Conglomerate Deposits Research Articles

New fossil anhingids from the upper Acre River (Late Miocene of southwestern Amazon).

Four extant species of anhingids are found worldwide, all belonging to a single genus (Anhinga). However, the fossil record reveals a much greater diversity of this group in the past. The oldest known anhingids date back to the upper Oligocene period in Australia, but during the Miocene epoch in South America, they achieved their most remarkable diversity. This study describes newly discovered anhingid fossils from the Late Miocene period in South America. These fossils were extracted from the Acre conglomerate member, part of the Upper Miocene deposits in the southwestern Amazon region. The described fossils consist of two fragments of pelvic girdles, two femora, and two vertebrae belonging to a giant anhingid (Macranhinga sp.), as well as a vertebra from Anhinga minuta, the smallest of all darters. The examination of these fossils suggests the presence of potentially three distinct anhingid taxa within the same locality. The environment in which the conglomerate deposits were formed was ecologically complex. It is likely that these three species coexisted within the same ecosystem but avoided direct competition for food and reproductive sites by not fully exploiting their fundamental niche.

Devonian to Mississippian strata of the Shine Jinst region revisited: Facies development and stratigraphy in southern Mongolia (Gobi Altai Terrane)

AbstractThis report provides new stratigraphical and facies data from Devonian and Carboniferous rocks in the Shine Jinst region (Trans Altai Zone, southern Mongolia) with a special focus on the Lower Devonian Chuluun Formation, the Middle Devonian Tsagaankhalga Formation, and the Upper Devonian to Mississippian Heermorit Member of the Indert Formation. Facies development in the Shine Jinst region exhibits a fundamental break in the carbonate platform evolution in the Lower Devonian as reef building organisms were affected by a major regression and deposition of several metres-thick conglomerates at the base of the Tsakhir Formation (Lower Devonian). The overlying Hurenboom Member of the Chuluun Formation is composed of fossiliferous limestones. Reef building organisms, such as colonial corals and stromatoporoids show low diversity and exhibit limited vertical growth and lateral extension of individuals. Thus, they do not represent a real reef as proposed in previous publications but biostromal limestones instead. One reason might be the isolated position of the Shine Jinst region between an unknown continent and a volcanic arc in the early Middle Devonian that hampered the successful colonization in shallow-water areas. Bivalves of the Alatoconchid family were once grouped into reef builders or biostrome builders and they are known only from Permian rocks. The found bivalve biostomes in Mongolia may represent precursors, which would document the oldest record of Alatoconchids found in the world. Remarkable thicknesses of massive crinoidal grainstones (“encrinites”) are documented in many parts of the succession, which suggest rather stable environmental conditions of a carbonate ramp setting at different times. The occurrence of thick-bedded conglomerates in the Shine Jinst section is not restricted to the Lochkovian to Pragian interval (Tsakhir Formation), but also occurs in the Eifelian. A thick-bedded conglomerate, which is interpreted to represent braided fluvial or fan-delta to shallow-marine deposits occurs at the base of the Tsagaankhaalga Formation. A steep relief associated with uplift and volcanism seems to be a realistic scenario for deposition of these sediments. This succession points to a remarkable tectonic uplift or sea-level fall in the Middle Devonian. Conodont findings of the studied section confirm the occurrence of time-equivalent strata of the Choteč Event, the Dasberg Crisis, and the Hangenberg Event found elsewhere in the world, which are described from Mongolia for the first time. Sedimentological descriptions, revised biostratigraphical data, and U-Pb dating by LA ICP-MS of some volcaniclastic rocks from the Chuluun Formation are presented in this report. The studied section records a complex interaction of sedimentation, regional tectonics, sea-level changes and coeval volcanism, which is very similar to other regions in Mongolia. The new data provide the background for further scientific studies in this region. This is a contribution to the Special Series on “The Central Asian Orogenic Belt (CAOB) during Late Devonian: New insights from southern Mongolia”, published in this journal.

