Paleosol-sedimentary sequences of the Glacis de Buenavista, Central Mexico: interaction of Late Quaternary pedogenesis and volcanic sedimentation

Quaternary International 106–107 (2003) 185–201

Paleosol-sedimentary sequences of the Glacis de Buenavista, Central Mexico: interaction of Late Quaternary pedogenesis and volcanic sedimentation E. Solleiro-Rebolledoa,*, S. Sedova, J. Gama-Castroa, D. Flores Roma! na, G. Escamilla-Sarabiab b

a ! ! 04510 D.F., Mexico Departamento de Edafolog!ıa, Instituto de Geolog!ıa, Universidad Nacional Autonoma de M!exico, Del. Coyoacan ! ! 04510 D.F., Mexico Departamento de Ciencias de la Tierra, Instituto de Geolog!ıa, Universidad Nacional Autonoma de M!exico, Del. Coyoacan

Abstract Two sections (Buenavista and Ahuatenco) of Late Quaternary paleosol-sedimentary sequences of the Glacis de Buenavista, Morelos, Central Mexico, were studied and interpreted as a regional record of climatic change and interaction of pedogenesis and volcanic sedimentation. Buenavista is the deepest section and includes seven pedostratigraphic units, with Andosol (surface unit) and Luvisol (all underlying units) types of pedogenesis. Both types indicate a humid forest environment, the divergency being related to differences in pedogenesis duration. Albeluvisol with a bleached E horizon in unit 5 indicates cooler but still moist conditions in the final Late Pleistocene, whereas strong vertic properties in unit 4 indicate a climate with contrasting seasonal variation of precipitation during the first part of the Holocene. In the upper part of the Ahuatenco section, Bt horizons of different pedogenetic events form a welded Luvisol profile instead of a set of separate paleosols due to less intensive sedimentation. Material of indurated Cm horizons (tepetates) in the lower part of the Ahuatenco section is a mixture of fresh coarse volcanoclastic components and redeposited Luvisol clayey material, formed by mass movement associated with volcanic events. The resulting granulometric composition of these layers is favourable for structural collapse and hydroconsolidation, which caused hardening together with subsequent pedogenic illuviation and redoximorphic processes. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.

1. Introduction Buried and non-buried paleosols (relict soils), though occurring frequently within the Transmexican Volcanic Belt (TMVB), are still underestimated as a record of Quaternary environmental change in Central Mexico. Paleosols do not give as high temporal resolution as lacustrine records, which now form the base for regional Quaternary paleoclimate reconstruction (Lozano-Garc!ıa et al., 1993; Lozano-Garc!ıa and Ortega-Guerrero, 1998; Caballero-Miranda et al., 1999; Ortega-Guerrero et al., 2000) but are more susceptible to paleoenvironment spatial diversity. Due to regular eruptions, covering extensive areas with tephra, paleosol sequences were *Corresponding author. Instituto de Geolog!ıa, Circuito de la ! Cient!ıfica, Cd. Universitaria, C:P. 04510, Mexico, D.F. Investigacion Tel.: +52-55-56-22-42-86 ext. 142; fax: +52-55-56-22-43-17. E-mail address: [email protected] (E. Solleiro-Rebolledo).

formed on different geomorphic position, providing unique information about local variations of ancient climate and ecosystems. However, local differences in sedimentation rate as well as accompanying processes of erosion and redeposition produce considerable variability in the relation between paleosols and sedimentary units within the sequences. Different paleosols can be completely separated by unaltered sediments merging into ‘‘welded soils’’ (Ruhe and Olson, 1980) or pedocomplexes (Smolikova, 1967). In case of minimal or no sedimentation, more than one phase of pedogenesis can overlap within one profile, giving rise to polygenetic soil profiles. These variations, occurring along short distances, can obscure pedostratigraphic paleoenvironmental interpretations and sometimes give a false climatic signal. We studied the paleosol-sedimentary sequence of the Glacis de Buenavista in the southern part of the TMVB to reconstruct regional bio-climatic conditions of paleosol formation as well as interaction of pedogenesis

