Oligocene Sequences Research Articles

Salt sheet extrusion and emplacement within the South-Central Pyrenean fold-and-thrust belt: the Les Avellanes Diapir case-of-study

ABSTRACT A detailed geological map of the Les Avellanes salt Diapir (South-Central Pyrenees, Spain) that includes both the diapir body and adjacent areas is presented to understand the diapir evolution and geometry. Structural, stratigraphical, and sedimentary data north and south of the diapir is used to infer the timing of its emplacement. The northern diapir boundary is characterized by a set of extensional faults oblique to the main Pyrenean trend, while the southern boundary is an extrusive salt sheet that overlays the late Eocene-early Oligocene sequence in three adjacent sub-basins. Salt extrusion occurred due to synorogenic folding. The topography created as salt extruded trapped the arrival of external sediments from the north, blocking the transport pathways southward. Low sedimentation rates southwards allowed for the lateral salt extrusion, advancing southwards from the feeder. The salt sheet emplacement was postdated by Oligocene conglomerates, indicating that the salt extrusion was a relatively quick event.

Benthic foraminiferal biostratigraphy of the Oligocene Sequence of the Asmari Formation in the Interior Fars Sub-Zone, Zagros Belt, SW Iran

Benthic foraminiferal biostratigraphy of the Oligocene Sequence of the Asmari Formation in the Interior Fars Sub-Zone, Zagros Belt, SW Iran

Interaction between salt and mobile shale in the East Breaks foldbelt, northwestern Gulf of Mexico

The East Breaks foldbelt comprises a series of dominantly slope-parallel folds and thrusts detached on a mostly evacuated salt canopy on the northern Gulf of Mexico slope. The canopy was emplaced near the end of the Eocene. The foldbelt formed mostly in the Oligocene, although minor amounts of shortening continued through the Miocene. We attribute the anomalous trend of structures in the foldbelt to convergent gravity gliding produced by the bend in topography in the northwest corner of the Gulf of Mexico. We suggest that convergent deformation was localized in the East Breaks foldbelt because the area comprised a shale-dominated slope between two sand-prone slope aprons during the Oligocene. These aprons were stronger than the shalier East Breaks area in between, allowing convergent deformation to be partitioned into weak shales.The fact that the Oligocene sequence in the area is shale prone means that shortening was accommodated on multiple detachment levels with fishtail thrusts, as well as in mobile shales, which occur alongside salt diapirs sourced from the canopy. The coincidence of salt structures and mobile shales allows a comparison of their behavior during shortening. Mobile-shale deformation is most intense near a minimally shortened salt massif, leading us to conclude that salt behaved more competently than mobile shale during shortening. This conclusion is counter to the general assumption that salt, where present, is the weakest part of any geologic system.

Lithostratigraphy of a long, fossiliferous Oligocene sequence: Revisiting Saint Jacques, Nei Mongol, China

For a hundred years the Saint Jacques area has been known to produce rich Oligocene vertebrate fossils, yet only a handful of previous studies have focused on this area. Since 2010, we have conducted 12 field expeditions to Saint Jacques, and here we report findings from our paleontological excavations and stratigraphical investigations. Twenty-two fossiliferous blocks across the area are recognized and a chronostratigraphic framework has been established to aid fossil collection. Fossil-mammal materials have been recovered in situ from 1635 localities and additionally from surface sediments. Fossiliferous blocks in the area are correlated by lithological similarity and lateral tracing. Lithologically, the area is mainly composed of reddish silty mudstone and muddy siltstone, with three distinctive layers of grayish white sandstone. The measured composite stratigraphic column spans 239 meters and are divided into 12 lithostratigraphic units. Contrary to previous knowledge that Saint Jacques contains two Oligocene mammalian assemblages, our preliminary biostratigraphic analysis of small mammals shows that the area documents successive faunal transition from the Eocene to possibly the early Miocene. The hyracodontid perissodactyl Ardynia, the ctenodactyloid rodent Gobiomys, and the basal Glires Gomphos from the bottom litho-units imply the presence of the Eocene–Oligocene boundary, while small mammal assemblage of the top units is similar to Miocene faunas in northern China and Mongolia. Thus, rock strata in Saint Jacques likely span the Eocene through the early Miocene, bracketing an entire Oligocene sequence within. In sum, our re-exploration of Saint Jacques has greatly expanded the chronostratigraphic and taxonomic coverage of the mammalian fossil collection from this area. This long, successive Oligocene sequence makes an important record for studying the Eocene–Oligocene Transition. Further study in this area will contribute to a range of paleontological and paleoenvironmental questions.

