Origin and stratigraphic architecture of the Middle Permian lower and upper Quanzijie low-order cycles, Bogda Mountains, NW China

This study is carried out in the Bogda Mountains, NW China, and aims to provide a detailed sedimentary, stratigraphic, environmental, and paleoclimatic reconstruction in the paleo-mid-latitude along the east coast of Pangea. The Middle Permian lower and upper Quanzijie low-order cycles (QZJ LCs) provide a nearly complete sedimentary record in an area were little is known. In the first part of this work, the previously-established QZJ LC is divided into two LCs on the basis of regional stratigraphic correlation and major changes in depositional environments and tectonic and climatic processes. The newly-established lower QZJ LC is mainly composed of meandering streams and associated overbank deposits that formed under a semiarid climate with strong precipitation seasonality and its delineation demonstrates that a process-response approach is effective in time-stratigraphic analysis of complex nonmarine fluvial-lacustrine strata. In the second part of this work, the depositional environments of the upper QZJ LC are interpreted as a mixture of meandering and ephemeral stream and loess deposits. Loess in the region is poorly-documented and its presence indicates persistent arid to semiarid conditions during upper QZJ LC. The uppermost part of the upper QZJ LC records a major climatic change to humid-subhumid as a result of the global icehouse to hothouse transition. Finally, the fluvial conglomerates and sandstones in the lower and upper QZJ LCs are interpreted as being derived from the northern Tian Shan volcanic arc to the south and from local rift shoulders. This study provides a detailed sedimentologic, stratigraphic, environmental, and paleoclimatic reconstruction in an area were little is known and aims to provide a record in an important time period in the Earth\u27s history when major climatic and biotic changes occurred. --Abstract, page iv

Nonmarine Time-Stratigraphy in a Rift Setting: An Example from the Mid-Permian Lower Quanzijie Low-Order Cycle Bogda Mountains, NW China

Sedimentological and stratigraphic studies of seven stratigraphic sections of Permian Hongyanchi (HYC) and Quanzijie (QZJ) low-order cycles (LCs) in the Tarlong-Taodonggou half graben and Dalongkou area in Bogda Mountains, NW China, demonstrate effective approaches and methodology in cyclo- and time-stratigraphic analyses of complex fluvial-lacustrine deposits in an intracontinental rift setting. A new synchronous stratigraphic unit, the lower QZJ LC is defined. The lower and upper boundaries of this cycle include a regionally correlative disconformity, erosional unconformity, and conformity, across which significant and abrupt changes in palaeoenvironments and tectonic and climatic conditions occurred. The lower boundary is an erosional unconformity and disconformity with a high-relief topography that juxtaposes lacustrine deposits of the underlying HYC LC with the overlying meandering stream deposits of the lower QZJ LC, and was caused by a regional tectonic uplift. The upper boundary is a disconformity and local erosional unconformity and conformity, juxtaposing stacked paleosols developed on fluvial sediments with overlying fluvial and loessial deposits of the upper QZJ LC. The paleosols indicate landscape stability and a prolonged period of subaerial exposure and minimal deposition and suggest that climatic conditions were semi-arid with strong precipitation seasonality in the Tarlong-Taodonggou half graben and subhumid in the Dalongkou area. The fluvial-loessial deposits indicate a renewed tectonic uplift and a change in the atmospheric circulation pattern. The newly-defined lower QZJ LC facilitates accurate palaeogeographic reconstruction in the study area during a period of major tectonic and climatic changes. The interpreted tectonic and climatic conditions provide a critical data point in the mid-latitude east coast of NE Pangea during the Mid-Permian icehouse-hothouse transition. The results demonstrate that a process-response approach is effective in time-stratigraphic analysis of complex fluvial-lacustrine strata in a highly-partitioned rift basin

Petrographic Comparison and Contrast of Fluvial and Deltaic Sandstones, Upper Pennsylvanian Oread Cyclothem, NE Oklahoma

Upper Pennsylvanian sandstones have been the target in petroleum exploration and production in Kansas and Oklahoma. However, the petrographic characteristics of these sandstones have been rarely reported. A good understanding of the characteristics and depositional environments of those sandstones will help explore the correlative subsurface petroleum reservoirs efficiently

Incoherency in Central American Hydroclimate Proxy Records Spanning the Last Millennium