1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 2 ) 0 0 1 7 2 - 6

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and sedimentation, caused by volcanism. A particular question regarding the latter problem is the genesis and the paleoenvironmental interpretation of subsurface indurated horizons, known as tepetates, which fit into the definition of Duripans or Fragipans in the international soil classification WRB (1998). These horizons are spread over major part of the Glacis de Buenavista and often appear on the surface due to erosion. A number of previous studies about these specific layers were devoted mostly to identification of their cementing agents and ecological and land use significance (Zebrowski, 1991; Oleschko et al., 1992; Flores-Roma! n et al., 1996). In these papers tepetates were considered as pedogenic phenomena, however, being often interlayered with or overlain by paleosols, they were never related to the past soil forming processes and environmental conditions. 2. Materials and methods 2.1. Geological and environmental setting The study area is located in the TMVB, central Mexico (Fig. 1), in the Glacis de Buenavista, located at 85 Km SW of Mexico City (181450 –191000 N and 991250 – 991050 W), at 2200–1600 m a.s.l. Present day climate is warm and humid with an annual rainfall of 1147 mm; and summer rains of 993 mm (from June to September). Annual mean temperature is 20:71C; and maximum average temperatures reach 23:21C and minimums 18:71C (Garc!ıa, 1988). The natural vegetation at the highest elevations ð> 1900 mÞ is temperate moist forest, described as pine and oak forest according to Rzedowsky (1978). At 1800 m a.s.l., temperate rain forest with oak (Quercus spp.) and juniper (Juniperus spp.) is dominant. Nowadays, the lack of clearly diversified altitudinal zonation of vegetation belts and the human impact that begin with prehispanic societies have modified plant communities drastically. Deforestation is strong and land use has changed. The Glacis de Buenavista is a geomorphic feature formed by repeated sedimentation and degradation periods that allowed colluviation processes during the Pleistocene. Conglomerates and detrital materials derived from andesitic volcanic rocks of Zempoala Formation (Tertiary) constitute the Glacis substrate. Both conglomerates and detrital materials interlayered with lava flows, ashes and lahar deposits of variable thickness (1–100 m) constitute the Cuernavaca Formation of Late Pleistocene to Recent age (Fries, 1960; Ortiz-Pe! rez, 1977). They are also interdigitated with volcanic materials from the Chichinautzin Formation, constituted by basaltic–andesitic lava flows, pyroclastic deposits, mainly basic tephras, and reworked ashes of Pleistocene age with the lower limit at 700; 000 yr BP (Mooser et al., 1974). In the Chichinautzin volcanic

field, located at the north of the Glacis de Buenavista, a number of volcanic events, younger than 40; 000 yr BP were recorded (Mart!ın del Pozzo, 1982). The youngest dated volcanic deposits in the vicinity are basaltic lava flows of Chichinautzin volcano, o4000 yr BP (Mart!ın del Pozzo et al., 1997). The modern soil cover is differentiated according to altitude. At elevations higher than 2200 m; Cambisols and Andosols are found under a pine forest. Luvisols occur between 2200 and 1900 m; sometimes having andic horizons on top, developed in a thin layer of recent volcanic ash. At elevations between 1900 and 1800 m; Luvisols are associated to cemented soil horizons— tepetates. Two sections were studied: Buenavista, located at 2000 m a.s.l. (181570 4000 N; 991180 4000 W) and Ahuatenco, at 1850 m a.s.l. (181560 1500 N; 991190 5500 W). 2.2. Laboratory analyses Bulk samples for soil physical and chemical analyses as well as undisturbed samples for thin sections were collected from genetic horizons of paleosols and modern soils. Soil colour was determined using Munsell soil color charts (1975). Particle size distribution analysis was performed by sieving and pipette methods (Soil Conservation Service, 1984) with preliminary destruction of aggregating agents: organic matter with 15% H2 O2 and iron oxides with dithionite–citrate–bicarbonate (DCB) extraction. Organic carbon content and amount of Fe, Al and Si, extracted with DCB and oxalate solutions were evaluated according to USDA (1996). Chemical analysis for the total oxide composition in soil samples was carried out by X-ray fluorescence. All results were calculated on a weight basis of oven-dry ð1051CÞ soil. Thin sections were made according to the method of FitzPatrick (1984), studied under a petrographic microscope and described following the terminology of Bullock et al. (1985). Dating of studied sequences with instrumental methods appeared to be difficult. Non-buried humus horizons with sufficient content of organic material or charcoal fragments of proper size for radiocarbon dating are present. However, the ion-beam technique, particularly Rutherford Backscattering (RBS) showed that occluded carbon is present in hard Fe–Mn concretions of the E horizon in the pedostratigraphic unit 5 (see below). This carbon was used to obtain a C14 date by AMS method in the NSF-Arizona AMS Laboratory at the University of Arizona. 3. Results 3.1. Buenavista section The Buenavista profile was studied in a deep road cut. An extensive paleosol-sedimentary sequence with seven

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G. OF MEXICO

Texcoco lake

TMVB

PAC IFI

2200

CO CE

AN 100˚

Sierra

105˚

19˚30’

