Thick Miocene Research Articles

A Volcanic Ash Layer in the Nördlinger Ries Impact Structure (Miocene, Germany): Indication of Crater Fill Geometry and Origins of Long‐Term Crater Floor Sagging

AbstractSince its recognition as an impact structure 60 years ago, no volcanics were anticipated in the circular depression of the 14.8 Ma old Nördlinger Ries. Here, we describe for the first time a volcanic ash‐derived clinoptilolite‐heulandite‐buddingtonite bed within the 330 m thick Miocene lacustrine crater fill. Zircon U‐Pb ages of 14.20 ± 0.08 Ma point to the source of the volcanic ash in the Pannonian Basin, 760 km east of the Ries. The diagenetically derived zeolite‐feldspar bed occurs in laminated claystones of the Ries soda‐lake stage and represents the first unequivocal stratigraphic marker bed in this basin, traceable from marginal surface outcrops to 218 m below surface in the crater center. These relationships demonstrate a deeply bowl‐shaped geometry of crater fill sediments, not explainable by sediment compaction and corresponding stratigraphic backstripping alone. Since most of the claystones formed at shallow water depths, the bowl‐shaped geometry must reflect 134 +23/−49 m of sagging of the crater floor. We attribute the sagging to compaction and closure of the dilatant macro‐porosity of the deeply fractured and brecciated crater floor during basin sedimentation and loading, a process that lasted for more than 0.6 Myr. As a result, the outcrop pattern of the lithostratigraphic crater‐fill units in its present erosional plane forms a concentric pattern. Recognition of this volcanic ash stratigraphic marker in the Ries crater provides insights into the temporal and stratigraphic relationships of crater formation and subsidence that have implications for impact‐hosted lakes on Earth and Mars.

Evolution of the Miocene megalake in the western Qaidam Basin, northwestern China

Thick Miocene lacustrine deposits have been identified across the Qaidam Basin on the northeast Tibetan Plateau, indicative of a relatively unified megalake then. How this megalake evolved at its final stage and associated controlling factors, e.g., global climate or tectonic uplift, remain largely elusive. Here we use the KC-1 well, drilled at the depo-center of the western Qaidam Basin, to investigate the megalake evolution over the mid- and late Miocene. A set of lipid biomarker indices, namely β-carotane/n-C36, gammacerane index, terrigenous/aquatic ratio and long-chain saturated ketone content, altogether indicate substantial lake level fluctuations, with lower lake level at the intervals of ~18–17 Ma, 13.5–11.5 Ma, 10.5–8 Ma and 7–6 Ma, and higher level at the intervals of ~17–13.5 Ma, 11.5–10.5 Ma, 8–7 Ma and 6–5 Ma, superimposed on the long-term shrinking trend. Direct comparison with existing regional and global temperature records suggests that such lake dynamics was largely associated with global climatic conditions, i.e., shrinking under relatively cool conditions and vice versa, for both its long-term evolution and secondary fluctuations. It thus appears that global climatic conditions had controlled the megalake status during the mid- and late Miocene, whereas tectonic activities then might have also contributed to its long-term gradual demise.

Differences in the Tectonic Evolution of Basins in the Central‐Southern South China Sea and their Hydrocarbon Accumulation Conditions

