Late Cretaceous-Early Tertiary sedimentation in a semi-arid foreland basin (Neuquén Basin, Western Argentina)

Sedimentary Geology, 66 (1990) 255-275 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

255

Late Cretaceous-Early Tertiary sedimentation in a semi-arid foreland basin (Neuquen Basin, western Argentina) C L A U D I O A. B A R R I O Earth Sciences and Resources Institute, University of South Carolina, Columbia, SC 29208 (U.S.A.) Received August 28, 1989; revised version accepted December 7, 1989

Abstract Barrio, C.A., 1990. Late Cretaceous-Early Tertiary sedimentation in a semi-arid foreland basin (Neuqu6n Basin, western Argentina). Sediment. Geol., 66: 255-275. A Late Cretaceous transgression and a subsequent Paleocene regression is recorded in the Neuqu~m Basin of western Argentina in the sediments of the Upper Malarghe Group. Seven facies associations (D through J) representing different depositional subenvironments have been recognized in the younger depositional sequence. Facies association E, green calcareous mudstones, was deposited in the outer shelf. Facies association F, bioclastic carbonates, represents inner-shelf deposits to littoral sediments. Facies association G, sandy calcarenites, corresponds to nearshore to coastal deposits. Facies association H, oolitic calcarenites, and facies association D, evaporites, composed a shallowing upward carbonate sequence. Facies association I, cross-bedded lithic sandstones, and facies association J, red mudstones, are characterized as meandering fluvial deposits. The lateral and vertical distribution of facies association changes from north to south in the basin, according to several depositional controls. The foreland setting of the Neuqurn Basin at this time, with different sediment input and paleoslopes on the retro-arc vs. the cratonic side of the basin, influenced sedimentation. Basin embayment geometry caused differences in tidal regime, which in turn affected depositional environments. Westerly winds generated clockwise tidal currents inside the basin, which affected alongshore lateral facies association variation. The semi-arid climate favoured deposition of mixed carbonate-siliciclastic lithologies and evaporites.

Introduction

The Neuqurn Basin (Fig. 1) is a sedimentary basin in western Argentina with deposits ranging in age from Upper Triassic to Tertiary. Three major sedimentary supercycles are represented in the filling of the basin: Jur/tsico (Late TriassicLate Jurassic), Andico (Late Jurassic-Early Cretaceous) and Riogrhndico (Late Cretaceous-Paleocene) (Groeber, 1929, 1946, 1953). This paper focuses on the upper sedimentary cycle of the Malargiie Group which forms the upper part of the Riogrhndico cycle. The upper Malargiie Group is represented by the Jagiiel, Roca and Carrizo (Pircala) Formations (late Maastrichtian-Paleo003%0738/90/$03.50

© 1990 Elsevier Science Publishers B.V.

cene) (Fig. 2). After facies association differentiation and paleoenvironmental interpretation of the individual facies association, a paleogeographical reconstruction of the Neuqurn Basin during late Maastrichtian-Paleocene was attempted. The study area is located in the Neuqurn Province of western Argentina between 37000 ' and 39°00'S latitude and 70000 , and 67°30'W longitude, where the Malargiie Group is well exposed (Fig. 1). During the field work in the months of October 1985, February 1986 and October 1986, a total of eight sections in three areas (see below) were measured and described (see Fig. 1 for location): (1) Lago Pellegrini-General Roca (LP), where

256

C.A. B A R R I O

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Fig. 1. Location map for the study area, western Argentina. A. Regional background of the Neuqu6n Basin. B. The three areas with good exposures for the Upper Cretaceous-lower Tertiary strata in the northern Neuqu6n Basin. Numbers indicate study areas: 1 = Lago Pellegrini (GR = General Roca section; EC = E1 Carrizo section; Y = La Yesera section); 2 = Auca Mahuida (R = Jagiiel de los Rosauros section;

LC= Lomas Coloradas section); 3 = Sierra de Huantraico ( A T = Aguada del Tuco section; V = Cerro Villegas section; P = Puesto Pizarro section).

LATE CRETACEOUS-EARLY TERTIARY SEDIMENTATION IN A SEMI-ARID FORELAND BASIN

W ANDEAN AREA

257

EMBAYMENT AREA

Carrere Fro.

Vaca Mahuida Fro.

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sandstones mudstones • ,',',

limestones evapodtes

calcareousmudstones calcareous sandstones ~J

bioruditic limestones volcaniclastic sandstones

Fig. 2. S t r a t i g r a p h y o f t h e M a l a r g i i e G r o u p .

a continuous belt of Upper Cretaceous-lower Tertiary outcrops are found (El Carrizo (EC), La Yesera (Y), and General Roca (GR) sections). (2) Auca Mahuida (AM), 116 km northwest of the first area, with facies association and thicknesses different than those of the Lago PellegriniGeneral Roca area (Lomas Coloradas (LC) and Jagiiel de Los Rosauros (R) sections). (3) Sierra de Huantraico (SH), 133 km northwest of the Auca Mahuida area, where excellent exposures of the Malargiie Group show facies association and thicknesses that are unique to this area (Cerro ViUegas (V), Puesto Pizarro (P) and Aguada del Tuco (AT) sections).

Regional geology The Neuqu6n Basin is bordered to the northeast by the Sierra Pintada massif and to the south by the North Patagonian massif (Fig. 1A). The northeastern and southern margins of the basin closely approximate to the original depositional

limits of the Jurassic and Cretaceous marine rocks. The present western flank is formed by the fold and thrust belt of the Andean Cordillera (Principal Cordillera). The Neuqu6n Basin formed initially in the Late Permian-Triassic in an intra-arc setting that continued until Triassic-Jurassic boundary. After the Rio Atuel orogeny (Early Jurassic), deposition in the basin took place in a back-arc basin setting that continued until the Early Cretaceous. The back-arc history of the Neuqu6n Basin is more intensively studied (Digregorio and Uliana, 1980; Mitchum and Uliana, 1985) due to the hydrocarbon potential of the basin in this tectonic setting. During the Late Cretaceous, the tectonic evolution of the Neuqu~*n Basin changed abruptly with the Mirano orogenic phase (Cenomanian; also known as the Peruvian, sub-Hercynian or Intersenonian phase), a compressional event in which uplift of the back-arc region transformed the basin into a foreland province, dominated by molassic

