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Complex bedding geometries in proximal deposits of the Castelnuovo Member, Rocchetta Formation, Tertiary Piedmont Basin, NW ltaly R.D.A. Smith KSEPL, Shell Research BV, Volmerlaan 6, 2288 GD Rijswijk, The Netherlands

lntroduction The Castelnuovo Member of the Oligo-Miocene Rocchetta Formation of the Tertiary Piedmont Basin, NW Italy (Fig. 35.1) is a relatively small 02km lang) sandstone-rich turbidite system. 1t developed in a marine basin which was characterized by intrabasinal topography caused by tilting of basement fault blocks. This is manifest in the deposits by tabular sandstone bodies showing onlap relationships with syndepositional intrabasinal slopes ( Cazzola et al. 1981, Cazzola, Mutti and Vigna 1985). The Rocchetta Formation contains turbidite sandstone bodies enclosed in a mudstone background. Sandstone bodies lowest in the succession exhibit strong lateral restriction and onlap against syndepositional slopes at the margins of sub-basins. Later sand-rich systems tend to be larger and were less affected by basin-floor topography (Fig. 35.2). This contribution presents geometric information at the bed scale from sandstone bodies in the proximal portion of one of the later systems, the Castelnuovo Member (Cazzola et al. 1981), Photostratigraphie Unit C of Sgavetti 0991).

Two exposures which allow good lateral control of bedding geometries are described and interpreted here. One exposure, approximately 1 to 2 km from the apex of the turbidite system, exhibits complex, shingling, pinch and swell bedding geometries in both structureless sandstones and associated mudstones with thin sandstones. A second exposure, approximately 3 km from the system apex, exhibits bed geometries that are less variable laterally, but still show subtle depositional relief, scouring and soft­sediment deformation features.

The Castelnuovo Member Deposits of the Castelnuovo Member of the Rocchetta Formation can be mapped over about 12 km in a SW-NE direction up to 4 km in SE-NW direction (Fig. 1). The unit reaches approximately 60--70m thick (Cazzola et al. 1981). Palaeocurrents record flow towards the north in proximal parts of the system and towards the NE in more distal deposits (Fig. 35.1). Amalgamated conglomerates and pebbly sandstones with erosive bases are patchily exposed in the most proximal part of the system, adjacent to a basin­bounding fault. Tabular alternating sandstone and mudstone deposits are exposed in the distal part of the unit, towards the northeast.

Two exposures provide good lateral control of bedding geometries at a scale of approximately 100m. These are 0) a roadside exposure araund a hairpin bend between the villages of Castelnuovo and Priero and (2) a roadside exposure west of the village of Strada (Fig. 35.3).

Lithofacies in the Castelnuovo Member have been described by Cazzola et al. 0981). Conglomerates with erosive bases and normal grading are exposed in the vicinity of the village of Castelnuovo. Over most of the exposure, however, normally graded or structureless sandstones are present, vertically separated by finer­grained intervals of mudstones with thin sandstones. The latter only rarely exhibit Iamination which is defined by plant debris where present.

Sandstones are typically highly bioturbated. Trace fossils include the following forms: Ophiomorpha, 1balassinoides, Muensteria, Palaeophycus, Planolites, Rhizocorallium and Zoophycos with rare Chondrites and extremely rare Paleodictyon (Smith 1990).

Hairpin exposure

Descrlption The section, located approximately 1 to 2 km from the system apex, is approximately 5m thick, 100m lang and is sigmoidal in form (Fig. 35.4). A fine-grained interval with wedging sandstones is overlain by an internally complex thickening-up bundle of medium to thick-bedded sandstones (Units A to D in Figs 35.5 and 35.6 (a) and (b)). This sandstonepacket maintains a

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, R.N. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

nearly constant thickness of 2.65 to 2.8m along the outcrop. Individual beds, however, pinch and swell. Sandstone bed thickness ranges from approximately 1 cm up to 2.15m.

A subtle mound can be seen in mudstones with thin sandstones beneath Unit A between stations 27 and 37 (Fig. 35.5). Unit A thickens away from this positive feature. Compensational thickening and thinning can be seen within the composite Unit A at the eastern end of the exposure. Unit B, which is also composite varies dramatically in thickness, its thickest portians being offset with respect to the thickest parts of Unit A. Unit D compensates in thickness for the underlying Unit C. At its top is a mudstone-filled scour, 0.5 m deep and 1. 5 m wide, which records the passage of a bypassing current.

Interpretation of bedding geometries Lateral thickness variations in individual beds could be produced in the following ways:

(a) compensation (by depositional smoothing) for depressions on the sediment surface;

(b) differential deposition i.e. mounding. This is clearly documented in the Laga Formation of central Italy (Mutti, Nilsen and Ricci Lucchi 1978). Subtle depositional relief at the top of modern depositional sandstone lobes of the Urnnak Turbidite System, Bering Sea, has been recently documented by Kenyon et al. 0990) in the form of long-wavelength, very low­amplitude braid bar-like bedforms. Such low­relief bedforms may be common at the tops of turbidite sheet sands.

