Late Eocene Early Oligocene Research Articles

Structural history of hinge fault system of the Malay Basin

The curvilinear Hinge fault system demarcates the western boundary of the Malay Basin: it trends northerly in the north and this direction changes progressively to WNW-ESE in the south. This 20-kilometre wide fault system is traceable for more than 450 km. Based on the trends and structural styles, this fault system can be subdivided into five major segments: northerly trending Ular-Kuda fault segment, northwest-southeast trending Kabut-Tembikai fault segment, WNW-ESE trending Angsi­ Duyong fault segment and NNW-SSE Dungun fault segment. During ?Late Eocene-Early Oligocene, this fault system experienced right lateral movement. In this wrenching period, strings of rhomboid pull-apart grabens were developed within its northern segments. The fault system was probably active until Late Middle Miocene. During Middle Miocene, reversal of wrench movement (left lateral) produced transpressional anticlines within the pull-apart grabens. Based on the abundance of faults and sedimentation rates within the fault segments, it is interpreted that the dominant activities within this fault system shifted progressively northwards from Early Miocene to Late Miocene. This system may represent a major fault system in the western margin of the Malay Basin since ?Triassic.

Late Eocene-Early Oligocene paleofloristic patterns in southern Mississippi and Alabama, US Gulf Coast

Climate, vegetation and sediment supply are fundamental controls on the palynofloras of depositional sequences. In ancient depositional environments, the diversity and abundance of palynomorphs being transported into, and preserved in the basin of deposition are dependent on these factors as well as burial conditions. Preserved palynomorph assemblages can be used to recognize past floral patterns and climates, and such a model is used here to interpret the Late Eocene Shubuta Member of the Yazoo Formation and Early Oligocene Vicksburg Group in southern Mississippi and southwestern Alabama. Two hundred and sixty spore and pollen taxa have been identified in the strata. Eleven groups representing about 50% of the taxa have been recognized on the basis of abundance, occurrence in most samples and ecological significance. The groups are spores, bisaccate pollen, Sequoiapollenites-Cupressacites, Momipites, Carya, Quercoidites, Fraxinoipollenites, Cupuliferoidaepollenites, Cupuliferoipollenites, Cyrillaceaepollenites, and Siltaria. Statistical analysis on presence-absence data produced five significant cluster groups which were dominated by species of the eleven groups, as well as by Ulmipollenites thompsonianus and Salixipollenites parvus. These cluster groups show some ecological significance because they appear to represent upland, swamp and marsh taxa. The prominence of Quercoidites, Sequoiapollenites, Cupressacites and Cyrillaceiopollenites in the rich palynomorph assemblage, which contains some tropical to subtropical elements, suggests a warm temperate paleoclimate during the Late Paleogene in the eastern Gulf Coast. Although Momipites was generally more abundant in the Eocene and Quercoidites increased during the Oligocene, there was no significant paleofloristic change across the Eocene-Oligocene boundary.

Strain analysis of the Ninemile fault zone, western Montana: insights into multiply deformed regions

By means of detailed field mapping and a systematic strain analysis we investigated the development of the Ninemile fault in western Montana. The fault has been considered to be either a Mesozoic or mid-Tertiary strike-slip fault, or a Late Eocene-Early Oligocene normal fault. The rock of the study area experienced a Mesozoic to early Tertiary NE-SW shortening expressed by thrusts, folds, and pervasive cleavage. We estimated total strain by measuring preferred orientation of chlorite grains in samples from within and near the Ninemile fault zone and by calculating the strain according to the March theory. Our goal was to explain how strain accumulated by successive superposition of strains during the two deformation events onto an early compaction strain. In all samples the minimum-elongation direction (= maximum shortening) is nearly vertical. The maximum-elongation directions of all the samples outside the Ninemile shear zone are subperpendicular to the strike of the shear zone, whereas in the shear zone they are either parallel to the strike of the shear zone or form an angle of about 45°. The systematic variation in the principal strain directions is the result of two deformational events expressed by local structures created by early NE-SW shortening and later NE-SW extension. The observed strain requires that the Ninemile fault, a normal fault during the Tertiary, had earlier been either a thrust or a zone of contractional shear deformation, along which there was fault-perpendicular shortening and fault-parallel extension. Although the average value of maximum elongation is greater within the Ninemile shear zone than outside, it varies significantly inside the fault zone, suggesting a highly inhomogeneous strain distribution created by a complex deformation history.

