.J:-'lA/1 c. t':.. t}C!-"' tocr·c....o

"?~ --fi .. dfio o BF rr n to •1P .)'" t\.10 1 e L.w J

54.810 P~ru 1

996 . DUQN~--~

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Abstract Three zones, each with a different •tructurol style, have been recognized in the altiplono of the Andes in southern Peru . The western and eastern zones are char­acterized by folds, the central zone by the chaotic juxta­position of blocks up to 500 m long (Titicaca melange). Previous authors supposed that the chaotic structure had been caused by folding and overthrusting, but more probably the chaos is the result of . subaerial sliding. The melange is thus not of tectonic, but of sedimentary origin, and is hence an olistostrome. large olistostromes, such as the Titicaca melange, indicate sedimentary-tectonic mass transport and are transitional between structures formed by gravity tectonics (tectonic mass transport) and clastic sediments (sedimentary mass transport). The close association of folds, a large olistostrome, and a thick pile of clostic sed im ents suggests that all three proce sses have been ill)partant in the area near laga (lake) Titicaca.

The great similarity between the Titicaca olistostrome and th<: Amargosa chaos in Death Valley, California , sup­parts the landslide explanat ion of this chaos.

INTRODUCTION

For more than 2,000 mi, from southern Chile northward, the Andes form a straight north-south mountain belt, which turns 50° west at the boundaries between Chile, Bolivia, and Peru. At this bend the Andes can be di­vided into the western cordillera, altipl ano, and eastern cordillera (Fig. 1, inset) . The geologic structure in the altiplano has been related to the emplacement of igneous rocks in the west­ern cordillera (James, 1971 ), and also to the abrupt change in direction of the Andes (Newell, 1949) .

The structure of the altiplano does not seem to be typical for that of the Andes, as the typical Andean structure is characterized by the absence of large subhorizontal movements and the dominance of vertical displacements (Zeit, 1970; Ahlfeld, 1970; -"Cobbin~ 1972). According to Newell (1949) , compression has been important in the altiplano , and horizontal mass transport was extensive. He wrote (p. 20):

The rocks of the altiplano have been greatly deformed and modified by compression from the southwest. ... This compression is expressed in a number of thrust faults southwest of the axis of Lake Titicaca, and in hundreds of broken, overturned, and recumbent folds which in the great majority of cases have been over­thrown toward the northeast.

Since Newell's monograph was published, .new concepts such as those on gravity sliding

TA 0 have been developed . pphc . ton of these con­cepts to the northern Titicaca area suggests strongly that the structure of this part of the altiplano is also typically Andean: vertical movements are dominant, and subhorizontal movements subordinate.

RocKs

The major contribution on the stratigraphy of the northern Titicaca area is by Newell, who published in 1949 the results of detailed field work. His stratigraphic data on the Puno- r

Santa Lucia area and the Puno Province were revised by PortugaL ( 1964) and Zambrano eta!. (1965).

The oldest rocks are Paleozoic and sedi­mentary. They underlie large areas of the eastern cordillera. The overlying sedimentary and volcanic rocks (mainly redbeds, andesites, and basalts) of Mesozoic and Tertiary age can be very thick: Portugal ( 1964) mentioned an aggregate maximum thickness of 7,255 m in the Puno area. The volcanic rocks, dominant in the upper part of the sequence, are char­acteristic of the western cordillera.

The stratigraphic units are described briefly in the following paragraphs (see also Fig. 2) . All localities mentioned in the text are shown in Figure 1.

© 1974. The American Association of Petroleum Geol­ogists. All rights reserved.

'Manuscript received, April 16, 1973; accepted, Oc­tober 18, 1973.

'Department of Geology, University of Cincinnati. This paper is No. 21 in a series of papers on South American geology by members of the Department of Geology of the University of Cincinnati.

I thank the American Philosophical Society for grant No: 6090 from the Penrose Fund which made this study possible. I also thank A Rodriguez and A Parodi of the Universidad Nacional de San Agustin in Arequipa for introducing me to the geology of the altiplano; J.uan Schoutens (Collegia Juan Bosco, Puna) , R. P . Ramon Leon (Caritas de Puno) , and Julio Velazco (Universidad Nacional del Altiplano, Puna) for arranging field transportation for a period of 6 days; W. F. Jenks (University of Cincinnati), P. J. Coney (Middlebury College), and S. P. Fisher (Ohio University, Athens) for comments on the manu­script; Ruth Scott for preparing the drawings; and Wanda Osborne and June Betts for typing the manu­script.

