Literature DB >> 29900393

Lithostratigraphic and magnetostratigraphic data from late Cenozoic glacial and proglacial sequences underlying the Altiplano at La Paz, Bolivia.

Nicholas J Roberts1, René W Barendregt2, John J Clague1.   

Abstract

We provide lithostratigraphic and magnetostratigraphic data derived from a Plio-Pleistocene continental sediment sequence underlying the Altiplano plateau at La Paz, Bolivia. The record comprises six sections along the upper Río La Paz valley, totaling over one kilometre of exposure and forming a ~20-km transect oblique to the adjacent Cordillera Real. Lithostratigraphic characterization includes lithologic and stratigraphic descriptions of units and their contacts. We targeted gravel and diamicton units for paleomagnetic sampling to address gaps in the only previous magnetostratigraphic study from this area. Paleomagnetic data - magnetic susceptibility and primary remanent magnetization revealed by progressive alternating field demagnetization - are derived from 808 individually oriented samples of flat-lying, fine-grained sediments. The datasets enable characterization of paleo-surfaces within the sequence, correlation between stratigraphic sections, and differentiation of asynchronous, but lithologically similar units. Correlation of the composite polarity sequence to the geomagnetic polarity time scale supports a range of late Cenozoic paleoenvironmental topics of regional to global importance: the number and ages of early glaciations in the tropical Andes; interhemispheric comparison of paleoclimate during the Plio-Pleistocene climatic transition; timing of and controls on inter-American faunal exchange; and the variability of Earth's paleomagnetic field.

Entities:  

Keywords:  Altiplano; Central Andes; Detrital remanent magnetization; Glacial stratigraphy; Magnetic susceptibility; Magnetostratigraphy; Mid-Piacenzian warm period; Plio-Pleistocene transition; South America

Year:  2018        PMID： 29900393      PMCID： PMC5997878          DOI： 10.1016/j.dib.2018.05.038

Source DB:  PubMed          Journal:  Data Brief        ISSN： 2352-3409


Specifications Table Value of the data Provides detailed lithostratigraphic and magnetostratigraphic records of the earliest known tropical glaciation in the Cenozoic Era. Enables comparison with global records of paleoenvironmental change during the Plio-Pleistocene climatic transition. Presents a detailed record supporting magnetostratigraphic comparison with late Cenozoic sequences underlying other parts of the Altiplano plateau. Provides detailed chronostratigraphic constraints of paleoenvironmental change and spatio-temporal variability of land mammal assemblages related to biotic exchange between the Americas. Contributes to the growing paleogeomagnetic record for central South America.

Data

Geologic sequences underlying the Altiplano plateau in the South American Andes provide extensive, but underexplored records of late Cenozoic continental paleoenvironments. Due to the Altiplano's long history as an internally drained basin [1], [2], its sequence of up to 12 km of Tertiary sediments is relatively complete [3]. The low-energy depositional environments represented by many units [3], [4], [5] makes these sediments suitable recorders of variations of the ancient geomagnetic field on a wide range of time scales [6], as demonstrated by the results of the small number of paleomagnetic investigations in the region [7], [8], [9], [10]. Despite the importance of records from the sub-Altiplano fill sequence, their ages are generally poorly constrained. Current chronologic control is based largely on radiometric dating of volcanic beds within the sequence [11], [12], [13], but many of the ages are unreliable [14]. Magnetostratigraphy at a few localities across the Altiplano constrains ages of non-volcanic units as well as their accumulation rates [7], [8], [9], [10]. Here we present chronostratigraphic data from the upper part of the fill sequence at La Paz, Bolivia, where it is extensively exposed. The data come from six sections within and adjacent to the city of La Paz (Fig. 1 and Table 1). Our lithostratigraphic descriptions and magnetostratigraphic data, respectively, build on the Plio-Pleistocene stratigraphic framework developed by previous workers [5], [15], [16], [17], [18], [19], [20], [21] and greatly expand upon the only previous paleomagnetic study in the area [8]. The data provide new insights into several aspects of the Altiplano and adjacent Cordillera Real area during the late Pliocene and Early Pleistocene [24]. Specifically, these data can be used to: demonstrate facies relationships of coeval units fining away from the high cordillera; constrain the number and ages of recurrent Pliocene and Early Pleistocene glaciations of the tropical Andes; revise the ages of several gravel sequences; quantity rates of sediment accumulation, the spatial variability of which help to characterize syndepositional tectonism; constrain the onset of incision of the local Altiplano surface and thus the approximate time of drainage capture of the eastern Altiplano by headwaters of the Amazon River system; and compare the timing of environmental change related to early glaciation of the Central Andes with patterns of Plio-Pleistocene faunal evolution and dispersal in the Americas, including occasional migrations leading up to the Great American Biotic Interchange.
Fig. 1

Location map. A. Extent of the Altiplano plateau within South America. B. Physiographic setting of the Cordillera Real and adjacent Altiplano margin. C. Locations of lithostratigraphic and magnetostratigraphic sections. Stratigraphic sections mentioned in the text are: PWT, Patapatani West; PTE, Patapatani East; TNG, Tangani; MIN, Minasa; PUR, Purapura; and JKT, Jacha Kkota. Terrain is from the ASTER GDEM 2 produced by METI and NASA.

Table 1

Summary of stratigraphic sections.

SectionSection height (m)
Sample sitesAverage sample spacing (m)Samples
Remanence calculation
TotalaSampledbTotalUsefulcUsedcPCAdGCe
Patapatani West232229474.93282842512510
Patapatani East5036.5103.7664841410
Tangani250190.506.41931811601600
Minasa4104101429.3847161610
Purapura25091146.5837066606
Jacha Kkota1755996.6545345414
Overall136710169410.880870762461410

Includes covered slopes and exposures that were not described.

Includes only described, sampled exposures.

