Climate Changes and Cyclic Sedimentation in the Mid-Late Permian: Kennedy Group, Carnarvon Basin, Western Australia

Climate has an important direct and indirect influence of sedimentation, and especially on the development of cyclic sedimentation. Climate influences both accommodation and supply, the major controls on the architecture of sedimentary sequences. The Permian paleoclimate is the subject of increasing controversy, giving rise to numerous differing models developed and an expanding database of fossil evidence for climatic conditions.Western Australian basins, in particular the Carnarvon Basin, are unique among nearby Gondwanan basins in that they do not have extensive coal measures within their Permian successions. The Kennedy Group, the uppermost unit in the onshore Carnarvon Basin Permian succession, has a detrital composition indicative of arid weathering conditions. Within the Kennedy Group, even lagoonal and very nearshore sediments are devoid of plant debris or indications of the nearby presence of extensive flora. Presumed surfaces of subaerial exposure do not show paleosol development or root-traces. There has been little development of clay and chemical grain degradation is almost entirely related to diagenetic cementation and dissolution phases, rather than transport and source weathering. Nearby basins, at similar latitudes, in India and East Australia contain coal measures, consistent with the humid climates that are predicted for this latitude. It is suggested therefore that the inferred aridity in the climate of the Carnarvon Basin and other Western Australian Basins is due to local climatic effects, probably related to an interruption in atmospheric circulation caused by tectonic rifting and uplift to the west.Cyclicity in the Kennedy Group indicates regular Late Permian, Milankovitch scale eustatic sea-level change, and may signify the presence of some ice at the poles. The development of cycles may have been enhanced by shifting climate belts controlled by Milankovitch cyclicity.     

Temporal patterns of arid-humid transitions in the south-eastern Arabian Peninsula based on optical dating

The Rub Al Khali aeolian system of the Arabian Peninsula is the world's largest erg. While the region is presently hyper-arid, evidence for multiple past humid periods has been documented in previous studies. Radiocarbon dating of lacustrine, travertine and palaeogroundwater deposits suggests that the climate was humid at 10–6 and 35–25 ka. These phases have been associated with summer rainfall increases occurring in tandem with an intensification and northward migration of the monsoon system during Northern hemisphere precessional maxima. Published optical dating studies have focused on preserved sand dune and aeolianite deposits to establish ages for the intervening arid phases. Here we present a summary of this work and additional optical dating results for samples of aeolian sediment from a deep drill core, which provides insights into the style of barchan dune accretion in the Liwa region of the United Arab Emirates. The large dune accreted rapidly during the mid-late Holocene. The results suggest that the transitions from humid to arid conditions and resulting accumulation of aeolian sediment in the form of large bedforms occurred abruptly at around 6 ka. Further aeolian sedimentation has not been constant since that time, with initial gradual dune growth followed by rapid vertical accretion, and possible termination of accumulation around c. 2 ka.     

Abstract databases as information sources in economic geography

The limitations of existing information sources on mergers/acquisitions within the EU are a serious obstacle to the study of the restructuring of the European production system. The experience of using a computerized abstracting service as a source of information on mergers/acquisitions in the chemical industry is reviewed. This experience informs an assessment of the strengths and weaknesses of similar databases as information sources in economic geography. The proliferation of such databases, together with advances in text management software, suggests that their potential has not been fully appreciated. This potential is considerable in providing easy access to detailed factual information on economic events and processes at continental and global scales. It is, however, strictly limited when it comes to interpreting the corporate strategies and motivations responsible for these developments.     

The Mackenzie Delta: sedimentary processes and facies of a high-latitude, fine-grained delta

The Mackenzie Delta is a large fine‐grained delta deposited in a cold arctic setting. The delta has been constructed upon a flooding surface developed on a previous shelf‐phase delta. There are three principal depositional zones: the subaerial delta plain, the distributary channel mouth region and the subaqeous delta. The subaerial delta plain is characterized by an anastomosing system of high‐sinuosity channels and extensive thermokarst lake development. This region is greatly influenced by the annual cycle of seasonal processes including winter freezing of sediments and channels, ice‐jamming and flooding in the early spring and declining river stage during the summer and autumn. Deposition occurs on channel levees and in thermokarst lakes during flood events and is commonly rhythmic in nature with discrete annual beds being distinguishable. In the channel mouth environment, deposition is dominated by landward accretion and aggradation of mouth bars during river‐ and storm surge‐induced flood events. The subaqeous delta is characterized by a shallow water platform and a gentle offshore slope. Sediment bypassing of the shallow‐water platform is efficient as a result of the presence of incised submarine channels and the predominance of suspension transport of fine‐grained sediments. Facies of the shallow platform include silty sand with climbing ripple lamination. Offshore facies are dominated by seaward‐fining fine sand to silt tempestites. Sea‐ice scouring and sediment deformation are common beyond 10 m water depth where bioturbated muds are the predominant facies. The low angle profile of the shallow‐water platform is interpreted to be the combined response of a fine‐grained delta to (1) storm sediment dispersal; (2) autoretreat as a result of the increasing subaerial and subaqeous area of deposition as the delta progrades out of its glacial valley; (3) limited water depth above the underlying flooding surface; and (4) efficient nearshore bypassing of sediment through subice channels at the peak of spring discharge. Several indicators of the cold climate can be used as criteria for the interpretation of ancient successions, including thermokarst lake development, submarine channel scours, freeze–thaw deformation and ice‐scour deformation structures. Permafrost inhibits compaction subsidence and, together with the shallow‐water setting, also limits autocyclic lobe switching. The cold climate can thus influence stratal architecture by favouring the development of regional‐scale clinoform sets rather than multiple, smaller scale lobes separated by autocyclic flooding surfaces.     

