Transitional Horizon Research Articles

Mineralogia da fração argila de solos vermelhos com horizontes superficiais brunados do Planalto de Lages (SC)

Em regiões de altitudes elevadas do sul do Brasil, predominam perfis de solos com coloração bruna ou bruno-amarelada, em conseqüência do clima frio e úmido, que favorece a formação e, ou, persistência de óxidos de ferro na forma de goethita, causando o amarelecimento completo dos solos. Entretanto, em alguns locais, constatam-se perfis de solos com horizontes superficiais brunados, mas que, em profundidade, apresentam cores avermelhadas, o que parece constituir relíquia de clima pretérito mais seco e quente que o atual. O presente trabalho objetivou estudar a gênese das feições citadas, por meio da avaliação das características estruturais dos óxidos de ferro goethita e hematita nos horizontes brunados e vermelhos. Para tanto, foram descritos e amostrados no Planalto de Lages dois perfis de Nitossolos Vermelhos (Podzólicos Vermelho-Escuros), com horizontes superficiais brunados. Além da caracterização analítica usual, avaliaram-se, através de difratometria de raios X, as características estruturais dos óxidos de ferro de horizontes dos perfis, bem como em crosta laterítica, pedotúbulos e nódulos de gibbsita. Diferenças no padrão da goethita entre os horizontes brunados e vermelhos, bem como similaridade no padrão da hematita ao longo dos perfis, evidenciam que a hematita e, possivelmente, parte da população de goethita com menor substituição de ferro por alumínio tenham sido preferencialmente dissolvidas nos horizontes superficiais. O amarelecimento superficial parece ter sido resultante da persistência da população de goethitas com maior substituição de ferro por alumínio e da neoformação de goethitas, cujas características de maior substituição isomórfica de ferro por alumínio indicam terem sido formadas em condições ambientais diferentes das que prevaleceram na formação dos horizontes subsuperficiais vermelhos.

ALTERAÇÃO SUPERGÊNICA DO DEPÓSITO DE COBRE-OURO DO SALOBO, SERRA DOS CARJÁS-PA - ÊNFASE NO COMPORTAMENTO DO COBRE

The weathering mantle of the Salobo deposit was developed upon metagraywackes with interbedded lens of iron formations. The profile is of the truncated type and constituted, from the base to the top, of a transition horizon, a saprolitic horizon and colluvial deposits. The two former horizons show lateral, textural and mineralogical variations which reflect the heterogeneous nature of the parent rock. Thus, alteration of the barren graywackes resulted essentially in the formation of a saprolite rich in clay minerais (hydrobiotite, smectite, kaolinite). On the other hand, weathering of the iron formations, host-rocks of copper sulfides (bornite, calcocite, chalcopyrite), originated a gossan type profile composed from the base to the top of a supergene sulfide zone, an oxidized minerais zone and a ferruginous saprolite. The supergene sulfides are digenite, covellite and bornite (?) while cuprite, malachite, azurite and native copper constitute the oxidized copper assemblage. Within the saprolite, smectite and goethite are the main copper enriched minerais. The evolution of such supergene setting involved oxidization and dissolution of primary sulfides with mobilization of Cu and Fe as soluble sulfatos. The downward and lateral transfer of these metais caused the formation of new mineral associations according to the Cu/S ratio in the solutions and the ambient pH and Eh conditions.In the reducing zone the ionic exchange between copper sulfate and primary sulfides yielded mainly digenite and covellite, while in the water-table fluctuation zone and immediately above, in more oxidizing conditions, copper precipitated as cuprite and native metal. Besides, reaction of copper sulfate with calcite from veins that cuts iron formations resulted in the fixation of Cu as malachite and azurite in neutral to alkaline conditions. In the Carajas region, the lateritic mantle was formed during the Lower Tertiary, but, during the Upper Tertiary and Quaternary, underwent dissection due to regional uplift. Thus, in the Salobo area, the duricrust and the upper saprolite were removed. Each phase of lowering of the hydrostatic levei resulted in the deepenning of the weathering profile and alteration, in oxidizing conditions, of previously formed supergene oxides and sulfides. One parte of the released copper then underwent downward leaching and reprecipitation in deeper reconstituted cementation and oxidation zones, while another part was fixed by neoformed goethite and clay-minerals.