AbstractTo reveal the causes of differences in the hydrocarbon accumulation in continental marginal basins in the central‐southern South China Sea, we used gravity‐magnetic, seismic, drilling, and outcrop data to investigate the tectonic histories of the basins and explore how these tectonic events controlled the hydrocarbon accumulation conditions in these basins. During the subduction of the Cenozoic proto‐South China Sea and the expansion of the new South China Sea, the continental margin basins in the central‐southern South China Sea could be classified as one of three types of epicontinental basins: southern extensional‐foreland basins, western extensional‐strike slip basins, and central extensional‐drift basins. Because these basins have different tectonic and sedimentary histories, they also differ in their accumulated hydrocarbon resources. During the Cenozoic, the basin groups in the southern South China Sea generally progressed through three stages: faulting and subsidence from the late Eocene to the early Miocene, inversion and uplift in the middle Miocene, and subsidence since the late Miocene. Hydrocarbon source rocks with marine–continental transitional facies dominated by II–III kerogen largely developed in extremely thick Miocene sedimentary series with the filling characteristics being mainly deep‐water deposits in the early stage and shallow water deposits in the late stage. With well‐developed sandstone and carbonate reservoirs, this stratum has a strong hydrocarbon generation potential. During the Cenozoic, the basin groups in the western South China Sea also progressed through the three developmental stages discussed previously. Hydrocarbon source rocks with lacustrine facies, marine‐continental transitional facies, and terrigenous marine facies dominated by II2–III kerogen largely developed in the relatively thick stratum with the filling characteristics being mainly lacustrine deposits in the early stage and marine deposits in the late stage. As a reservoir comprised of self‐generated and self‐stored sandstone, this unit also has a high hydrocarbon generation potential. Throughout those same three developmental stages, the basin groups in the central South China Sea generated hydrocarbon source rocks with terrigenous marine facies dominated by III kerogen that have developed in a stratum with medium thicknesses with the filling characteristics being mainly sandstone in the early stage and carbonate in the late stage. This reservoir, which is dominated by lower‐generation and upper‐storage carbonate rocks, also has a high hydrocarbon generation potential.

Four-dimensional analog and geometrical modeling of the Hides anticline, Papua New Guinea: Structure of a giant gas field

The Hides anticline in the Papua New Guinea fold belt is the core part of the Papua New Guinea liquified natural gas project that commenced production in 2014, yet little is known about its internal structure, particularly at depth. The structure of the upper 3 km (9843 ft) is relatively well constrained from surface data, recent development wells, and poor-quality two-dimensional seismic data, but the deep structure has been obscure. Seismic data and gravity modeling suggest an underlying large normal fault in basement. Forward geometrical modeling using MoveTM and four-dimensional analog sandbox modeling under an x-ray tomography scanner have greatly constrained the structural interpretation. The modeling confirms the presence of the underlying basement normal fault and suggests that it was mildly inverted prior to the onset of pure compression deformation in the sedimentary section. The main detachment level was just above basement approximately 8–10 km (∼5–6 mi) beneath the surface, but this ramped up at the basement fault creating an array of triangle-zone faults through the overlying sedimentary section, building the Hides anticline. The early inversion was instrumental in initiating a back thrust and hinterland-verging tight fold in the Mesozoic section, including the source, reservoir, and seal. Deformation in the thick Miocene carbonates at the surface was detached from that in the underlying reservoir along an Upper Cretaceous mudstone horizon that varies strongly in thickness such that the 2–4-km (6562–13,123-ft)-high Mesozoic structure verges northeast and the overlying Miocene structure verges southwest. Analog modeling shows that a ductile stratigraphy is required with slow strain rates of approximately 1–3 km/m.y. (∼3281–9843 ft/m.y.).

Lacustrine stromatolites: Useful structures for environmental interpretation – an example from the Miocene Ebro Basin

AbstractThe significance of stromatolites as depositional environmental indicators and the underlying causes of lamination in the lacustrine realm are poorly understood. Stromatolites in a ca 600 m thick Miocene succession in the Ebro Basin are good candidates to shed light on these issues because they are intimately related to other lacustrine carbonate and sulphate facies, grew under variable environmental conditions and show distinct lamination patterns. These stromatolites are associated with wave‐related, clastic‐carbonate laminated limestones. Both facies consist of calcite and variable amounts of dolomite. Thin planar stromatolites (up to 10 cm thick and less than 6 m long) occurred in very shallow water. These stromatolites represented first biological colonization after: (i) subaerial exposure in the palustrine environment (i.e. at the beginning of deepening cycles); or (ii) erosion due to surge action, then coating very irregular surfaces on laminated limestones (i.e. through shallowing or deepening cycles). Sometimes they are associated with evaporative pumping. Stratiform stromatolites (10 to 30 cm high and tens of metres long) and domed stromatolites (10 to 30 cm high and long) developed in deeper settings, between the surge periods that produced hummocky cross‐stratification and horizontal lamination offshore. Changes in stromatolite lamina shape, and thus in the growth forms through time, can be attributed to changes in water depth, whereas variations in lamina continuity are linked to water energy and sediment supply. Growth of the stromatolites resulted from in situ calcite precipitation and capture of minor amounts of fine‐grained carbonate particles. Based on texture, four types of simple laminae are distinguished. The simple micrite and microsparite laminae can be grouped into light and dark composite laminae, which represent, respectively, high and low Precipitation/Evaporation ratio periods. Different lamination patterns provide new ideas for the interpretation of microbial laminations as a function of variations in climate‐dependent parameters (primarily the Precipitation/Evaporation ratio) over variable timescales.