Depositional and diagenetic processes in travertines: A comprehensive examination of Tocomar basin lithotypes, Northwest Argentina

The use of travertines in active fault studies is known as travitonics. Their geobody shape, veins, faults and fractures, are commonly used as neotectonic proxies. The surficial travertines in the pull apart Tocomar Basin (Puna Plateau, Argentina) occur in the center of the Calama – Olacapato – El Toro (COT) transcurrent fault zone and have never been substantially buried. The Tocomar area exhibits complex tectonic with N–S thrust faults and NW-SE transfer faults, crucial in shaping its travertine deposits and geothermal activity. The travertine bodies are characterized in terms of geometry as a fissure ridge type, coexisting with ash tuff, siliciclastic sandstone, and conglomerate deposits. The travertines mainly consist of crystalline crusts, with occasional occurrences of granular, arborescent, and porous textures. Mineralogical analyses by X-ray diffraction (XRD) reveal the predominance of magnesian calcite, along with trace minerals identified as dolomite, anhydrite, gypsum, halite, fluorite, bassanite, and barite. Petrographic analyses, including cathodoluminerscence (CL), indicate intense diagenesis in the travertine samples. Isotopic signatures of C and O in these samples exhibit normally a negative trend, ranging from −6.38 to +2.45 ‰ VPDB for δ13C and from −20.48 to −11.21 ‰ VPDB for δ18O. Laboratory analysis and field observations allow to classify three distinct lithotypes in the Tocomar travertine: (1) crystalline crust, further divided into three sub-lithotypes: (1.1) laminated crystalline crust (LCC), (1.2) filamentous-like crystalline crust (FlCC) and (1.3) fiber-like crystalline crust (FbCC); (2) granular travertine (GT) and (3) porous travertine (PT). These different lithotypes likely reflect variations in fluid velocity during deposition, although some of them may result from diagenetic processes. The three lithotypes give rise to four facies association (steep slope, marshy/pools, drainage, and fracture-associated facies), forming two distinct depositional systems (slope and channel), which comprises the Tocomar travertine system.

Morphotectonic Analysis in the Sombe Lewara Sub-Watershed in the Region of Palu City and Sigi Regency, Central Sulawesi Province

A study on the tectonic activity index (IAT) was conducted in the Sombe Lewara Sub-Watershed, encompassing the area of Palu City and Sigi Regency, Central Sulawesi Province. The objective of this research was to determine the level of tectonic activity in the Sombe Lewara Sub-Watershed. The analysis employed in this study was morphotectonic analysis, utilizing parameters such as Hypsometric Integral (HI), Basin Shape (BS), Asymmetry Factor (AF), Stream Length-gradient Index (SL), Area Factor (AF), and Valley floor width-to-height ratio (Vf ratio). The tectonic activity index (IAT) value in the Sombe Lewara Sub-Watershed was 1.6, which classifies it within the high tectonic activity category. This high level of tectonic activity in the research area influences the landscape conditions and the types of rocks present. The predominant rock types in the study area are sedimentary rocks such as conglomerate deposits, sandstone, siltstone, and claystone. These rocks form the character of an expansive alluvial fan landscape that nearly covers the entire plain. One of the primary factors contributing to liquefaction in the Balaroa Region is that the area is composed of sand sediment deposits and granite boulders that constitute the alluvial fan.

Site effect evaluation for Culiacan, Sinaloa, Mexico

Using the spectral ratio technique (HVSR), we performed an analysis of 120 seismic noise records obtained at different points distributed throughout the city of Culiacán, Sinaloa, México. The results show a clear relationship of dominant periods between 0.2 to 0.7 for the alluvium zone in the central and western area of the city and minor periods for points near outcrops of igneous rocks south and southeast of the city. The higher relative amplitudes were found along the riverbed, in the transition zone of conglomerate deposits to alluvium. We performed a multichannel analysis (station arrays) at some points in which we also measured H/V. Afterward, we produced velocity profiles were obtained and transfer functions were calculated. Five of these values were compared with the HVSR technique. Three of them could be compared with the surficial startigraphy obtained in soil mechanics studies to which access was granted.