Paleosols in upper permian sedimentary rocks, Sukhona river (Severnaya Dvina basin)

Paleosols crop out in the Sukhona River valley as several members, up to 10 m thick, embedded into the Salarevo Formation sediments. Principal characteristics of paleosols include a dense network of root channels, indications of eluvial gley alteration, redistribution and formation of secondary carbonates represented by several generations, and formation of block-prismatic soil structure with specific clayey films at structural jointing faces. The paleosols are divided into a number of genetically interrelated horizons (from top to bottom): presumably organogenic accumulation (AElg), eluvial gley horizon (Elg), illuvial horizons (B1 and B2), illuvial gley horizon (Bg), and transitional horizons (ElBg and BElg). Paleosols were formed under conditions of a semiarid climate with sharp seasonal or secular and multisecular oscillations of atmospheric precipitation. Such soils point to specific ecological environments, which were existing in the northern semiarid belt of the Earth before greatest (in the Phanerozoic) biospheric crisis at the Permian-Triassic boundary.

Porosity Factors that Control the Hydraulic Conductivity of Soil‐Saprolite Transitional Zones

AbstractSlowly permeable transitional horizons separate soil and saprolite, but these horizons cannot be identified easily in the field. The objectives of this study were to determine why the soil‐saprolite transitional zone (BC and CB horizons) is slowly permeable, and to evaluate ways for identifying it in the field. Two saprolite deposits were studied in the North Carolina Piedmont. At each site, saturated and unsaturated hydraulic conductivities (Ksat and Kunsat) were measured for major horizons. Volume fractions of water‐conducting pores were also compared with the changes in hydraulic conductivity with depth. Horizon mean Ksat values at both sites ranged from virtually 0 to approximately 3 cm h‐1. The lowest Ksat values (<0.3 cm h‐1) occurred in or near the transitional horizons that were directly below the Bt horizons. Changes in the volume of pores within or between mineral grains (termed inter/intraparticle pores) with depth corresponded to changes in both Ksat and Kunsat. In the transitional horizons, the inter/intraparticle pores were plugged with clay and this caused the horizons to have low K values. In situ measurements of Ksat with depth were the most accurate technique to use for identifying transitional zones in the field. Examination of both the soil and rock structures in pits was also an acceptable technique. Texture and consistence were not considered reliable for pin‐pointing transitional horizons.

Micromorphology of the Soil‐Saprolite Transition Zone in Hapludults of Virginia

AbstractMost residual soils occurring in the southeastern USA have formed in saprolite. Between the saprolite and the argillic horizon lies a zone that divides material undergoing pedogenesis from that undergoing geochemical processes. We examined the physical, chemical, mineralogical, and micromorphological properties and characteristics of representative soils and saprolite formed from schistose or gneissic rocks of Virginia in order to describe and define the soil‐saprolite transition zone. Argillic horizons showed considerable oriented clay (3–16%), strong to moderate subangular blocky microstructure, porphyric coarse/fine related distribution patterns (c/f RDP), low (<0.5) coarse/fine ratios separated at 20 µm (c/f20µ ratios), and many planar voids. Oriented clay was absent from C horizons and microstructure was termed rock controlled to describe the apedal nature of these horizons. The c/f RDP was chitonic or gefuric, with the c/f20µ ratio at least double that of the argillic horizon. Microstructure of transition horizons was moderate subangular blocky to weak subangular blocky/rock controlled. The c/f20µ ratios and c/f RDP were intermediate between the argillic horizons and the saprolite. Transition horizons were designated as BCt, BC, or CB depending on percent oriented clay, rates of change with depth of clay, dithionite‐citrate‐bicarbonate (DB) extractable Fe, and sand contents. Weathering of mica grains showed slight to almost total pellicular and parallel linear alteration to hydroxy‐interlayered vermiculite (HIV), kaolinite, and an interstratified component of these minerals. Alteration of feldspar grains was generally <25% and occurred in the form of pellicular, or parallel, or cross‐linear patterns to kaolinite or gibbsite.