258

C.A. BARRIO

sedimentation derived from the uplifting cordillera to the west (Ramos, 1981, 1985; Malumi/m et al., 1983; Digregorio et al., 1984). The "Riogrhndico" supercycle, which marks the foreland stage, is divided into the "Neuqueniano" cycle (midCenomanian to lower Campanian?), whose sediments were deposited in a continental fluvial environment with erosive unconformity over the Rayoso Formation and older units (Cazau and Uliana, 1972), and the "Malalhueyano" cycle (Upper Campanian-Paleocene), which represents the first Atlantic transgression into the basin (Wichmann, 1927; Bertels, 1979; Uliana and Dellap& 1981) during a time of relative tectonic quiescence and epeiric flooding resulting from a eustatic rise of sea level (Ramos and Ramos, 1979; Uliana and Biddle, 1988).

tions, forms a depositional sequence bounded below by a disconformity at the top of the evaporites of the Allen (Loncoche) Formation and above by an erosional disconformity upon which lie upper Eocene to Oligocene units (Carrere Formation and Vaca Mahuida Formation) (Fig. 2). The Jagtiel Formation (Andreis et al., 1974) is characterized by a homogeneous pelitic sequence with a fauna which indicates a late Maastrichtian to Danian age (Uliana and Dellap& 1981). The type locality of the Jagtiel Formation is located in Jagiiel de Los Rosauros in the Auca Mahuida area (Uliana, 1979). An outer shelf depositional setting was assigned to the Jagiiel Formation by Bertels (1969), Andreis et al. (1974) and by Uliana and Dellap6 (1981). The Roca Formation (Weaver, 1927) has been differentiated by several authors based on the abundance of skeletal fragments. The type locality lies at the eastern side of the basin twelve kilometers north of the General Roca town (Bertels, 1969). Lithologically the formation consists of

Stratigraphy The Upper Malargiie Group which comprises the Jagi~el, Roca and Carrizo (Pircala) Forma-

TABLE 1 Summary of facies associations for the second depositional sequence of the Malargiie Group (left) and stratigraphic arrangement of the facies associations in the study area (right; LP = Lago Pellegrini; A M = Auca Mahuida; SH ~ Sierra de Huantraico). The letters in the lower comer of each facies association are representing facies association relationships FACIES ABSOC.

J

I

U1HOLOGY

massive laminated mudstones sandstones gravels carb. shales oolitic

H

G F

E

D

calcareniles

COLOR reddish orange (10 R 6/6)

SED4MENTARY MINERALOGY B~RUCTURES

montmoriIlonite

very pale quartz, leld., orange volcanics, (10 YR 8/2) zeolites yellowish orange (to YR 8/6)

calcite quartz ooids,peloids

sandycarbonates mudstonee

greyish yellow green (5 GY 7/2)

skeletal fragments quartz, volc.

coquinas biociastic limestones

very pale orange (10 YR 8•2)

skelelal fragments quartz, feld.

massive marlslones and rare sandstones

yellowish green (10 Y 8/2)

calcite montm, zeolites

evaporites

white

gypsum

F~SILS

DEPOSITIONAL ENVIRONMENT

massive lamination nodules

t u r tie s charophytes estracodes

allu vial

X-bedding ripples inlraclasts

plant debris tree trunks

meander channel

ripples diss. cracks collapse breccia

--

X-bedding brachiopods bivalves massive forams

inner and outer shelf beach

chicken-wire lamination massive

brachiopods bivalves gastropods forams ~

o

u-

2

tidal shoal lagoon

inner shelf nearshore

outer to inner shelf

sabkha/ salina

Facies Asaoc,

J

plain

bivalves X-bedding bent. forams clay drapes ostracodes burrows

massive lamination

REGIONAL Un|!

g

H

2

A F

~LL

DISTRIBUTION

LP

AM

SH

13 m caliche

17 m caliche

7m f.g, ss coal lens

6m t.g. ss.

57 m intraclasts scour

8m

199 m burrows

35 m volcanic ash

4,5 m 6m transitional rod

3m coquinas F-1 23 m

5m 20 m bioclastic less. packs. grainstones F-3 F-2 26 m

23 m

LATE C R E T A C E O U S - E A R L Y T E R T I A R Y S E D I M E N T A T I O N IN A SEMI-ARID F O R E L A N D BASIN

carbonates and evaporites with a thickness ranging from 45 to 50 m. The Roca Formation on the western side of the basin (Sierra de Huantraico area) was studied by Bertels (1968; upper Huantraico Member), Marcrn (1975) and by Ramos (1981). They recognized a sequence 150 m thick of thick fossiliferous limestones and calcareous sandstones forming the major components of the formation (Marc6n, 1975). A MaastrichtianDanian age for the Roca Formation is based on foraminiferal and macrofossil studies (Bertels, 1969; Ramos, 1981; Mance~do and Damborenea, 1984; Rossi de Garc[a and Levy, 1984). Uliana and Dellap6 (1981) considered that the Roca Formation was formed in a shallow marine environment passing upwards to a sabkha in the east. Ramos (1981), for the western sections, proposed a shallow-marine depositional environment. The Carrizo Formation (Uliana, 1979) is the accepted name assigned to the redbeds overlying the Roca Formation on the eastern side of the Neuqurn Basin. The type locality red beds, 30 m thick, are located in a quarry of the same name north of Allen town (Uliana, 1979). Uliana and Dellap6 (1981) regarded the basal section as lacustrine followed above by a fluvial meandering system at the top of the unit. The Pircala Formation (C. Bohem, 1938, in Ramos, 1981) is the formational name of the equivalent unit in the Sierra de Huantraico area. The Pircala Formation, on the western side of the basin, has a measured thickness of 140 m (Marc6n, 1975). The Paleocene to Eocene age proposed for the Carrizo (Pircala) Formation is based on its stratigraphic position (Marc6n, 1975; Uliana and Dellapr, 1981; Ramos, 1981). Facies associations description and depositional environments

The Upper Malargiie Group can be divided into seven facies associations each composed of genetically related sediments (Reading, 1986). The next section is a description followed by an interpretation of the depositional environment of each individual facies association. Within the Jagiael Formation, a single facies association E is recognized. The Roca Formation

259

contains the four facies associations F, G, H and D, and the Carrizo (Pircala) Formation two (facies association I and J) (Table 1). Facies associations are defined as follows: D: evaporites, E: green calcareous mudstones, F: bioclastic limestones, G: sandy calcarenites and calcareous mudstones, H: oolitic calcarenites, I: cross-bedded lithic sandstones, and J: red massive mudstones.