Paleomagnetism of latest Anisian (Middle Triassic) sections of the Prezzo Limestone and the Buchenstein Formation, Southern Alps, Italy

A paleomagnetic study was carried out at six stratigraphic sections (309 specimens) in the latest Anisian (Middle Triassic) Prezzo Limestone and the overlying Buchenstein Formation. These units outcrop over a wide area in the western Southern Alps, although most of the sampled sections are in the vicinity of the Late Eocene-Early Oligocene Adamello batholith. Three sites suffered a complete remagnetization induced by the Adamello, whereas a characteristic component with a positive fold test has been isolated at the three other sites. The mean pole of the characteristic component (Lat. 63.2°N, Long. 229.3°E, N = 3, A 95 = 8° , k = 236) is in agreement with the Triassic portion of the West Gondwana apparent polar wander path (APWP), supporting the use of paleopoles from well-dated rocks in the Southern Alps as useful proxies for the African APWP. The characteristic component, where isolated in the sampled sections, is of normal polarity only, corresponding to the latest Anisian on the basis of well-defined ammonoid and conodont biostratigraphy, but the present results suggest that there are good opportunities for extending Middle Triassic magnetostratigraphy in these Southern Alps rock units. The mean pole of the Adamello-induced component (Lat. 74.5°N, Long. 172.1°E, N = 4, A 95 = 7.6° , K = 145) lies close to the Early Tertiary portion of the APWP for stable Europe. The post-folding, Adamello-induced directions confirm that the 30–42 Ma Adamello batholith intruded after Alpine deformation and that no further deformation apparently occurred in post-emplacement times.

African origin of caviomorph rodents is indicated by incisor enamel microstructure

The same three subtypes of derived multiserial Hunter-Schreger bands are found in the incisor enamel of African phiomorph rodents from the late Eocene-early Oligocene and the oldest South American Caviomorpha from the Deseadan (late Oligocene). The synapomorphies contained therein, especially arrangement and orientation of interprismatic matrix, make an African origin of the Caviomorpha very probable. A North American origin of the Caviomorpha is thus rejected, as only primitive pauciserial Hunter-Schreger bands have been observed in possible ischyromyoid caviomorph ancestors. A multiserial Schmelzmuster apparently never evolved in the North American rodent fauna.

Development of Paleogene Depressions and Deposition of Lacustrine Source Rocks in the Pearl River Mouth Basin, Northern Margin of the South China Sea

A more accurate, integrated chronostratigraphic framework is applied to the analysis of the development of Paleogene depressions in the Pearl River Mouth basin. The results of our study show that the development of these depressions was characterized by at least three rifting or basin-forming phases occurring during these periods: late Paleocene (Late Cretaceous?)-middle Eocene, late Eocene-early Oligocene, and middle Oligocene-late Oligocene. The transition from rifting stage to postrifting stage in the basin is about 10 m.y. later than the initial spreading of the South China Sea. The prologue of the spreading of the South China Sea began as early as the end of the middle Eocene. Lacustrine source rocks deposited during the basins first rifting phase are thick and of go d quality; source rocks deposited during the last two phases, which had a sharp increase in sedimentation rate, are of lesser quality, with the exception being those areas where deposits were out of reach of sediment from the northern mainland.

Structural and Stratigraphic Analysis of the Dinaride Thrust Belt: A Frontier Exploration Province in Central-Southern Europe

The Dinarides are a 200-300-km-wide southwest vergent fold and thrust belt extending along the eastern margin of the Adriatic Sea. The complex and varied structural and stratigraphic relationships can be used to define three major tectonic units: the internal, central, and external Dinarides. In the Internal Dinarides, platform sequences deposited in the Early and Middle Triassic underwent rapid subsidence and drowning in the Late Triassic and Early Jurassic along with rifting and subsequent formation of oceanic crust. Melange and flysch were deposited during the late Jurassic through Cretaceous as the developing thrust belt encroached upon the northeastern margin of the Dinaride carbonate platform. The Dinaride carbonate platform forms the cores of the central and external Dinarides and is composed primarily of Permian-Triassic clastics and evaporites overlain by Middle Triassic through early Eocene platform carbonates. The entire sequence is overlain by late Eocene and early Oligocene synorogenic flysch. In the central Dinarides, late Jurassic and Early Cretaceous unconformities suggest structural uplift prior to the onset of thrusting. Deformation involves Paleozoic basement and includes a major decollement in the Permian Triassic clastic and evaporite unit. Thrusting in the external Dinarides occurred in the late Eocene-early Oligocene and is restricted to Middle Triassicmore » and younger units with major detachments forming near the base of the Ladinian and within a Late Jurassic-Early Cretaceous evaporite. Important oil source and seal lithofacies occur within intraplatform basins and lagoons in the Mesozoic sequences of the central and external Dinarides. Widespread dolomitized units within the Mesozoic carbonate sequence are potential reservoir zones. The presence of surface hydrocarbon seeps and of existing production on trend to the Dinarides in the thrust belt of Italy and Albania suggest the potential for hydrocarbon discoveries in this underexplored area.« less

