Siliciclastic Basin Research Articles

Screening for AVA Anomalies in Siliciclastic Basins: Testing a Seismic Inversion Method in the Mississippi Canyon, Gulf of Mexico

Screening for AVA Anomalies in Siliciclastic Basins: Testing a Seismic Inversion Method in the Mississippi Canyon, Gulf of Mexico

Correlation of Boreholes through Well Logs: Application to the Western Sector of Madrid

This study correlates the results obtained from the resistivity and spontaneous potential well logs in six boreholes for water extraction, located in the multilayer siliciclastic basin in the Madrid region, in the center of the Iberian Peninsula. Given the small lateral continuity that the layers considered in isolation show in this type of multilayer aquifer, geophysical stretches, with their corresponding average lithological assignments, have been established to achieve this objective from the well logs. These stretches allow for mapping the internal lithology in the studied area, obtaining a correlation of greater geological scope than that provided by the correlation between layers. Subsequently, the possible correlation of the lithological stretches selected in each of the boreholes was analyzed, verifying their lateral continuity and establishing an NNW-SSE section in the study area. In this work, the transcendence of the correlation of wells up to great distances (about 8 km in total, and average distance of 1.5 km between wells) is focused on the fact that, if there is a presence of pollutant in certain aquifer stretches in a part of the studied area, overexploitation in the Madrid basin will cause the mobilization of this pollutant to the whole basin, with a possible impact on non-polluted areas.

Automatic Zonation and Correlation of Well Logs for a Tectono-Sedimentary Interpretation in the Continental Siliciclastic Basin of Duero River (Iberian Peninsula)

Abstract A procedure to automatically correlate well logs measured in boreholes that are located in continental siliciclastic basins by using two different methods is shown. The first method is applied to the parametric layers that were determined in each borehole starting from the values of their geophysical parameters and consists of correlating, by cross-association, the columns formed by these layers. The second method consists of cross-correlating the geophysical stretches or units, which are established as sets of layers with similar characteristics that are sufficiently different from the average values in the adjacent stretches. The evaluation of the correlation results requires showing the criteria that are used for determining the parametric layers that are obtained from the well logs, the result of which is called segmentation in this study. This evaluation also requires to show the techniques that are used to determine the geophysical stretches by a process that is called stretching in this study. The reason for using different correlation methods is that cross-association of layers provides high resolution but relatively smaller spatial extent, whereas cross-correlations of geophysical stretches provide higher spatial extent but lower resolution. Thus, the cross-association results have been used both to assess the correlations in boreholes that are relatively close (distances<10 km) and to support the establishment of the stretch correlation criteria. The developed methodology is applied to a set of boreholes located in the Duero Basin (Spain). From the results obtained, an evaluation of the correlations with respect to the distances between boreholes was carried out. Furthermore, it is shown that the correlations between geophysical stretches enable identifying the correspondences between these and the tectono-sedimentary sequences (activation-relaxation of a tectonic phase) that are established in the literature.

Characterization and evolution of a carbonate basin floor fan on a leeward side of an isolated carbonate platform, Sirte Basin, Libya

The recognition of carbonate basin floor fans and the interpretation of their formation and evolution are extremely crucial to the hydrocarbon exploration since they form important oil and gas reservoirs. There has been little research on the geometries and controlling factors of the carbonate basin floor fans from subsurface three-dimensional (3D) seismic and wireline log data. Most studies have been conducted on siliciclastic basin floor fans. Thus, this study characterizes the internal configuration and processes controlling the deposition of a lower Eocene carbonate basin floor fan system in the Sirte Basin in a sequence stratigraphic context through integrated 3D seismic and wireline log interpretations. The carbonate fan complex shows hummocky to undulated seismic geometries on vertical seismic sections and bounded by an unconformable sequence boundary at the base and a downlap surface at the top. It has an aerial extension of around 9 km from south to north along depositional dip and at least 8 km from east to west along strike. In well data, the fan system predominantly comprises limestone units intercalated with thin shale intervals. Log curves display an overall funnel-shaped trend, suggesting progradational stacking pattern. The vertical thickness of the fan complex varies from 120 to 165 m. The evolution of the carbonate fan was influenced by the interplay of accommodation space and sediment supply. A pre-existing topography, tectonic subsidence, and sea-level rise generated accommodation space for the deposition of the fan system, whereas sediment-gravity flows transported the fan sediments from the platform interior and margin to the basin floor. This study provides sedimentological and stratigraphic criteria to recognize and interpret the evolution of basin floor fans along isolated carbonate platforms from 3D seismic and wireline log data and may serve as an analog for ongoing and future studies on carbonate basin floor fans elsewhere.

