FIRST DATA ON CLAY MINERAL ASSEMBLAGES AND GEOCHEMICAL CHARACTERISTICS OF TOARCIAN SEDIMENTATION IN THE UMBRIA- MARCHE BASIN (CENTRAL ITALY)

Clay Minerals (1993) 28, 297-310 FIRST DATA ON CLAY MINERAL ASSEMBLAGES AND GEOCHEMICAL CHARACTERISTICS OF TOARCIAN SEDIMENTATION IN T H E U M B R I ...
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Clay Minerals (1993) 28, 297-310

FIRST DATA ON CLAY MINERAL ASSEMBLAGES AND GEOCHEMICAL CHARACTERISTICS OF TOARCIAN SEDIMENTATION IN T H E U M B R I A MARCHE BASIN (CENTRAL ITALY) M. O R T E G A - H U E R T A S ,

P. M O N A C O * A N D I. P A L O M O

Departamento de Mineralogia y Petrologia, 1AGM, Universidad de Granada, Spain and *Dipartimento Scienze della Terra, Universitgtdi Perugia, Italy (Received 19 December 1991; revised 21 September 1992) ABSTRACT: A moderate to high rate of clay sedimentation is characteristic of the Toarcian sequences in the Umbria-Marchebasin. The clay mineralassemblagesand geochemicalcharacteristics indicate that the Marne di Monte Serrone Formation and the Rosso Ammonitico UmbroMarchigianoUnit were depositedin a shallowmarineenvironment.In this generalpalaeogeographic scheme, a reducing subenvironment must have exsisted in which black shale-type facies were deposited resultingin geochemicalanomaliesin As, Sb, Zn, Co, Cu, Pb, V, Cr, and Ba, amongother elements. The high values of both the detrital index and the Ce/Ce* ratio reveal the influenceof proximal emerged reliefs. It is suggestedthat the palaeosoilsthat developedon the Liassiccarbonate Laziale-Abuzzeseplatform were the source area of these sediments,followinga palaeogeographic scheme analogousto that proposed for the Betic Cordilleras(Spain)duringthe MiddleDomerian to Middle Toarcian. The positiveand negative Eu anomaliesare due to the decisiveinfluenceof the weathering process in which sedimentswith heterogeneousEu anomalysize were mixed. The final distributionpattern of REE is the resultof the differentenvironmentsto whichthe claymineralswere subjected and the differencesin intensity of weathering.

The Toarcian sequences of the Umbria-Marche basin are characterized by a moderate to high rate of clay sedimentation. According to the variability of local subsidence and bottom morphology, "condensed", "intermediate" or "extended" clayey sequences were formed (Colacicchi et al., 1988). The "condensed" sequences were deposited in areas of elevated structural zones where the formations are absent or very thin. "Intermediate" sequences, which are defined as normal because they are very c o m m o n , were located on slightly tilted fault blocks, forming wide, relatively stable structural surfaces, and on the slightly sloping sea-bottom. The extended sequences, which present the greatest thickness and probably the deepest facies, are scarcer than the intermediate ones; they indicate areas with rapid subsidence and represent structural, but not morphological depressions. During the Toarcian, a b u n d a n t clay materials entered the basin and were deposited in this type of depression. Some of these argillaceous sequences contain a b u n d a n t detrital beds 0.5-100 cm thick. In some cases these detrital intervals are represented by calcareous turbidites showing the classic B o u m a BC sequence. In other cases, sharp-based hummocky cross-stratified (HCS) calcarenites are interbedded or overlie turbiditic materials (Monaco, 1992). Locally, slumps and pebbly mudstones are present. The depositional e n v i r o n m e n t of HCS beds usually seems not to exceed the major storm wave base depth, unlike turbiditic deposits. Vertical 9 1993 The MineralogicalSociety

M. Ortega-Huertas et al.

298

transition from turbidites to H C S deposits should therefore prove to be useful in the reconstruction of the variations in basin depth and the relations with tectonics and sea-level changes in regressive shelf sequences. The detailed study of mineralogical characteristics, particularly of clay minerals and of the geochemistry of the M a r n e di M o n t e Serrone (MS) F o r m a t i o n and the Rosso A m m o n i t i c o U m b r o - M a r c h i g i a n o ( R A U M ) Unit has contributed significantly to the exact determination of the depositional environment and the p a l a e o g e o g r a p h y of the depositional basin. GEOLOGICAL

