Scanning electron microscopy of quartz grains in supraglacial debris, Adishy Glacier, Caucasus Mountains, USSR

The surface microtopography of quartz grains in glacial depostis has often been attributed to mechanical release from source rocks during weathering and prior to entrainment by glaciers. Fractures on quartz particles, including subparallel and conchoidal features, often attributed to cryostatic pressure in ice and stick-slip processes at the base of glaciers, have been considered, in part, to result from mechanical weathering of source rocks. To test this hypothesis we studied 15 samples of supraglacial debris from the Adishy Glacier in the central Caucasus Mountains, USSR. Clasts in transport on the surface of the glacier originate primarily from the cirque headwall by weathering and mass wasting processes; a minor amount is presumed to have been derived from aeolian influx following reworking of older sediments down valley. The results show unequivocally that mechanical weathering is only likely to produce grains with pronounced fracture faces that lack subparallel and conchoidal fracture features. No fresh glacial-crushing type features were observed on these samples. Grains that had been glacially crushed, weathered and retransported, presumably by aeolian processes, were observed in four samples of the data set.     

The Rio Dell Formation: a Plio-Pleistocene basin slope deposit in Northern California

The Rio Dell Formation (Pleistocene and Pliocene), exposed south of Eureka, California, is a prograded sequence of basinal turbidites overlain by basin slope and shelf deposits. The slope deposits studied in the Centerville Beach section accumulated in a steadily shallowing environment delineated by analysis of palaeobathymetrically significant benthonic foraminiferal biofacies in turn suggesting deposition at depths of 1000–100 m. Lower slope deposits interfinger with basinal turbidites derived from the Eel River delta to the north. Slumped blocks of silty mudstone, and associated silt and mud beds, are common. The middle slope deposits are mudstones; coarser sediments bypassed this zone. Mudstones and muddy siltstones alternate on the upper slope. Shallow depressions, probably slump scars, that have been rapidly filled by upper slope sediment are common. The transition to shelf deposits is marked by an increase in sediment grain size, in the degree of oxidation, and in the abundance of megafossils. High percentages of benthonic foraminifera displaced from shelf depths indicate that resedimentation processes are most important on the upper slope.     

Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia

The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment-water interface. The calcite is biologically precipitated as ostracod valves. In addition to the carbonates in the modern offshore (deep-water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high-Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units. Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.     

Quantitative relationships between numbers of fluvial cycles, bulk lithological composition and net subsidence in a Scottish Namurian basin

This detailed quantitative basin analysis of fluvial deposits in a subsiding Namurian structural basin is aimed at discovering underlying statistical relationships between numbers of fluvial cycles, bulk lithological composition and net subsidence which could be used as a background to future sophisticated computer simulation experiments and would also facilitate comparison with other ancient basins. The succession studied lies between two widespread marine bands, one of Arnsbergian (E₂) and one of Kinderscoutian (R₁) age, and is dominated by upward-fining cycles: 94% of the semi-cycles containing sand, here termed grain-size cycles, are upward-fining and the ratio of fine members (mudstone+siltstone) to coarse members ranges from 0·23 to 5·0. Trend-surface analysis reveals basement structures, which influenced sedimentation, including a Caledonoid graben. Correlation coefficient values and results of principal component analysis demonstrate that the numbers of rooty horizons and grain-size cycles, together with the total thicknesses of sandstone and of mudstone+siltstone, all tend significantly towards a linear relationship with the total thickness of strata and hence net subsidence. The average thickness of grain-size cycles tends towards an inverse linear relationship with net subsidence. This probably reflects the presence of stacked, relatively thin, channel-fills within persistent channel belts. These belts tend to be localized in the areas of greatest subsidence within the basin and follow courses basically similar to channels already discovered in the underlying deltaic sediments. The highest concentrations of mudstone+siltstone tend to lie on the flanks of the basin, but high local sand concentrations found at points on the margin where channels entered the basin from the NW and NE and left to the SW, effectively disrupt any significant relationship between net subsidence and the proportion of sand. Coal has been selectively preserved on the flanks of the basin and the number of rooty horizons is greatest in an area of low subsidence which lay somewhat remote from the main channel belts.     