Interpreter's Corner

Global empirical relationships of P-wave to S-wave and density for sandstones and shales are used to model two-term amplitude variation with angle at various depths of burial in a typically compacting siliciclastic basin. Data from the normally pressured Tertiary strata of Judd Basin, Atlantic Margin, West of Shetland, are used as a control. For a typical prospect depth of 1750 m below mudline, forward models of angle-dependent reflectivity reveal that discrimination of lithology (shale and brine sand) and fluid (oil sand) is optimally resolved at a 47° incidence angle (θ). This is equivalent to an angle of 28° on an intercept-gradient crossplot. Repeat experiments at other depths produce similar results but with the angle for optimal lithology and fluid determination shifting slightly with increasing depth. Background trends in seismic data crossplots of intercept versus gradient are typically overprinted by noise that has a disproportionate effect on the gradient. This study suggests that the difference between the noise and background rock-property trend is relatively small, such that in most modern seismic data sets, anomalies should be identifiable on time-windowed crossplots and equivalent weighted stacks. It is proposed that a seismic inversion for relative extended elastic impedance at a 45° incidence angle should capture most anomalies of interest in frontier basins with simple burial histories. An example is illustrated from a seismic line in Mozambique.

Stress states of isolated carbonate platforms: Implications for development and reactivation of natural fractures post burial

We use 2D finite element numerical geomechanical modeling to interrogate how different scenarios of far field stress regimes, rock mechanical properties and margin morphology affect stresses post burial within a generic isolated carbonate platform. We test a range of stress scenarios that include normal and strike-slip faulting conditions, under low and high differential far field stress regimes. Rock mechanical properties are representative of both young (weak) and ancient (strong) end-member reservoir rocks and distributed within two distinct domains (rimming reef and platform interior). Results show that platform geometry is a first order control on the magnitude and orientation of stresses. Characteristic features of carbonate margins like scallops or promontories tend to preferentially concentrate stresses, and can become sweet spots for fracture development. In low differential far field stress scenarios, the maximum horizontal stress rotates to mimic the margin orientation, while no rotations are observed under high differential stress scenarios. The stiffer nature of the carbonate rocks, compared to the surrounding siliciclastic basin infill, amplifies the far field differential stresses within the carbonate platform. In high differential far field stress scenarios, the local differential stresses can increase to the point that shear failure can be achieved within the platform interior, resulting in the potential development of fracture corridors. Some margin parallel pre-existing fractures show a high likelihood of reactivation. We postulate that some early margin syndepositional fractures are long lived conduits for fluid flow as they are prone to undergo multiple reactivation phases during and post burial, thus potentially controlling diagenetic overprints and present-day fluid flow in hydrocarbon reservoirs. Results from this type of modeling can be used to predict areas of increased fracture density within a carbonate reservoir (i.e. reservoir characterization), as well as to optimize trajectories of highly deviated wellbores (i.e. wellbore stability).

Age, provenance and tectonic setting of metasedimentary sequences of the Gurupi Belt and São Luís cratonic fragment, northern Brazil: Broadening the understanding of the Proterozoic-Early Cambrian tectonic evolution