SETTING

AND STRATIGRAPHY

The U m b r i a - M a r c h e (U-M) basin is located in the N o r t h e r n A p e n n i n e s (Central Italy) between the front of the T r a s i m e n o - F a l t e r o n a - C e r v a r o l a nappe and the Pliocene A d r i a t i c f o r e d e e p (Fig. l-I). It consists of a Mesozoic and Cenozoic carbonatic-siliciclastic sequence which varies greatly in thickness from South to North (between 2000 m and ~3500 m) and

m

A

B

C

D

E

F

G

H

t

FI6. 1. Location of the sequences studied. I. Schematic geological setting of the Umbria-Marche area (Monaco, 1989). A: Tosco-Emiliani Allochthonous Complex; B: Toscana facies; C: Trasimeno-Falterona-Cervarola allochthonous sediments; D: Mesozoic and Cenozoic sediments of the Umbria-Marche basin; E: Upper Tertiary syn- and postorogenic siliciclastic rocks; F: Laziale-Abruzzese carbonate platform; G: PliocenePleistocene postorogenic sediments; H: Tosco-Laziale volcanic province; L: Major thrusts; AN: Ancona; PG: Perugia; TR: Terni. II. Jurassic outcrops and sequences studied (Monaco, 1992). 1: Jurassic sediments including the COR, RAUM and CP Units, and MS Formation from Sinemurian to Aalenian. 2: Early Jurassic carbonate platform (Hettangian to Sinemurian). 3: Major thrusts. 4: Studied sequences. VD: Valdorbia; PLV: Pale Vallone; MS: Monte Serrone; PO: Pozzale.

Clay minerals, geochemistry and sedimentation

299

is at present organized in an acute fold belt. The basin was rapidly downwarped in the Early Lias, when extensional faulting fragmented a nearly isolated passive continental margin (a large southern Tethyan carbonate platform) commonly referred to as Adria (Channell et al., 1979). This brought about a turning point in the type of Liassic sedimentation, changing from neritic-type carbonates (Calcare Massiccio Formation), where sediment production occurs mainly on the shallow-sea bottom, to sediments produced in the water mass (from the Corniola to Calcari a Posidonia Units) (see the extensive literature summarized in Colacicchi et al., 1970, 1988; Centamore et al., 1971; Farinacci et al., 1981; Farinacci & Elmi, 1981; Cresta et al., 1988, 1989). The marly and marly-argillaceous Marne di Monte Serrone (MS) Formation (Fig. 1-II) (Late Domerian-Middle Toarcian) (Pialli, 1969) represents a moderate to thick clayey deposit. Black shales with organic rich intervals formed during the "Toarcian anoxic event" (Early Toarcian) are common (Jenkins & Clayton, 1986; Jenkins, 1988; Bartolini et al., 1992). The MS unit ranges in thickness from a few metres to 80 m in the intermediate and extended sequences of the Umbria-Marche area. The microfacies (Cresta et al., 1988) indicate a circalittoral to upper bathyal environment in the type section (Monte Serrone). The deposition depth and the thickness of the MS Formation are consequently extremely variable in the Umbria-Marche area and reflect a differentiated topography of the basin. The MS Formation is overlain by reddish nodular marls with ammonite-rich horizons of the Rosso Ammonitico Umbro-Marchigiano (RAUM) (Colacicchi et al., 1988), which is mainly Middle-Late Toarcian in age (Cresta et al., 1988; Cecca et al., 1990). The microfacies of the RAUM in general indicate a shallower and more oxygenated environment than that of older deposits, which range from the Late Sinemurian to the Early Toarcian. Figure 2 shows the location of the samples studied in stratigraphic correlation between Pozzale (PO), Valdorbia (VD), Monte Serrone (MS) and Pale Vallone (PLV) sequences. Thirty two representative samples were studied in palaeontologically well-dated stratigraphical sequences, thus premitting accurate correlations between them.

METHODS The whole samples and clay fractions were examined by X-ray diffraction (XRD) using a Philips PW 1710 diffractometer equipped with graphite monochromator and automatic slit (Department of Mineralogy and Petrology, University of Granada). The reflecting factor, calculated for this equipment and its instrumental conditions on the basis of the data by Schultz (1964) and Barahona (1974) were for random powder samples--phyllosilicates, 0.09; quartz 1.43; calcite 1-05; dolomite, 1.08; feldspar, 1-03; for oriented powder samples--illite, 0.36; smectites, 0.93; chlorite, kaolinite, 0.98. The morphological study of the minerals was carried out by scanning electron microscopy (SEM) using a Zeiss DSM 950 (Technical Services of the University of Granada). The microanalyses of the clays were obtained using a Jeol JSM-820 fitted with a Link Analytical microanalysis system (University of Cadiz, Spain). Analyses of the major and minor elements and the rare-earth elements (REE) were carried out using X-ray fluorescence (XRF), neutron activation (NA), inductively coupled plasma (ICP) and atomic absorption spectrometry (AAS) at the X-Ray Assay Laboratories in Ontario, Canada.

