A high‐resolution relative time scale for the Viséan Stage (Carboniferous) of the Kulm Basin (Rhenish Mountains,Germany)

The Viséan (Carboniferous) sedimentary succession of the basinal Kulm facies (Rhenish Mountains) was investigated in detail in order to achieve a high‐resolution stratigraphic subdivision and correlation. Additionally, the ranges of fossil index taxa (ammonoids), fossil marker beds, volcaniclastic horizons and sedimentary features (e.g. colour changes) were integrated in the correlation. As a result, a comprehensive database was compiled, which contains 190 stratigraphic events of the Viséan interval of this area. Several sections are almost completely composed of shales, which are regarded to represent a slow but constant basinal background sedimentation of the Kulm facies. The thickness of lithological homogeneous sections thus indicates an approximately linear record of time and the average thicknesses of biozones and positions of stratigraphic events can easily be calculated from the compiled database. The result is an approximately time‐linear biostratigraphic scale for the Viséan Stage of the Kulm Basin. Given a numerical length of the Viséan Stage of ca. 19 Ma, 190 stratigraphic events give a mean resolution of 100 000 years. This is unique in Palaeozoic stratigraphy. Copyright © 2008 John Wiley & Sons, Ltd.     

Primary versus secondary and subaerial versus submarine hydrovolcanic deposits in the subsurface of Jeju Island, Korea

Jeju Island is a Quaternary shield volcano built upon the Yellow Sea continental shelf off the Korean Peninsula. Decades of borehole drilling reveals that the shield‐forming lavas of the island are underlain by extensive hydrovolcanic deposits (the Seoguipo Formation), which are about 100 m thick and show diverse depositional features. This study provides criteria for distinguishing between hydrovolcanic deposits formed by primary (pyroclastic) and secondary (resedimentation) processes in subaerial and submarine settings based on the observations of several selected cores from the formation. Five facies associations are identified, including: (i) primary hydrovolcanic deposits formed by pyroclastic surges and co‐surge fallouts in tuff rings (facies association PH_TR); (ii) primary hydrovolcanic deposits formed by Surtseyan fallout and related pyroclastic transport processes in tuff cones (facies association PH_TC); (iii) secondary hydrovolcanic deposits formed by debris flows, hyperconcentrated flood flows, sheet floods and rill flows in subaerial settings (facies association RH_AE); (iv) secondary hydrovolcanic deposits formed in submarine settings under the influence of waves, tides and occasional mass flows (facies association RH_MAR); and (v) non‐volcaniclastic and fine‐grained deposits formed in nearshore to offshore settings (facies association NV_MAR). The primary hydrovolcanic facies associations (PH_TR and PH_TC) are distinguished from one another on the basis of distinct lithofacies characteristics and vertical sequence profiles. These facies differ from the secondary hydrovolcanic and non‐volcaniclastic facies associations (RH_AE, RH_MAR and NV_MAR) because of their distinctive sedimentary structures, textures and compositions. The depositional processes and settings of some massive and crudely stratified volcaniclastic deposits, which occur in many facies associations, could not be discriminated unambiguously even with microscopic observations. Nevertheless, these facies associations could generally be distinguished because they occur typically in packets or sequences, several metres to tens of metres thick and bounded by distinct stratigraphic discontinuities, and comprise generally distinct sets of lithofacies. The overall characteristics of the Seoguipo Formation suggest that it is composed of numerous superposed phreatomagmatic volcanoes intercalated with marine or non‐marine, volcaniclastic or non‐volcaniclastic deposits. Widespread and continual hydrovolcanic activity, together with volcaniclastic sedimentation, is inferred to have persisted for more than a million years in Jeju Island under the influence of fluctuating Quaternary sea‐levels, before effusion of the shield‐forming lavas. Extensive distribution of hydrovolcanic deposits in the subsurface of Jeju Island highlights that there can be significant differences in the eruption style, growth history and internal structure between shelfal shield volcanoes and oceanic island volcanoes.     