Deposition Environment Interpretation of Lemat Formation in the West Tanjung Jabung, Jambi Province

The study of facies analysis and interpretation of the depositional environment of the Lemat Formation located on Bukit Tigapuluh aimed to clarify the problems found in Paleogene sediments in the South Sumatra Basin. The results of research on facies analysis and interpretation of the depositional environment of the Lemat Formation will provide an explanation and description of the history of the formation and stratigraphic evolution of Paleogene-aged sedimentary rocks in the South Sumatra Basin as reflected in their lithological and facies characteristics. A detailed analysis of fluvial facies was carried out in the West Tanjung Jabung area, Jambi Province. A stratigraphic cross-section was prepared with an accumulated thickness of ± 25 m. The stratigraphic section included two rock deposits, namely sandstone-quartz deposits and conglomerate deposits. Analysis of lithofacies and architectural elements of the study area indicated a fluvial depositional environment. The resulting architectural elements consisted of six associations: channel (CH), scour hollows (HO), gravel bedform (GB), sediment gravity flows (SG), sandy bedform (SB), and floodplain fines (FF). The dominance of sedimentary material, which tends to be coarse, and the many facies associations GB, SG, and SB indicated many river bars and were characteristic of braided rivers. The obtained interpretation of the depositional environment was Conglomerate deposits in the Alluvial Fans System with Gravity Flow River environment or gravel bed braided river, and sandstone-gravel deposits in the shallow environment or gravel bed braided river.

The missing sedimentological section linking the Neogene and quaternary of the Tandilia system and the Salado basin, Buenos Aires province, Argentina

The Tandilia system and the Salado basin are two adjacent morphostructural units located within the Argentine Pampean plain, each exhibiting distinct and independent geological histories. The Tandilia system preserves outcrops of Argentina's oldest Proterozoic rocks, while the sedimentary record of the Salado basin began during the Jurassic period. However, starting in the Cenozoic era, both units began to display common continental sedimentary fill characteristics. Despite this connection between the Tandilia system and the Salado basin, there is no unified lithostratigraphic framework for the Cenozoic that varies according to the studied localities. This study aims to define the stratigraphy of a sedimentary section that demonstrates the link between these morphostructural units from the Neogene onwards and to correlate it with the formal lithostratigraphic frameworks proposed in the literature. The analysis of well records led to the description of lithofacies and the establishment of facies associations, which were integrated to generate three correlation transects between the wells. Four facies associations were identified, three of which were fluvial and one aeolian. The analysis of these facies associations revealed that the sedimentary fill could be divided into two sedimentary sequences, lower sedimentary sequence and upper sedimentary sequence, separated by a sequence boundary marked by conglomeratic deposits. The definition of a stratigraphic framework for the middle section of the Del Azul creek basin and its correlation with the formal lithostratigraphic frameworks of the region indicate that from the Neogene onwards, both morphostructural units, the Tandilia system and the Salado basin, share sedimentological and tectonic characteristics. This study highlights the significance of subsurface information in understanding the geology of plain areas and its influence on regional geological evolution.

Geochemistry of the North Gondwana Paleozoic Araba and Naqus formation siliciclastics, Sinai: implications for provenance, paleoweathering, and tectonic setting

ABSTRACT The Araba and Naqus formations in Ras Milan, Sinai, constitute part of the major continuous Paleozoic clastic belt stretching across northern Gondwana. They are sandstone-dominated successions, resting on Precambrian granitic basement, with lowermost thin basal polymictic conglomerate deposits (1–2 m), marking the unconformity. The Araba Formation section is composed mainly of varicoloured violet to reddish brown, fine to coarse-grained laminated and planner and overturned cross-bedded sandstones, while the overlying Naqus Formation is characterized by chaotically distributed poorly sorted quartz pebbles to cobbles within a moderately-sorted coarse- to medium-grained quartz arenite and by abundance and variety of large-scale tabular and trough cross bedding. Modal QFL classification of Araba and Naqus formation sandstones revealed that they are merely quartzose sandstones (quartz arenites). Geochemistry of the succession shows extremely wide compositional ranges for almost all major elements, but the Araba Formation is relatively enriched in SiO2, TiO2, Fe2O3*, Na2O, P2O5, and depleted in Al2O3, MgO, CaO and K2O relative to Naqus Formation. Only the Araba Formation samples are depleted in the transition elements Ni, Cr and Co and oppositely possess extreme enrichment in the HFSEs (i.e. Zr, Hf, Y, Nb, Ta, Th, U), whereas both formations show variable depletion in the LILE Rb, K and Ba, and excessive REE abundances relative to the PAAS and UCC, and LREE-enriched patterns with marked negative Eu anomalies. The provenance of the Naqus and Araba formations could be an earlier felsic magmatic source, which has undergone extensive sedimentary recycling. They possess high compositional maturity and were subjected to intensive to moderate weathering during humid climatic conditions. Passive margin tectonic setting is implied which denotes the derivation of Araba and Naqus formation siliciclastics from stable continental regions or plate interiors and deposition in intracratonic basins.