Useful samples are of sufficient quality to enable identification of polarity, but in some cases yield low-precision remanence directions or remanence directions that are statistical outliers with respect to other samples within the same group. Used samples are those considered in statistical analysis, and do not include samples yielding low-precision remanence directions or remanence directions that are statistical outliers with respect to other samples within the same group.

Primary remanence directions determined by Principal Component Analysis (PCA) for samples included in overall statistics (Fig. 3A–C and Table 1 of Ref. [14]) and for mean directional data by stratigraphic section (Fig. 3D and Table 1 of Ref. [14]).

Primary remanence directions determined by intersection of great circles (GC) used for group mean directional data (Fig. 3D and Table 1 of Ref. [14]), but not in overall statistics of entire sample collection (Fig. 3A–C and Table 1 of Ref. [14]).

Location map. A. Extent of the Altiplano plateau within South America. B. Physiographic setting of the Cordillera Real and adjacent Altiplano margin. C. Locations of lithostratigraphic and magnetostratigraphic sections. Stratigraphic sections mentioned in the text are: PWT, Patapatani West; PTE, Patapatani East; TNG, Tangani; MIN, Minasa; PUR, Purapura; and JKT, Jacha Kkota. Terrain is from the ASTER GDEM 2 produced by METI and NASA. Summary of stratigraphic sections. Includes covered slopes and exposures that were not described. Includes only described, sampled exposures. Useful samples are of sufficient quality to enable identification of polarity, but in some cases yield low-precision remanence directions or remanence directions that are statistical outliers with respect to other samples within the same group. Used samples are those considered in statistical analysis, and do not include samples yielding low-precision remanence directions or remanence directions that are statistical outliers with respect to other samples within the same group. Primary remanence directions determined by Principal Component Analysis (PCA) for samples included in overall statistics (Fig. 3A–C and Table 1 of Ref. [14]) and for mean directional data by stratigraphic section (Fig. 3D and Table 1 of Ref. [14]). Primary remanence directions determined by intersection of great circles (GC) used for group mean directional data (Fig. 3D and Table 1 of Ref. [14]), but not in overall statistics of entire sample collection (Fig. 3A–C and Table 1 of Ref. [14]).

Section details

Patapatani West

Location

The Patapatani West section (16° 25.49′ S, 68° 08.03′ W, 232 m in height; Table 2; Figs. 5 and 6A of Ref. [14]) is located on the west bank of Río Kaluyo, where it curves east above the Limanpata landslide. It is the farthest upstream exposure in the Río Kaluyo/Choqueyapu valley (Fig. 1C) and consists of two exposures, each containing a 10-m-thick tuff at 4260–4270 m a.s.l. The upper exposure (137 m) extends from the base of the Chijini Tuff to the Altiplano surface and is exposed in a gully entering the west side of Quebrada Aquatiña. The lower part of the exposure [22] was uncovered during recent (ca. 2007) roadwork just downvalley of Quebrada Aquatiña. The top of this lower section aligns with Dobrovolny's [5], [18] 7-m type section of the Patapatani Drift on the opposite side of the valley and thus greatly extends exposure of the Patapatani Drift. Nineteen metres of the lower exposure are covered by spoil dumped downslope during road construction. A 3-m-high exposure on the west side of Quebrada Aquatiña, which does not appear to have slumped [22], provides the only details on stratigraphy and paleomagnetism in this largely covered zone (Fig. 6A of Ref. [14]). The sequence below the base of the exposed section (4165 m a.s.l.) is buried beneath Late Pleistocene and Holocene glacial and colluvial deposits down to Río Kaluyo (4125 m a.s.l.).

Stratigraphy

aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded. bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation. cSee Fig. 2 for clast fabric.
Fig. 2

Diamicton clast fabrics from the (A) Patapatani West and (B) Patapatani East sections. The fabrics are each based on trends and plunges of 50 elongate pebbles and cobbles (long:short axis ratio of ≥2:1) and are represented by Fisher distributions on equal-angle stereonets. Orientation densities range from 0% (white) to 10% (red).

Patapatani East

The Patapatani East section (16° 25.89′ S, 68° 08.06′ W, 50 m in height; Table 3; Fig. 6B of Ref. 14) is located between ~4060 and 4110 m a.s.l., low on the east slope of the Río Kaluyo valley [28], ~1 km south of the Patapatani West section (Fig. 1C). Dobrovolny [5], [18] describes the upper ~25 m of this section and assigns the diamicton below the tuff to the Patapatani Drift. A road cut created in 2003 or 2004 forms the lower 10 m of the Patapatani East section, 15 m below and 40 m southwest of the base of Dobrovolny's [5], [18] natural exposure of the Patapatani Drift. Colluvial deposits cover the steep slope between the natural exposure and the road cut. aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded. bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation. cSee Fig. 2 for clast fabric.

Tangani

The Tangani section (16° 27.33′ S, 68° 08.78′ W, 250 m in height; Table 4; Fig. 6C of Ref. [14]) is located in Quebrada Capellani, a deeply incised network of gullies on the east bank of Río Choqueyapu, 0.5 km upstream from the hairpin curve on the autopista (Fig. 1C). The uppermost 80 m of the main section are exposed only in vertical inaccessible cliffs; exposures in branches of Quebrada Tangani, ~300 m to the southeast, provide access to units 10–12, which we correlate to the upper part of the Tangani section by elevation and by a thick, laterally persistent silt bed 185 m above the base of the main section. aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded. bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

