Miocene Carbonate Research Articles

Study of a TX field Miocene carbonate build-up: Integration of sedimentological data with modern carbonate analogue and 3D seismic characterization to establish carbonate facies modelling workflow

Study of a TX field Miocene carbonate build-up: Integration of sedimentological data with modern carbonate analogue and 3D seismic characterization to establish carbonate facies modelling workflow

Presence of microporosity in Miocene carbonate platform, Central Luconia, offshore Sarawak, Malaysia

The Luconia Province—offshore Sarawak—is a key hydrocarbon province in Malaysia. However, the gas reservoirs in Central Luconia pose unique problems and challenges as they have partially water-filled microporosity that overprint wireline logs. Microporosity in Central Luconia occurs throughout the Miocene carbonates and is a crucial element that influences fluid flow properties and ultimately the recovery of hydrocarbons. Quantification of macroporosity was achieved using petrographic analysis of thin sections and the FESEM images. The point counting technique was used to estimate the amount of macroporosity from the thin section, which was then subtracted from the total porosity to derive microporosity. The qualitative investigation of the Miocene carbonates indicates the presence of three different types of microporosity namely grain-based microporosity, matrix-based microporosity, and cement-based microporosity. Quantification of microporosity showed that the microporosity varies from sample to sample, ranging from 10 to 60% of the total measured porosity. The depositional texture, mineralogy, and microtexture control this microporosity variation including its abundance and type. The microporosity in Central Luconia is diagenetically controlled based on four major diagenetic mechanisms namely (1) mechanical process/endolithic grains/marine diagenesis; (2) leaching/meteoric diagenesis; (3) cementation/shallow diagenesis; and (4) deeper diagenesis environment.

Strike-Slip Fault Deformation and Its Control in Hydrocarbon Trapping in Ketaling Area, Jambi Subbasin, Indonesia

Geologically, Ketaling area consists of a local high considered as flexure margin of Tempino-Kenali Asam Deep in west part and graben in east part also known as East Ketaling Deep. Numerous proven plays were established in Ketaling area with reservoir in early Miocene carbonate and middle Miocene sand. This area underwent several major deformations. Faults are developed widely, yet their geometrical features and mechanisms of formation remained so far indistinct, which limited exploration activities. With new three-dimensional seismic data acquired in 2014, this area evidently interpreted as having strike-slip mechanism. The objective of this study is to examine characteristic of strike slip fault and its affect to hydrocarbon trapping in Ketaling Area. Structural pattern and characteristic of strike slip fault deformation was examined with integration of normal seismic with variance seismic attribute analysis and the mapping of Syn-rift to Post-rift horizon. Seismic flattening on 2D seismic cross section with NW-SE direction is done to see the structural pattern related to horst (paleohigh) and graben. Typical flower structure, branching strike-slip fault system and normal fault in synrift sediment clearly showed in section. An echelon pattern identified from map view as the result of strike slip mechanism. Detail structural geology analysis show the normal fault development which has main border fault in the southern of Ketaling area dipping to the Southeast-East with NE-SW lineament. These faults related to rift system in Ketaling area. NW-SE folds with reactive NE-SW fault which act as hydrocarbon trapping in the shallow zone. This polyphase tectonic formed local graben, horst and inverted structure developed a good kitchen area (graben) and traps (horst, inverted structure). Subsequently, hydrocarbon accumulation potentials such as basement fractures, inverted syn-rift deposit and shallow zone are very interesting to explore in this area.

Petrography and stable isotope geochemistry of Oligocene-Miocene continental carbonates in south Texas: Implications for paleoclimate and paleoenvironment near sea-level

