9.1 8.3 8.4 11.4 14.6 24.2 28.8 21.6

0.4 0.2 0.1 0.2 0.2 0.4 0.3 0.7 1.4 5.0

12.8 14.1 23.3 14.9 13.4 6.5 9.4 3.6 1.7 0.5

14 64 73 75 39 42 56 50 49 9

0.19 0.087 0.083 0.094 0.106 0.085 0.075 0.050 0.043 0.056

30

P i e v e di G e s s o T186

plagioclase 32.4 mg

Total gas age 9 10.2 no plateau

+1

t~

I

Ma

q) m 20

750~

900~

1013~

1085 ~

1175~

1250 ~

1350 o

t~ t,n Q,.

10

-J I 9G S O .

94. 10

L

I

I

I

I

I

I

I

I

20

30

40

50

60

70

80

90

100

3 9 A r release C u m u l a t i v e p e r c e n t

Fig. 5.4~

age spectrum for sample T186 plagioclase.

section corresponds to part of the pre-conomiozea wide-spaced anoxic interval, clearly distinguished from the overlying narrow-spaced ones in the eastern sections (Vai et al., 1993). In fact, only foul" dark-grey, partly anoxic, shaly layers are found here between the two dated samples, with a large portion of the interval characterized by reddish to maroon dysoxic pe/ites interlayered with the usual grey ones in agreement with observations in the Lower part of the Monte del Casino I section (Vaiet al., 1993). The interval between samples T186 and T185 is about 8.5 m. The total thickness up to the 'Calcare di base' is slightly less than 50 m, to be compared to about 40 m totalling the distance from the first biotite-rich layer to the 'Calcare di base' in the Monte del

460

G.S. Odin et al.

Table 5 Results of 4~ GSO in Lausanne)

step heating measurements on biotite T185 (92.6 mg) from Pieve di Gesso (data by

~

Apparent age (Ma)

4-2o-

%39A1-

%rad.

K/Ca

650 750 850 925 975 1025 1065 1100 1140 1170 1230 1300

2.0 6.3 7.32 7.39 7.42 7.36 7.34 7.38 7.34 7.4 7.36 7.1

0.5 0.2 0.08 0.08 0.06 0.08 0.06 0.06 0.06 0.3 0.06 0.2

2.8 1.0 8.0 9.4 8.2 13.4 13.7 12.5 17.1 6.3 7.2 0.6

3 23 60 73 79 77 83 87 88 93 83 29

5 34 60 67 66 72 117 106 107 61 71

Step 1170~ is technically dubious due to gas lost during analysis (the apparent age is a maximum one). Preheating 450~ for 10 min under vacuum.

Pieve

850 ~

~-

925 ~

975 ~

_jF. . . .

'

1025 ~

1

'

1065 ~

t

1140 ~

11 O0 ~

b--

I

_

di

Gesso

1170 ~ 1230 ~

--iiii:iL

_

.....

plateau" 87.7 % t~

B

+1 r 3Z

6

r G) L

m

Q. 12.

T185 biotite 92.6 rng Total gas age : 7.2 M a Plateau a g e K20 %

I 9

.94. i o

;o

,o

I

: 7.37 + 0 . 0 6 (20) = 9.6

I 8 0'

9o

100

39At release Cumulative percent

Fig. 6. 4~

age spectrum for sample T185 biotite.

Casino-Monte Tondo area. Assuming a simultaneous occurrence of the first biotite-rich layer in the two nearby areas, the resulting 20% increase of the sedimentation rate would be consistent with (1) the regional increase of thickness of the Mamoso-arenacea