Oligocene moisture variations as evidenced by an aeolian dust sequence in Inner Mongolia, China

The aridification of Central Asia since the Eocene has widespread evidence, but climate-controlled environmental reorganizations during the Oligocene remain ambiguous. We employed environmental magnetic, mineralogical and geochemical methods on a latest Eocene to late Oligocene terrestrial sequence in Inner Mongolia, China, to examine how global climatic trends and regional factors influenced the evolution of moisture and weathering in the region. Highlighting the climatic influence, our weathering and rainfall proxy data document the drawdown of atmospheric CO2 and global cooling during the early Oligocene semi-arid phase, which culminated in the Early Oligocene Aridification Event at 31 Ma. Moreover, for the first time in the terrestrial eastern Central Asian setting, we provide geochemical and geophysical evidence for a second major Oligocene aridification event nearly synchronous to the mid-Oligocene Glacial Maximum at around 28 Ma. These aridification events were interrupted by periods of increased rainfall and weathering and can be associated with the terminations of glacial events seen in marine oxygen isotope records.

Cenozoic Marine Basin Evolution in the Western North Aegean trough Margin: Seismic Stratigraphic Evidence

This study investigates the interplay of evolving tectonic and submarine sedimentation processes in the northwest Aegean Sea using marine multichannel seismic profiles. We identify an extensive basin developing in the Thermaikos Gulf inner shelf, outer shelf, and slope leading to the 1500 m deep West North Aegean Trough (NAT). We establish the unconformable extent of Eocene and Oligocene sequences on the upper shelf and trace their continuation in the deeper shelf and slope of Thermaikos Gulf. The start of the Miocene and Middle Miocene developed below the well-established Messinian bounding reflectors that are mostly erosional. Important lateral variations are observed within the Messinian sequence, which is up to 0.8 s thick. Messinian prograding clinoforms are identified on the Thermaikos Gulf shelf and southeast of Chalkidiki, and a zone of irregular reflectors is attributed to the Messinian salt layer. The transpressional deformation of the Messinian in the southwestern margin constrains the timing of westward progradation of the North Anatolian Fault during Messinian. The Pliocene-Quaternary sediments are 0.6–1.8 s thick, showing the overwhelming effect of tectonics on sedimentation plus the northwards Quaternary activation at the Thermaikos apron.

Oligocene Planktic Foraminiferal Taxonomy and Evolution: An Illustrated Revision of Ocean Drilling Program Site 803

ABSTRACT The Oligocene (33.9–23.0 Ma) has historically proven to be a difficult interval to examine with respect to planktic foraminifera; the tendency for many of the taxa to be basically globigerine in shape, with 4 or 5 chambers in the final whorl means differences between species are limited. Recently, an international working group has attempted to clarify the Oligocene planktic foraminiferal taxonomy, with the goal of establishing phylogenetically-consistent generic and species concepts. A relatively expanded and continuous Oligocene section recovered at Ocean Drilling Program Site 803 in the western equatorial Pacific was previously studied by Leckie et al. (1993) using fairly conservative species concepts. Since 1993, foraminiferal biostratigraphic datum age calibrations have changed, and so revised sedimentation rates for the 220-m thick Oligocene sequence are actually more constant than previously thought. As a part of the recent taxonomic revision, this site was reevaluated and numerous additional taxa are recorded at this location. Macroevolutionary rates are calculated from the occurrences, and increased extinction is found within the late Oligocene, counter to the expectations laid out in broader-scale macroevolutionary studies. An effort is made here to describe the diagnostic features, which can be used to distinguish all taxa under a standard binocular microscope. Finally, several figures of scanning electron microscope photomicrographs (from Site 803 and tropical Atlantic Ocean ODP Site 628) depict features used to describe and differentiate important, but difficult or homeomorphic taxa, with the hope that these figures can be used by other workers at the microscope attempting to do Oligocene taxonomy-based studies.