The aggradation of alluvial fans in response to monsoon variability over the last 400 ka in the Hajar Mountains, south-east Arabia

The preservation and interpretation of past external forcings within sedimentary archives can be complicated. Marine proxy records show that, throughout the Mid-Late Quaternary, monsoon rainfall in south-east Arabia has varied at precessional timescales (∼ 23 ka). By contrast, terrestrial environmental records from the region, such as speleothems and palaeolake sequences, generally only capture rainfall variability at eccentricity (∼ 100 ka) timescales and geomorphological archives rarely record events over multiple glacial cycles. Previous work has suggested that the alluvial fan systems of the Hajar Mountains record aggradation coincident with precessional peaks because of more northerly Indian Ocean Summer Monsoon (IOSM) rainfall, relative to present. However, the alluvial record is complex: there is a growing body of ‘anomalous’ aggradation age data from periods of precessional minima and it is spatially uneven, mainly being derived from fans at either the northern- or southern-most extents of the western side of the mountains.We present optically stimulated luminescence (OSL) ages of alluvial aggradation from new sites in the central portion of the western and eastern sides of the Hajar mountains. Sedimentary units that show evidence of more sustained flow conditions, such as coarse conglomerate deposition and the development of thick floodplain deposits, typically date to periods of higher IOSM rainfall at precessional timescales. Sites where there is evidence of deposition under more ephemeral flow conditions, like today, yield ages corresponding to precessional minima. We synthesise these data with similarly interpreted alluvial and lacustrine deposits from across the region, employing a hierarchical clustering approach to generate clusters of ages representing periods of increased hydrological activity. This objective approach to regional chronological data suggests that ten periods of more sustained hydrological activity are evidenced across the Hajar region, broadly aligning to precessional and eccentricity peaks over the last 400 ka. These clusters show that Hajar fans predominately aggrade because of increasing rainfall over their catchments, with the central ages of clusters often aligning with the onset of precessional forcing. Hajar alluvial systems preserve important records of Quaternary climate variability over long timescales in an arid region where terrestrial records are scarce due to preservation issues.

Paleocene rapid exhumation of Hong Kong: Implications for the Cenozoic rifting along the northern margin of the South China Sea

Unraveling the rifting history along the southeastern margin of the South China Block has important implications for establishing the origin and evolution of the South China Sea (SCS). However, the onset time of rifting associated with the opening of the SCS remains unclear. The Littoral Fault Zone is the largest boundary fault between the South China Block and the SCS, and its fault activity provides information about the evolutionary history of the SCS and crucial constraints on the opening of the SCS. Here we present new apatite (UTh)/He (AHe) ages obtained from the Sunset Peak and Tai Mo Shan, Hong Kong, along the footwall of the extensional Littoral Fault Zone, to constrain the timing of fault activity. The AHe age-elevation relationships and thermal modeling results document an accelerated exhumation during ∼60–55 Ma, which we interpret to be associated with extensional movement along the Littoral Fault Zone. The activity of this extensional fault at ∼60–55 Ma is considered to be coeval with the initial deposition of conglomerate at the bottom of the Pearl River Mouth Basin in the hanging wall of the Littoral Fault Zone. Integration of thermochronological data, magmatic characteristics, and seismic and drilling data support that the Paleocene extension occurred widely in the southeastern margin of the South China Block, representing a prelude to the seafloor spreading in the SCS.

Morphotectonic Processes in Seismic Zones of Moderate Intensity, Argentina

This study deals with the neotectonic deformation in seismic zones of moderate intensity in some areas of the Sierras Pampeanas, Santa Bárbara System and Chaqueña Plain of Argentina. In the study region, the location of earthquakes of ≥ three, ≥ four, and ≥ six Richter magnitudes agree with the traces of regional faults, evidencing their neotectonic activity. Seismic energy is generally transmitted over inherited faults, such as in the Metan basin. Minor new faults occur in Quaternary terraced deposits,and others move, taking advantage of the Neogene sedimentary strata. The folds are generally buried, but the younger sequences are gently undulating. The seismic energy dissipated through fewer cohesion materials that form the valleys' fill, developing discrete fault scarps and strongly folded conglomerate strata. The foothills deposits and basins absorbed most of the seismic energy released during the reactivation of the faults. Tectonic activity is deforming 630 BP deposits in the Cumbres Calchaquíes piedmont. Considering deformation values, neotectonic deformation in the outcrops and seismic profiles evidence, it is possible to interpret that the seismic activity in the study area from the Pleistocene to the present remained between the current records. This seismic activity, sustained over time, is sufficient to produce deformation in the foothills, reactivate pre-existing faults with small displacements and generate minor new faults in recent conglomeratic deposits. Many faults are blind. However, the deformation of the landscape and the modification of the drainage network are direct indicators of the activity of these structures.

