Eocene Basin Research Articles

Time, place and mode of propagation of foreland basin systems as recorded by the sedimentary fill: examples of the Late Cretaceous and Eocene retro-foreland basins of the north-eastern Pyrenees

Abstract The relationship between tectonics and sedimentary fill has been studied in two syncontractional basins of the western Corbières (eastern North Pyrenean retro-foreland basin). The Late Cretaceous basin formed during c. 10–12 Ma as a result of left-lateral transpressional deformation, and is composed of forward-younging sub-basins characterized by reworking of the forelimbs of growing fold-propagation folds. Thrust-wedge advance and cratonward migration of the platform are recorded by a deepening-upward stacking pattern indicating increased regional subsidence with a limited contribution of the submarine orogen. Tectonic quiescence and erosional unloading lasting 29–30 Ma are recorded by a shallowing-upward stacking pattern, and fluvial sedimentation issued from widespread sources in the emerging inner orogen. The Early to Middle Eocene basin formed as a result of pure shortening normal to the range. The marine Early Eocene basin developed during c. 2 Ma by widening of a single basin provoked by the two-step propagation of a basement duplex. This is recorded by growth-stratal patterns and coarsening-upward depositional sequences indicating the increasing contribution of the emerged orogen. The Middle Eocene continental deposits infilled two sub-basins working synchronously and were transported by alluvial fans with a provenance in the inner orogen, during decreasing thrust-wedge advance and increasing erosional unloading.

The rise and fall of the Faroe–Shetland Basin: evidence from seismic mapping of the Balder Formation

Seismic mapping of the Faroe–Shetland Basin from three-dimensional surveys provides striking evidence for uplift of the southern Faroe–Shetland Basin in the late Palaeocene and rapid subsidence in the early Eocene. The seismic reflector at the base of the Balder Formation follows a regional unconformity surface, which records erosion of a major branching drainage network into the underlying Palaeocene section, following a major base level fall. The topography of this surface was subsequently in-filled following marine transgression, and deep-water conditions returned to the basin in the early Eocene. We attribute base level changes to the influence of the proto-Iceland mantle plume beneath the lithosphere, providing support until continental break-up west of the Faroe Islands.

Paleocene–Eocene syncontractional sedimentation in narrow, lacustrine-dominated basins of east-central Tibet

Sedimentologic, stratigraphic, compositional, and structural data from four elongate basins ( 30 km long) in the Nangqian-Yushu region of east-central Tibet (near the headwaters of the Mekong and Yangtze Rivers) indicate nonmarine sedimentation synchronous with Paleocene–Eocene northeast-southwest shortening. Sedimentation in the Nangqian, Niuguoda, Xialaxiu, and Shanglaxiu basins involved (1) mud and carbonate accumulation in offshore to nearshore lacustrine environments and (2) gravel and sand deposition in fan-delta to alluvial-fan environments localized near basin margins. Growth strata in both fine- and coarse-grained deposits, primarily in upper levels of basin fill, provide evidence for sedimentation contemporaneous with motion on fold-thrust structures. Provenance data collected from 25 measured stratigraphic sections, including >1500 paleocurrent measurements and >150 conglomerate compositional analyses, show that each basin was fed sediment from several directions by proximal source areas composed of Carboniferous–Triassic rocks. The source-area proximity and a stratigraphic variability among the basins suggest that each basin evolved independently and was filled by sediment derived from relatively small drainage networks (<103 km2). Age control for basin fill is based on Paleogene fossils, 38–37 Ma 40Ar/39Ar ages from volcanic rocks interbedded with uppermost strata of the Nangqian basin, and 51–49 Ma 40Ar/39Ar ages from igneous rocks that intrude and unconformably overlie strata of the Shanglaxiu basin. Strata containing middle Cretaceous palynomorph and ostracod assemblages are present only locally in the lowermost part of the Nangqian basin. Although the tectonic setting for Cretaceous sedimentation is unclear, early Tertiary basin development was controlled by thin-skinned fold-thrust deformation. We interpret the narrow widths of Paleocene–Eocene basins to be a result of thrust spacing, which in turn may have been controlled by the depth to the decollement (∼5 km deep according to our balanced cross section) from which imbricate thrusts ramped up through the Carboniferous–Triassic section. Sedimentologic and provenance evidence for internal drainage, limited unroofing, and relatively low average sediment-accumulation rates in these syncontractional, plateau-interior basins indicates generally small drainage systems, short main-stem rivers, shallow regional slopes, and limited denudation in east-central Tibet during early Tertiary time. Such geomorphic conditions, which are similar to the modern low-relief interior of the Tibetan plateau, suggest that the deeply incised Mekong and Yangtze Rivers of eastern Tibet were not established until after the termination of Paleogene basin development in the region.