Glaciomarine turbidite and current controlled deposits in Prydz Bay, Antarctica

A thick sequence (up to 2200 m) of presumed post late Eocene/early Oligocene glaciomarine sediments is inferred to be present on the Prydz Bay continental rise. In the absence of information from drillholes, we correlate to ODP Leg 119 drillsites on the shelf and compare with the seismic reflection pattern of glaciomarine sequences in the Weddell Sea. The inferred glaciomarine sediments in Prydz Bay appear to be deposited in a complex manner, suggesting interaction by both turbidity cur rents and strong bottom currents. Reflection seismic profiles from the lower continental slope and rise shows an abundance of current influenced deposits, such as sediment waves and large sediment ridges with similarities to contourite drifts. In addition, large channel-levee complexes are abundant, suggesting deposition by turbidity currents and other massflow processes. Large channels and sediment ridges trend oblique to the continental margin. The geometry and character of the seismic reflection pattern suggest that the ridges have been deposited under the combined influence of overflow from downslope channelized turbidity currents and strong bottom water flow. The observed sediment waves and the difference along eastern and western channel margins suggest that bottom currents are flowing towards the west. We suggest that the initiation of turbidite sedimentation occurred in the late Eocene-early Oligocene, when the Amery Ice Shelf reached the shelf edge for the first time. Onset of current controlled deposition may possibly be related to the opening of the Drake Passage at the Oligocene/Miocene boundary.

Palaeobathymetry and palaeogeography of the Bakony Eocene Basin in western Hungary

The authors studied two new outcrops and the cores of 10 new bore holes drilled through the marine Eocene of the Bakony Hills. The main purpose was to recognize palaeoenvironments and their changes in the Bakony Eocene Basin. Earlier research rather neglected this aspect. Palaeoecologic analyses of different groups of fossils (nannoplankton, foraminifera, molluscs, ostracods) and to a lesser extent, sedimentologic observations and field studies served as tools for the reconstruction of bathymetric conditions and subsidence history, by studying the trends in lateral facies changes. The reconstruction of paleaogeographic patterns was made possible. Vertical facies successions are clear, due to the excellent core material and the rather simple tectonic structure. Accurate datings have been possible, using the sophisticated stratigraphy worked out for the Bakony Eocene by a combination of magnetostratigraphic and planktonic zonations, published elsewhere. In part 1 the authors present the description and palaeoecologic analyses of the studied sections. The vertical succession of the main facies units in the SW Bakony Basin is as follows (bottom to top): coastal sandy pelites, pebbly sands, biogenic shelf limestone, glauconitic calcareous marl, hemipelagic globigerina marl with turbiditic sandstone intercalations. The same sequence in the NE Bakony Basin is as follows: alluvial-coastal sediments with coal seams, nummulitic shelly sands, glauconitic sands and marls, laminated clay, globigerina marl and bathyal clays. The authors discuss palaeonvironments and their evolution in part II: The coastal facies in the SW Bakony Basin is a few million years older than in the NE. Nevertheless, this facies indicates in both areas flat coastal plains and seashores probably covered by mangrove. A carbonate shelf developed in the SW Bakony Basin with very low sedimentation and subsidence rates. The shelf period was much shorter in the NE. The shelves were influenced by intense currents that were active in the whole basin. Since Mid Bartonian times, retrogradation of the shelves is demonstrated by the sequence. First, sediments of shelf break and upper basin-slope environment overlie the shallow marine deposits. Strong glauconitization indicates current activity. The glauconitic facies is overlain by a globigerina marl and silts, described earlier as shelf sediments! The authors set out to prove that the maximum depth of its deposition has been at least 800 m. Turbidites are intercalating with the globigerina marl in the SW. In part III mainly palaeogeographic conclusions are discussed. The authors recognized on the basis of lateral facies changes that shallow marine conditions prevailed to the east and south of the Bakony Basin during the entire Bartonian Priabonian. The position of erosional source areas and the direction of dense turbidity currents suggest the same conditions. The correlation with the Late Eocene-Early Oligocene Zala-Velence andesitic arc is also discussed in some detail and is based on tephra layers, found in the globigerina marl. The authors recognized a remarkable, Middle Bartonian coincidence of several different events, such as increasing andesitic volcanism, sudden acceleration of basin subsidence, increase of sea depth, beginning of turbidity current activity. This may indicate a coeval increase in tectonism. The transgressions are younging towards the NE. The basin generation shows a similar shift. A complete palaeogeographic and probably a tectonic “inversion” took place in the Priabonian. The Southern alpine-Dinaric character of the Eocene fauna and lithofacies changed gradually to an Eastern Alpine-Carpathian one during the Priabonian. From the beginning of the Oligocene, only the latter influence did exist. Also during the Priabonian, a new marine basin was formed in Hungary to the NE of the Balkony Hills.