Cenozoic sedimentary rocks in the southern Texas Gulf Coastal Plains contain abundant continental carbonates that are useful for reconstructing terrestrial paleoclimate and paleoenvironment in a region near sea-level. Our field observations and thin section characterizations of the Oligocene and Miocene continental carbonates in south Texas identified three types of pedogenic carbonates, including rhizoliths, carbonate nodules, and platy horizons, and two types of groundwater carbonates, including carbonate-cemented beds and carbonate concretions, with distinctive macromorphologic and micromorphologic features. Based on preservations of authigenic microfabrics and variations of carbon and oxygen isotopic compositions, we suggest these carbonates experienced minimal diagenesis, and their stable isotopic compositions reflect paleoclimate and paleoenvironment in south Texas. Our Oligocene and Miocene carbonate clumped isotope temperatures (T(Δ47)) are 23–28 °C, slightly less than or comparable to the range of modern mean annual and mean warm season air temperature (21–27 °C) in the study area. These T(Δ47) values do not show any dependency on carbonate-type, or trends through time suggesting that groundwater carbonates were formed at shallow depths. These data could indicate that air temperature in south Texas was relatively stable since the early Oligocene. The reconstructed paleo-surface water δ18O values are similar to modern surface water which could indicate that meteoric water δ18O values also remained stable since the early Oligocene. Mean pedogenic carbonate δ13C values increased ~ − 4.6‰ during the late Miocene, most likely reflecting an expansion of C4 grassland in south Texas. This study provides the first mid- and late Cenozoic continental records of paleoclimate and paleoecology in a low-latitude, near sea-level region.

Seismic analysis of early-mid Miocene carbonate platform in the southern Qiongdongnan Basin, South China Sea

The southern uplift of the Qiongdongnan Basin is a deepwater area in which no wells have beens drilled. The Miocene-Quaternary strata in the Xisha Islands, which are located 40–100 km to the south, are composed of carbonate reef formations. Paleotectonic and paleogeographic analyses of the basin suggest that the southern uplift experienced favorable geological conditions for the development of carbonate reefs during the Miocene. The high-impedance carbonates have high amplitudes and low frequencies on seismic profiles. The reefs are distributed on paleotectonic highs and are thicker than the contemporaneous formations. A forward model of the variation in carbonate thickness based on lithological and velocity information from wells in nearby regions can simulate the seismic response of carbonates with different thicknesses. We identified several important controlling points for determining the thickness of carbonates from seismic profiles, including the pinchout point, the λ/4 thickness point, and the λ/2 thickness point. We depict a carbonate thickness map in the deepwater area of the southern Qiongdongnan Basin based on this model. The carbonate thickness map, the paleotectonic and paleogeographic background, and the seismic response characteristics of reefs suggest that the carbonates that developed on the southern uplift of the Qiongdongnan Basin during the Miocene were mainly an isolated carbonate platform peninsula and ramp deposits. It consisted of gentle ramp platform, steep slope platform, platform depression, gravity flow, and reef bank facies.

Quantification of architectural variability and controls in an Upper Oligocene to Lower Miocene carbonate ramp, Browse Basin, Australia

Integration of seismic geomorphology and trajectory analysis of the Oligocene through Lower Miocene distally-steepened carbonate ramp system in the subsurface Browse Basin of the Northwest Shelf of Australia introduces a new way to parameterize carbonate slope channel systems and their stability, and sheds light on how basinward sediment transport is influenced by allogenic and autogenic controls. Seismic geomorphologic analysis identified large-scale prograding clinoforms with an extensive slope channel system and slope angles of up to 12°. Clinoforms have an average slope angle of 8°, a height of ∼500 m, and prograded 9 km during the Upper Oligocene through the Lower Miocene. Relative sea level changes overprinted the effect of intermediate-scale and small-scale antecedent topography, and determined progradation, aggradation and slope angles of the system. We introduce cumulative channel cross sectional area (CCCSA) as the product of number of slope channels, slope channel depth, and slope channel width. CCCSA quantifies the capability of the slope channel system to transport sediment basinward and highlights phases of autogenic slope system re-organization in response to changes in relative sea level. Abruptly increasing values of CCCSA correlate with slope sediment bypass through the incision of new slope channels during system re-organization phases at slope angles of 10°. Thus, we propose that CCCSA can be used as a proxy for slope system stability. Incision of new slope channels into the ramp margin created strike-parallel variability of the ramp margin trajectory. Our seismically-derived relative sea level curve and subsidence rates of 9–30 m/Myr indicate a good preservation of the subsidence and sea level signal in the data.

