Palaeomagnetic Stratigraphy of Pliocene Continental Deposits of the Bolivian Altiplano
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I, f' af 33 1 '0- '0- IS, o1 PALAEOMAGNETIC STRATIGRAPHY OF PLIOCENE k- i CONTINENTAL DEPOSITS OF THE BOLIVIAN ALTIPLANO m :y al N.THOUVENY' and hf. SERVANT' J. LGQ/CIL'RS, Luminy, Case 907, 13288 Marseille Cedex O9 (France) 11- 'UR 103 ORSTON, 70-74 Route d'Aulnay, Bondy 9.3 (France) n- (Received April 28, 19%; revised and accepted September '26, 19SS) e, .5. 1 Abstract. 31 2: Thouveny, N.and Servant, LI., 19S9. Palaeomagnetic stratigraphy of Pliocene continental deposits of the Bolivian .Altiplano. Palaeogeogr., Palaeoclimatol., Palaeoecol., 'io: 331-344. Fluvio-lacustrine post-Miocene deposits of the Bolivian Altiplano have been sampled in 400 m of section in the La Paz and Ayo Ayo basi'ns in order to provide a magnetostratigraphic scheme for chronostratigraphic interpretations. Stable characteristic components were isolated after stepwise .AF or thermal demagnetizations. The directions of cleaned magnetization were compared to the normal and reverse dipole field directions at the site. The plot OE the mean directions calculated for each straiigraphic level define successive magnetozones which can be identified on the standard geomagnetic polarity scale with the help of the K/Ar ages obtained on tuff layerã. The deposition of n the middle and upper part of the La Paz Formation covers the Gauss epoch. The Gauss/bIatuyama (2.4s Ma) and the GilbertiGauss (3.4 Ma) limits are located in the top and the middle OF the formation. The occurrence of the first g!acial/interglacial oscillation can be placed in the early Matuyama (Ca 2.2 Ma) in agreement with the oxygen iãotopic record from the equatorial ocean. Introduction Ayo basins, 100 km south from La Paz (Evern- den et al., 1977). This can be considered as a The Altiplano plateau at 4000 m spreads maximum age since remains of Pliocene mam- between the Western and the Eastern cordil- mals were discovered at Ayo Ayo, in the lower lera of the Andes. The La Paz basin (Bolivia) part of the Umala Formation, correlatable to situated at 16'30'S, 6S'W (Fig.1) was cut in the the La Paz Formation (Hoffstetter, 1971), and Altiplano by a regressive erosional phase dur- at La Paz in the lower part of the La Paz I l ing the Late Pleistocene related to the setting formation (Villarroel, 1978). The K/Ar age was I of the upper hydrologic basin of the Amazon. recently confirbed on a correlatable tuff layer I First reported by Troll and Finterwalter (1935), near the base of the La Paz Formation called the lacustrine, fluvial and glacial deposits were Toba Cota Cota: the age given by Servant et al. described in detail by Dobrovolny (1962). The (1988) is 5.5 I: 0.02 Ma. stratigraphy was recently reinterpreted by Servant (1977) and Balliviàn et al. (1978). Stratigraphy and recent chronological I The first chronological data obtained on the data I post-hfiocene deposics (5.5 Ma) results from i E(/& analyses on a tuff layer (Toba Umala) The lower sequence (La Paz Formation sensu 1 underlying the deposits in the Umala and Ayo stricto) is more than 500m thick, and can be I i' oo:~l-ols~js9/so:3.so c, 1989 Elsevier Science Publishers B.V. AC : Achocalla PU: Purapura O 3 km u 'i .2900 Fig.1. Geographical situation of the La Paz Basin and sampling localities. described at successive outcrops (Figs2 and 6). Ayo and 2.SF0.1 at La Paz (Lavenu et al., It consists of lacustrine silty and clayey beds 19SS). interlayered with fluvio-lacustrine sandy and Above the Chijini tuff, the La Paz Formation gravelly layers. The sediments are grey, often is enriched with conglomeratic units. The lightly coloured in yellow (silts) or green upper part of the formation is affected by a (clays). Some clayey levels are brightly col- major discontinuity (erosional phase) overlain oured (red, white or black). by fluvio-glacial or glacial deposits: till In the upper part of this sequence, a white (50-100 m thick) of the Calvario glaciation on bed (2-12m thick) was recognized as a tuff which lies a thick (100-200 m) interglacial (Toba Chijini, sometimes presenting an ignim- conglomeratic bed (Purapuråni Formation). X britic facies), which was formerly reported as second tuff layer (Toba Sopari) can be observed 1 overlying the La Paz Formation (Dobrovolny, in the upper valley of the Rio Kaluyo (North 1962) but can clearly be observed as inter- from La Paz) interlayered between the Cal- I bedded in the upper part of it. After unsuccess- vario till and the Purapurani conglomerates. i full K/Ar dating attempts (ages ranging Dobtovolny (1962) did not differentiate this i from 0.5 to 11 Ma), two coherent ages were tuff from the Chijini tuff and considered the finally obtained by K/Ar analyses on bio- lowest till older than the Chijini tuff but he tite by MacIntyre (quoted in Clapperton, noted that in other sections of the basin, the I 1979): 3.2'7k0.14 and 3.2840.13 Ma. Other Chijini tuff directly overlies the La Paz Forma- ! i ages now available are 2.840.1 Ma at Ayo tion. Clapperton (1979) also claimed to have i 1 ! 