Upper Rhine Graben: Role of preexisting structures during rift evolution

The evolution of the Cenozoic Upper Rhine Graben was controlled by a repeatedly changing stress field and the reactivation of a complex set of crustal discontinuities that had come into evidence during Permo‐Carboniferous times. A comparison of the spatial and temporal thickness distribution of synrift deposits with preexisting fault patterns permits to infer a sequence of distinct basin subsidence phases that can be related to changes in the ambient stress field. Reactivation of a system of late Palaeozoic fault systems, outlining troughs and highs, controlled the nucleation of initially separated middle and late Eocene basins, the depocenters of which coincided with a preexisting WSW‐ENE trend. During Oligocene crustal extension the individual basins coalesced, resulting in the development of the SSW‐NNE striking Upper Rhine Graben. During the late Oligocene (Chattian) change in stress field, the Upper Rhine Graben was probably reactivated as a dextral strike‐slip system with the central graben segment forming a releasing bend. During the early Miocene (Aquitanian), a major reorientation of the regional stress field is held responsible for the main subsidence phase of the northern parts of the Upper Rhine Graben. This is reflected by a counterclockwise rotation and northeastward shift of the depocenter axis and later by the middle Miocene uplift and erosion of the southern parts of the Upper Rhine Graben. During the Plio‐Quaternary, the Upper Rhine Graben was reactivated as a sinistral strike‐slip system with the central graben segment forming a restraining bend.

Magnetic lineations constraints for the back-arc opening of the Late Neogene South Banda Basin (eastern Indonesia)

The South Banda Basin is located within eastern Indonesia near the triple junction between the Eurasian, Pacific and Indo-Australian plates. It is underlain by oceanic crust, but its origin and age were not well established. It has been interpreted as a Mesozoic trapped piece of Indian ocean or as a Cretaceous–Eocene basin related to the Celebes and Sulu basins, but a Neogene back-arc origin was also considered. Recent geochemical and geochronological studies strongly support the latter hypothesis.In this paper we present a new analysis of the magnetic data from the eastern part of the South Banda Basin, the Damar Basin. We used magnetic field measurements collected during nine oceanographic cruises from various institutions. Looking for magnetic correlation in the time span given from recent geochronological data, the comparison between measured profiles and theoretical profiles deduced from the reversals of Earth's magnetic field during Neogene time allows us to infer an opening of South Banda Basin during Late Miocene–Early Pliocene time, from 6.5 to 3.5Ma. Magnetic lineations 2An, 2Ar, 3n, 3r, 3An and possibly 3Ar are recognized, with an extinct spreading centre trending ENE–WSW. At least five segments are identified, each segment being separated by inactive transform faults perpendicular to the extinct spreading centre. A half spreading rate of about 3cm/yr is calculated, based on spacing of magnetic lineations.The opening history of the basin is discussed. The cause of cessation of spreading is likely the arc-continent collision dated at about 3Ma. However the onset of opening is less determined. We suggest that Damar Basin began to open at about 6.5Ma during magnetic period 3An as an intra-arc basin, separating the Banda volcanic arc to the south from the incipient Lucipara volcanic arc to the north. The latter was probably created at the beginning of rifting of Damar Basin, as shown by both magnetic and geochronological data.The young age of the South Banda Sea Basin is contradictory with its great depth. We discuss this problem in view of thermal and tectonic considerations. We finally conclude that our magnetic model allows us to more precisely describe the opening of the basin.