Integrated stratigraphy of the late Tortonian Pieve di Gesso section

461

turbidite body NW-ward, (2) the regional pinch-out of many arenaceous turbidite layers SE-ward (Ricci Lucchi and Valmori, 1980), and (3) the local occurrence of faint, fine-grained turbidites in the Pieve di Gesso western (relatively depressed) area at higher stratigraphic levels as compared with the eastern (relatively raised) area. The abrupt occurrence of volcaniclastic layers in about ten previously studied sections appears quite well confined within or close to the short biostratigraphic interval between G. humerosa (and/or G. suterae) and Amaurolithus sp. first occurrences. The local survey has shown that the first biotite-rich layer is found at sample T186; Amaurolitus sp. occurs immediately beneath; G. praemargaritae and G. humerosa are present in the upper T186-T185 interval together with Amaurolithus sp. Therefore, an assignment of both T186 and T185 layers to the 'first cluster' of similar horizons described by Vai et al. (1993) appears quite appropriate. Assignment to the second cluster cannot be excluded and closer correlation of individual layers on the base of pure biostratigraphic and field lithostratigraphic data is still difficult (Vai et al., 1993, p. 1411). The separated phases appear reliable geochronometers according to their location in dominantly volcanogenic layers; the actual significance of the layers is not well understood, but we suspect a near contemporaneity between volcanic eruption and inclusion in the sedimentary succession and no mixture with extraneous pyroclastic components. Biotite shows a good shape, favourable X-ray diffraction patterns, generally good homogeneity and high K content. T185 biotite appears a priori better than T186 biotite concerning these criteria. Geochronologically, the two successive layers give apparent ages at 7.47 (4-0.08 Ma, pseudo plateau) and 7.37 (4-0.04 Ma, excellent plateau). According to the age spectra and overall quality of the selected flakes, this is most probably representative of their respective crystallization age. The very good internal consistency of the apparent ages calculated from the heating steps measured for T185 and the external consistency of the two biotite separates versus the stratigraphical relation (7.47 > 7.37) is an additional criterion for the geological meaning of the measured ages. The comparison between the plagioclase and the biotite ages for sample T186 emphasizes both the inappropriate nature of the particular plagioclase separate as a geochronometer and the probable incorporation of extraneous argon in the mineral at the time of crystallization or eruption. The analytically precise ages of the biotite separates are related to the accepted value of 24.21 Ma for the monitor allowing calibration of the neutron flux. However, the actual age of HD-B 1 is known with an error bar (-1-0.6 Ma or +2.6% of the age, 2or) which should be propagated to the analytical age to derive realistic geological ages comparable to data obtained using other monitors or analytical techniques and methods. Accordingly, the radioisotopic ages of biotite from T186 and T185 are 7.47 4-0.28 and 7.37 + 0.26 Ma (2or interlaboratory), respectively. Comparison with the study by Vai et al (1993) can be made via the proposed respective stratigraphic position of the dated layers. The new 4~ ages obtained from biotite separates from samples T186 and T185 fall within the age range previously found for the group of biotite-rich layers close to the suterae FO (K-Ar conventional ages at 7.29 4- 0.76 Ma, 7.90 4- 0.58 Ma, and 7.75 4- 0.42 Ma). The mean age value of this early group of volcanic events located 10 to 13 m below the FO of G. conomiozea (7.72 5:0.30 Ma) is analytically consistent with our new analytical data. However, the

462

G.S. Odin et al.

younger group of volcanic events located about 5 m below the FO of G. conomiozea in the Monte del Casino and Monte Tondo sections has a mean age at 7.35 4- 0.16 Ma according to Vai et al. (1993) and is also consistent with our data from the Pieve di Gesso section. The favourable crystallographical nature, the geochemical homogeneity, and the nearly undisturbed plateau of sample T185 points to an acceptable reliability of the measured ages. The Pieve di Gesso ages indicate that the FO of G. humerosa is older than 7.4 to 7.7 Ma in this section and that the FO of Amaurolithus sp. is about this age; this conclusion is not analytically distinct from that obtained from the previously dated sections and confirms that the Tortonian/Messinian boundary is older than 7 Ma. SOMMAIRE - - STRATIGRAPHIE INTEGRI~E DU PROFIL TORTONIEN SUPI~RIEUR DE PIEVE DI GESSO (ROMAGNE, ITALIE) (Manuscrit soumis: Fgvrier 1994; rgvisg: Fgvrier 1995; r~dacteur responsable: GSO) Nous avons travaill6 sur la premiere section de la rfgion de Faenza (Romagne, Italie) dans laquelle des niveaux volcanoclastiques ont 6t6 identififs autour de la limite Tortonien/Messinien. Par comparaison lithostratigraphique et 6tude biostratigraphique, deux niveaux fiches en biotite ont 6t6 localisfs au sommet du Tortonien. L'ftude gfochronologique a d'abord permis de caractfriser ces niveaux ~ biotite; ce sont sont des turbidites - - c'est ~ dire des dfp6ts secondaires - - renfermant encore plagioclase, apatite et zircon; la biotite est bien prfservfe et gfochimiquement homog~ne. Les analyses isotopiques ont utilis6 la technique 39Ar/a~ avec chauffage par 6tape appliqufe deux 6chantillons l'un avec plagioclase et biotite, l'autre avec biotite seule. Le spectre d'~ge perturb6 du plagioclase a rfvf16 que ce gfochronombtre n'ftait pas favorable. La biotite correspondante a donn6 un spectre plus rfgulier avec presque un plateau (~ge 7,47 4- 0,08 Ma). La deuxi~me biotite a livr6 un plateau acceptable permettant le calcul d'un ~ge de 7,37 4-0,06 Ma (incertitude analytique interne). Ces ~ges confortent les rfsultats gfochronologiques de Vai et al. (1993) indiquant, pour la base du Messinien, dans sa dffinition originelle, un ~ge un peu supfrieur h 7,2 Ma. (Sommaire des auteurs) ACKNOWLEDGEMENTS The radiometric measurements were achieved in Lausanne thanks to the facilities made available in the Institute of Mineralogy by the Swiss National Funds. No funding was provided by French organizations for this research.