Cooling,degassing and compaction of rhyolitic ash flow tuffs: a computational model

Previous models of degassing, cooling and compaction of rhyolitic ash flow deposits are combined in a single computational model that runs on a personal computer. The model applies to a broader range of initial and boundary conditions than Riehle's earlier model, which did not integrate heat and mass flux with compaction and which for compound units was limited to two deposits. Model temperatures and gas pressures compare well with simple measured examples. The results indicate that degassing of volatiles present at deposition occurs within days to a few weeks. Compaction occurs for weeks to two to three years unless halted by devitrification; near-emplacement temperatures can persist for tens of years in the interiors of thick deposits. Even modest rainfall significantly chills the upper parts of ash deposits, but compaction in simple cooling units ends before chilling by rainwater influences cooling of the interior of the sheet. Rainfall does, however, affect compaction at the boundaries of deposits in compound cooling units, because the influx of heat from the overlying unit is inadequate to overcome heat previously lost to vaporization of water. Three density profiles from the Matahina Ignimbrite, a compound cooling unit, are fairly well reproduced by the model despite complexities arising from numerous cooling breaks. Uncertainties in attempts to correlate in detail among the profiles may be the result of the non-uniform distribution of individual deposits. Regardless, it is inferred that model compaction is approximately valid. Thus the model should be of use in reconstructing the emplacement history of compound ash deposits, for inferring the depositional environments of ancient deposits and for assessing how long deposits of modern ash flows are capable of generating phreatic eruptions or secondary ash flows.     

Cyclic facies architecture as a key to depositional controls in a distal foredeep: Campanian Mesaverde Group,Wyoming, USA

The Mesaverde Group consists of a thick wedge of fluvial, littoral-deltaic and shallow marine clastics shed into the Cretaceous Western Interior Seaway of North America. The western parts of the seaway lay within the strongly subsiding foredeep of the active Sevier fold and thrust belt further to the west. The study area is located east of the axis of maximum subsidence and is thus in a favourable position to record competing effects of eustasy, sediment supply and thrust-load induced subsidence. Facies and sequence analysis carried out on high quality outcrop and well log data led to the recognition of a complex depositional cycle hierarchy within the typical storm- and wave-dominated inner shelf/shoreface/strand plain and delta systems of the Mesaverde. Fourth-order parasequences and parasequence bundles of estimated 100–400 ka duration are the best recognizable, ubiquitous and most useful stratigraphic units. Their arrangement with respect to sequence boundaries, however, varies with their overall stratigraphic position and also differs from the Exxon models. Mesaverde progradation was interrupted by a major transgression that occurred out of phase with the aggradational to progradational stacking trend of third-order sequences. A proposed genetic model relates large-scale (second-order) sequence architecture to tectonics: a Sevier thrust event as well as Laramide uplift within the foredeep controlled non-linear changes in the accommodation/supply ratio. Parasequence stacking patterns and sequence boundary formation, in contrast, were the product of (global?) eustasy enhanced by short-term, perhaps local, changes in the rates of subsidence and detrital influx.     

Upper Campanian–Maastrichtian holostratigraphy of the eastern Danish Basin

One of the most expanded upper Campanian–Maastrichtian successions worldwide has been cored in a series of boreholes in eastern Denmark. A high-resolution holostratigraphic analysis of this part of the Chalk Group has been undertaken on these cores, notably Stevns-1, in order to provide a record of changes in chalk facies, water depths and sea-water temperatures. Combined lithological data, a suite of petrophysical logs including gamma ray (GR) logs, nannofossil and dinoflagellate palaeontology, stable carbon isotopes, seismic reflection and refraction sections form the basis for the definition of two new formations and six members, three of which are new, and for recognition of Boreal nannofossil subzones UC15e^BP to UC20d^BP. The upper Campanian–lowermost Maastrichtian Mandehoved Formation is subdivided into the Flagbanke and Boesdal Members and the Maastrichtian Møns Klint Formation is subdivided into the Hvidskud, Rørdal, Sigerslev, Kjølby Gaard Marl and Højerup Members. The Boesdal and Rørdal Members show high GR values and a pronounced chalk-marl cyclicity. The Rørdal and the thin Kjølby Gaard Marl Members have a regional distribution and can be traced over most of the Danish Basin, whereas the Højerup Member is restricted to the easternmost part of Sjælland. The other members consist of rather featureless white chalk.     