729

730

500 kr<l

Kees A. De Jong

c=J OIIUVIOI depOSitS

~ S•llop~;Je~io'}erflory

Puna Group B Put1no Group c:J ~-Upper Cretaceous-

Lower Tert1ory

I Tit icoco Group

c=J Lower and Mtddle Cretaceous

v:::::.:·: :·1 Lagunil las Group JuraSSIC

LAGO

T/ TIC A CA 38 12 m

@ -:4monlom

FIG. !-Geologic map of area around northern end of Lago Titicaca. Mainly after Newell ( 1949) , Portugal (1964), and Zambrano eta/. ( 1965). In inset, st ipple pattern indicates Andes.

Cabanil/as Group (Newell, 1949; Boucot and Megard, 1972)-Shale, micaceous sandstone. Bryozoa, brachiopods, and trilobites indicate Si lurian-Devonian age. Thickness more than 2,000 m.

Copacabana Group (Newell, 1949)-Mainly in Ccarccoyo M assif. Sandstone , shale, carbonate rock. Early Permian age is documented by brachiopod­fusulinid-gastropod fauna (Newell et a/., 1953). North-

east of Munani , 1,800 m thick. Mitu Group-Red sandstone and conglomerate along

road between Arapa and Chupa were interpreted as part of Puna Group (Tertiary) by Newell ( 1949) and Zambrano et a/. (1965). But their similarity to rocks of the Mitu Group (Newell et a/. , 1953) suggested to the present writer that Arapa-Chupa rocks are Paleo­zoic instead of Terti ary. Thickness more than 900 rn.

' ,

'l

Melange !Olistostrome) Near Logo Titicaca, Peru 731

W± E± ~, Si!lopaca Fm (

V v 'v' ' v ·v U!per Tertiary) \1 'I V v V \1 \1 \· \1

Puna Group v v v v V V v v v v (Upper Cretaceous- " Tocaza Fmv V V v + ,; v v ,, ·v v v ~v.v+ Lower Tertiary)

Putina Group (Upper Cretaceous)

T1ticaca Group (Lower B Middle Cretaceous)

Copacabana Group (Lower Permian)

" VVV: volcanic rocks /V /VV : unconformities

FIG. 2-Diagram of stratigraphic units in Titicaca area.

Lagunil/as Group (Portugal, 1964)-Limestone, shale, sandstone. Group is of Jurassic age according to ammonite fauna. Thickness more than I ,050 m.

Titicaca Group-Sipin, Muni, Huancane, and Moho Formations can be combined in a group, here referred to as Titicaca Group. Type sections of all four for­mations are near shore of Lago Titicaca; Huancane and Moho type sections are at northeastern end of lake, and Sipin and Muni sections at northwestern end. Formations are conformable with one another, whereas upper and lower boundaries of Titicaca Group are unconformable. Redbeds are predominant, but limestones are present as well.

Sipin Formation (Newell, 1949)-Limestone, com­monly brownish-gray, but lighter in color than Ayavacas limestone. With diabasic andesite flows (Zambrano eta/. , 1965). Thickness about 50 m.

Muni Formation (Newell , 1949)-Mainly redbeds (shale-sandstone-conglomerate), with some carbonate beds. Thickness more than 350 m in Puno area.

Huancane Formation (Newell, 1949; Portugal, 1964) -Red, pink, and white sandstone and conglomerate. Northeast of Puno, hematite bed is present near top of formation . Thickness of formation in that area about 300m.

Moho Fo rmation (Newell, 1949; Portugal, 1964; Zambrano et a/. , 1965)-Red shale and sandstone with limestone intercalations. Most persistent of these lime­stone units is Ayavacas limestone. Other limestone units with lower stratigraphic position are similar to Sipin Formation which may lead to confusion (Por­tugal, 1964). Ayavacas limestone is massive with many beds thicker than 1 m, but total thickness does not exceed 30 m. Fossils in Ayavacas limestone indicate Cenomanian age. With this information, and compa r­ing T iticaca Group with similar rocks in adjoining areas, Newell ( 1949) assigned early-Middle Cretaceous age to rocks of Titicaca Group. Gypsum beds in Moho Formation are quarried near Saman. Their strati-

. graphic position is probably below that of Ayavacas limestone.

Putina Group-Sequence of mainly redbeds and volcanic rocks overlies Titicaca Group unconformably. Near village of Putina, this sequence can be divided into three formations (Newell, 1949) which have been combined in this paper into Putina Group. Portugal (1964) recognized formations of this group also in western part of area of Figure 1. Trachytes are pres­ent at base of oldest unit, but bulk of this Cotacucho Formation. consists of pink sandstone and conglom­erate. Red shales of Vi/quechico Formation, with plant fossil s of Late Cretaceous affinities (Newell, 1949), are overlain by red sandstone and conglomerate of Munani Formation. Thickness of Putina Group in western part of area is 3,640 m (Portugal, 1964), and in east 2,500 m (Newell, 1949).