Minasa

The Minasa section (16° 27.85′ S, 68° 07.11′ W, 410 m in height; Table 5; Fig. 6D of Ref. [14]) is located along Río Minasa, a west-bank tributary of Río Orkojahuira in barrio Villa El Carmen (Fig. 1C). It extends from Puente Colonial (~3910 m a.s.l.) to Huari Pampa (4320 m a.s.l.), an undeveloped section of the Altiplano surface between ríos Kaluyo/Choqueyapu and Chuquiaguillo/Orkojahuira. Low natural exposures and road cuts along the south bank of Río Minasa form the lower half of the section. High natural exposures along the steep westernmost gully provide continuous exposure of the upper half of the section. Río Minasa follows the trace of a high-angle fault, but there is no obvious displacement of the Huari Pampa surface along this structure. aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded. bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

Purapura

The Purapura section (16° 27.74′ S, 68° 09.23′ W, 250 m in height; Table 6; Fig. 6E of Ref. [14]) mostly follows the old railway ascending to the Altiplano along the west slope of the Río Choqueyapu valley (Fig. 1C). The lower part of the section crosses this slope obliquely between quebradas Jacha and Pantisirca along the old railway route where it passes under the aqueduct. The upper part of the section follows the railway route on its final (60-m elevation gain) approach to the plateau. This section roughly coincides with the ‘Pura Pura’ (Aqueducto) section of Bles et al. [19] and Ballivián et al. [20], particularly the lowest 80 m. The Purapura section is ~2 km up-valley of the approximate location Thouveny and Servant [8] give for their Purapura magnetostratigraphic section. aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded. bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

Jacha Kkota

The Jacha Kkota section (16° 34.67′ S, 68° 10.25′ W, 175 m in height; Table 7; Fig. 6F of Ref. [14]) is exposed in the gullies of a ridge west of Laguna Jacha Kkota, rising from the floor of the Achocalla basin to the Altiplano surface (Fig. 1C). The ridge is an intact remnant of the fill sequence below the Altiplano, which was left behind when a gigantic, early Holocene earthflow created the Achocalla basin [23] shortly before 11,485–10,965 cal yr BP [24]. The section includes the Chijini Tuff and 42 m of fine-grained sediments directly below it. The Achocalla magnetostratigraphic section of Thouveny and Servant [8] extends from the base of the tuff 80 m upslope along the same ridge, but does not reach the Altiplano surface. We included the magnetostratigraphy of the uppermost 10 m of the sedimentary sequence at a nearby section (~3.2 km to the southeast); Fig. 1 to extend the Jacha Kkota section to the Altiplano surface. The exact stratigraphic alignment of the two sections is uncertain, but in view of the similar elevations of the Altiplano surface and the Chijini Tuff at both sites, the upper part of the sequence probably starts 40–45 m above the top of Thouveny and Servant's [8] Achocalla section, in agreement with the stratigraphy of the Achocalla basin margins reported by Bles et al. [19] and Ballivián et al. [20]. aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded. bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

Paleomagnetic data

See Table 2, Table 3, Table 4, Table 5, Table 6, Table 7.
Table 2

Means of paleomagnetic directional data for the Patapatani West section.

UnitLithologyχn
DIkα95apa
Position (m)Material(mean)CollectedUsefulUsed
PTW-20Altiplano gravelNot sampled
PTW-19Sorata Drift
 230.5 Paleosol779*1212121.0−27.0106.034.2N
 220.51896653.2−33.165.269.5N
 220.02136665.2−36.371.788.0N
 214.5294665333.6−35.579.458.6N
303028357.7−32.137.534.5N
PTW-18Kaluyo Gravels
 213.543166617−32.149.499.6N
N
PTW-17Kaluyo Gravels
 187.5299121210173.625.814.7013.0R
R
PTW-16Purapurani Gravels
 183.5 Sand lens114666174.020.175.797.7R
R
PTW-15Calvario Drift
 176.0 Diamict73600Indeterminate polarity−
 175.5 Silt bed104622222.545.3----R
1222R
PTW-14Calvario Drift
 165.5 Paleosol1054*666152.35518.5316R
 162.0 Silt bed700333161.437.0319.486.9R
999156.148.920.8111.6R
PTW-13Calvario Drift
 161.0 Paleosol2455*333348.6−35.7105.5412.1N
N
PTW-12Calvario Drift
 153.0 Silt bed (likely ash)2532333357.7−22.335.9220.9N
N
PTW-11Calvario Drift
 152.5 Paleosol1633*333353.7−42.9228.288.2N
 143.0 Sand lens3866663.7−35.2289.363.9N
 139.5 Silt/sand lense168666350.5−3.510.5921.6N
 134.0 Diamict & silt lens1641211843.3−22.221.7312.2N
 114.5 Silt lens154665346.4−47.230.2714.1N
 109.5 Silt lens18166612.6−38.134.7511.5N
 109.0 Diamict138665331.0−62.7124.936.9N
 108.0 Sand lens180666354.9−28.845.0510.1N
 107.5 Sand lens151611356.5−46.8−−N
5751464.8−35.48.827.6N
PTW-10Chijini Tuff
 103.5 Cliff-forming ash1509b66618.3−38.6120.906.1N
 101.0 Cliff-forming ash57366610.8−37.290.257.1N
 100.0 Cliff-forming ash6866630.9−27.1364.376.5N
 98.0 Cliff-forming ash84666510.3−29.882.668.5N
 97.0 Cliff-forming ash72966615.6−28.769.208.1N
 95.5 Silt-sized ash4076656.7−44.170.099.1N
36363111.5−34.956.793.5N
PTW-09Patapatani Drift
 94.5 Diamict1196660.5−13.237.0211.2N
 92.0 Diamict746668.3−29.628.0412.9N
 91.0 Diamict284666355.9−36.965.838.3N
1818181.7−26.724.157.2N
PTW-08Patapatani Drift
 89.5 Paleosol formed in diamict2368*6666.0−43.6250.614.2N
 88.0 Diamict8566528.4−27.420.6617.2N
12121117.1−36.922.219.9N
PTW-07Patapatani Drift
 85.5 Paleosol formed in diamict5556*66619.8−28.0343.833.6N
 81.5 Diamict155643347.7−23.266.7315.2N
121099.0−27.226.5510.2N
PTW-06Patapatani Drift
 77.5 Paleosol formed in diamict2641*664182.442.582.4310.2R
 74.5 Diamict125665166.214.6143.256.4R
 70.0 Sand lens168643217.055.641.5119.4R
 62.0 Silt lens162664182.71.631.3216.7R
242216180.726.89.1712.9R
PTW-05Patapatani Drift
 57.5 Paleosol (weakly developed)154644213.370.07.9334.8R
 52.5 Silt lens1001496187.628.819.8215.4R
201310192.644.97.0919.5R
PTW-04Patapatani Drift
 38.0 Silt lens149886356.3−28.455.679.1N
N
PTW-03Patapatani Drift
 19.0 Silt lens1271299191.126.07.8019.7R
 9.5 Sand lens & diamict146161210164.967.530.588.9R
282119182.249.66.8813.8R
PTW-02Pre-Patapatani stratified diamict
 7.0 Sand lens293666358.2−32.4173.295.1N
 4.5 Silt lens & diamict158121185.4−15.730.8910.1N
1817142.5−23.030.887.3N
PTW-01Pre-Patapatani gravels
 1.5 Gravel matrix113654359.1−15.214.9324.6N
N