Geological history, Ichnofacies and Sequence Stratigraphy of the Eocene – Oligocene Boundary at Sinjar Area, Northwestern Iraq

Three outcrop sections for the Eocene- Oligocene boundary have been studied in Sinjar anticline northwestern Iraq. This study includes the lithostratigraphy, biostratigraphy and the identification of trace fossils and related ichnofacies in order to report the geological history of this boundary in the frame of sequence stratigraphy. Jaddala Formation, which is composed of marl and marly limestone, represents deep (outer shelf-upper bathyal) Eocene facies, Palani and Tarjil formations represent deposition in deep basinal environments during Early and Late Oligocene sequences. The Eocene- Oligocene boundary in Sinjar area represents a disconformity surface, where the deposition ceased in the Middle Eocene (Lutetian) at the end of Jaddala Formation, forming the firmground Glossifungites Ichnofacies which embrace Thalassinoides ichnotaxa in Jaddala section, and the hardground Trypanites Ichnofacies in Sharafiddin section, while at Sinjar section, a chalky limestone bed deposited at the Early Oligocene (Rupelian) Palani Formation representing Skolithos Ichnofacies. A rapid wide transgression took place at the end of Early Oligocene (Rupelian) leading to the deposition of Palani Formation, and continued through the Late Oligocene (Chattian) leading to the deposition of Tarjil Formation.

Sequence stratigraphic framework of the Upper Eocene–Oligocene succession, northwest Birket Qarun, Fayum-Egypt

The Upper Eocene–Oligocene sequence exposed northwest Birket Qarun; Fayum district covers two rock units, Qasr El-Sagha (Temple and Dir Abu Lifa members) and Gebel Qatrani formations were re-evaluated by sequence stratigraphy. Six depositional sequences were recognized based on facies geometry, and sedimentological aspects. The first sequence (Temple Member) was accumulated under marginal intertidal flat environment indicating highstand deposits. The second (the lower Dir Abu Lifa Member) displays a regressive sedimentary facies starting with progradational delta front environment reflecting lowstand deposits followed upward by a lagoonal environment indicating transgressive/highstand deposits. The third (the top of the Dir Abu Lifa Member) was accumulated under a low sinuosity stream reflecting lowstand deposits. The fourth which constitutes the topmost of Dir Abu Lifa Member initiating near-shore shallow marine points to transgressive sediments, followed upward by fluvial sediments reflecting highstand deposits. The fifth (the Lower of Gebel Qatrani Formation) accumulated as point-bar deposits demonstrating lowstand deposits. The last occupies the main of Gebel Qatrani Formation initiates with restricted riverine marking lowstand deposits, followed upward by several fining upward cycles that were accumulated in fluvial and flood plain conditions with evidence of tidal influence on sedimentation near its top denoting transgressive deposits. Several minor transgressive episodes accompanied by tidal influence on sedimentation are noticed near the top, which may reflect gradual subsidence of the Oligocene alluvial plain. The area is subjected to relative sea-level changes that show a similarity with the global sea level. The base-level fluctuations are mainly controlled by the regional tectonic setting.

Origin of hydrocarbons in the Bach Ho field (The Vietnamese shelf)

This article presents the results of a geochemical study of oil on the Vietnam shelf (Cuu Long basin), including those in the crystalline basement. The Bach Ho field oils in the basement have a hydrocarbon distribution that is no different than oil of numerous accumulations in Oligocene and Miocene sedimentary sequences. Similar to the organic origin oils of the world, oils from the Bach Ho field lack regular isoprenanes С12 and С17 and cheilanthanes (tri-cyclic terpanes) С22 and С27. A distinctive feature of these oils is a large amount of cheilanthanes С19-С29, and large neo-adiantane to adiantane and hopanes to steranes ratios. All these parameters indicate a large bacterial contribution in the generation of these oils. Studies have shown the similarity between oil biomarker parameters and the organic matter of sedimentary rocks, which supports the organic nature of the oils in the basement fields on the Vietnam shelf. It is shown that the hydrocarbon accumulations in the basement complexes of the Cuu Long basin are in a secondary occurrence, and their origin was the organic matter of the sedimentary source rocks.