Regional geologic setting of epithermal gold deposits, Chile

Epithermal gold deposits in Chile are Cretaceous to Cenozoic in age. The ores and the volcanic rocks with which they are associated were generated at a noncollisional, ocean-continent convergent plate margin. They are related to long, slightly sinuous north-northeast-trending magmatic belts roughly parallel to the Pacific coast. Most of the deposits are associated with eroded volcanic centers and subvolcanic porphyries. North-south variations in the nature, extent, and timing of magmatism control the succession of mineralization events.In the Late Cretaceous, vein districts (e.g., El Bronce) and hydrothermal alteration zones were formed in central Chile. Paleocene-Eocene volcanic rocks constitute a large, diffuse volcanic field in the Antofagasta region (21 degrees -26 degrees S), which contains epithermal precious metal and porphyry copper deposits associated with coeval extrusive rocks (e.g., Faride, San Cristobal, Guanaco, and Cachinal de la Sierra). In the late Eocene-early Oligocene, volcanism diminished and finally ceased in northern Chile, probably as a consequence of highly oblique plate convergence. Oligocene magmatism was restricted to emplacement of porphyries carrying copper deposits along transtensional zones within a strike-slip fault system. One of these deposits (Potrerillos), hosted by sedimentary rocks, developed epithermal gold mineralization in its cupola (El Hueso).The late Oligocene to mid-Miocene magmatic province of north-central Chile (26 degrees -31 degrees S) coincides approximately with the modern, nonvolcanic, fiat slab segment of the Chilean Andes and hosts the largest number of known epithermal developments. Deposits such as El Indio-Tambo, Nevada, La Pepa, Lobo, Marte, and La Coipa are hosted by Oligocene-Miocene volcanics. The deposits are associated with large zones of argillic and advanced argillic alteration, in some places related to shallow-level stocks and flow domes. A younger event of alteration-mineralization is recognized in the eroded cores of middle Miocene composite volcanoes, which cap some of the older altered areas (e.g., Refugio, La Laguna). The main alteration event began at 23 Ma, continued after a major deformation event initiated at 18 to 16 Ma, and ended at 11 to 8 Ma. The precious metal deposits and mineralized zones of the Oligo-Miocene province were exposed by erosion after the cessation or eastward shift of volcanism in the late Miocene, which was probably due to shallowing of the subduction zone between 26 degrees 30 and 33 degrees S.Late Miocene to Holocene magmatism in Chile is present in two segments beneath which the dip of the Benioff zone remains steep: north of 27 degrees S (Central Volcanic zone, CVZ) and south of 33 degrees S (Southern Volcanic zone, SVZ). The Central is characterized by widespread Neogene hydrothermal alteration zones and native sulfur deposits associated with composite volcanoes, calderas, subvolcanic intrusions, and active geothermal fields which are superimposed on the Oligocene-Miocene volcanic belt. These features are believed to represent a very shallowly eroded epithermal environment. In this setting the late Miocene Choquelimpie deposit is located in the exposed core of a volcano.