Near‐isochemical evolution of clay mineral assemblages in differentially buried volcaniclastic sediments,the Late Miocene Upper Red Formation,Iran

The Upper Red Formation (URF) comprises over 1–5 km of late Miocene siliciclastic sediments in the Central Iran Basin. The formation is dominated by volcaniclastic conglomerates and arenites. The prevailing arid conditions during most of the basin's history resulted in deposition of predominantly organic‐poor, red sediments with gypsum and zeolites. This investigation concentrates on the mineralogy and geochemistry of the URF in the southern and northern margins of the basin where the formation was buried to depths of 2.4 and 6.6 km, respectively. Fine fraction mineral separates from the southern margin consist of nearly pure smectite and zeolites at a depth of 400 m and smectite with minor quartz and calcite at 1800 m. Shallow samples (1350 m) from the northern section are rich in smectite, illite/smectite with some discrete illite and chlorite. This assemblage is progressively replaced by discrete illite and chlorite with increasing burial depth so that only these two minerals are found at depths greater than 4300 m. The initial alteration process involved replacement of glass and volcanic lithics by smectite and zeolites in both margins of the basin. Increased depth of burial in the northern margin resulted in the progressive isochemical alteration of smectite to discrete illite and chlorite. Diagenesis of clay assemblages occurred essentially in a closed system. Solute products of glass hydrolysis reactions were retained in highly alkaline, saline ground waters from which zeolites, carbonates and oxides precipitated as cements. It is unlikely that these sediments were ever significantly leached by meteoric waters or by organic acids generated during burial diagenesis. Copyright © 2010 John Wiley & Sons, Ltd.     

Tidal sedimentation preserved in volcaniclastic deposits filling a peripheral seaway embayment (early Miocene,Sardinian Graben)

The Sardinian Graben System was a part of a NE-SW-oriented extensional basin, rotated counter-clockwise into a N-S-elongate basin, as consequence of the eastward migration of the Apennine orogenic front, in the western Mediterranean during the Neogene.Starting from the early Miocene, the Sardinian Graben was inundated by marine waters, turning progressively into a seaway, characterized by a tidal circulation as consequence of the connection between the Atlantic Ocean to the west and the Paratethys Ocean to the east.In this work, we investigate an area located marginally to the mid-seaway, whose well-exposed volcaniclastic deposits record the local expression of a tidal amplification occurring in a coastal peripheral embayment of the wider Sardinian Seaway.The studied succession is ca. 140 m thick and includes three main units: (i) the 20-m-thick lowermost unit consists of fluvio-lacustrine sandstones and conglomerates belonging to lower delta-plain and delta-platform environments; (ii) the second unit is 60–70 m thick and includes heterolithic sandstones and mudstones, exhibiting a variety of tidal sedimentary structures, and lies on the previous deposits through a tidal ravinement surface; these two units are mostly volcaniclastic in composition, reflecting the dominance of a magmatic source over other extrabasinal components; (iii) the uppermost unit is ca. 50 m thick, erosionally overlies the previous deposits and is made up of shoreface sandstones and open-shelf mudstones, whose composition indicates even less volcaniclastic elements and the prevalence of other clastic alongshore-derived components.Based on the results of the facies analysis, the study succession is interpreted as the infill of an incised valley along the southern flank of a structural high. The valley was excavated during a phase of relative sea-level lowstand (Aquitanian?) preceding a subsequent stage of major transgression (Burdigalian). Initially, a fluvial system impinged the valley from the west favoring the progradation of a deltaic system in a shallow-marine embayment. During an early stage of transgression, the isolation of a part of this coastal area generated by the building of a barrier island, produced the onset of a tidal-flat sedimentation over the previous deposits. A late transgression occurred through the inundation of this coastal area by marine waters and the consequent back-stepping of beach-barrier and open-shelf strata.The sedimentological features of this stratigraphic succession indicate as this valley was filled in a tectonic setting with a high rate of accommodation, where the tidal influence progressively increased during sediment accumulation, possibly due to the marginal position respect to a wider tide-dominated marine conduit.The present paper thus: (i) documents for the first time a tidal signature in the lower Miocene strata of Sardinia; (ii) indicates new possible relationships with other, coeval seaway successions of the western and northern Mediterranean area; (iii) suggests constrains for palaeogeographic reconstructions; (iv) and throws the basis for future researches on the Sardinian Seaway.