Basal Sediments Research Articles

Prospects of hydrocarbon exploration in the Laptev sea basin

Background. The geological structure of the Laptev Sea basin is characterized by a rift nature, thus providing no favorable conditions for the presence of hydrocarbon accumulations. However, the lack of seismic information and the absence of well drilling indicate an incomplete stage of the basin exploration.Aim. To analyze the results of geological modeling based on structural mapping and to identify promising areas of hydrocarbon exploration. A geological hydrocarbon study of the Laptev Sea basin was carried out in order to recommend areas for licensing, as well as to identify exploration drilling sites and estimate the total initial prospective geological resources according to the conducted numerical basin modeling. To design sedimentation models and predict reservoir properties in the Laptev Sea basin.Materials and methods. The analysis was based on the data retrieved from production reports for individual large objects in the water area. A model developed by Equinor specialists was used for the basin analysis. The results of hydrocarbon system modeling, the prediction of hydrocarbon saturation obtained from sections of two sample profiles while structural mapping of the basement and top of Oligocene-Miocene sediments were used.Results. Prospective areas of accumulation of hydrocarbons and their confinement were determined. The probability of hydrocarbons in non-anticlinal traps was estimated. Priority areas of hydrocarbon exploration within the unallocated subsoil area were identified. Recommendations for further geological exploration across the license area were formulated.Conclusion. Further exploration of the Yuzhno-Laptevomorsky promising area is recommended by means of analyzing the available data, processing of new geophysical data, updating the geological model and designing 3D seismic surveys within the identified promising areas. In addition, 2D seismic surveys at the regional level should be carried out to define the sedimentary boundaries of the Eastern Artic water area.

Paleoenvironmental record from lake San Lorenzo, Monetbello, Chiapas, Mexico

Lake San Lorenzo is one of the lakes at the NW karstic plain of the “Parque Nacional Lagunas de Montebello”, Chiapas, Mexico, that currently have eutrophic, turbid waters. To understand the recent history of this lake, multi-elemental contents determined by X-ray fluorescence analysis, organic matter and carbonate contents -determined by loss on ignition-, Corg/N ratios and diatom analyses from a 41 cm-long sedimentary sequence, were performed. The chronology of the sequence was established by one 14C date from the basal sediments, which allowed establishing that the studied record began in 1956. These new data are consistent with previously published analysis of a profile from the nearby Lake Balamtetik, that could not be dated by 14C, therefore this work provides a better chronological resolution of events. The diatom community along the Lake San Lorenzo sequence was dominated by high nutrient tolerant species, which allows inferring that the lake was already turbid and eutrophic since 1956. Erosion indicators (titanium and magnetic susceptibility) allowed identifying two episodes of basin-wide erosion associated with long distance sediment transport to the lake (very likely through the ”Río Grande de Comitán”) the first episode from 1970 to 1985 and the second one from 1990 to 2000.

Three-layer structure of the Carpathian sedimentary prism from the results of seismic migration on the PANCAKE and RomUkrSeis WARR profiles

The method of finite-difference migration of reflected/refracted waves, applied to the PANCAKE and RomUkrSeis WARR seismic profiles, made it possible to form wave images of sedimentary layer and crystalline basement under the Carpathian orogen to a depth of 25 km. The study area belongs to Ukrainian Carpathians, which consist of Outer Carpathians — the Cretaceous-Neogene accretionary prism and Inner Carpathians —fragments of Alkapa and Tissia-Dakia microplates. The Carpathian belt is thrusted over the Neogene Carpathian Foredeep, which was laid on the basement of the Eastern/West European platforms. We used a technique specially developed for WARR study to generate a wave image of basement and deep sedimentary basin. The wave images obtained for two profiles show the presence of deep troughs (accretionary prism) under the Carpathian orogen and reveal their similarities and differences due to the peculiarities of the tectonic development in the junction zone of East- and West European platforms. The sedimentary prism reaches a depth of 20 km and consists of three nested troughs distinguished well in the wave field and characterized by different velocities on velocity models. The upper two layers up to ~15 km belong to the allochthon of the Ukrainian Carpathians and the underlying sequence. Whereas the lower one (15—21 km) could represent the older complexes of the basement, up to the Neoproterozoic age (Ediacaran), associated with accretion of young plates from the west to the East European platform and formation of the Trans-European suture zone. The deep trough along the RomUkrSeis profile is significantly narrower than along the PANCAKE one, which indicates a stronger shortening of the sedimentary basin (and possibly the upper crust) in the southeastern part of Ukrainian Carpathians. The sedimentary prism on both profiles is bounded from both sides by steeply dipping faults — from the east by the Forecarpathian fault, and from the west by the Transcarpathian fault along PANCAKE profile and by the Dragos Voda fault on RomUkrSeis profile, which may indicate active strike-slip tectonics.