Continued Global Warming is Expected to Result in Reduced Precipitation and a Drier Climate in Central America. Projections of Future Changes Are Highly Uncertain, However, Due to the Spatial Resolution Limitations of Models and Insufficient Observational Data Coverage Across Space and Time. Paleoclimate Proxy Data Are Therefore Critical for Understanding Regional Climate Responses during Times of Global Climate Reorganization. Here We Present Two Lake-Sediment based Records of Precipitation Variability in Guatemala Along with a Synthesis of Central American Hydroclimate Records Spanning the Last Millennium (800–2000 CE). the Synthesis Reveals that Regional Climate Changes Have Been Strikingly Heterogeneous, even over Relatively Short Distances. Our Analysis Further Suggests that Shifts in the Mean Position of the Intertropical Convergence Zone, Which Have Been Invoked by Numerous Studies to Explain Variability in Central American and Circum-Caribbean Proxy Records, Cannot Alone Explain the Observed Pattern of Hydroclimate Variability. Instead, Interactions between Several Ocean-Atmosphere Processes and their Disparate Influences Across Variable Topography Appear to Have Resulted in Complex Precipitation Responses. These Complexities Highlight the Difficulty of Reconstructing Past Precipitation Changes Across Central America and Point to the Need for Additional Paleo-Record Development and Analysis Before the Relationships between External Forcing and Hydroclimate Change Can Be Robustly Determined. Such Efforts Should Help Anchor Model-Based Predictions of Future Responses to Continued Global Warming

A Prograding Margin during Global Sea-Level Maxima: An Example from Mahajanga Basin, Northwest Madagascar

The Mesozoic shelf margin in the Mahajanga Basin, northwest Madagascar, provides an example where inherited palaeobathymetry, coupled with sea-level changes, high sediment supply and fluctuations in accommodation influenced the stacking patterns and geometry of clinoforms that accreted onto a passive rifted margin. Two-dimensional (2D) seismic profiles are integrated with existing field data and geological maps to study the evolution of the margin. The basin contains complete records of transgression, highstand, regression and lowstand phases that took place from Jurassic to Cretaceous. Of particular interest is the Cretaceous, Albian to Turonian (ca. 113-93 Ma), siliciclastic shelf margin that prograded above a drowned Middle Jurassic carbonate platform. The siliciclastic phase of the shelf margin advanced ca. 70 km within ca. 20 My, and contains 10 distinct clinoforms mapped along a 2D seismic reflection data set. The clinoforms show a progressive decrease in height and slope length, and a fairly constant slope gradient through time. The successive shelf edges begin with a persistent flat to slightly downward-directed shelf-edge trajectory that changes to an ascending trajectory at the end of clinoform progradation. The progressive decrease in clinoform height and slope length is attributed to a decrease in accommodation. The prograding margin is interpreted to have formed when siliciclastic input increased as eastern Madagascar was uplifted. This work highlights the importance of sediment supply and inherited palaeobathymetry as controls on the evolution of shelf margins and it provides a new understanding of the evolution of the Mahajanga Basin during the Mesozoic

Sedimentological And Geochemical Characterization Of A Varved Sediment Record From The Northern Neotropics

Annually resolved sedimentological records (including annual varves) can be used to develop precise chronologies for key climatic and tectonic events. Varved records, however, are most common in high latitude lakes, resulting in a spatial bias with respect to annually resolved records in tropical regions. Here we report on the sedimentology of two sediment cores from Lake Izabal, eastern Guatemala, that contain a well-preserved thinly laminated section spanning ca. 2200 years of the mid-Holocene. We integrate radiocarbon age-depth modeling, sedimentological observations, laminae counting, µX-ray fluorescence scanning, and multivariate statistical analyses to constrain the nature and chronology of the laminations. Our sedimentological and geochemical results suggest that the alternating clastic (dark) and biogenic (light) laminae couplets were deposited annually. Dark laminae are characterized by an abundance of detrital grains, organic detritus, total organic carbon, and terrigenic elements, and most likely formed during times of increased discharge during the rainy season. In contrast, light laminae are characterized by a decrease in detrital grains and total organic carbon, and an increase in biogenic silica constituents, and were likely deposited at times of increased lake productivity during the dry season. We compare a floating varve chronology that spans ca. 2200 years with three radiocarbon-based age-depth models. Consistency between the varve chronology and one of the models partially supports the annual character of the laminated section in Lake Izabal. This laminated section, one of the first annually resolved sedimentological records from Central America, can help explore mid-Holocene hydroclimate variability and regional tectonic processes in this understudied region

Timing of Deformation along the Iron Springs Thrust, Southern Sevier Fold-and-Thrust Belt, Utah: Evidence for an Extensive Thrusting Event in the mid-Cretaceous