Cosmogenic nuclide chronological constraints on the late Cenozoic strata of the Linxia Basin, northeast Tibetan Plateau

The thick Cenozoic sedimentary sequence of the Linxia Basin contains abundant well-preserved mammalian fossils, which provide significant information on the paleoenvironmental processes and mammalian evolution associated with the uplift and expansion of the Tibetan Plateau. Ambiguous age estimations have been made for some coarse-grained sedimentary units in the basin, which have led to the misinterpretation of biostratigraphic and magnetostratigraphic age models. To resolve these issues, we present radiometric dating of three late Cenozoic sedimentary units (“Liushu”, Hewangjia, and Jishi formations) in the eastern Linxia Basin using cosmogenic 26Al/10Be and 10Be/21Ne burial methods. isotope measurements for the upper “Liushu” Formation indicate a 10Be/21Ne burial age of 11.70 ± 0.17 Ma. This age is older than, but not contrary to, recently revised bio-magnetostratigraphic work. The 26Al/10Be burial age of the Hewangjia Formation is 3.06 (+0.36/−0.31) Ma and had been underestimated by post-burial muon production possibility, while the 26Al/10Be burial age of the Jishi Formation is 2.50 (+0.20/−0.18) Ma and had been overestimated due to inherited cosmogenic nuclides from recycled deposits. The new age from the Jishi Formation delays the initial deposition of conglomerates in the Linxia Basin to no earlier than 2.5 Ma and constrains an important episode of tectonic uplift of the northeastern Tibetan Plateau to 2.5–3.1 Ma. The age of the Equus fauna of Longdan is revised to the Early Quaternary (ca. 2.1–1.9 Ma). Our new age determinations significantly refine the terrestrial sequence chronology of the Linxia Basin and associated mammalian faunas of the late Miocene to the early Pleistocene.

The Miocene record of the Betic Cordillera uplift in Los Guájares valley (Granada province, S Spain)

The Los Guájares valley is located about 35 km south of Granada in the Internal Zones of the Betic Cordillera. The Miocene deposits in this area consist of six sedimentary units separated by unconformities, which are exposed in small and laterally discontinuous outcrops overlying metamorphic rocks of the Alpujárride Complex. The lowest unit comprises upper Serravallian marls with planktonic foraminifera filling neptunian dykes and covering Triassic dolomitic marbles of the Alpujárride Complex. These marls reflect deep marine deposition on the Betic basement under a local extensional regime. Uplift of the region led to emergence and deposition of continental red alluvial-fan conglomerates and foothill breccias. The overlying unit, a shoreline conglomerate with small oyster banks, indicates relative sea-level rise. Shallow-marine conditions continued during the deposition of the following unit, early Tortonian in age, which consists of calcareous sandstones with hermatypic corals. Small patch reefs developed in the overlying unit composed of sandy limestones with corals (Porites, Tarbellastraea, Thegioastraea) and oysters, and sandy limestones with coralline algae in the most distal areas. The coral reefs in Los Guájares and some coral heads in Albuñuelas, a near locality, are the only record of early Tortonian coral buildups in the Mediterranean. The last marine unit comprises lower Tortonian conglomerates, sandstones and siltstones accumulated in the front of a small delta. Oyster banks and concentrations of Turritelines suggest high nutrient levels in the delta-front paleoenvironments. The Los Guájares valley area emerged afterwards, following the onset of a compressional geodynamic regime in the central Betic Cordillera and since the early Tortonian only small bodies of foothill and alluvial-fan deposits formed in a predominantly erosional context.

