Majie Fan

Flexural subsidence by 29 Ma on the NE edge of Tibet from the magnetostratigraphy of Linxia Basin, China

Abstract This study provides a detailed magnetostratigraphic record of subsidence in the Linxia Basin, documenting a 27 Myr long sedimentary record from the northeastern edge of the Tibetan Plateau. Deposition in the Linxia Basin began at ∼29 Ma and continued nearly uninterruptedly until ∼1.7 Ma. Increasing rates of subsidence between 29 and 6 Ma in the Linxia Basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to Late Oligocene–Late Miocene time. By Late Miocene–Early Pliocene time, a decrease in subsidence rates in the Linxia Basin associated with thrust faulting and a ∼10° clockwise rotation in the basin indicates that the deformation front of the Tibetan plateau had propagated into the currently deforming region northeast of the plateau.

Tectonic uplift and sedimentary evolution of the Jiuxi Basin in the northern margin of the Tibetan Plateau since 13 Ma BP

Sediments shed from the northern margin of the Tibetan Plateau, the Qilian Mountains, are widely deposited in the foreland basin, the Jiuxi Basin, archiving plenty of information about the mountain surface uplift and erosion history. The Laojunmiao section, 1960 m thick, representing the upper sequence of the Cenozoic basin sediments, is paleomagnetically dated to about 13-0 Ma BP. Detailed sedimentary study of this sequence has revealed five sedimentary facies associations which determine four stages of sedimentary environment evolution. They are: (I) the half-deep lake system before 12.18 Ma BP, (II) the shallow lake system between 12.18 and 8.26 Ma BP, (III) the fan delta dominated sedimentary system in dry climate between 8.26 and 6.57 Ma BP, and (IV) alluvial fan system since 6.57 Ma BP. The associated mountain erosion and uplift are suggested to have experienced three phases, that is, tectonic stable (13-8.26 Ma BP), gradual uplift (8.26-<4.96 Ma BP), and rapid intermittent uplift (>3.66-0 Ma BP). The uplift at ∼3.66 Ma BP is of great importance in tectonics and geomorphology. Since then, tectonic uplift and mountain building have been accelerated and become strong intermittent. At least three significant tectonic events took place with ages at <1.80-1.23, 0.93-0.84 and 0.14 Ma BP, respectively. Thus, the uplift of the northern Tibetan Plateau is a complex process of multiple phases, unequal speed and irregular movements.

Sedimentology, detrital zircon geochronology, and stable isotope geochemistry of the lower Eocene strata in the Wind River Basin, central Wyoming

Shallow subduction of the Farallon plate beneath the western United States has been commonly accepted as the tectonic cause for the Laramide deformation during Late Cretaceous through Eocene time. However, it remains unclear how shallow subduction would produce the individual Laramide structures. Critical information about the timing of individual Laramide uplifts, their paleoelevations at the time of uplift, and the temporal relationships among Laramide uplifts have yet to be documented at regional scale to address the question and evaluate competing tectonic models. The Wind River Basin in central Wyoming is fi lled with sedimentary strata that record changes of paleogeog raphy and paleoelevation during Laramide deformation. We conducted a multidisciplinary study of the sedimentology, detrital zircon geochronology, and stable isotopic geochemistry of the lower Eocene Indian Meadows and Wind River formations in the northwestern corner of the Wind River Basin in order to improve understanding of the timing and process of basin evolution, source terrane unroofi ng, and changes in paleoelevation and paleoclimate. Depositional environments changed from alluvial fans during deposition of the Indian Meadows Formation to low-sinuosity braided river systems during deposition of the Wind River Formation. Paleocurrent directions changed from southwestward to mainly eastward through time. Conglomerate and sandstone compositions suggest that the Washakie and/or western Owl Creek ranges to the north of the basin experienced rapid unroofi ng ca. 55.5‐54.5 Ma, producing a trend of predominantly Mesozoic clasts giving way to Precambrian basement clasts upsection. Rapid source terrane unroofi ng is also suggested by the upsection changes in detrital zircon U-Pb ages. Detrital zircons in the upper Wind River Formation show age distributions similar to those of modern sands derived from the Wind River Range, with up to ~20% of zircons derived from the Archean basement rocks in the Wind River Range, indicating that the range was largely exhumed by ca. 53‐51 Ma. The rise of these ranges by 51 Ma formed a confi ned valley in the northwestern part of the basin, and promoted development of a meandering fl uvial system in the center of the basin. The modern paleodrainage confi guration was essentially established by early Eocene time. Carbon isotope data from paleosols and modern soil carbonate show that the soil CO 2 respiration rate during the early Eocene was higher than at present, from which a more humid Eocene paleoclimate is inferred. Atmosphere pCO 2 estimated from paleosol carbon isotope values decreased from 2050 ± 450 ppmV to 900 ± 450 ppmV in the early Eocene, consistent with results from previous studies. Oxygen isotope data from paleosol and fl uvial cement carbonates show that the paleoelevation of the Wind River Basin was comparable to that of the modern Great Plains (~500 m above sea level), and that local relief between the Washakie and Wind River ranges and the basin fl oor was 2.3 ± 0.8 km. Up to 1 km of post-Laramide regional net uplift is required to form the present landscape in central Wyoming.