Paleomagnetism in the Tajikistan: continental shortening of European margin in the Pamirs during Indian Eurasian collision

A stable and well defined magnetization with D = 350°, I = 55.5° and α 95 = 4° , carried by hematite, is isolated in the red beds of the Tajik basin. The fold test is positive (99% probability) and this magnetization predates the Late Eocene-Early Oligocene folding phase in the Tajik basin. The coordinates of the VGP are: 82°N, 323°E, dp = 4, dm = 6 with a paleolatitude of Dushanbe (38°38N, 68°51E) of 35.6 ± 5°. A high blocking temperature component with weak intensity and reverse polarity has been demonstrated in the site of Ragoon, but it has not been isolated. This component predates the Long Cretaceous Normal Period and is compatible with the older stratigraphic age attributed to the formation (Valanginian); these arguments indicate that this component is likely of primary origin. The other component, of higher intensity and clearly isolated in all the sites, is a pre-folding remagnetization. The process of remagnetization is attributed to the tectonic events, such as fluids migration and slight heating during a few million years, which took place just before the first Late Eocene-Early Oligocene phase of folding An anticlockwise rotation of 26° encountered by the region of Dushanbe is in good agreement with the indentation model of collision leading to anticlockwise rotation West of the Indian wedge and clockwise rotation to the east. A comparison of the paleolatitude of Dushanbe, computed from the direction of magnetization, and the paleolatitude expected from the apparent polar wander path for Eurasia, gives an estimate of the post-collision continental shortening north of the Tajik basin. This shortening did not exceed a few hundred kilometers and was probably between 100 and 300 km. An estimate of the roles of the different classical mechanisms of crustal shortening is discussed and a paleogeographic reconstruction giving the location of the southern margin of Eurasia at 45 Ma is proposed. The paleolatitude deduced from the prefolding remagnetization of the Red Beds of the Tajik basin is then compared with the results obtained by Soviet workers in Tertiary sediments of the same area, revealing strong contradictions in interpretation. This contradiction emphasizes the problem of the rigidity of European plate.

Evolution and biogeography of Paleogene microperforate planktonic foraminifera

Microperforations with a maximum diameter less than 1 μm characterize some Paleogene planktonic foraminifera and 5 microperforate groups are distinguished. Differences occur in pore density and wall textures as well as in the coiling mode. These groups are unrelated phylogenetically and the microperforate wall represents a morphological character of convergence. Parvularugoglobigerina, the earliest group, with smooth surface and scattered microperforations, evolved immediately after the K/T boundary in many localities. The Globoconusa group is characterized by a heavily pustulate wall and relatively coarser pores of lower density. Derived from Guembelitria, members of this group occur in almost every marine sequence of the lower Paleocene with no latitudinal preference, but they tend to dominate shallow water assemblages in epicontinental seas and possibly are related to coastal upwelling. Together with its immediate ancestor Woodringina and the evolutionary offshoots Zeauvigerina and Tubitextularia, the biserial Chiloguembelina contains the largest number of species, falling into 4 subgroups as here recognized. The chiloguembelinids dominated the early Paleocene, the high latitude middle Eocene, and the late Eocene-early Oligocene. The middle Eocene Guembelitria triseriata-Cassigerinelloita amekiensis lineage is defined by triserial to triserial-pseudoplanispiral coiling and occurs mainly in marginal seas and over oceanic rises in southern oceans. The tenuitellids, the latest group, first occur in the late Eocene, become common in the Oligocene-Miocene, and extend into the Quaternary and modern oceans. With a high frequency in high latitudes of the Oligocene, tenuitellids are considered to be cold water indicators.

Late eocene—early oligocene integrated isotope stratigraphy and biostratigraphy for paleoshelf sequences in southern Australia: paleoceanographic implications

A late Eocene—early Oligocene planktonic foraminiferal oxygen-isotope stratigraphy derived for two overlapping sections (Browns Creek and Castle Cove) through paleoshelf sequences in southern Victoria, Australia, contains a pronounced late Eocene depletion (warm) peak (∼1.2%PDB) centred at about 41 Ma, followed by a two-stage enrichment (cooling) pattern, the later one coinciding with the well known Terminal Eocene Event (∼37 Ma), exhibiting ∼1.0%PDB) erinchment. The early Oligocene isotope stratigraphy shows fluctuations of about 1.0%PDB, with prominent depletion peak immediately following the terminal Eocene δ 18O enrichment. The start of the terminal Eocene δ 18O enrichment coincides with the LAD of the nannofossil indicator of the Eo-Oligocene boundary, Discoaster saipanensis (early NP21 Zone). On this basis the traditional southern Australia planktonic foraminiferal Eocene-Oligocene boudary markers (LAD Globigerapsis index, LAD Subbotina linaperta Subbotina linaperta) locally survived into Zone NP22, well into the early Oligocene. Sea-surface paleotemperature estimates derived from the δ 18O data assuming a δW value of −1.2%PDB, show that the late Eocene ingression of the tropical planktonic foraminiferal genus Hankenina coincided with peak surface-water tempertures of ∼24°C. The temperature decline in the southern Australia shelf waters associated with the Terminal Eocene Event may have been as much as 7°C, reaching a minimum value of ∼13°C. The later survival of G. index and S. linaperta coincides with a rapid recovery of water temperatures over the south Australian shelf following the Terminal Eocene Event. This is attributed to the local warming of waters in shallow broad embayments where circulation patterns were decoupled from the developing Circum-Antartic Current to the south.