Glacial geology of the Torngat Mountains, Labrador

Two late Quaternary alloformations are identifed throughout the Torngat Mountains by degree of soil profile development in till with a granitic composition, morphologic expression of deposits, and their relative areal distribution. The two alloformations are interpreted to represent two, successively limited glaciations of the Torngat Mountains by the Laurentide Ice Sheet. Deposits of the Shoal Cove Alloformation mantle hillslopes distal to deposits of the younger Saglek Alloformation. Deposits are primarily soliflucted till found around coastal regions. Soils developed in till of the Shoal Cove Alloformation are characterized by well-expressed Bw horizons ≥ 19 cm thick. Deposits of the younger Saglek Alloformation are characterized by prominent, well-preserved moraines, outwash fan surfaces, and ice-marginal channels. Soils described in till are poorly developed, with A horizons directly overlying Cox horizons, or separated from Cox horizons by thin, transitional (AC, CA) horizons or poorly expressed Bw horizons. During deposition of this alloformation, the Laurentide Ice Sheet was confined to major valleys and fiords, leaving large areas of the mountains ice free. Old radiocarbon dates on shells from diamictons on the Iron Strand coast do not provide a unique age for glacial events on the Labrador coast. Age of marine limit on the coast suggests deglaciation from the event depositing the Saglek Alloformation began 9 000 – 10 000 years BP.

An early Holocene Solodic paleosol near Priddis, Alberta

An early Holocene paleosol was found beneath Mazama Ash (ca. 6600 BP) near Priddis, Alberta. The paleosol pedon has strong textural and morphological differentiation, with an eluvial surface horizon, thick prismatic transitional horizon, and a Btb horizon containing both illuvial clay and organic matter. A Solonetz to Solod genetic sequence is inferred, probably related to changes in groundwater regime caused by stream entrenchment.

Classic to Postclassic in Highland Central Mexico

The data and argument we have presented converge on three points. 1) With the decline and abandonment of Teotihuacan by the end of the Metepec phase (Teotihuacan IV), the valleys of Mexico and of Puebla-Tlax-cala witnessed the development of a ceramic culture that was represented, on the one hand, by obvious Teotihuacan derivations in presumably ritual ware and possible Teotihuacan derivations in simpler pottery of red-on-buff, and, on the other hand, by elements that seem to represent a resurgence of Preclassic characteristics. Whether the development is explained through a measure of outside influence or as a local phenomenon, the direct derivation of a substantial portion of the complex from Classic Teotihuacan is unmistakable. This transitional horizon predated the arrival of plumbate tradeware in highland central Mexico. 2) The transitional horizon coincided with (and no doubt was an integral part of) an alteration of Classic settlement patterns so drastic that it must bespeak political disruption. Nevertheless, there is no evidence that the Postclassic center of Tula represented a significant force in the highlands at that time. There is no evidence that the center of Cholula, which may even have been substantially abandoned during the previous period, was able to exert any force at this juncture; it appears more likely that Cholula was largely reoccupied after the abandonment of Teotihuacan. There is no direct evidence of domination by Xochicalco or any other known major foreign center, although some ceramic traits suggest that relatively minor influences may have emanated from Xochicalco; unfortunately, the state of research at that center does not permit a determination at this time. Thus the most reasonable view on the basis of present evidence is that the abandonment of Teotihuacan was not the direct result of the strength of another centralized power, although some outside populations may have been involved in a minor way. Whatever the proximate cause, however, it is now clear that the abandonment of Teotihuacan led to a period of Balkanization in which no single center, or pair of centers, were dominant in the highlands. 3) The transitional horizon saw the immediate development of a cultural distinction between the Valley of Mexico and the Valley of Puebla-Tlaxcala, a distinction in which differential degrees of outside cultural influence may have played a part. This distinction was magnified in the early Postclassic, with the rising power of Tula on the west and of Cholula on the east, and Balkanization ended with the growth of empire.

THE K HORIZON

The designation K is proposed for soil horizons so strongly carbonate-impregnated that their morphology is determined by the carbonate. Tough these horizons display a variety of macroscopic forms, and range in consistence from soft to extremely hard, they all have a peculiar and diagnostic soil fabric, the K-fabric. In material with K-fabric, fine-grained authigenic carbonate coats or engulfs skeletal pebbles, sand, and silt grains as an essentially continuous medium. The material breaks down or is markedly softened by acid treatment. The designation K2 is proposed for carbonate horizons of 90 percent or more, by volume, of K-fabric, and K1 and K3 designations are proposed for upper and lower transitional horizons containing 50 percent or more of K-fabric. Other soil horizons containing carbonate accumulations are noted by the ca symbol in combination with the appropriate master horizon designation. /Author/