Geological, U-Pb (detrital zircon), whole-rock Nd isotopes, and geophysical information constrain the timing, provenance, and depositional and tectonic settings of sedimentary successions from the Gurupi Belt (GB) and São Luís cratonic fragment (SLCF) in northern Brazil. The results, along with published information about the magmatic events are used to discuss the crustal framework and regional correlations. We defined the following stages of sedimentation related to the Rhyacian and Neoproterozoic-Early Cambrian evolution. (1) 2240–2160 Ma –metavolcano-sedimentary sequences formed in arc- (SLCF) and back-arc (GB) basins. (2) 2165–2120 Ma – two syn-orogenic, siliciclastic (turbidite) sequences; the oldest (>2149 Ma) formed in a forearc or intra-arc setting, the youngest (>2116 Ma) formed in forearc or distal intra-arc system. (3) 2120–2088 Ma – post-collisional, continental, siliciclastic basin(s) formed in fluviatile setting, and representing a foreland system, which was split into two sub-basins in more recent times. These Rhyacian sequences correlate with similar successions in the West-African and Amazonian cratons, and formed during the agglutination of a continental block in pre-Columbia times (>2000 Ma), which remained joined through the assembly (~1800 Ma) and breakup (~1300 Ma) of Columbia, being only fragmented during Rodinia breakup (~750 Ma). This breakup produced, probably distal, intracontinental effects that led to the deposition of (4) 1130–740 Ma – siliciclastic sequences formed in rift and passive margin basins; no evidence of oceanization is observed, and the sequences correlate in time with sedimentary formations of the Rokelide Belt in West Africa. The closure of the epicontinental sea took place during intracontinental orogeny, which represents a distal effect of the collisional orogenies that assembly West-Gondwana at the Neoproterozoic-Early Cambrian boundary. The last sedimentation event comprises (5) < 591 Ma – a post-orogenic/post-collisional siliciclastic basin formed in a rift that might also have been acted as a precursor rift of the Phanerozoic intracratonic basins.

The origin of the Pliocene to recent succession in the Levant basin and its depositional pattern, new insight on source to sink system

The Pliocene-to-Recent succession in the deep Levant basin is coeval to the development of the Nile River delta and to the progradation of the thick (~1500 m) Sinai-Israel shelf. It hides a series of paleo-channels exhibiting transportation and sedimentation patterns revealing a world class source to sink system feeding a deep (>1500 m) siliciclastic basin. The general agreement that the Pliocene-to-Recent succession originates from the Nile Delta dispersing sediments via a system of counterclockwise currents does not reveal how the sediments were transported to the deep basin. Particularly, how sediments originating from the Nile Delta could have bypassed the ~50 km wide Sinai-Israeli shelf. Here, we examine the various sources that contributed to the accumulation of the Pliocene-to-Recent succession in the deep Levant basin, and the temporal and spatial contribution of each source. Analysis of a unique seismic data set covering the shelf, slope and deep basin enable us to track submarine sediment transport systems; we map channel sets, analyze their morphological features and interpret their erosional and depositional patterns. We argue that sediments sources vary from eastward remnant Arabian drainage network at the onset of the Pliocene, to Nilotic origin during the Pliocene. Since the Late Pleistocene reworked sediments, deriving from the Israeli shelf and northern Sinai provide a major source to the deep basin. Furthermore, our results demonstrate an increase in channel's complexity since the Early Pliocene to Recent, suggesting a gradual transition from sporadic submarine flow events, carrying fewer sediments to the deep basin at the Early Pliocene, to more frequent events during the Late Pleistocene-to-Recent characterized by an increase in sediment load. The gradual increase of channel complexity from Pliocene-to-Recent is discordant to the general trend of sea-level fluctuation, suggesting that sea-level has a minor effect on sediment accumulation in the deep basin.

Bottomset and foreset sedimentary processes in the mixed carbonate-siliciclastic Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Picún Leufú Area, Argentina

The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation at the Picún Leufú area constitutes bottomset and foreset marine deposits comprising open bay, siliciclastic basin and mixed carbonate-siliciclastic shelf facies. Detailed sedimentological, ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences. Depositional Sequence 1 (DS1) begins with lowstand eolian deposits included in the Quebrada del Sapo Formation, and continues with thin, retrograding, transgressive open bay facies of the Vaca Muerta Formation encompassing coastal sand sheet and dune, marginal marine, bay margin and distal bay facies. Above the transgressive deposits, the regressive hemicycle of DS1 consists of bottomset deposits representing a siliciclastic basin facies association. Lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen-deficient environment. A relative sea-level fall generated a sequence boundary, which coincides with an angular unconformity that marks the base of Depositional Sequence 2 (DS2). This sea-level fall triggered the formation of an extensive lowstand channel-fill and lobe complex at the base of the slope, followed by retrogradational lobe facies during the subsequent transgression. The regressive hemicycle of DS2 represents foreset mixed carbonate-siliciclastic shelf facies, forming a slope mud belt and slope sand bodies. Near the top, sedimentation in a foreset-topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits. Sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary. The combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition, underscoring the role of wave-influenced hyperpycnal flows, and the effect on organic matter dilution in one of the most important unconventional reservoirs from Argentina.

