Continental Red Beds Research Articles

Discovery of ≥ 105 Ma continental redbeds in the Qiangtang Block: Implications for the early uplift of central Tibet

Discovery of ≥ 105 Ma continental redbeds in the Qiangtang Block: Implications for the early uplift of central Tibet

Mesozoic clastic provenance during post-rift evolution of the Essaouira Agadir Basin, Northern Morocco

This study combines several provenance tools, analysis of published structural and geodynamic data, integrated with Low-Temperature Thermochronology (LTT) and time-Temperature Modelling (tTM) to reconstruct the evolution of source-to-sink systems feeding the Essaouira-Agadir Basin (EAB) during the Jurassic (Toarcian, Bathonian, and Kimmeridgian) and Lower Cretaceous (Hauterivian and Barremian).LTT and tTM define timing and rate of subsidence and exhumation of the hinterland and allows modelling of the predicted age and lithology of eroding rock units from the most-likely source locations through time. Extrapolation of predicted surface geology allows recognition of the lithology of sedimentary overburden in the hinterland, much of which has been subsequently eroded and is not preserved in the modern surfical geological record.Heavy mineral, petrography and detrital zircon data analysis was carried out on fluvial and shallow marine sandstones sampled from Jurassic and Cretaceous sections across in the EAB. The results document changing sediment source terrains through time. In the Early and Middle Jurassic, the heavy mineral and detrital zircon signature correlates with a Palaeozoic source, suggesting provenance was dominantly from erosion of Cambrian and Ordovician sandstone in the Central and Western Anti-Atlas. From the Late Jurassic to Early Cretaceous, the heavy mineral and zircon signatures have a strong affinity with Triassic sediments. This indicates a provenance switch to the exhuming West Moroccan Arch (MAM and Western Meseta), interpreted to have been largely covered by Triassic continental red beds at the time.The results help in predicting sediment delivery offshore, into the deep-water basin, where sandstones are a target for hydrocarbon exploration. Defining timing of input, location and composition helps to de-risk exploration. All the intervals examined contained discrete fluvial systems entering in the EAB, suggesting multiple periods of clastic delivery. Results suggest the Middle Jurassic and Hauterivian and Barremian intervals offer the optimum time for delivery of coarse clastics to the shelf margin, and potentially into the deep basin. The source to sink maps developed in this study further characterize these systems, their provenance and timing.

Investigating the impact of paleoclimatic conditions and diagenesis on the genesis of Permian Continental Red Beds: A case study from the Bohemian Massif, Czechia

The processes responsible for reddening of Continental Red Beds (CRBs) and the relationship between color variation and paleoenvironmental conditions are presented focusing on a comprehensive multi-proxy study of Permian sediments in the Bohemian Massif, Czechia. The investigation incorporates facies analysis, quantitative color assessment using diffuse Vis-spectral reflectance (DRS), optical and electron microprobe microscopy, bulk-rock (XRF and XRD), and in-situ geochemistry (laser-ablation ICP-MS). Results indicate a progressive drying trend from the Cisuralian to Guadalupian series in studied continental red sediments. Different facies indicate the change of the sedimentary environment from a deep lacustrine environment (lower part of Rudník Member, Cisuralian) to a fluvial floodplain and eolian environment (Trutnov Formation, Guadalupian). Examination of the three major categories (white, gray–green and red sediments) identified in the studied continental red beds indicates that diagenetic alteration of clay minerals and biotite was the main source of iron fueling the growth of hematite responsible for their red color. Early diagenetic processes and paleoenvironmental conditions, particularly the oxidizing or reducing conditions play a key role in the red sediment formation. It is suggested that later diagenetic stages are incapable of coloring non-red, iron-rich sediments formed in deep anoxic lacustrine environments. Microbial activities and reducing fluids have been identified as the main factors in the formation of gray–green sediments forming distinct reduction zones. The reduction spots formed during the early stages of diagenesis (eodiagenesis), and they were likely never red. In contrast, reduction strips, initially exhibiting a red hue, underwent a color change during more advanced stages of diagenesis (mesodiagenesis).