Syn-sedimentary deformation as a mechanism for the initiation of submarine gullies on a carbonate platform to slope transition, Browse Basin, Australian North West Shelf

Syn-sedimentary deformation and submarine channel initiation are both important processes influencing the stratigraphic development and reservoir properties of carbonate platforms. Based on a high-resolution 3D seismic dataset our study shows that platform-margin parallel syn-sedimentary faulting in the vicinity of the clinoform breakpoint can lead to the indention of channel heads that subsequently grow and expand in headward and downslope direction. The carbonate-dominated Miocene succession of the Browse Basin on the Australian North West Shelf reveals a distinct submarine channel system during a time of overall platform progradation. The progradation occurs over a drowned Eocene to Lower Miocene carbonate ramp, which formed a large scale inflection. Previous studies investigated the architecture of the carbonate platform, containing a large Neogene barrier reef system. We present a high-resolution 3D seismic study of the contemporaneous platform to slope transition. Landward of first significant gullies, a distinct network of margin parallel, small scale arcuate faults is observed. Active faulting during appearance of the first gullies is exhibited by growth strata, which are located directly basinward of the faults. These steep (65°–85°), small-scale faults are characterized by throws of 5–25 ms TWT and show a tendency to merge laterally and vertically with adjacent faults. Syn-sedimentary deformation associated with these faults likely triggered slope failure and the generation of turbidity currents finally leading to the growth of the gullies by headward and downslope erosion.

Entobia ichnofacies from the Middle Miocene carbonate succession of the northern Western Desert of Egypt

A bed of Middle Miocene (Serravallian) lagoonal facies with well-developed patch reefs is described from a section at the Siwa Oasis, northern Western Desert of Egypt. It is well-exposed in the middle Siwa Escarpment Member of the Marmarica Formation and displays remarkable bioerosion structures that show abundant ichnofossils. Nine ichnotaxa, belonging to four ichnogenera, were identified: two correspond to the clionaid sponge boring Entobia ( E . laquea and E . ovula ), five to the bivalve boring Gastrochaenolites ( G . lapidicus , G . torpedo , G . cluniformis , G. hospitium and G . cf. orbicularis ) and two to the annelid-worm boring Maeandropolydora ( M. sulcans ) and Trypanites ( T. weisei ). In addition, traces of the boring polychaete worm Caulostrepsis and the boring acrothoracican barnacle Rogerella were recorded. These ichnoassemblages have been assigned to the Entobia ichnofacies. The organisms bored into a hard, fully lithified carbonate substrate in a low-energy, shallow-marine environment. The ichnotaxa associations indicate water depths of a few metres (<10 m) and a very low sedimentation rate in a lagoonal setting during a Serravallian regressive cycle.

Miocene Oceanographic Evolution Based on the Sr and Nd Isotope Record of the Central Mediterranean

AbstractThe Miocene is a key interval in the geodynamic and oceanographic evolution of the Mediterranean marking the transition from a wide open basin to the modern closed basin. We used the Sr and Nd isotope records of two Miocene carbonate successions in the Adriatic to document that the evolution of the Mediterranean Basin controlled its seawater chemistry. During the late Aquitanian (~21 Ma), a time of glaciation and sea level lowstand, increased runoff affected the Sr isotope ratios of Mediterranean waters, whereas during the Burdigalian (20.44–15.97 Ma) volcanism in the circum‐Mediterranean area mainly influenced the Sr isotopic signature. During the Langhian (15.97–13.82 Ma), a time of sea level highstand associated with the Middle Miocene Climatic Optimum, the Nd isotope values indicate that waters exchanged between the Paratethys and the Central Mediterranean. The Central Mediterranean was well connected with the Atlantic Ocean between the Langhian and the early Tortonian (15.97–11.5 Ma), but exchange of water with the Paratethys declined. In the Messinian (6.3 Ma), connections between some marginal Mediterranean basins, for example, the proto‐Adriatic basin, and the Central Mediterranean, became restricted. In this basin, the Sr isotope values fell below the global reference line, while Nd isotope ratios show a strong affinity with the Atlantic Ocean and also indicate freshwater input. We conclude that the Mediterranean Nd isotope signature differs from that in the open oceans and reflects the basin physiography, reflecting a mix of signals derived from the adjacent oceans and local signals.