Development characteristics and formation mechanism of two-phase transfer zone during rifting in Wushi Sag, Beibuwan Basin, South China Sea

Based on the three-dimensional seismic dataset, we consider a key area in Beibuwan Basin, South China Sea, combined with quantitative analysis method, in order to define the features of transfer zone, and its temporal spatial development mechanism is revealed. The two boundary faults in the study area constitute an S-type fault system which does not connect. During the Paleogene rifting, the fault systems and Structuralstratigraphic characteristics between Eocene and Oligocene rifting sequences are different, the ways and quantities of transfer displacement are also distinctly different: (1) The two-phase of Eocene and Oligocene generally developed transfer zones, which were controlled by the structure background and boundary conditions, the formation mechanism of the transfer zone was different in time and space, the distribution of the transfer zone was distinctly different; (2) The Ailaoshan-Honghe strike slip fault, the extension derived from the expansion of the South China Sea have a significant impact on the transfer zone formation and characteristics. The segmented growth of the fault has a direct impact on the formation of the transfer zone. This study reveals the formation mechanism of the transfer zone of superimposed basins in China offshore, which is beneficial to hydrocarbon accumulation in Eocene and Oligocene sequences.

Planktonic Foraminiferal Biosatratigraphy of the Eocene – Oligocene Boundary at Sinjar Area, NW Iraq

The present study deals with the lithostratigraphy, biostratigraphy and chronostratigraphy of the Eocene- Oligocene boundary in three surface sections in Sinjar anticline, northwestern Iraq. Jaddala Formation composed of marl, marly limestone represents deep Eocene facies, Palani, and Tarjil formations represent Early and Late Oligocene sequences. Sixty-two planktonic foraminiferal species belonging to sixteen genera have been recognized which permit the recognition of four Zones; these are from older at the base: Globorotalia opima opima Total- Range Zone (P21) Globigerina ampliapertura partial- Range Zone (P20) Pseudohastigerina micra Interval Zone (P 19) Globigerinatheka subconglobata subconglobata Partial Range Zone (P11) (Part). The Eocene- Oligocene boundary in Sinjar area represents disconformity surface, where the deposition ceased in the Middle Eocene (Lutetian) at the end of Jaddala Formation, then deposition started at the Upper Early Oligocene due to rapid wide transgression, leading to the deposition of Palani (Rupelian) and Tarjil (Chatian) formations, which consist of thinly laminated hard white-pale brown intercalation of marly limestone and marl beds.

New insights on interpretation of velocity anomalies in the offshore Nile Delta basin, Egypt

Several issues associated with existing velocity models and their anomalies affecting the petroleum geology of the offshore Nile Delta basin are described. The paper aims to present velocity and pressure variations and their associated pitfalls in the deeper Nile Delta, where the ambiguities exist in the interpretation of depth structures, especially in between sea bottom and Oligocene sequences. Different conceptualized geological, velocity, and seismic models are generated. We make use of data cubes to generate data views for geological interpretation. The connectivity is established between velocity anomalies and corresponding seismic times, ascertaining that the depth structures have geological implications. The visualization of representative schematic data views of velocity anomalies facilitates the interpretation of new knowledge and the connectivity between seismic and geological models. The velocity attributes significantly influence the seismic data interpretation in the offshore Nile Delta basin and have an impact on the overall cycle of the exploration and production entities.