The mechanism of phreatomagmatic maar-diatreme eruption in the Lei-Hu-Ling volcanic area of northern Hainan Island, China: Insights from magnetotelluric and magnetic data

A maar-diatreme forms in response to the phreatomagmatic eruption caused by violent interaction between magma and externally derived water. Experimental work on phreatomagmatic eruptions has yielded considerable knowledge on maar-diatremes. However, the mechanism and process at natural scale remain poorly understood, especially when surface deposits are present. This paper presents a magnetotelluric and magnetic study of the Late-Pleistocene maar-diatreme volcanoes in the Lei-Hu-Ling area of northern Hainan Island, China. The regional 3D electrical structures show shallow high resistivities below craters and a low resistivity layer overlying the deep high-resistivity basement. The local magnetic structure of maar Luo-Jing-Pan reveals a cylinder-like diatreme with high susceptibility. The shallow low-resistivity zone, which coincides with the Fushan depression controlled by the Changliu-Xiangou Fault, is associated with the accumulation of high-salinity fluids with high porosity. The considerable volume of water stored in the Fushan depression and the sedimentary basement ridges beneath maars could have enabled a high level of magma-water interaction. This indicates a fault-controlled type for the phreatomagmatic eruption in the Lei-Hu-Ling volcanic area. The explosion depth is at least 600 m for Ru-Huang-Ling and Luo-Jing-Pan maar-diatreme volcanoes, indicating deep phreatomagmatic eruptions with less contribution to surface deposits. In addition, the explosion energy of the Ru-Huang-Ling maar with a diameter of 1900 m is about 7–8 times that of Luo-Jing-Pan maar with a diameter of 900 m.

Geological and geophysical transect of the Middle Tien Shan across the Naryn and Atbashi depressions

The article presents the results of integrated geological and geophysical research works dealt with detailed magnetotelluric (MT) sounding and the study of the morphology and spatial position of the sedimentary cover and basement structures along the key transect of the Middle Tien Shan crossing the Naryn and Atbashi depressions. The data on the distribution of deep electrical conductivity of the crust and upper mantle were compared with the seismic profiling data. The compilation of the results of structural-geological and geophysical studies provided the opportunity to draw 2D upper-crust geological structure, consistent with the structure of electrical conductivity of the crust to depths of about 10 km. The detailed geological cross-sections and the structural and geological data allow us to characterize the deformations of the Cenozoic sedimentary complex and Paleozoic basement surface associated with the alpine activation of the key segment of the Tien Shan. It is shown that the Cenozoic structural parageneses emerged during a relatively short-term phase of deformation and orogeny under conditions of horizontal compression and transpression, which did not manifest themselves during sagging that occurred previously.

