The gas temperature in the surface layers of protoplanetary disks

Models for the structure of protoplanetary disks have so far been based on the assumption that the gas and the dust temperature are equal. The gas temperature, an essential ingredient in the equations of hydrostatic equilibrium of the disk, is then determined from a continuum radiative transfer calculation, in which the continuum opacity is provided by the dust. It has been long debated whether this assumption still holds in the surface layers of the disk, where the dust infrared emission features are produced. In this paper we compute the temperature of the gas in the surface layers of the disk in a self-consistent manner. The gas temperature is determined from a heating-cooling balance equation in which processes such as photoelectric heating, dissociative heating, dust-gas thermal heat exchange and line cooling are included. The abundances of the dominant cooling species such as CO, C, C+ and O are determined from a chemical network based on the atomic species H, He, C, O, S, Mg, Si, Fe (Kamp & Bertoldi 2000). The underlying disk models to our calculations are the models of Dullemond, van Zadelhoff & Natta (2002). We find that in general the dust and gas temperature are equal to withing 10% for A_V >~ 0.1, which is above the location of the `super-heated surface layer' in which the dust emission features are produced (e.g. Chiang & Goldreich 1997). High above the disk surface the gas temperature exceeds the dust temperature and can can become -- in the presence of polycyclic aromatic hydrocarbons -- as high as 600 K at a radius of 100 AU. This is a region where CO has fully dissociated, but a significant fraction of hydrogen is still in molecular form. The densities are still high enough for non-negligible H_2 emission to be produced.....(see paper for full abstract)Comment: 28 pages, 8 figures, accepted for publication in Ap

Sequence stratigraphy and architectural elements of the Giant Foresets Formation, northern Taranaki Basin, New Zealand

The modern continental margin in northern Taranaki Basin is underlain by a thick, mud-dominated, Pliocene and Pleistocene succession (Giant Foresets Formation, GFF) clearly imaged in seismic reflection datasets. A study focusing on the geometry and internal reflection character of the GFF has revealed structural, sedimentological, and eustatic controls on its accumulation. Isopach maps prepared for northern Taranaki Basin show shifts through time in the main loci of sediment accumulation of the Mangaa Formation and Giant Foresets Formation. During the Early Pliocene (Opoitian Stage) deposition was focused in the southern part of the Northern Graben. The prograda¬tional front moved into the vicinity of Arawa-1 and Taimana-on the Western Platform during the early-Late Pliocene (Waipipian and Mangapanian Stages), forming large mounded slope fans. Through the latest Pliocene (Mangapanian - lower Nukumaruan Stages) the progradational front moved rapidly to the north and west through and across the Northern Graben to form a distinct shelf-slope depositional front. During the Pleistocene (upper Nukumaruan Stage – Recent), the progradational front straightened out, reaching the present position of the shelf-slope break. Even during the Pleistocene, broad subsidence persisted in the Northern Graben, trapping a proportion of the sediment flux being delivered to this part of the basin. The Late Pliocene part of the GFF, particularly where it prograded on to the Western Platform, displays classic clinoform profiles, with over steepening having resulted in mass-failure of paleoslopes. Major degradation of the shelf edge and slope occurred during the Early Pleistocene, reflecting a change in the calibre and flux of sediment sourced to the continental margin. Detailed examination of part of the GFF not significantly affected by mass-failure indicates that small-scale channel levee and overbank deposits dominate slope deposition, while basin floor deposits are characterised by slope-disconnected muddy and silty basin floor fans, with little lateral continuity between systems. In a sequence stratigraphic context, many of the dominant components of each seismic unit (slumps, fans, and channel-levee complexes) were deposited during the falling (RST) and low (LST) sea level parts of a relative sea level cycle, resulting in highly asymmetric sequences. While the GFF is considered to have minor reservoir potential in terms of containing sandstone-dominated stratigraphic traps, it does afford the opportunity to study in detail how deep-water clastic systems evolved in response to the various factors that control depositional architectures, particularly in a rapidly prograding muddy continen¬tal margin system

New insights into the condensed nature and stratigraphic significance of the Late Neogene Ariki Formation, Taranaki Basin