Formation and evolution of the Solander Basin, southwestern South Island, New Zealand, controlled by a major fault in continental crust and upper mantle

Seismic reflection and refraction data from the Solander Basin, southern New Zealand, show that its structural evolution has been controlled by a major fault, named here the Tauru Fault, that cuts the entire crust and splays into a diffuse zone in the upper mantle. The tectonic setting of the Solander Basin has evolved from Eocene‐Oligocene extension and transtension to Miocene‐Quaternary transpression and subduction. The Tauru Fault is 100 km east of the active Puysegur subduction zone thrust and is part of the overriding plate. On the basis of lower crustal reflectivity, the base of the crust beneath the adjacent Stewart Island shelf is at ∼30 km depth (∼9 s two‐way time (TWT)), and rises to ∼20 km (∼8 s TWT) beneath the Solander Basin. This is consistent with gravity data. Prominent dipping reflections show that the Tauru Fault can be traced to ∼30 km depth (∼12 s TWT), where it merges with a zone of subhorizontal reflectors in the upper mantle. The Tauru Fault dips ∼30° northeast and appears to offset the Moho in a reverse sense. Stratigraphic relationships show that the Tauru Fault was active as a normal fault during Eocene extension, when Solander Basin crust was thinned and ocean crust was generated farther south in the Solander Trough. It has been reactivated as a reverse fault during at least two phases of Miocene‐Quaternary compression and is still active. The strike of the Tauru Fault, which is parallel to Paleozoic‐Mesozoic structures and was poorly oriented for the known Eocene extension direction, strongly suggests that it formed prior to Eocene time. The Tauru Fault significantly influenced the geometry of Eocene basin formation, producing a strongly asymmetric basin dominated by east dipping normal faults, with a single eastern boundary fault. Our data demonstrate that Miocene‐Quaternary simple shear associated with the Tauru Fault cuts the whole crust and continues into the upper mantle. We conclude that variations in strength of the lithosphere, particularly associated with inherited structures in the crust and upper mantle, may control many aspects of basin development, passive margin formation, and the kinematics of continental deformation zones.

The transition from Tethys to the Himalaya as recorded in northwest Pakistan

Early Cenozoic sedimentary rocks exposed in the Kohat Plateau of northwestern Pakistan record tectonic closure of the Tethys sea and development of a restricted marine basin that formed during Himalayan collision between India, Asia, and a series of microplates. In the Paleocene, initial subsidence of the basin was caused by the downward deflection of the Indian plate in response to loading of the Asian plate. The western margin of the early Eocene basin was dominated by deposition of shale, sandstone, and conglomerate derived from microplates located to the north and west of the Indian continental margin. The eastern margin of the basin was a carbonate shelf and sabkha flat. Salt deposition occurred subaqueously in the central parts of the basin. In the late early Eocene, redbeds derived from the northwest were deposited by a fluvial and/or deltaic system. This influx of clastic sediments marks the earliest record of terrestrial foreland-basin deposition in northwest Pakistan. During the middle Eocene, the basin was reflooded and a carbonate shelf developed. Relative sea-level rise may reflect subsidence of the Indian plate in response to continued crustal loading in the Himalayan suture zone. Uplift and erosion occurred between the late Eocene and Miocene, possibly related to a peripheral bulge south of the Himalayan suture zone. The main phase of Himalayan foreland-basin fluvial deposition began in the Miocene. Renewed uplift related to final collision of India and Afghanistan during the Pliocene is recorded by a thick sequence of conglomerate in the western Kohat Plateau. Correlation with Eocene sedimentary rocks from southern Pakistan to northern India delineates the depositional systems that developed as the Tethys sea closed during Himalayan convergence.