Facies association E: Green calcareous mudstones

Facies association E, recognized in three areas as the Jagiiel Formation (Fig. 3 and Table 1, right), is characterized by a homogeneous sequence of massive and laminated fossiliferous calcareous mudstoneswith minor intercalations of fine-grained calcareous sandstones (Fig. 4). This association (facies association E) was found in the Lago PeUegrini area in the upper part of the Y section (10 m), in the lower part of the EC section (15 m), and at the base of the GR section (23 m). In the Auca Mahuida area facies association E was recognized in both the LC (26 m), and R sections (18 m). Twenty-three meters of facies association E were described in the Sierra de Huantraico area (V section). This association is composed of yellowish-green massive or laminated calcareous mudstones, intercalated with finegrained calcareous sandstones, and with coquina beds at the top of the section. X-ray diffraction of the mudstones samples from the three areas shows calcite, quartz and feldspars as the main minerals. Montmorillonite, the most common clay mineral, is locally associated with zeolites. Fossils, which are common and usually broken, include bivalves (Odontogryphaea, Cubitostrea), gastropods, brachiopods and serpuhds. Echinoid spines and brachiopods were also described by Andreis et al. (1974). The paucity of sedimentary structures and the abundant fauna suggests that the mudstones of facies association E may have been intensively bioturbated.

260

C.A. BARRIO

NW

AM Sierra de Huantraico

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LP

Auca K Lago Mahuida--- 116 m - - - P e l l e g r i n i

SE

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0 n 0 N

".Fo

E

It.

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Fig. 3. Simplified stratigraphic and facies association correlation of measured sections of the Upper Cretaceous-lower Tertiary sediments of the Lago Pellegrini (LP), Auca Mahuida (AM) and Sierra de Huantraico (SH) areas.

Middle Maastrichtian to lower Tertiary planktonic and benthonic forams and ostracodes were described by Bertels (1974, 1975, 1978) and Kielbowicz (1980) in the Lago PeUegrini area. In the Auca Mahuida area, Maastrichtian and Danian benthonic and planktonic foraminiferids and ostracodes were described by Bertels (1978). More recently, calcareous nannoplankton of Maastrichtian (Malumihn et al., 1984) and Danian age (Angelozzi, 1987) were identified in this area. No macrofauna has yet been found in the Sierra de Huantraico area, but the abundant microfauna is composed of ostracodes and planktonic and benthonic forams of Maastrichtian age (Bertels, 1978).

Depositional environment interpretation The fine grain size, paucity of physical sedimentary structures, and the presence of planktonic and benthonic microfossils indicates that facies association E was deposited in an outer-shelf environment (Andreis et al., 1974; Uliana and Dellap6, 1981; Fig. 5). No evidence of currents is present, indicating that facies association E was deposited below normal-weather wave base, largely from suspension. Based on the microfauna from the Jagiiel Formation (facies association E), Bertels (1974, 1975) proposed a marine environment with normal salinity, warm temperatures, depths between 150 and

L A T E C R E T A C E O U S - E A R L Y TER.T]ARY S E D I M E N T A T I O N IN A SEMI-ARID F O R E L A N D BASIN

261

Facies association F: Fossiliferous carbonates

23 m (SH) 26 m (AM) 23 m (LP)

Facies Association E Forams Fossil debris Fig. 4. Representative facies association partial section of the Jagtiel Formation (facies association E) with total thickness in the three study areas. See Fig. 3 for additional legend.

300 m, clear water, and a muddy substratum with normal pH (7.8-8.1).

The characteristic feature of facies association F, mapped in three areas as part of the Roca Formation is the abundance of skeletal fragments. The occurrence of different structures and subordinate lithological differences permit subdivision into three types (facies sub-association F-l, F-2 and F-3). F-I: Coquinas. This is a ledge-forming unit is best exposed in the Lago Pellegrini area (EC and GR sections, respectively 3 and 1.5 In) but it also appears in the Auca Mahuida area (R section, 0.4 m). Facies sub-association F-1 is composed of grayish yellow, coarse abraded skeletal fragments in medium-bedded tabular beds with scarce matrix and secondary gypsum veins, interbedded with calcareous mudstones of facies association E (Fig. 6). Different skeletal fragments were noted, mainly bivalves (oysters being particnlarly abundant), echinoids and gastropods. Studies of the macrofauna of the coquina beds exposed in the Lago Pellegrini area, indicate that

Basin

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Fig. 5. Paleoenvironmental and paleogeographical model for the deposition of the Jagiiel (facies association E), Roca (facies association F, G, H and D) and the Carrizo (Pircala, facies association I and J) Formations in the Neuqu6n Basin during the Late Maastrichtian-Paleocene. The Late Cretaceous Abanico Formation in Chile is sketched after Aguirre et al. (1974).

262

C.A. BARRIO

,..,.~D~ ~.~_,

.

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5.74 m (SH)

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micritic

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low-angle x-bedding planar bedding

intraelasts asymmetric rypples wave-ripples

G ..=,...,--.

flaser bedding unit geometry

Z=T

birdseye structure

J

burrows

T

6 m (LP)

"alabastrine" 4.5 m (LP)

....... ~ : -.- ~- ".- -.=-.-~1

H

,,=,

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"chicken° wire"

D

Fig. 6. Representative facies associations partial sections of the Roca Formation, with average thickness in the three study areas. See Fig. 3 for additional legend.

these deposits are mainly Danian in age but include some Maastrichtian (Schiller, 1922; Bertels, 1964, 1969, 1970; Rossi de Garc~a and Levy, 1984; Mance~do and Damborenea, 1984). F-2: Bioruditic grainstones with terrigenous grains. The facies sub-association F-2 (Roca Formation), seen only in the Auca Mahuida area in the LC (5 m) and R (2 m) sections, differs from F-1 and F-3 by the presence of abundant terrigenous grains. It consists of pale orange and yellowish gray bioruditic limestones with abraded skeletal fragments, terrigenous grains, and intraclasts of micritic carbonates (Fig. 6). Several fining-upward

trends were recognized in the field. One is characterized by sets, ranging from 0.2 to 0.5 m in thickness, of basal fine-grained conglomerates that fine upward to medium-grained trough cross-bedded sandstones at the top. Another fining-upward trend is characterized by thickness from 0.4 to 0.8 m that starts with "foreshore accretion-bedding", which passes upwards to trough cross-bedding and is capped by planar bedding. Large-scale tabular cross-bedding was seen in this facies associated with the coarsest beds. Thin section revealed that this unit contains abundant skeletal fragments, mainly mollusks, associated with bryozoans, ostracodes and gastro-