Puna G roup-Puno Group, apparently absent in eastern area, consists of clastic sedimentary rocks of Saracocha Formation and volcanic rocks of Taca za Formation. Portugal (1964) measured aggregate thick­ness of 2,400 m. Newell ascribed all redbeds above Titicaca Group in west to Puno Group, but Portugal found that lower part of redbed sequence around Puno belongs to Putina Group instead. Part of Puno Group which overlies the Putina Group must be younger than Late Cretaceous, but part which overlies Titicaca Group may be lateral equivalent of Putina rocks and consequently of Late Cretaceous age.

Sil/apaca Formation (Portugal, 1964 )-Formation consists of essential ly horizontal andesite and besalt flows, tufts, breccias, and agglomerates which overlie truncated older rocks in western cordillera. Maximum thickness is about 400 m.

Intrusive rocks-Mostly small stocks of rhyolite, diorite, granodiorite, monzonite, and related igneous rocks. Similarity between intrusive rocks suggested to Portugal (1964) that stocks have common magmatic origin, representing apophyses of large batholith. In­trusive rocks cut Tacaza Formation of probable Terti­ary age and are thus younger than rocks of coast batholith near Arequipa, which are of Late Cretaceous age (Jenks, 1948).

732 Kees A . De Jong

Titicoco Group (LS.MCret.)

Paleozoic Rocks

Laqo

FIG. 3-Map showing location of structural zones I, II, and III.

STRUCTURE

In the area aro und the northern end of Lago Titicaca the rocks of the Titicaca Group have been deformed more intensely than the sed i­ments of the Putina Group, Puno Group, and the Paleozoic formations. The Titicaca rocks crop out in a long belt northeast of the lake, and in smaller areas west and south . Three zones, I, II, and III, each with its own char-

sw

\

acteristic structure, have been distinguished (Fig. 3).

In Zone I large folds characterize the struc­ture. The folds can be traced over many kilo­meters, and the direction of the fold axes is invariably northwest-southeast. The folds are tight, locally even isoclinal, and the axial planes dip northeast (Fig. 4). Southwest of the Ccarccoyo Massif the sediments of the Upper Cretaceous Putina Group also are folded, but in a much more open and symmetric way. Newell (1949) ascribed this difference in tec­tonic style to a difference in the behavior of the Putina and Titicaca Groups during deforma­tion , but his assumption that both groups were deformed at the same time is not necessarily correct. Possibly the Titicaca Group in Zone I was folded before the deposition of the Putina Group.

Folds are also characteristic in Zone II , but they are distinctly different from those in Zone I. It is almost impossible to follow them over more than 1 or 2 km; the folds have varied shapes as shown in Figure 5, and the attitude of the axial planes, though generally inclined, varies from fold to fold , from upright to overturned and recumbent. Portugal (1964) explained this type of fold as the result of downward sliding of the rocks of the Titic;11ca Group from the uplifted area of the Coachico Massif shortly after the deposition of the Moho Formation. Locally the rocks in Zone II be­come chaotic: the attitude of the sedimentary reeks changes abruptly and recurrently, and rocks of widely different stratigraphic position are juxtaposed.

The structural style of Zone Ill is similar to that of Zone II . In both, small irregular folds

NE

F IG. 4- Example of structu re in Zone I, northeast of Azangaro (see Fig. 1 for location). Isoclinal folds.

Melange !Olistostrome) Near Logo Titicaca, Peru 733

s N

FIG. 5-Example of structure in Zone II, southwest of Juliaca (see Fig. I for location) . Irregular folds of Ayavacas limestone.

and a chaotic structure are present; but in Zone II the folds are more important, whereas chaotic structure dominates in Zone III. In the northern part of Zone III folds are almost totally absent, and this part of Zone III is the place to which Newell principally referred when he wrote ( 1949) about crustal compression in the altiplano. Within this zone the writer mapped a small area immediately north of Puno (Figs. 6-10) and reinterpreted the structure of the oil field district of Pirin (Figs. 11-16).

WSW

The extension of Zone III is unknown, but might be very large. At Cusco, 200 km north­west of Lago Titicaca, rocks of the Yuncaipata limestone (correlated with the Ayavacas lime­stone by Kalafatovich , 1970) are also chaotic, and the structure is remarkably similar to that of Zone III .