Overpressure development in sedimentary basins induced by chemo-mechanical compaction of sandstones

Abstract Mechanisms of overpressure build-up resulting from chemically-induced compaction are investigated by considering that intergranular pressure-solution, controlled by temperature and effective stresses, is the dominant process of chemical deformation in sandstones. The numerical simulations are performed using a thermo-poro-mechanical tool based on the finite element method specifically devised to deal with sedimentary basin modeling. At the material level, purely mechanical and chemo-mechanical deformations are respectively addressed by means of plastic and viscoplastic constitutive components. Porosity data of the Middle Jurassic Garn Formation from the Haltenbanken area of the Mid-Norwegian Continental Shelf are taken as reference for calibration of the sandstone model to be used on synthetic cases of a siliciclastic basin in oedometric conditions. Two situations are proposed involving different depositional sequences of sandy and shaly sediments with the aim to assess the permeability effect in the numerical model. The results have shown that early overpressure development in low permeability formations preserves sandstone porosities by reducing effective stresses and thus retarding pressure-solution compaction, whereas higher effective stresses associated with permeable depositional environments may lead to important chemo-mechanical deformation, resulting in low porosity sandstones and significant overpressure generation at later stages of basin history. An additional case is finally analyzed to investigate the consequences of pressure-solution inhibition due to diagenetic grain coating in a sandstone reservoir. The simulation resulted in porosity preservation and lower overpressure values, showing that a sedimentary basin submitted to an important level of chemo-mechanical compaction can present substantially higher overpressure distribution than basins where this phenomenon did not take place. The paper highlights the importance of integrating stresses, fluid pressure and temperature to represent the porous material evolution in order to describe the geological state of sedimentary basins.

Checklist for the use of potassium concentrations in siliciclastic sediments as paleoenvironmental archives

Weathering indices quantify the release of mobile elements from source units of soils and sediments. They have been employed in geosciences for more than three decades, but their performance in sedimentary archives can still be improved by deciphering other forcing factors than climatic. From the most common alkali and alkaline earth metals included in the weathering indices, K seems to be most suitable for deciphering paleoclimate records of temperate to subtropical non-arid climates, assuming the interpretation respects the complex of controlling factors on sediment geochemistry. As a case study, normalised K concentrations in siliciclastic sediments of the Most Basin (Miocene period, central Europe) are revisited here and variations compared with modern fluvially transported solids. Existing knowledge on manifold controls of K concentrations in sediments is summarised and used to propose list of possible controlling factors, in particular Al/Si and Zr/Rb as grain-size proxies, Ti/Al as provenance proxy relevant for K concentration in parent rocks, and K/Rb as a measure for possible effects of metasomatism. The K/Al or K/Ti element ratios may be efficient in chemostratigraphic correlations in siliciclastic basins and efficient proxies for chemical weathering in the source area. The proposals in this work should stimulate future studies to overcome the underrepresentation of continental sediments in paleoclimate reconstructions.

Spatial and temporal variation in detrital zircon age provenance of the hydrocarbon-bearing upper Roper Group, Beetaloo Sub-basin, Northern Territory, Australia