Eocene-Oligocene succession at Kıyıköy (Midye) on the Black Sea coast in Thrace

A belt of Upper Eocene-Lower Oligocene marine sedimentary rocks extends from Kiyikoy on the Black Sea coast to Pinarhisar in the Thrace Basin, suggesting a marine connection between the Black Sea and the Thrace Basin during this period. The Cenozoic succession of this marine corridor was studied in the vicinity of Kiyikoy along two measured stratigraphic sections. The sequence lies unconformably over metamorphic basement rocks and consists of ~75 m of bioclastic limestone and sandstone of the Sogucak Formation, overlain by ~40 m of limestone, marl, mudstone, sandstone, and acidic tuff, which are assigned to the newly defined Servez Formation. Larger benthic foraminifera indicate that the lower part of the succession is Late Eocene in age, and nannoplankton from the upper part of the succession suggest an Early Oligocene age; these age determinations are also supported by the Sr-isotope data. A U-Pb age from zircons from a tuff bed is 33.9 ± 0.4 Ma, which falls on the Eocene-Oligocene boundary. The Kiyikoy Upper Eocene- Lower Oligocene sequence was deposited in shallow marine conditions below 50-m water depth. The depositional setting, as well as the relatively reduced thickness of the sequence, shows that any marine connection between the Black Sea and the Thrace Basin along the Kiyikoy-Pinarhisar corridor was not significant. The Late Eocene-Early Oligocene marine connection between the Black Sea and the Thrace Basin occurred along the Catalca gap southeast of Kiyikoy. In the Catalca gap the Upper Eocene-Lower Oligocene sequence is much thicker (350 m) and was deposited at much greater water depth.

The Oligocene Reference Section in the South of the East European Platform (Northern Ergeni)

This work presents the results of the integrated study of the sequence of Oligocene (Maikop) and underlying Eocene deposits uncovered in borehole 768 (362 m). This section can be regarded as the reference one as for Northern Ergeni, as for the entire northern part of the Eastern Paratethys. The data obtained on lithology, mollusks, and benthic foraminifers enabled us to subdivide the Oligocene interval of the Maikop Group into formations and subformations (Tsimlyanian, Solenovian, and Kalmyk), to correlate it with adjacent and remote areas of the Paratethys and with the scale of regional stages, and to reconstruct the main sedimentation conditions. On the basis of nannoplankton and dinocysts, these subdivisions are correlated with the zonal stratigraphic scale (Gradstein et al., 2012), and according to the palynological data, the evolution of climatic conditions at the Paratethys northern margin is reconstructed. The phytoplankton, foraminiferal, and paleomagnetic data show a significant reduction of the section in its Eocene interval. The latter includes only negatively magnetized intervals of the Keresta, Kuma, and Belaya Glina formations, separated by discontinuities, as well as the complete set of biotic and paleomagnetic zones of the Lower Oligocene. The lower subformation of the Solenovian Formation (50 m), eroded in most of the sections of Ciscaucasia, is characterized by unusual completeness. It is probable that the Kalmyk Formation in this section is incomplete and is represented only by the first half of the Chattian.

Development of two-phase transfer zones during multiphase rifting and their influence on sedimentation in the Baxian Sag, Bohai Bay Basin, northern China

AbstractWe investigate the formation and deformation of transfer zones and their impact on sedimentation during multiphase rifting using a three-dimensional seismic dataset in the Baxian Sag, the onshore part of the Bohai Bay Basin, northern China. The fault system in the study area is dominated by two arcuate, opposing boundary faults, that is, the Niudong and Maxi faults, which form an S-type fault system which does not link together. The fault system and structural-stratigraphic features between the Eocene and Oligocene syn-rift sequences were distinctly different during the Palaeogene rifting. These differences allow us to identify the two-phase transfer zones: (1) a NW–SE-trending Eocene transfer zone linking the NW-tilted Baxian Block and the SE-tilted Raoyang Block , and (2) the N–S-trending Oligocene transfer zone forming along the central part of the S-type fault system between the two inward kinks, and linking S-tilted and N-tilted fault blocks. The two-phase transfer zones comprise transverse boundary fault segments and fault styles which are related to strike-slip motion. The strike-slip faults occurred in the sequence where the transfer zone formed. The transfer zones significantly influenced the syn-rift sediments, drainage catchments and reservoir properties during the periods when they formed, and the two-phase transfer zones represent favourable positions for hydrocarbon accumulation in the Eocene and Oligocene sequences, respectively.