The Ariki Formation is a distinctive Late Miocene – Early Pliocene marl facies rich in planktic foraminifera, reaching thicknesses in the range 70 - 109 m in most exploration holes drilled into the Western Platform northwest of Taranaki Peninsula. In Awatea-1 and Mangaa-1 in the Northern Graben, however, there are two marl units separated by the Mangaa “B” Sands. The lower unit has the same upper Tongaporutuan and Kapitean age as the lower part of the marl on the Western Platform, and the upper marl has an Upper Opoitian - Waipipian age, similar to the upper part of the Ariki Formation on the platform. In other holes located on the margins of the graben there can be one thin marly horizon, which usually correlates with the upper marl unit in Awatea-1 and Mangaa-1. The presence of two marly units in the Northern Graben, which are probably amalgamated on the western Platform, suggests two periods of late Neogene condensed sedimentation in northern Taranaki Basin arising from siliciclastic sediment starvation, separated by a period of submarine fan accumulation (Mangaa ‘B’ sands) following subsidence of the Northern Graben. We attribute the initial interval of marl accumulation mainly to a marked landward shift in the position of coastal onlap in central and southern Taranaki and in the region east of the Taranaki Fault Zone (southern King Country and northern Wanganui regions), which effectively shut-off the supply of siliciclastic sediment to northern Taranaki Basin, thereby enabling marl to accumulate. The start of accumulation of the upper part of the Ariki Formation and its marly correlatives in and around the Northern Graben, is attributed to a younger (upper Opoitian) landward shift in the position of coastal onlap, this time involving the formation of the Wanganui Basin depocentre and Toru Trough, which trapped the contemporary siliciclastic sediment being supplied from the south. A lower Opoitian phase of progradation between these two phases of retrogradation led to accumulation of the lower part of the Mangaa Formation (Mangaa ’B’ sands), which was limited in its extent to the Northern Graben because bounding normal faults had by then developed sea floor relief precluding mass-emplaced siliciclastic sediment from being deposited on the higher standing Western Platform. The accumulation of Ariki Formation marl in northern Taranaki Basin ended during the mid-Pliocene due to progradation of a thick continental margin wedge (Giant Foresets Formation) across the Northern Graben and Western Platform

Eastern Taranaki Basin field guide.

Linking the onshore and offshore parts of Eastern Taranaki Basin: Insights to stratigraphic architecture, sedimentary facies, sequence stratigraphy, paleogeography and hydrocarbon exploration from the on land record

Evolution of the Giant Foresets Formation, northern Taranaki Basin, New Zealand

Plio-Pleistocene aggradation and progradation has resulted in the rapid outbuilding of the continental shelf margin, northern Taranaki Basin. Seismic reflection profiles reveal that this outbuilding is characterised by bold clinoforms which offlap in a basinward direction. This stacked succession of clinoforms, collectively termed the Giant Foresets Formation, obtains thicknesses of over 2 km in places, and has had a significant effect on the thermal regime of the region. This integrated study was initiated to document the Late Neogene evolution of this formation, and thereby gain insights on sedimentary distribution patterns, timing of sedimentation, and controls on progradation and aggradation. Latest Miocene extension in the northern Taranaki Basin, related to rotation of the Hikurangi subduction zone, greatly influenced sedimentation patterns in the Pliocene. Palinspastic reconstruction shows that initial extension of the Northern Graben occurred before Giant Foresets Formation sedimentation began. Sediment, sourced from erosion to the east, was preferentially funneled into the newly created Northern Graben during the late Miocene and early Pliocene, while areas to the north and west underwent a period of sediment starvation. During the late Pliocene, and into the Pleistocene, sediment accumulation outpaced graben extension, and by the end of the Mangapanian, the graben was overtopped. During this period, the progradational front associated with the outbuilding of the continental shelf-slope margin advanced northwards. Throughout the Nukumaruan, continuing to the present day, shelf migration was extremely rapid. While at least seven cyclical sea level changes, with an approximate periodicity of 400 ka (fourth-order) have been identified, overall, depths shallowed from dominantly bathyal, to dominantly shelfal

Geological structure of the forearc basin in central Hawke’s Bay, eastern North Island