See Fig. 6A of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity.

Magnetic enhancement of paleosol compared to the parent material in which it formed.

Error between 10° and 20° underlined; error greater than 20° double underlined.

Apparent magnetic enhancement at top of tuff unit.

Table 3

Means of paleomagnetic directional data for the Patapatani East section.

UnitLithologyχn
DIkα95apa
Position (m)Material(mean)CollectedUsefulUsed
PTE-08Calvario Drift
 39.0 Fine sand lens344666349.8−24.175.997.7N
N
PTE-07Calvario Drift
 37.0 Silt lens206664339.2−32.356.3112.4N
N
PTE-06Chijini Tuff
 31.5 Pumacious ash636666359.7−38.2159.305.3N
N
PTE-05Patapatani DriftNot sampled
PTE-04Patapatani Drift
 28.0 Silt lens13012121010.9−33.226.389.6N
121210N
PTE-03Patapatani Drift
 PaleosolNot sampled
PTE-02Patapatani Drift
 6.5 Silt lens152665146.919.445.9311.4R
R
PTE-01Patapatani Drift
 2.5 Fine sand lens131666185.019.724.8413.7R
 2.5 Fine sand lens131664211.232.3162.567.2R
121210194.625.419.0311.4R

See Fig. 6B of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity.

Error between 10° and 20° underlined; error greater than 20° double underlined.

Table 4

Means of paleomagnetic directional data for the Tangani section.

UnitLithologyχn
DIkα95apa
Position (m)Material(mean)CollectedUsefulUsed
TNG-13Valley-slope coverb
 Variable Gravel matrix76663328.0−28.918.0129.9N
N
TNG-12Purapurani Gravel
 193.0 Silt bed112664175.230.7112.728.7R
 186.0 Silt bed128665180.728.849.910.9R
 183.5 Silt lens271666180.132.552.89.3R
181815179.030.864.704.8R
TNG-11Purapurani Gravel
 179.0 Gravel matrix286666349.0−18.660.428.7N
N
TNG-10Purapurani Gravel
 158.0 Sand lens439666165.825.689.577.1R
 142.0 Sand lens76665175.839.892.628.0R
121211169.932.147.426.7R
TNG-9Purapurani Gravel
 141.0 Sand lens1248*666138.143.072.847.9R
 129.0 Sand lens23664167.221.395.669.4R
121210151.535.218.6911.5R
TNG-08Calvario Drift, diamict
 127.5 Clay lens & diamict matrix403*121211187.526.322.689.8R
 125.0 Silt lens93600Indeterminate polarity–
181211187.526.322.689.8R
TNG-07Calvario Drift, diamict
 121.0 Diamict matrix97600Indeterminate polarity–
–
TNG-06Calvario Drift, gravel
 101.0 Silt lens42666181.942.347.429.8R
 108.0 Fine-sandy silt bed78121210187.216.135.388.2R
 105.0 Silt & sand lens46666177.824.646.609.9R
 102.5 Sand lens85998173.919.442.188.6R
333330180.723.925.015.4R
TNG-05Calvario Drift, diamict
 101.0 Fine sand lens173666177.724.9245.954.3R
 105.0 Silt & sand lens153662181.427.9––R
12128178.625.7254.773.5R
TNG-04Calvario Drift, diamict
 104.5 Sand bed804*665342.9−41.621.1917.0N
 86.0 Silt lens218666354.7−11.476.117.7N
 74.0 Silt lens218665336.3−20.523.9815.9N
 70.0 Silt lens184666339.6−18.662.978.5N
 66.0 Sand lens1135554.5−23.972.339.1N
 52.5 Silt lens1335549.1−37.443.9514.0N
 49.0 Silt lens1696660.3−13.636.5011.2N
 44.0 Silt lens1726652.60.136.2012.9N
 20.0 Silt lens1786640.6−2.837.8315.1N
525246354.1−18.816.295.4N
TNG-03Calvario Drift, gravel
 13.0 Silt lens686666352.3−29.785.027.3N
N
TNG-02Calvario Drift, diamict
 8.0 Silt bed165666346.5−33.8425.253.3N
N
TNG-01Calvario Drift, gravel
 4.5 Silt lens188666356.3−19.046.479.9N
 2.5 Fine sand lens2846653.8−35.762.399.8N
121211359.5−26.232.958.1N

See Fig. 6C of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity.

Magnetic enhancement of paleosol compared to the parent material in which it formed.

Error between 10° and 20° underlined; error greater than 20° double underlined.