Sediment colour as a marker of syn-depositional and early diagenetic processes in glaciofluvial sediments

The continental red beds, encompassing a broad spectrum of genetic types, can serve as important palaeoclimatological and palaeoenvironmental archives. The origin of sediment colouration is a complex process involving abiotic processes (e.g., breakdown of original and precipitation of newly-formed minerals), which, together with biogenic factors, lead to mobilisation of redox-sensitive elements and precipitation of Fe- and Mn-(oxy)hydroxides. There is still discussion about the interpretation of the continental red beds as palaeoclimatological archives or the colour patterns reflecting ancient redox gradients. The layers coloured in red, yellow or black can be found in the Quaternary glaciofluvial sediments in the Czech Republic. We are using a combination of field study with multi-spectral petrophysical, petrological and geochemical analyses to investigate the mechanism and timing of the origin of coloured coatings in glaciofluvial sediments, and causes of cycling of Fe, Mn, and other redox sensitive elements and isotopes. The results show that both syn-depositional and early diagenetic processes are responsible for the origin of colour patterns in the Quaternary glaciofluvial sediments. The stable molybdenum and iron isotope fractionation is primarily driven by the breakdown of the primary Fe and Mn-bearing silicates and the precipitation of the secondary Fe- and Mn-(oxy)hydroxides, such as goethite and birnessite. These precipitates are the main components of colouring coatings on the detrital grains and are able to bind other redox-sensitive elements, such as Cu, As, Mo, U, and REEs. The textural patterns and geochemistry suggest that the colour features were developed in the time range of decades to several thousand years after the deposition along ancient subsurface redox gradients due to changes in groundwater flow associated with primary lithology, glaciotectonics, and seasonal changes in the active layer of permafrost. The coatings show morphological features (rods, botryoids) and geochemical signatures (e.g., increased P contents) suggesting involvement of microorganisms to their precipitation.

Geology of the Late Carboniferous, Permian and Early Triassic basins of the Eastern Pyrenees

ABSTRACT We present a detailed geological map of the Upper Carboniferous, Permian and Triassic rocks of the southern border of the Axial Zone in the Eastern Pyrenees. These units were deposited in three intra-mountain, terrestrial basins, characterized by the emplacement/deposition of volcanic, sub-volcanic and volcaniclastic units (ca. 1000 m thick) together with continental red beds. The map is based on unpublished drafts, sketches and structural data. This previous information was complemented with new structural, petrologic and stratigraphic data. The geological map is presented together with three cross-sections, a detailed lithological description, and a stratigraphic correlation panel to help understand the stratigraphy, volcanism and structure and, ultimately, the geometry and origin of the highly subsident areas of this segment of the chain in post-Variscan times. The geological map gives new clues for reconstructing the geometry of these basins and the distribution of the volcanic episodes during the Late Carboniferous and Permian.

Sedimentary characteristics and model of seasonal deltaic sandstone: a case study on the continental red bed from the Cretaceous Baxigai Formation, Tarim Basin, NW China

Seasonal delta is a sandy fan-shaped sedimentary system formed by seasonal river flooding in continental basins. Modern sedimentary finds that seasonal deltas mainly develop in an arid-semi-arid climate environment with gentle terrain. However, the study of seasonal deltas in ancient rock records is still insufficient, leading to misinterpretations as other sedimentary systems, which poses challenges to the correct understanding of the origin and spatial distribution of sand body deposits. In this study, the red bed deposits of the Cretaceous Baxigai Formation in the west of the Tabei Uplift were selected as the research object. Based on core observation and description, a comprehensive analysis of the sedimentary environment, sedimentary characteristics and sedimentary model of the Baxigai Formation was carried out using data such as well-logging and analytical testing, combined with modern depositional characteristics. Research shows that the sedimentary sequence of the Baxigai Formation red bed developed a seasonal delta sedimentary system in the arid climate. The seasonal delta deposits provide an opportunity to understand the influencing of river flow and total sedimentary loading vary greatly on the sedimentary model, sedimentary mechanism, and distribution of sand bodies in arid climates. The sedimentary process of the seasonal delta in the Baxigai Formation was controlled by the dual-stage sedimentation process in an arid paleoclimate. During the flood period, the supercritical flow sedimentary structures are widely developed in the major distributary channels, and the distributary channels migrate and bifurcate frequently. On the delta plain, crevasse channels are formed through the avulsion of major distributary channels. During the dry period, due to the consumption of water seepage and evaporation transfer, terminal distributary channels are abandoned, crevasse channels are not developed, and the sediment is exposed. The density of the distributary channels is reduced, and the total sedimentary loading is low. The distributary channels sand body is the skeleton sand body of the seasonal delta. This study is of great significance for accurately explaining the formation process of seasonal deltas and provides a useful reference for further exploration and deployment of oil and gas reservoirs and paleogeographic reconstruction in the study area.