Structure of Puno Area

The geologic structure of the area just north of the city of Puno (Fig. 6) changes drasticaliy

ENE

LAGO TITICACA (3812 ml

FIG. 6-Geologic map and section of Puno area. Notice distinction between observation and interpretation in section. Topographic base of this map was a 1.20,000-aerial photograph, kindly provided by Julio Velazco of Universidad Nacional del Altiplano. Dip symbols in olistostrome blocks do not show whether blocks have been

overturned. Legend: see names of strati graphic units in cross section.

734 Kees A. De Jong

FIG. 7- Qua rry north of Puna in block of Sipin(?) limestone. Breccia layers separate limestone slabs.

from east to west. In the east the rocks have a fairly constant attitude, and some individual layers, such as a hematite bed at the top of the Huancane Formation, can be traced over at least 2 km. The presence of a large overturned syncline is likely, though a hinge zone was not observed.

West of the syncline the structure is typically chaotic. Here the formations of the Titicaca Group are in large blocks (50-500 m) which appear to flo at in a softer matrix. Adjacent blocks may be of formations widely separated in the stratigraphic column. Although some blocks are extremely shattered, others are not fractured. Movement zones in the blocks indi-

cate that some formations must have undergone internal deformation as well (Fig. 7).

In some blocks the bedding is nearly hori­zontal , but in many others it is steep or sub­vertical, and probably a few blocks are actually overturned. Bedding in adjoining blocks can have a similar or a strongly different attitude, as shown, for example, in the contrasted atti­tudes of Ayavacas blocks west and east of the University buildings. A similar attitude in ad­joining blocks may be the result of the frag­mentation of a large block shortly before its final emplacement, whereas highly variable atti­tudes suggest that such fragmentation occurred at a much earlier stage.

The matrix around the blocks is well exposed in road-metal quarries and in cuts behind the University buildings. It is a breccia with boul­ders and smaller clasts of fractured limestone and sandstone between extremely fragmented smaller particles of multicolored shale and sandstone (Fig. 8). The boulders may reach 12 m in size, and a continuous transition in size between the smallest clast in the breccia and the largest block is common . Stratification never was observed.

In the western part of the map area blocks of shale and sandstone are overlain by basalts of the Sillapaca Formation and coarse sand­stone of the Puno Group (Fig. 9) . A few meters above the unconformable contact the

FIG. 8- Matrix of mel ange (olistostrome). Extremely fragmented boulders of limestone between smaller particles which are fragmented equally.

' .

Melange !Olistostrome) Near Lago Titicaca, Peru 735

Puno- road

FIG. 9-Unconformable contact between sandstone of Titicaca Group and sedimentary rocks of Puno Group. North of Puno.

sandstone of the Puno Group gives way to conglomerate, with many pebbles of limestone and sandstone from the Titicaca Group; ande­site pebbles become abundant about 30 m above the contact.

Puno Area: Interpretation The blocks in the Puno area are in a litho­

logic association essentially similar to that in which the blocks were formed , and they are thus not exotic, but native (Berkland et a!. , 1972) . The blocks are part of a "mappable body of rock characterized by the inclusion of fragments and blocks of all sizes ... 3 em­bedded in a fragmented and generally sheared matrix of more tractable material." Such a body is named melange by Berkland et al. (1972) , and the name Titicaca melange will be used

3 Both "exotic" and "native" have been omitted from the Berkland et a/. ( 1972) definition because they would have restricted the term mel ange unnecessa rily in this case. As the blocks are exclusively native in the Puno area, the correct name for the chaotic rock unit would have been "broken formation " (p. 2296) , whereas the chaotic units in the Pirin area should have been named a melange because both exotic and native blocks are present. To avoid two different terms for essentially the same unit, the requirement that blocks should be both exotic and native was omitted from the definition of a melange by Berkland et a/. ( 1972).

henceforth to indicate the chaotic rocks of Zone III.

The Titicaca melange reminded me of large olistostromes in the northern Apennines. The term olistostrome (Flores, 1955; Abbate et al. , 1970) is applied in Italy to breccias and con­glomerates which are derived from previously coherent rocks by fragmentation , sliding, and flowage . Olistostromes in the northern Apen­nines frequently were precursory to large nappes (Elter and Trevisan, 1973), as they bad been deposited in a tectonically active environ­ment. Slide breccias in a tectonically quiet en­vironment, such as the platform of the Sahara, also have been named olistostromes (Beuf et al. , 1971). In both the Sahara and the Apennines the olistostromes are products of submarine prdcesses, and they almost always are inter­calated in a sequence of marine sediments. Landslides which have been "fossilized" (i .e. , they were not eroded, but buried under sedi­ments) could be named olistostrome as well, and the Titicaca melange may be an example of such an olistostrome.