Facies Development of Carbonate Rock and System Tract Influence Based on Fullbore Formation Microimager (FMI) and Well Log Analysis on Carbonate Reservoir Tuban Formation, North East Java Basin

The North East Java Basin is known to be one of the basins that consist of Miocene carbonate rocks, like the reef carbonate of Tuban Formation. It has the potential hydrocarbons that can be explored. Therefore, the FMI log analysis is very important to identify carbonate rocks of Tuban Formation to know facies and characteristics of that carbonate rocks. The method used descriptive and analysis process of FMI and Gamma Ray log to determine facies of the carbonate rock and the system tract. Based on the result of FMI log analysis, there are variations lithofasies include mudstone, wackestone, packstone, grainstone, floatstone, rudstone and claystone. A collection of rock associations can interpret the reef facies. It involves back reef facies, reef core facies, and fore reef facies. The changes of lithofasies and reef facies are caused by sea level fluctuations and subsidence resulting in the system tract. The system tracts generated in the research area include transgressive and highstand system tract.

Structural Restoration of Carbonate Platform in the Southern Part of Central Luconia, Malaysia

Carbonates of Central Luconia, Malaysia, had been serving as hydrocarbon reservoirs for more than 25 years. However, the relationship and impacts of extensive tectonic events to the growth of Miocene carbonates in Central Luconia have not been revealed sufficiently. In this work, two carbonates platforms in the southern part of Central Luconia were used for detailed interpretation and seismic based structural restoration. This work had provided new insight for the interpretation of carbonate growth in Central Luconia and its association with the surrounding tectonic. This work suggested three possible tectonic evolutions from Late Oligocene to Pliocene are responsible for the growth of carbonates in Central Luconia. These stages are equivalent with pre-carbonate stage (Late Oligocene–Early Miocene), syn-carbonate stage (Middle–Late Miocene) and post-carbonate stage (Pliocene). Rifting of the South China Sea and subduction of proto-South China Sea are believed to be responsible for the development of faulting during pre-carbonate stage, while movement of the ancient Baram Line is thought to control the parallel striking direction of normal faults during syncarbonate stage. Finally, subsidence and compaction due to the overburden clastic materials from the prograding deltas is considered as the main reason for the impacts of gravitational tectonics in this area, which corresponding to the post-carbonate stage.

Genesis and evolution of the Watukosek fault system in the Lusi area (East Java)

Detailed analysis of two-dimensional seismic lines acquired in the NE Java basin has been performed to unravel the subsurface geology of the region around the Lusi mud eruption. This work revealed the existence of a system consisting of a complex set of faults, here called the Watukosek fault system, forming triangular deformation zones converging at the top of the early Miocene Carbonates. This system continues downwards with vertical individual fault segments, often bordering the steep margins of the carbonate platforms. The analysis of data includes the interpretation of seismic lines, regional structural data inferred from basement gravity maps and present-day main direction of stress. Results suggest that a possible rotation of stress direction from N40E-S40W to N-S occurred during the post-Miocene history of the Java back arc tectonic evolution. The Watukosek fault system was first generated as a tensional lineament during the E-W sinistral transpressive strain, which involved the basement. In this phase, synthetic and antithetic Riedel faults formed, the former controlling the NW-SE orientation of the structural highs represented by Oligo-Miocene carbonate platforms. As a consequence of the rotation of the main principal stress direction to a N-S direction, the Watukosek fault and similar parallel lineaments became sinistral Riedel shears, developing intense triangular deformation zones. Based on the stratigraphic position of gentle anticlinal deformations with axis corresponding to the N40E-S40W oriented triangular deformation zones, the transpressive strain linked to the N-S main stress compression occurred likely in the Late Pliocene-Early Pleistocene.The detailed examination of a) stratigraphy at the wells BJP-1 and Porong-1 as well as b) the seismo-stratigraphic architecture of the entire succession in the study area, allowed a new subsurface interpretation and revision of the stratigraphic units below Lusi. The thick Early Miocene Tuban Formation is found sandwiched between the coheval Upper Kalibeng Formation and the Early Miocene Carbonate of the Kujung Formation, which in turn overlain the older Ngimbang Formation.