Paleogene sediment provenance in the Gaize Basin: Implications for early Cenozoic paleogeography of central Tibet

A comprehensive study of the sediment source-to-sink history of basins arranged along the Bangong-Nujiang Suture Zone (BNSZ) is critical for understanding the early Cenozoic topography and drainage systems of central Tibet and its implications for the uplift history of the Tibetan Plateau. Nevertheless, previous provenance analyses work mainly focused on the middle-eastern segment of the BNSZ, those Paleogene basins in the western segment have received less attention, which hinders a full understanding of the early Cenozoic topography of central Tibet. In this study, based on a chronostratigraphic framework developed for the lower Cenozoic continental redbeds of the Gaize Basin, we applied detrital zircon U-Pb geochronology to the well-exposed Paleogene Suonahu Formation in order to reconstruct the basin's source-to-sink history. Integrating these new data with detrital zircon U-Pb results from the underlying Kangtuo Formation, we recognize a major secular shift in sediment provenance characterized by inputs of Lhasa-derived clasts during the Late Eocene. Moreover, we applied statistical analysis to compare the U-Pb age spectra of Eocene strata in the Gaize-Nima-Lunpola basins in order to assess similarities between them and potential source regions. Our study confirms a paleogeographic scenario in which these Paleogene basins most likely were part of a low-elevation intermontane valley.

Color Origin of Red Beds within the Danxia Basin, Southern China

Sedimentary facies color is an important paleoclimate indicator, but may be unreliable in thick continental red beds. The Danxia Formation is the landscape strata of the Danxia basin, and its color fluctuates between reddish-brown and reddish-purple vertically. This study examined the ferric oxides characteristics, distribution, and mineral composition of the Danxia Formation using a variety of analytical techniques, including optical microscopy, high-resolution scanning electron microscopy, electron probe microanalysis, X-ray diffraction, and micro-Raman spectroscopy. The results indicate that the reddish-brown mineral is composed primarily of fine hematite with traces of goethite, while the reddish-purple mineral consists solely of fine hematite. These hematites exhibit a particle size range of submicron to micron and display various morphologies, including spherical, flake, and fibrous forms. Goethite particles are needle-shaped and often form star-shaped aggregates containing twins. Whole rock geochemical analysis reveals a strong positive correlation between iron and iron group elements, as well as phosphorus and rare earth elements. The findings suggest that the reddening of the Danxia Formation occurs during diagenesis, with fine hematite forming as a result of the alteration of iron-bearing detrital grains. In addition, vertical color variations in Danxia Formation are not attributable to the paleoclimate records, but rather the result of the differentiation of sediment sources. This study provides a novel viewpoint for examining the coloration of thick continental red beds in other regions.