The Titicaca melange is the result of frag­mentation of lithified rocks of the Titicaca Group, and of the sliding of these rocks under the influence of gravity . Movement by an ex-

736 Kees A. De Jong

ternal force, exerted for example by an over­thrust sheet, seems to be precluded. A slope of sufficient steepness must have preceded sliding, and probably this topographic relief was re­duced not only by sliding of the Titicaca Group, but also by erosion, and subsequent sedimentation of the Puno Group. The youn­gest rocks of the Titicaca Group are shallow marine, and the clastic rocks of the Puno Group are continental (Newell, 1949; Portugal, 1964); the differential vertical movements which originated the sliding were thus upward relative to sea level. Upward movement after the deposition of the shallow-marine Moho Formation suggests strongly that the Titicaca melange represents a subaerial slide deposit (landslide) or, as has been argued above, an olistostrome.

There are no indications how far the melange (olistostrome) has moved in the Puno area, although the side-by-side position of blocks of quite different stratigraphic level is suggestive of a movement of at least a few kilometers. That movement was eastward is indicated by blocks derived from the older formations in the western part of the area, and also by the facing position of the syncline if olistostrome and syn­cline are related genetically. Such a relation has been assumed in the reconstruction of the events which led to the structure of the Puno area (Fig. 10).

Both fold and olistostrome indicate mass transport: coherent mass in the fold , frag­mented mass in the olistostrome. The sedi­mentary rocks of the Puno Group represent a further stage of fragmentation and are the product of sedimentary mass transport; the fold

FOLD: te c t oni c

moss- transp o rt

FrG. I 0-Genetic sections across Puno area.

was formed during tectonic mass transport, and the olistostrome during sedimentary-tectonic mass transport . The size and coherence of the major components in the deposit determine the adjective, the hybrid adjective stressing the continuity between the extremes in size. Such a continuity has been defended for many years by proponents of gravity tectonics , notably Van Bemmelen (1954).

The distinction between mass displacement by tectonic processes and mass displacement by sedimentary processes is thus scale-dependent when the tectonic processes are gravitational. Mass propelled by an external force is not emplaced gravitationally and consequently is not genetically transitional to sedimentary masses. An external force (push f10m behind) would result in a fabric which shows a clear relation with that force, whereas the emplace­ment of masses by gravity would produce a more chaotic fabric by virtue of the largely independent motion of the components, result­ing in a structure such as that of the Titicaca melange.

Structure of Pirin Area

The development of an oil field near Pirin began in 1875 and ended in 1944. Although the total production was small (about 3 7,500 metric tons, according to Newell, 1949) , the field received considerable interest from geolo­gists, perhaps because it is by far the highest oil field in the world . One of the wells was started at an elevation of 3,991 m above sea level .

Several of the geologists active in this area during the exploration for oil published their findings (Cabrera La Rosa and Petersen, 1936 ; Heim , 1947. 1948; Newell, 1946, 1949) , and a map and a cross section were published by three members of the work group of French geologists in Peru ( Chanove et al. , 1969). The structural interpretations of the Pirin area are highly divergent, but they can be compared easily, as three of the figures in the publications mentioned previously show interpretative cross sections (Fig. 11) along the same line (Fig. 12). The sections of Figure 11 a, b, c are very different from each other, and also from Fig­ure 11d which shows my interpretation. In each section an explanation has to be given of the same facts, i.e., the juxtaposition of rocks of different age, the variable attitude of the bed­ding, and the presence of the oldest rocks (Paleozoic and Lower Cretaceous) near the top of the hills.

..

Melange !Olistostrome) Near Logo Titicaca, Peru 737

Puno ·Group·. ·. ·.··

Moho - Ayovocu c. Siptn C. ()u6!1oro

HElM (1947) a Huoncorul

~;:~~ Jiti!l;t;t;~) Cobonollos

0 b-e-d

Fm. It-Sections across Pirin area, showi ng fo ur different interpretations (c/. Gerth, 1955, Fig. 26). a, Heim (1947, sec. 5 of Plate II ); b, Newell ( 1949, sec. D of Plate 2 1); c, Chanove et a/. ( 1969, Fig. I); d, this paper.

According to Heim (Fig. lla), it is not necessary to assume major tectonic movements other than block faulting to explain the struc­ture of the Pirin area. Intense erosion was fol­lowed by the deposition of Sipin limestone of

upperport ~Sipinlst. ol is tostrome Devonian

Ayovocos 1st. lower port Huoncone' sst.