Colluvium draping incised valley slope (possible mass flow deposit).

Table 5

Means of paleomagnetic directional data for the Minasa section.

UnitLithologyχn
DIkα95apa
Position (m)Material(mean)CollectedUsefulUsed
MIN-15Gravel grading to diamict
 410.0 Modern soil (in diamict)1354*6651.5−33.7183.395.7N
N
MIN-14Gravel grading to diamict
 406.0 Paleosol (in sand lens)1309*666354.9−24.8118.906.2N
N
MIN-13Diamict
 402.5 Sand lens3236666.1−21.557.258.9N
N
MIN-12Gravel grading to diamict
 378.0 Sand lens111666350.7−22.626.1713.3N
N
MIN-11GravelNot sampled
MIN-010Gravel grading to diamictNot sampled
MIN-09Gravel grading to diamictNot sampled
MIN-08Diamict
 324.0 Sand lens125665356.1−24.476.908.8N
N
MIN-07GravelNot sampled
MIN-06DiamictNot sampled
MIN-05Kaluyo gravel & Sorata Drift
 324.0 Silt lens91654350.3−24.852.1512.8N
N
MIN-04Kaluyo gravel & Sorata DriftNot sampled
MIN-03Purapurani Gravel
 278.0 Silt bed337600Indeterminate polarity–
 275.0 Silt bed71664187.331.250.1413.1R
 234.0 Silty sand lens71666176.638.965.888.3R
 214.0 Sand bed82664176.840.073.2510.8R
241814179.937.154.775.4R
MIN-02Multi-lithic gravel
 144.0 Paleosol (in gravel)1926*665143.738.292.858.0R
 112.0 Silt lens102664175.824.7211.716.3R
 10.5 Medium sand lens107600Indeterminate polarity–
18129159.133.221.9111.2R























 0.0 Silt-sized ash85666259.6−35.828.9112.7N
N

See Fig. 6D of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity.

Magnetic enhancement of paleosol compared to the parent material in which it formed.

Error between 10° and 20° underlined; error greater than 20° double underlined.

Table 6

Means of paleomagnetic directional data for the Purapura section.

UnitLithologyχn
DIkα95apa
Position (m)Material(mean)CollectedUsefulUsed
PUR-15Altiplano surface gravelsNot sampled
PUR-14Sorata Drift, diamicton
 246 Silt lens210888345.8−25.0112.375.2N
 244 Silt lens5256661.0−29.0317.463.8N
141414352.2−26.971.564.7N
PUR-13Sorata Drift, diamicton
 240 Palsosol1461*888358.0−30.2230.973.7N
N
PUR-12Sorata Drift, diamictonNot sampled
PUR-11Sorata Drift, gravelNot sampled
PUR-10Sorata Drift, diamicton
 192121666351.6−21.648.689.7N
PUR-09Purapurani Gravel
 58.556600Indeterminate polarity–
PUR-08Calvario Drift
 5811565430.1−39.539.8414.7N
PUR-07Calvario Drift
 57.5 Palsosol3426643.2−38.860.3811.9N
 57.5 Palsosol417666343.5−53.324.1413.9N
121210352.8−47.922.6610.4N
PUR-06Calvario Drift
 54 Palsosol1631*333351.2−28.9401.546.2N
 23.5 Possible ash1360*776337.5−16.41.5Nb
10109Mix of PCA and GCN
PUR-05Calvario Drift
 13.584600Indeterminate polarity–
–
PUR-04Calvario Drift, finesNot sampled
PUR-03Chijini Tuff
 3.5 Tuff1727666357.6−54.972.687.9N
N
PUR-02La Paz Formation, possible pyroclastic flow
 2.5 Silt bed10233316.1−29.5234.828.1N
 2 Silt bed91666355.4−30.2281.724.0N
9992.3−30.365.006.4N
PUR-01La Paz Formation, gravel

See Fig. 6E of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity.

Magnetic enhancement of paleosol compared to the parent material in which it formed.

Error between 10° and 20° underlined; error greater than 20° double underlined.

Remanence directions obtained by the intersection of great circles.

Table 7

Means of paleomagnetic directional data for the Jacha Kkota section.

UnitLithologyχn
DIkα95apa
Position (m)Material(mean)CollectedUsefulUsed
JKT-99Altiplano surface gravel
 178.0 Silt lens194665355.4−26.4129.446.8N
N
JKT-98Diamicton
 175.0 Silt lens133664352.4−7.47.9Nb
N
JKT-06upper La Paz FormationNot sampled
JKT-05Chijini Tuff
 43.5 Cemented tuff2122666357.0−37.3108.486.5N
 42.5 Loose ash1976662.3−28.9258.184.2N
121212359.8−33.1103.104.3N
JKT-04
 39.5 Silt bed108664349.1−36.81239.452.6N
 32.5 Fine sand bed280666344.6−26.153.919.2N
121210348.3−30.463.276.1N
JKT-03upper La Paz Formation
 22.0 Paleosol565*665354.7−34.549.299.6N
N
JKT-02upper La Paz Formation
 13.0 Silt lens262665353.7−23.298.058.2N
N
JKT-01upper La Paz Formation
 5.5 Silt bed157654187.938.295.209.5R
R

See Fig. 6F of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity.

Magnetic enhancement of paleosol compared to the parent material in which it formed.

Error between 10° and 20° underlined; error greater than 20° double underlined.

Remanence directions obtained by the intersection of great circles.