The temporal and spatial distribution of strain associated with the Sevier orogeny in western North America is significantly different in the southern end of the belt, at the latitude of Las Vegas, Nevada, than farther to the north at the latitude of Salt Lake City, Utah. Reasons for these differences have been speculative as a lack of temporal constraints on thrusting in the intervening region hindered along-strike correlation across the belt. We determined a crystallization age of 100.18 ± 0.04 Ma for zircons extracted from a recently recognized dacite lapilli ash-fall tuff near the base of the synorogenic Iron Springs Formation. We propose the name “Three Peaks Tuff Member” for this unit, and identify a type stratigraphic section on the western flank of the “Three Peaks,” a topographic landmark in Iron County, Utah. Field relationships and this age constrain movement on the Iron Springs thrust and the end of the sub-Cretaceous unconformity in the critical intervening area to latest Albian/earliest Cenomanian. Movement on the Iron Springs thrust was synchronous with movement on multiple Sevier thrusts at ~100 Ma, indicating that the mid-Cretaceous was a period of extensive thrust-fault movement. This mid-Cretaceous thrusting event coincided with a period of global plate reorganization and increased convergence, and hence an increased subduction rate for the Farallon Plate beneath North America. The accelerated subduction contributed to a Cordilleran arc flare-up event and steepening of the orogenic wedge, which triggered widespread thrusting across the retroarc Sevier deformation belts. Additionally, based on temporal constraints and the strong spatial connection of mid-Cretaceous thrusts to lineaments interpreted as pre-orogenic transform faults, we suggest that temporal and spatial variations along the strike of the orogenic belt reflect tectonic inheritance of basement structures associated with the edge of the rifted Precambrian craton

Distinct Modes Of Aged Soil Carbon Export In A Large Tropical Lake Basin Identified Using Bulk And Compound-Specific Radiocarbon Analyses Of Fluvial And Lacustrine Sediment

The 14C content of sedimentary organic matter (OM) and specific organic molecules provide valuable information on the source and age of OM stored in sediments, but these data are limited for tropical fluvial and lake sediments. We analyzed 14C in bulk OM, palmitic acid (C16), and long-chain n-alkanoic acids (C24, C26, and C28), within fluvial and lake sediments in the catchment of Lake Izabal, a large tectonic lake basin in Guatemala. We combined these measurements with bulk and compound-specific δ13C measurements, as well as sediment organic carbon to nitrogen (OC:N) ratios, to understand the source and age of sedimentary OM in different regions of the lake catchment. Most fatty acid and bulk OM samples were characterized by pre-modern carbon, indicating important input of aged carbon with residence times of hundreds to thousands of years into sediments. We identified two mechanisms leading to aged carbon export to sediments. In the high-relief and deforested Polochic catchment, older OM and fatty acids are associated with low % total organic carbon (TOC) and low OC:N, indicating aged OM associated with eroded mineral soil. In the smaller, low-relief, and largely forested Oscuro catchment, old OM and fatty acids are associated with high %TOC and high OC:N ratios, indicating export of undegraded aged plant biomass from swamp peat. The age of bulk OM and fatty acids in Lake Izabal sediments is similar to the ages observed in fluvial sediments, implying that fluvial input of aged soil carbon makes an important contribution to lake sediment carbon reservoirs in this large tropical lake

Incoherency in Central American hydroclimate proxy records spanning the last millennium

During the last 40 years, precipitation across much of Central America has decreased, creating problems for a region that depends heavily on rainfall for agriculture. Observations suggest, however, that precipitation changes are spatially complex with large differences over relatively short distances. This differs from climate model projections that indicate uniform drying for the entire Central American region. Using precipitation records derived from sediment and speleothems, we set out to explore whether over the past millennium Central American hydroclimate was characterized by relatively uniform changes across the region or if more complex patterns of hydroclimate change occurred. We acquired two new hydroclimate records from lakes in Guatemala and compare them with existing records from Central America. Our analysis indicates that precipitation has been highly variable across space and time and suggests that complex spatiotemporal patterns of precipitation variability in response to external forcing should be expected in the region

Sedimentology, Depositional Systems, and Sequence Stratigraphy of Mixed Fluvial-Eolian-Lacustrine Deposits in an Intracontinental Rift Basin, Bogda Mountains, NW China

The Greater Turpan-Junggar Basin in NW China is a mature petroliferous basin that has been extensively explored and exploited for hydrocarbons. Future development will require a thorough understanding of the basin fill and evolution. However, the complexity of the basin fill, with abundant erosional surfaces and regional thickness and lithological changes, creates a challenge for such understanding. The study focuses on the sedimentology, environmental interpretation, and sequence stratigraphy of mid-Permian (Capitanian) fluvial-eolian-lacustrine deposits of the Quanzijie Formation exposed in the Bogda Mountains. These superbly exposed strata offer a unique opportunity to interpret the processes and factors controlling basin filling, such as tectonics, climate, provenance, and conditions of the catchment basin. Five complete stratigraphic sections of the Quanzijie Formation have been measured in the southern and northern slopes of the Bogda Mountains. Sedimentary processes vary greatly between the two areas; fluvial-eolian deposits dominate in the south, and fluvial deposits dominate in the north. Nevertheless, both areas recorded similar tectonic and climatic changes. Evidence from sandstone petrography suggests that the two areas had different provenance based on compositional variations. However, detrital zircon age dates suggest a similar provenance for the two areas. Additional petrographic and geochemical data is needed to understand the roles of provenance, tectonic, climatic, and depositional processes in the deposition of Quanzijie. The strategies developed in this study will be used to understand the evolution of the Greater Turpan-Junggar basin and other similar basins around the world