Analisis Angkutan Sedimen Pada Sungai Galang-Galang Kampung Paku Kabupaten Polewali Mandar

The Galang-Galang River or also known as the Binanga Karaeng River is a border river between the Polewali Mandar Regency (Polman) which was formerly known as Polmas before separating from South Sulawesi Province. During the rainy season, the water in this river often overflows and can even pose a danger to plantation and rice field farmers around the two border areas. Some of the rainwater that falls on the surface erodes the soil so that it is carried by the flow of water into the river downstream to make the sediment deposits increase. Sediment formed in the flow of the Galang - Galang river will cause a lack of river capacity during the dry season and in the rainy season, it will cause flooding. In this study, direct measurements and sampling of bottom sediment and floating sediment were carried out at three river points. The purpose of the study was to analyze deductions of basic sedimentary transports and sediments floating in the galange-Galang river. The study conducted direct measurements as well as retrieval of basic sedimentary samples and sediment rising at three points of the river. Basic sedimentary transport was calculated using the van rjin method, and for levitating sediments, it was calculated by a momentary method. From the results of the analysis, the bottom sediment transport discharge in the upstream is 4x10-4 tons/day, in the middle, it is 0.0891 tons/day, and in the downstream, it is 1x10-4 tons/day. Meanwhile, the floating sediment transport discharge is 0.208 tons/day upstream, 0.278 tons/day in the middle, and 0.121 tons/day downstream, where floating sediment transport is measured at a very small discharge so that the results are obtained are also small.

ADJUSTMENT OF THRUSTING STRUCTURE IN THE KOVYKTA-KHANDINSKAYA REFLECTED FOLDING ZONE

The Kovykta-Khandinskaya zone, which includes the Kovykta gas condensate field, is a giant located in the junction zone of the Angara-Lena stage with the craton edge – the Cis-Baikal trough. Here, according to the results of long-term geological exploration, the Verkhnelenskoe uplift was identified, the northern part of which is reflected on tectonic maps as the Kovykta ledge. At the same time, the geological model of the field today is based on the standard two-member model, which includes a slightly disturbed sedimentary cover and basement. However, new geophysical studies revealed that the sedimentary cover of the eastern part of the zone is intensely deformed and has a two-tiered nodular-thrust structure (the lower layer is autochthon, the upper layer is allochthon). The main object for gas exploration within the Kovykta gas condensate field is the Vendian formation. The middle (halogen-carbonate) section has been studied fragmentarily, mainly as an object of geological geohazards when drilling deep wells. The involvement of new data from 3D seismic and 3D transient electromagnetic methods made it possible to clarify the Kovykta gas condensate field tectonic structure to assess the potential of secondary carbonate reservoirs characterized by intense fluid (natural gas, brines) inflows.

Fault rupture and aseismic creep accompanying the December 26, 2018, Mw 4.9 Fleri earthquake (Mt. Etna, Italy): Factors affecting the surface faulting in a volcano-tectonic environment

On December 26, 2018 (2:19 UTC), during a volcanic eruption on the Mt. Etna eastern flank (Sicily, southern Italy), the largest instrumental earthquake ever recorded in the volcano ruptured the Fiandaca Fault, with epicenter between Fleri and Pennisi villages (hypocenter at ca. 300 m a. s. l., Mw 4.9). This was the mainshock of an earthquake swarm and it was accompanied by widespread surface faulting and extensive damage along a narrow belt near the fault trace. Few hours after the mainshock, an episodic aseismic creep event occurred along the Aci Platani Fault, a SE extension of the Fiandaca Fault, which caused several damages in the Aci Platani village. We surveyed and mapped the coseismic and aseismic ground ruptures, and collected structural data on their geometry, displacement, and fault zone fabric. We compared the mapped surface ruptures with topography, lithology, and morphology of the buried top of the sedimentary basement. We conclude that the geometry of the volcanic pile influenced the surface expression of faulting during the December 26, 2018 event. The top surface of the marly clay basement should be considered as a detachment surface for shallow sliding blocks. The earthquake occurred on top of a depression of the sedimentary basement forcing the sliding eastward, causing at surface the re-arrangement of the fault strand pattern and deformation style, switching from shear faulting to a tensile failure. The Fleri earthquake therefore provides an unprecedented dataset for 1) understanding active faulting in the European largest onshore volcano, 2) modeling its complex dynamics, and 3) contributing to a more refined surface faulting hazard assessment at Mt. Etna. Results from this investigation might be useful for characterizing capable faulting in similar volcano-tectonic settings worldwide.

