Late Palaeogene Research Articles

Late Palaeogene–Quaternary geology of Halmahera, Eastern Indonesia: initiation of a volcanic island arc

The Late Palaeogene–Quaternary stratigraphy of Halmahera is described, and new formation names are proposed, based on recent field investigations of the NE and central part of the island. This stratigraphic information provides new insights into the Neogene history of Halmahera and the development of the present island arc. The Late Palaeogene and younger rocks rest unconformably on an ophiolitic Basement Complex which formed part of a Late Cretaceous–Early Tertiary fore-arc. After volcanic arc activity ceased in the Eocene the former fore-arc terrane was uplifted and deeply eroded in the Late Palaeogene. Some of the Late Palaeogene–Early Miocene river valleys are currently being re-excavated by the present rivers. Slow subsidence began in the mid–late Oligocene and by the end of the Miocene all eastern Halmahera was the site of shallow-water carbonate deposition. There is no evidence for arc volcanism in central Halmahera at this time and the reported Oligo-Miocene volcanism in nearby regions is interpreted as volcanism related to the Sorong Fault system. The Miocene shallow water region subsided rapidly in the Early Pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites, with increasing amounts of calc-alkaline volcanic debris from a Pliocene volcanic arc built on the western arms of Halmahera, probably on the eroded Early Tertiary arc. This phase of rapid subsidence in the Pliocene back-arc region resulted from the initiation of subduction of the Molucca Sea lithosphere eastwards beneath Halmahera. Differential subsidence on NW–SE and NE–SW sets of faults in the region immediately behind the active arc led to the formation of deep sediment-filled basins adjacent to the eastern arms. A major deformation event in the Pleistocene resulted in folding and local thrusting at the junction between eastern and western Halmahera and volcanism ceased in the Pliocene arc. The third Halmahera arc, the Quaternary arc, currently active in the northern part of the islands, began activity within the last 1 Ma and is built upon the deformed and partly eroded Pliocene arc. The Pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward-dipping Molucca Sea plate with adjacent plates, either with a fragment of the Australian continent in the Sorong Fault zone and/or with the Philippine Sea plate beneath northern Halmahera.

Mesozoic extensional history of the southern Tethyan continental margin in the SE Aegean

The islands of Rhodes, Alimnia and Symi are situated at the eastern end of the Hellenic arc. Well exposed on these islands are a series of nappes composed of Mesozoic-early Tertiary, principally carbonate, sequences which formed the sedimentary cover of the southern Tethyan margin and were stacked during Alpine deformation in the late Palaeogene. The stratigraphy of these sequences is described and interpreted as preserving a transition from a carbonate platform through slope to basin. The sequences formed part of the cover of a passive continental margin but this margin differed from typical modern margins in recording several episodes of extension with no clear evidence of ocean crust formation in the SE Aegean. Three important episodes of extension are recorded: a platform-basin topography was initiated by Mid-Late Triassic rifting, late Liassic extension led to platform collapse and development of carbonate slopes, and renewed Late Cretaceous extension further reduced the size of carbonate platforms. The Late Cretaceous extension was probably linked to the formation of ocean crust further east, in Cyprus and Turkey. Sedimentological evidence indicates that the slope was eastward-facing and the platform-basin margin was orientated approximately north-south. This evidence is not consistent with previously proposed palaeogeographic reconstructions of the region based on tracing the isopic zones of the Hellenides through the arc. A new two-dimensional reconstruction of the form of the southern Tethyan margin in the SE Aegean is proposed.

Magnetic polarity dating of tectonic events at passive continental margins

Palaeomagnetic studies of continental margin sediments drilled during IPOD Legs 47a (NW African margin), 47b (NW Portugal margin) and 48 (NE Bay of Biscay and SW Rockall margins) have provided magnetic polarity records for the late Neogene, early Palaeogene and Cretaceous periods. The general pattern of geomagnetic field polarity reversals during late Neogene times is now well established, and the absolute age of major polarity transitions during the past 5 Ma or so has been determined from palaeomagnetic studies of radiometrically dated igneous rocks (see, for example, McDougall et al. 1977). Consequently, a comparison of the late Neogene polarity sequences identified in the sediments of NE Biscay and the NW African margins, with this ‘standard’ polarity reversal time scale, allows the precise determination of sediment accumulation rates, and the times at which significant changes in these rates occurred. These comparisons have led to the recognition of a short but significant hiatus in early Pleistocene times in both areas. Such information has importance in evaluating the recent geological evolution of these margins. For earlier geological periods the correlation between the magnetic polarity time scale and the geological time scale is less well established but studies of the type described here can still provide important information on the timing of certain geological events recorded in the magnetic anomaly patterns of the oceanic lithosphere. For example, in the early Palaeogene sediments cored at Sites 403-405, off the SW Rockall continental margin, a sequence of magnetic reversals has been identified, which shows a good correlation with marine magnetic anomalies 22-24. Since anomaly 24 is the oldest recognizable anomaly in the Atlantic, and lies immediately adjacent to the continental margins, the biostratigraphic age of this anomaly, determined from the nannofossil and dinocyst zonal determinations at Site 404, provides important information on the date of initial rifting of Rockall (together with the rest of NW Europe) from Greenland. The Site 404 results indicate that this important tectonic event occurred in early Eocene times, at about 52 Ma B.P., rather than at 60 Ma B.P. as was originally proposed by Heirtzler et al .(1968). A further example of the potential value of this type of study is provided by the Cretaceous sediments cored at Sites 397, 398, 400A and 402A. A long section of predominantly normal polarity sediments at the latter three sites appears to correlate with the long Cretaceous interval of dominantly normal polarity identified in marine magnetic anomaly patterns. The combination of palaeomagnetic and biostratigraphic studies allows useful constraints to be placed on the maximum duration of this interval, and on the age of short reversals within and below it. This information has direct relevance to the interpretation of Mesozoic marine magnetic anomaly patterns in terms of the history of seafloor spreading and evolution of continental margins during the early stages of opening of the South Atlantic in Cretaceous times.