Muddy and sandy conglomerates: Mixing between gravelly slurry flow and substrates in a fan-delta system, Junggar Basin, Northwest China

Mixing of a gravity flow with substrate over which it travels can control flow transformations with implications for the structure of the deposit that is left; however, the effects of mixing in conglomerate-dominated gravelly underflow in lacustrine environments remain poorly understood. This study documents two types of interactions between gravelly underflow and substrate and the resulting conglomerate deposits across a lacustrine fan-delta system in the Junggar Basin, northwest China. The resulting conglomerates are typically massive, poorly sorted, clast-supported without reserve grading; rich in mudstone clasts; and having been emplaced by river-derived hyperpycnal flow. Based on the character of the matrices between the clasts, conglomerates are divided into three sub-categories: (i) muddy (mud-rich) conglomerates with a mainly fine-grained muddy matrix, (ii) sandy (mud-poor) conglomerates with a mainly sandy matrix, and (iii) mud-bearing sandy conglomerates with a coexisting mud-sand matrix. Furthermore, two distinct lithological associations were identified: (i) a regularly recurring tripartite conglomerate unit interlayered with mudstone (S I) and (ii) a regularly recurring tripartite conglomerate unit interlayered with sandstone (S II). Both lithological associations contained a recurring tripartite conglomerate unit with a basic tripartite structure consisting of muddy conglomerates in the lower division, mud-bearing sandy conglomerates in the middle division, and sandy conglomerates in the upper division. There was more sand in the upper layers of the tripartite conglomerate unit matrix. Muddy conglomerates with large amount of mudstone clasts are thought to form through extensive erosion and mixing between the lower part of the flow with mud-substrate on the lake bottom, whereas central mud-bearing conglomerates and upper sandy conglomerates are considered to be generated by successively weaker mixing in the upper part of the flow. The consequent sandy trailing flow resting upon the previously-emplaced tripartite conglomerate unit forms a thinner sandstone bed that acts as a “sand mat” for the next incoming gravelly flow. A single sandy conglomerate bed subsequently forms above when gravel mixes with the sand substrate, as opposed to forming muddy conglomerates that were generated by mud from eroded substrate. Our results show that the erosion and mixing of sediment flow with substrate are important factors in flow transformation and deposition and that the substrate type (muddy or sandy) along the flow path plays a crucial role in controlling flow behavior and reservoir characteristics.

Comparison of three grain size measuring methods applied to coarse-grained gravel deposits

The size of grains in gravel and conglomerate deposits is most easily measured on photos taken from related outcrops. However, the occlusion of grains by the sedimentary matrix or other grains, and possible distortions of photos, could introduce a bias in such datasets. Here, we explore the uncertainties associated with datasets where the lengths of the grains were measured on photos. To this end, we analysed coarse-grained (>2 mm) fluvial material from a gravel pit (Bern, Switzerland). We compared grain size data collected from digital photos with the results where the same material was measured with a calliper and mechanically sieved. Our analyses reveal that the percentile values such as the D16, D50 and D84 of datasets where the grains' longest visible axes were measured on digital photos best correlate to the corresponding percentile values of data collected through sieving. We also find that the longest visible axes of grains measured on digital photos are c. 17 % smaller than the lengths of the intermediate b-axes of grains measured with a calliper. We therefore suggest to measure the longest visible axes on digital photos, and to correct the data by a corresponding factor such as +17 % for the target grain size percentiles.

Facies architecture of the Plio-Quaternary alluvial fan deposits from south-eastern margin of the Saïss foreland basin (Morocco): Paleoclimatic and neotectonic implications