Tertiary climate and phytogeography in southeastern Australia

Palynofloras of late Eocene to Pliocene-Pleistocene age have been used to reconstruct vegetation and climate in southeastern Australia. The vegetation was forest, becoming open only in the late Pliocene-early Pleistocene. Precipitation was the highest in the late Eocene-early Oligocene, decreasing in the late Oligocene-early Miocene and mid Miocene, with a brief increase in the late Miocene-early Pliocene and subsequent further decreases. Patterns of change show a retreat of rainforest taxa from a formerly widespread distribution to their present restricted habitats, e.g. the Huon Pine (pollen type Phyllocladidites mawsonii) to Tasmania and the Araucariaceae to the northern part of the east coast. Other taxa may be abundant, under certain ecological conditions, e.g. Nothofagus flemingii on well drained soils and Casuarinaceae in marginal marine regions. The Miocene was a time of great change and maximum variation, when widespread rainforest disappeared from inland regions and burning on a regular basis became an integral part of the environment. Throughout the time period under review, there was a climatic gradient parallel to that of today.

A Sequence Stratigraphic Study of the Tangaroa Sandstone, Taranaki Basin, New Zealand: ABSTRACT

Seismic sequence interpretation of the Tangaroa Sandstone, combined with detailed biostratigraphic analysis, well log interpretation, and core descriptions, confirm the Tangaroa as a late Eocene-early Oligocene, sand-rich, submarine fan complex within the Taranaki basin of northwest New Zealand. The Tangaroa Sandstone is underlain by deep-water shales (Eocene Kaiata Formation) and overlain by a thick deep-water limestone (Oligocene Te Kuiti Formation), and typically consists of two, 25-150-m-thick vertically stacked sandstones, separated by a thin (8 m) limestone. Seismic isochron thickness maps of the Tangaroa interval display a fan-shaped geometry. Internal seismic facies are dominated by erosional channels and progradational wedges. The erosional channels, to 1-5 km wide and approximately 70-150 m thick, are interpreted as upper to mid-fan feeder channels. Thin progradational wedges are located basinward of the channels. Paleobathymetic data, based on micropaleontology, indicate a shelf-to-deep-water genesis of the Tangaroa. Conventional core data suggest that the Tangaroa comprises fine- to coarse-grained clastics that were deposited by debris flows, liquefied flows, and turbidites. Using seismic sequence techniques, verified by biostratigraphic control, the Tangaroa Sandstone is subdivided into two sequences: the Lower Tangaroa sequence and the Upper Tangaroa sequence, which formed during two distinct relative lowstands in sea level. The intervening limestonemore » and the overlying Te Kuiti Limestone, apparently were deposited during periods of relative highstands in sea level. The Eocene Oligocene boundary is located in the thin limestone of the Lower Tangaroa sequence.« less

Short Paper: Accretion and uplift in the Shimanto Belt, SW Japan

K-Ar cleavage dating and fission track cooling ages have been combined with existing stratigraphic data to constrain the evolution of the Shimanto Belt, a Cretaceous-Neogene accretionary complex. Cleavage ages record two major phases of accretion during the late Cretaceous and late Eocene-early Oligocene. The minimum time periods between sedimentation and deformation are constrained to 9, 5 , 9 and 0Ma intervals within four tectonic units containing youngest sediments of Maastrichtian, late Eocene, late Eocene and early Oligocene ages respectively. One phase of middle to late Miocene uplift is indicated for the whole study area by the fission track cooling ages. Frontal underplating combined with out-of-sequence thrusting are suggested as mechanisms for this regional uplift. The Cretaceous-Neogene Shimanto Belt (Fig. la) forms the outermost, onshore exposure of accreted material in SW Japan. The boundaries of constituent accreted units of the Shimanto Belt are defined by a detailed stratigraphy (Taira et al. 1980Taira et al. 1988) providing an opportunity to test the application of cleavage dating and fission track studies to accretionary complex evolution. The Butsuzo Tectonic Line (Fig. la) juxtaposes the Shimanto Belt to the Jurassic Sanbosan Belt in the north. Younger accreted material extends offshore to the south as far as the present-day boundary of the Eurasian Plate, the Nankai Trough (Fig.la). Continued accretion seaward of the Shimanto Belt has back-rotated older accreted units to their present-day, steeply dipping orientations. The type section of the Shimanto Belt (Taira et al. 1980) lies in the eastern Hata Peninsula (Fig. 1b) which contains the study