3000

20˚

187

95˚

Mexico City

Nevada

Chalco lake

3500

19˚15´ 4000 5230

Iztaccíhuatl Chichinautzin volcanic field

2500

2000

Popocatépetl

19˚00’

2200

1600 Buenavista

Cuernavaca City

Ahuatenco

0

Glacis of Buenavista

99˚15’

99˚00’

10 Km

98˚45’

Study area Glacis de Buenavista Cities TMVB

Transmexican Volcanic Belt Study sections Fig. 1. Location of study area.

pedostratigraphic units (surface modern soil and six paleosols) was found there (Fig. 2). The upper unit consists of Ah–AB–Bw horizons, loose, with fine granular structure. All underlying units are characterised by well-developed light yellowish brown to brown (10YR 6/4, 5/4, 4/3, dry) Bt horizons, compact, clayey, with coarse angular or subangular blocky to prismatic structure. The units 4–7 are separated from each other by BC and C horizons slightly transformed by weathering and pedogenesis. The upper unit boundaries are only marked with thin EB horizons, which have lighter colours and coarser textures than the Bt horizons below and above. The pedostratigraphic unit 5 has a well-developed pale (7.5YR 7/4, dry) E

horizon with tonguing along the lower boundary, containing hard dark-brown Fe–Mn nodules (Fig. 3). In Btss horizons of units 4–7, we observed intensive cracking. The planes often meet at B301 and the angular wedge-like blocks, formed by these planes, have slickensides. These features, associated with vertic properties (Ahmad, 1983), are more developed in the pedostratigraphic unit 4. A thick layer of conglomerate forms the base of the sequence. Relying on field observations (later confirmed by micromorphological and analytical data), we defined the upper pedostratigraphic unit as Andosol, units 2–4, 6 and 7 as Luvisols (specifically unit 4 as Vertic Luvisol) and unit 5 as Albeluvisol, according to WRB (1998).

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Buenavista exposure

Ahuatenco exposure

Depth (cm)

Unit 1

Depth (cm)

0

0

25

25

55

Unit 2 105 120

Luvisol section

Unit 3 205

215 225

259

Unit 4 284 325

Tepetate section

316 349

385 371 411 431

AA39627 12,160 140

480

Unit 5

567

607 627 652

Unit 6

757

827 867

Other features

AB

Redeposited material of Bt-horizons Charcoal

B

Vertic properties

Bt

Hard Fe-Mn nodules

BC

Redox features

Ah 707

Unit 7

Soil genetic horizons

EB

Sedimentary strata Fresh volcanic material Weathered volcanic material

907

Conglomerate

Fig. 2. Schematic sections of the Glacis de Buenavista.

Micromorphological observations (summarised in Table 1) reveal differentiated distribution of features, indicating various soil forming processes throughout the section. Microstructure in Ah and Bw horizons is granular, in some areas with spongy fabric (Fig. 4a). Bt horizons are formed by subangular to angular blocks and are more compact. Interrelation of primary minerals in sand and silt fractions and secondary components, clay and iron oxides in fine material, reflects the weathering status of paleosols. Ah and Bw of the unit 1 still contain slightly weathered volcanoclastic components, among them plagioclases, pyroxenes and rock fragments. On the contrary, the upper Bt horizons (units 2–4) are strongly weathered, containing very few coarse grains, mostly of quartz. The major part of groundmass in these horizons

is made up of clay, coloured with brown iron oxides. Round clay bodies, found in the Bt horizon, are probably pseudomorphs after pumice fragments. Below, in the unit 3-Bt2 horizon, some remains of primary components appear. Plagioclases are strongly etched: when they are located close to depositional clay coatings, illuvial clay often enters dissolution pits (Fig. 4b) (these infillings have the same morphology as the fine material of nearby coatings and sometimes are connected to them, indicating their illuvial genesis). Boxwork of iron oxides is developed instead of Fe–Mg silicates (Fig. 4c). Even in BC and C horizons, commonly rich in coarse material, etching of plagioclases and weathering of mica (decrease of birefringence, fragmentation along cleavage) were observed (Fig. 4d). Lower Bt horizons have some unstable primary

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Fig. 3. Buenavista profile, pedostratigraphic unit 5; Albeluvisol: (E) bleached E-horizon, (Bt) -Bt horizon.