Eruption and emplacement of a laterally extensive, crystal-rich, and pumice-free ignimbrite (the Cretaceous Kusandong Tuff, Korea)

The Cretaceous Kusandong Tuff, Korea, is a thin (1–5 m thick) but laterally extensive (~ 200 km) silicic ignimbrite emplaced in a fluviolacustrine basin adjacent to a continental volcanic arc. The tuff has been used as an excellent key bed because of its great lateral continuity and unique lithology, characterized by the virtual absence of juvenile clasts and an abundance of quartz and feldspar crystals (up to 55–73 vol.%). The tuff is mostly massive and ungraded and locally shows crude internal layering, basal inverse grading and near-top normal grading of crystals, either erosional or non-erosional lower surfaces, and flat-lying to imbricated grain fabrics. Fragile intraformational clasts of mudstone and tuff are also included. These features provide only ambiguous information on the properties of the responsible pyroclastic density currents: i.e. whether they were dense and laminar or dilute and turbulent. The overall lateral continuity and sheet-like geometry of the tuff suggests, however, that the transport system of the currents was highly expanded, dilute, and turbulent. A plug-flow or slab-flow model cannot explain the origin of crude internal layering, imbricated grain fabrics, and the high crystal content, which is most likely the result of vigorous sorting processes within a dilute and turbulent current. Features indicative of deposition from a dense and laminar transporting medium are locally present, suggesting that a dense and laminar depositional system could develop locally at the base of the dilute and turbulent transport system. The virtual absence of juvenile clasts in the tuff is interpreted to be due to rapid ascent, sudden decompression, and full fragmentation of silicic magma into fine glass shards and crystals. Scarcity of basement-derived accidental components together with the absence of pumiceous fallout deposits beneath the tuff is interpreted to be due to shallow-level fragmentation of magma followed by immediate generation of pyroclastic density currents from shallow-level blasts at the onset of eruption. The eruption occurred through multiple vent sites in a short period of time, producing a seemingly single but actually composite ignimbrite unit. Such an eruption was probably possible because of a regional tectonic event within the basin or in its vicinity. It is proposed that a composite ignimbrite with the characteristics of the Kusandong Tuff can be an exemplary product of syntectonic volcanism that can provide an insight into the interpretation of structural and stratigraphic evolution of a sedimentary basin.     

Calcareous nannofossil biostratigraphy of Campanian strata (Abtalkh Formation) from the eastern Koppeh-Dagh Basin,NE Iran

The Campanian of the eastern Koppeh-Dagh Basin (NE Iran) is generally considered to be represented by the upper part of the Abderaz and the Abtalkh formations. The Abtalkh Formation, which is studied here, reaches thicknesses of up to 1750 m in the area. The formation is characterized by abundant, diverse, and poor to moderately well preserved calcareous nannofossil assemblages of Tethyan affinity. The assemblages were studied in detail in two sections in eastern Koppeh-Dagh, allowing construction of a precise biozonation for this stage. The Abtalkh Formation at sections in Abtalkh village and Padeha spans biozones CC20 to CC23a (UC15b^TP to UC16). The results of this study indicate a late early to late Campanian age for the formation in the area. The most complete Campanian sequence is in the southeast, where the Padeha section is located. Nannofossil abundance and diversity decreases upwards, showing a trend from the base to top of the formation. Dominance of warm water taxa, and low abundance of high latitude taxa, confirm placement of the basin in low to mid palaeolatitudes during deposition of the formation.     