A monogenetic, Surtla-type, Surtseyan volcano from the Eocene-Oligocene Waiareka-Deborah volcanics, Otago, New Zealand: A model

The relics of a small, monogenetic, continental-shelf, Surtseyan volcano are preserved on the North Otago coast, South Island, New Zealand, in the late Eocene-early Oligocene Waiareka-Deborah volcanics. The succession consists of two parts, i. e. a lower interval of bedded lapilli tuffs and lapillistones, representing the eruptive, aggradational-cone-building phase, and an upper epiclastic sequence, representing the post-eruptive degradational phase. All of the preserved succession appears to have been deposited below storm wave base. The lapilli tuffs and lappillistones are subaqueous fall deposits, modified contemporaneously by downslope grain flow and turbidity current redeposition, and perhaps by local reworking caused by turbulent thermal eddies. The absence of major discordances in the lapilli tuffs suggests that the active eruptive period was very short-lived, perhaps lasting only a few days. The epiclastic succession consists of redeposited volcanic, skeletal, lime mud and glauconitic detritus, transported by debris flows and other mass flows. The initial epiclastic unit, a debris flow, appears to represent the sector collapse of a significant part of the cone. The appearance of fossils and rounded clasts low in the epiclastic succession coincides with stabilisation of the top of the submarine volcanic edifice, development of a wave-planed top, and its colonisation by a diverse fauna. Periodic storm activity swept material off the platform, redepositing it as marginal talus ramps. Surtla, a wholly submarine satellite volcanic centre of the 1963–1967 eruptive activity of Surtsey, is an excellent modern analogue for both the eruptive and post-eruptive phases of the Bridge Point-Aorere Point volcanic centre. By analogy with Surtla, the 120 metres of lapilli tuffs and lapillistones exposed on Bridge Point and Aorere Point accumulated in only several days. The 25 metres of reworked, glauconitic and fossiliferous volcaniclastics, represent thousands of years based on the time required for glauconite to form.

Middle Tertiary calcareous nannofossils from Aire district, Victoria; A comparison with equivalent assemblages in South Australia and New Zealand

Late Eocene-Early Oligocene calcareous nannofossil assemblages in southwestern Victoria reflect changes in the paleo-climatic history of the region. Similar assemblages can be recognised in South Australia and New Zealand and a comparison of key nannofossil datums reveals a homotaxial sequence of biostratigraphic events. These datums allow correlation with international nannofossil schemes and enable a comparison to be made with equivalent correlations based on the local planktonic foraminiferal biostratigraphy. Such comparisons show a marked difference between nannofossil and planktonic foraminiferal Eocene/Oligocene boundary markers in southern Australia and New Zealand. In southern Australia the traditional foraminiferal boundary markers (LAD Globigerapsis index, LAD Subbotina linaperta) occur in the Helicosphaera reticulata (NP22) Zone, after the LAD of Coccolithus formosus. In New Zealand these foraminiferal events occur prior to the LAD of Coccolithus formosus within the lowermost Ericsonia subsistichus (NP21) Zone, immediately following the LAD of Discoaster saipanensis. The difference between these nannofossil and foraminiferal events in southern Australia is significantly greater than that generally reported elsewhere, including results following completion of IGCP Project 174: Terminal Eocene Events (Pomerol and Premoli-Silva, 1986). This “above the average” variation is considered due to a local difference related to southern Australian Lower Oligocene paleogeography.