minerals, etched and partly substituted by iron-clay plasma. They are similar to lower Bt of unit 4 and less weathered than the uppermost Bt horizons. Fine material in unit 1 is completely isotropic indicating predominance of amorphous compounds. In Bt horizons, the clay is partly birefringent, especially in illuvial pedofeatures. Low interference colours could indicate that 1:1 clay minerals prevail, agreeing with preliminary results of X-ray diffraction analysis of clay ( minerals. Illuvial separates showing abundance of 7 A pedofeatures are absent in unit 1. They are detected in Bt horizons, being few in pedostratigraphic units 2 and 3, and are more frequent in lower Bt horizons. However, in many cases, they are deformed and partly incorporated in groundmass (Fig. 4e). In C horizons of units 5–7, fragments of clay coatings (papules) are redeposited and mixed with fresh volcanogenic material (Fig. 4f). Monostriated and porostriated b-fabric around planar voids, interpreted as the result of alternating shrink/ swell processes and associated with vertic properties,

189

often referred as stress cutans (Ahmad, 1983), were observed in all Bt horizons except in units 2 and 3, being more common in the upper Bt horizon of unit 4. Redoximorphic features are frequent throughout the sequence, except in the upper Ah, AB and Bw horizons. Ferruginous mottles with diffuse boundaries as well as coatings, superimposed or juxtaposed with clay pedofeatures are found in Bt horizons (Fig. 4g). In the bleached E horizon of unit 5, Fe–Mn rounded nodules, distinctly separated from groundmass, are common (Fig. 4h). The results of granulometric analysis show considerable accumulation of clay in Bt horizons compared to underlying parent materials. In the upper Luvisol (units 2–4), clay content reaches 70–80%. In the lower units (5–7), it ranges 40–50% (Fig. 5). Bulk chemical composition of Bt horizons is characterised by accumulation of Fet and Alt at the expense of Sit : The values of dithionite-extractable Fe ðFed Þ (which is interpreted as Fe of fine iron oxides, mostly produced by weathering) are also high in Bt horizons of upper Luvisols (units 2–5). The Fed =Fet ratio reaches its maximum in these horizons, indicating that the major proportion of iron is included in secondary iron oxides. The values of Feo =Fed ratio are rather low in all Luvisol Bt horizons, meaning that crystallised components prevail among fine iron oxides. In contrast, in the A and AB horizons of Andosol (unit 1), this ratio is the highest in the sequence, indicating the presence of larger proportions of amorphous or poorly crystallised iron oxides. Oxalate-extractable Si ðSio Þ and Al ðAlo Þ; belonging to amorphous compounds (mostly allophanes), have a sharp maximum in surface Andosol (unit 1). In Luvisols of units 2–4, the values fall below 1% (Fig. 5). A radiocarbon date of 12; 1607140 yr BP was obtained from carbon of Fe–Mn concretions of the E horizon in pedostratigraphic unit 5. 3.2. Ahuatenco section The Ahuatenco sequence was exposed in a deep gully. The section is divided in two morphologically different parts (Fig. 6). The upper one consists of a set of welldeveloped clayey Bt horizons, capped by a thin dark yellowish brown (10YR 3/4) humus Ah horizon and underlain by 4BC0 ; less transformed by pedogenesis. Some dissimilarities were observed among Bt horizons: colour varies from dark brown (7.5YR 4/4, dry) in upper Bt horizons (Bt0 and 2Btss0 ) to light brown (7.5YR 6/4, dry) in the lower Bt ð3Bt0 Þ: Maximum cracking and slickenside formation was registered in 2Btss0 : The lower part of the sequence is constituted by seven indurated horizons: tepetates, labelled 5Cm to 11Cm: They have coarser texture than Bt horizons, are much harder, and in sites with active erosion, they form terraces. Deep vertical planes break them into polygonal

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Table 1 Selected morphological and micromorphological characteristics of Buenavista section a

a

blocks, which can be observed on horizontal surfaces of the erosion terraces. Variations among tepetates are related to texture (the upper ones are coarser), colour and hardness. As in the Buenavista section, a thick conglomerate exposed in the deepest part of the gully forms the base of the sequence. Observations from thin sections (Table 2) show the presence of few thin illuvial clay coatings in the Ah horizon (Fig. 7a). Micromorphological features of the three Bt horizons are different. In the upper, continuous laminated clay coatings and infillings are present (Fig. 7b). On the contrary, in the medium 2Btss0 horizon, most illuvial pedofeatures are deformed and partly incorporated in the groundmass. Numerous stress cutans and cracks, crossing at B301 indicate vertic properties (Fig. 7c). All Bt horizons show advanced grades of weathering: a few coarse particles mainly of stable minerals and clay plasma coloured with iron oxides dominate. We were surprised to find numerous opal phytoliths in the 4BC0 horizon. They make up the major part of silt material in the groundmass, whereas the few clay coatings, penetrating scarce cracks, are free of them (Fig. 7d). The layers of indurated horizons, tepetates, also have differences in thin sections. Upper layers, especially 6Cm and 7Cm; are rich in sand materials formed by fresh volcanogenic minerals and rock fragments, which constitute a large proportion of the groundmass. The lower layers 8–11Cm; although they also contain some fresh primary minerals in coarse fractions, are richer in clay. In these horizons, numerous fragments of