333 sw NE / IR : lrpavi section VS : Viscachani section AC: Achocalla section PU: Purapura section Fig.?. Summary stratigraphy of the post->riocene deposits of the La Paz basin. I =Paleozoic or Cretaceous basement. 2=La Paz formation s.s. 3=Tills. .i=interglacial conglomerates. 5= tuff layers. LP= La Paz. C= Calvario. FP= Purapurani formation. K= Kaluyo. S=Sorata. Ch= Choqueyapu. I-IV design the erosional phases. observed glacial deposits below the Chijini integrated in the dated samples and so falsify tuff. Following stratigraphical arguments and the results. As such inclusions were not using the ages provided by McIntyre in Clapper- observed in the Chuquiaguillo valley, the age ton (19í9) Clapperton dated this glacial phase obtained from this site might be considered as prior to 3.2 Ma. However, Clapperton did not more reliable than the other two. present any precise description or stratigraphic Other successive tills can be observed in log to support his arguments. All the clear the upper part of the Kaluyo valley attributed and complete sections of the Basin clearly to the Kaluyo, Sorata and Choqeyapu gla- show that the Chijini Tuff not only overlies the ciations. These tills are separated by inter- La Paz formation, but is interlayered in its glacial deposits, erosional phases and palaeo- upper part (note that Clapperton’s new section soils. is situated between the Capellani section and the Viscachani section of Ballivian et al., 1978) Magnetostratigraphic study This problem is further complicated by con- tradictory KIAr ages obtained on the Sopari Magnetostratigraphic investigations of post- tuff 2.7 f0.1 and 2.8 f0.1 Ma (remarkably con- Miocene deposits from Bolivia were success- sistent with the ages of the Chijini tuff) on fully carried.out by MacFadden et al. (1983) on two samples from the Kaluyo valley and the Quaternary Tarija Formation (south 1.650.1Ma on one sample from the Chuquia- Bolivia). The palaeomagnetic investigations of guillo valley. However, the observation of the La Paz Formation were initiated by Ser- multiple inclusions of granitic materials vant in 1981: specimens were tested for the (pebbles) in the outcrops of the Kaluyo valley intensity and sta-lsility of the remanence at the suggests that old biotites might have been LGQ. A complete sampling program was car- , 334 ried out in May 19S4 on five different sections laminated clayey levels only provided one or of the La Paz basin (Figs2 and 6). two specimens each. (1) The IRPAVI section (Fig.6a). 130 m thick. comprises the bottoni part of the La Paz Palaeomagiiet ic ai1 a ljses Formation: 31 levels were sampled. Unfortu- nately. the link with the middle and upper part Measurements were made with a Balanced of the formation could not be determined until Flusgate Spinner Magnetometer (Molyneux. now. 19ïl) connected to a microcomputer (Tucholka (2) The VISCACHANI section (Fig.Gb), about and Tessier, 1957). Natural Remanent Magneti- 210 ni high. comprises the middle aiid upper zation (NRM) intensities range from 0.5 to part of the La Paz Forgation (Chijini tuff 20 mA ni- (1 millihinpere/nieter = 1 niicro- included) and the Calvario till. However, for Gauss). The highest values (0.35.4 ni-’) are reasons of accessibility. the levels above the recorded in the tuff layers. The NRlI values Chijini tuff could not be sampled. Twenty-nine are not plotted on the figures because un- levels were sampled in the l‘ioni below the cleaned remanent magnetizations are not tuff. representative: furthermore, characteristic (3) The levels above the Chijini tuff were remanent magnetizations were isolated at dif- sampled in the basin of the Rio Achocalla ferent thermal steps providing a large and (tributary to the La Paz basin) (Fig.6~).Twelve meaningless spectrum of intensity values. levels were sampled in a Som section called Susceptibility nieasurements were carried Achocalla. out with a Bartington Als2 meter: the values (4) Rare and thin clayey layers could be range from 5 to 200 x SI units (Fig.6a-c): sampled in the Calvario till and in the Purapu- the highest values (>SO x IO-’ SI) have not rani interglacial complex (Pura Pura section. been recorded in rhe tuff layers but in the silty Fig 6d). The scarcity of the subsampled levels and clayey layers of the Irpavi section. is due to the difficulty of finding fine-grained The uncleaned NRM directions seem to be layers and safe sampling sites. strongly influenced bh7 the present-day field.