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Erosional, depositional and post -depositional features of a turbidite channel-ftll, Jurassic, Neuquen Basin, Argentina R.D.A. Smith1 and L.A. SpallettP 1Kaninklijke/Shell Exploratie en Produktie Laboratorium, Shell Research BV, Volmerlaan 6, 2288 GD Rijswijk, The Netherlands 2Centro Investigaciones Geologicas, Universidad de La Plata, Argentina

lntroduction This contribution describes and interprets features of a turbidite channel-fill in basinal fades of the Lower­Middle Jurassie Los Molles Formation, part of the syn­rift stage in the evolution of the Neuquen Basin, western Argentina (Mitchum and Uliana 1985; Uliana and Biddle 1988; Uliana et al. 1990).

The Neuquen Basin (Fig. 24.1) is one of the most prolifie hydrocarbon produdng basins in Argentina. The basin evolved from a late Triassie to early Jurassie rift through a late Jurassie to early Cretaceous sag phase (Mitchum and Uliana 1985), and subsequently into a late Cretaceous foreland basin (Barrio 1990). The basin is bounded to the NE by the Sierra Pintada Massif and to the SE by the North Patagonian Massif. The present western border to the preserved basin-fill is formed by the fold and thrust belt of the Andean Cordillera (Fig. 24.1).

The deep-water fades discussed here occur basinwards of the north-east-south-west-trending south-eastern margin of the basin. This trend is reflected in both the gross distribution of lithofades (Mitchum and Uliana 1985; Uliana and Biddle 1988) and our palaeocurrent and palaeowave data from the shallow marine Lajas Formation whieh overlies the Los Molles Formation (Fig. 24.2).

Mesozoie deposits of the study area have been deformed by compressional tectonies into open folds trending from N-S to E-W. E-W structures are assodated with features indieating a transpressional origin at a constraining bend in a local dextral strike-slip system (e.g. Bettini 1984; Ploszkiewicz et al. 1984).

Figure 24.3 summarizes the Jurassie stratigraphy of the Neuquen Basin-fill (based on Gulisano et al. 1984, modified). Triassie volcaniclastic deposits of the Choiyoi Group are overlain by the Lapa and Chachil Formations and then by dark mudstones containing turbidite sandstone bodies at the Los Molles Formation. An upward-shallowing succession from the deeper marine Los Molles through shallow marine Lajas to continental Chacaieo Formation is diachronaus from south to north due to progradation of the basin margin during much of Jurassie time.

The channel-fill described and interpreted here occurs within the Los Molles Formation whieh is of Toardan to lowermost Bajodan in age within the study area. It is partially exposed in a roadcut at Fortin Primero de Maya, c. 65 km south-west of Zapala (Fig. 24.2). Palaeocurrents measured from flutes and grooves indicate a SW-NE orientation of the channel­fill, with flow towards the north-east, parallel to the strike of the SE Basin margin (Fig. 24.2).

Channel-fill and sandstone lobe deposits in the Los Molles Formation have been identified from cores and wireline log signatures in an exploration well, Barda Colorada Este (Hinterwimmer and Jauregui 1984), located 30 km south-west of Plaza Huincul, some 120km ENE of the Fortin Primerode Maya exposure described here.

Sedimentology of the Fortin Primero (1 °) de Mayo channel-fill

Lithofacies

Sandstones and pebbly sandstones The sandstones and pebbly sandstones in the channel­fill exhibit normal grading or are non-graded (Fig. 24.5). Floatingmudstone rip-up clasts are common. Bed thieknesses range from c. 0.3 to 1.6m. In the lower part of the section bed bases are erosive and lateral thiekness changes marked, but in the higher part of the sandstone body, where not deformed by

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

wet-sediment folding, beds tend tobe closer to parallel sided and may be separated by thin mudstones (Figs 24.5 and 24.9).

Mud-clast conglomerates Intraformational mud-clast conglomerates occur as lenticular bodies with widths of 4-11 m and thieknesses of up to 1m (Figs 24.6-24.8). They have concave-up bases and smooth or irregular top surfaces. They are interpreted to be basal parts of the deposits of high density turbidity currents a short distance downstream from an area of active erosion of a mud substrate.

Mudstones Mudstones occur: 1. in the section beneath the channel-base; 2. asthin layers within the channel-fill sandbody, both laterally extensive (tens of metres) and laterally discontinuous; 3. as a thieker unit above the channelized sandbody representing channel abandonment. This last unit contains sandstone injection structures and is locally, above vents in the underlying sandstone body, intensely deformed.

Mudstones were probably deposited from the wakes of currents whieh deposited sand in the channel and, in at least one case, perhaps from flows which were travelling too quiekly to deposit coarser grain sizes from the body.

Erosional structures

Three types of erosional structure are present in the channel-fill: 1. erosional surfaces cut in mudstones and overlain by sandstones; 2. erosional surfaces cut in sandstone and overlain by mudstone; 3. erosional surfaces cut in sandstone and overlain by sandstone or mud-clast conglomerates.