LATE C R E T A C E O U S - E A R L Y T E R T I A R Y S E D I M E N T A T I O N IN A SEMI-ARID F O R E L A N D BASIN

pods. Both embayed and polycrystalline quartz is abundant. Zoned plagioclases, microcline and andesite fragments are common. Chert is sporadically abundant. F-3: Bioclastic packstones and grainstones. Facies sub-association F-3 (Roca Formation), in the Sierra de Huantraico area (V, AT and P sections, respectively 18, 20 and 15 m), is differentiated based on the abundant carbonate matrix. It is characterized by an olive gray to white packstone and minor grainstone (Dunham, 1962), with sparse terrigenous grains (Fig. 6). The skeletal fragments are usually abraded and occasionally well preserved. The rocks are very tightly cemented. Burrows and plant fragments occur towards the top of the sequence. The terrigenous clasts are mainly quartz and rare lithic grains. The finest grained calcareous beds are characterized by the presence of breccioid stylolites (Logan and Semeniuk, 1976). Thin-sectioned samples from this facies sub-association show an abundance of skeletal fragments, mainly mollusks (particularly oysters). The most common terrigenous material are embayed quartz and zoned plagioclase and locally lithic volcanic (andesite) fragments are abundant. The F-3 facies sub-association contains planktonic, benthonic forams and ostracodes of Maastrichtian and Danian age (Bertels, 1968, 1969, 1970). Weaver (1927) and later Camacho (1968) assigned a Cretaceous age to the Roca Formation based on the occurrence of Baculites in the macrofauna. More recently, G. Blasco and R. de Caminos (1975, in Ramos, 1981) have defined a Maastrichtian-Danian zonation for the Roca Formation based mainly on mollusks. Depositional environment interpretation Facies association F, is interpreted as tempestite deposits based on the abundance of abraded skeletal fragments in tabular beds. The textural differences of the individual facies sub-associations point to considerable differences in their wave energies (see the discussion section). In F-1 facies sub-association, the abundant skeletal fragments are found in tabular beds, with non-scoured bases. The fragments are surrounded by open marine calcareous mudstones of facies

263

association E and suggest that facies sub-association F-1 consists of storm deposits formed in an outer to inner-shelf environment (Fig. 5). In origin they are similar to the storm lag deposits generated by storm-induced waves affecting muddy sediments containing skeletal fragments (Brenner and Davies, 1974; Specht and Brenner, 1979). The alternative interpretation is that the beds were transported by gradient currents from the coastal area to the shelf, and then concentrated in tidal channel distributary mouths (Aigner, 1985). The tabular shape of the beds favors the "storm lag" interpretation for facies sub-association F-1. Facies sub-association F-2, based on bed amalgamation, abundant cross-bedding, channeled bottoms of individual structures and the coarse-grained terrigenous fraction suggests deposition in a carbonate beach/nearshore setting affected by wave action (Fig. 5). Such deposits are characterized as proximal tempestites by Aigner (1985). The closest analogues to this facies sub-association are found in the Cow Creek Limestone in central Texas (Inden and Moore, 1983), the Holocene carbonate beaches described in the northeastern Arabian Gulf (Shinn, 1973), and in southern Australia (Gostin et al., 1988). The latter examples developed in arid climates with low rainfall and low sediment input similar to the interpretation proposed here. Facies sub-association F-3 characterized as bioclastic limestones with a fauna indicative of a relatively shallow water setting, occasionally disturbed by the bottom waves which controlled the fabric of the limestone (Fig. 5). The wackestones and packstones thus formed in areas where the wave action was not strong enough to winnow out the mud-sized grains (Specht and Brenner, 1979). Well preserved fossils are characteristic of such an environment and of the high sedimentation rate (Rhoads, 1975). Facies association G: Sandy calcarenites and mudstones with minor carbonates

Facies association G (Roca Formation) is well represented towards the west, in the Sierra de Huantraico area, and although it was less developed, it was also recognized in the Auca Mahuida

264

area (Fig. 3 and Table 1, right). In the Sierra de Huantraico area, facies association G was described in sections V (114 m), AT (82 m) and P (199 m). In the Auca Mahuida area, it was distinguished in the R (8 m) and LC (9 m) sections. Facies association G is composed of grayish yellow to pale orange fine to medium-grained calcareous-terrigenous sandstones intercalated with calcareous mudstones (Fig. 6). The sandstones show medium-scale tabular and lenticular cross-bedding, locally with clay drapes in the foresets. Pelitic intraclasts are common, usually associated with skeletal fragments and plant debris. In the P section, at the base of the sequence, facies association G shows a fining-upward trend in sets 4 - 8 m thick. Fine-grained basal conglomerates, with abundant well rounded sandy and pelitic intraclasts, quartz and opaques grains, are succeeded by cross-bedded calcareous sandstones with burrows, and finally by fine-grained sandstones with ripples and flaser, wavy and lenticular structures (Reineck and Singh, 1975), with thin intercalated volcanic ash and veins of secondary gypsum. In these fining-upward sets, clay drapes in the foresets were found in the middle part of the sequence. The calcareous mudstones in facies association G are more common towards the top and contain very thin sandy layers with abundant burrowing. Almost pure carbonates with isolated terrigenous clasts are also represented in facies association G: a dolomitic breccioid bed with some quartz, and beds with algal lamination were observed at section P. The P and AT sections contain a domal algal mat (Gobulic, 1976), composed of an oolitic grainstone with an internal lamination parallel to the external shape of the bed. In the AT section, an isolated boulder with abundant branching corms was seen. In thin section, the sandy carbonates are characterized by a framework of fine- to mediumgrained sparry calcite grains with abundant terrigenous grains and sparite cement. Most of the terrigenous clasts are volcanic fragments of andesite and embayed quartz. Skeletal fragments are present in some samples, mainly oysters and gastropods. X-ray diffraction analyses indicated abundant montmorillonite.

C.A. BARRIO

Depositional enuironment interpretation The sandy calcarenites and mudstones of facies association G were probably deposited in a innershelf to nearshore carbonate-terrigenous setting (Fig. 5), based on the presence of sand-size grains, abundant cross-bedded structures, marine fauna, plant debris at the top of the section, and a transitional passage to redbeds of the overlying Carrizo (Pircala) Formation. Scoured bottom sets and the presence of intraclasts point to a high-energy environment with active sediment transport. Clay drapes are typical of currents in tidal environments (Visser, 1980; Terwindt, 1988). The conditions for the formation of the algal mat described in the AT section was probably in a lower intertidal setting, similar to that of the columnar, smooth algal mats of the Shark Bay area in Australia (Hoffman, 1976). The scarcity of algal mats is related to terrigenous contamination on the sublittoral shelf (Hoffman, 1976).