Diagenetic history of late Oligocene-early Miocene carbonates in East Sabah, Malaysia

Limestones are particularly susceptible to drastic early diagenesis modifications, mainly cementation and dissolution. During the early Miocene, a major tectonic deformation has caused a widespread of uplift in Sabah. This has resulted change in depositional environment from deep to shallow marine, which favours the deposition of Gomantong Limestone. This study aims to investigate the diagenetic history of Gomantong Limestone in East Sabah. Thorough understanding of the diagenetic processes may provide data to unravel the tectonic activities which affected the reservoir quality of the carbonates. Combining the data from comprehensive petrographic analysis, and Scanning Electron Microscopy (SEM) of 30 samples, two main cements type were identified. These are microcrystalline cement and Mg-calcite cement of granular and blocky mosaics which are dominantly seen in all samples. The sequence of diagenesis events are determined as (1) micritization; (2) grain scale compaction; (3) cementation (pore-filling); (4) mechanical compaction and cementation infilling fractures and (5) chemical compaction. These diagenetic events are interpreted as reflection of changes in diagenetic environment from shallow marine to deep burial. The massive cementation in the Gomantong Limestone has resulted into a poor reservoir quality.

Comment on “Tectonic and environmental factors controlling on the evolution of Oligo-Miocene shallow marine carbonate factories along a tropical SE Circum-Caribbean” by Silva-Tamayo et al. (2017)

Silva-Tamayo et al. (2017) study the Chattian to Langhian carbonate succession of the Siamana Formation in the Cocinetas Basin (La Guajira, Colombia). They identify a change in carbonate factory from mixed photozoan-heterozoan and photozoan associations dominated by corals in the Chattian-early Burdigalian to a heterozoan rhodalgal association in the late Burdigalian-Langhian. To validate the regional scale of this shift in carbonate-producing biota along the southeastern Circum-Caribbean realm, Silva-Tamayo et al. compare the Siamana Formation with the San Luis carbonate succession in the Falcón Basin (NE Venezuela) and the Perla carbonates in the Urumaco Trough (Gulf of Venezuela). Referring to Albert-Villanueva (2016) they state that, as in the case of the Siamana Formation, the carbonates of the San Luis Formation also recorded a change in carbonate-producing biota, from a photozoan/heterozoan carbonate factory in the late Oligocene-early Miocene to a heterozoan/rhodalgal carbonate factory in the middle Miocene. Notwithstanding, Albert-Villanueva (2016) interprets the carbonate units cropping out in the Falcón Basin (San Luis and Churuguara formations) as early Miocene in age, and the passage from photozoan to heterozoan carbonate factory as a lateral change of facies within the lower Miocene carbonate platforms of the Falcón Basin.

Late Miocene climate and time scale reconciliation: Accurate orbital calibration from a deep-sea perspective

Accurate age control of the late Tortonian to early Messinian (8.3–6.0 Ma) is essential to ascertain the origin of benthic foraminiferal δ18O trends and the late Miocene carbon isotope shift (LMCIS), and to examine temporal relationships between the deep-sea, terrasphere and cryosphere. The current Tortonian–Messinian Geological Time Scale (GTS2012) is based on astronomically calibrated Mediterranean sections; however, no comparable non-Mediterranean stratigraphies exist for 8–6 Ma suitable for testing the GTS2012. Here, we present the first high-resolution, astronomically tuned benthic stable isotope stratigraphy (1.5 kyr resolution) and magnetostratigraphy from a single deep-sea location (IODP Site U1337, equatorial Pacific Ocean), which provides unprecedented insight into climate evolution from 8.3–6.0 Ma. The astronomically calibrated magnetostratigraphy provides robust ages, which differ by 2–50 kyr relative to the GTS2012 for polarity Chrons C3An.1n to C4r.1r, and eliminates the exceptionally high South Atlantic spreading rates based on the GTS2012 during Chron C3Bn. We show that the LMCIS was globally synchronous within 2 kyr, and provide astronomically calibrated ages anchored to the GPTS for its onset (7.537 Ma; 50% from base Chron C4n.1n) and termination (6.727 Ma; 11% from base Chron C3An.2n), confirming that the terrestrial C3:C4 shift could not have driven the LMCIS. The benthic records show that the transition into the 41-kyr world, when obliquity strongly influenced climate variability, already occurred at 7.7 Ma and further strengthened at 6.4 Ma. Previously unseen, distinctive, asymmetric saw-tooth patterns in benthic δ18O imply that high-latitude forcing played an important role in late Miocene climate dynamics from 7.7–6.9 Ma. This new integrated deep-sea stratigraphy from Site U1337 can act as a new stable isotope and magnetic polarity reference section for the 8.3–6.0 Ma interval.