New Stigmaphronidae and Megaspilidae (Hymenoptera: Ceraphronoidea) from Canadian Cretaceous amber

Three new species within the stigmaphronid genus TagsmiphronEngel and Grimaldi, 2009, and one new species within the megaspilid genus ConostigmusDahlbom, 1858 are described from Upper Cretaceous (Campanian) amber originating at the Grassy Lake locality in Alberta, Canada. New taxa include Tagsmiphron spiculum sp. nov., Tagsmiphron leucki sp. nov., Tagsmiphron exitorum sp. nov., and Conostigmus cavannus sp. nov. The new Conostigmus species is a rare discovery. It is the third megaspilid species to be found in Cretaceous amber, with the two specimens described herein effectively doubling the number of known Mesozoic exemplars for the family. We provide the first comprehensive report of known Ceraphronoidea within Canadian amber, and contrast this against other Cretaceous amber assemblages, discussing the potential palaeobiogeographic and palaeoenvironmental implications of the Canadian amber assemblage.     

Molluscan biostratigraphy and paleomagnetism of Campanian strata, Queen Charlotte Islands, British Columbia: implications for Pacific coast North America biochronology

A previously uncollected fauna of ammonites, bivalves, and other molluscs, associated with radiolarian microfossils, has been newly recognized near Lawn Hill on the east coast of central Queen Charlotte Islands, British Columbia. The regional biostratigraphic zonation indicates that the Lawn Hill fauna is correlative with the Nostoceras hornbyense zonule of the Pachydiscus suciaensis ammonite biozone, recognized in the Nanaimo Group of southeast Vancouver Island. The Nostoceras hornbyense Zone (new) is herein proposed for strata of Pacific coast Canada containing the zonal index. Several molluscan taxa present in the Lawn Hill section are new to British Columbia and the ammonite fauna suggests that the Nostoceras hornbyense Zone is late Campanian in age, supported by radiolarian taxa present in the section. Strata sampled in the Lawn Hill section preserve reversed-polarity magnetization, considered likely correlative with Chron 32r. The presence of the Nostoceras hornbyense Zone on Queen Charlotte Islands is the first recognition of this zone in Canada north of central Vancouver Island and represents the youngest Cretaceous known in this region. Campanian radiolarians identified from the Lawn Hill section are also the first recognized from the Pacific coast of Canada.     

The Austin Chalk-Taylor Clay succession in Texas and the Santonian-Campanian boundary

Re-evaluation of samples from several Texas localities over the Austin Chalk-Taylor Clay boundary for planktic Foraminifera, and comparison with recent work on European and North African successions confirms the general correlation of the Texan formations with the European Stage succession. The sedimentary hiatus indicated by Pessagno (1967) does not appear to be present: the Late Santonian-Early Campanian zonal marker Globigerinita elevata (Brotzen) occurs in the Lower Taylor Clay associated with late representatives of Marginotruncana. The Lower Taylor Clay may belong to the Late Santonian in some localities.     

Inter-regional correlation of transgressions and regressions in the Cretaceous period

Well investigated platforms have been selected in each continent, and the history of Cretaceous transgressions and regressions there is concisely reviewed from the available evidence. The factual records have been summarized into a diagram and the timing of the events correlated between distant as well as adjoining areas.On a global scale, major transgressions were stepwise enlarged in space and time from the Neocomian, via Aptian-Albian, to the Late Cretaceous, and the post-Cretaceous regression was very remarkable. Minor cycles of transgression-regression were not always synchronous between different areas. Some of them were, however, nearly synchronous between the areas facing the same ocean.Tectono-eustasy may have been the main cause of the phenomena of transgression-regression, but certain kinds of other tectonic movements which affected even the so-called stable platforms were also responsible for the phenomena. The combined effects of various causes may have been unusual in the Cretaceous, since it was a period of global tectonic activity. The slowing down of this activity followed by readjustments may have been the cause of the global regression at the end of the Cretaceous.