Facies association H: Oolitic calcarenites Facies association H in the Roca Formation was only recognized in the Lago Pellegrini area as a 6 m outcrop at the G R section and a 5 m truncated unit at the EC section (Fig. 3). It consists of yellowish, fine-grained, calcareous sandstones with symmetrical ripples, intercalated with grayish siltstones near the base (Fig. 6). The siltstones intercalations commonly show wavy, flaser and lenticular bedding (Reineck and Singh, 1975). Amorphous silica concretions filling small holes were seen in the calcareous sandstones throughout the section. Lenticular silica nodules are present in the G R section. At the top of facies association H in the EC section, a distinctive micritic lenticular body (flat bottom, convex top) with widespread "birds-eye structures" overlies calcareous sandstones. Elsewhere desiccation cracks are common. In thin sections the calcarenites appear as oolitic-peloidal calcitic grainstone, with some calcite grains showing a rhombic outline. X-ray diffraction shows abundant quartz and scarce feldspars in the lower part of the section. Halite was also identified. Towards the top of the unit calcite is the main mineral, with scarce feldspars

L A T E C R E T A C E O U S - E A R L Y T E R T I A R Y S E D I M E N T A T I O N IN A SEMI-ARID F O R E L A N D BASIN

and montmorillonite. No fossils were reported for this facies association.

Depositional environment interpretation The presence of oolites, ripples, desiccation cracks, birds-eye structures in facies association H belong to a "shoaling upward cycle" deposited in a tidal flat environment. StratigraphicaUy it overlies open marine calcareous shales of facies association E and is overlain by evaporites (facies association D, Fig. 5). Shoaling-upwards cycles are characterized by the presence of a normal marine unit at the base (facies association E) followed by a subtidal to intertidal unit affected by tidal action (facies association H), and finally by a supratidal unit (facies association D) covered only during flood tides (James, 1984). Facies association H, the lowermost subtidal to intertidal interval, is dominated by wave action that formed wave-rippled oolitic calcarenites. At the top, desiccation cracks are clear evidence of subaerial exposure in an intertidal setting and the presence of halite suggests desiccation in the oolitic calcarenites. Silica nodules as described are evidence of hypersaline waters (Fliigel, 1982) and the concentration of silica in rugs is interpreted to be the replacement of anhydrite crystals by quartz or chalcedony (James, 1984). The fine-grained, flat-based limestone beds with convex tops which overlie the oolitic calcarenites are regarded as lagoonal deposits, based on their finer grain size compared to the oolitic calcarenites, and on their stratigraphic position. Birds-eye structures in the fine-grained limestones is indicative of algal mat proliferation in an intertidal to supratidal zone, with pores forming after burial by entrapped gas and mat shrinkage (Ham, 1952). This interpretation parallels that of the Guadalupe Mountains (Kendall, 1969), and Carboniferous example in South Wales (Wright, 1986), all of which are examples of the facies belt model of Wilson (1975), where the oolitic calcarenites represent a barrier developed basinward of the lagoonal deposits. The process of de-dolomitization is typical of carbonate-evaporite sequences affected by fresh water. Carbonates found under such conditions

265

are dominantly dolomitic due to the precipitation of evaporites which elevates the M g / C a ratio allowing for dolomite precipitation, which may subsequently be replaced by calcite through fresh groundwater intrusion. Thin-sections of facies association H shows calcite to be the main mineral (stained with Alizarina red), indicating possible de-dolomitization due to the fresh-water influence (James, 1984; Fliigel, 1982), but the euhedral rhombic outline of some of the grains are evidence for the prior existence of dolomite.

Facies association D: Evaporites Facies association D caps the Roca Formation in the Lago Pellegrini area and was recognized in the EC section where it is 4.5 m thick (25 m of this facies association were measured by Uliana and Dellap~, 1981) and was also reported in General Roca with a thickness of 6 m (Vall6s, 1987). The contact of facies association H and D is marked by a collapse breccia of large gypsum clasts in a carbonate matrix. Three distinct evaporitic beds were observed (Fig. 6). The lowest beds are characterized by "chicken-wire" or nodular structure, the nodules decreasing in size towards the top of the section. The layers of gypsum nodules are separated by light gray, laminated mudstones. The second bed is a 0.5 m thick massive "alabastrine" gypsum. At the top, the third bed is 1.1 m thick of laminated gypsum with very thin interbedded claystones ends the section. The setting of facies association H and D shows similarities to carbonate-evaporite shoaling-upward sequences leached by fresh water as described by James (1984), the collapse breccia providing the clearest evidence of fresh-water leaching (see Kendall, 1969).

Depositional environment interpretation Facies association D an evaporitic deposit capping the shoaling-upward carbonate sequence in the Lago Pellegrini, resembles the examples of the Arabian Gulf (Shinn, 1973) and Shark Bay in Australia (Logan et al., 1974). Regionally, the oolitic shoals form the barriers behind which evaporites developed.

266

C,A. BARRIO

The planar stratification found at the top of facies association D is attributed to the cyclic repetition of subaerial and subaqueous phenomena due to the fluctuating level of hypersaline water (Nurmi and Friedman, 1977). The nodular or "chicken-wire" structure is evidence of a supratidal setting (Shearman, 1966; Bosellini and Hardie, 1973). However, some authors have claimed that nodular structure is related more to early diagenesis in the hosting sediments than to depositional conditions (Murray, 1964; Warren and Kendall, 1985). The previous paragraphs show the difficulty of attributing a primary environment to structures in evaporites, which in many cases have a diagenetic origin. Warren and Kendall (1985) have pointed out that the presence of a sabkha environment is shown by the "trinity" of subtidal, intertidal and supratidal deposits, including algal mats and an upper evaporitic bed eroded at the top. In the depositional sequence that constitutes the Upper Malargiie Group, the supratidal position of facies association D is demonstrated by its occurrence as a culmination of a shoaling-upward sequence from the basal calcareous mudstones deposited in the outer shelf to inner shelf (facies association E and F) passing upwards to subtidal to intertidal calcarenites (facies association H). However, a subaqueous salina environment for the evaporites of facies D is evidenced by the considerable thickness (up to 25 m) of the section (Warren and

T

Kendall, 1985; Elliott and Warren, 1989). It is more likely that the evaporites of facies association D represent a lagoonal infilhng as was described for the Lower San Andres Formation (E1liott and Warren, 1989), where both salina (subaqueous) and sabkha (subaerial) settings are represented.

