Textural and geochemical alternations in Late Cenozoic Bahamian dolomites

Dolomitized intervals of a core from San Salvador Island, Bahamas, exhibit variations of two texturally and geochemically distinct end-members. In the Pliocene section of the dolomitized interval, the two end-members alternate in a pattern that may reflect originally and/or diagenetically modified depositional facies. Formerly mud-free intervals, locally capped by exposure surfaces are massive crystalline, mimetic dolomites (CM). Muddier sediments are replaced by friable microsucrosic dolomites (MS). CM and MS dolomites also differ in porosity (< 10% vs > 30%), permeability (< 10 md vs > 100 md), mol% MgCO₃ (44–9 vs 47–7) mol%), oxygen isotopic composition (1–7 vs 2–7‰) and strontium content (241 vs 106 ppm). These data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types. Differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks.     

Basal Waitemata Group lithofacies: rapid subsidence in an early Miocene interarc basin, New Zealand

The abrupt transition from coastal and shallow shelf sediments to bathyal sediments provides a record of rapid subsidence and deepening of the early Miocene Waitemata basin. Basal shallow marine strata (Kawau Subgroup) accumulated upon a highly dissected surface that overlies deformed Mesozoic metagreywacke. The early Miocene coast was characterized by an embayed and cliffed shoreline with numerous sea stacks and islands. Kawau Subgroup lithofacies, which include pocket beach, shallow shelf and base-of-cliff talus deposits, reflect rapidly changing coastline configuration and water depths as the rugged bedrock surface was buried. The response to continued rapid subsidence and transgression in Waitemata basin was a decrease in the supply of coarse clastic sediment. Beach gravels were locally displaced to greater water depths by avalanching down steep bedrock slopes. The first bathyal turbidite facies, which abruptly overlie the shallow-water Kawau Subgroup, include locally derived sediment gravity flows commonly ponded by remnant bedrock submarine highs. When this local supply of sediment had been exhausted, coarse sediment starvation ensued and bathyal muds accumulated. With the resumption of sediment supply and gradual burial of submarine bedrock relief, submarine fans coalesced and increased in lateral extent. Subsidence of the Waitemata basin to bathyal depths is thought to have occurred in less than a million years. From the above hypothesis, a general model of sedimentation is proposed.     

Palaeokarst-influenced depositional and diagenetic patterns in Upper Permian carbonates and evaporites, Karstryggen area, central East Greenland

The Karstryggen area of eastern Greenland represents the western edge of sedimentation in the Jameson Land Basin, an arm of the northern Zechstein seaway. Upper Permian strata of this area were deposited as two major sequences. The first marine incursion transgressed largely peneplaned Lower Permian strata and deposited thin, paralic conglomerates, sandstones and shales (the Huledal Formation) followed by a thick package of carbonates and evaporites (the Karstryggen Formation). Although the Karstryggen Formation represents the transgressive maximum of this sequence, it contains only marginal or restricted marine strata, including micritic, stromatolitic and peloidal carbonates and thick, but localized, bedded gypsum deposits. These lithofacies indicate that relatively arid climates prevailed in this basin, as in most of the Zechstein region. A major regression, associated with a change to a more humid climate, terminated Karstryggen sedimentation. Pre-existing evaporites and carbonates underwent diagenetic alteration, including widespread calcitization and dissolution of gypsum. More importantly, topographic relief in excess of 120 m was generated by fluvial drainage systems and karstic sinkholes. A second marine incursion, accompanied by a return to a semi-arid climate, drowned this high relief topography, producing a complex sequence of strata (the Wegener Halvø Formation) in which sedimentation was greatly influenced by the rugged underlying terrain. Marine cemented algal-molluscan grainstones draped pre-existing palaeotopography during the initial stages of flooding. Continued drowning led to differential sedimentation on ‘highs’ and in ‘lows’. Oolitic and bryozoan-brachiopod grainstones formed as shoals on the crests of most prominences, whereas shales, conglomeratic debris flows, evaporites, or oolitic turbidites were deposited in the lows. More restricted sedimentation took place in the westernmost areas which lay closest to the mainland shoreline and were situated to the west of a palaeotopographic ridge. There, oolitic, stromatolitic and evaporitic strata were deposited under hypersaline conditions indicative of a return to more arid climatic conditions. Three subcycles mark smaller scale relative changes of sea level that occurred during deposition of the Wegener Halvø Formation; they are delimited by regional surfaces with moderate relief (5–20 m) developed during subaerial exposure. Widespread diagenetic changes, including leaching of aragonitic grains, dissolution/collapse brecciation of evaporites and meteoric calcite cementation, occurred in association with these smaller scale sequence boundaries, again reflecting climatic oscillations. Relative sea level fluctuations, coupled with regional climate changes, played a dominant role in determining both depositional and diagenetic relations in these strata. These features undoubtedly extend into subsurface parts of this basin as well as into yet unexplored areas of the northern Zechstein Basin and Barents Shelf, and may have economic significance for the localization of hydrocarbons.     