re-deposited material of Bt horizons were observed. Both small clasts of fragmented clay coatings and large blocks with well-preserved illuvial clay pedofeatures (Fig. 7e) are incorporated in the groundmass, and neighbouring unweathered volcanic mineral and rock particles. Phytoliths and few small charcoal fragments are also present in tepetates. In all indurated layers, some undeformed in situ clay coatings filling planar voids were found. The number of these illuvial pedofeatures is even higher than in the overlying 4BC0 horizon. Redoximorphic features (ferruginous mottles and nodules) can also be observed in all tepetates (Fig. 7f). Analytical characteristics of Bt horizons are similar to those of the units 2–3 of the Buenavista sequence (Fig. 8). They are characterised by extensive accumulation of clay (up to 70%) and DCB-extractable Fe (more than 4%). As in the upper Bt horizons of Buenavista section, values of Fed =Fet ratio are rather high ð> 0:6Þ; showing that most of the iron is already included in secondary iron oxides, whereas the content of Feo and Feo =Fed values are low (Fig. 8), indicating the high degree of weathering and a small proportion of poor crystalline oxides. Even if the high contents of total Alt ; and Fet and the low values of oxalate-extractable Al and Si are registered in the ‘‘Luvisol unit’’, in the Ah horizon that forms the upper part of the sequence the content of oxalate-extractable Alo ; Sio and Feo as well as Feo =Fed ratio are as low as in the Bt horizons. This differs from the upper Andosol Ah and Bw horizons of the Buenavista section, which, in contrast to the underlying paleosol Bt horizons, have high values of these

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parameters. Tepetates have less clay and Fed and higher total Sit content. The amount of elements, extracted by oxalate, is the lowest in both studied sequences, showing that they are poor in amorphous materials. The lower tepetate layers, especially 8Cm and 9Cm have some higher clay, Fed and Fet content, that corresponds to the presence of re-deposited Bt material, as observed in thin sections.

4. Discussion 4.1. Correlation and chronology of sections The specific combination of properties of the uppermost part of Buenavista section (Ah–AB–Bw horizons

191

of pedostratigraphic unit 1): fine granular structure, very high porosity, high quantities of oxalate-extractable Al and Si, have no analogues in the Ahuatenco section. On the contrary, the underlying sequence of Bt horizons has similarities in both sections, indicating that they were formed along the same pedogenetic and sedimentation events. Although Bt horizons in Ahuatenco are not separated by BC, C or E horizons as in Buenavista, the differences in their properties suggest that they can be assumed to represent different phases of pedogenesis. Morphological and micromorphological observations as well as analytical results support correlation of the sequence of Bt horizons in Ahuatenco with the upper Luvisols of units 2–4. In both sections these parts are characterised by strong weathering, extensive clay accumulation (clay content up to 80%) and high

Fig. 4. Micromorphology of Buenavista section: (a) Fine granular structure of unit 1-Ah horizon; (b) Etched plagioclase, illuvial clay filling etch pits (unit 2-Bt2 horizon); (c) Boxwork of iron oxides, substituting pyroxene (unit 2-Bt2 horizon); (d) Weathered mica (m) and etched plagioclase (p) (unit 4-C horizon); (e) Deformed clay coatings (marked with arrow) (unit 3-Bt2 horizon); (f) Fragments of laminated clay coatings (marked with arrows) incorporated in the groundmass (unit 6-C horizon); (g) Ferruginous coatings (unit 4-Bt2 horizon); and (h) Fe–Mn nodule (unit 5-E horizon).

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Fig. 4 (continued).

contents of dithionite-extractable iron (up to 4%). We suppose that the upper Bt0 of Ahuatenco could be correlated to units 2 and 3 of Buenavista, which do not have vertic properties, whereas 2Btss0 of Ahuatenco to the 4Btss of Buenavista. In both, vertic properties are most pronounced on macro- and micromorphological levels. Anomalous high quantities of phytoliths in 4BC0 horizon in Ahuatenco suggest that it was exposed for considerable time on the surface with vegetation cover. Therefore, it marks an interval (a hiatus) between the formation of polygenetic ‘‘Luvisol’’ (Ah–Bt–2Btss0 – 3Bt0 –4BC0 ) and ‘‘Tepetates’’ (5–11Cm). We further speculate that the set of indurated horizons—tepetates (5–11Cm) in Ahuatenco, corresponds to the lower Luvisol pedostratigraphic units 5–7 in Buenavista. Tepetates correlate with BC and C horizons having similar composition: a mixture of fresh