Bransfield Strait and South Shetland Islands Sedimentary Basement and Upper Crustal Structure: An Analysis of Gravity and Magnetic Data at the Northwestern Area of the Antarctic Peninsula

Bransfield Strait and South Shetland Islands Sedimentary Basement and Upper Crustal Structure: An Analysis of Gravity and Magnetic Data at the Northwestern Area of the Antarctic Peninsula

Multiproxy approach to track changes in the ecological condition of wetlands in the Gunbower Forest, a Ramsar site

Gunbower Forest is bordered by the Murray River and Gunbower Creek and hosts several floodplain wetlands listed under the Ramsar Convention. Sediment cores were retrieved from three wetlands to trace changes to their ecological state over time. The basal sediments of the wetlands date back to the beginning of river regulation in the 1930s, suggesting that only after then were they inundated sufficiently often to allow for net sediment accumulation. The diatoms preserved in the lower levels of all cores suggest clear, freshwater conditions prevailed during that period. Increased sediment and nutrient loads are inferred by increased epiphytic forms and nutrient indicators. Over recent decades the wetlands have transitioned to plankton dominance, reflecting greater connectivity to the river and distributary, and a reduced light environment. This pattern resembles to that recorded both upstream and downstream, suggesting a regional-scale change in the wetlands of the southern Murray–Darling Basin.

Pre- and post-industrial levels of polycyclic aromatic hydrocarbons in sediments from the Estuary and Gulf of St. Lawrence (eastern Canada)

The concentrations of 23 polycyclic aromatic hydrocarbons (PAHs; 16 parent PAHs and 7 alkyl-PAHs) were determined in 45 surface sediment and 7 basal sediment box core samples retrieved from the Estuary and Gulf of St. Lawrence in eastern Canada. The concentration sums of 16 priority PAHs (Σ16PAHs) in the surface sediments (representing modern times or at least younger than the last decade) ranged from 71 to 5672 ng g−1. Σ16PAHs in the basal sediments ranged from 93 to 172 ng g−1 among the pre-industrial samples (pre-1900 common era or CE) and from 1216 to 1621 ng g−1 among the early post-industrial samples (~1930s and ~1940s CE). The highest Σ16PAH values occurred in samples retrieved from the Baie-Comeau-Matane area, an area affected by intense industrial anthropogenic activities. Source-diagnostic PAH ratios suggest a predominance of pyrogenic sources via atmospheric deposition, with a minor contribution of petrogenic seabed pockmark sources. The PAH concentrations in the sediments from the study areas reveal low ecological risks to benthic or other organisms living near the water-sediment interface.

Sedimentary and crustal structure of the US Gulf Coast revealed by Rayleigh wave and teleseismic P coda data with implications for continent rifting

We have developed an S-wave model of the south-central US focusing on the Gulf Coast sedimentary basin and its crust to understand continental rifting and regional tectonics. The model was derived by a joint inversion of Rayleigh wave phase velocities, Z/H ratios and P-coda data. The surface- and body-wave measurements were made, respectively, from ambient noise and teleseismic earthquakes recorded by 215 USArray stations in a rectangular area of 100°–87° west and 28°–37° north. We employed a cross-convolution function and H-κ analysis to better constrain sedimentary and Moho structure. We find that the southern edge of the Ouachita fold-and-thrust belt (OFTB) appears as a boundary in measured phase velocities, Z/H ratios, sediment basement depths, Moho depths, average crustal Vs and Vp/Vs ratios. The model shows southeastward thickening of the sedimentary basin, accompanied by thinning of the crystalline crust. The Moho gradient suggests that early rifting between North America and the Yucatan block commenced in a SE direction and involved most of the Pangea crust south of the OFTB boundary (i.e., Gondwana crust). We believe that a high velocity feature in the lowermost crust and upper mantle parallel to the southeast Texas coast was emplaced as a mafic body and is the source of the Houston magnetic anomaly. The seismic structures of the crust and uppermost mantle observed beneath the Mississippi Embayment and the Mississippi Valley Graben are consistent with plume induced Cretaceous uplift of the Mississippi Embayment as North America passed over the Bermuda hotspot.