Groundwater red beds in Holocene fluvial sediments as a product of iron and manganese redox cycling; Morava River, Czechia

ABSTRACTModern fluvial deposits can be affected by early diagenetic element mobilization at redox boundaries between oxygenated and oxygen‐free zones near the groundwater level. The visible product of redox transformation of Fe is the sediment colour. Distinct red and black sediment layers have been studied in eight shallow (<4.2 m depth) sections in the floodplain of Morava River, Czechia. The aim was to investigate the composition, origin, rates of formation and stratigraphic significance of the red strata using bulk‐rock analytical methods, radiocarbon dating, optical and scanning electron microscopy coupled with in situ (energy‐dispersive X‐ray scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry) geochemistry. The coloured layers, in places well‐cemented, developed in permeable sands and gravels above the water table, close to the boundary with overlying less permeable floodplain sandy silts. Their colour is due to Fe and Mn oxyhydroxides (goethite, haematite, todorokite and birnessite) coatings of framework grains. Black, Mn‐rich layers occur stratigraphically higher than the red, Fe‐rich ones. The coatings are a few‐hundred microns thick at maximum, often botryoidal, composed of alternating Fe‐rich and Mn‐rich laminae enriched in As, Mo, Sb, P, Cu and U. The coatings formed under suboxic conditions due to Fe and Mn oxyhydroxide cycling, driven partly by microbial activity, at the groundwater‐related redox boundary which resulted in the distinct vertical arrangement of the black and red layers. Being markedly enriched in As bound to Fe‐oxyhydroxides and exposed to fluctuating redox conditions, the coloured strata can potentially release As to groundwater and represent serious environmental hazards. Radiocarbon ages suggest that the groundwater‐type red beds can occur extremely quickly, over a few hundred years. It is assumed that, under suitable circumstances, this reddening can be preserved in the rock record, and represent a specific model for the development of continental red beds.

Paleoclimate quantitative reconstruction and characteristics of continental red beds: a case study of the lower fourth sub-member of Shahejie Formation in the Bonan Sag

Varied origins have the ability to construct the continental red beds, such as paleoclimate, provenance, drainage status, etc. Reconstructing paleoclimate is the key to investigate the origin. Thus, this paper outlines the normal distribution constrained method (NDCM) to reconstruct paleoclimate quantitatively and accurately during the lower fourth sub-member of Shahejie (Es4x) in the Bonan Sag, which is in low requirement of data and environmental conditions. Based on the NDCM, the paleoclimate is still in a long-term arid background as the potential evapotranspiration rates (PERs) are larger than 6 in the transgressive system tract (TST) and regressive system tract (RST) during the period of Es4x. The decreasing PERs that range from 6.28 to 6.04, decreasing atmospheric temperature range from 14.37 to 13.95 °C and increasing mean precipitation of the wettest month and mean precipitation of the driest month indicate the paleoclimate is breaking away from the hot and arid background from TST to RST. Meanwhile, the paleoclimate fluctuation develops decreasing frequency and increasing amplitude which can be inferred from the increasing standard deviation (std), Th/K and Th/U curves. The hot and arid background is still the main origin of red color during the Es4x. Moreover, the other origin may also develop since there is no clear law between red coloration and the developing location of sediments.

NEW 254 ma PALEOMAGNETIC POLE OF THE EAST EUROPEAN PLATFORM: MOSCOW SYNECLISE, SUKHOBORKA AND SOSNOVKA SECTIONS

The paper presents the results of paleomagnetic and rock magnetic studied of upper Permian continental red beds of the Sukhoborka and Sosnovka sections of the south-eastern part of the Moscow syneclise on the right bank of the Vetluga river in Nizhniy Novgorod region, Russia. Arguments in favor of the primary nature of the characteristic component of magnetization are given. A first securely dated upper Permian (254 Ma) paleomagnetic pole of the east part of the East European Platform is calculated: slat = 56.731°N, slong = 45.748°E, N = 44, plat = 55.3°, plong = 158.1°, dp/dm = 3.5°/5.1°, paleolat = 34.6°.