(c) Erosional cuts at the tops of sheet sands generated by bypassing currents, commonly in the form of flute-shaped scours, (mud-draped scours of Mutti and Normark 1987).

In the hairpin exposure there is evidence of processes (a) and (b), but not (c) with the exception of a bypassed scour at the top of the sandstone packet (Figs 35.4 and 35.5). Comparable 'shingling' geometries have been previously described at a larger scale by Enos 0969). At a more comparable scale pinch and swell of fine-grained turbidites can be seen in the Lower Silurian Y strad Meurig Grits of the Welsh Basin

(Smith, Waters and Davies 1991). Since there is no measurable erosion at bed bases and no systematic axis-margin relationships the exposure is unlikely to represent a channel-fill.

Strada exposure

Descrlption The measured section is approximately 6 m thick, 130m long and slightly sinuous in form. Sandstones are typically normally-graded with grain sizes ranging from granule grade to medium sand.

The first impression of the exposure is one of great lateral continuity and tabular geometries of beds (Figs 35.6 (c) and (d). However, on closer inspection, significant lateral variations are apparent. The following features can be observed:

(a) thick beds are composite, individual beds ranging from 5 cm to 1 m (Units A to C, Fig. 35.7). In the upper, approximately 2m thick, amalgamated unit (Unit C) discontinuous mudstones occur as erosional relics;

(b) depositional relief occurs at the tops of some sandstone beds, in one case (at the base of Unit C) approximating asymmetric dune form sets (although no internal structures are apparent), in another (Unit B) taking the form of a subtle mound;

(c) soft sediment folding is locally present, small wavelength at bed contacts within units A and B and Ionger wavelength within Unit A (Fig. 35.7);

(d) lateral wedging outofthin beds occurs; (e) deeper erosional scour is in evidence in the

lower part of the section. An interval of interbedded sandstones and siltstones is replaced laterally by a body of amalgarnated sandstones.

Interpretation of bedding geometries The strata exposed along the section are interpreted to be mainly sandstone lobe deposits in the sense of Mutti and Ghibaudo 0972) (Cazzola et al. 1981). They are primarily the deposits of unchannellized high­concentration sediment gravity flows poor in mud. The surfaces of some of the resulting sheet-like deposits were gently mounded, in one case (at the base of Unit C)

Study area

Genova

30 km

",....- Onlap stratal p1nchout accordmg 10 Sgavett1 1991

y Sinke and d1p duect10ns

/ Faults

D Turb1d1le sandstone bod1es

D Marine Mudstone

Alluv1allan and fan della conglomerates

c:::::::> General palaeocurrent d1rect10n

Fi . 35.1. Loc:Hion map and geological :kctch map ·howing

distribution of sandstonc members in the R ·chetw Formation,

palae currem dir ction:-. and onlap. C after azzola et ul L 98 I anti

gaveni 1991 ).

L

1000 m

500

0 0 5 10 km

Fig. 35.2. Cross-section through the basin-fill showing the relationship between the distribution of sand-rich turbidite and tilted basement fault blocks (after Cazzola et al. 1981 and Sgavetti 1991).

Fig. 35.3. lap sh ' ing locati m:-. < f two det·riled ·e tions

in Lhe ' IStelnu0\'0 lembers ur Lhe Roccheua Formation

pre. enl ·d in Fig . .)S . .:; anti 3S.6.

Palaeocurrents. n = 4

0 10m t I , I I I I I I I I

Fig. 35.4. Map of the 'Hairpin Section ' exposure (see Fig. 35.3), showing locations of measured profiles (Fig. 35.5) and palaeocurrent directions measured from sole structures.

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being ornamented by regular long-wavelength (approximately 20m), low relief (0.35 m) asymmetric bedforms.

The basal part of the section contains part of a small-scale channel-fill. Whilst broad scours at the bases of tabular sandstone bodies, with depths of erosion up to 3m, have been documented in the Miocene Laga Formation of centralltaly (Mutti, Nilsen and Ricci Lucchi 1978) and the Marnoso Arenacea (Ricci Lucchi and Pignone 1978), the Strada section differs in that it exhibits filling of a depression by multiple events.

Conclusions Proximal sandstone lobe deposits of the Castelnuovo Member exhibit lateral variations in bed thicknesses and degree of amalgamation over lateral distances (oblique to palaeoflow) of tens of metres. These result from lateral variations in the depth of scour beneath individual turbidity currents and lateral variations in the thickness of sand deposited from sheet flows.

Acknowledgement The work presented here was carried out in the summer of 1987 as part of a field season funded by a Tarquin Teale Award.

Heferences Cazzola, C., Fonnesu, F., Mutti, E. et al. 1981. Geometry and

fades of small, fault-controlled deep sea fan systems in a transgressive depositional setting (Tertiary Piedmont Basin, Northwestem Italy). In: Ricci Lucchi (ed.) International Association of Sedimentologists, 2nd European Regional Meeting, Bologna, 1981. Excursion Guidebook, pp. 7-53.