Middle Cenozoic uplift and concomitant drying in the central Rocky Mountains and adjacent Great Plains

When and how the central Rocky Mountains (Rockies) of western North America gained modern topography remain controversial questions. We reconstruct the middle and late Cenozoic topography along a transect that extends from the Great Plains of western Nebraska across the central Rockies in Wyoming, based on reconstructed surface-water δ D values from volcanic glass δ D values. Our data show gradual increases of surface-water δ D values in both the central Rockies and adjacent Great Plains during middle and late Cenozoic time, and the establishment of a similar-to-present surface-water δ D gradient between the central Rockies and western Great Plains before earliest Oligocene time. These observations suggest that the region underwent differential uplift to form relief similar to that of today before earliest Oligocene time. This uplift has caused regional drying and the gradual increase of surface-water δ D values over the past 35 m.y. When placed in the context of other paleoaltimetry studies, our work suggests that along our transect, the central Rockies and adjacent Great Plains underwent uplift during the late Eocene, and have not undergone any large-magnitude (>∼500 m) uplift since that time.

Terrestrial paleoenvironmental reconstructions indicate transient peak warming during the early Eocene climatic optimum

Major changes in climate and ecology occurred during the early Eocene climatic optimum, sometime between 52 and 50 Ma. Recent work suggests that the timing and duration of the event are characterized by different responses in the marine and terrestrial realms, and that traditional causal mechanisms may not adequately explain such differences. We applied high-resolution paleopedology, geochemical analysis, and phytolith biostratigraphy techniques to paleosol suites within the well-described Wind River Formation of western Wyoming, USA. This multiproxy record indicates a short (<1 m.y.) peak period of carbon isotopic enrichment (up to 2‰ higher) and elevated pCO2, high temperatures (up to 8 °C higher), increased precipitation (up to 500 mm yr –1 higher), and shifts in fl oral composition (up to 10%). Terrestrial climatic and ecological changes of this kind during the early Eocene climatic optimum are consistent with changes in contemporaneous records that have been ascribed to high atmospheric pCO2, but a transient peak interval suggests that the cause of high atmospheric pCO2 during the early Eocene was likely not increased volcanism or decreased silicate weathering, which operate on longer timescales. Instead, terrestrial records from across western North America agree that early Eocene climatic optimum changes may have been caused by other sources, such as a combination of increased ventilation of oceanic carbon and increased petroleum generation in sedimentary basins. The climatic and environmental changes exhibited by this and other North American terrestrial records also defi ne a pattern of regional response that is relevant for understanding the impacts of global climate change events.

Widespread basement erosion during the late Paleocene-early Eocene in the laramide rocky mountains inferred from 87sr/86sr ratios of freshwater bivalve fossils