Middle Permian basic and acidic volcanism in the Istanbul zone (NW Turkey): evidence for post-variscan extensional magmatism

ABSTRACT The Istanbul Zone (NW Turkey) forms the eastward extension of Avalonia and was subjected to deformation, uplift and erosion for a time period of 40–50 Ma following the collision with the Sakarya Zone during Early to Late Carboniferous. This paper deals with the petrology and age of the volumetrically minor basic and acidic volcanism at the lowermost horizons of Middle Permian continental red beds, which are overlain by Lower Triassic marine sedimentary rocks in the Kocaeli Peninsula. The volcanic activity is represented mainly by amygdaloidal basalt, rhyolite and minor trachydacite. The amygdaloidal basalt was derived from near-primary middle-K calc-alkaline mantle melts with negligible crystal fractionation. On the other hand, the rhyolite and trachydacite compositionally resemble A2-type rhyolites and underwent low-pressure crystal fractionation as indicated by the presence of a significant Eu anomaly. Initial ɛNd values of amygdaloidal basalt range from 0.0 to 1.5 and those of rhyolite-trachydacite are between −0.4 and −3.4. Amygdaloidal basalt and rhyolite-trachydacite are not directly related to each other by crystal fractionation. Amygdaloidal basalt probably represents the product of the near-primary mantle melts from low-degree melting of a spinel peridotitic source, and the rhyolite-trachydacite originated from highly-fractionated products of basic magmas that are slightly more alkaline than amygdaloidal basalt. However, basic and intermediate products of alkaline basic magmas are unknown in this region to date. U-Pb dating of zircons from a rhyolite sample yielded an igneous crystallization age of 261 ± 3 Ma (2σ), suggesting that the date of deposition of the continental red beds goes back to the latest Middle Permian. Based on the transgressive nature of the Permian-Triassic sequence that starts from the Middle Permian continental red beds and grades into Lower Triassic marine deposits, we suggest that the volcanism likely occurred in an extensional setting. This extension was concurrent with the northward subduction of the Palaeo-Tethys beneath the Sakarya and Istanbul zones after the Variscan orogeny. Therefore, the latest Middle to Late Permian volcanism might have occurred during the initial stage of a back-arc extensional setting

Mid‐Cretaceous Exhumation of the Central Qiangtang Mountain Range Metamorphic Rocks as Evidenced by the Abushan Continental Redbeds

AbstractMetamorphic rocks and surrounding Paleozoic strata in central Tibet compose the core of an anticlinorium associated with the Central Qiangtang Mountain Range. The analysis of Mesozoic strata flanking the anticlinorium provides insights into the geological and landscape evolution of the range, which is important in determining the Mesozoic evolution of central Tibet and the paleo‐elevation of the Tibetan Plateau. Here we present a detailed stratigraphic study of the Shuanghu Conglomerate and over 1‐km‐thick mid‐Cretaceous continental redbeds of the Abushan Formation, which non‐conformably overlie the Qiangtang metamorphic belt. Pollen assemblages and detrital zircon ages along with published magnetostratigraphic results indicate a mid‐Cretaceous depositional age for the Abushan Formation. The Shuanghu Conglomerate is undated but may have been coeval with the Abushan Formation as suggested by foraminiferal assemblages contained within limestone clasts. The Abushan Formation shows a fining‐ and deepening‐upward megasequence, changing from braided river delta‐plain conglomerate and sandstone to lacustrine delta‐plain and delta‐front sandstone, siltstone and carbonate rocks. Both the Abushan Formation and Shuanghu Conglomerate received metamorphic detritus, indicating that the exhumation of the Qiangtang metamorphic belt to the surface took place at or before mid‐Cretaceous time. The lower member of the Abushan Formation contains dominant limestone clasts, whereas quartz, metamorphic, and volcanic clasts increase in percentage up‐section reflecting an unroofing sequence. The stratigraphic, sedimentologic, and provenance evolution suggests that exhumation of the Central Qiangtang Mountain Range metamorphic rocks may have occurred primarily by tectonic extension after the end of a long period of late Mesozoic Lhasa‐Qiangtang convergence.