Fig.l4

alluvium

Tertiary age. Newell (Fig. llb) made a dis­tinction between Lower Cretaceous Sipin lime­stone and middle Cretaceous Ayavacas lime­stone, and attributed the high position of the P aleozoic rocks to folding and thrusting toward

1000 2000

FIG. 12-Geologic map of Pirin area; reinterpretation of Newell 's (1949) 1:30,000 geologic map. Notice location of cross sections (Fig. !la-d).

738 Kees A. De Jong

FIG. 13-Biocks of Ayavacas limestone and Huancane sandstone in lower part of olistostrome (melange) in Pirin area . Notice abrupt termination of bedding aga inst matri x (see Fig. 12 for location ).

the northeast. It is interesting to note that the stratigrapher Newell attempted to explain the complex structure of the Pirin area tectonically, whereas the structural geologist Heim provided an essentially stratigraphic explanation .

Chanove et al . (Fig. llc) assumed the pres­ence of folds with inverse flanks of several kilometers and suggested that tectonic trans­port had been southwest instead of northeast. A steep fault near Pusi should have been the

E

result of a second tectonic phase. I noticed that the Pirin terrane was very similar to that of the P uno area: large blocks with highly vari­able attitudes within a softer matrix (Fig. 13). At a site where a fold hinge should be present according to Newell's ( 1949) geologic map, the attitude of the blocks has been determined (Fig. 14) . The attitude in a single block is fairly constant, but abrupt changes between blocks occur. A fault was not observed be-

~60 w

'···' I . I 1 1

Fto. 14--B1ocks of Ayavacas limes tone in lower part of o li stostrome (melange ) in Pirin area . Strike symbols have been drawn with respect to no rth arrow (see Fig. 12 fo r loca tion) .

••

I

·'4

Melange (Olistostrome) Near Lago Titicaca, Peru 739

/ 1 NW I

I I \ \ \

\

/ (

I \

...-/

I I \

FIG. 15-Sedimenta ry rocks of Puno Group in overturned position; topographi cally below, but stratigraphically above olistostrome. This is Pusi fl exure. Compare Figure 2, Plate 8 in Newell (1949).

tween the Puno Group and the older forma­tions (Fig. 15) , though draping over a fault in the subsurface is suggested by the overturned position of the Puno rocks.

Pirin Area: Interpretation

The absence of folds and faults and the presence of a chaotic structure, such as that of the Puno area, indicate a melange ( olisto­strome). The Paleozoic and Lower Cretaceous rocks (exotic blocks at the hilltops represent, together with the younger Cretaceous rocks in the valleys, a kind of reversal of the strati­graphic sequence, similar to the diverticulation in the western Alps, described by Debelmas and Kerckhove (1973 ) and Lemoine (1973). According to these authors, the sliding of the younger rocks occurred before sliding of the older rocks, eventually resulting in a partly inverted stratigraphic sequence. Continuing dif­ferential vertical uplift in the Pirin area re­sulted in the deposition of the thick clastic sequence of the P~no Group (Fig. 16) . This deposition was not continuous, but was inter­rupted east of Pirin (see F ig. 12) by renewed sliding of the chaotic rocks which became intercalated in the Puno rocks, indicating that a considerable topographic relief was still i existence during the first phase of Puno sedi­mentation.

Although tectonic disturbances did occur before and after the deposition of the Puno Group (Portugal, 1964) , the major tectonic movement in Zones II and III of the alti­plano in southern Peru is demonstrated by the presence of the folds , the olistostrome, and the thick clastic sequence of the Puno Group. These three features are caused by demolition of a relief generated by differential vertical

movement. This vertical movement is the major orogenic movement in the northern altiplano, and is not indicated by angular unconformities (Steinmann, 1929 ; Newell , 1949 ; Portugal, 1964), but by a thi ck pile of sedimentary and tectonic deposits. That orogenesis is reflected locally by sedimentation and not by uncon­formities has been emphasized by many geolo­gists, e.g., Price (1973 ) .

Amargosa Chaos: Melange (Olistostrome)?

Searching in the literature for a deposit simi­lar. to the Titicaca melange, my attention was called to the Amargosa chaos in the mountain

FIG. 16-Genetic sections across Pirin area.

740 Kees A. De Jong

ranges which border Death Valley in California (L. H. Lattman, personal commun.) . Drewes ( 1963, p. 57) describe<,l the Amargosa mass as follows. The chaotic blocks . . . are large and disordered, but are internally coherent .. . Adjacent blocks may be of rocks from two widely separated forma!ions. Bedding attitudes may also vary widely from block to block. Most of the blocks are internally unshattered, but some blocks of quartzite and dolomite are shattered and healed to their original strength.

This description is indeed similar to that of the Titicaca olistostrome!