Geology of the eastern slopes of the Simbruini Mts. between Verrecchie and Capistrello (Central Apennines – Abruzzo, Italy)

ABSTRACTThis paper presents the results of a geological mapping project across the eastern Simbruini Mts. and the higher Roveto Valley (Central Apennines). The study area belongs to the Latium-Abruzzi Domain, which is characterized by a Cretaceous and Miocene carbonate platform succession followed by a thick upper Miocene terrigenous succession. A unique feature of the study area is the existence of a thick clastic unit, the ‘brecce della Renga fm.’; this unit was produced by the dismantling of the margins of a large pre-orogenic extensional structural high, which rose within the foredeep basin starting in the early Tortonian. Following the Messinian-Pliocene Apennine chain building phase, the area was subjected to post-orogenic Quaternary extension, related to the opening of the Tyrrhenian sea. A geological map on the 1:20,000 scale illustrates the main stratigraphic and structural features of the area.

Folding, thrusting and diapirism: Competing mechanisms for shaping the structure of the north Dezful Embayment, Zagros, Iran

AbstractDifferentiating compressional growth strata from halokinetic sequences is not straightforward in foreland fold‐and‐thrust belts where compressional and diapiric processes were coeval. Although there are numerous studies on the role of salt layers in fold‐and‐thrust belts, very few focus on syntectonic evaporites where they are thick enough to develop diapirism. The tectonic structures of the northern Dezful Embayment, along the footwall of the Zagros Mountain Front Fault, show a complex evolution interpreted in this study by the concurrence of folding, thrusting and pre‐ and syn‐shortening diapirism of the lower Miocene Gachsaran evaporites. We explore this tectonic–diapiric–sedimentary interplay by combining field studies and remote‐sensing mapping, detailed stratigraphy and sedimentology, high‐quality seismic data, and step‐by‐step balanced tectono‐sedimentary restorations. Initial diapirism occurred soon after the deposition of at least 1.5 km thick Miocene Gachsaran evaporites during the sedimentation of up to 5 km thick Aghajari, Lahbari and Bakhtyari formations, which display syntectonic mixed growth strata and halokinetic sequences. These sedimentary sequences filled salt‐related minibasins, from which the Azanak Minibasin is the deepest, limited by growing salt walls and salt anticlines (Azanak thrust‐weld and the Saland–Lali anticline). Palaeocurrents and clasts compositions document fluvial diversions and exposure and erosion of Gachsaran evaporites along the active salt walls. Although, by further shortening, the precursor salt walls squeezed and reactivated as thrust‐welds structuring a duplex of overriding minibasins. During the deposition of the middle‐late Miocene Aghajari Formation, Kazhdumi and Pabdeh source rocks were buried to oil‐expulsion depths, and Sarvak and Asmari reservoirs were mildly folded in large and open anticlines. The Gachsaran system in the northern Dezful Embayment is collated with the structures of preshortening Hormuz salt and syn‐shortening Fars salt to highlight the important role of these ductile units in the tectonic evolution of the Zagros fold belt that is compared to syn‐compressional salt‐related Pyrenees, Carpathians, Sivas and Kuga fold belts.