Facies association I: Cross-bedded lithic sandstones In the Lago Pellegrini area, facies association I (Carrizo (Pircala) Formation) is 7 m thick in the EC sectie~i. Six meters of facies association I was described in the LC section of the Auca Mahuida area. In the Sierra de Huantraico area, 57 m of facies association I was measured in the V section. Facies association I consists mainly of pale orange and whitish, fine- to very fine-grained, thick- to medium-bedded, trough cross-bedded sandstones (Fig. 7). The sandstones in the paleochannels grade up to carbonaceous mudstones (EC section). In the Auca Mahuida area a consistent trend in the cross-bedding was from medium-scale trough cross-bedding at the base to trough cross-lamination and parting lineation at the top. The beds pass laterally to medium- to large-scale trough cross-bedded sandstones with red pelitic intraclasts. In the Sierra de Huantraico area (V section) medium- to coarse-grained lithic sandstones and fine-grained medium-bedded lithic conglomerates

::::':::::" ~ i nodules ~caliche

19.8 m (SH)

5.75m (AM) 3.45m (LP)

Facies Association I

[

•conglomerates ] volcanic ash

l 12.79m (SH) 9.09 (AM) 7.02m (LP)

Facies Association J

~ Fig. 7. Representativefacies associationspartial sections of the Carrizo (Pircala) Formation (facies association I and J) with average thickness in the three study areas. See Figs. 3 and 5 for additional legend.

LATE C R E T A C E O U S - E A R L Y T E R T I A R Y S E D I M E N T A T I O N IN A SEMI-ARID F O R E L A N D BASIN

were described towards the top. The cross-bedding in this facies association is better developed in this coarse-grained beds, where it occurs as large-scale tabular cross-bedding. Cross-bedding in this area shows two main paleocurrent directions, N 90-75 ° and N 144-145 °. Pelitic intraclasts 0.35 m in diameter showing low erosion are present in the conglomeratic beds. Also, lithic sand-size intraclasts are common. A consistent fining-upward trend is present in facies association I in the Sierra de Huantraico area. Thin-section samples of the sandstones shows abundant monocrystalline quartz with volcanic embayments and an euhedral habit, polycrystalline quartz, zoned plagioclases and lithic fragments (mainly basic volcanic clasts). Point-counting four samples, shows that the sandstones in the EC section may be classified as quartzose arkose (Qm: 50.6; F: 13.5; Lt: 35.9) and litharenite (Qm: 47.3%; F: 13.8%; Lt: 38.8%) in the EC section. And as feldspathic litharenite (Qm: 10.0%; F: 19.1%; Lt: 70.9%) and litharenites (Qm: 17.4%; F: 3.47; Lt: 79.1%) following Folk (1974), for the Sierra de Huantraico sections. Two samples have been identified as tufts with some non-volcanic grains, usually polycrystalline quartz and chert. X-ray diffraction in the sandstones shows abundant montmorillonite, with scarce illite and clinoptilolite. Facies association J: Red massioe mudstones

Facies association J (Carrizo (Pircala) Formation) occurs as a uniform red mudstone across the basin (Fig. 7). In the Lago Pellegrini area, facies association J is 13 m in the EC section. Seventeen meters was measured in the Auca Mahuida area (LC section). In the Sierra de Huantraico area, 35 m of this association was described in the V section. Facies association J is composed of reddish and yellowish gray mudstones, with very thin beds of fine-grained sandstones. Locally in the Lago Pellegrini and Auca Mahuida areas, horizon of caliche nodules are abundant. The mudstones are apparently massive, although parallel lamination and flaser bedding were observed in the sandy-shaley beds. Scarce conglomeratic beds and micritic carbonates

267

were seen in the V section. Bands of very thin, light gray volcanic ash are common. Desiccation cracks are common and some burrowing was evidenced by the mottled texture of the beds. Turtles bones were described by Uliana and Dellap6 (1981). Continental charophytes and ostracodes were recovered by Musacchio and Moroni (1983). X-ray diffraction for facies association J shows that montmorillonite is associated with quartz, feldspars and calcite. Depositional environment interpretation Facies association I and J (Carrizo (Pircala) Formation) are interpreted as formed by a meandering river system, based on the occurrence of fining-upward cycles (Jackson, 1978), the thick fine-grained member (Jackson, 1978), and crossbedding on the planar base (Collinson, 1986). The sand to gravel grain size of facies association I and abundant cross-bedding suggest bed-load deposits (Fig. 5). The thick red shales of facies association J were deposited on the alluvial mud plain (Fig. 5). The measured sections show sectors with predominance of facies association I (bed load) in amalgamated bodies interpreted as channelled areas of the main fluvial distributaries. Conglomeratic beds interbedded with the mudstones of the alluvial plain (section V) are presumably crevasse splays of the main channels. In channel deposits of facies association I, the scale of the cross-bedding and the average grainsize decreases towards the east, indicating a paleoslope dipping from northwest to southeast. The inference is confirmed by the paleocurrent trends which points to the southeast. Channel scouring is inferred from pelitic intraclasts described in the CV section, which were derived from erosion of the alluvial plain (Collinson, 1986) or erosion of clay plugs common in meandering systems (Galloway, 1985). In the LC section, fine-grained sandstones with upper-flow regime sedimentary structures (parting lineation, plane bedding) were described adjacent to the major channels, corresponding to laminated sand sheets (Miall, 1985), from overbank flooding of the main channels. The typical point-bar structure of the meander-

268

ing channels was not found in facies association I. The absence of lateral aggradation structures can be explained by clay plugs which would have restricted the lateral movement (Collinson, 1978; Walker and Cant, 1984), or to rapid subsidence that caused channels to be isolated in a large volume of fine-grained sediments of the alluvial plain (Shuster and Steidtmarm, 1987; Kraus and Middleton, 1987). Facies association J, characterized by reddish fine-grained deposits, was deposited on the flood plain by vertical aggradation (Miall, 1985) and shows important paleoclimatic indicators. The reddish coloration was probably caused by the decomposition of biotite and hornblende to iron hydroxides during pedogenesis, a process th~it is better developed in semi-arid and high-temperature environments (cf. McBride, 1974). However, this interpretation of red coloration is not widely accepted and a late diagenetic origin regardless of the original environment has been proposed by other authors (Walker et al., 1978; Reif and Slatt, 1979). The caliche deposits were probably formed in a subhumid to arid climate with slow aggradation on a stable landscape, followed by periods of non-deposition or very slow deposition during which pedogenesis occurred (Gustavson and Winlder, 1988). Mud cracks found in facies association J point to the partial subaerial exposure of the flood plain. Coal beds were described in the EC section, showing seasonal periods of a relatively wet climate which permitted continued plant growth in the wetter areas of the alluvial plain.

C.A. B A R R I O

Depositional controls To explain a particular facies association arrangement illustrated in Fig. 5, the inferred depositional controls are modified by tectonic factors, basin geometry, tidal processes influenced by the Coriolis effect and climatic variables including aridity and strong winds. These controls are summarized in Fig. 9, and will be discussed further in the following paragraphs.