Oligocene and Miocene Carbonate Platform Development In the Browse Basin, Australian Northwest Shelf

Abstract: Extensive 2D and 3D seismic-reflection data calibrated by wells provide new insights into the lateral extent, morphology, internal architecture, and age of carbonate platforms in the subsurface of the Browse Basin, Australian Northwest Shelf (NWS). The oldest carbonate build-ups are interpreted as a giant bryozoan build-up complex of Oligocene age (34.03–27.8 Ma). In the late Burdigalian, tropical reef growth began, and numerous reef-rimmed carbonate platforms progressively coalesced into an extensive barrier reef. From the mid-Langhian to the early Tortonian, the Browse Basin contained an elongate (along-margin) barrier-reef system over 500 km long. This barrier reef possibly extended in the southwest into the Northern Carnarvon Basin. After the early Tortonian, reefs on top or landward of this major barrier-reef system were smaller and less connected. This significant reduction in area covered by reef-rimmed platforms likely resulted from cooling following the Mid-Miocene Climate Optimum and relatively high subsidence rates. The reef-rimmed carbonate platforms that formed between the late Burdigalian and the early Tortonian are very similar throughout the Browse Basin. Platform development after the early Tortonian was regionally diverse. High subsidence rates in the northern Browse Basin are interpreted to have promoted the development of thick, aggrading platforms. Lower subsidence in the south led to thinner, more widespread platforms, recurrently shifting location in response to eustatic change. The final phase of reef decline around 6 Ma coincides with an increase in current-driven drift sedimentation. Current-related winnowing of sediments from the lagoons and slopes of the remaining carbonate platforms likely contributed to their final drowning.

The identification of Oligo-Miocene mammalian palaeocommunities from the Riversleigh World Heritage Area, Australia and an appraisal of palaeoecological techniques.

Fourteen of the best sampled Oligo-Miocene local faunas from the Riversleigh World Heritage Area, north-western Queensland, Australia are analysed using classification and ordination techniques to identify potential mammalian palaeocommunities and palaeocommunity types. Abundance data for these faunas are used, for the first time, in conjunction with presence/absence data. An early Miocene Faunal Zone B and two middle Miocene Faunal Zone C palaeocommunities are recognised, as well as one palaeocommunity type. Change in palaeocommunity structure, between the early Miocene and middle Miocene, may be the result of significant climate change during the Miocene Carbon Isotope Excursion. The complexes of local faunas identified will allow researchers to use novel palaeocommunities in future analyses of Riversleigh’s fossil faunas. The utility of some palaeoecological multivariate indices and techniques is examined. The Dice index is found to outperform other binary similarity/distance coefficients, while the UPGMA algorithm is more useful than neighbour joining. Evidence is equivocal for the usefulness of presence/absence data compared to abundance.