300

M. Ortega-Huertas et al.

MINERALOGY A graphic representation of the results obtained is presented in Fig. 3. The whole mineralogy consists of calcite, feldspars, quartz and clay minerals; gypsum (VD sequence), or dolomite and celestite (PLV sequence) were detected exceptionally in some samples. The clay minerals found in the sequences and lithofacies studied were illite (I), smectites (Sm) and kaolinite (K). Chlorite (Chl) was detected exceptionally in the PLV-11.5 sample in quantities < 5 % (Fig. 3). The Marne di Monte Serrone Formation is characterized by the assemblages illitesmectite-kaolinite. Illite is the most abundant mineral, its mean proportions ranging from 76% (VD and MS sequences) to 93% (PLV sequence). Scanning electron microscopy suggests the existence of inheritance and diagenetic transformation processes, following the hypotheses of Pollastro (1985) and Ortega Huertas et al. (1991a). Dioctahedral smectite (Fe/A1 = 0-4; A I > F e > M g ) is the next most abundant mineral and is present in all the stratigraphic sequences, except the PLV sequence. The mean contents and range of variation are: VD sequence (19%, 7-25%), PO sequence (13%, < 5 - 2 3 % ) , MS sequence

~GE

POZZALE

VALDORBIA

TO5

MONTE SERRONE

PALE V A L L O N E

160~ ...... ~ * 161 (Aalenian)

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185

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360-

* 359.8 9 362.8 * 364.15

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"MJtmedi M.Sen'one"Fm. MS)

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9 8.60 samples

Fro. 2. Stratigraphy of the sediments studied. Informal subdivisions of the Domerian and Toarcian intervals are after Cantaluppi (1970) and Venturi (1990). a: The sequence is continuous and the difference in metres is not real but is due to the paleomagnetic sampling that is indicated here (Channell et aL, 1979).

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Clay minerals, geochemistry and sedimentation

301

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(18%, 10-30%). Kaolinite is also present in all samples studied, the mean contents usually being not > 7 % , although they can reach 17% (PO-25). In some sequences, the black-shale lithofacies appears within the Marne di Monte Serrone Formation. It is characterized by large quantities of illite, ranging from 70-76% in the VD sequence and from 70-85% in the PO sequence; significant amounts of smectite in the VD sequence (17-25%, average value 22%) and in the PO sequence (10-22%, average value 17%), and minor quantities of kaolinite, which are invariably < 5 % , except for sample PO-7.40, whose content is 8%. Overall this lithofacies is homogeneous from a mineralogical point of view. By SEM the smectite presents characteristic crinkly, ridged, honeycomb-like textures. Figure 4 represents a general view of the quantitative variations of the whole mineralogy and the clay minerals during the Early Toarcian in the different stratigraphic sequences studied. The mean calcite content varies from 30% in VD to 81% in PLV, and is very similar in the PO (39%) and MS (41%) sequences. The values of the variation coefficient (V) reveal great diversity in the calcite content, which is high in the VD sequence (V = 55), normal in PO (V = 17) and low (V = 8) in the MS and PLV sequences. The detrital contribution (quartz, feldspare and most of the clay minerals) is more homogeneous throughout the different stratigraphic sequences. The illite proportion is very homogeneous in all the sequences, whereas those of smectite and kaolinite are very heterogeneous within all the sequences and also comparatively between them. The Rosso Ammonitico Umbro-Marchigiano Unit shows a very homogeneous clay

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50 25+ 1~55 ~ 2 3

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Fl6. 4. Quantitative variations in the whole mineralogy(A) and the clay minerals (B) in the MS Formation duringthe EarlyToarcian. The standard deviation is indicated by the bar, the arithmetic average by the cross-bar and the coefficient of variation by number. 1: calcite; 2: quartz; 3: clay minerals; 4: illite; 5: kaolinite; 6: smectite. VD, PO, MS and PLV: stratigraphic sequences.

Clay minerals, geochemistry and sedimentation

303

m i n e r a l c o n t e n t w h e t h e r its l i t h o l o g y is c a l c a r e o u s or m a r l y - l i m e s t o n e (Fig. 3). This facies is v e r y largely m a d e up of illite ( f r o m 82% in P O to 92% in P L V ) , with m i n o r p r o p o r t i o n s of smectites ( 5 - 1 0 % ) and k a o l i n i t e ( 5 - 8 % ) . This fact, t o g e t h e r with the a f o r e m e n t i o n e d m i n e r a l o g i c a l h o m o g e n e i t y o f the c a r b o n a t e , q u a r t z and clay m i n e r a l c o n t e n t , s e e m s to indicate s e d i m e n t a t i o n o c c u r r i n g u n d e r h o m o g e n e o u s p a l a e o g e o g r a p h i c and t e c t o n i c conditions, in which m o d i f i c a t i o n s to the s e d i m e n t a r y p a r a m e t e r s w o u l d h a v e c a u s e d the p r e s e n c e o f c a r b o n a t e o r m a r l y levels.