coarse volcanic minerals and rocks and re-deposited fragments of clay-illuvial horizons. The tepetates are not interlayered with paleosol B-horizons. However, they contain clay coatings and infillings in higher quantities than the overlying 4BC0 horizon. We suppose that these clay-illuvial pedofeatures are not the ‘‘roots’’ of the overlying Ah–Bt0 ð1; 2; 3Þ-4BC0 profile, but are related to different events of Luvisol formation. B-horizons, corresponding to these events, were eroded and more resistant indurated BC and C-horizons (tepetates) were only preserved. Finally, the thick layer of colluvial conglomerates is present in the lower part of both sections providing the uniform basal layer for these paleosol-sedimentary sequences. The only radiocarbon date was obtained from the carbon occluded in Fe–Mn nodules from the 5E horizon of the Buenavista section. The formation of such nodules in Luvisols is related to the water-logging

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193

Fig. 4 (continued).

process above the Bt horizon, when the latter is already formed. This date corresponds to the end of unit 5-soil formation. Units 1–4 of Buenavista, and the correlative Bt horizon sequence of Ahuatenco were formed during the last B12; 000 years, covering the Pleistocene– Holocene transition period and the complete Holocene. Conglomerates at the base of paleosol-sedimentary sequences are interlaced with the volcanic rocks of Chichinautzin Formation of early-middle Pleistocene. We conclude that pedostratigraphic units 5–7 of Buenavista section and the corresponding set of indurated horizons in Ahuatenco are of late Pleistocene age. 4.2. Pedogenetic and paleoenvironmental interpretation of paleosols Two different types of pedogenesis are detected in Buenavista sequence judging from the specific combina-

tion of soil properties. Pedostratigraphic unit 1 displays the ‘‘Andosol type’’, characterised by well-developed fine granular structure, high porosity, moderate weathering and extensive accumulation of amorphous components of Al, Si, and Fe. Both Ah and AB horizons of this unit have more than 2% of Alo þ 12Feo ; one of the diagnostic criteria for an Andic horizon according to the international soil classification WRB (1998). This profile is the result of modern pedogenesis under present humid climate on recently deposited volcanic ash. Pedostratigraphic units 2–7 belong to the ‘‘Luvisol type’’ of pedogenesis, having an E(EB)–Bt–BC profile. In units 2, 6 and 7 the eluvial part is absent, most probably due to erosion before burial. Absence of humus and eluvial horizons is also typical of buried Pleistocene Luvisols in loess–paleosol sequences (Bronger and Heinkele, 1989). Bt horizons are characterised by blocky-prismatic structure, advanced weathering

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Fig. 4 (continued).

(reflected also in higher quantities of bulk Fe and Al and lower of Si), and extensive accumulation of iron oxides and clay, although with relatively little proportion of amorphous compounds (according to micromorphological observations and low values of Alo and Sio ). Clay coatings and infillings are observed in these horizons, confirming that clay illuviation and weathering contributed to high clay content. Iron mottles and coatings indicate that migration of Fe occurred simultaneously with clay, due to redoximorphic processes. Some differences in soil properties among these pedostratigraphic units reflect variations in pedogenetic processes within the ‘‘Luvisol type’’. Illuvial pedofeatures are fewer in Bt horizons of the upper Luvisol units, which have the highest grades of clay accumulation. Pedoturbation due to shrink–swell processes is supposed to be responsible for this. Disappearance of clay

coatings was observed earlier in illuvial horizons with advanced clay accumulation (Nettleton et al., 1969; Bronger and Sedov, 1997). Deformation and incorporation of illuvial pedofeatures in the groundmass, observed in all studied Bt horizons, confirm that these features are subjected to destruction. These phenomena are associated with vertic features, related to shrink/ swell processes (intensive cracking, often forming angle B301; slickensides and stress cutans). These features are developed in units 4 and 7, and are stronger in unit 4, which was defined as a Vertic Luvisol. In unit 5, a rather thick bleached E horizon is present, containing hard Fe– Mn nodules and having a tonguing low boundary. These features are related to periodical excess of moisture in the upper part of the profile and associated to redoximorphic processes. They allow association of this paleosol with the Albeluvisol group of WRB (1998). In

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A AB Bw

0

(c)

(b)

(a) 50

100 % 0 Feo

(d)