Subsurface controls on the development of the Cape Fear Slide Complex, central US Atlantic Margin

Abstract The Cape Fear Slide is one of the largest (>25 000 km 3 ) submarine slope failure complexes on the US Atlantic margin. Here we use a combination of new high-resolution multichannel seismic data (MCS) from the National Science Foundation Geodynamic Processes at Rifting and Subducting Margins (NSF GeoPRISMS) Community Seismic Experiment and legacy industry MCS to derive detailed stratigraphy of this slide and constrain the conditions that lead to slope instability. Limited outer-shelf and upper-slope accommodation space during the Neogene, combined with lowstand fluvial inputs and northwards Gulf Stream sediment transport, appears to have contributed to thick Miocene and Pliocene deposits that onlapped the lower slope. This resulted in burial of an upper-slope bypass zone developed from earlier erosional truncation of Paleogene strata. These deposits created a broad ramp that allowed accumulation of thick Quaternary strata across a low-gradient (<3.5°) upper slope. Upslope of one of the larger headwalls, undulating Quaternary strata appear to downlap onto a buried failure plane. Many of the nested headwalls of the upper-slope portion of slide complex are underlain by deformed strata, which may be the result of fluid migration associated with localized subsidence from salt migration. These new data and observations suggest that antecedent margin physiography, sediment loading and substrate fluid flow were key factors in preconditioning the Cape Fear slope for failure.

Tectonostratigraphic and sedimentary evolution of the Ubur–Orabi sub-basin, southeast Nile Delta, Egypt

Core study of two wells drilled in the southeastern part of the Nile Delta reveals a sub-basin in northern Egypt from the Carboniferous–Permian up to the post-Miocene. The drilled sections are approximately 2600 m thick in the Ubur well in the east and about 3238 m thick in the Orabi well to the west. Basement rocks are recorded in the eastern side of the sub-basin, whereas drilling reached only Lower Cretaceous rocks in the west. The faulting movements played the most important factor determining the distribution, lateral facies changes and unconformities between the different rock units across the sub-basin. The reversal of the faulted blocks over time led to different thicknesses of the drilled units. During the Paleogene, the downfaulted western Orabi part of the sub-basin received up to 1163 m of Paleocene–Eocene sediments, whereas only 203 m were deposited in the eastern uplifted Ubur part. During the Miocene, reversal of the sub-basin taphonomy took place leading to deposition of a thick Miocene section (898 m) in the downfaulted Ubur eastern part, whereas minor thickness (387 m) of the Miocene sediments were laid down in the high Orabi western part of the sub-basin. The stratigraphy of sections drilled is more related to the north Eastern Desert and Sinai rather than to the Western Desert facies. During the Miocene, stratigraphy of the rock units and their facies are a continuation of the Mediterranean subsurface in northern Sinai and eastern Nile Delta facies.

RELATIONS BETWEEN KARST AND THE NEOGENE PALEO-BELAYA VALLEY NETWORK, SOUTHERN CIS-URALS REGION

Miocene-Pliocene tectonic movements transformed considerably the landscape of South Urals and drove spatial patterns and intensity of Earth surface processes. Deepening of river valleys in Paleo-Volga and Paleo-Ural drainage systems facilitated ground water action. Geological section in paleo-valleys of Belaya, Ufa, Bystriy Tanysh, Dioma and other rivers includes up to 200 m thick Late Miocene – Pliocene clayey unit that may favor both drainage and screening of aquifers depending on local hydrogeological conditions. In the former case, karst processes are intensified due to the increased pressure gradients and ground water velocities along the sides of paleovalleys that are composed of Carboniferous and Lower Permian gypsum and limestones. In case of shielding, rising flows of highly mineralized water are discharged in the form of large karst springs (“Krasniy Klyuch”, and others.). Along with natural control factors (lithology, tectonics, hydrogeology, etc.), present-day karstic processes development are controlled by economic activity. Karst and suffosion (underground erosion) are often activated by growth of cities and development of new lands. Neogene and modern valleys promote karst development in Lower Permian gypsums in the Ufa agglomeration where 63 karstic fieds were found. In the last 25 years, 70–80% of hazardous cases related to kasrt intensification were induced by technogenic waterlogging. Karstic and suffusion processes threaten the exploitation of Yumaguzinskaya and – Pavlovskaya Hydropower stations that were constructed on karst prone carbonate rocks at the Belaya and Ufa palevalleys.