The effect of sampling scale on actualistic sandstone petrofacies

Empirical correlations between plate tectonic setting and sand/sandstone composition have been the basis for large scale petrological models. These models do not explicitly treat sampling scale. Four areas from the western USA with diverse tectonic settings and rock types provide a natural laboratory for sampling sand at three different scales: talus piles to small drainages (first order), streams and rivers draining mountain ranges (second order), and large rivers and marine environments (third order). Existing plate tectonic petrofacies models should only be applied to third order settings because the data were derived from studies of such settings. This is especially true in tectonic settings with diverse source rocks (e.g. continental rifts and transform settings). On the other hand, some settings, such as active magmatic arcs and foreland fold-thrust belts, provide uniform results at any sampling scale because of homogeneity of source rocks. The Rio Grande drainage area is especially complex, with diverse igneous, metamorphic and sedimentary source areas. Some components (e.g. basalt) are destroyed with minimal transport, whereas others (e.g. quartz) are relatively enriched with greater transport. In this complex continental rift setting, first and second order sand is diverse and heterogeneous due to input from tributaries. The Santa Clara River of southern California also has heterogeneous sand due to diverse source rocks in this transform setting. It is only after considerable homogenization and stabilization due to weathering and mixing with more stable components, and/or considerable transport, that homogeneous compositions are produced in these two settings. In contrast, the Cascade magmatic arc and the Canadian Rocky Mountain fold-thrust belt have uniform source rocks (dominantly volcanic in the former and dominantly sedimentary in the latter). Uniform sand composition that is unique to each of these tectonic settings results at any sampling scale in these two cases. Uniformity of data collection and analysis is essential for reproducible results. Use of the Gazzi-Dickinson point counting method allows direct comparison among source rocks (zero order samples), modern sand of any order and ancient sandstone of unknown provenance. Lack of recognition of the effect of sampling scale in the development of actualistic petrofacies models has led to incorrect rejection of many existing models. Third order sands are excellent predictors of plate tectonic setting, but first and second order sands can provide ambiguous plate tectonic interpretations in many settings. More complex actualistic petrofacies models based on diverse sampling scales are needed.     

Major Addition of Magma at the Pyroxenite Marker in the Western Bushveld Complex, South Africa