7% 0 Fed

2% Alo

(e) 50 % 0

0

Sio Fet

195

Alt

Sit

0.7

Fed*

Feo*

2Bt 3EB 3Bt1 3Bt2 4EB 4Btss 4Bt2 4BC

4CB 5E 5Bt 5BC 5C 6E 6Bt nd

nd

nd

nd

6C 7Bt1 7Bt2 7C

nd- not determined Fig. 5. Analytical results of Buenavista section. Legend for Figs. 5 and 8: (a) grain size distribution: sand (2.0–0:05 mm), silt (0.05–0:002 mm), clay ðo0:002 mmÞ; (b) content of Fe extracted by oxalate ðFeo Þ and DCB solutions ðFed Þ; and total iron ðFet Þ; (c) content of aluminium ðAlo Þ and silicon ðSio Þ extracted by oxalate solution; (d) total aluminium ðAlt Þ and silicon ðSit ); and (e) Feno ¼ Feo =Fed ratio, Fend ¼ Fed =Fet ratio.

general, lower Luvisols (units 5–7) are characterised by less advanced weathering and lower accumulation of secondary components, clay and free iron oxides. In the Ahuatenco section, we only find evidence of ‘‘Luvisol type’’ pedogenesis (intensive weathering, clay illuviation, redoximorphic features). It is detected not only in the upper sequence of Bt horizons but also in tepetates. Variations between Bt horizons are mostly related to colour and distribution of vertic features. Ah horizon in this sequence is not correlative to Andosol unit 1 in Buenavista, having a completely different set of morphological and analytical properties. We consider that the Ah horizon is genetically associated with the

upper Bt horizon. The presence of clay coatings in this horizon indicates that clay illuviation is not restricted to ancient pedogenesis but also to a modern, current process. ‘‘Luvisol type’’ pedogenesis indicates a humid forest paleoenvironment, which is likely to persist in the studied region throughout Late Pleistocene–Holocene. However, differences in Luvisol properties suppose some paleoclimate variations. The Albeluvisol of unit 5 in the Buenavista section is interpreted as evidence of cooler conditions during the final part of the Late Pleistocene. Modern analogues of this soil are formed mostly under moist temperate climates. This

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Fig. 6. Ahuatenco profile: Bt-sequence, Cm-tepetates.

Table 2 Selected morphological and micromorphological characteristics of Ahuatenco section a

a

interpretation agrees with the less advanced weathering status of this paleosol. However, lower grade of weathering in underlying units 6 and 7 can be related to erosion of the more weathered upper horizons or due to shorter duration of pedogenesis. Vertic properties, culminating in unit 4, are characteristic of warmer (subtropic–tropic) climate with contrasting seasonal variations, including a well-expressed dry season (Ahmad, 1983). According to the position of this unit in the sequence, these paleoclimatic conditions belong to the first half of the Holocene.

We conclude that the presence of ‘‘Andosol’’ and ‘‘Luvisol’’ types of pedogenesis in Buenavista sequence reflects differences in duration of pedogenesis rather than climatic changes. Evolution of Andosols towards soils with clay illuviation was shown in a number of chronosequences of soils formed on volcanic sediments (Martini, 1976, Delvaux et al., 1989). It is associated with crystallisation of amorphous materials, formed on initial stages (Shoji et al., 1993). In the Ahuatenco section no recent sedimentation of ash took place, causing the absence of Andosol unit.

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These results agree with the paleopedological record from the Nevado de Toluca, which also indicate the predominance of humid climate throughout Late Quaternary with some drier phases in the early Holocene and with Andosol/Luvisol alternation, due to differences in the pedogenesis duration (Sedov et al., 2001). Cooler but still humid conditions during the final part of Late Pleistocene, indicated by Albeluvisol (unit 5), correspond to the glacial advance at higher elevations of the TMVB, dated 15,000–8; 000 yr BP (White and Valastro, 1984) or 12; 000 yr BP (Heine, 1984). However, most lacustrine records indicate cooler but relatively dry conditions in Central Mexico during Last Glacial Maximum. This disagreement in paleopedolo-

197

gical and lacustrine records regarding the humidity of the final part of the late Pleistocene is partly attributed to the spatial variations of mesoclimate (for details, see Sedov et al., 2001). 4.3. Interaction of pedogenesis and erosion/sedimentation processes; genesis of tepetates Several events of sediment deposition, interrupting pedogenesis, are reflected in both sections. In Buenavista thick depositional strata were formed, separating clearly pedostratigraphic units. In Ahuatenco, sedimentation was in general less intensive, resulting in minor profound sequence. The most recent tephra layer, from

Fig. 7. Micromorphology of Ahuatenco section; (a) thin clay coatings (marked with arrows) in Ah horizon; (b) clay-illuvial pedofeatures in the upper Bt horizon; (c) stress cutan (marked with arrow) in 2Btss0 ; (d) phytoliths in the groundmass of BC horizon (two marked with arrows); clay cutans do not contain phytoliths; (e) fragment of Bt horizon (Bt) and fresh volcanic rock fragment in 8Cm horizon; and (f) Fe–Mn nodule in 7Cm horizon.