Extensional basin development and subsequent inversion in Greymouth Coalfield

The validity of the history of basin development in the east of Greymouth Coalfield, as broadly accepted for the past 60 years, is examined through reinterpretation of the structure and stratigraphy. Rather than a Late Cretaceous–Paleocene fault-angle basin (extensional half-graben) west from the Mount Davy Fault Zone, a downwarping basin extended across the location of that future fault zone. This basin developed within regionally extensive deposition of Late Cretaceous sediments. Uplift and erosion in the east was followed by the development of an Eocene fault-angle basin extending from the Montgomerie Fault west across the developing Mount Davy Fault Zone. A thick Miocene sequence accumulated in the Grey Valley Trough to the east, and a thinner sequence is inferred to have covered the whole coalfield. Inversion in the coalfield, rather than being intermittent throughout the whole Palaeogene, began c. 4.5 Ma ago with development of the Brunner Anticline and reversal of movement on both the Mount Davy Fault Zone and the Montgomerie Fault.

Source rock potential and paleoenvironment of the Miocene Rudeis and Kareem formations, Gulf of Suez, Egypt: An integrated palynofacies and organic geochemical approach

A study on source rock characterization and paleoenvironmental interpretation has been carried out on a relatively thick Miocene sedimentary succession from three wells (GH 404-2A, GH 420-1, SA-E6A) in the southern Gulf of Suez, Egypt. The compilation of detailed optical investigations, including palynofacies analysis, spore coloration index (SCI), vitrinite reflectance (VRr) and source rock geochemical data enables a detailed evaluation of organic matter (OM) richness, kerogen type, and thermal maturation level. Despite their high content of calcium carbonate (up to 52%), the studied sediments yielded sufficient palynologic residue for a detailed palynofacies and organic petrographic study. The palynomorph content comprises mainly marine components, essentially dinoflagellate cysts and accessorily microforaminiferal linings, while terrestrial-derived material is relatively rare. Persistent occurrence of amorphous organic matter (AOM) indicates that dysoxic to anoxic conditions prevailed throughout the deposition of the investigated sediments.The main objective of this study is to combine organic geochemical and palynological data in an attempt to assess the source rock potential of the Rudeis and Kareem formations in the Gulf of Suez. These data, in addition, also allow for the reconstruction of the depositional environment.The present investigation employs multimode microscopic methods using transmitted white light along with incident light fluorescence microscopy. For the first time, geochemical and petrographic analyses performed on bulk rock samples and palynological residues demonstrate that both the Rudeis Formation and the Kareem Formation are mainly of kerogen type III or mixed type II/III. Independent thermal maturation specific parameters such as Rock-Eval Tmax, VRr and SCI indicate an immature to early mature stage. Palynofacies analyses, combined with organic geochemical data indicate that the Rudeis Formation was deposited under shallow to open marine conditions, whereas the overlying Kareem Formation was formed mainly in an open marine setting.

Early diagenetic quartz formation at a deep iron oxidation front in the Eastern Equatorial Pacific – A modern analogue for banded iron/chert formations?