Dense brine refluxing: A new genetic interpretation of widespread anhydrite lumps in the Oligocene–Lower Miocene Asmari Formation of the Zagros foreland basin, NE Iraq

Abstract The Asmari Formation in the Zagros Basin of NE Iraq contains a thick sequence of Oligocene–Lower Miocene carbonates, and it forms one of the best-known carbonate reservoirs in the world. The formation mainly consists of dolostones, limestones, sandstones, and mudstones, and there is a general upward increase in the abundance of evaporites (generally in the form of anhydrite lumps) in the dolostones of the upper part. The anhydrite types and textures provide evidence on the salinity, depth, temperature, environment, and sea-level fluctuations which can be used for paleoenvironmental and climatic interpretations. And they also exert a significant control on reservoir quality, which has attracted the attention of many petroleum industries. Previous studies suggested that the anhydrite lumps in the Upper Asmari Formation (Aquitanian–Burdigalian) were synsedimentary deposits in a hypersaline setting of a sabkha (salt flat). However, this paper shows that 1) the Upper Asmari Formation was deposited in an inner-to mid-ramp setting, based on the sedimentological, petrographical, and paleontological characteristics; 2) the anhydrite lumps are irregular and variable in size, and are generally found within or associated with karst systems; and 3) fossil assemblages indicate that the synsedimentary salinity was normal or only slightly hypersaline, which means that the anhydrite was not directly deposited from the synsedimentary marine water. After comprehensive analysis of the anhydrite lumps, it is proposed in this paper that the anhydrite lumps were formed within the Upper Asmari Formation (Aquitanian–Burdigalian) due to downwards seepage of the overlying hypersaline water, which is termed as “dense brine refluxing”, a new genetic model. It is suggested 1) that the Upper Asmari Formation (Aquitanian–Burdigalian) was deposited in an inner-to mid-ramp setting and subjected to eogenetic karstification generating karst systems in local area with relative high landscape due to sea level change; 2) that during the middle Miocene tectonic compression resulted in the Zagros Basin isolated and the seawater becoming hypersaline with higher salt concentrations and a high density; and 3) the hypersaline water seeped downwards through the underlying porous sediments, forming anhydrite lumps and resulting in a decrease in the porosity of the Upper Asmari Formation (Aquitanian–Burdigalian). This paper provides new perspectives on the origin of anhydrite lumps and the resulting effects on reservoir quality in the Zagros foreland basin.

„Depositional setting of the Oligocene sequence of the Western Carpathians in the Polish Spisz region – a reinterpretation based on integrated palynofacies and sedimentological analyses” – Reply

The research on the Oligocene succession of the Central Carpathian Paleogene Basin (CCPB) in the Kacwin region focused on sedimentological and palynofacies analyses. Observations were carried out in natural exposures along three streams flowing in the Polish Spisz: Kacwinianka, Łapszanka and Kacwinski. Three main groups of lithofacies have been distinguished: coarse-grained, mixed and sandy-grained, and fine-grained. The lithofacies are characterized by variable sedimentary structures, e.g. massive structure, horizontal lamination, ripple cross-lamination, hummocky cross-stratification, deformation structure. Sedimentological analyses showed that structures typical for turbidity currents and those characteristic of relatively shallow deposition (HCS, wave ripples) could coexist in the investigated succession interpreted as typical for turbidity currents influenced by storm waves. This indicates relatively shallow-water environments. The palynofacies analysis enabled identification of the following components: black wood, brown wood, cortex, resin, sporomorphs, cuticle, algae, dinoflagellates and AOM. A high proportion of black wood and low diversity of components point to an origin related to turbidity currents. Furthermore, results of the palynofacies analysis have allowed determining that, during the deposition of the CCPB sediments in the Polish Spisz area, the distance between the deposition area and the source area became relatively large. The biostratigraphic analysis of dinoflagellate cysts from the Podhale Basin (Szaflary, Zakopane, and lower Chocholow beds) indicates an Early Rupelian age. Moreover, kerogen analysis in the UV was applied for the first time to study the CCPB succession. As a result, reworking was documented, so far unrecognized by other methods, and the mutual verification of the obtained results was possible