A depositional model for storm- and tide-influenced prograding siliciclastic shorelines from the Middle Devonian of the central Appalachian foreland basin, USA

A stratigraphic motif observed in many foreland basins is the development of basinward tapering siliciclastic wedges characterized by various scales of depositional cycles. The Middle Devonian (Givetian) Mahantango Formation in the central Appalachian foreland basin is such an example. It consists of both small-and large-scale thickening- and coarsening-upward cycles; the small-scale cycles are typically less than 10 m thick whereas larger-scale cycles are generally a few tens of metres thick and commonly contain several of the smaller-scale cycles. Outcrop-based facies analyses indicate that the depositional cyclicity resulted from episodic progradation of a regionally straight, tide-dominated shoreline onto a storm-dominated, shallow marine shelf. The depositional model for this ancient shallow marine system consists of a vertical facies succession in which storm-dominated offshore marine mudstone and fine sandstone pass gradationally upward into storm-dominated nearshore marine shelf and shoreface sandstone overlain by, in proximal sections, tide-dominated shoreline sandstone, pebbly sandstone and mudstone. Transgressively reworked lag deposits cap most of the thickening- and coarsening- upward packets. In this model, coarse-grained rocks, rather than implying basinward shifts of facies, are a consanguineous part of the stacked shoaling cycles. Lateral facies relationships show that the dominance of storm- vs. tide-generated sedimentary features is simply a function of palaeogeographical position within the basin; proximal sections contain tidally influenced sedimentary features whereas more distal sections only display evidence for storm-influenced deposition. These results suggest caution when inferring palaeoceanographic conditions from sedimentological datasets that do not contain preserved examples of palaeoshorelines.     

Deformation structures and an alteration zone linked to deposition of volcanogenic sulphate in an ancient playa (Oligocene of Nebraska, USA)

Historic, sulphur-rich volcanic eruptions have altered global climate for as much as five years, and much larger events are known from the geologic record. At Scotts Bluff, Nebraska, Early Oligocene strata of the lower Arikaree Group contain a tephra bed with abundant calcite pseudomorphs after gypsum. Previous work has shown sulphate from the pseudomorphs in this tephra bears a high ¹⁷O anomaly indicative of oxidation of sulphur gases by ozone or hydrogen peroxide in the atmosphere. Possible sources of the tephra were caldera eruptions at about 28 Ma in the San Juan volcanic field of south-western Colorado (∼500 km SW of the study site) and the eastern Great Basin (∼1000 km WSW). The present sedimentological study shows that tephra and volcanogenic sulphate were deposited and preserved within a small, surface-discharging playa that developed on the irregular upper surface of aeolian siltstones of the subjacent White River Group. Sulphate solutions (including perhaps sulphuric acid) percolated downward within the vadose zone, dissolving early formed smectite cement within underlying volcaniclastic sandstones, reddening these rocks along an irregular alteration front. Preserved fine-scale stratification within the sandstones precludes the possibility that reddening took place during pedogenesis. Displacive growth of gypsum at the playa centre folded tephra beds and forced tephra into underlying sandstones, forming elongate cones. The large mass fraction of gypsum (now replaced by calcite) in the playa sediments suggests a huge, long-distance delivery of sulphate aerosols. Some of the sulphate and tephra may have come from the same eruption, or the fine-grained tephra may simply have aided preservation of dry-fog sulphate derived from an unrelated, effusive eruption of lava.     

Sedimentology of a saline playa in the northern Great Plains, Canada

Ceylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present-day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite. Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate-clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na-rich solution to a mixed Mg-Na system. The most important post-depositional processes affecting Ceylon Lake sediments are mud diapirism and salt karsting.