The mechanisms of early diagenetic quartz formation under low-temperature conditions are still poorly understood. In this study we investigated lithified cherts consisting of microcrystalline quartz recovered near the base of a 420m thick Miocene–Holocene sequence of nannofossil and diatom ooze at a drill site in the Eastern Equatorial Pacific (Ocean Drilling Program Site 1226). Precipitation seems still ongoing based on a sharp depletion in dissolved silica at the depth of the cherts. Also, palaeo-temperatures reconstructed from δ18O values in the cherts are in the range of adjacent porewater temperatures. Opal-A dissolution appears to control silica concentration throughout the sequence, while the solution remains oversaturated with respect to quartz. However, at the depth of the sharp depletion in dissolved silica, quartz is still saturated while the more soluble silica phases are strongly undersaturated. Hence, precipitation of quartz was initiated by an auxiliary process.A process, previously observed to assist in the nucleation of quartz is the adsorption of silica on freshly precipitated iron oxides. Indeed, a deep iron oxidation front is present at 400m below seafloor, which is caused by upward diffusing nitrate from an oxic seawater aquifer in the underlying oceanic crust. Sequential iron extraction showed a higher content of the adsorbed iron hydroxide fraction in the chert than in the adjacent nannofossil and diatom ooze. X-ray absorption near-edge structure (XANES) spectroscopy revealed that iron in the cherts predominantly occurs in illite and amorphous iron oxide, whereas iron in the nannofossil and diatom ooze occurs mainly in smectite. Mössbauer spectroscopy also indicated the presence of illite that is to 97% oxidized.Two possible mechanisms may be operative during early diagenetic chert formation at iron oxidation fronts: (1) silica precipitation is catalysed by adsorption to freshly precipitated iron oxide surfaces, and (2) porewater silica concentration is locally decreased below opal-A and opal-CT saturation allowing for precipitation of the thermodynamically more stable phase: quartz. This mechanism of chert formation at the iron oxidation front in suboxic zones may explain why early-diagenetic microcrystalline chert only occurs sporadically in modern marine sediments. It may also serve as a modern analogue for the deposition of much more abundant banded iron/chert formations at the time of the great oxidation event around 2.4Ga BP, which was probably the largest iron oxidation front in Earth's history.

The Stratigraphic, Mineralogical, and Geochemical Characterization of the Beypazarı Oil Shales, Central Anatolia, Turkey

The Beypazarı-Çayırhan oil shale deposit is situated 100–130 km northwest of Ankara in central Anatolia, within a northeast-southwest-oriented Neogene basin that is approximately 70 km long and 25 km wide. The oil shale beds occur in a thick Late Miocene sequence within the Hırka formation, and were deposited in an extended lake. Besides oil shale, the Neogene basin of Beypazarı also contains lignite, trona, and thenardite as valuable raw materials. The vegetation of the Beypazarı lacustrine basin was dominated by trees, and palynological analysis reveals the existence of a swamp-forest developed in a subtropical to warm-temperate humid climate. Finally, the oil shale of Beypazarı-Çayırhan basin is, on average, of low quality (the average upper calorific value is 840 kcal/kg [= 3.5 MJ/kg] and the oil content is 5.4% respectively 60 l/t), and, due to the tectonic situation, low cost open-cast mining is precluded. Therefore, the industrial utilization of the oil shale is not economical under present conditions and is thus not recommended. This is due to the (1) on-average poor oil shale quality, (2) complicated geologic structure, and (3) necessity of expensive underground mining.

Species-specific responses of late Miocene Discoaster spp. to enhanced biosilica productivity conditions in the equatorial Pacific and the Mediterranean

Census data of a major Cenozoic calcareous nannofossil genus (Discoaster) have been acquired from Site U1338, located near the Equator in the eastern Pacific Ocean and drilled in 2009 during Integrated Ocean Drilling Program (IODP) Expedition 321. The investigated 147.53 m thick upper Miocene sediment sequence is primarily composed of biogenic carbonate and biogenic silica. Diatom biostratigraphic data were used to develop a revised biomagnetostratigraphic age model, resulting in more variable late Miocene sedimentation rates. Carbonate content variations mainly reflect dilution by biogenic silica production, although intense carbonate dissolution affects a few shorter intervals. Abundance variations of discoasters show no distinct correlation with either carbonate or biosilica contents. The two dominant Discoaster taxa are D. brouweri and D. variabilis, except for a 12 m thick interval where D. bellus outnumbers the sum of all other discoasters by a factor of 4.6. Data presented indicate that first D. hamatus and then D. berggrenii both evolved from D. bellus. Three unusual morphotypes, here referred to as Discoaster A, B and C, increase in relative abundance during episodes of enhanced biosilica production in the upper half of the investigated sequence (Messinian). Strikingly similar morphotypes have been observed previously in Messinian age sediments from the Mediterranean, characterized by alternating deposition of biogenic carbonate and biosilica. This suggests a species-specific response among some of the late Miocene discoasters to broader oceanographic and climatic forcing that promoted episodes of enhanced deposition of biogenic silica.