The subsurface Beetaloo Sub-basin of the McArthur Basin, Northern Territory, Australia, comprises a succession of shallow-water, dominantly marine, clastic sedimentary rocks that formed in the main depocentre of the Mesoproterozoic Roper Group. This group contains the oldest commercial hydrocarbons known, whose presence has been linked to changing nutrient flux controlled by a changing provenance. LA-ICP-MS detrital zircon U–Pb age data presented here provide new age constraints on the upper Roper Group and reveal spatial and temporal provenance variations illustrating the evolution of the basin and its margins that are linked to a major provenance change caused by the coeval collision of the combined South Australian Craton/North Australian Craton with the West Australian Craton.The maximum depositional ages of the Bessie Creek Sandstone and the Velkerri Formation of the Roper Group are constrained to 1386 ± 13 Ma and 1308 ± 41 Ma, respectively, whereas the overlying Moroak Sandstone has no younger detrital zircons, so its maximum depositional age is also constrained as 1308 ± 41 Ma. The Kyalla Formation was deposited after 1313 ± 47 Ma, and two, as yet, informally defined and ungrouped latest Mesoproterozoic to Neoproterozoic sedimentary units, the lower and upper Jamison sandstone, have maximum depositional ages of 1092 ± 16 Ma and 959 ± 18 Ma, respectively. Large detrital zircon age datasets (of 1204 near-concordant analyses) indicate that zircons from the Maiwok Subgroup were originally sourced from Palaeoproterozoic and earliest Mesoproterozoic rocks. These are consistent with derivation from the surrounding exposed basement. Detrital zircon age variations up-section suggest a systematic temporal change in provenance. The oldest formation analysed (Bessie Creek Sandstone) has a major source dated at ca. 1823 Ma. Rocks of this age are common in northern basement exposures. Samples from the overlying Velkerri Formation, show derivation from a ca. 1590 Ma source, consistent with rocks exposed in Queensland, or the Musgrave Province. The Moroak Sandstone and the Kyalla Formation show progressively more ca. 1740 Ma detritus, which we suggest likely reflects new sources in the Arunta Region to the south.We suggest that the provenance variation initially records exposure and denudation of western Queensland rocks at ca. 1400 Ma due to rifting between Laurentia and the North Australian Craton. From then until at least ca. 1320 Ma, the increased ca. 1740 Ma detritus suggests uplift of the Arunta Region that we interpret as reflecting collision between the southern North Australian Craton and the West Australian Craton as ca. 1300–1400 Ma. This tectonically-controlled provenance change is interpreted to have included erosion of nutrient rich arc-rocks that may have caused a bacterial bloom in the Roper Seaway. The Jamison sandstone and overlying Hayfield mudstone represent a marked change in provenance and were deposited after the Musgrave Orogeny, representing a newly-recognised siliciclastic basin that may have formed a shallow, long wavelength foreland basin to areas uplifted during the Musgrave Orogeny.

Geochemical constraints on the temperature and timing of carbonate formation and lithification in the Nankai Trough, NanTroSEIZE transect

Information about diagenetic processes and temperatures during burial of sediments entering the subduction zone is important for understanding changes in physical properties and seismic behavior during deformation. The geochemistry of authigenic carbonates from accretionary prisms can serve as proxies for conditions during carbonate cementation and resultant lithification. We report results from the Nankai accretionary prism recovered from Integrated Ocean Drilling Program (IODP) sites C0011 and C0012 and we document continued cementation of deep sediment sections prior to subduction. Elemental and isotope data provide evidence for complex mixing of different isotopic reservoirs in pore waters contributing to carbonate chemical signatures. Carbon stable isotope values exhibit a broad range (δ13CV-PDB=+0.1‰ to −22.5‰) that corresponds to different stages of cement formation during burial. Carbonate formation temperatures from carbonate-clumped isotope geochemistry range from 16°C to 63°C at Site C0011 and 8.7°C to 68°C at Site C0012. The correspondence between the clumped-isotope temperatures and extrapolations of measured in situ temperatures indicate the carbonate is continuing to form at present. Calculated water isotopic compositions are in some cases enriched in 18O relative to measured interstitial waters suggesting a component of inherited seawater or input from clay-bound water. Low oxygen isotope values and the observed Ba/Ca ratios are also consistent with carbonate cementation at depth. Strontium isotopes of interstitial waters (87Sr/86Sr of 0.7059–0.7069) and carbonates (87Sr/86Sr of 0.70715–0.70891) support formation of carbonates from a mixture of strontium reservoirs including current interstitial waters and relic seawater contemporaneous with deposition. Collectively our data reflect mixed sources of dissolved inorganic carbon and cations that include authigenic phases driven by organic carbon and volcanic alteration reactions. Physical properties of input sediments continue to undergo modification by carbonate cementation at present. Due to ongoing recrystallization, temperatures from carbonate-clumped isotopes reflect the modern geothermal gradient and may serve as useful measures of geothermal gradients in other siliciclastic basins where carbonate cementation occurs. We conclude that clumped-isotope signatures in authigenic carbonates from accretionary prisms are important proxies for the timing and conditions of cementation in active margins. Our results highlight the importance of using multi-proxy approaches to elucidate the history of carbonate cementation, particularly to establish carbonate precipitation at depth and its potential impact on the physical and mechanical properties of the sediment prior to subduction.