Impact of Upward Oxygen Diffusion From the Oceanic Crust on the Magnetostratigraphy and Iron Biomineralization of East Pacific Ridge-Flank Sediments

Recent measurements of pore-water oxygen profiles in ridge flank sediments of the East Pacific Rise revealed an upward-directed diffusive oxygen flux from the hydrothermally active crust into the overlying sediment. This double-sided oxygenation from above and below results in a dual redox transition from an oxic sedimentary environment near the seabed through suboxic conditions at sediment mid-depth back to oxic conditions in the deeper basal sediment. The potential impact of this redox reversal on the paleo- and rock magnetic record was analyzed for three sediment cores from the Clarion-Clipperton-Zone (low-latitude eastern North Pacific). We found that the upward-directed crustal oxygen flux does not impede high quality reversal-based and relative paleointensity-refined magnetostratigraphic dating. Despite low and variable sedimentation rates of 0.1–0.8 cm/kyr, robust magnetostratigraphic core chronologies comprising the past 3.4 resp. 5.2 million years could be established. These age-models support previous findings of significant local sedimentation rate variations that are probably related to the bottom current interactions with the topographic roughness of the young ridge flanks. However, we observed some obvious paleomagnetic irregularities localized at the lower oxic/suboxic redox boundaries of the investigated sediments. When analyzing these apparently remagnetized sections in detail, we found no evidence of physical disturbance or chemical alteration. A sharp increase in single-domain magnetite concentration just below the present lower oxic/suboxic redox boundary suggests secondary magnetite biomineralization by microaerophilic magnetotactic bacteria living as a separate community in the lower, upward oxygenated part of the sediment column. We therefore postulate a two-phased post-depositional remanent magnetization of ridge flank sediments, first by a shallow and later by a deep-living community of magnetotactic bacteria. These findings are the first evidence of a second, deep population of probably inversely oriented magnetotactic bacteria residing in the inverse oxygen gradient zone of ridge flank sediments.

Identification of and Favorable Conditions for a New Long-Offset Seismic Phase from Offshore–Onshore Seismic Surveys

Abstract Offshore–onshore seismic survey is one of the main methods to study crustal structures in offshore–onshore transitional zones. At present, the seismic waves commonly used in imaging are the crustal refraction (Pg), the crustal reflection from the Moho (PmP), and the upper-mantle refraction (Pn) waves. The propagation distances of Pg and PmP are commonly less than 210 km, and Pn propagates with an apparent velocity of ∼8 km/s. In 2015, two offshore–onshore wide-angle seismic lines with a length of ∼350 km were acquired in the Pearl River Estuary. In addition to Pg, PmP, and Pn, a new seismic phase was observed, which has a long propagation distance (offset of ∼170–290 km), low apparent velocity (∼5.85 km/s), and low frequency (∼4–7 Hz). Similar seismic phases have been widely reported in previous offshore–onshore and reservoir seismic surveys, but the understanding of these phases is still limited. Herein, we used both raytracing and waveform modeling methods to identify the new seismic phase as the secondary Pg phase, which reflects from the surface (named Pg2Pg). We also discuss favorable conditions for Pg2Pg, including (1) a thin sedimentary layer with low velocity at the surface in which the reflection of Pg occurs, which can reduce the incidence angles and hence increase the energy of the reflected waves; (2) a sedimentary basement dipping toward the sea at the positions of the air gun shots, which focuses seismic waves; (3) relatively smooth interfaces of the medium, which can reduce the scattering of Pg2Pg; and (4) air guns that can excite low-frequency signals, which can reduce the attenuation of seismic waves. Checkerboard tests and practical applications show that Pg2Pg can significantly improve upper-crustal resolution, especially for onshore areas. Our research promotes the data mining of offshore–onshore seismic surveys, which is important for obtaining more detailed crustal structures.

Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the 2011 Tohoku Earthquake Rupture Area

Multi-channel seismic reflection and sub-bottom profiling data reveal landward-dipping normal faults as potential conduits for mantle-derived fluids in the coseismic slip area of the 2011 Tohoku earthquake (Mw9.0). Normal faults below the helium isotope anomaly sites appear to develop through the forearc crust (i.e., the seafloor sedimentary section and Cretaceous basement) and to evolve to lower dip angles as extension progresses deeper, potentially extending down to the mantle wedge, despite their intermittently continuous reflections. The faults are characterized by high-amplitude, reverse-polarity reflections within the Cretaceous basement. Moreover, deep extension of the faults connecting to a low-velocity region spreading from the Cretaceous basement into the mantle wedge across the forearc Moho suggests that the faults are overpressured by local filling with mantle-derived fluids. The locations of the normal faults are roughly consistent with aftershocks of the 2011 Tohoku earthquake, which show normal-faulting focal mechanisms. The 2011 Tohoku mainshock and subsequent aftershocks can lead the pre-existing normal faults to be reactive and more permeable so that locally trapped mantle fluids can migrate up to the seafloor through fault fracture zones. The reactivated normal faults may be an indicator of shallow coseismic slip to the trench. Locally elevated fluid pressures can decrease the effective normal stress for the fault plane, facilitating easier slip along the fault and local tsunami. The landward-dipping normal faults developing from the seafloor down into the Cretaceous basement are predominant in the middle slope region of the forearc. A possible shear zone with high-amplitude, reverse-polarity reflections above the plate interface, which is almost localized to the middle slope region, suggests more intense basal erosion of the overlying plate in that region.

Sedimentary Processes Within the Canadian Arctic Archipelago: Relationships Among Sedimentological, Geochemical, and Magnetic Sediment Properties

AbstractThe sedimentological, geochemical, physical and magnetic properties of 40 surface and basal sediment samples of box cores collected throughout the Canadian Arctic Archipelago (CAA) from the Canadian Beaufort Shelf to Lancaster Sound were analyzed to determine the sedimentary processes that operate within the CAA during the pre‐ and post‐industrial periods (i.e., before and after 1900 Common Era or CE). In addition, the chronology of seven selected regional cores was established using 210Pb measurements, where the base is dated between 1550 and 1820 CE. These cores provide an opportunity to robustly compare post‐1900 sedimentary conditions with those of the colder Little Ice Age period (LIA; ∼1500–1900 CE). The different properties combined with multivariate statistical analyses result in the identification of three regional provinces with distinct sedimentary characteristics: (a) the West province (Mackenzie Shelf/Slope, west Banks Island and M'Clure Strait) typified by detrital associations (Fe‐Rb‐Ti‐Zn), high organic matter inputs, dominance of magnetite and low‐coercivity minerals and high aluminosilicate contents; (b) the Intermediate Zone (Amundsen and Coronation gulfs) distinguished by Si‐Al‐Zr‐Sr‐K associations, Mn oxyhydroxides precipitation, constant high magnetic grain concentrations and a mixture between marine and terrigenous organic matter; and (c) the East Province (Queen Maud Gulf, Victoria and Barrow straits, and Lancaster and Eclipse sounds) described by high detrital carbonate inputs, marine organic matter, a dominance of high‐coercivity minerals and high magnetic concentrations in pre‐industrial samples. Our results confirm that the pre‐ and post‐industrial sedimentary dynamics are controlled by sediment supplies from the river discharges in the West and Intermediate provinces, whereas the East province is more influenced by sea ice and coastal erosion. Basal sediment samples from the seven 210Pb‐dated cores suggest an intensification of the Mackenzie, Coppermine, and Ellice rivers runoff, extensive sea‐ice cover and consequently sediment transport by the latter during the LIA period.