198

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Fig. 7 (continued).

which the modern Andosol of Buenavista section is derived, is absent in Ahuatenco. The earlier deposited layers were thin enough, to be completely reworked by pedogenesis and produce a continuous set of Bt horizons forming a ‘‘welded’’ profile (Ruhe and Olson, 1980). During the formation of the lower part of the Ahuatenco sequence, sedimentation was stronger, producing a set of tepetates rich in fresh volcanic material. Sediment deposition was accompanied by erosion, which is responsible for the absence of part of Bt horizons in the majority of the observed profiles. The composition of BC and C-horizons in both sections presents a peculiar mixture of fresh volcanic rocks and minerals and re-deposited material of different soil horizons. We suppose that erosion/ sedimentation events do not represent a climatic signal

but are associated with volcanic processes in the adjacent Chichinautzin volcanic field that was characterised by impulsive eruptive activity throughout the Late Quaternary (Mart!ın del Pozzo et al., 1997). The surface geological processes in the Glacis de Buenavista, at the southern margin of Chichinautzin field, were likely to include deposition of pyroclastic material, associated with intensive mass movement which sometimes occurs during and immediately after eruption in the forms of lahars, mud flows, etc. (Fisher and Schmincke, 1984; Smith and Lowe, 1991). These movements involved both fresh pyroclastics and weathered surface soil materials, producing a specific mixture of coarse (originated by eruptions) and fine (from paleosols) materials. The distance from the centres of volcanic activity determined the rate of sedimentation, explaining

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199

Fig. 7 (continued).

why it was more intensive in Buenavista, closer to the Chichinautzin volcanic field, than in Ahuatenco, located further to the south. Incorporation of already pre-weathered clay-rich material in parent sediments speeds up the formation of Luvisols, forming as many as three Luvisol units after 12; 000 yr BP. In a number of chronosequences it was shown that, in most cases, the formation of a profile with well-developed Bt horizons requires 10,000 years or more (Birkeland, 1984). However, when sufficient clay content or additional sources of it are already available, this time can be shorter, thousands or even hundreds of years (Birkeland, 1984; Alexandrovskiy et al., 1998). An important conclusion is that sedimentation processes, which gave rise to tepetates in the studied

area, produced the most favourable material with granulometric composition for structural collapse and hydroconsolidation of this layers when moistened and loaded (Bryant, 1989; Assallay et al., 1998). It has relatively high silt and fine sand content, whereas the amount of clay lies in the limits B5–B40%; being appropriate for hydroconsolidation processes. Further clay illuviation and redoximorphic segregation of iron oxides filled the few pores left after primary consolidation and thus contributed to induration of these strata. Finally, we conclude that the whole soil/sedimentary sequence of the Glacis de Buenavista, including tepetates, is the result of humid pedogenesis interaction and impulsive volcanogenic sedimentation processes during a long period of the Late Pleistocene–Holocene (Ten thousand years).

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(a) 0

50

Ah

(b) 100 % 0 Feo

Fed

(c) 7% 0 Fet Sio

(d) 0.4 % 0 Alo Alt

(e) 50 % 0 Sit

0.8

Feo* Fed*

Bt 2Btss´

3Bt´

4BC´ 5Cm 6Cm 7Cm 8Cm 9Cm 10Cm 11Cm

Legend for figures 5 and 8 (a) Grain size distribution:

(b) Content of Fe extracted by oxalate (Feo)

and DCB (Fed) solutions, and total iron (Fet) Sand (2.0-0.05 mm) Silt (0.05-0.002 mm) Clay (< 0.002 mm)

(c) Content of aluminum (Alo) and silicon (Sio)

extracted by oxalate solution (d) Total aluminum (Alt) and silicon (Sit) (e) Feo*= Feo/Fed ratio

Fed*= Fed/Fet ratio Fig. 8. Analytical results of Ahuatenco section.

Acknowledgements This work has been supported financially by CONACYT(Grant No. 32337-T) and DGAPA (Grant No. IN104600). We would like to thank D. Donahue who ! supplied the radiocarbon date, E. Vallejo-Gomez, A. ! ! Gonzalez-Velazquez, A. Rocha-Torrallardona, H. Carmona and L. Huerta for collaboration in the laboratory analyses, A. Altamira for processing of photographs, and E. Jime! nez for preparing of thin sections. Finally to ! who performed the X-ray R. Lozano and P. Giron fluorescence analyses.

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