Influence of plate tectonics on the distribution of late Palaeogene to early neogene larger foraminiferids in the Australasian region

The distribution of late Palaeogene to early Neogene larger Foraminiferida in the Australian—Pacific region is related to climate and the interaction of tectonic plates. Biostratigraphic studies suggest that Cycloclypeus, Miogypsina and Lepidocyclina (Nephrolepidina) invaded the region during the Middle Oligocene. At this time the northern margin of Australia (New Guinea) was close enough to the Indonesian region and was warm enough to permit the larger foraminiferids to invade the Australian, New Zealand and the Solomon—Fiji Islands chain. Climatic and oceanic current factors had little influence on the larger foraminiferal distribution within the north Australian region, but strongly influenced the faunal distribution in Southern Australia and New Zealand. Of these three important genera Lepidocyclina (Nephrolepidina) shows the greatest amount of evolutionary differentiation; by the latest Early Miocene three distinct lineages can be recognised. The difference between the Australian and New Zealand faunas is reflected from biometric studies, although the non-biometric characteristics of the equatorial chambers indicate a close relationship. The equatorial chambers of the lineage from the Solomon—Fiji Islands chain indicate a more distant genetic relationship to the two other lineages. Plate reconstructions show that the island groups were well separated from the rest of Australasia; and that the presence of the Tasman Sea effectively isolated New Zealand stocks from the others.

Brachyodonty and hypsodontyin some Palaeogene Eurasiatic Lagomorphs

Palaeogene Eurasiatic lagomorphs show more or less different degrees and kinds of brachiodonty and hypsodonty in the cheek teeth. No fully brachyodont lagomorphs are known up to the present. The probably Paleocene Mimolagus from Western Kansu shows the less advanced stage against partial hypsodonty in the upper cheek teeth. Partial hypsodonty is more advanced in the late Eocene Mongolian genera Shamolagus, Gobiolagus and Lushilagus. Roots, however, are still well developed in the cheek teeth. Some recent discoveries from earlier Oligocene sites in France and Germany seem to represent a comparable degree of hypsodonty. The Asiatic Oligocene Desmatolagus is partiallyhypsodont with root formation at later age. Compared with the Eocene taxa partial hypsodonty is, however, somewhat more advanced so far reduction of roots and extension of unilateral enamel covering is concerned. Lagomorphs with a similar degree of partialhypsodonty are known from the Lower Miocene of Kasachstan ( Agispelagus) and the later Miocene of Anatolia and the Mongolian Peoples Republic. In Europe partial hypsodont lagomorphs similar to but not identical with the Desmatolagus level and more advenced than the earlier Oligocene specimens appear at the late Stampian and have a continuous, structurally rather stable record up into the Vallesian. The earliest nearly complete hypsodont lagomorph in Eurasia is the central Asiatic medial and late Oligocene Sinolagomys. Primary structures disappear after some wear in the upper teeth, the hypostria is short, root formation is vestigial or absent. Decendants of Sinolagomys may be represented by some Miocene Asiatic ochotonids ( Bellatona Alloptox). European late Palaeogene lagomorphs demonstrate severalways in the acquisition of hypsodonty. Examples are the Marcuinomys-Lagopsis lineage, in hypsodonty near to Sinolagomys, the Amphilagus-Titanomys lineage and the Piezodus-Prolagus lineage. The two latter are independent from the Sinolagomys type of hypsodonty. Thus, evolution towards hypsodonty, one of the very characteristics in lagomorphous evolution, is not a one way phenomenon but shows a rather diversified pattern with differend levels of completion.

Late Cretaceous and Palaeogene palaeolatitudes of the Ontong Java Plateau

THE Pacific plate has undergone a substantial northward displacement during the late Mesozoic and the Cainozoic. Here we give additional documentation for such motion based on palaeomagnetic measurements of a sequence of sedimentary and basalt samples collected from middle Oligocene to Aptian sections of Deep Sea Drilling Project (DSDP) site 289 (ref. 1; 00° 29.92′S, 158° 30.69′E) drilled on the Ontong Java Plateau.