Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

The detailed analysis of the microfacies succession on a stratigraphic sequence representative of the upper part of the Cupido Formation, cropping out in Durango State, provide additional data for the paleoecological and micropaleontological significance of the Barremian–Aptian transition in northeast Mexico. The stratigraphic section herein studied is composed of a series of deposits typical of a shallow carbonate platform, which was apparently exposed to different paleobathymetric changes of short duration and low magnitude through the Barremian–Aptian transition. The section is characterized by a rich micropaleontological association mainly composed by benthonic foraminifera of the families Ataxophragmiidae, Lituolidae, Textularidae, Orbitolinidae, Nezzazatidae (Subfamily Nezzazatinae) and Miliolidae. These foraminifera are commonly associated to calcareous algae, mollusk and ostracod fragments, pellets, and echinoid spines. The prevailing textural composition and allochemical content throughout the sedimentary facies succession are indicative of Standard Microfacies typical of different sites of deposition, which vary from lagoon or back-reef environments to sand bars or shoals, and into high energy environments in fore-reef positions. The limit of the Barremian–Aptian transition in the area of study is defined by the first stratigraphic appearance of Palorbitolina cf. lenticularis (Blumenbach), and is coincident with an apparent short period of deepening of the deposition environment to a local extent.

The seismically imaged subsurface stratigraphy of the Arcabuz–Culebra area in the Burgos Basin (Nuevo León State, northeast Mexico) consists of a probably metamorphic basement of late Paleozoic age unconformably overlain by a sequence of sedimentary rocks which spans from the Middle Jurassic (?) to the early Oligocene. This stratigraphic sequence can be subdivided into two packages with different structural behavior that define a detached extensional deformation. The lower sequence appears undeformed, and includes the basement as well as the mesozoic marine strata which dip shallowly to the east. The upper sequence includes the paleogene units and is deformed by a series of eastward dipping normal faults with listric geometry, which sole out at the Mesozoic–Paleogene boundary. The upper sequence also exhibits shale diapir structures. Two structurally distinct domains can be identified within the detached upper plate: the deformation within the western domain is dominated by listric faulting defining a detached normal fault style, while in the eastern domain, shale mobility or flow appears to be the dominant deformation mechanism defining a shale structure style. The truncation of fault systems by paraconformities constrains their age to the Eocene. In detail, the first deformational phase, of early Eocene age, is post-depositional, while the latter two phases in middle Eocene and late Eocene–early Oligocene time, respectively, correspond to syn-sedimentary events. These last two pulses are also spatially and temporally associated with three periods or stages of shale mobility: a middle Eocene reactive stage, a late middle Eocene active stage and a new late Eocene–early Oligocene reactive stage. Toward the middle of the seismic section, the intersection of the two structural styles corresponds with the development of an important depocenter, in which important thicknesses of Queen City, Weches, and Cook Mountain strata accumulated.

Sequences of buried soils were studied in the floodplains of the Oka and Moscow rivers. These soils were formed in the periods of low floods, when alluvial sedimentation was inactive. Numerous 14C and archaeological dates indicate the following intervals of intensive soil formation on the floodplains of the Oka River: 6,500–4,500; 4,000–3,000; and 2,200–800 yr BP. In many cases, buried Gleysols and Phaeozems of the middle Holocene are replaced by Luvisols of the late Holocene. Deforestation and extensive tillage in the river basins during the last 700–900 years has caused an increase in the rates of spring runoff, slope erosion, and alluviation on the floodplains under conditions of active and high-level floods. As a result, Luvisols have been buried under recent alluvium, on top of which weakly developed Fluvisols are forming.

The concept of palimpsest-wise soil memory and record is generally characterized in comparison with the book-wise memory and record of sedimentary rocks. Soil systems have a capacity for storing information about environmental factors and pedogenic processes that have been acting for a period of pedogenesis. The main mechanisms of soil memory and record formation are those sets of pedogenic processes that generate the solid phase products and features within the multiphase soil system. The main types of solid-phase carriers of soil memory and their spatial/temporal hierarchy within the soil system are briefly described. The phenomena of isomorphism and polymorphism of soil record carriers in regard to the pedogenic processes should be realized when we try to decode and understand the information stored in soil memory. Each specific type of climate could not be imprinted in only one type of pedon. When reading the record in the soil system under one type of climate, it is obligatory to account for the diversity of solid phase soil horizons and pedons induced by the diversity of parent materials (lithodiversity), topography (topodiversity), biota (biodiversity) and duration of pedogenesis (chronodiversity).

A mineralized locality with Ni–Co values that could be economic is reported for the first time in Sonora. The La Esperanza ore deposit, located at 12 km NW from Bacanora, is a skarn type deposit developed at the contact between a laramidic granitoid and Paleozoic limestones. The elements Ni and Co are present as siegenite with the composition (Ni2.05 Fe0.19Co0.75)2.99S3.97. Copper, zinc and iron sulphides are also present. The economic preliminary potential at La Esperanza is about some millions of tons with 1 gr Au/ton, 1–2 % Cu, 1–2 % Zn, 0.1–0.2 % Ni y 0.1–0.2 % Co. The Co–Ni anomalies recently found in stream sediments of the region indicate new potential targets for the regional exploration.  