Paleomagnetism and Cyclostratigraphy of the Permian-Triassic Boundary Interval of the Staroe Slukino Section, Vladimir Region

The paper presents the results of paleomagnetic, rock magnetic, and cyclostratigraphic studies of continental red beds that compose the Permian-Triassic boundary interval of the Staroe Slukino section in the Vladimir Region, Russia. Based on the directions of the characteristic components of the natural remanent magnetization of the studied rocks in the sampled stratum, the intervals of normal and reverse polarity related to the regional magnetozones r2RnP, r3RnP, and N3P-T were identified. Within the r3RnP zone, there is an interval of anomalous paleomagnetic directions, which has similar characteristics to those identified earlier in the coeval intervals of the Nedubrovo, Zhukov Ravine, and Okskiy Siyezd sections. Revising the biostratigraphy allows us to assume that the two zones of anomalous paleomagnetic directions in the composite magnetic polarity scale of the upper Permian of the Russian Platform are a reflection of the same epoch of an anomalous geomagnetic field configuration. It is estimated by the cyclostratigraphic method that the studied 16 m interval of the Staroe Slukino section took 900±20 thousand years to accumulate, which limits the duration of the geomagnetic field anomalous state near the Permian-Triassic boundary to ~110 000 years. A new Permian-Triassic (~252 Ma) paleomagnetic pole of the East European Platform is calculated: plat = 36.3°; plong = = 155.0°; dp/dm = 2.8°/4.8°.

Micro-Mechanisms and Implications of Continental Red Beds

Continental red beds, widely formed at various geologic timescales, are sedimentary rocks and sediments with red as the main color. Geoscientists have analyzed the geomorphology, paleomagnetism, paleoenvironments, paleontology, energy, and minerals in continental red beds. Despite the agreement that fine-grained hematite is closely related to the color of continental red beds, controversies and problems still exist regarding the micro-mechanism of their formation. As a review, this paper details the composition and color properties of pigmentation in red beds, analyzes the existence and distribution of authigenic hematite, and summarizes the iron sources and the formation of hematite. In addition, we introduce the fading phenomenon observed in continental red beds, including three types of secondary reduction zones: reduction spots, reduction strips, and reduction areas. Lastly, this paper summarizes the evolution of color in continental red beds, emphasizes the relationship between authigenic hematite and the diagenetic environment, and proposes possible research directions for future red bed-related issues.

Origin of the Bleaching in Lower Cretaceous Continental Red Beds in the Uragen Zn–Pb Deposit, Xinjiang, NW China, and Its Implications for Zn–Pb Mineralization

The Uragen giant sandstone-hosted Zn–Pb deposit has a proven reserve of 5.90 Mt metals in the southern ore zone and potentially 10 Mt metals for the whole deposit, and orebodies are strictly confined to the bleached clastic rocks of the Lower Cretaceous red beds. The bleaching has been used to guide lead–zinc exploration; however, its nature and origin, as well as the relationship with Zn–Pb mineralization, remains unclear, although it is closely related to regional oil–gas infillings. Detailed field investigation and petrographic observation, TESCAN-integrated mineral analyzer (TIMA), and X-ray fluorescence (μ-XRF) analysis of the red and bleached sandstone at the same sedimentary layer demonstrate that the bleaching is mainly caused by the reductive dissolution of hematite pigment, which probably resulted from the interaction with H2S formed by in situ sulfate reduction during hydrocarbon migration. The calcite cements in the bleached sandstones show δ13C and δ18O values of −5.36~−5.94‰ and 20.94~27.91‰, respectively, and these samples fall close to the evolution line of decarboxylation of organic matter in δ13C-δ18O diagram, also suggesting a genetic relationship between the bleaching and hydrocarbon-bearing fluids. Petrol–mineral composition changes and sulfide characteristics of red, bleached, mineralized zones, as well as pyrite locally replaced by coarse-grained galena in the mineralized zone, imply that the bleaching may occurred before Zn–Pb mineralization. Mass balance calculation and μ-XRF analysis indicate that large amounts of Fe and minor Zn were extracted from red beds with little or no sulfates; however, the red beds with abundant sulfates may be a sink for leached ore metals during the bleaching process. We therefore propose that the former accumulations of iron sulfides and reduced sulfur in the bleached zones may provide an ideal chemical trap for later Zn–Pb mineralization, and the bleached zones with high ∑S contents are the favorable prospective targets of the Uragen-style sandstone-hosted Zn–Pb deposits.