The main difference between the chaotic masses near Lago Titicaca and the Amargosa chaos appears to be the ratio between matrix and blocks: in Peru the blocks float in an abundant matrix, whereas the width of the matrix between the blocks in the Death Valley area is Jess than a few meters (Drewes, 1963). But if certain fragmented shale "blocks" in the Amargosa chaos (Noble, 1941, Plates 7, 8, 10) are considered as matrix, the matrix-block ratio would become comparable in both chaotic masses. The Titicaca and Amargosa chaos both overlie the basement on which the sedimentary rocks had been formed, and both are covered by a thick pile of sedimentary and volcanic rocks which were deposited in a subaerial en­vironment, and later intruded by igneous rocks.

The structure of the Amargosa chaos has been related to compressional faulting (thrust faults, Noble, 1941), to extensional faulting (normal faults , Wright and Troxel , 1969, 1973), to strike-slip faulting (Drewes, 1963), and to landsliding (Sears, 1953; Bucher, 1956; Hunt and Mabey, 1966). The similarity between the Titicaca melange and the Amargosa chaos sug­gests that the Amargosa chaos is also a me­lange, produced by landsliding.

CONCLUSIONS

1. In the altiplano of southern Peru three zohes, each with its own characteristic struc­ture, can be distinguished (Fig. 3). The large asymmetric and locally isoclinal folds of Zone I must have been formed by the detachment of the Lower and middle Cretaceous formations of the Titicaca Group. Basement rocks do not participate in the folding ; this indicates folding is a result of sliding (gravity tectonics) . ·

2. The structure of Zones II and III is simi­lar. Irregular folds, characteristic for Zone II, were the result of sliding immediately after the deposition of the Titicaca Group (Portugal, 1964). The chaos, characteristic for Zone III,

is that of a melange, produced by subaerial • sliding.

3. The melange (olistostrome) in Zone III and the overlying clastic rocks of the Puno Group are demolition products of a relief generated by differential, vertical, upward movements. The olistostrome is transitional be­tween the sedimentary rocks and the (tectonic) folds; the distinction between sedimentary and tectonic is determined by the size of the com­ponents of the displaced mass.

4. Exploration for oil in the Pirin area has been based on an incorrect understanding of the structure of that area . Probably, the very small production per well in the Pirin oil field was the result of the well being drilled into a single block of the olistostrome.

5. Major orogenic movements in southern Peru are vertical. They are not indicated by angular unconformities in the altiplano, but by the formation of a melange (olistostrome) , and of clastic rocks.

6. Interpretations of the Amargosa chaos (Death Valley, Calltothia) are many, but the similarity between the chaos and the Titicaca melange enhances the landslide interpretation.

REFERENCES CITED

Abbate, E., V. Bortolotti, arid P. Passerini, 1970, Olistostromes and olistoli ths: Sediment. Geology, v. 4, p. 521-557.

Ahlfeld, F ., 1970, Zur Tektonik des andinen Bolivien: Geol. Rundschau, v. 59, p. 1124-1140.

Berkland, J. 0., L. A. Raymond, J. C. Kramer, E. M. Moores, and M. O'Day, 1972, What is Franciscan?: Am. Assoc. Petroleum Geologists Bull., v. 56, p. 2295-2302.

Beuf, S., B. Biju-Duval, 0. de Charpal, P. Rognon, 0. Gariel, and A. Bennacef, 1971 , Les gres du Paleozoique inferieur au Sahara: Paris, In st. Fran­cais Petrole, 419 p.

Boucot, A. J., and F. Megard, 1972, Silurian of Peru, in Correlation of the South American Silurian rocks: Geol. Soc. America Spec. Paper 133, p . 51.

Bucher, W. H., 1956, Role of gravity in orogenesis : Geol. Soc. America Bull., v. 67, p . 1295-1318.

Cabrera La Rosa, A., and G. Petersen, 1936, Recon­ocimiento geologico de los yacimientos petroliferos del departamento de Puno: Cuerpo lngenieros Minas

, Petroleo Peru Bol. 115, 100 p. .· Chanove, G ., M. Mattauer, and F. Megard, 1969,

Precisions sur Ia tectonique tangentielle des ter­rains secondaires du massif de Pirin (Nord-Ouest du lac Titicaca, Perou) : Acad. Sci. Comptes Rendus, ser. D, v. 268, p. 1698-170 I.

Cobbing, E. J., 1972, Tectonic elements of Peru and the evolution of the Andes, in Tectonics: 24th Inter­nat. Geol. Cong., Proc., sec. 3, p. 30~315.

Debelmas, J., and Cl. Kerckhove, 1973, Large gravity nappes in the French-Italian and French-Swiss Alps, in Gravity and tectonics : New York, Wiley-Inter­science, p. 189- 200.