Permian reefs in Sierra Plomosa, Chihuahua are well exposed and contain shallow-water faunas similar to the better-studied rocks of the Permian Reef Complex in Texas and New Mexico. The reefs are dominantly mudstones with subordinate algal boundstones, packstones, and grainstones. Fusulinids indicate the reefs are Late Wolfcampian through at least Early Leonardian. The reefs are found in complicated juxtaposition to basinal clastic rocks of the Plomosas Formation. Thrusting is clearly responsible for the structural association in the Sierra Santo Domingo at the east end of the range. Sliding of reefal blocks into adjacent deep water may have occurred. Reefal olistoliths of shallow water origin are embedded in deep marine terrigenous clastics.  

Paleosol sequences of the Nevado de Toluca volcano (NTV) preserve a detailed record of late Quaternary environmental change. In fact, some studies have been used the NTV paleosols to interpret paleoenvironmental conditions, however some uncertainties still remain concerning their stratigraphic position and correlation. In this paper, we present the results of the NTV pedostratigraphy by using different paleosol units and based on present day tephrostratigraphy. We recognized eight Pleistocene paleosols, labeled PT0–PT7 which cover the 100,000–10,000 yr BP interval. These paleosols are located in four exposures: Arroyo La Ciervita, in the northern flank of the volcano, Zacango, in the northeast, and San Pedro Tlanisco and Barranca Cieneguilla, in the eastern part. The younger five paleosols correspond to different intergrades of Andosols (PT0–PT4) while the older sequence consists of three well developed Luvisols (PT5–PT7). All Andosols have humic horizons allowing radiocarbon dating. PT0 and PT1 were formed at the end of the Pleistocene and correlate with the oxygen isotope stage (OIS) 1 and to the middle-late OIS-2, respectively; the time for PT2 formation corresponds to the end of OIS- 3 and the beginning of OIS-2, PT3 to the OIS-3, and PT4 to the OIS-4. PT5–PT7 have not yet been dated, but we correlate them with the OIS-5a and 5b. A more strongly developed paleosol (PT1) was formed during the Last Glacial Maximum (18,000 yr BP) in comparison with other paleosol sequences where weakly developed paleosols were reported or even no soil formation was achieved in this period. We conclude that during late Pleistocene to Holocene, OIS 5 to 1, paleosol sequences of the Nevado de Toluca provide good environmental resolution, similar to or even higher than loess-paleosol sequences, although stratigraphic correlation with other paleoenvironmental records is more difficult because of the heterogeneity of volcanic sediments.

Ten to twelve pedosedimentary stages were identified within the Upper Permian red clayey sediments of the Sukhona and North Dvina drainage basins. Paleosols formed at these stages are generally similar in their profile arrangement and pedogenic features. The main pedogenic processes occurring in the area during the Upper Tatarian appear to have been: (1) eluviation and gleying (reduction and mobilization of non-silicate-bound iron and removal of some fine clay) in the upper parts of soil profiles; illuvial accumulation of iron, and some clay in the subsoil; (3) formation of secondary carbonates; and (4) generation of complex soil These processes are interpreted to have taken place in a semiarid climate with sharp seasonal and long-term fluctuations in precipitation. Upper Tatarian plant communities at that time probably consisted mostly of shoreline hygrophytes and halophytes, and xerophyllous conifers living on relatively elevated areas.

Bignell Loess is a Holocene loess unit that has accumulated in the central Great Plains since 9,000 14C yr BP. The Brady Soil separates Bignell Loess from last glacial Peoria Loess and thick sections, which can reach 6 m thickness, may contain four buried soils beneath the modern surface soil. The Brady Soil has A/B/C horizonation, including a thick dark A and a blocky or prismatic structured B horizon. The Brady soil is intensely burrowed, probably by nymphs of cicada species, which require woody vegetation. Secondary carbonate is present in all genetic horizons and was emplaced post burial. The Holocene soils are dominated by A horizon characteristics, namely dark colors, but blocky or prismatic structured B horizons occur in several sections. Secondary carbonate is present in most, but not all Holocene soils. Three phases of loess deposition and soil formation are evident from the thick proximal stratigraphic sections presented here. Initial rates of Bignell Loess deposition were low relative to the rate of soil formation, producing a basal aggraded A horizon above the Brady Soil, that is capped by a increment of loess with minimal pedogenic features. The middle portion of the Bignell Loess is finer textured and contains multiple dark buried soils. The final increment of loess is coarser and contains the weakly expressed modern surface soil. Climatically, termination of Brady Soil formation marks a shift to drier Holocene climates with a decrease in woody species, episodes of new loess deposition and soil formation, and secondary carbonate accumulation.

  Specific features of the polygenetic mountain soils of northern Tian-Shan (Kyrgystan) are due to the action of present-day and relict soil processes that varied in age and intensity. These properties can be used as indicators of paleoclimatic changes. Diagnosis of ancient pedogenesis was based on criteria with the longest response time, namely soil morphology, humus characteristics, isotope composition of humus and carbonates, and the soil age. Results indicate a glacial climate with mean annual temperature about -25° C during the late Pleistocene, a dry and cold climate during the early Holocene, warm and dry conditions of soil formation in the middle Holocene, and a humid climate of the late Holocene.