A major, but gradual, reversal in the cryptic variation patternof the plagioclase and pyroxenes, of 13 mol% anorthite and 10mol% Mg/ (Mg + Fe) respectively, is documented in the Main Zoneof the western Bush veld Complex. These changes are accompaniedby a decrease in initial ⁸⁷Sr/⁸⁶Sr ratio from > 0.708 to< 0.707. The Pyroxenite Marker, a distinctive orthopyroxenitelayer, occurs close to the top of this reversed differentiationsequence. This is attributed to addition of less differentiatedmagma. On the basis of a mass balance calculation of the initial⁸⁷Sr/ ⁸⁶Sr ratios, it is estimated that the volume of magmaadded was comparable to that of the resident magma. Increases in the Fe₂O₃, TiO₂, Al₂O₃, and Na₂O contents of thepyroxenes above the level of magma addition indicate that thenew magma had a lower silica activity and higher fO₂ than theresident magma. Quantification of the trace element and REEcontent of the two magmas is hampered by the very low proportionof trapped intercumulus component in these adcumulate rocks.However, semi-quantitative modelling indicates that the traceand REE signatures of the two magmas were similar, with moderateLREE enrichment and flat HREE profiles. The new magma had aslightly higher La/ Sm ratio than the resident magma, consistentwith its more alkaline nature. The new magma was probably added gradually, while 100–150m of cumulates formed. It probably intruded at an intermediatelevel within an existing stratified magma chamber, where itcooled and crystallized, and composite packets of liquid pluscrystals plunged to the base of the chamber. The cores of plagioclasegrains formed during this mixing interval show a wider rangeof compositions than in other sections, and plagioclase primocrystsfrom both magmas may be preserved within single samples. Therefore,although intimate physical mixing of packets of unknown sizeof the two magmas occurred, re-equilibration of the major oxidecomposition of the plagioclase primocrysts was not achieved.However, the data and calculations based on diffusion ratesindicate that partial Sr isotopic resetting of plagioclase mayhave occurred.     

Petrology and Geochemistry of the Rhobell Volcanic Complex: Amphibole-Dominated Fractionation at an Early Ordovician Arc Volcano in North Wales

The Rhobell Volcanic Complex is a remnant of a late Tremadoc,dominantly calc-alkaline, arc volcano. It is the only substantiallypreserved representative in the southern British Caledonidesof an early phase of Ordovician ensialic arc volcanism whichfollowed the onset of southeasterly subduction of Iapetus oceaniclithosphere beneath the northern margin of Gondwanaland. Thecomplex includes extrusive basalts and associated breccias,known as the Rhobell Volcanic Group, which rest unconformablyon folded, uplifted and eroded rift-basin sediments of the earlierpassive margin of the Iapetus Ocean. Amongst the basalts arerelatively primitive pargasite-bearing varieties which containcognate cumulate blocks, dominantly of pargasite but also withcalcic clinopyroxene, Ti-magnetite, and (rarely) plagioclase.Basaltic rocks also occur in an associated feeder-sheet intrusioncomplex, and as numerous minor sills and dykes. In the intrusioncomplex, basaltic sheets are cut by microdiorites and scarcemicrotonalites. The compositional range in the volcanic complex,from low-SiO₂ basalts to microtonalites (SiO₂ 45–66 wt.per cent), is attributable to fractional crystallization, earlystages of which were dominated by removal of pargasite at (water-undersaturated)pressures close to 10 kb, within the mantle. The parental magmawas derived by hydrous partial melting of a supra-subductionzone mantle wedge. Trace-element patterns indicate that themantle was slightly depleted relative to the putative primordialcomposition (Ta/Yb = 0?1), prior to metasomatism by componentsfrom the subduction zone. Textural variations in cumulate blocks,and various phenocryst forms in basalts, are interpreted asindicating that the erupted magmas came from a thermally andcompositionally stratified magma chamber with associated layeredcrystal accumulations, and that materials from initially separatelayers were mixed prior to eruption.     

COSMOGENIC 10BE-AGES FROM THE STORE KOLDEWEY ISLAND, NE GREENLAND

Earlier work in northeast Greenland has suggested a limited advance of the Greenland Ice Sheet during the Last Glacial Maximum (LGM). However, this concept has recently been challenged by marine geological studies, indicating grounded ice on the continental shelf at this time. New ¹⁰Be‐ages from the Store Koldewey island, northeast Greenland, suggest that unscoured mountain plateaus at the outer coast were covered at least partly by cold‐based ice during the LGM. It is, however, still inconclusive whether this ice was dynamically connected to the Greenland Ice Sheet or not. Regardless of the LGM ice sheet extent, the ¹⁰Be results from Store Koldewey add to a growing body of evidence suggesting considerable antiquity of crystalline unscoured terrain near present and Pleistocene ice sheet margins.