Humid Paleoclimate Research Articles

Climatic control on clay mineral formation: Evidence from weathering profiles developed on either side of the Western Ghats

Many physico-chemical variables like rock-type, climate, topography and exposure age affect weathering environments. In the present study, an attempt is made to understand how the nature of clay minerals formed due to weathering differs in tropical regions receiving high and low rainfall. Clay mineralogy of weathering profiles in west coast of India, which receives about 3 m rainfall through two monsoons and those from the inland rain-shadow zones (<200 cm rainfall) are studied using X-ray diffraction technique. In the west coast, 1:1 clays (kaolinite) and Fe—Al oxides (gibbsite/goethite) are dominant clay minerals in the weathering profiles while 2:1 clay minerals are absent or found only in trace amounts. Weathering profiles in the rain shadow region have more complex clay mineralogy and are dominated by 2:1 clays and kaolinite. Fe—Al oxides are either less or absent in clay fraction. The kaolinite—smectite interstratified mineral in Banasandra profiles are formed due to transformation of smectites to kaolinite, which is indicative of a humid paleoclimate. In tropical regions receiving high rainfall the clay mineral assemblage remains the same irrespective of the parent rock type. Rainfall and availability of water apart from temperature, are the most important factors that determine kinetics of chemical weathering. Mineral alteration reactions proceed through different pathways in water rich and water poor environments.

STRATIGRAPHY OF RHAETIAN TO LOWER SINEMURIAN CARBONATE PLATFORMS IN WESTERN LOMBARDY (SOUTHERN ALPS, ITALY): PALEOGEOGRAPHIC IMPLICATIONS

A stratigraphical revision of the Upper Triassic-Lower Jurassic succession of the western Southern Alps (Varese area) leads to the introduction of one new lithostratigraphical unit: the ?upper Hettangian-lower Sinemurian Alpe Perino Limestone and the recognition, also in the western Lombardy, of the Rhaetian Zu Limestone, consisting of subtidal cycles, with inner platform facies at the base (Campo dei Fiori Dolomite). These formations represent lagoonal-peritidal to subtidal carbonate depositional systems. A locally angular unconformity between the Rhaetian (or the lower Hettangian) and the upper Hettangian-Sinemurian formations, characterised by ‘terra rossa’ paleosols and pedogenetic carbonate breccias, can be correlated through all of the investigated profiles. The Lower Jurassic paleogeography of the western Lombardy Basin was characterised by an emerged area from Hettangian until earliest Sinemurian times in a warm humid paleoclimate. The Alpe Perino Limestone represents a small peritidal to subtidal carbonate platform developed only in a shallow-water gulf of the M. Nudo area. The platform was surrounded by a wide emerged area (island or peninsula ?), in the west to southeast, and by open subtidal and basinal environments, in the east to northeast (M. Generoso basin).

Micromorphology of calcitic features in highly calcareous soils of Fars Province, Southern Iran

The micromorphology of calcitic pedofeatures in highly calcareous arid and semiarid soils of the Fars Province, Southern Iran has been studied in order to determine their genesis and distribution in different climatic regions. Available soil moisture, soil temperature, soil texture, vegetation and availability of calcium carbonate are amongst the main factors determining the type and morphology of calcitic features. Size and frequency of microcrystalline calcite nodules and coatings increase from aridic to xeric, but decrease again towards the areas with a ustic moisture regime. Cytomorphic and acicular calcite are observed mainly in areas with an ustic moisture regime and a denser vegetative growth, but rarely in xeric areas. Layered pendents of calcite are common beneath coarser fragments, but are considered as relicts of a more humid paleoclimate. Calcite depletion pedofeatures are observed in xeric–mesic areas with higher available soil moisture. Ultraviolet (UV) violet (VL) and blue light (BL) fluorescence and cathodoluminescence (CL) studies proved to be useful in differentiating in this region pedogenic from geogenic calcite. All types of pedogenic calcite were UV and BL-fluorescent but didn't show CL, whereas, limestone fragments showed CL, but no UV–BL-fluorescence. The absence of CL in the studied pedogenic calcite samples is suggested to be related to the oxidative environment, hindering mobilisation of Mn ++.

Rocky Mountain Tertiary coal-basin models and their applicability to some world basins

Tertiary intermontane basins in the Rocky Mountain region of the United States contain large amounts of coal resources. The first major type of Tertiary coal basin is closed and lake-dominated, either mud-rich (e.g., North Park Basin, Colorado) or mud plus carbonate (e.g., Medicine Lodge Basin, Montana), which are both infilled by deltas. The second major type of Tertiary coal basin is open and characterized by a preponderance of sediments that were deposited by flow-through fluvial systems (e.g., Raton Basin, Colorado and New Mexico, and Powder River Basin, Wyoming and Montana). The setting for the formation of these coals varies with the type of basin sedimentation, paleotectonism, and paleoclimate. The mud-rich lake-dominated closed basin (transpressional paleotectonism and warm, humid paleoclimate), where infilled by sandy “Gilbert-type” deltas, contains thick coals (low ash and low sulfur) formed in swamps of the prograding fluvial systems. The mud- and carbonate-rich lake-dominated closed basin is infilled by carbonate precipitates plus coarse-grained fan deltas and fine-grained deltas. Here, thin coals (high ash and high sulfur) formed in swamps of the fine-grained deltas. The coarse-clastic, open basins (compressional paleotectonism and warm, paratropical paleoclimate) associated with flow-through fluvial systems contain moderately to anomalously thick coals (high to low ash and low sulfur) formed in swamps developed in intermittently abandoned portions of the fluvial systems. These coal development patterns from the Tertiary Rocky Mountain basins, although occurring in completely different paleotectonic settings, are similar to that found in the Tertiary, Cretaceous, and Permian intermontane coal basins in China, New Zealand, and India.

Paleoclimatic evidence from the Bauru Group, Cretaceous of the Paraná Basin, Brazil

Numerous papers have been published on the Bauru Group, which represents the most important Cretaceous continental deposits in Brazil. Nevertheless, paleoclimatic evidence was rarely discussed. This work deals with the calcrete deposits, atapulgite clay mineral, rudaceous deposits, immaturity of sediments and geochemical properties as evidence of warm and dry (semi-arid) paleoclimate during the greatest part of the Bauru Group sedimentation. On the other hand, its fossiliferous content must be considered as an argument in support of intervals of warm and humid paleoclimate.