Clay-rich Zones Research Articles

Chemical and mineralogical heterogeneities of weathered igneous profiles: implications for landslide investigations

Abstract. Landslides in tropical and sub-tropical regions are generally associated with weathered rock profiles which often possess chemical and mineralogical heterogeneities at material- and mineral-scales. Such heterogeneities reach a climax by the occurrences of oxyhydroxide- and clay-rich zones. Weakness and low permeability of these zones makes them ideal for the development of slip zones along which landslides take place. This paper describes the nature and distribution of chemical and mineralogical heterogeneities within weathered profiles developed from felsic igneous rocks in Hong Kong. It sets out the use of integrated geochemical and mineralogical studies to improve understanding of the development of critical heterogeneities and hence to predict their types and presence in a given weathered profile.

Subglacial deformation of trees within overridden foreland strata, Bering Glacier, Alaska

The foreland stratigraphy overridden during recent Bering Glacier surges bears evidence of subglacial deformation. The pre-existing, fine textured substrate (till and diamicton) experienced diminished strength because of saturation, thus resulting in shallow mobilization and the formation of a new till of limited thickness. Glacial coupling with well drained sediment resulted in ploughing that generated a diamicton that retains vestiges of outwash sorting and stratification. The outwash sequence extending decimeters beneath the surface till contains four prominent sub-meter sand beds. Each sand bed holds multiple small, fossil trees still rooted in underlying layers of gravel. Virtually all trees in the upper two sand beds are deformed. Several are offset by centimeter to decimeter horizontal shears confined to thin, silt, and clay-rich zones at the base of each sand bed. Trees that escaped shearing are warped and kinked. Deformed trees are present at depths that range from 15.76 to 5.31 m. beneath potential ice/substrate interface surfaces. The most likely source of deforming stress in this foreland setting is related to glacial advance. The style and orientation of tree deformation are consistent with the direction of ice movement. Therefore, the occurrence of deformed trees is attributed to stress applied by overriding ice.

Water geochemistry of the Seferihisar geothermal area, İzmir, Turkey

The Na–Cl thermal waters of Seferihisar have measured temperatures of 30–153°C, and conductivities of 7400–55200 μS/cm. They issue from Upper Cretaceous to Paleocene Bornova melange rocks, which are made up of sandstone–shale intercalations, conglomerate, mafic submarine volcanics, limestone lenses, serpentinite and limestone bodies, and their complexes, through the intersection of faults. The clay-rich zones of the overlying Neogene terrestrial sediments cap the system. The heat source is the high geothermal gradient caused by graben tectonics of the area. Na and Cl ions mainly dominate the chemistry of thermal waters, thus thermal waters of the Seferihisar area appear to be mixtures of groundwaters and seawater. The seawater contribution in thermal waters varies from 10% to 80%. Assessments from empirical chemical geothermometers, mineral equilibria and silica-mixing model applied on the thermal waters of the Seferihisar geothermal area suggest that the temperatures of the reservoir systems vary between 60 and 180°C. The Seferihisar geothermal area has been physically divided into five main groups and estimated reservoir temperatures provided by the entire methods give the increase as: Doğanbey Burnu, Karakoç spa, Doğanbey, Tuzla and Cumali spa areas in ascending order of reservoir temperature. Hydrogeochemical assessments indicate that thermal waters were mixed with cold waters before and/or after heating at depth in different proportions. The results of mineral equilibrium modeling indicate that the thermal waters of Seferihisar are undersaturated with respect to gypsum and amorphous silica, oversaturated with respect to calcite, dolomite and aragonite, and undersaturated or oversaturated with respect to quartz, chalcedony and anhydrite.

Alleged vertebrate eggs from Upper Cretaceous redbeds, Gobi Desert, are fossil insect (Coleoptera) pupal chambers: Fictovichnus new ichnogenus

Small egg-like structures from the Upper Cretaceous (Campanian) Djadokhta Formation of Mongolia were previously interpreted as casts of crocodile, lizard, and turtle eggs, or as inorganic nodules. Identical structures from coeval redbeds at Bayan Mandahu in northern China indicate the structures are not vertebrate eggs, nor of vertebrate origin. Comparisons with Recent and Quaternary beetle (Coleoptera) pupal chambers show close similarities in size, shape, and the presence of a round to irregular exit hole for the adult beetle. Most importantly, the Cretaceous structures are enveloped by a thin clay-rich zone, which is expected if constructed by beetle larvae but inexplicable in any egg or inorganic nodule model. Additional evidence contradicting a vertebrate egg origin for the structures includes (i) the structures are too small to have been laid by turtle or crocodile species occurring in the Bayan Mandahu redbeds; (ii) the structures are isolated, not in clutches or pairs; and (iii) unlike newly hatched soft-shelled lizard eggs, the Cretaceous structures are not collapsed and show a round to irregular exit hole rather than a slit. It is concluded that the egg-like structures are sand casts (steinkerns) of beetle pupal chambers, probably of Scarabaeidae, Tenebrionidae, or Curculionidae. The Cretaceous pupal chambers are assigned to a new ichnogenus, Fictovichnus, and new ichnospecies, Fictovichnus gobiensis and Fictovichnus parvus.