Means of paleomagnetic directional data for the Patapatani West section. See Fig. 6A of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity. Magnetic enhancement of paleosol compared to the parent material in which it formed. Error between 10° and 20° underlined; error greater than 20° double underlined. Apparent magnetic enhancement at top of tuff unit. Means of paleomagnetic directional data for the Patapatani East section. See Fig. 6B of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity. Error between 10° and 20° underlined; error greater than 20° double underlined. Means of paleomagnetic directional data for the Tangani section. See Fig. 6C of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity. Magnetic enhancement of paleosol compared to the parent material in which it formed. Error between 10° and 20° underlined; error greater than 20° double underlined. Colluvium draping incised valley slope (possible mass flow deposit). Means of paleomagnetic directional data for the Minasa section. See Fig. 6D of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity. Magnetic enhancement of paleosol compared to the parent material in which it formed. Error between 10° and 20° underlined; error greater than 20° double underlined. Means of paleomagnetic directional data for the Purapura section. See Fig. 6E of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity. Magnetic enhancement of paleosol compared to the parent material in which it formed. Error between 10° and 20° underlined; error greater than 20° double underlined. Remanence directions obtained by the intersection of great circles. Means of paleomagnetic directional data for the Jacha Kkota section. See Fig. 6F of Ref. [14] for stratigraphy and stratigraphic positions of sample groups. Position, sampling height in metres above base of section; χ, mean magnetic susceptibility of collected samples (×10−6 SI units); n, number of samples; D and I, mean declination and inclination, respectively; k, precision parameter; α95, circle of confidence (P=0.05); p, polarity. Magnetic enhancement of paleosol compared to the parent material in which it formed. Error between 10° and 20° underlined; error greater than 20° double underlined. Remanence directions obtained by the intersection of great circles.

Experimental design, materials and methods

Lithostratigraphic characterization

We measured and described six sections along the western margin of the La Paz and Achocalla basins, totaling 1100 vertical metres of exposure of the sediment sequence underlying the Altiplano plateau (Table 1). The sections are exposed in steep valley slopes, gullies, and road cuts and form a ~20-km-long transect through the eastern Altiplano margin, oblique to the trend of the Central Andes (Fig. 1). Sedimentologic and stratigraphic characterization include texture, structure, lithology, colour, clast size and shape, sorting, weathering features, and the nature of contacts. We divided units on the basis of major changes in material properties and on the occurrence of major hiatuses indicated by paleosols or erosional contacts. We measured unit thicknesses using a TruPulse 200 Laser Range Finder, and stratal thicknesses and sizes of clasts using a graduated metric scale. We measured the long-axes orientations (trend and plunge) of 50 elongate clasts from each of eight units at the two sections closest to the Cordillera Real (seven units at the Patapatani West section and one unit at the Patapatani East section: Fig. 2). Diamicton clast fabrics from the (A) Patapatani West and (B) Patapatani East sections. The fabrics are each based on trends and plunges of 50 elongate pebbles and cobbles (long:short axis ratio of ≥2:1) and are represented by Fisher distributions on equal-angle stereonets. Orientation densities range from 0% (white) to 10% (red).

Materials

The sample collection comprises 808 oriented cylindrical samples (2.1 cm diameter, 1.8 cm length) collected typically in groups of six (ranging from three to 16) from 124 stratigraphic levels at the sections (Table 1). Sampling gaps due to limited exposure, inaccessibility, or unsuitably coarse sediments were filled where possible by sampling closely aligned units at nearby exposures. We collected larger numbers of samples in gravel and diamicton units to provide a more complete magnetostratigraphic record; coarse units are more likely to yield problematic paleomagnetic results [25] and are thus less commonly sampled in magnetostratigraphic studies, including the only previous paleomagnetic study in the La Paz area [8]. During subsequent field visits, we re-sampled sites that produced indeterminate polarity or incoherent magnetization characteristics. Samples were typically taken in horizontally bedded zones of predominantly silt and fine to medium sand. Where these were not available, we collected samples from the matrices of gravel and diamicton units, avoiding granules and pebbles. Where possible, sampling included material both above and below unit boundaries. Samples were stored in magnetic shields at the University of Lethbridge following transport from the field and between measurements.

Magnetic susceptibility

Prior to demagnetization, we measured bulk magnetic susceptibility of each sample with a Sapphire Instruments SI-2B magnetic susceptibility meter.