Central Hawke’s Bay lies within an extensive forearc basin in eastern North Island that developed during the Late Miocene to Pleistocene. The onshore structural elements of Hawke’s Bay can be classified into four structural domains, each reflecting differing styles and scales of deformation. These domains are from west to east, the axial range domain, the range front con¬tractional domain, the central forearc basin domain, and the eastern contractional domain. Some degree of the oblique-interaction of the Australia and Pacific plates on the subduction thrust is inferred to be partitioned across the four structural domains and to be expressed dominantly as oblique-(dextral) slip on faults bordering the axial ranges, and as short¬ening on reverse faults and folds in more eastern parts of the forearc. The axial range domain involves the eastern parts of the North Island axial ranges where there is marked oblique-slip displacement on major faults. Some dextral offest is accommodated in the range front contractional domain, although dip-slip displacement is more significant. The central forearc basin domain is comparatively undeformed with only minor reverse faulting and (fault-force driven) folding. By comparison, the ad¬jacent eastern contractional domain, which comprises an accretionary wedge, is characterised by imbricate reverse and thrust faulting and associated folding. A small degree of dextral-slip is also accommodated in this domain. The uppermost parts of the inboard margin of the accretionary wedge, particularly the part onshore, is currently undergoing gravitationally-driven collapse expressed as deep-seated landslides and normal faulting. Many folds in the basin are fault-cored, several of which have been targeted in recent years by petroleum exploration companies (e.g. Hukarere-, Whakatu-and Kereru-). Most deformation of the forearc basin fill in central Hawke’s Bay is post early Nukumaruan (2.4 Ma) and much of this has occurred since the early Pleistocene (.8 Ma). Dextral-slip on Mohaka and Ruahine Faults since the Early Pliocene is likely to be less than 0 km. Significant unconformities in the basin fill reflect early phases of development of oblique-slip faults in the axial ranges. New dextral oblique-slip faults are developing in the basin fill to the east of the main oblique-slip faults bordering the ranges

The Late Miocene Southern and Central Taranaki Inversion Phase (SCTIP) and related sequence stratigraphy and paleogeography

We present a new sequence stratigraphic scheme for Taranaki Basin that identifies four 3rd order duration (3 - 4 m.y.) sequences of Middle Miocene to Pleistocene age. These include: (i) the late-Middle Miocene (upper Lillburnian to uppermost Waiauan) Otunui Sequence; (ii) the Late Miocene (lower and lowermost-upper Tongaporutuan) Mt Messenger Sequence; (iii) the latest Miocene (uppermost-upper Tongaporutuan) to Early Pliocene (lower Opoitian) Matemateaonga Sequence, and (iv), the Late Pliocene (upper Opoitian) to Late Pleistocene (Castlecliffian) Rangitikei Sequence, which includes the Giant Foresets Formation offshore in northern Taranaki Basin. Full sequence development can be observed in the parts of these four sequences exposed on land in eastern Taranaki Basin and in Wanganui Basin, including the sequence boundaries and component systems tracts; the character of the various depositional systems and their linkage to correlatives in subsurface parts of Taranaki Basin can be reasonably inferred, although we do not develop the detail here. Our sequence framework, with its independent age control, is integrated with established evidence for the timing of Late Miocene structure development in southern Taranaki (the Southern Inversion Zone of King & Thrasher (1996)) and new evidence presented here for the extent of Late Miocene unconformity development in central Taranaki. This shows that the Mt Messenger Sequence, particularly its regressive systems tract, results from a major phase of tectonism in the plate boundary zone, the crustal shortening then extending into the basin at c. 8.5 Ma and differentially exhuming parts of the sequence and underlying units in southern and central Taranaki Basin. This Southern and Central Taranaki Inversion Phase (SCTIP) peaked at around 7.5 Ma (mid-upper Tongaporutuan). At that time it extended across the whole of the area presently covered by Wanganui Basin, all of southern Taranaki Basin (Southern Inversion Zone), west to the Whitiki and Kahurangi Faults, and across southern parts of Taranaki Peninsula. We have also identified in outcrop sections, wireline logs for Peninsula exploration holes, and selected seismic reflection profiles, the occurrence of forced regressive deposits of the Mt Messenger Sequence. These deposits are mainly preserved beneath distal parts of the unconformity and basinward of it in central Taranaki Peninsula and west to the Tui Field, and need to be distinguished from the much younger Giant Forests Formation within the 3rd-order Rangitikei Sequence, which also shows clinoform development. The new sequence framework with its inferred stratal patterns also helps clarify understanding of the lithostratigraphic nomenclature for Late Miocene – Pliocene units beneath Taranaki Peninsula

Rapid progradation of the Pliocene-Pleistocene continental margin, northern Taranaki Basin, New Zealand, and implications