Coal geology, chemical and petrographical characteristics, and implications for coalbed methane development of subbituminous coals from the Sorgun and Suluova Eocene basins, Turkey

Abstract The Sorgun and Suluova basins contain a thick and laterally extensive Lower Eocene coal seam. The stratigraphy of both basins comprises basement rocks, the Yozgat Granitoid of Pre-Eocene age in the Sorgun basin and Jurassic-Cretaceous limestones in the Suluova basin, overlain unconformably by the coal-bearing Lower Eocene Celtek Formation and by transgressive units of Middle-Upper Eocene age. In the Sorgun basin the Artova Ophiolitic Complex tectonically overlies the transgressive units. Neogene and Quaternary deposits cover all the older units unconformably in both basins. The single coal seam is located at the base of the Celtek Formation and reaches a thickness of 6 m in the open-cast mine at Sorgun and 8 m in boreholes in the Suluova basin. Some coal is produced by opencast mining, but most is produced by underground mining methods. A total of 75 coal samples was collected from both basins and analysed chemically and petrographically. The coals are characterized by a relatively low moisture content (0.7–9.3 wt% on an air-dried basis) and variable ash yields (2.4–67.8 wt% on an air-dried basis) and huminite contents (28.4–89.6 vol%). Measurements of the mean random reflectance of ulminite (mainly eu-ulminite A) vary between 0.46 and 0.60% (0.53% average). These reflectance values and the chemical analyses show that the rank is subbituminous A or transitional to high-volatile bituminous C coal according to the ASTM classification. The coalbed methane potential of both basins is at present unknown. No borehole has been drilled to explore for this resource and no gas capacity value has been obtained for the coal seam and bituminous shales. The methane explosions which have occurred during coal production in the underground mines imply that both basins may have been an important potential for coalbed methane.

Rotational deformation during Palaeogene thrusting and basin closure in eastern central Greece: palaeomagnetic evidence from Mesozoic carbonates

SUMMARY Palaeomagnetic studies in Greece have concentrated upon Neogene sequences, with Mesozoic rocks of mainland Greece receiving little attention. The Neogene data base is now sufficiently extensive, however, to allow meaningful interpretation of older units. This paper presents new palaeomagnetic data from Mesozoic and Tertiary sequences of eastern central Greece, which document a pre-Neogene phase cf rotational deformation. Mesozoic carbonates have been sampled within the classic Pelagonian Zone in the Argolis Peninsula, Beotia and Evvia. Pelagonian sequences record the development, drowning and subsequent re-establishment of a carbonate platform separating two strands of the Neotethys Ocean (the Pindos and Vardar basins). Most platformal facies are tdo weakly magnetized for palaeomagnetic analysis. However, pelagic horizons consisting of pink condensed limestones have been deposited periodically in response to platform subsidence. These Middle-Late Triassic, Jurassic and Upper Cretaceous sequences yield high-quality palaeomagnetic data, which reveal a consistent clockwise rotation of about 94. The constancy of rotation angle deduced from the Mesozoic rocks suggests that the total Tertiary rotation is uniform throughout the area studied. This implies that observed variations in declination within the Neogene sequences result from sampling of rocks of different ages deposited during a period of rapid overall rotation. Removal of the largest Neogene rotation (45) from the Mesozoic data set reveals a pre-Middle Miocene, post-Late Cretaceous clockwise rotation of the southern Pelagonian Zone of 50. The major tectonic event during this time period was the closure of the Pindos Ocean during the Eocene. A large carbonate platform within the Pindos basin, now preserved in the Parnassos Zone, presented a major obstacle to the advancing Pelagonian nappes during SW-directed thrusting (present coordinates). Impingement of the nappes on to the Parnassos platform resulted in pinning and subsequent clockwise rotation of thrust units around the south-eastern margin of the platform. The southernmost tip of the Pelagonian Zone (southern Argolis) was detached along a pre-existing line of weakness along the present Migdhalitsa Graben, and experienced a different rotation history. Additional palaeomagnetic data from the Pindos nappes in the eastern Peloponnesos also provide evidence for significant rotation of thrust sheets during Eocene basin closure. The Pindos sequences formed by pelagic sedimentation within the Pindos Ocean. An overall anti-clockwise rotation with respect to the underlying Gavrovo-Tripolitza autochthon is implied by the data. This probably reflects oblique emplacement of the Pindos thrust sheets on to the adjacent platform. Variations in rotation angle probably result from variations in footwall topography.