The hydrology of glacier‐bed overdeepenings: Sediment transport mechanics, drainage system morphology, and geomorphological implications

AbstractEvacuation of basal sediment by subglacial drainage is an important mediator of rates of glacial erosion and glacier flow. Glacial erosion patterns can produce closed basins (i.e., overdeepenings) in glacier beds, thereby introducing adverse bed gradients that are hypothesized to reduce drainage system efficiency and thus favour basal sediment accumulation. To establish how the presence of a terminal overdeepening might mediate seasonal drainage system evolution and glacial sediment export, we measured suspended sediment transport from Findelengletscher, Switzerland during late August and early September 2016. Analyses of these data demonstrate poor hydraulic efficiency of drainage pathways in the terminus region but high sediment availability. Specifically, the rate of increase of sediment concentration with discharge was found to be significantly lower than that anticipated if channelized flow paths were present. Sediment availability to these flow paths was also higher than would be anticipated for discrete bedrock‐floored subglacial channels. Our findings indicate that subglacial drainage in the terminal region of Findelengletscher is dominated by distributed flow where entrainment capacity increases only marginally with discharge, but flow has extensive access to an abundant sediment store. This high availability maintains sediment connectivity between the glacial and proglacial realm and means daily sediment yield is unusually high relative to yields exhibited by similar Alpine glaciers. We present a conceptual model illustrating the potential influence of ice‐bed morphology on subglacial drainage evolution and sediment evacuation mechanics, patterns and yields, and recommend that bed morphology should be an explicit consideration when monitoring and evaluating glaciated basin sediment export rates.

The Effect of Differential Weathering on The Magnetic Properties of Paleosols: A Case Study of Magnetic Enhancement vs. Magnetic Depletion in the Pleistocene Blackwater Draw Formation, Texas

The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.

A simple but efficient non-linear method for 2D inversion of magnetic field data based on Ridge-Regression algorithm

In geophysical exploration, inversion is carried out on the observed data to generate a geophysical model, approximating the subsurface geological struc­ture. In the interpretation of magnetic data, the subsurface model parameters are found by a proper inversion scheme. Hence, it will be possible to obtain the entire parameters of any features (e.g. Dike) including depth, width, and location. In this paper, theoretical and field studies were carried out to interpret the total components of magnetic anomalies of dikes at the finite depth. Moreover, a least-squares approach was used for depth determination using anomalous magnetic data. Potential field data inversion can be achieved through many optimization techniques. This study, however, it is attempted to develop an efficient two-dimensional (2D) inversion algorithm based on the Ridge Regression routine. The developed method was programmed using Matlab software and applied to three sets of synthetic magnetic data containing different percent of random noise to find out how good the results are. It was found that the proposed 2D inversion method can produce an accurate subsurface model that precisely explains the synthetic data in each case of data inversion. Finally, the method was applied to the real total magnetic field (TMF) data of Moghan Sedimentary basin. In that case, the estimated sedimentary basement depths were found to be in good agreement with that of the seismic data acquired before.

Unravelling hidden glacial effects in the Cryogenian marine depositional settings of the Tandilia Basin, Argentina

The Cryogenian Period (720–635 Ma) is wide known by the occurrence of two global glaciations, Sturtian and Marinoan, responsible of dramatic climate and sea level changes as well as seawater oxygenation events. Constrained 87Sr/86Sr ratios from 0.7064 to 0.7071 and δ13C excursion are consistent with Cryogenian cap carbonates as documented in the upper Villa Mónica Formation. This unit constitutes the basal sedimentary cover of the Río de La Plata Craton in Central Argentina which begins its deposition in a shallow marine siliciclastic shelf, turning to a carbonate succession with exceptionally well-preserved stromatolites. The detailed facies analysis added to stromatolite morphologies included in paleoenvironmental models of deposition remain unknown. The later are considered necessary to establish the stratigraphic framework and the regional correlation of the unit across the basin as a piece for the Cryogenian assembly. The entire Villa Mónica Formation in the Tandilia Basin was deposited on a shelf with sedimentary facies variations observed from the shallow margins to the offshore transition zone. The transition from the lower to the upper sections implies the conquer of microbial communities to paleogeographically controlled chemocline fluctuations where the stromatolites evidence the development from a post-glaciation phosphorus-rich ocean. This drastic shift in paleoenvironments represented on the two distinctive marker sections was influenced by changes in climatic and oxygenation conditions. In this context, presumed glacial influence is predictable in a partially ice-covered Cryogenian Earth surface.