The significance of oil shale fabric in primary hydrocarbon migration

Scanning electron microscopy (SEM) and thin-section analysis of the fabrics (particle orientation) of some well known ‘oil shales’ reveal they all share two common characteristics: (1) even though preferred platy particles dominate, there is present an open network of numerous interconnecting pores; and (2) fine lamination of alternating silt and organic clay-rich zones is common. Shales selected for analysis include the Woodford Shale, Sunbury Shale, torbanite, Rundle Shale and Ohio Shale (Huron Member), West Falls Formation (Rhinestreet Member), New Albany Shale (Clegg Creek Member) and the Exshaw Formation. A model is proposed, based upon fabric information, for the migration of hydrocarbons from source rocks. Hydrocarbons generated in organic-rich zones migrate through the micropores until reaching adjacent silt laminae, which act as ‘pipelines’ to facilitate further migration out of the source rock.

Actinide and rare earth element characteristics of deep fracture zones in the Lac du Bonnet granitic batholith, Manitoba, Canada

The history of major, trace, and actinide element mobility and fluid infiltration has been studied in two deep fractures (>1 km) in the Lac du Bonnet batholith as part of the Canadian Nuclear Fuel Waste Management Program. Core samples collected from two fracture zones, FR1 and FR2 (~1175 m deep), containing saline groundwater (TDS = 50 g/L) were investigated mineralogically, chemically, and isotopically ( 238U-series, O and H). Several sequentially overprinting alteration stages were identified, from early high-temperature to later low-temperature hydrothermal alteration. K-feldspar, illite, chlorite, and later kaolinite formed during these stages. Subsequent infiltration of oxidizing fluids produced alteration of the chlorite to hydrous iron oxides. Fracture zone FR1 contains predominantly hematite coating; fracture zone FR2 is characterized by the formation of a breccia and by an intense alteration of the granite in contact with this breccia to illitic clay. Alteration occurred during infiltration either of formation brines or of isotopically evolved meteoric water where δ 18 O = 8 to 12‰ and δD = −65 to −20‰, at calculated temperatures between ~250 and ≤25°C. Loss of Ca, Na, and Sr and, to some extent, Ba was observed in all the altered samples relative to fresh granite. A sharp increase in K, Mg, U, Th, and REE was observed in the illitic clay rich zone. Pronounced disequilibria of 234U 238U (<0.5) , 230Th 234U (~0.7) , and 226Ra 230Th (~0.9) exist in the illitic clay, indicating loss of 234U, 230Th, and 226Ra to the groundwater within the last 1.5 Ma. In contrast, an excess of 234U, 230Th, and 226Ra was measured in the brecciated samples. The disequilibria are consistent with a model involving loss of 234U, 230Th, and 226Ra to groundwater by α-recoil from U deposited on the illitic clay surfaces. These radionuclides were deposited subsequently in the nearby brecciated zone.

Mineralogical and fluid inclusion features of rock alterations in the Seigoshi gold-silver mining district, western part of the Izu Peninsula, Japan

Propylitic and advanced argillic alterations occur in the Seigoshi gold-silver mining district, in the western part of the Izu Peninsula, Japan. The propylitic alteration has the following zonal arrangement from the deeper portion to the shallower portion: an epidote-prehnite-K-feldspar-chlorite zone; a wairakite-laumontite zone; and a stilbite-heulandite-montmorillonite zone. Lateral and vertical zoning is conspicuous in the advanced argillic alteration. The inner zone is silica-rich and this grades laterally and vertically through alunite- to clay-rich zones. Fluid inclusions from the epidote-prehnite-K-feldspar-chlorite zone are liquid-dominated and filling temperatures of this zone are in the range of 225–285°C. Filling temperatures for the zeolite zone are variable, being in the range of 240–380°C. This wide range suggests that boiling of the fluids was responsible for the zeolite zone. Filling temperatures of the advanced argillic alteration range widely from 210° to 430°C. This wide range and the coexistence of liquid- and vapor-dominated fluid inclusions in a given sample suggest that liquid-vapor separation simultaneously occurred during the advanced argillic alteration process. It is deduced that the ranges of gaseous fugacity are quite different for each type of alteration. For instance, f S 2 and f O 2 of the advanced argillic alteration are estimated to be higher than those of the propylitic alteration. Based on the alteration mineral assemblage, chemical composition and mode of occurrence of alteration minerals, fluid inclusions, estimated ranges of gaseous fugacities, and comparison of these features with those of active geothermal systems, it is concluded that the propylitic alteration minerals were probably precipitated due to the loss of gases such as CO 2, and that the formation of advanced argillic alteration was caused by subsurface mixing of volcanic gas and/or condensed hot water with groundwater. These coexisting propylitic and advanced argillic alterations are commonly found in the other AuAg mining districts in Japan.