Modeling the sulfur and oxygen isotopic composition of sulfates through a halite-potash sequence: Implications for the hydrological evolution of the upper eocene Southpyrenean basin

The isotopic composition of sulfates sampled throughout a complete evaporite sequence has contributed to the understanding of the evolution of the Southpyrenean Upper Eocene basin. The δ 34S and δ 18O values appear to be constant in thick segments of the sequence. This pattern is clearly different from the continuous decrease in δ values predicted by the evaporation processes in a closed system. The observed trend of the δ 34S and δ 18O values can be successfully modeled by an evaporation process in an open system, where steady state conditions are reached, separated by periods of significant changes in the hydrological regime of the basin. Moreover, in the segments of the sequence where they are constant, the δ 34S and δ 18O values help to constrain the hydrological parameters of the model. A first marine stage of the evolution of the basin corresponds to the formation of the Basal Anhydrite and Lower Halite Units. The mineral sequence corresponds to evaporation in a basin with a progressive degree of restriction and evaporation compensated by equal inflow of major marine origin. The sharp decrease in the δ 34S and particularly the δ 18O values, recorded at the end of the Basal Anhydrite Unit, illustrates a limited reservoir effect produced by the increase of restriction leading to the halite precipitation. The modeling of the constant isotopic values through the Lower Halite Unit confirms the isotopic composition of Eocene marine dissolved sulfate (δ 34S = +20.0%0; δ 18O = +8.7%0) deduced from the average values of marginal gypsum. The second continental stage of basin evolution corresponds to the formation of the Potash Unit and the Upper Halite Unit. The mineral sequence and lateral extension indicate that the basin evolved with a decreasing brine volume after disconnection from the sea. The dilution of the residual brine stopped the precipitation of camallite, and the basin then evolved as a perennial endorreic lake with continental recharge replacing evaporation. The sharp decrease in the δ 34S and δ 18O values in the sylvite member is consistent with the massive precipitation of sulfate occurring during the decrease in the volume of water. The δ values of the sulfates intercalated in the camallite and the Upper Halite Unit indicate the influence of the recycling of both the already deposited Upper Eocene sulfates and the Upper Triassic sulfates carried by continental recharge. The constant δ 34S and δ 18O values from the uppermost half of the Upper Halite Unit is used to constrain the sulfate source as 80% recycled Upper Triassic and 20% recycled Eocene gypsum.

The chemical and hydrological evolution of an ancient potash-forming evaporite basin as constrained by mineral sequence, fluid inclusion composition, and numerical simulation

The chemical evolution of the brine in a potash evaporite basin has been investigated by X-ray microanalysis of frozen primary inclusions trapped in halite. A Computer program based on thermodynamic equilibrium and mass balance principles has been used to simulate evaporation paths. The comparison between the results of calculations, the observed mineralogy and mineral sequence, and the solute content in fluid inclusions has placed constraints on the hydrological evolution of the basin. The upper Eocene basin of Navarra, southern Pyrenees, Spain, began as a marine basin, evolving from a moderate to a high degree of restriction, depositing first a basai anhydrite horizon, and then a thick sequence of massive halite. An additional inflow of CaCl 2 in the basin during seawater evaporation is proposed as the process responsible for the sulfate depletion required for sylvite instead of Mg-sulfates to form. Mixing of seawater with continental waters, bacterial sulfate reduction and “in situ” dolomitization are discarded. The basin subsequently closed to the sea and evolved with decreasing volume. Alternating bands of clays-halite-sylvite and then clays-halite-carnallite were deposited under the influence of seasonal continental recharge. Before reaching total desiccation the residual brine was diluted by continental water. The basin then evolved under an endoreic regime, where continental recharge and the recycling of previously-formed halite led to deposition of alternating beds of clays and halite.