Drewes, H., 1963, Geology of the Funeral Peak quad-

. ' .

.·.

(

!I

••

Melange !Olistostrome) Near Lago Titicaca , Peru 741

rangle, California, on the east flank of Death Val­ley: U.S. Geol. Su rvey Prof. Paper 413, 78 p.

Elter, P., and L. Trevisan, 1973, Olistostromes in the tectonic evolution of the northern Apennines, in Gravity and tectonics: New York, Wiley-lnterscience, p. 175-188.

Flores, G. , 1955, Discussion : 4th World Petroleum Cong. Proc., v. A2, p. 120-121.

Gerth, H ., 1955, Der geologische Bau der sudamerikan­ischen Kordillere: Berlin, Gebruder Borntraeger, 264 p.

Heim, A. , 1947, Estudios tectonicos en Ia region del campo petrolifero de Pirin, !ado NW del Lago Titicaca: Peru, Direccion Minas y Petroleo Bol. , ano 26, no. 79, 45 p.

--- 1948, Die Petrolfelder von Pirin und Ganso Azul in Peru : Ver. Schweizer Petroleum-Geo1ogen u.- Jngenieure Bull., v. 15 , no. 48, p. 10-1 3.

Hunt , C. B., and D. R. Mabey 1966, General geology of Death Valley, California-strat igraphy and struc­ture: U.S. Geol. Survey Prof. Paper 494-A, p. A l­A165.

James, D. E., 1971, Plate tectonics model for the evolution of the central Andes: Geol. Soc. America Bull., v. 82, p. 3325-3346.

Jenks, W. F ., 194&, Geology of the Arequipa quad­rangle of the Carta N acional del Peru: Peru I nst. Geol. Bol. 9, 204 p.

Kalafatovich, C., 1970, Geologia del Grupo arqueo­]ogico de Ia Forteleza de Saccsayhuaman y sus vecindades: Revista Patronato Dept. Arquelogia Cuzco, ano I, v. 1, p. 61-68.

Lemoine, M., 1973, About gravity gliding tectonics in the western Alps, in Gravity and tectonics: New York, Wiley-lnterscience, p. 201-216.

Newell, N. D ., 1946, Geological investigations around Lake Titicaca: Am. Jour. Sci., v. 244, p. 357-366.

1949, Geology of the Lake Titicaca region ,

Peru and Bolivia: Geol. Soc. America Mem. 36, Ill p.

- -- J. Chronic, and T . G. Roberts, 1953, Upper Paleozoic of Peru: Geol. Soc. America Mem. 58, 276 p.

Noble, L. F., 1941, Structural features of the Virgin Spring area, Death Valley, California: Geol. Soc. America Bull., v. 52, p. 941 - 1000.

Po rtugal, J . A. , 1964, Geology of the Puno-Santa Lucia area , Department of Puno, Peru: Ph.D. thesis, Univ. Cincinnati.

Price, R. A., 1973, Large-scale gravitational flow of supracrustal rocks, southern Canadian Rockies, in Gravity and tectonics: New York , Wiley-lntersci­ence, p. 491-502.

Sea rs, D. H. , 1953, Origin of Amargosa chaos, Virgin Spring area, Death Valley, California: Jour. Geol­ogy, v. 61, p. 182-186.

Stei nmann, G., 1929, Geologie von Peru : Heidelberg, Carl Winters Universitatsbuchhandlung, 448 p.

Van Bemme1en, R. W., 1954, Mountain building: Den Haag, Martinus Nijhoff, 177 p.

Wright, L. A., and B. W. Troxel, 1969, Chaos struc­ture and Basin and Range normal faults--evidence for a genetic relationship (abs.): Geol. Soc. Amer­ica Abs. with Programs, pt. 7, p. 242.

--- and --- 1973 , Shallow-fault interpretation of Basin and Range st ructure, southwestern Great Basin, in Gravity and tectonics: New York, Wiley­Interscience, p. 397-407.

Zambrano, R., P . Lavi, and R. Eyzaguirre, 1965, Pro­grama de inventario y evaluacion de los recursos nat­urales del Departamento de Puno. Capitulo III : Geologia y recursos mineros: Puno Oficina Nac. Evaluation Recursos Naturales Corp. (ONERN & CORPUNO), v. 2, 42 p.

Zeil, W. , 1970, Zur Geologie der An den: Geol. Rund­schau, v. 59, p. 827-834.

Reprinted for private circulation from THE AMERICAN ASSOCIATION OF PETROL EUM GEOLOGISTS bULLETIN

Vol. 58 , No. 4, April , 1974