A new Paleoproterozoic tectonic history of the eastern Capricorn Orogen, Western Australia, revealed by U–Pb zircon dating of micro-tuffs

The Earaheedy Basin is one of several Paleoproterozoic siliciclastic basins in the eastern part of the Capricorn Orogen in Western Australia. In the absence of an adequate chronostratigraphic framework, multiple conflicting models have been proposed for the tectonic history of the basin. We present in situ SHRIMP U–Pb zircon dates of 1971±11Ma, 1954±5Ma and 1949±10Ma from three thin tuff horizons (“micro-tuffs”) in the Yelma Formation at the base of the basin succession. These micro-tuffs are only slightly younger than the 1990±6Ma Imbin porphyry, which has been interpreted as a basement inlier. Our new dates, combined with field mapping, suggest that the Imbin porphyry is a volcanic–subvolcanic complex within the basin. Micro-tuffs in the Yelma Formation are more than 50m.y. older than c. 1890Ma micro-tuffs within the overlying Frere Formation. Deposition of the Yelma Formation was coeval with continent–continent collision farther west in the Orogen marked by the 2005–1950Ma Glenburgh Orogeny, and the formation is possibly correlative with peripheral foreland deposits of the Padbury Group. The Frere Formation was deposited in shallow marine conditions within an intracratonic basin about 60m.y. after the Glenburgh Orogeny. The Frere Formation is unconformably or disconformably overlain by the Miningarra Group, the youngest unit of which is the Mulgarra Sandstone. This sandstone was deposited after c. 1865Ma, and derived either from exposed Frere Formation or from a now-buried source within the Capricorn Orogen that was uplifted by inception of the 1820–1770Ma Capricorn Orogeny. Therefore, the Earaheedy Basin comprises at least three distinct, unconformity-bound successions deposited in contrasting tectonic settings.

Basement geology beneath the northeast Thelon Basin, Nunavut: insights from integrating new gravity, magnetic and geological data

ABSTRACTCurrent models for unconformity‐associated uranium deposits predict fluid flow and ore deposition along reactivated faults in &gt;1.76 Ga basement beneath Mesoproterozoic siliciclastic basins. In frontier regions such as the Thelon Basin in the Kivalliq region of Nunavut, little is known about the sub‐basin distribution of units and structures, making exploration targeting very tenuous. We constructed a geological map of the basement beneath the unconformity by extrapolating exposed features into the subsurface. The new map is constrained by detailed geological, geophysical, and rock property observations of outcrops adjacent to the basin and by aeromagnetic and gravity data over the geophysically transparent sedimentary basin. From rock property measurements, it is clear that the diverse magnetic and density characteristics of major rock packages provide quantitative three‐dimensional constraints. Gravity profiles forward modelled in four cross sections define broad synforms of the Amer Belt and Archean volcanic rocks that are consistent with the structural style outside the basin. Major lithotectonic entities beneath the unconformity include: supracrustal rocks of the Archean Woodburn Lake group and Marjorie Hills meta sedimentary gneiss and associated mixed granitoid and amphibolitic gneiss; the Amer Mylonite Zone and inferred mafic intrusions oriented parallel and sub‐parallel; other igneous intrusions of 2.6 Ga, 1.83 Ga, and 1.75 Ga vintage; and the &lt;2.3 Ga to &gt;1.84 Ga Amer Group. Four main brittle regional fault arrays (040°–060°, 075°–90°, 120°, and 150°) controlled development and preservation of the basin. The reactivated intersections of such faults along fertile basement units such as the Rumble assemblage, Marjorie Hills assemblage, Nueltin igneous rocks, and Pitz formation are the best targets for uranium exploration.

Evolution of the Late Cretaceous Yongfeng-Chongren Basin in Jiangxi Province, southeast China: insights from sedimentary facies analysis and pebble counting

The Gan-Hang Belt in Southeast China is characterized by several igneous and siliciclastic basins associated with crustal extension during Late Mesozoic. The sedimentary evolution of the red basins is still poorly understood. In this study, sedimentary facies analysis and pebble counting were performed on outcrop sections of the Late Cretaceous Guifeng Group inthe Yongfeng-Chongren Basin in central Jiangxi Province. Thirty-five conglomerate outcrops were chosen to measure pebble lithology, size, roundness, weathering degree and preferred orientation. Results show that gravels are mostly fine to coarse pebbles and comprise dominantly quartzites, metamorphic rocks, granitoids and sandstones. Rose diagrams based on imbricated pebbles indicate variable paleocurrent directions. Combining with typical sedimentary structures and vertical successions, we suggest that the Guifeng Group were deposited in alluvial fan, river and playa lake depositional systems. The proposed depositional model indicates that the Hekou Formation represents the start-up stage of the faulted basin, accompanied by sedimentation in alluvial fan and braided river environments. Then this basin turned into a stable expansion stage during the deposition of the Tangbian Formation. Except for minor coarse sediments at the basin margin, the other area is covered with fine-grained sediments of lake and river environments. The Lianhe Formation, however, is once again featured by conglomerates, suggesting a probable tectonic event. Therefore, the study region possibly suffered two tectonic events represented by the conglomerates of the Hekou and Lianhe formations in the context of the crustal extension in Southeast China.