Tectonic setting The Neuqu6n foreland basin was divided into a shallower, eastern, cratonic side bounded by the Sierra Pintada Massif, and a deeper, western, retro-arc side limited by a volcanic arc (Aguirre et al., 1974; Digregorio et al., 1,984) (Fig. 5). This tectonic setting directly influenced sediment input and paleoslopes (Fig. 9); the greater paleoslope on the western, retro-arc side of the basin favored higher sediment input. Low sediment input on the cratonic side of the basin favored the development of carbonate and evaporite deposition over the broader cratonic area. Sediment input on the retro-arc side introduced terrigenous material into the carbonate basin and probably was responsible for flexural loading resulting from the higher sedimentation rate. The varying paleoslopes also affected facies association development. On the gentler paleoslopes on the cratonic side of the basin, only minor channel scouring is found. The lower slopes permitted pedogenesis to the extent of forming caliche deposits. In the west, the steeper paleoslopes and high sediment input resulted in the formation of major channels by the more powerful currents carrying coarser-grained sedimentary loads.

Discussion

Embayment geometry The Neuqu6n Basin at the time of deposition of the Upper Cretaceous-lower Tertiary sediments of the Malargiie Group was a foreland basin closed off to the north and west creating an embayment, and dominated by tidal currents under an overall arid climate. Related to this particular setting, marked differences in depositional environments existed between the three study areas (Figs. 5 and 8).

The embayment geometry of the Neuqu6n Basin (Fig. 8) controlled the tidal regime, which in turn affected depositional processes (Barrio, 1990). The tidal range has been shown to be a function of shelf width and shoreline geometry (Hayes, 1975). The greatest tidal ranges occur when a tidal wave is funnelled along a narrowing embayment (Langbein, 1963). According to the Hayes's model for medium energy embayments, a microtidal range

269

LATE C R E T A C E O U S - E A R L Y TERTIARY S E D I M E N T A T I O N IN A SEMI-ARID F O R E L A N D BASIN

(0-2 m) is expected in the outer portion of an embayment increasing from mesotidal (2-4 m) further into the embayment to maerofidal (up to 4 m) at the apex of the funnel. In the Neuqu6n Basin, a microtidal setting (mixed energy, wave dominated) is inferred in the LP area (Fig. 8), a mesotidal area (mixed energy, tide dominated) in the AM region, while the SH area is presumed to have been lain in the region dominated by a macrotidal tide range (Figs. 5 and 8) (Barrio, 1990).

This may have affected sedimentation processes in the Neuqu6n Basin (Figs. 5 and 8). Wind directions were determined (by paleowinds measurements of directions from eolianites in the Cenomanian Candeleros Formation; Spalletti and Gazzera, 1989), and are considered with respect to the latitudinal position of the basin. The wind stress (in the southern hemisphere) created a northeast swell and a north mean Ekman flow (Pond and Pickard, 1983). Tides propagated along the coast from the southeast and the entering tidal surges (flood tide) would have flowed close to the west embayment shore, causing erosion on the western side of the basin (Fig. 8). The ebb tide reflecting the Coriolis effect would have lain to the

Tidal currents

Tidal currents were subject to the Coriofis effect amplified by predominantly westerly winds.

|

ARGENTINA

/

AM

(0

4O S

Atlantic Ocean 0 100 •

///

200

"~,~ 300kin

igneousactivity non-marine sedimentation

( ~

dw

marine sedimentation emergentcratonic

Mi.- microtidal range Ms.- mesotidal range Ma.- macrotidal range After Uliana and Biddle (1988)

Fig. 8. Paleogeography(after Uliana and Biddle, 1988) and environmentalprocessesin the Neuqu6n Basin at the Late CretaceousEarlyTertiary.

270

C.A. BARRIO

Controls of the Neuqudn Foreland Basin carbonate [ CRATONIC J.,.,~low sediment input --~ evaporite " ~ gentle paleoslope _~,.minor scouring caliche terrigenous ~[RETRO-ARC high sediment inputC supply Flexural loading ~steep paleoslope [ Basin Geometry

J--~iT|dal Reglme}---

"

major scouring higher competence

Micro to macrotidal

[ Coriolis Force J [_W_~.,~. asymmetric tidal currents

in an area located further to the south within the warm temperate belt. The aridity of the Neuqurn Basin in the study area was enhanced by a rainshadow effect behind the volcanic arc to the west. Seasonal rains brought sediments to the basin in aperiodic patterns under a dominant dry climate under similar conditions to the Gulf of California in Mexico, where the Peninsular Ranges-Sierra Juarez and Sierra San Pedro Mhrtir range acts as a barrier to the westerly Pacific winds (Glennie, 1970). Depositional processes in the three study areas

,f

~

'~

Z ,~

high evaporation

,,

coastal evaporites

aperiodic sediment input--~ terrigenous input Fig. 9. Summary of the controls in the deposition of the Neuqurn foreland basin. See text for further explanation.

The distribution and interrelationship of the depositional controls (Fig. 9) described above resulted in distinctive characteristic variations in the facies association in the study areas. These can be related to the three areas examined in the Neuqurn Basin: S H area

east (Fig. 8) (Carter, 1988). Draining flow (ebb tide) was probably less energetic than the entering tide (C. Medeiros, verbal communication, 1989), transporting sediment over shorter distances. The model proposed here is similar to the tidal currents in the northern Arabian Gulf basin (Gunatilaka, 1986, 1987). Climate

Arid conditions during Roca time are indicated by the presence of evaporites (facies association D) and extensive carbonate sedimentation. Aridity favored the high evaporation rates (with negative E - P budgets) necessary for evaporite deposition on sabkhas. Sporadic or seasonal rains generated aperiodical terrigenous input punctuating periods of carbonate deposition. Caliche deposits suggests hot and dry conditions (McBride, 1974). On the other hand, the coal beds which were described for facies association I in the EC section point to adequate water to support vegetation in low-level swamp or lagoonal conditions, indicating at least a sub-arid climate. Humid conditions were inferred for correlative deposits of the Malargiie Group south of the study area (Casamiquela, 1978; Volkheimer, 1984)

The SH area is located at the apex of the embayment on the retro-arc side of the basin in a macrotidal regime (Fig. 8). The tempestite facies represented in this area by facies sub-association F-3, show less energetic conditions compared to the other areas, pointing to reduced wave action and dominance by tidal currents. The inner-shelf to nearshore deposits of facies association G with a greater sediment input from the north of the basin (Figs. 3 and 5) are better developed. The same pattern of high sediment input along the basin axis from north to south was seen in the Allen (Loncoche) Formation (Barrio, 1990). Flexure in the foreland basin caused thickening of facies association G in the SH area (Fig. 3). The presence of clay drapes in facies association G is evidence of tidal currents dominating sedimentation. Seasonal sediment input indicating a semi-arid climate also influenced facies association G deposition. Carbonate sedimentation was predictably almost continuous in the basin, but was disturbed during the rainy season(s), when terrigenous clasts were brought into the basin causing mixed carbonate-terrigenous deposition in the nearshore environments. A similar example of