Reconstruction of the timing and pattern of exhumation of basement-involved uplifts during the Laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region. We use reconstructions of the 87 Sr/ 86 Sr ratios of Late Cretaceous–early Cenozoic river water from fossil shells in six basins of the Rocky Mountains contextualized by modern river-water Sr chemistry studies to trace the erosion of Precambrian basement cores in the Laramide ranges. The 87 Sr/ 86 Sr ratios and Sr concentration of modern river water in the Rocky Mountains are controlled by river bedrock lithology. Weathering of Precambrian silicate rocks in the cores of Laramide ranges produces high 87 Sr/ 86 Sr ratios in highland rivers. Weathering of Paleozoic and Mesozoic carbonates along the basin margins reduces the 87 Sr/ 86 Sr ratios and increases Sr concentration of rivers as they flow basinward. Lowland rivers that have headwaters in Precambrian basement mostly have 87 Sr/ 86 Sr ratios >0.711, whereas rivers confined to or with very long reaches in basins have 87 Sr/ 86 Sr ratios between 0.709 and 0.711. River-water δ 18 O values do not change in response to changes in catchment elevation. Our results from fossil shells show that carbonate weathering dominated the river-water Sr chemistry in the Alberta Foreland, Williston, and Crazy Mountains Basins, and that Proterozoic low-grade metamorphic carbonates in the Belt-Purcell Supergroup were not exposed in the Canadian Rocky Mountains during Late Cretaceous–early Paleocene time. Silicate weathering influenced the Sr chemistry of surface waters in the Powder River Basin during the late Paleocene and the Washakie Basin during the early Eocene, suggesting Precambrian silicate basement rock was extensively exhumed and eroded. The observation that Precambrian basement was eroded earlier around the Powder River Basin than around the Washakie Basin is consistent with a previous study that suggests Laramide ranges in northeast Wyoming reached high elevation earlier than the ranges adjacent to the Sevier thrust belt. Widespread basement erosion in late Paleocene–early Eocene time was controlled by tectonic exhumation, and may have been intensified by the wet and warm global climate.

Middle Cenozoic diachronous shift to eolian deposition in the central Rocky Mountains: Timing, provenance, and significance for paleoclimate, tectonics, and paleogeography

Eolian sedimentation was widespread in the Rocky Mountains (Rockies herein) during the middle and late Cenozoic. Although changes to eolian depositional environment have significance for tectonics, paleoclimate, and paleogeography, little is known regarding the timing of initiation and the provenance of these eolian sedimentary rocks in the Rockies. Here we study the timing of a transition to eolian depositional environments in the central Rockies and the adjacent Great Plains during the middle Cenozoic, and use sandstone petrography and detrital zircon U-Pb geochronology to constrain the provenance of the eolian sedimentary rocks. Our samples have compositions of Qm44F27Lt29 (Qm is monocrystalline quartz, F is feldspar, and Lt is total lithics), Q47F27L26 (Q is total quartz, L is total non-quartzose lithics), and Qm64P28K8 (P is plagioclase and K is potassic feldspar), and zircon age populations of 17–44 , 45–218 Ma, 220–708 Ma, 948–1326 Ma, 1332–1816 Ma, and 1825–3314 Ma. The youngest zircon population was derived from distal volcanism in western and southwestern North America, and the other populations were derived directly from local Precambrian basements on Laramide ranges and recycled from Paleozoic–lower Cenozoic strata distributed along flanks of Laramide ranges and on the Sevier hinterland. The maximum depositional ages, based on the mean U-Pb ages of the youngest clusters of zircon grains, are generally consistent with the available ash radiometric dates for the latest Eocene–early Miocene samples, confirming that detrital zircon maximum depositional ages can be used to constrain depositional ages when ash beds or dateable minerals in ash beds were not present and when synchronous magmatic activity was intense. The occurrence of eolian deposition initiated during the latest Eocene–early Oligocene and became younger eastward, suggesting eastward progressive drying in the central Rockies. The diachronous drying may have resulted from the combined effect of renewed uplift of the Cordilleran hinterland and central Rockies during the late Eocene and global cooling at the Eocene-Oligocene boundary. The provenance data presented here suggest that during the latest Eocene–early Oligocene, the westerlies and possibly the dry summer monsoon winds transported unlithified fluvial sediments and pyroclastic materials eastward and northeastward, and formed massive eolian deposits in the central Rockies and the adjacent Great Plains. The eolian sedimentation continued into the Miocene and largely blanketed the Precambrian basement cores on Laramide ranges. The unlithified Oligocene eolian sediments were further eroded and recycled into the latest Oligocene–Miocene eolian sedimentary rocks.