Epochs of intrusion-related copper mineralization in the Andes

Seventy-four copper deposits and prospects related intimately to intrusive activity in the Andes have been dated radiometrically during the last 18 years by many different investigators, most of whom used the KAr method. The results are summarized and some of their local and regional implications are reviewed. A number of copper deposits, mainly of the porphyry type, were emplaced in, or near to, premineral volcanic sequences and (or) equigranular plutons. Such precursor volcanism lasted for as long as 9 Ma, and preceded mineralization by intervals of from less than 1 Ma to as much as 9 Ma. Precursor plutons were emplaced no more than 2 to 3 Ma prior to mineralization at several localities in Chile, but possibly as long as 10 to 30 Ma earlier in parts of Colombia and Peru. The time separating emplacement of progenitor stocks and hydrothermal alteration and accompanying copper mineralization, and the duration of alteration-mineralization sequences generally are both less than the analytical uncertainty of the KAr method. However, on the basis of a detailed study of the Julcani vein system in Peru and less clearcut evidence from elsewhere, it may be concluded that alteration and copper mineralization followed stock or dome emplacement by substantially less than 1 Ma and lasted for 0.5 to 2 Ma and, locally, possibly as long as 3 Ma. At several localities, post-mineral magmatic activity could not be separated by the KAr method from the preceding alteration-mineralization events. As many as nine epochs of copper mineralization, ranging in age from late Paleozoic to late Pliocene-Pleistocene, are recognizable in the central Andes of Chile, Peru, Bolivia, and Argentina, and at least four somewhat different epochs characterize the northern Andes of Colombia. Each epoch coincides with a discrete linear sub-belt, some of which extend for more than 2000 km along the length of the orogen. More than 90% of Andean copper resources, mainly as porphyry deposits, are confined to three Cenozoic sub-belts of Paleocene-early Eocene (66-52 Ma), late Eocene-early Oligocene (42-31 Ma), and middle Miocene-early Pliocene (16-5 Ma) ages in southern Peru and Chile. The scarcity of porphyry copper deposits and the increase of pluton-related copper veins in Jurassic and Cretaceous sub-belts farther west in the central Andes is believed to be due to deeper levels of erosion. In the central Andes, copper sub-belts became progressively younger eastward in response to the effects of a compressional tectonic regime imposed on the overriding South American plate, following mid-Cretaceous opening of the South Atlantic Ocean basin. A marked eastward breakout of middle Miocene-early Pliocene copper mineralization up to 300 km into western Argentina and Bolivia is attributed to rapid subduction of young buoyant oceanic lithosphere during the Miocene. A contrasting migration pattern of copper sub-belts characterized the northern Andes, where an early Cretaceous to early Eocene trenchward shift is thought to have been caused by accretion of oceanic terranes to the continental edge. The observed distribution of copper sub-belts reflects the tectonic segmentation of the Andes. Three first-order segments are defined by the well-known Huancabamba and Abancay Deflections, at latitudes 5°S and 14°S, respectively. Each first-order segment is characterized by a profoundly different distribution, timing and, in part, style of copper mineralization, and by distinct copper endowments. Anomalous patterns of sub-belts abut both deflections. Several second-order boundaries appear to have caused more subtle changes along individual copper sub-belts. Additional programs of radiometric dating, on both local and regional scales, are required before the migration and segmentation of copper (and associated metallogenic) sub-belts in the Andes are properly defined and understood.

Plate boundary evolution in the Halmahera region, Indonesia

Halmahera is situated in eastern Indonesia at the southwest corner of the Philippine Sea Plate. Active arc-arc collision is in process in the Molucca Sea to the west of Halmahera. New stratigraphic observations from Halmahera link this island and the east Philippines and record the history of subduction of the Molucca Sea lithosphere. The Halmahera Basement Complex and the basement of east Mindanao were part of an arc and forearc of Late Cretaceous-Early Tertiary age and have formed part of a single plate since the Late Eocene-Early Oligocene. There is no evidence that Halmahera formed part of an Oligo-Miocene arc but arc volcanism, associated with eastwards subduction of the Molucca Sea beneath Halmahera, began in the Pliocene and the Pliocene arc is built on a basement of the early Tertiary arc. Arc volcanism ceased briefly during the Pleistocene and the arc shifted westwards after an episode of deformation. The present active arc is built upon deformed rocks of the Pliocene arc. The combination of new stratigraphic information from the Halmahera islands and models of the present-day tectonic structure of the region deduced from seismic and other geophysical studies is used to constrain the tectonic evolution of the region since the Miocene. Diachronous collision at the western edge of the Philippine Sea Plate which began in Mindanao in the Late Miocene impeded the movement of the Philippine Sea Plate and further motion has been achieved by a combination of strike-slip motion along the Philippine Fault, subduction at the Philippine Trench and subduction of the Molucca Sea lithosphere beneath Halmahera.