LATE C R E T A C E O U S - E A R L Y T E R T I A R Y S E D I M E N T A T I O N IN A SEMI-ARID F O R E L A N D BASIN

aperiodical sedimentation in an arid setting is described in the Northern Red Sea by Purser et al. (1987) and by Roberts and Murray (1988) with infrequent terrigenous input, allowing interim periods for carbonates to develop. This contrasts with sedimentation in temperate climates, typically associated with more uniform terrigenous input and consequently decreased carbonate sedimentation (see Walker et al., 1983; Pigram et al., 1989). Meandering channels of facies association I in the SH area show major scouring and large pelitic intraclasts and coarser grain-size of the channel deposits probably related to steeper paleoslope on the retro-arc side of the basin and to the closer location to the mountain front (Fig. 8). The lack of point-bar accretion on this side of the basin is attributed to a high subsidence rate from flexural loading (Kraus and Middleton, 1987; Shuster and Steidtmann, 1987). Rapid subsidence and sediment accumulation inhibited pedogenesis in this area (Kraus and Middleton, 1987).

A M area

To the southeast, the AM area is located in the transition zone between the retro-arc and the cratonic sides of the basin (Figs. 5 and 8). Inferred conditions in this area included a mesotidal regime, with low sediment input, gentle paleoslope, and tidal currents under an arid climate. Beach deposits of facies sub-association F-2 were probably laid down by asymmetric ebb tide currents. Eroded shells from the northern area (SH) were deposited as facies sub-association F-2 along the eastern side, as the flow lost its carrying capacity (Fig. 5). The fossils probably originated in the northern embayment area (SH), where less energetic waves created appropriate conditions for faunal development. Once these shell-rich sediments were carried south to this area, they were reworked, mainly by storms which washed the fine-grain size sediments away, concentrating the biodetritus. Similar process of beach accumulation by currents parallel to the shoreline in an arid setting are described by Shinn (1973) for the Arabian Gulf and by Gostin et al. (1988) for southeast Australia.

271

Weaker tides in this area allowed the dominance of wind driven waves, which produced spits, beaches and barriers (Pethick, 1984). Lithologic homogeneity and the scarcity of terrigenous sediments in this area were possibly caused by proximity to the stable cratonic side of the basin, which restricted continental sediment input. Facies association I and J, representing continental meandering rivers, clearly show the tectonic controls active during their deposition. Minor scouring, finer grain-size and caliche deposits in the AM area, are clear evidence of reduced paleoslope and tectonic stability (Leeder, 1975; Fliigel, 1982; Ghosh, 1987). On the cratonic side of the basin, areas not affected by recurrent overbank deposition underwent more rapid pedogenesis (Alexander and Leeder, 1987) which favored the caliche development.

L P area

The LP area is located on the cratonic side of the basin (Fig. 8). During marine sedimentation it was presumably affected by a microtidal regime, with low sediment input and a gentle paleoslope under a semi-arid climate. Facies sub-association F-l, as F-2 in the Auca Mahuida area, shows an increase in wave activity compared to facies subassociation F-3, and may be related to the microtidal setting dominant in this area. The development of oolitic shoals or barriers in facies association H (Fig. 5), was the result of a microtidal regime on the open side of the embayment, which was more conducive to developing barriers along the coastline (Hayes, 1975). High wave energy is inferred by the dominance of oolites in the shoal. The barrier was probably generated as a series of spits formed by southerly ebb-tidal currents, similar to the Arabian Gulf (Shinn, 1973) and to the Suez Gulf (Sneh and Friedman, 1984) where accretion along the coast takes place mainly on spit complexes. Climate also controlled the sediment input, with periods of sediment starvation during the dry season leading to the development of evaporite deposits (facies association D, Fig. 5). The meandering deposits of facies association I

272

and J in this area had the same controls that were active in the AM area.

Conclusions The Neuqutn Basin at the Late CretaceousEarly Tertiary was a funnel-shaped foreland basin, with an active volcanic arc to the west and a low-lying craton to the east, within the westerly wind belt. The mountains created rain shadow to the east resulting in semi-arid chmatic conditions, The tidal range in the funnel shaped basin change from micro to macrotidal due to the northwards narrowing of the basin. Related to the particular setting, major paleoenvironmental changes have been described for the sediments of the Malargiie Group. Depositional controls have interacted to produce a particular facies association arrangement in the Neuqutn Basin. Foreland tectonics produced an asymmetrical basin which controlled sediment input and paleoslopes, and affected tidal current pathways inside the basin. Tidal currents entered the basin on the deeper, western, retro-arc side. The tidal currents affected by the Coriohs force associated with westerly winds generated a clockwise transport that governed sediment movement in the basin. The embayment geometry of the Neuqutn Basin affected the tidal regime, which in turn controlled depositional environments along the shoreline. The predominant arid climate was also an important factor in the development and distribution of facies associations. The evaporites are evidence of high rates of evaporation under a negative E-P budget. Aperiodic terrigenous input to the basin allowed the development of carbonate sedimentation during long periods, disturbed occasionally by terrigenous influx, which generating mixed carbonate-clastic facies associations.

Acknowledgements This contribution is a part of a Doctoral Thesis at the University of South Carohna under the direction of Dr. A.E.M. Nairn. Financial assistance for the field work was provided by CONICET (National Research Council of Argentina; PID 390450285 to Dr. Luis A. Spalletti). The

C.A. BARRIO

Centro de Investigaciones Geol6gicas (La Plata, Argentina) provided the logistical support for the earlier research. Ces~r Gazzera was a dependable friend in the field work. At the Earth Sciences and Resources Institute, Dr. William H. Kanes encouraged my studies in the Neuqutn Basin and allowed the time to complete the study. John Reed, Tom DeVries, Luis A. Spalletti, Carlos E. Macellari, Richardson Allen and Steven Schamel, provided some valuable ideas related to the topic of this paper. The advice of Dr. Carmen Medeiros (Baruch Institute, U.S.C.) about tidal currents was extremely clarifying. My wife Cindy Barrio prepared some of the figures and shared enthusiasm through my Ph.D. dissertation. The help of all these persons is greatly appreciated. Reviews by David Krinsley (Arizona State University) and an anonymous reviewer improved the final manuscript.

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