A single shallow concave-up erosional surface overlain by mudstone occurs close to the base of the channel-fill (Figs 24.6 and 24.7). Far more common, however, are erosional surfaces overlain by sandstone or mud-clast conglomerates. These exhibit relief of up to 1m (Figs 24.6-24.8).

The erosional scour overlain by mudstone is

interpreted as a feature eroded by a current which bypassed the area of the Fortin Primero de Maya outcrop, depositing its coarse-grained sediment load downstream to the north-east. The remaining erosional structures at the bases of sandstones and pebbly sandstones are attributed to erosion by the heads of high-velodty turbidity currents whieh subsequently deposited their suspended load of sand through en masse frietional freezing or rapid fallout from suspension.

Soft-sediment deformation

Soft-sediment folds and associated deformation The upper part of the channel-fill sandbody is strikingly thrown into large amplitude folds (up to c. 5 m). Fold crests are assodated with zones of intense deformation in the overlying mudstone unit (Figs 24.9 and 24.10). Same folds are demonstrably conieal in form (Fig. 24.10). Whilst most of the soft-sediment folds are upright at least one is recumbent, its hingeline oriented between 340° and 30°, oblique to palaeocurrents (Fig. 24.4).

The folding is interpreted to be a consequence of liquefaction, post-dating deposition of the abandonment phase mudstone. This mudstone unit formed an impermeable cap to the channel-fill sandstone. Fluidization of the mudstone occurred above vents in the top of the sandstone body. A seismie origin is considered likely in the absence of any evidence for loading due to rapid deposition above this interval.

Injection structures Injection structures are represented by sandstone dykes and sills. Centrimetre-scale sandstone dykes occur in thin intervals of mudstone. A sill, c. 6m lang and up to 1m thiek occurs in the mudstone interval overlying the main channel-filling sandbody (Fig. 24.9). Sandstone dykes and sills are produced when sand is fluidized under a non-uniform confining load (Owen 1987). The presence of sills is consistent with a shallow depth of intrusion (Hiscott 1979).

Key features The key features of the channel-fill are as follows: 1. a sharp erosive base above locally highly deformed mudstarres beneath; 2. a crude fining- and thinning-up sequence Cover a distance of c. 11m; 3. internal erosional surfaces in the basal part of the channel-fill in the form of: (a) a shallow mudstone-draped scour and (b) deeper sand and mud-clast conglomerate-filled scours; 4. extensive soft-sediment deformation in the form of: (a) large-scale folds and cones in the upper part of the channel-fill sandbody, (b) inverted cones or wedges of liquidized muds above 'vents' in underlying sands, and (c) injection structures (sandstone dykes and sills).

Sequence of events The following sequence of events can be inferred from the observed channel-fill:

1. Channel cutting and deformation of underlying mudstones.

2. Deposition from high density cohesionless turbidity currents. At least one current bypassed this location without depositing any sand, but leaving a scoured surface subsequently draped with mud.

3. Channel abandonment and deposition of mudstarres with thin sands from dilute turbidity currents.

4. Liquefaction of the top 4-5 m of channel-fill sands beneath the impermeable mudstarre plug resulting in formation of soft-sediment folds and zones of fluidized mudstone above water escape vents. Sills were injected into the overlying mudstone unit.

Acknowledgements Financial support from the Royal Society for fieldwork in 1989 is gratefully acknowledged. Cesar Gazzera is thanked for assistance in the field and Rick Hiscott for his useful comments.

Heferences Barrio, C.A. 1990. Late Cretaceous-early Tertiary

sedimentation in a semi-arid foreland basin (Neuquen Basin, western Argentina). Sedimentary Geology, 66, 255-275.

Bettini, F.H. 1984. Pautas sobre cronolgia estructural en el area del Cerro Lotena, Cerro Granito y su implicancia en el signifieado de Ia Dorsal del Neuquen, Provincia del Neuquen. Noveno Congreso Geologicio Argentino, Actas II, 342-361.

Gulisano, C.A. Gutierrez Pleimling, A.R. and Disgregorio, R.E. 1984. Esquema estratigrafico de Ia secuencia jurasiea del oeste de Ia Provincia del Neuquen. Noveno Congreso Geologico Argentino, Actas I, 236-259.

Hinterwimmer, G.A. and jauregui, J.M. 1984. Analisis de facies de los depositos de turbiditas de Ia Formacion Los Molles en el Sandeo Barda Colorada Este, Provincia del Neuquen. Noveno Congreso Geologico Argentino, San Carlos de Bariloche, 1984. Actas pp. 124--135.

Hiscott, R.N. 1979. Clastie sills and dikes associated with deep-water sandstones. Tourelle Formation, Ordovician, Quebec. Journal ofSedimentary Petrology, 49, 1-10.