Progradation and aggradation of the modern continental margin in northern Taranaki Basin has resulted in the deposition of a thick and rapidly accumulated Pliocene-Pleistocene sedimentary succession. It includes the predominantly muddy Giant Foresets Formation, and the underlying sandy Mangaa Formation. Investigation of the internal attributes and depositional systems associated with the Giant Foresets Formation suggests that it would provide very little effective reservoir for hydrocarbon accumulations, although it does provide essential seal and overburden properties. While the sand-dominated Mangaa Formation could be a hydrocarbon reservoir, drilling so far has yet to reveal any significant hydrocarbon shows. Undoubtedly the most significant contribution that the Giant Foresets and Mangaa Formations have had on petroleum systems in northern Taranaki Basin is the cumulative effect that rapid and substantial accumulation has had on maturation and migration of hydrocarbons in the underlying formations. Palinspastic restoration of a seismic reflection profile across the Northern Graben, together with isopach mapping of stratigraphic section for biostratigraphic stages, indicates that the thickest part of the Pliocene-Pleistocene succession is along the central axis of the Northern Graben. Deposition of this succession contributed substantially to subsidence within the graben, providing further accommodation for sediment accumulation. Isopach and structure contour maps also reveal the extent to which submarine volcanic massifs were exposed along the axis of the graben and the timing of movement on major faults

Late Miocene-Early Pliocene Matemateaonga Formation in eastern Taranaki Peninsula: A new 1:50,000 geological map and stratigraphic framework

In recent years the Matemateaonga Formation has become an additional exploration play in Taranaki Basin. Exploration interest has been stimulated by the success of Swift Energy Company in the Rimu/Kauri prospect (38719), located near south Taranaki Coast. At this location, sandstone lithofacies, commonly termed “Manutahi Sandstone” in the lower parts of the Matemateaonga Formation have been intersected by the Kauri-A2 and Kauri-A3 wells at depths of ~1100-1200 m and are yielding commercial quantities of oil. As part of a FRST-funded sedimentary basins research programme, we have geologically mapped in detail Matemateaonga Formation within an 1800 km2 area of the eastern peninsula region (Fig. 1), incorporating license areas 38739, 38718, 38753, 38138, 38139, 38141, 38140, 38716, 38758, 38728 and 38760. Mapping at 1:50,000 scale has revealed an ~1100 m-thick succession of cyclothemic, unconformity bounded shelfal strata of Late Miocene-Early Pliocene (Late Kapitean to Early Opoitian) age (c.5.5-4.7 Ma). This succession formed as a result of the interplay between climatically-driven 6th-order (41 k.y.) eustatic sea-level changes, high rates of basin subsidence and a substantial southerly-derived sediment flux. Individual sequences or groups of sequences are the fundamental mapping entities. The mapping area sits astride the southward-plunging Whangamomona Anticline, which has deformed the Late Neogene succession, producing a regional dip on its western flank of 2 to 4 degrees to the southwest. Northeast-southwest trending normal faults are relatively common and offset Matemateaonga Formation strata with throws of 2-50 m. This improved knowledge of Matemateaonga Formation stratigraphy enhances the understanding of the distribution and geometry of potential reservoir sandstone units and associated mudstone seal units in the region

Nylon 6 polymerization in the solid state

The postcondensation of nylon 6 in the solid state was studied. The reactions were carried out on fine powder in a fluidized bed reactor in a stream of dry nitrogen in the temperature range 110-205°C and during 1-24 h. The solid-state polymerization (SSP) did not follow melt kinetics, but was found to be limited by the diffusion of the autocatalyzing acid chain end group. Factors thought to influence SSP were studied, e.g., heat treatment, starting molecular weight, and remelting. Surprisningly, heat treatment had little effect, but the starting molecular weight had a strong effect on the reaction rate. The higher the starting molecular weight, the faster the reaction. This could be explained as a changing concentration distribution of the reactive groups in the solid state on SSP. The kinetics of the SSP had more than one region, and the rate of reaction for conversions of over 30% could be expressed as - dc/dt = k(c/t), where k is a dimensionless constant independent of temperature with a value of 0.28. The integrated form has the form - In(c/co) = k In(t/), where co is the acid end-group concentration at the start, t is the reaction time, and is the induction time. The value of is both dependent on the starting concentration co and the reaction temperature and has an activation energy of 105 kJ/mol