The Boscan Field -- Venezuela's Giant Stratigraphic Trap With A Complex History

The Boscan Field lies in the northwest quadrant of the Maracaibo Basin, Venezuela. It is a large stratigraphic trap that has produced more than 800 million barrels of heavy oil since 1946, principally from Eocene fluvial-deltaic clastic reservoirs. These are faulted, folded, and are truncated by a pre-Oligocene unconformity. The geochronology of the Boscan Field has been interpreted as follows: (1) Eocene fluvial-deltaic deposits prograded east and southeast across the Boscan Field Area. A subsiding area northeast towards the State of Falcon received tremendous quantities of Eocene fine grained organic-rich deposits in pro-delta and deep water environments. (2) The Eocene shales in the deep basin began to generate hydrocarbons which migrated towards the west and southwest. (3) In the Boscan Area, Eocene deposits were folded, faulted, and tilted towards the east and southeast and an active fluvial flood-plain truncated the deposits resulting in subcrop bands that form the pre-Oligocene unconformity topographic surface. (4) Hydrocarbon generation and migration continues in the deep basin. (5) Oligocene fluvial sandstones deposited on the underlying unconformity often cut into and are in direct contact with truncated Eocene sandstones. This allowed some oil to leak up into the Oligocene deposits. (6) By the end of themore » Oligocene, migration of oil into the gross Boscan trap was well underway. The east-west and northwest southeast trending fluvial conduits carried the oil out of the Eocene basin northwestward until it was blocked by reservoir truncation on the west and northwest sides of the Boscan Field. (7) Regional subsidence of the Maracaibo Basin has caused the Boscan reservoirs to dip towards the southwest resulting in realignment of fluids. The present-day Boscan Field is a composite of stacked, linear, faulted, and truncated reservoir segments. They may be isolated or interactive with other segments and form a complex [open quotes]plumbing[close quotes] tangle.« less

Origin and Evolution of the Tertiary Hydrocarbon-Bearing Basins in Kalimantan (Borneo), Indonesia (1)

During the middle Eocene, a large extensional basin formed in Kalimantan and began to be filled by transgressive middle Eocene and lower Oligocene nonmarine and shallow marine clastics, carbonates, and deep marine clastics. This episode was followed by regressive late Oligocene-Miocene deposition. Oligocene tectonism resulted in uplift, erosion, and structural segmentation of the large Kalimantan basin into several smaller separate basins. Deltaic sedimentation began in the latest Oligocene in the upper Kutei basin and prograded eastward, so that by the end of the early Miocene, deltas were near the present Kutei coast, where deltaic sedimentation continued to the present. Lower(?)-middle Miocene deltaic sediments also filled the Barito, Asem Asem, and Pasir basins and we e probably contiguous with those of Kutei. A separate Miocene deltaic depocenter developed in the Tarakan basin (northeastern Kalimantan). Carbonate sedimentation prevailed in the shallow areas in between these deltas, and shales were deposited in the deep basins. Eocene basin inception and Oligocene tectonism are interpreted to have resulted from plate rearrangements in southeast Asia. Eocene plate rearrangement is associated with the collision of India with Asia, and the middle Oligocene tectonism and magmatism are interpreted to be due to the collision of northern Australia with island arcs and opening of the South China Sea. Inversion of the upper Kutei basin and uplift of the Meratus Mountains started in the early part of the middle Miocene and are related to a third major plate readjustment in southeast Asia. Regionally synchronous Miocene-Pliocene tectonic phases in the region are probably related to collisions of microcontinents along Sulawesi and resulting strike-slip faulting extending into Borneo. Original recoverable hydrocarbon reserves from the Eocene of the Barito basin are 180-200 million bbl; 5 billion bbl of oil and oil-equivalent gas may be recoverable from Miocene deltaics in the lower Kutei basin, and 200 million bbl oil from those of the Tarakan basin.

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.