Paleozoic sediment dispersal before and during the collision between Laurentia and Gondwana in the Fort Worth Basin, USA

We report detrital zircon U-Pb ages in the Fort Worth Basin (southern USA) aimed at understanding sediment dispersal patterns on the southern margin of Laurentia before and during the Laurentia-Gondwana collision. The ages from two Cambrian fluvial-marginal marine sandstone and six Pennsylvanian deltaic-fluvial sandstone samples span from Archean to early Paleozoic time. In the Cambrian sandstones, 80% of zircons are of Mesoproterozoic age (1.451– 1.325 Ga) and 18% are of Grenvillian age. The high abundance of the Mesoprotero zoic population suggests that the grains were dispersed by a local river draining the midcontinent granite-rhyolite province located in the Texas Arch to the northwest of the Fort Worth Basin. In the Pennsylvanian sandstones, 26% of zircons are of Archean–early Mesoproterozoic age, 47% are of Grenvillian age, 15% are of Neoproterozoic–earliest Paleozoic age (800–500 Ma), and 10% are of early Paleozoic age (500–318 Ma), indicating a different dispersal pattern during the Pennsylvanian relative to the Cambrian. Compared to other early Paleozoic detrital zircon records on the southern margin of Laurentia, our Pennsylvanian sandstones have a distinct age peak at ca. 650–550 Ma, which we interpreted to be a result of transport by local rivers draining a peri-Gondwana terrane, most likely the Sabine terrane in the Ouachita orogen. The high abundance of Grenvillian zircons reflects either direct transport from the Appa lachians by an axial river or recycling from Mississippian–Pennsylvanian sedimentary rocks incorporated in the Ouachita orogenic front. The similarity of detrital zircon age distributions in the Fort Worth Basin, the Arkoma Basin, and the southern Appalachian forelands seems to favor sediment dispersal by a major river with headwaters in the southern Appalachians.

Hydrogen isotope measurement of bird feather keratin, one laboratory's response to evolving methodologies

Hydrogen in organic tissue resides in a complex mixture of molecular contexts. Some hydrogen, called non-exchangeable (Hnon), is strongly bound, and its isotopic ratio is fixed when the tissue is synthesized. Other pools of hydrogen, called exchangeable hydrogen (Hex), constantly exchange with ambient water vapor. The measurement of the δ2Hnon in organic tissues such as hair or feather therefore requires an analytical process that accounts for exchangeable hydrogen. In this study, swan feather and sheep wool keratin were used to test the effects of sample drying and capsule closure on the measurement of δ2Hnon values, and the rate of back-reaction with ambient water vapor. Homogenous feather or wool keratins were also calibrated at room temperature for use as control standards to correct for the effects of exchangeable hydrogen on feathers. Total δ2H values of both feather and wool samples showed large changes throughout the first ∼6 h of drying. Desiccant plus low vacuum seems to be more effective than room temperature vacuum pumping for drying samples. The degree of capsule closure affects exchangeable hydrogen equilibration and drying, with closed capsules responding more slowly. Using one control keratin standard to correct for the δ2Hex value for a batch of samples leads to internally consistent δ2Hnon values for other calibrated keratins run as unknowns. When placed in the context of other recent improvements in the measurement of keratin δ2Hnon values, we make recommendations for sample handing, data calibration and the reporting of results.

Late Paleozoic Subsidence and Burial History of the Fort Worth Basin

ABSTRACT The Fort Worth basin in northcentral Texas is a major shale-gas producer, yet its subsidence history and relationship to the Ouachita fold-thrust belt have not been well understood. We studied the depositional patterns of the basin during the late Paleozoic by correlating well logs and constructing structure and isopach maps. We then modeled the one-dimensional (1-D) and two-dimensional subsidence history of the basin and constrained its relationship to the Ouachita orogen. Because the super-Middle Pennsylvanian strata were largely eroded in the region, adding uncertainty to the subsidence reconstruction, we used PetroMod 1-D to conduct thermal-maturation modeling to constrain the post-Middle Pennsylvanian burial and exhumation history by matching the modeled vitrinite reflectance with measured vitrinite reflectance along five depth profiles. Our results of depositional patterns show that the tectonic uplift of the Muenster uplift to the northeast of the basin influenced subsidence as early as the Middle Mississippian, and the Ouachita orogen became the primary tectonic load by the late Middle Pennsylvanian when the depocenter shifted to the east. Our results show that the basin experienced 3.7–5.2 km (12,100–17,100 ft) of burial during the Pennsylvanian, and the burial depth deepens toward the east. We attributed the causes of deep Pennsylvanian burial and its spatial variation to flexural subsidence that continued into the Late Pennsylvanian in response to the growth of the Ouachita orogen and southeastward suturing of Laurentia and Gondwana. The modeling results also suggest that the Mississippian Barnett Shale reached the gas maturation window during the Middle–Late Pennsylvanian.