Eight Plio-Quaternary alluvial fans from south-eastern margin of the Saïss foreland basin were the subject of a thorough sedimentological study based upon detailed facies analysis. This study allowed distinguishing 16 sedimentary facies. The typology of the latter, their spatial distribution and sedimentary organization afforded the classification of the studied fans into three groups. Group 1 is dominated by non-cohesive debris flow and cohesive to very poorly-sorted blocky conglomerate deposits that are associated with silt-clayey and calcrete. The setting up of the deposits of this group occurred in a tectono-karstic depression whereby a WNW progradation of the deposits and an important incision into the previous consolidated deposits would have been controlled. Group 2 is characterized by a predominance of the muddy fraction, presumably linked to the large surface area of the watershed and the river activity. Group 3 is dominated by coarse deposits with rare mudflows interpreted to be the direct consequence of an important neotectonic activity which is expressed by localized incremental pulses of the Agourai-El Hajeb-Ain Cheggag faults system and Middle Atlas directional faults (i.e., NE-SW) along toe fan during the Plio-Quaternary period. Groups 2 and 3 are also characterized by deposits whose the related processes are dominated by cohesive and non-cohesive debris flows, mud-rich debris, channelized debris flows and non-cohesive flows, hyper-concentrated flows and sheet-flood flows. The morphogenesis of these alluvial fans and the emplacement of their deposits are neotectonic- and climate-controlled. Tectonics have played a major role in the rejuvenation of the Middle Atlas relief and, through controlling the base-level, created the space available for the establishment and development of alluvial fans onto the south-eastern margin of the Saïss basin. This extensive tectonics has resulted in normal faultings and layer tiltings. The climate played an important role in the formation of facies by controlling (i) the mechanical properties of paleoflows, (ii) sedimentary dynamics and (iii) the formation of paleosols and calcrete.

Identification of favorable underwater reservoirs in a conglomerate fan delta

Conglomerates are the main components of fan delta-front and usually the important reservoirs in an oilfield, but the cyclicities of the conglomerate deposition readily lead to stratigraphic multiplicities. Therefore, the accurate prediction of the distribution patterns and scales of the conglomerate fan delta has great significances for the oil and gas exploration and production. The conglomerate fan delta of the Triassic Baikouquan Formation (TBF) in the west slope of the Mahu Sag, Junggar Basin is a research hotspot because billions of petroleum resources have been discovered in this Formation. However, the stratigraphic correlation of the conglomerate of the TBF is still unclear for the particularly heterogeneous of the conglomerate strata, which caused the paleogeomorphology and favorable underwater reservoirs of the TBF are yet to be determined. In this study, the integrated methods of seismic interpretation, well logging, well curves and heavy minerals are applied to study the paleocurrent directions, ancient trench distribution and shoreline positions of the lake. The results show: (1) There are two ancient trenches developed in the west slope of the Mahu Sag. (2) The shoreline positions are generally distributed along the shape of ancient trenches. And (3) the favorable underwater reservoirs in a conglomerate fan delta are distributed in the front of the shoreline as a progradation deposits of the fan delta. The lake level elevation and paleotopography jointly controlled the lithology and reservoir physical properties, and the underwater facies belt in the ancient trench are the favorable exploration zones.

Trapped in a graben: deposition of Huronian gold-bearing conglomerates in a fault-influenced, valley-confined, fluvial system in the southern Cobalt Basin, Ontario, Canada

Isopachs of Huronian strata of the Elliot and Hough Lake groups in the southern part of the Cobalt Basin can be used to define the geometry of a 4 km wide valley system that directly influenced the location of gravel-bed rivers bearing detrital gold and auriferous pyrite in the Mississagi Formation. Distribution and thickness of these and underlying formations can be directly linked to initial valley formation parallel to existing north-south-oriented faults in the Archean basement. Thickness distributions were directly influenced by active subsidence associated with transverse, east-south-east (ESE)-oriented, normal faults, related to extension along the Huronian transform-rift margin further south. Strata underlying the Mississagi Formation were largely removed by erosion in the northern part of the paleovalley system, but thickened and then thinned south of the ESE faults. Pyrite and detrital gold in the Mississagi Formation may have been concentrated from reworking of coarse clastic rocks of the Matinenda and Ramsay Lake formations, along with significant contributions from erosion of proximal Archean basement within 3–5 km of the preserved basin margins. There is strong evidence to suggest that stream flow was initially concentrated in three main structurally influenced valley systems in the north, with one lateral tributary in the south-eastern part of the basin. The fluvial systems merged, and thickened, south of the Tee Lake fault, possibly reflecting trans-tensional influences on the basin margin faults.