Tectonics and geology of spreading ridge subduction at the Chile Triple Junction: a synthesis of results from Leg 141 of the Ocean Drilling Program

An active oceanic spreading ridge is being subducted beneath the South American continent at the Chile Triple Junction. This process has played a major part in the evolution of most of the continental margins that border the Pacific Ocean basin. A combination of high resolution swath bathymetric maps, seismic reflection profiles and drillhole and core data from five sites drilled during Ocean Drilling Program (ODP) Leg 141 provide important data that define the tectonic, structural and stratigraphic effects of this modern example of spreading ridge subduction.A change from subduction accretion to subduction erosion occurs along-strike of the South American forearc. This change is prominently expressed by normal faulting, forearc subsidence, oversteepening of topographic slopes and intensive sedimentary mass wasting, overprinted on older signatures of sediment accretion, overthrusting and uplift processes in the forearc. Data from drill sites north of the triple junction (Sites 859–861) show that after an important phase of forearc building in the early to late Pliocene, subduction accretion had ceased in the late Pliocene. Since that time sediment on the downgoing oceanic Nazca plate has been subducted. Site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis. Here, thick and intensely folded and faulted trench slope sediments of Pleistocene age are currently involved in the frontal deformation of the forearc. Early faults with thrust and reverse kinematics are overprinted by later normal faults.The Chile Triple Junction is also the site of apparent ophiolite emplacement into the South American forearc. Drilling at Site 862 on the Taitao Ridge revealed an offshore volcanic sequence of Plio-Pleistocene age associated with the Taitao Fracture Zone, adjacent to exposures of the Pliocene-aged Taitao ophiolite onshore. Despite the large-scale loss of material from the forearc at the triple junction, ophiolite emplacement produces a large topographic promontory in the forearc immediately after ridge subduction, and represents the first stage of forearc rebuilding.     

The determination of biomarker distributions by tandem mass spectrometry

The rapid development of biomarker geochemistry and its application to petroleum exploration problems has been due in no small part to gas chromatography-mass spectrometry (GC-MS). The recent availability of hybrid and triple stage quadrupole mass spectrometers have introduced a number of novel ways for the detection and identification of biomarkers present in extremely complex mixtures of organic compounds from various sources including crude oils and source rock extracts.A triple stage quadrupole mass spectrometer has been used in the MS/MS mode with both a gas chromatograph and direct insertion probe as inlet systems, to develop alternative methods for biomarker analyses. Selected ions formed in the ion source of the mass spectrometer are permitted to enter the collision cell where, following collision with an inert gas, the daughter ions are separated using the third quadrupole. In this manner specific parent/daughter ion relationships are utilized to monitor and resolve classes of biomarkers or individual components from within a complex mixture. Furthermore, deuterated analogues of naturally occurring biomarkers can be utilized for quantitation purposes since the parent/daughter relationship for the standard will differ, depending on the number of deuterium atoms present, even though the relative retention times are identical.The ability of MS/MS to resolve components spectrometrically as well as chromatographically greatly enhances the analytical short-column capabilities of this system. In this paper the utilization of GC-MS/MS and the direct insertion probe MS/MS to determine biomarker distributions in crude oils both qualitatively and quantitatively are discussed. The results from the study show that it is possible to use the probe data to rapidly correlate oils on the basis of their source materials and if necessary select samples for more detailed analysis by GC-MS/MS. A major advantage of the direct insertion probe method is speed of analysis although some component resolution is lost. In an effort to minimize this problem and still maintain the rapid analysis time, the use of short columns in the GC is also described. This approach provides the ability to undertake relatively rapid analyses with limited chromatographic resolution but at relatively high levels of sensitivity.