Palaeoenvironmental and chronological constraints on the Tuğlu Formation (Çankırı Basin, Central Anatolia, Turkey)

The Çankırı Basin, located in the northern part of the Central Anatolian Plateau, is a large Tertiary basin where thick Miocene to Quaternary continental sediments overlay the Cretaceous-Tertiary units. This investigation focuses on the Tuğlu Formation, an Upper Miocene succession mainly composed of dark grey silty and organic rich clays. The type section of Tuğlu has been sampled for an array of multidisciplinary analyses. The palaeontological proxies included ostracod, foraminifer, nannoplankton, pollen, molluscs, charophytes, small mammal assemblages, fish, and crab remains. The abiotic parameters studied were: palaeomagnetism and environmental magnetism, stable carbon and oxygen isotope ratios (on ostracods and bulk sediment samples), strontium isotope ratios (on ostracods and foraminifera), and major elemental composition of the sediments. All analysed proxies point to a continental setting characterised by permanent water bodies affected by strong salinity oscillations. A shallow saline lake developed in a permanent freshwater lake. Barren layers, potentially linked to a short subaerial exposure, mark the end of the saline lake and the transition to a fluvial environment. Geochemical analysis confirms aridity-humidity oscillations as recorded by the micropalaeontological proxies. Analysis of small mammal assemblages refined the chronology of the Tuğlu Formation, with the onset of the deposition at the base of the mammal zone MN9 (around 11 Ma) and continuous deposition until the MN11 zone (around 8 Ma). The stable oxygen isotope records from the Tuğlu section point to Miocene delta^{18}O water values consistent with subdued topography where no prominent mountain belts were yet developed at the northern plateau margin. If correct, this suggests that at least until the end of the deposition of the Tuğlu Formation, the Çankırı Basin did not yet experience rain shadow conditions, and the regional surface uplift of the area most likely occurred after 8 Ma.

Stable isotopic composition of soil calcite (O, C) and gypsum (S) overlying Cu deposits in the Atacama Desert, Chile: Implications for mineral exploration, salt sources, and paleoenvironmental reconstruction

Soils overlying two porphyry Cu deposits (Spence, Gaby Sur) and the Pampa del Tamarugal, Atacama Desert, Northern Chile were collected in order to investigate the extent to which saline groundwaters influence “soil” chemistry in regions with thick Miocene and younger sediment cover. Soil carbonate (calcite) was analyzed for C and O isotopes and pedogenic gypsum for S isotopes. Soil calcite is present in all soils at the Spence deposit, but increases volumetrically above two fracture zones that cut the Miocene gravels, including gravels that overlie the deposit. The C isotope composition of carbonate from the soils overlying fracture zones is indistinguishable from pedogenic carbonate elsewhere at the Spence deposit; all δ13CVPDB values fall within a narrow range (1.40–4.23‰), consistent with the carbonate having formed in equilibrium with atmospheric CO2. However, δ18OVPDB for carbonate over both fracture zones is statistically different from carbonate elsewhere (average δ18OVPDB=0.82‰ vs. −2.23‰, respectively), suggesting involvement of groundwater in their formation. The composition of soils at the Tamarugal anomaly has been most strongly affected by earthquake-related surface flooding and evaporation of groundwater; δ13CVPDB values (−4.28‰ to −2.04‰) are interpreted to be a mixture of dissolved inorganic C (DIC) from groundwater and atmospheric CO2. At the Spence deposit, soils only rarely contain sufficient SO4 for S isotope analysis; the SO4-bearing soils occur only above the fracture zones in the gravel. Results are uniform (3.7–4.9‰ δ34SCDT), which is near the middle of the range for SO4 in groundwater (0.9–7.3‰). Sulfur in soils at the Gaby Sur deposit (3.8–6.1‰ δ34SCDT) is dominated by gypsum, which primarily occurs on the flanks and tops of hills, suggesting deposition from SO4-rich fogs. Sulfate in Gaby Sur deposit gypsum is possibly derived by condensation of airborne SO4 from volcanic SO2 from the nearby Andes. At the Gaby Sur deposit and Tamarugal anomaly, pedogenic stable isotopes cannot distinguish between S from porphyry or redeposited SO4 from interior salars.The three sites studied have had different histories of salt accumulation and display variable influence of groundwater, which is interpreted to have been forced to the surface during earthquakes. The clear accumulation of salts associated with fractures at the Spence deposit, and shifts in the isotopic composition of carbonate and sulfate in the fractures despite clear evidence of relatively recent removal of salts indicates that transfer from groundwater is an ongoing process. The interpretation that groundwaters can influence the isotopic composition of pedogenic calcrete and gypsum has important implications for previous studies that have not considered this mechanism.