Sedimentary facies, depositional environments, and sequence stratigraphy of the Neogene continental redbeds in the southwest of Alborz Province, N Iran

Sedimentary facies, depositional environments, and sequence stratigraphy of the Neogene continental redbeds in the southwest of Alborz Province, N Iran

E/I-corrected inclination shallowing in Cenozoic redbeds from the northern Tarim Basin, NW China: Possible causes and paleogeographic implications

AbstractBecause of their widespread occurrence and ability to carry stable remanence, continental redbeds in central Asia are frequently used in paleomagnetic studies. However, the paleomagnetic inclinations recorded by redbeds are much shallower than the expected values, as redbeds are usually subjected to inclination shallowing. To recognize and correct the inclinations recorded by the Cenozoic redbeds, the paleomagnetic data that were used for magnetostratigraphic studies in the Kuqa Depression, northern Tarim Basin, are reanalyzed in this study. The mean inclinations of the four groups of samples (Eocene, Oligocene, Miocene, and Pliocene) are systematically ~20° shallower than the expected values calculated from the apparent polar wander paths (APWPs) of Eurasia, indicating the presence of inclination shallowing. We apply the elongation/inclination (E/I) method to correct the inclination shallowing. The mean inclinations of the Eocene, Oligocene, Miocene, and Pliocene sediments are corrected from 40.5° to 63.1°, 41.0° to 63.8°, 42.0° to 63.8°, and 44.7° to 63.2°, within 95% confidence limits between 55.1° and 71.6°, 53.7° and 70.4°, 51.5° and 72.7°, and 52.2° and 71.3°, respectively, which are indistinguishable from the expected inclination values. Our results suggest that inclination shallowing in the redbeds of central Asia can be reasonably corrected using the E/I method, and sedimentary processes such as compaction and/or imbrication in the very early stage of burial are important causes for inclination shallowing. Paleolatitudes calculated from the E/I-corrected inclinations show that the Tarim Basin should have reached or been at least close to its current latitude since the Cretaceous. The Cenozoic crustal shortening estimate of the northern Tarim Basin is not detectible for paleomagnetic study.

Ichnological Model of the Upper Triassic Continental Red Bed from India: Implications for Palaeoenvironment and Palaeoclimate

Ichnological Model of the Upper Triassic Continental Red Bed from India: Implications for Palaeoenvironment and Palaeoclimate

POTENTIAL ICE CRYSTAL MARKS FROM PENNSYLVANIAN–PERMIAN EQUATORIAL RED-BEDS OF NORTHWEST COLORADO, U.S.A.

ABSTRACTThe Pennsylvanian–Permian Maroon Formation of northwest Colorado is an up to 4,600 m thick succession of mainly siliciclastic continental red-beds deposited in equatorial intermontane basins of the Ancestral Rocky Mountains. Sedimentary surfaces of fluvio-lacustrine to eolian siltstones and fine-grained sandstones from various stratigraphic levels within the Maroon Formation preserve cm-sized straight to gently curved sediment-filled acicular structures referred to five morphological groups: single, branched, stellate, rosette, and bunched. Depositional environment, shape, and size of the structures are most similar to ice crystal marks that result from freezing of water-saturated fine-grained substrate at the sediment-air interface. They differ from other syngenetically produced crystals and crystal pseudomorphs in sedimentary rocks mainly by crystal shape and environmental conditions. The potential ice crystal marks of the Maroon Formation are notable for the fidelity and morphological diversity of the crystal casts and could be a key for the understanding of similar but hitherto often only called enigmatic structures of the sedimentary rock record. The ice crystal mark occurrences in the Maroon Formation suggest that night frost affected lower elevation equatorial areas during the climax of the Late Paleozoic Ice Age and may stimulate research on evolutionary adaptations of early terrestrial biota to overcome significant air temperature fluctuations.