Oceanic crust of the Grenada Basin in the Southern Lesser Antilles Arc Platform

Seismic refraction data permit the southern Lesser Antilles arc and surrounding regions to be divided by the velocity of their basement. We propose that high‐velocity basement of the arc platform beneath the Grenadine islands and below a part of the Tobago Trough forearc basin is oceanic and continuous and was originally connected with oceanic crust of the Grenada Basin. Low‐velocity basements of the Tobago terrane and the arc platform from St. Vincent north lie south and north, respectively, of the high‐velocity basement of the arc platform. An oceanic origin of this high‐velocity crust in the Grenadines is argued to be more plausible than an origin as unroofed lower arc crust. The segment of probable oceanic crust in the arc platform was greatly uplifted during development of the present island arc, mainly in late Neogene time, relative to the Grenada Basin and Tobago Trough. Accepting the proposition of shallow oceanic crust in the Grenadines, early middle Eocene and possibly older pillow basalts of Mayreau, the oldest rock unit of the southern Lesser Antilles arc platform, may be an exposure of such basement. Major and minor element compositions of Mayreau Basalt are indicative of a spreading rather than arc origin. The stratigraphy of the pillow basalts indicates extrusion in an open marine environment, distant or shielded from sources of arc or continental sediment, followed by a period of pelagic sedimentation above the carbonate compensation depth. The Eocene basalt and pelagic cover formed a relatively deep floor of a marine basin in which arc‐derived turbidites and pelagic sediments accumulated over the succeeding 25–30 ma. Such basalts thus indicate a probable spreading origin of the Grenada Basin and an age of cessation of spreading in the region of Mayreau in Eocene time. The configuration of the Eocene basin and the direction of spreading, however, are unknowns. Regional structural relationships imply the spreading was probably backarc, an origin also hinted at by Ce/Zr ratios of Mayreau Basalt. Although the Aves Ridge probably was the remnant arc, the existence and position of an early middle Eocene frontal arc are unresolved. The early Paleogene tectonic architecture of the southeastern Caribbean differed substantially from that of the Neogene.

Jurassic through Oligocene Pre-Basin Stratigraphy in the Santa Maria Basin Area, California

Compilation from published records of 30 pre-Miocene stratigraphic columns in the Santa Maria basin area of California (west of the Sur-Nacimiento fault and north of the Santa Ynez fault) reveals two basement units and 22 overlying sedimentary units. This article displays the stratigraphic columns and includes descriptions and environmental interpretations of the 24 rock units. The basement rocks include an Upper Jurassic ophiolite sequence and the Lower Jurassic through Upper Cretaceous Franciscan Complex. Most of the 22 sedimentary units were deposited along a subduction-type margin prior to development of the late Tertiary Santa Maria basin. Overlying and generally in fault contact with the basement rocks are four Upper Jurassic through Lower Cretaceous units that were deposited in basin plain and out continental margin environments. Unconformably overlying these units are eight Upper Cretaceous units that were deposited in a wide range of environments that ranged from trench, slope, and submarine fan up through shelf and nonmarine fluvial environments. Lower Tertiary units onlap unconformably onto the Cretaceous rocks and were deposited only in the southernmost part of the area. These rocks include lower Eocene basin plain and outer submarine fan deposits; middle Eocene mid-fan and slope deposits; upper Eocene inner fan, shelf,more » shoreface, and foreshore deposits; and Oligocene shoreface, foreshore, and nonmarine fluvial deposits.« less

Eocene sedimentation and volcanism in the Fig Lake Graben, southwestern British Columbia

The Fig Lake Graben is a narrow, complex Eocene basin that developed along part of the Coldwater fault system in southwestern British Columbia. Its origin as a pull-apart basin is probably related to dextral wrench faulting along the Fraser Fault and low-angle normal faulting of the Okanagan shear zone. Within the graben are Kamloops Group volcanic and sedimentary rocks, the thickness of which implies that one fault block has been downthrown at least 4.5 km. Geochemical interpretation of previously published analyses of Kamloops Group volcanic rocks indicates that magma production was genetically related to both extension and subduction.