Magnetic remanence

We measured natural remanent magnetization of each sample with an AGICO JR-6A spinner magnetometer. We re-measured remanence after stepwise alternating field (AF) demagnetization with an ASC Scientific D-2000 alternating-field demagnetizer in fields up to 200 mT. One or two pilot samples, having either representative or relatively high magnetic susceptibility, were selected from each group. These pilots were demagnetized at 10 to 16 closely spaced steps (intervals of 2.5–10 mT up to 80 mT, and 10–30 mT above 80 mT). The remaining samples from each group were then demagnetized at 4 to 10 steps (5–30 mT spacing) guided by characteristic magnetizations of pilot samples. Each sample was demagnetized to 20% or less of the natural remanent magnetization. Median destructive fields for most samples range from 10 to 80 mT, although a small number of samples included hard components of magnetization that remained following demagnetization at 200 mT AF (the limit of the equipment used). We determined remanence directions for most samples by principal component analysis [26] and for a small number of samples (<2%) by the intersection of great circles [27] (Table 1). We calculated mean remanence directions by group (Table 2, Table 3, Table 4, Table 5, Table 6, Table 7), stratigraphic unit (Table 2, Table 3, Table 4, Table 5, Table 6, Table 7), and polarity (Table 1 and Fig. 3 of Ref. [14]). Sample-specific and mean remanence directions were calculated using AGICO's Remasoft v. 3.0.
Subject areaGeology
More specific subject areaPlio-Pleistocene tropical glaciation, landscape evolution, and paleoclimate
Type of dataTables, figures
How data was acquiredIn-field lithostratigraphic characterization; Survey; Sapphire Instruments SI-2B magnetic susceptibility meter; AGICO JR-6A spinner magnetometer; ASC Scientific D-2000 alternating-field demagnetizer
Data formatRaw and analyzed
Experimental factorsSamples were dried then stored in a magnetic shield prior to and between magnetic measurements
Experimental featuresLithostratigraphic characterization includes texture, structure, lithology, colour, clast size and shape, sorting, weathering features, diamicton fabric, and the nature of contacts. We collected groups of typically six individually oriented cylindrical samples from 124 sample locations and processed them at the University of Lethbridge, Alberta, Canada. Magnetic susceptibility was measured with a Sapphire Instruments SI-2B magnetic susceptibility meter. Remanent magnetization was measured with an AGICO JR-6A spinner magnetometer prior to and after stepwise demagnetization using an ASC Scientific D-2000 alternating-field demagnetizer (4 to 16 steps at 2.5–30 mT spacing). Remanence directions were determined for most samples by principal component analysis and for a small number of samples (<2%) by the intersection of great circles. We calculated remanence directions of samples and mean remanence directions by group, stratigraphic unit, and polarity using AGICO's Remasoft v. 3.0.
Data source locationCity of La Paz, Department of La Paz, Bolivia (16°30′ S, 68°9′ W)
Data accessibilityData are within this article and in related references
Related research articleRoberts et al. (2017, 2018)
UnitDescriptionaThickness (m)
20Poorly sorted, clast-supported, pebble-cobble gravel1
Strong paleosolb
19Massive to weakly stratified, matrix-supported, pebble-cobble-boulder diamicton17
18Weakly stratified, clast-supported, pebble-cobble-boulder gravel (10% granitic clasts)
(unit 17+unit 18)27
17Weakly stratified, clast-supported, pebble-cobble-boulder gravel (10% granitic clasts); sharp wavy basal contact with incorporated clasts from underlying unit
16Weakly stratified, clast-supported to clast-supported, pebble-cobble-boulder gravel (90% granitic clasts)10
Weak paleosolb
15Massive, matrix-supported, pebble-cobble-boulder diamicton11
Strong paleosolb
14Massive, matrix-supported, pebble-cobble-boulder diamicton4.5
Strong paleosolb
13Weakly stratified, matrix-supported diamicton grading to gravel2.5
12Massive, matrix-supported, pebble-cobble-boulder diamicton6
Weak paleosolb
11Massive to weakly stratified, matrix-supported, pebble-cobble-boulder diamictonc48
10Rhyolitic tuff with rare granite and argillite pebbles; locally faulted10
9Massive to very weakly stratified, matrix-supported, pebble-cobble-boulder diamicton (~90% granitic clasts)c5
Strong paleosolb
8Massive, matrix-supported, pebble-cobble-boulder diamicton (~90% granitic clasts)c4
Strong paleosolb
7Massive, matrix-supported, pebble-cobble-boulder diamicton (~90% granitic clasts)c8
Strong paleosolb
6Massive, matrix-supported, pebble-cobble-boulder diamicton with rare sandy silt
lenses (~50% granite clasts)c20
Weak paleosolb
5Massive, matrix-supported, pebble-cobble-boulder diamicton (~50% granitic clasts)c>13
–Covered5.5
4Massive, matrix-supported, pebble-cobble-boulder diamicton (~50% granitic clasts)>3
–Covered13.5
3Massive, matrix-supported, pebble-cobble-boulder diamicton (~50% granitic clasts)>15
2Inclined stratified matrix- supported diamicton with rare silt beds (<10% granitic clasts)c6
1Weakly stratified, poorly sorted, clast-supported, pebble-cobble gravel with some striated clasts (<10% granitic clasts)2

aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded.

bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

cSee Fig. 2 for clast fabric.

UnitDescriptionaThickness (m)
8Weakly stratified, matrix-supported, pebble-cobble-boulder diamicton1
7Horizontally stratified, clast-supported, pebble-cobble gravel with some striated clasts1
6Rhyolitic tuff (40Ar/30Ar step-heating on sanidine biotite recovered from a ~2-kg bulk sample yields an age of 2.74±0.04 Ma; Roberts et al. [14])5
5Massive, matrix-supported, pebble-cobble-boulder diamicton (80% granitic clasts)3
4Massive, matrix-supported, pebble-cobble-boulder diamicton1.5
Strong paleosolb
3Massive, matrix-supported, pebble-cobble-boulder diamicton (60% granitic clasts)c>1.5
–Covered15
2Poorly sorted, very weakly stratified, clast-supported, pebble-cobble-boulder gravel with uncommon contorted silt lenses>3.5
1Massive, clast-supported, pebble-cobble diamicton with massive silt lenses near upper contact3.5

aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded.

bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

cSee Fig. 2 for clast fabric.

UnitDescriptionaThickness (m)
13Gently dipping, stratified, contorted clast-supported gravel (<10% granitic clasts) with a silty sand matrix; unit 13 cuts across underlying units, parallel to valley slope and thus differs in thickness across the exposure~6
12Weakly stratified, clast-supported, pebble-cobble-boulder gravel (>80% granitic clasts) with rare laterally extensive silt beds62
11Weakly stratified, clast-supported, pebble-cobble-boulder gravel (>80% granitic clasts); location of lower contact is uncertain~5
10Weakly stratified, clast-supported, pebble-cobble-boulder gravel (>80% granitic clasts); location of upper contact is uncertain~35
Strong paleosolb
9Weakly stratified, clast-supported, pebble-cobble-boulder gravel (>80% granitic clasts)13
Strong paleosolb
8Weakly stratified, matrix-supported pebble-cobble diamicton (>60% granitic clasts)7
Strong paleosolb
7Weakly stratified, matrix-supported, pebble-cobble diamicton (>60% granitic clasts)5.5
6Sub-horizontally stratified, pebble-cobble-boulder gravel (>80% granitic) with numerous silt lenses; gravel ranges from matrix- to clast-supported14
Strong paleosolb
5Weakly stratified, matrix-supported, pebble-cobble diamicton (~50% granitic clasts); grades upward into weakly stratified gravel then silt7
Weak paleosolb
4Massive, matrix-supported, pebble-cobble-boulder diamicton; largest clasts are granitic (>60% granitic clasts)79
3Weakly stratified, clast-supported, pebble-cobble-boulder gravel (>80% granitic clasts)3.5
2Massive, matrix-supported, pebble-cobble-boulder diamicton (>50% granitic clasts)3.5
1Horizontally stratified, clast-supported, pebble-cobble gravel (>80% granitic clasts)8

aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded.

bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

UnitDescriptionaThickness (m)
Strong paleosolb
15Weakly stratified, matrix-supported diamicton with thin basal zone of poorly sorted pebble-cobble gravel (10% granitic clasts)4
Strong paleosolb
14Weakly stratified, matrix-supported diamicton with thin basal zone of poorly sorted pebble-cobble gravel (10% granitic clasts)3.5
13Weakly stratified, matrix-supported, pebble-cobble diamicton with thin basal zone of poorly sorted pebble-cobble gravel (10% granitic clasts)11
12Weakly stratified, matrix-supported, pebble-cobble-boulder diamicton with thin basal zone of poorly sorted pebble-cobble gravel (10% granitic clasts)16.5
Strong paleosolb
11Weakly stratified, clast-supported, pebble-cobble gravel7
10Weakly stratified, matrix-supported diamicton with thin basal zone of poorly sorted pebble-cobble gravel (10% granitic clasts)4
9Weakly stratified, matrix-supported, pebble-cobble diamicton with thin basal zone of poorly sorted pebble-cobble gravel (10% granitic clasts)3
8Massive matrix-supported diamicton; includes a pebble-cobble gravel bed8.5
7Horizontally stratified, clast-supported, pebble-cobble gravel6.5
6Weakly stratified, matrix-supported, pebble-cobble-boulder diamicton15
5Horizontally stratified, poorly sorted, clast-supported, pebble-cobble-boulder gravel (<20% granitic clasts)8
4Matrix-supported, weakly stratified, pebble-cobble diamicton (<20% granitic clasts)6
3Weakly stratified, poorly sorted, clast-supported, pebble-cobble-boulder gravel (granitic
clast content decreases from >50% in the lower part of unit to <20% in the upper part of the unit)173
Strong paleosolb
2Weakly stratified, poorly sorted, clast-supported, multi-lithic gravel, coarsening upward from pebble-cobble to cobble-boulder (granite content increases in the upper part of the unit from <20% to >50%); 10-m-thick zone near the base of the unit is covered134
1Rhyolitic tuff10

aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded.

bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

UnitDescriptionaThickness (m)
15Weakly stratified, poorly sorted, clast-supported, pebble-cobble gravel (<10% granitic clasts)2
14Weakly stratified, poorly sorted, matrix-supported pebble-cobble diamicton (<10% granitic clasts)6
Strong paleosolb
13Massive, poorly sorted, matrix-supported pebble-cobble diamicton (<10% granitic clasts)>2
–Covered12
12Weakly stratified, matrix-supported pebble-cobble-boulder diamicton>12
–Covered22
11Stratified, poorly sorted, clast-supported, pebble-cobble gravel (<20% granitic clasts)>6
10Massive, matrix-supported, pebble-cobble-boulder diamicton with silt lenses>5
–Not sampled or systematically described125
9Weakly horizontally stratified, clast-supported, pebble-cobble-boulder gravel (90% granitic clasts)>7
8Massive, matrix-supported pebble-cobble diamicton (50% granitic clasts)0.5
Strong paleosolb
7Massive, matrix-supported pebble-cobble diamicton (50% granitic clasts)3.5
Strong paleosolb
6Massive, matrix-supported, pebble-cobble-boulder diamicton (<50% granitic clasts)30.5
5Weakly stratified, matrix-supported, pebble- cobble diamicton (50% granitic clasts)11
4Weakly laminated silt and sand4
3Rhyolitic tuff5
2Tuffaceous silt and sand with current structures; sharp lower contact1.5
1Weakly stratified, matrix-supported, pebble-cobble gravel (90% granitic clasts)1.5

aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded.

bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

UnitDescriptionaThickness (m)
8Poorly sorted, clast-supported, pebble-cobble gravel3.5
Strong paleosolb
7Weakly stratified, matrix-supported, pebble-cobble diamicton7
Not sampled or systematically described42
6Interbedded silt and sand (described by Thouveny and Servant [8] as clay and silt)31
5Rhyolitic tuff5.5
4Interbedded silt and silty sand19
Strong paleosolb
3Silt and underlying cross-bedded, medium to coarse sand; erosional basal contact5
2Interbedded silt, sand, and pebble gravel; erosional basal contact30
1Weakly laminated fine sandy silt10

aAll diamicton units contain striated and faceted clasts; most clasts are subrounded to subangular. Most clasts in gravel units are rounded to subrounded.

bThe paleosols are classified as either strong or weak, depending on the degree of development of soil horizons and pedogenic structure. Strong paleosols are those with B horizons thicker than 50 cm (e.g. Fig. 5C of Ref. [14]) and pedons coated with clay (e.g. Fig. 5I of Ref. [14]). In contrast, weak paleosols are those with B horizons thinner than 30 cm, and little or no clay translocation and pedon formation.

  2 in total

1.  Geochronology of type uquian (late cenozoic) land mammal age, Argentina.

Authors:  L G Marshall; R F Butler; R E Drake; G H Curtis
Journal:  Science       Date:  1982-05-28       Impact factor: 47.728

2.  Multiple tropical Andean glaciations during a period of late Pliocene warmth.

Authors:  Nicholas J Roberts; René W Barendregt; John J Clague
Journal:  Sci Rep       Date:  2017-02-07       Impact factor: 4.379
  2 in total

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