Detrital zircon ages from Archaean conglomerates in the Singhbhum Craton, eastern India: implications on economic Au-U potential

New U–Pb age and Hf isotope data obtained on detrital zircon grains from Au- and U-bearing Archaean quartz-pebble conglomerates in the Singhbhum Craton, eastern India, specifically the Upper Iron Ore Group in the Badampahar Greenstone Belt and the Phuljhari Formation below the Dhanjori Group provide insights into the zircon provenance and maximum age of sediment deposition. The most concordant, least disturbed 207Pb/206Pb ages cover the entire range of known magmatic and higher grade metamorphic events in the craton from 3.48 to 3.06 Ga and show a broad maximum between 3.38 and 3.18 Ga. This overlap is also mimicked by Lu–Hf isotope analyses, which returned a wide range in εHf(t) values from + 6 to − 5, in agreement with the range known from zircon grains in igneous and metamorphic rocks in the Singhbhum Craton. A smaller but distinct age peak centred at 3.06 Ga corresponds to the age of the last major magmatic intrusive event, the emplacement of the Mayurbhanj Granite and associated gabbro, picrite and anorthosite. Thus, these intrusive rocks must form a basement rather than being intrusive into the studied conglomerates as previously interpreted. The corresponding detrital zircon grains all have a subchondritic Hf isotopic composition. The youngest reliable zircon ages of 3.03 Ga in the case of the basal Upper Iron Ore Group in the east of the craton and 3.00 Ga for the Phuljhari Formation set an upper limit on the age of conglomerate sedimentation. Previously published detrital zircon age data from similarly Au-bearing conglomerates in the Mahagiri Quartzite in the Upper Iron Ore Group in the south of the craton gave a somewhat younger maximum age of sedimentation of 2.91 Ga. There, the lower limit on sedimentation is given by an intrusive relationship with a c. 2.8 Ga granite. The time window thus defined for conglomerate deposition on the Singhbhum Craton is almost identical to the age span established for the, in places, Au- and U-rich conglomerates in the Kaapvaal Craton of South Africa: the 2.98–2.78 Ga Dominion Group and Witwatersrand Supergroup in South Africa. Since the recognition of first major concentration of gold on Earth’s surface by microbial activity having taken place at around 2.9 Ga, independent of the nature of the hinterland, the above similarity in age substantially increases the potential for discovering Witwatersrand-type gold and/or uranium deposits on the Singhbhum Craton. Further age constraints are needed there, however, to distinguish between supposedly less fertile (with respect to Au) > 2.9 Ga and more fertile < 2.9 Ga successions.

U-Pb zircon-titanite-apatite age constraints on basin development and basin inversion in the Kiruna mining district, Sweden

To constrain the tectonothermal evolution of the type locality for iron oxide-apatite deposits, we have obtained U-Pb zircon, titanite, and apatite age data for the Kiruna mining district in northernmost Sweden. The results indicate that the host basin initiated in an overall extensional regime as indicated by the deposition of alluvial conglomerates and greywackes. A volcanic intercalation in a conglomerate unit approximately 1 km northwest of the Luossavaara iron oxide-apatite deposit yields a U-Pb zircon concordia age of 1887 ± 3 Ma representing the timing of the earliest Orosirian volcanism in the central Kiruna mining district coinciding with the onset of basin development. In-situ analysis of titanite sitting on hydrothermally altered fracture planes within a cataclastic fault damage zone (c. 270 m from the fault core system associated with the Luossavaara iron oxide-apatite deposit) yields complex U-Pb data. Applying a strict discordance filter yields seven analyses (<1.5 % relative discordance which included six common Pb-corrected analyses and one analysis where no significant common Pb was detected) that yield a 207Pb/206Pb weighted average age of 1889 ± 26 Ma that we interpret as the most probable minimum age of fault initiation. The overlap in age of the host volcanic rock and the fault implies that during basin development, contemporaneous volcanism and syn-volcanic faults may have played an important role during iron ore emplacement. A U-Pb lower intercept age of apatite from the Nukutus iron oxide-apatite deposit provides a first indication that the area was subsequently buried and metamorphosed under upper greenschist-facies conditions and later tectonically exhumed and cooled below the apatite closure temperature at 1805 ± 26 Ma. Further temporal constraints on exhumation and basin inversion is given by syn-tectonic titanite along a brittle-ductile reverse shear zone to the east of the study area that shows limited presence of significant common Pb and is part of a sodic-calcic + Fe + Cl hydrothermal alteration assemblage. Titanite grains that show a well-developed zonation yield a U-Pb concordia age of 1812 ± 3 Ma, which we interpret as the onset of basin inversion. Titanite grains from the same sample that show less pronounced zonation yield a U-Pb concordia age of 1802 ± 8 Ma, tentatively indicating that the tectonothermal activity lasted up to c. 20 m.y.