Failure mechanism of overhanging slopes in sedimentary rocks with dissimilar mechanical properties

The paper discusses a slope failure which occurred in December 2009 adjacent to a road in northern Greece. Thick Miocene limestone horizons are interbedded with relatively thin weak horizons of silty to sandy clays and loams which are more easily eroded, resulting in the formation of notches. The weak horizon acts as an aquitard, inhibiting downward percolation from both rainfall and house discharges. The two main areas of failure occurred where the notches were particularly deep (hence the overhang was greatest) at a time when heavy rainfall created a hydrostatic head within the bedrock to the west of the slope face. The stability of the notched benches is controlled by the tensile strength of rock bridges in the limestone’s discontinuities. The analysis showed that the limestone’s in situ tensile strength and cohesion are on average three times lower than those obtained from laboratory tests, hence their application in the failure analysis would lead to an overestimation of the slope stability.

Thick Miocene eolian deposits on the Huajialing Mountains: The geomorphic evolution of the western Loess Plateau

The geomorphic evolution of northwestern China during the Cenozoic has been a subject of much geological interest because of its link with the uplift of the Himalayan-Tibetan complex. Much information about these changes is recoverable from the sedimentary sequences of the region. We report here on the thick eolian deposits mantling the Huajialing Mountains, a relatively flat mountain range in the western Loess Plateau. Correlation of magnetic susceptibility stratigraphy with the QA-I Miocene eolian sequence dates a 134.7 m section (NL-VII) for the interval from 18.7 to 11.8 Ma, as confirmed by micromammalian fossils. These eolian deposits demonstrate a much wider distribution of the Miocene eolian deposits, and also indicate that the topography contrasts in the western Loess Plateau, including the uplifts of the Huajialing Mountains and the bedrock highlands in the Qinan region, were formed by the early Miocene. The near-continuous Miocene eolian sequence from 18.7 to 11.8 Ma indicates that the substratum of Huajialing had not experienced any intense tectonic changes during this time interval, which suggests further, the relative tectonic stability of the nearby Tibetan Plateau.

Structural styles in the Papuan Fold Belt, Papua New Guinea: constraints from analogue modelling

Abstract Cross sections, seismic data and centrifuge analogue modelling reveal the structural styles in the oil-producing areas of the Papuan Fold Belt. They include inverted basement faults, detachment faults in the Jurassic section 1–2 km beneath the Neocomian Toro Sandstone reservoir, and tight, overturned folds in the reservoir sequence with stretched and boudinaged forelimbs, cut by break-thrusts. Additional features include highly variable thicknesses in the Cretaceous Ieru Formation, the regional seal sequence, including through-going detachments that isolate the overlying thick Miocene Darai Limestone. Centrifuge analogue modelling of intact, plane-layered strata determined that the mechanical stratigraphy and the thickness of weak beds above the lower décollement horizon exert the greatest control on the structural style. Large-offset thrust faults were only produced in models with pre-cut faults, generating early inversion and then large ramp anticlines, similar to those in the Kutubu Oilfield, which has reserves of >350 million barrels. It is suggested that the Kutubu Oilfield trend was underlain by a large normal fault and that, by analogy with the Vulcan Sub-basin, oil-rich source rocks may be confined to the hanging wall or north side of this fault. Oil would have been generated and expelled during thin-skinned deformation.