GEOCHEMISTRY T h e c h e m i c a l analyses of the w h o l e samples are p r e s e n t e d in T a b l e s I and 2. In g e n e r a l , the trace e l e m e n t s are m a i n l y c o n c e n t r a t e d in the clay fraction, e x c e p t for Sr, Y and Zr. T h e Sr must be r e l a t e d to the a b u n d a n c e of c a r b o n a t e s , and the Y and Z r are m a i n l y t r a n s p o r t e d with the accessory m i n e r a l s (especially zircon). It is i m p o r t a n t to p o i n t out the significant positive c o r r e l a t i o n s h o w n by A s , Sb and Z n contents. T h e highest values a p p e a r in the s a m e samples ( f r o m V D - 3 6 8 . 5 0 to VD-362.80, P O - 7 , P O - 7 . 4 0 , P L V - 8 . 8 and P L V - 9 . 5 0 ) , all of which can be stratigraphically c o r r e l a t e d within the E a r l y T o a r c i a n . T h e s e g e o c h e m i c a l a n o m a l i e s are also clear for the s a m e TABLE1. Chemical analyses of the whole samples (wt%). Samples

Lithology* SiO2 A1203 CaO

MgO

NaaO

KzO

FezO3 MnO

TiO2

P205

LOI

VD-161 VD-185.30 VD-180 VD-177 VD-203.30 VD-362.80 VD-364.15 VD-368.50 VD-371

3 2 2 2 1 1 1 1 1

19.00 4-11 38.60 26-00 7.70 29.50 11.70 3.28 44-40 29.10 8-44 27-20 30-10 7-91 27.00 51-10 12-90 6-83 48.50 12.30 7.44 41.70 10.50 14.90 29-10 7.19 29.10

1.27 1.57 1-21 1-65 1.71 2.11 2-05 1.97 1.41

0.09 0.22 0.08 0.18 0.24 0-29 0.28 0.28 0-17

1.12 2-69 1.00 3-15 2.66 4-49 4.79 3.99 2-74

1-84 4-68 1.27 4.71 3-19 5.11 5.13 5-10 2.98

0.04 0.02 0.03 0.02 0.03 0.04 0.06 0.15 0.25

0-20 0-42 0-17 0.49 0.43 0.76 0-76 0-61 0.41

0-09 0.10 0.06 0.14 0.08 0.09 0.11 0.19 0-07

33.80 27.20 37-20 25.20 26-80 16-50 17.60 18.60 25.20

PO-13 PO-9.65 PO-9.40 PO-7.40 PO-7 PO-4

1 1 1 1 1 1

36.70 35.60 27-00 42.30 46.40 36.60

9-72 10.20 7.68 11.50 12-50 9-42

20-20 20.90 29.70 14.30 10-80 19.40

2-04 1.79 1.64 2.19 2-28 1.91

0.27 0.19 0.16 0-21 0.19 0.17

3.32 3.72 2.79 4-19 4-59 3.18

3.80 5-51 3-88 4.33 4.35 3-33

0.41 0-08 0.09 0-22 0.17 0-56

0-58 0.59 0.42 0.64 0-73 0.55

0.10 0.10 0.07 0.12 0.10 0.09

21-80 21.70 26.90 19.20 17-80 24.10

MS-60 MS-52 MS-40 MS-27 MS-8.60

1 1 l 1 1

36-10 9.75 35.70 9.85 37.10 9-51 30.70 7-60 39-60 11-30

22.10 21-80 20-80 27.00 17.60

1.90 1-93 1.95 1.63 2.08

0.20 0-24 0.30 0.18 0.28

2-87 2.91 2-82 2.11 3.36

3.75 3.86 4.47 3-19 4-38

0.02 0.03 0.06 0-06 0.03

0.50 0.51 0-54 0.41 0-61

0.30 0.41 0.09 0.08 0-09

22-80 22.90 22.80 26.70 21.10

PLV-13.70 PLV-11.5 PLV-10 PLV-9.50 PLV-8.80

2 2 1 1 1

18.50 22.40 21-90 25.70 40.10

37-90 32-30 34-90 31.40 16.70

1-20 1.59 1.29 1.47 2.50

0.14 0.17 0.20 0.14 0-28

2-04 2.54 2.45 3-29 4.76

2.91 3.63 2-08 2.59 4.88

0.03 0-03 0-03 0.03 0.02

0.31 0.38 0.36 0-43 0.70

0-08 0-07 0.07 0.09 0-13

31-50 29.30 29.80 26.40 16.20

5-76 7-12 6-88 7-94 12.70

* 1: Marne di Monte Serrone Formation; 2: Rosso Ammonitico Umbro-Marchigiano Unit; 3: Calcari a Posidonia Unit.

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