Tectono-sedimentary evolution of the Permian–Triassic extension event in the Zagros basin (Iran): results from analogue modelling

Since the 1970s, the largest oil and gas reservoirs have been discovered in the Permian–Early Triassic formations in Saudi Arabia. Thus, this time period is important for the discovery of new oil reserves in Iran. The Arabian passive continental margin has undergone lithospheric extension during the Permian–Triassic, which led to the formation of the Neo-Tethys. The aim of this paper is to describe the development of the continental rift basin in the Zagros region based on the tectono-sedimentological evolution. We have studied well-log data to specify the distribution of synrift deposits in the Zagros and have related this information to the modelling. Environmental changes indicated by various sedimentary sequences, from a siliciclastic basin to a carbonate platform setting, are described. The Cambrian Hormuz salt, which overlies the metamorphosed Precambrian basement, becomes effective as a basal detachment layer influencing the style of overburden deformation during the Permian–Triassic extension event. We have investigated the formation of various structures linked to the presence or absence of the Hormuz layer by analogue modelling and relating these structures to the Late Palaeozoic sedimentation. Based on results of the analogue modelling, we argue that the basal detachment layer (Hormuz series) has contributed to the various structural styles of the extensional basin development in the Fars domain and the Lorestan domain.

A high-resolution carbon-isotope record of the Turonian stage correlated to a siliciclastic basin fill: Implications for mid-Cretaceous sea-level change

Turonian strata of the Bohemian Cretaceous Basin, Central Europe, preserve a basin-scale record of shoreline transgressions and regressions, previously interpreted to have been strongly influenced by short-term eustatic cycles. Here, nearshore siliciclastic strata in two separate sub-basins are correlated to a multi-stratigraphic dataset generated from a new research core (Bch-1) drilled in offshore marine sediments of the central basin. A high-resolution δ13Corg record from Bch-1 is presented along with major- and minor-element proxies, TOC, carbonate content, terrestrial to marine palynomorph ratios, and detailed macro- and microfossil biostratigraphy. The 400m thick Turonian through Lower Coniacian interval permits correlation to the highest-resolution C-isotope curves available: all carbon-isotope events demonstrated by δ13Ccarb studies in the British Chalk, NW Germany and other reference sections in Europe are recognized in the δ13Corg curve from Bch-1.A number of short-term, basin-wide regressions in the Bohemian Cretaceous Basin, most likely reflecting eustatic falls, show a recurrence interval of 100kyr or less. This is an order of magnitude shorter than the timing of sea-level falls inferred from the New Jersey margin and the Apulian platform, interpreted to be driven by glacioeustacy. The estimated magnitude of the Bohemian Basin sea-level falls, typically 10–20m and generally <40m, indicates that the 2.4Myr period suggested by others to generate 3rd-order cycles, is too long to be the principal cycle generating unconformities in rapidly-subsiding basins, where the rate of eustatic fall must exceed the subsidence rate. Unconformities in low-accommodation settings such as passive margins most likely represent amalgamated records of multiple cycles of sea-level fluctuations of 100kyr scale, recognizable only in high-resolution datasets from expanded successions.Comparison of the δ13C excursions to the interpreted sea-level record has not yielded a clear causal link. A long-term ‘background’ δ13C cycle shows a duration close to the 2.4Myr long-eccentricity cycle, and shorter-term (1Myr scale) highs and lows in δ13C appear to broadly correspond to intervals characterised by more pronounced short-term sea-level highs and lows, respectively. However, on the scale of intermediate to short-term δ13C fluctuations, no systematic relationship between δ13C and sea-level change can be demonstrated.