Cazzola C., Mutti, E. and Vigna, B. 1985. Cengio Turbidite System, Italy. In: Bouma, A.H., Normark, W.R. and Bames, N.E. (eds) Submarine Fansand Related Turbidite Systems. Springer-Verlag, New York, pp. 179-183.

Enos, P.E. 1969. Anatomy of a flysch. Journal ofSedimentary Petrology, 39, 680-723.

Kenyon, N., Hunter, P., Weaver P. et al. 1990. High input and low input turbidite systems: examples from the Indus Fan and the Bering Sea basin. Abstracts of the 13th International Sedimentological Congress, Nottingham, England, p. 265.

Mutti, E., Nilsen, T.H. and Ricci Lucchi, F. 1978. Outer fan depositional Iobes of the Laga Formation (Upper Miocene and Lower Pliocene), east-central Italy. In: Stanley, D.]. and Kelling, G. (eds) Sedimentation in Submarine Canyons, Fans, and Trenches. Dowden, Hutehinsan &

Ross, Stroudsburg, Pa, pp. 210-223. Mutti, E. and Normark, W.R. 1987. Comparing examples of

modern and ancient turbidite systems: problems and concepts. In: Leggett, ].K. and Zuffa, G.G. (eds) Marine Clastic Sedimentology. Graham & Trotman, London, 1-38.

Ricci Lucchi, F. and Pignone, R. 1978. Ricostruzione geometrica parziale di un lobo de conoide sottomarina. Memoria della Societa Geological Italiana, 18, 125-133.

Sgavetti, M. 1991. Photostratigraphy of ancient turbidite systems. In: Weimer, P. and Link, M.H. (eds) Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems. Springer-Verlag, New York, pp. 107-125.

Smith, R.D.A. 1990. An atypical 'shallow marine' trace fossil suite in turbidite systems of the Tertiary Piedmont Basin. Abstracts ofthe 13th International Sedimentological Congress, Nottingham, England, pp. 509-510.

Smith, R.D.A., Waters, R. and Davies,]. 1991. Upper Ordovician and Lower Si/urian Turbidite Systems of the Welsh Basin. Field Guide No. 20, The International Sedimentological Congress, Nottingham, England, 1990, 45pp.

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Measuring Stations (see Figure 4 for locations)

1 5 10 15 20

Mudstone-tilied scour Bed compensates for underlying depositional relief

I I 1

1 r

Oblique to palaeoflow Near-perpendicular to palaeoflow

Grientation of the exposure with respect to mean palaeocurrent direction

Fig. 35.5. Outcrop panel for the 'Hairpin Section' showing shingling geometries. See Fig. 35.3 for location and Fig. 35.4 for map of exposure and locations of measurement stations.

25

Vertical exaggeration = x5

Oblique to palaeoflow

~ J

True scale

30 35 40 45

~ Covered

~'----

Near-perpendicular to palaeoflow

Fig. 35.6. (a) View of northern part of the 'Hairpin Section'. (b) Detail of shingling in the northern part of the 'Hairpin Section'. (c) and (d) Views of the exposure west of Strada.

(c)

(d)

1ao ---------Oblique to palaeoflow -------------- 360 160- Near-perpendicular to palaeoflow - 340 190 -- Oblique to palaeoflow -- 010

m Grooves ndicate I I --- - - Co~,., 040 palaeollow

I ""' 6_,.&. - - - - - - 0' 6

........... -:---

AmalgamatiOn Scour _/ Gap in exposure -- --:;:;::::-Amalgamation --- Co~ I

~ · ...- ...........

1 Wavelength is c 20 l ........._ ered

Losding al base ol be<i -- =>---- ~ Ama gamat1on --- - - -_L---- - - - -

~ assumlng 040 palae~ .,._ .... ;;i' thin t;;d 1'--- Wavybed - - - - -- - - - -

5

© 4 --• • • • - - - - =• • ~- - - - -

~ .. - - - -I - - - -- -

I - - - - - --- - - - - -=· - - - - - - - -I - - - - -Amalgamation _Soft sed ment ~!:, .......... --at contact two beds

- -Subtle depositional reliel - - -- -- - -- -

3

® - - -- -- - -- -

2 - - - - -- - - - -"'-'-' 1'- ............ ~

_!)<~!~ -- Soft sedlment .,.,,.,.m .. u"" beds ...... ~ - - -r-- Soft sediment - - -®

~~ 1 Scour .../ - delormatlon - - --- --? - - ---.::::: ..... --- ~ ___.,. ..? ----- - - ----- · - ~e\ ~ -- Multl-event -7 Roed\e amalgamated

Road Ievei sandstone body Scour --0 -Cavared Covered Covered ·-I I I

0 10 20 30 40 50 60 70 80 90 100 110 120

Vertical exaggeration = x5

True scale

-------------------~

Fig. 35.7. Outcrop panel from road section west of Strada. See Fig. 35.3 for location.