„Depositional setting of the Oligocene sequence of the Western Carpathians in the Polish Spisz region – a reinterpretation based on integrated palynofacies and sedimentological analyses” – Discussion

In their recent paper, Filipek et al. (2017) provided some data on age (dinoflagellate cyst biostratigraphy) and sedimentary setting (sedimentological and palynofacial analysis) of the Podhale Flysch (Central Carpathian Paleogene) from the Polish Spisz (their fig. 1). They carried out their integrated studies on the middle part of the Podhale Flysch succession – the upper part of the Szaflary beds through the lower part of the Chocholow beds. These studies, as stated in Introduction, “…allow for a new approach to the knowledge on the CCPB with regard to stratigraphy, depositional palaeoenvironment, and changes during deposition”. Authors accented the inconsistency of some of previous studies (biostratigraphical in particular), which provoked them to undertake their studies in order to reinterpret the depositional setting. In my opinion, however, it is hardly possible to find new reinterpretations, and those presented are frequently based on erroneous, commonly random and not complete data and interpretations; none of them are compared nor discussed with previous results. Below, these controversies are briefly presented.

Benthic foraminiferal fauna and sediment provenance of Eocene syn-rift sequences in Taiwan: implication for onset of Asian epi-continental marginal seas off China coast

Early Cenozoic rift basins developed commonly on the Mesozoic basement along the SE Asia Continent. However, Eocene–Oligocene sequences were only exposed widely onshore Taiwan. We study benthic foraminifera and determine U–Pb geochronology of detrital zircon grains in the Early Cenozoic sequence onshore Taiwan for understanding if there was any significant change in faunal content and sediment provenance between the Eocene syn-rift sequences and the overlying Oligocene post-rift strata across the breakup unconformity. The Eocene syn-rift sequences are mostly of thick, cross-bedded sandstones of fluvial-deltaic depositions without marine fauna. Intensive study on the Eocene stratigraphy in more than ten sections all over the Taiwan mountain belt in the last three decades, shallow marine benthic foraminifera of Gaudryina hayasakai-Hanzawaia asterizana-Planularia okinoshimaensis-Heterolepa praecinta assemblage were found only in very limited exposures in the Western Foothills, central Taiwan. This study shows that the Eocene benthic foraminiferal assemblage is similar to the Shihtsaoan fauna in the overlying Oligocene post-rift marine strata. This indicates no significant faunal break across the breakup unconformity. As a consequence, Gaudryina hayasakai Zone of the Shihtsaoan age can extend downward to the Eocene and the Shihtsaoan Stage would be equivalent to the Hsuehshanian Stage established previously from northern Taiwan. The Taiwan Eocene marine sequences could be deposited in a normal-fault bounded narrow seaway or gulf connecting southeastward to the open Huatung Basin. U–Pb geochronology of detrital zircons in the Taiwan Eocene syn-rift sandstones shows a prominent age peak centered in Yenshanian (100–155 Ma) similar to the Paleocene–Oligocene fluvial-deltaic deposits in the East China Sea and is also comparable to the U–Pb zircon age spectrum of modern river sands of the Fujian Province, SE China. Petrographic study also suggests a prevailing sediment provenance of the Cathaysia granites and volcanics. These Cathaysia detritals were transported to the Taiwan Eocene grabens through some SE-flowing rivers. Structural reconstructions in previous study indicated that the mountain belt with the Eocene sequences in NE Taiwan had been shortened by imbricated thrusting westward for 160–200 km wide during the arc-continent collision in the last 6.5 Ma. Integrating paleontology and sedimentology evidence reveal that the Eocene coast was located along the modern shelf break of the East China Sea ~ 350–400 km east off the modern Chinese coast line. This implies that in the Eocene time (~ 40 Ma) there was no wide Asian epi-continental marginal seas off Chinese coast until the onset sedimentation of the Oligocene post-rift strata during opening of the South China Sea in the Early Oligocene time (~ 33 Ma).