Quantitative simulation on the retrogradational sequence stratigraphic pattern in intra-cratonic basins using physical tank experiment and numerical simulation

A well studied intra-cratonic sedimentary sequence, the Lower Permian Shanxi Formation in the Ordos Basin, northern China, is characterized by a typical back-stepping or source-ward retrogradational stratigraphic stacking pattern and can be used to illustrate the conceptual sequence stratigraphic model developed for intra-cratonic siliciclastic sedimentary basins. Based on the basic geological parameters (initial topography, lake-level curve, and sediment supply) originated from the Lower Permian Shanxi Formation of the Ordos Basin, we simulated quantitatively and validated the source-ward retrogradational sequence stratigraphic pattern of intra-cartonic siliciclastic basin, and demonstrated the use of numerical simulation to replicate and extend a particular flume experiment. In Run 1 (SQ1) the result of numerical simulation was characterized by the fan-shaped distribution, Sequence SQ2 retrograded landwards and overlaid the deposits of Sequence SQ1 after Run 2, and Sequence SQ3 retrograded further landwards and overlaid the deposits of Sequence SQ2 after Run 3. The simulation results indicated that fan-shaped deposits of Sequences SQ1, SQ2, and SQ3 retrograded sequentially landwards. The sediment distribution of three sequences in the numerical simulation is in accordance with the deposits observed in the physical experiment. The simulated sequence stratigraphic pattern of three sequences also matches that from the geological model quite well. The proposed sequence stratigraphic pattern for the intra-cratonic basin should serve as a useful reference for the interpretation of sequence stratigraphic framework and stacking patterns, and the prediction of the distribution of potential reservoir sandbodies within such basins.

Weathering intensity in the Mesoproterozoic and modern large-river systems: A comparative study in the Belt-Purcell Supergroup, Canada and USA

The Mesoproterozoic Belt-Purcell Supergroup (BPS) preserves a thickness of 17km of dominantly siliciclastic rocks deposited between 1470 and 1400Ma. The total range of Chemical Index of Alteration (CIA) values, corrected for diagenetic K-addition, are 62–88 for argillites and 55–80 for sandstones, with averages of 72±6 and 68±7, respectively. More intense CIA values, in conjunction with low absolute contents of Sr, Ca and Na, and high Rb/Sr ratios (average 4), reflect an intensely weathered provenance in a hot-wet climate with hot-arid intervals resulting in evaporitic sediments. Covariations of CIA-Eu/Eu* and Sr-Eu/Eu* are consistent with a large catchment area including extensive provenance terranes of weathered recycled sedimentary rocks for the most extreme CIA and Eu/Eu* values, with smaller less intensely weathered juvenile terranes represented by lower CIA values. Accordingly, variations of CIA within the BPS stratigraphic sequence may record some combination of shifting catchment terranes and weathering intensity. Stratigraphic trends in CIA within the Appekunny and Grinnell Formations of 80±6 to 66±4 to 79±5 record the variation in this combination with time.Siliciclastic rocks record a first order trend of CIA values from CIA ∼80–100 in the Mesoarchean, through ∼80–90 in the Neoarchean, and ∼70–85 in the Proterozoic, to ∼72 for global Phanerozoic shales. These values reflect progressive drawdown of greenhouse gases that promote silicate weathering by their sequestration into carbonates and black shales, as preserved in the geologic record. Second order secular peaks in CIA values correlate in time with mantle plumes that emit greenhouse gases, which enhance silicate weathering. Some of the more intense CIA values in the BPS may also stem from release of volcanic gases during magmatism that accompanied rifting of Laurentia during breakup of the Supercontinent Columbia at ∼1.4Ga. Overall, CIA values are within the range of modern humid-temperate and humid-tropical climatic catchment areas drained by large river systems such as the Orinoco, Nile and Amazon rivers.Proterozoic rivers have been viewed as mostly braided systems due to the lack of influence of rooted vegetation, which resulted in fast channel lateral migration, high run-off rates, and low bank stability. Many large-scale Proterozoic siliciclastic basins have been preserved, formed by river systems up to pan-continental scale. However, their significance as archives of continental weathering intensity remains under-explored. This study suggests that BPS CIA values reflect more aggressive chemical weathering, since Proterozoic rivers had less sediment residence times due to a lack of vegetation cover, and therefore faster transport time than their modern counterparts. To achieve high CIA values in shorter periods of time without vegetation cover, more intense chemical weathering conditions must have been present.