Principal petrographic characteristics and K-Ar isotope-geochronological data on samples from the Keli Volcanic Highland, Greater Caucasus

The paper reports newly obtained stratigraphic, petrographic, and isotope geochronology data on modern moderately acid lavas from the Keli Highland of the Greater Caucasus and presents a geological map of the territory, in which 35 volcanoes active in Late Quaternary time were documented by the authors. Total duration of volcanic activity at the highland was estimated at 250 ka. Volcanic activity was discrete and occurred in three phases: Middle Neopleistocene (245-170 ka), Late Neopleistocene (135-70 ka), and Late Neopleistocene-Holocene (<30 ka). Newly obtained lines of evidence indicate that certain volcanoes erupted in the latest Neopleistocene-Holocene. The first phase of volcanic activity was connected mainly with lava volcanoes, and eruptions during the later phases of volcanic activity in this part of the Greater Caucasus produced mainly lavas. The most significant eruptions are demonstrated to occur in the territory during the second phase. The major evolutionary trends of volcanic processes during the final phase in the Keli Highland are determined. It was also determined that overwhelming majority of volcanoes that were active less than 30 ka BP are spatially restricted to long-liven local magmatic zones, which were active during either all three or only the final two phases of activity. These parts of the territory are, perhaps, the most hazardous in terms of volcanic activity.

Sedimentation and mineral analysis on sediment cores from the Lena Delta

Core and outcrop analysis from Lena mouth deposits have been used to reconstruct the Late Quaternary sedimentation history of the Lena Delta. Sediment properties (heavy mineral composition, grain size characteristics, organic carbon content) and age determinations (14C AMS and IR-OSL) are applied to discriminate the main sedimentary units of the three major geomorphic terraces, which form the delta. The development of the terraces is controlled by complex interactions among the following four factors: (1) Channel migration. According to the distribution of 14C and IR-OSL age determinations of Lena mouth sediments, the major river runoff direction shifted from the west during marine isotope stages 5-3 (third terrace deposits) towards the northwest during marine isotope stage 2 and transition to stage 1 (second terrace), to the northeast and east during the Holocene (first terrace deposits). (2) Eustasy. Sea level rise from Last Glacial lowstand to the modern sea level position, reached at 6-5 ka BP, resulted in back-filling and flooding of the palaeovalleys. (3) Neotectonics. The extension of the Arctic Mid-Ocean Ridge into the Laptev Sea shelf acted as a halfgraben, showing dilatation movements with different subsidence rates. From the continent side, differential neotectonics with uplift and transpression in the Siberian coast ridges are active. Both likely have influenced river behavior by providing sites for preservation, with uplift, in particular, allowing accumulation of deposits in the second terrace in the western sector. The actual delta setting comprises only the eastern sector of the Lena Delta. (4) Peat formation. Polygenetic formation of ice-rich peaty sand (''Ice Complex'') was most extensive (7-11 m in thickness) in the southern part of the delta area between 43 and 14 ka BP (third terrace deposits). In recent times, alluvial peat (5-6 m in thickness) is accumulated on top of the deltaic sequences in the eastern sector (first terrace).

(Table 1) Heavy minerals in silt and sand samples of IODP Site 301-U1301

The northwestern Cascadia Basin of western North America accumulated high-sedimentation-rate sequences during the Pleistocene sea-level low-stands. The continental shelf was largely exposed at that time, and rivers and estuaries delivered large sediment fluxes directly to the deep ocean. The IODP EXP1301 core, which was taken from the middle portion of the Cascadia Basin, is well preserved and exhibits the deeper and - more distal sedimentary facies. The lithology in this location is composed of two units, 1) hemipelagic mud with a thin sand layer and 2) thick, coarsening upward silt-sand turbidites with a small proportion of granules at the top. We will focus on the detailed sand-grain proportions in order to understand the origin of these sediments. We determined the modal proportions of the heavy minerals, and the chemical composition of olivine and orthopyroxene in fourteen samples. These are characterized by an abundance of amphibole, pyroxenes and epidote, and the presence of minerals derived from peridotite. There is no drastic change in the modal and mineral compositions of the sands and silts between the turbidite and hemipelagic sequences. There were two probable drainage systems on the continent, the Frazer and Columbia rivers, which shed turbidites into the Cascadia Basin after 1.6 Ma, especially at 0.46-0.76 Ma. Based on a comparison of the modal and mineral compositions, the Northern Cascadia Basin has been supplied with sediments, mainly from the Frazer River, through the Straits of Juan de Fuca, by Pleistocene to Holocene turbidites.

(Table 1) Coarse-fraction (0.1 to 0.05 mm) mineralogy at DSDP Leg 55 Holes

This chapter was previously intended to trace volcanic episodes through the Neogene and Pleistocene geological history recorded in the sedimentary sections drilled on the Emperor seamounts. Drilling disturbance, poor core recovery, and incomplete stratigraphic sections recovered from the seamounts have frustrated that plan, however. Moreover, the Leg 55 sedimentologists found in their smear-slide studies that transported island-arc tephra is scarce in the sediments, if present at all. So we have restricted our objective to description of the volcaniclastic admixture in sediments, as determined by mineralogical and geochemical data. We studied geochemistry of bulk samples (see Murdmaa et al., 1980), coarse-fraction mineralogy, and additional smear slides. The results obtained, however, do not tell much more about the volcaniclastic matter than did shipboard core descriptions.

(Table 1) Geochemistry of sector-zoned crystal at DSDP Hole 45-395A

Variations in crystal morphologies in pillow basalts and probable sheet flows sampled from the region of the East Pacific Rise drilled during Leg 54 are related both to differences in composition and to an extreme range of cooling rate experienced upon extrusion. The basalts range in composition from olivine-rich tholeiites to tholeiitic ferrobasalts, and include some more alkaline basalts. The kinetics of crystal growth in some samples appears to have been influenced by the amount of initial superheating (or supercooling) of the magma, or possibly by differential retention of volatiles. Olivine in quartznormative ferrobasalts apparently formed metastably at high undercooling. Despite these effects, reliable petrographic criteria are established to distinguish the principal rock types described regardless of the crystallinity and grain size. Microphenocrysts formed prior to pillow formation correspond closely to mineral assemblages inferred from normative plots and variation diagrams to control crystal fractionation at various stages. The details of spherulitic and dendritic growth also provide some clues about composition. Petrographic evidence for magma mixing is scant. Only some Siqueiros fracture zone basalts contain zoned plagioclase phenocrysts with glass inclusions similar to those used to infer mixing among Mid-Atlantic Ridge basalts. All basalts from the summit and flanks of the East Pacific Rise are aphyric. One possible petrographic consequence of mixing between olivine tholeiites and ferrobasalts - formation of clinopyroxene phenocrysts - is not evident in any fracture zone or Rise crest basalt. Highly evolved ferrobasalts with liquidus low-Ca clinopyroxene have not been sampled, nor does textural evidence indicate that any basalts sampled are hybrid compositions between such magmas and less fractionated compositions. Evidently the sampled ferrobasalts are close to the most evolved compositions that occur in any abundance on this portion of the East Pacific Rise.