Palaeoenvironmental significance of Miocene larger benthic foraminifera from the Xisha Islands, South China Sea

We studied 344 samples from Well XK-1 in Xisha Islands, South China Sea, and identified 66 species of larger benthic foraminifera, providing critical evidence for biostratigraphy and palaeoenvironmental interpretation of the Miocene reef carbonate sequence. Three assemblages are recognized, namely, Spiroclypeus higginsi–Borelis pygmaeus Assemblage (Letter Stage Te5, Early Miocene, 1256.28–1180.15m), Nephrolepidina–Miogypsina Assemblage (Tf, Middle Miocene, 1031.10–577.04m), and Cycloclypeus–Heterostegina Assemblage (Tg, Late Miocene, 468.13–380.42m). On the basis of the palaeoecological preference of the larger foraminifera, we interpret that the Miocene carbonate sequence was deposited mainly in a warm tropical shallow water environment, characterized by five stages of continuous long-term evolution: backreef lagoon to shelf in the Early Miocene, normal to frontal reef in the early Middle Miocene, backreef lagoon to shelf in the later Middle Miocene, normal to frontal reef in the early Late Miocene, and proximal forereef shelf in the later Late Miocene.

Cenozoic tectonic and stratigraphic development of the Central Vietnamese continental margin

The East Vietnam Boundary Fault Zone (EVBFZ) forms the seaward extension of the Red River Shear Zone and interacted with the extensional rift systems in basins along the Central Vietnamese continental margin. The structural outline of the central Vietnamese margin and the timing of deformation are therefore fundamental to understanding the development of the South China Sea and its relation to Indochinese escape tectonism and the India-Eurasia collision. This study investigates the structural and stratigraphic evolution of the Central Vietnamese margin in a regional tectonic perspective based on new 2-D seismic and well data. The basin fill is divided into five major Oligocene to Recent sequences separated by unconformities. Deposition and the formation of unconformities were closely linked with transtension, rifting, the opening of the South China Sea and Late Neogene uplift and denudation of the eastern flank of Indochina. The structural outline of the Central Vietnamese margin favors a hybrid tectonic model involving both escape and slab-pull tectonics. Paleogene left-lateral transtension over the NNW-striking EVBFZ, occurred within the Song Hong Basin and the Quang Ngai Graben and over the Da Nang Shelf/western Phu Khanh Basin, related to the escape of Indochina. East of the EVBFZ, Paleogene NE-striking rifting prevailed in the outer Phu Khanh Basin and the Hoang Sa Graben fitting best with a prevailing stress derived from a coeval slab-pull from a subducting proto-South China Sea beneath the southwest Borneo – Palawan region. Major rifting terminated near the end of the Oligocene. However, late stage rifting lasted to the Early Miocene when continental break-up and seafloor spreading commenced along the edge of the outer Phu Khanh Basin. The resulting transgression promoted Lower and Middle Miocene carbonate platform growth on the Da Nang Shelf and the Tri Ton High whereas deeper marine conditions prevailed in the central part of the basins. Partial drowning and platform retreat occurred after the Middle Miocene due to increased siliciclastic input from the Vietnamese mainland. As a result, siliciclastic, marine deposition prevailed offshore Central Vietnam during the Pliocene and Pleistocene.

The Importance of Microbial Binding In Neogene–Quaternary Steep Slopes

Abstract: The geometry and inclination of carbonate-platform slopes are typically described as the result of the sediment texture, components, and grain size occurring in the slope. Three carbonate slopes from the Holocene of the Tongue of the Ocean (Bahamas) and three slopes from an upper Miocene carbonate platform in SE Spain are investigated to identify the factors controlling dip angles. Although platforms differ in size and elevation with regard to the surrounding basin, both locations present slopes with linear profiles and steep angles over 35°. The slope facies in both locations are very similar, with the matrix and some bioclasts presenting microfabrics that evidence microbial activity during deposition. These include (1) clotted micrite patches locally connecting bioclasts or infilling primary pores, (2) porostromate structures in the micrite, (3) dense micritic masses, (4) binding structures, (5) micritic crusts, and (6) peloidal textures. The Holocene and Miocene depositional geometries and facies distribution are the response to various sedimentary processes including rockfalls, gravity flows, and in situ carbonate production. Extensive microbial binding is identified as the controlling factor for early stabilization of the slope, which in combination with subsequent rapid cementation produces and maintains the steep slope angles. Consequently, microbial-induced stabilization of modern and Neogene slopes is as significant as in Paleozoic and Triassic examples.