Mitchum, R.M. and Uliana, M.A. 1985. Seismie stratigraphy of carbonate depositional sequences, Upper jurassie-Lawer Cretaceous, Neuquen Basin, Argentina. Memoir of the American Association of Petroleum Geologists, 39, 255-274.

Owen, G. 1987. Deformation processes in unconsolidated sands. In: jones, M.E. and Preston, R.M.F. (eds) Deformation of Sediments and Sedimentary Rocks. Geological Society Special Publication, No. 29, pp. 11-24.

Ploszkiewicz, J.V., Orchuela, I.A., Vaillard, J.C. and Vines, R.F. 1984. Campresion y desplazamiento lateralen Ia zona de falla Huincul, estructuras associadas, Provincia del Neuquen. Noveno Congreso Geologico Argentino, Actas II, 163--169.

Uliana, M.A. and Biddle, K.T. 1988. Mesozoie-Cenozoie paleogeographie and geodynamic evolution of southern South Ameriea. Revista Brasileira de Geociencias, 18, 172-190.

Uliana, M.A., Biddle, K.T. and Cerdan,]. 1990. Mesowie

extension and the formation of Argentine sedimentary basins. In: Tankard, A.]. and Balkwill, H.R. (eds) Extensional Teetonics and Stratigraphy of the North Atlantic Margins. AAPG Memoir 46, 599-614.

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CHILE ARGENTINA

0 km

Fig. 24.1. Location map of the Neuquen Basin.

Chile

Aeserva lndlgena Auca Pan

Argentins

200

Fig. 24.2. Map showing location of channel-fill, palaeocurrents and wave-ripple crest trends for the Los Molles and Lajas formations and the trend of the SE basin margin.

EPOCH AGE GROUP FORIMTION

~retaceous Valanginian Muhdwlco

Berriasian Pkun LN~

"Tithonian Mendoza Veca Muena

Kimmeridg1an C>.Jflbrada del Sopc

Oxfordian Forlln l 0 de Moyo

Callovian ~KO

Jurassie Bathonian l.atllo

Bajocian Cuyo ~

Aalenlan Toarcian liEs MöiieSJ

Pliensbachian Qudw Slnemurian Hettanalan LIIP•

rriassic Permian

Choyol

Carbonlferous

? Piech S~r~ta

Fig. 24.3. Stratigraphie terminology in the Neuquen Basin.

N

Anticline

/ Palaeocurrents,

deep-marine Los Molles Formation

Palaeocurrents, shallow-marine Lajas Formation

/ Palaeowave data. shallow-marine Lajas Formalion

MAIHFACifS

u&Cil

Lm Sh

Cll

.... ~CQ

lh

slhlm

SI! (51)

Lm

co ""' \IOianiCI

or.n,..

metAmotpllcl

l:: 10m

----- I Covered / ... .....__~~-~-

Channel basa Ftgs 24.6 and 24 7

e Soll sedlment totd or cone crest

N

~·· 24.8

F IQS

Palaeocurrents lrom llut s and grooves

24 6 and 24 7 Log. F1g 24.5

I

sandstone lll

0 100m

N

t

Or ntauons of mall sandston

dykes a1 po1n1 A.

Fig. 24.4. Sketch map of the Fortin Primero (1 °) de Mayo outcrop showing locations of log and photographs.

m

~ne-gra11>ed

cna l·abandonrnetll 1n1erve1, localy ,",,. tt deto<med

- --

5

0 +--h---T-r--.--r-""1 m c vcg p e-s sand gravel

Fig. 24.5. Graphie log through channel-fill ; location shown in Fig. 24.4.

Fig. 24.6. Photomontage of basal part of channel-fill. See Fig. 24.4 for location.

Mudelast conglomerates

Soft-sediment folds in channel-fill sandstones

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Bypassed, shallow mudstone-filled scour

Erosional truncat1on of lntrachannel mudstone

Sharp erOSIVe base of channel-fill

Soft-sediment folds 1n mudstones beneath channel-fill base

Scour filled with mudclast conglomerate and sand

10m V=H

Fig. 24.7. Interpretive drawing of basal part of channel-fill. See Fig. 24.4 for location. Fig. 24.8. Photograph of right-hand part of Fig. 24.7 showing channel base and soft-sediment folds in the underlying mudstones.

(a)

(b

Unde ormed channel-abandonmen

Fig. 24.9. Photograph (a) and interpretive drawing (b) showing soft-sediment folds in the upper part of the channel-fill and overlying locally deformed channel abandonment mudstones which contain a sandstone sill. See Fig. 24.4 for location.

--

(a) (b)

Fig. 24.10. Photograph (a) and interpretive drawing (b) showing soft-sediment water-escape cone in the channel-fill and deformation of the overlying channel abandonment mudstone interval. See Fig. 24.4 for location.

Undeformed channel-abandonrnent