Present and recent sedimentation in the Loyalty Basin along the Thio-Lifou profile (New Caledonia, Southwest Pacific)

The Loyalty Basin, northeast of New Caledonia, is about 100 km wide and 2350 m deep in the transect between Thio (New Caledonia) and the island of Lifou. It is bounded to the southwest by New Caledonia and its surrounding barrier reef and to the northeast by the barrier reef of the Loyalty Ridge. Lifou Island, a raised atoll, is an emerged part of the Loyalty Ridge. The sediments cored in the Loyalty Basin during the BIOCAL mission in 1985 are bioturbated and composed of an alternation of turbidites and hemipelagites. The hemipelagites are brown-yellowish muddy oozes to calcareous muds; the turbidites are composed of sandy and silty oozes capped transitionally with muddy oozes. The hemipelagite and the T e division of the Bouma sequence have different characteristics but are not located in the core. However, they can be distinguished by carbonate composition and grain size. All the sediments originate from five sources: (1) New Caledonia (quartz, mud and heavy minerals), (2) the barrier reef (fragments of algae, madreporarian corals, foraminifera and mud of the periplatform, (3) plankton (foraminifera, coccoliths and pteropoda), (4) the bathyal community and (5) pyroclastic fragments. Turbidites, distributed in cores between Thio and Lifou, constitute a low-profile submarine fan fed by the main canyons of the slope opposite the Thio Pass. The fan developed from a rim of slumps and mud flows located near the base of the slope. The fan extends 50 km toward the northeast as far as the tectonic Lévi Ridge. The hemipelagites are predominant beyond this ridge. The modern sedimentation rate is 60 m/Ma on the proximal fan. It is 4–6 times less than the average sedimentation rate calculated for the period since the origin of the basin in the late Eocene.

Elemental chemical composition of the bivalvian shells from Gebel Mokattam, Cairo, Egypt, and their significance in classification and paleoecology

The Mg, Al, Si, Mn, Fe and Sr contents of 124 bivalvian shells from Gebel Mokattam, Cairo, Egypt, were determined by means of quantitative spectral analysis. The concentrations of Sr, Mn, Fe and Si increase from Middle to Upper Eocene, and vice versa with respect to Mg content. The concentration of Al is nearly stable in both the Middle and Upper Eocene bivalvian shells. This study showed that the Upper Eocene basin in Gebel Mokattam was warmer, shallower and more saline than that of the Middle Eocene. The postdepositional changes of the studied faunas were interpreted. Finally, an attempt was made to classify four of the studied families according to their chemical composition.

Iberian-Europe Convergence: Evolution of the Cretaceous and Eocene Basins in Pyrenees and Provence: ABSTRACT

During Cretaceous time the geodynamic evolution of Northern and Western Pyrenean basins was related to scissors-shaped rifting which evolved as a passive margin filled by thick flysch deposits. In Provence, the carbonate platform was marked since the late Albian by the arrival of significant detrital flows originated from an uplifted Paleozoic block situated in the Gulf of Lion. In Provence the northward migration of the basin from Cenomanian to Eocene and Oligocene indicates the growing of the Gulf of Lion-South Provence crustal uplift and its northward displacement. The Cretaceous opening of the western Pyrenean, Parentis, and Bay of Biscay basins is synchronous with the first stages of compression in the Gulf of Lion. These features are induced by the rotation of Iberia. During the Eocene the compression, resulting from the Iberian-Europe convergence, affected nearly the whole Pyrenean-Provencal area. In the southern part of the Pyrenees east of the Pamplona fault, the successive dislocations of carbonate platforms, migration of reefs, and filling of foreland basins became the signature of the intracontinental subduction of Iberia. The transform fault pattern, still well preserved in spite of the Eocene compression, prevents any important strike-slip movement between Europe and Iberia, especially along the so-called Northmore » Pyrenean fault zone, which shows several discontinuities in the western part of Pyrenees. The final evolution of Gulf of Lion crustal uplift generated a gliding of its cover (Provence overthrusts) and, during Oligocene, the opening of the Ligurian-Provencal basin by a propagating rift process.« less